Modeling and optimal designs for dislocation and radiation tolerant single and multijunction solar cells

NASA Astrophysics Data System (ADS)

Mehrotra, A.; Alemu, A.; Freundlich, A.

2011-02-01

Crystalline defects (e.g. dislocations or grain boundaries) as well as electron and proton induced defects cause reduction of minority carrier diffusion length which in turn results in degradation of efficiency of solar cells. Hetro-epitaxial or metamorphic III-V devices with low dislocation density have high BOL efficiencies but electron-proton radiation causes degradation in EOL efficiencies. By optimizing the device design (emitter-base thickness, doping) we can obtain highly dislocated metamorphic devices that are radiation resistant. Here we have modeled III-V single and multi junction solar cells using drift and diffusion equations considering experimental III-V material parameters, dislocation density, 1 Mev equivalent electron radiation doses, thicknesses and doping concentration. Thinner device thickness leads to increment in EOL efficiency of high dislocation density solar cells. By optimizing device design we can obtain nearly same EOL efficiencies from high dislocation solar cells than from defect free III-V multijunction solar cells. As example defect free GaAs solar cell after optimization gives 11.2% EOL efficiency (under typical 5x1015cm-2 1 MeV electron fluence) while a GaAs solar cell with high dislocation density (108 cm-2) after optimization gives 10.6% EOL efficiency. The approach provides an additional degree of freedom in the design of high efficiency space cells and could in turn be used to relax the need for thick defect filtering buffer in metamorphic devices.

Research on the Composition and Distribution of Organic Sulfur in Coal.

PubMed

Zhang, Lanjun; Li, Zenghua; Yang, Yongliang; Zhou, Yinbo; Li, Jinhu; Si, Leilei; Kong, Biao

2016-05-13

The structure and distribution of organic sulfur in coals of different rank and different sulfur content were studied by combining mild organic solvent extraction with XPS technology. The XPS results have shown that the distribution of organic sulfur in coal is related to the degree of metamorphism of coal. Namely, thiophenic sulfur content is reduced with decreasing metamorphic degree; sulfonic acid content rises with decreasing metamorphic degree; the contents of sulfate sulfur, sulfoxide and sulfone are rarely related with metamorphic degree. The solvent extraction and GC/MS test results have also shown that the composition and structure of free and soluble organic sulfur small molecules in coal is closely related to the metamorphic degree of coal. The free organic sulfur small molecules in coal of low metamorphic degree are mainly composed of aliphatic sulfides, while those in coal of medium and high metamorphic degree are mainly composed of thiophenes. Besides, the degree of aromatization of organic sulfur small molecules rises with increasing degree of coalification.

Highly Transparent Compositionally Graded Buffers for New Metamorphic Multijunction Solar Cell Designs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schulte, Kevin L.; France, Ryan M.; Geisz, John F.

The development of compositionally graded buffer layers (CGBs) with enhanced transparency would enable novel five and six junction solar cells, with efficiencies approaching 50% under high concentration. Here, we demonstrate highly transparent grades between the GaAs and InP lattice constants on both A- and B-miscut GaAs substrates, employing Al xGayIn 1-x-yAs and highly Se-doped Burstein-Moss (BM) shifted Ga xIn 1-xP. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted Ga xIn 1-xP on B-miscut substrates and Al xGayIn 1-x-yAs/Ga xIn 1-xP(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibitmore » Woc = 0.41 V at mA/ cm 2, performance comparable with the state-of-the-art Ga xIn 1-xP grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A GaAs/0.74cV GaInAs tandem cell was grown with a transparent BM-shifted Ga xIn 1-xP CGB to verify the CGB performance in a multijunction device structure. Quantum efficiency measurements indicate that the CGB is completely transparent to photons below the GaAs bandedge, validating its use in 4-6 junction IMM devices with a single-graded buffer. Furthermore, this tandem represents a highly efficient two-junction band gap combination, achieving 29.6% ± 1.2% efficiency under the AM1.5 global spectrum, demonstrating how the additional transparency enables new device structures.« less

Highly Transparent Compositionally Graded Buffers for New Metamorphic Multijunction Solar Cell Designs

DOE PAGES

Schulte, Kevin L.; France, Ryan M.; Geisz, John F.

2016-11-11

The development of compositionally graded buffer layers (CGBs) with enhanced transparency would enable novel five and six junction solar cells, with efficiencies approaching 50% under high concentration. Here, we demonstrate highly transparent grades between the GaAs and InP lattice constants on both A- and B-miscut GaAs substrates, employing Al xGayIn 1-x-yAs and highly Se-doped Burstein-Moss (BM) shifted Ga xIn 1-xP. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted Ga xIn 1-xP on B-miscut substrates and Al xGayIn 1-x-yAs/Ga xIn 1-xP(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibitmore » Woc = 0.41 V at mA/ cm 2, performance comparable with the state-of-the-art Ga xIn 1-xP grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A GaAs/0.74cV GaInAs tandem cell was grown with a transparent BM-shifted Ga xIn 1-xP CGB to verify the CGB performance in a multijunction device structure. Quantum efficiency measurements indicate that the CGB is completely transparent to photons below the GaAs bandedge, validating its use in 4-6 junction IMM devices with a single-graded buffer. Furthermore, this tandem represents a highly efficient two-junction band gap combination, achieving 29.6% ± 1.2% efficiency under the AM1.5 global spectrum, demonstrating how the additional transparency enables new device structures.« less

Inheritance, Variscan tectonometamorphic evolution and Permian to Mesozoic rejuvenations in the metamorphic basement complexes of the Romanian Carpathians revealed by monazite microprobe geochronology

NASA Astrophysics Data System (ADS)

Săbău, Gavril; Negulescu, Elena

2014-05-01

Monazite U-Th-Pb chemical dating reaches an acceptable compromise between precision and accuracy on one side, and spatial resolution and textural constraints on the other side. Thus it has a powerful potential in testing the coherence of individual metamorphic basement units, and enabling correlations among them. Yet, sensitivity and specificity issues in monazite response to thermotectonic events, especially in the case of superposed effects, remain still unclear. Monazite dating at informative to detailed scale in the main metamorphic basement units of the Carpathians resulted in complex age spectra. In the main, the spectra are dominated by the most pervasive thermal and structural overprint, as checked against independent geochronological data. Post-peak age resetting is mostly present, but statistically subordinate. Resetting in case of superposed events is correlated with the degree of textural and paragenetic overprinting, inheritances being always indicated by more or less well-defined age clusters. The lack of relict ages correlating with prograde structural and porphyroblast zonation patterns is indicative for juvenile formations. Age data distribution in the Carpathians allowed distinction of pre-Variscan events, syn-metamorphic Variscan tectonic stacking of juvenile and reworked basement, post-Variscan differential tectonic uplift, as well as prograde metamorphic units ranging down to Upper Cretaceous ages. In the South Carpathians, the Alpine Danubian domain consists of several Variscan and Alpine thrust sheets containing a metamorphic complex dominated by Upper Proterozoic to Lower Cambrian metamorphic and magmatic ages (Lainici-Păiuş), and several complexes with metamorphic overprints ranging from Carboniferous to Lower Permian. Any correlation among these units, as well as geotectonic models placing a Lower Paleozoic oceanic domain between pre-existing Lainici-Păiuş and Drăgşan terranes are precluded by the age data. Other basement of the South Carpathians contain lower Paleozoic or older units intruded by Ordovician granitoids, imbricated with juvenile Variscan slivers, the structural sequence differing in individual basement complexes. So, in the Leaota Massif the lowermost term of the sequence is prograde Variscan, tectonically overlain by reworked lower Paleozoic gneisses, supporting thrust sheets with very low- to low-grade Variscan schists. In the Făgăraş Massif a lower Paleozoic (Cumpăna) complex bearing a strong Variscan overprint, straddles Variscan juvenile rocks, and the lowermost visible structural level is assumed by upper Carboniferous to Permian juvenile medium-grade metamorphic schists. In the Lotru Metamorphic Suite of the Alpine Getic Nappe, the Variscan stacking is overprinted by post-orogenic differential uplift, documented by the correlation among younging ages, structural and metamorphic low-pressure overprints, recording often higher metamorphic temperatures. The most spectacular structure is Upper Jurassic in age, contains high-grade metamorphic rocks and peraluminous anatectic granitoids, is outlined by a deformed boundary evolving from ductile to brittle regime during cooling, and induces a thermal overprint in the neighbouring rocks. In the basement units thrust over the Getic Nappe, the Sibişel unit yielded Permian prograde peak metamorphic ages and Triassic post-peak overprints, while an adjacent gneissic unit (Laz) delivered an exclusively Cretaceous age pattern. Unexpectedly young metamorphic ages resulted also for the East Carpathians and the Apuseni Mountains. While most of the ages obtained so far correspond to Variscan retrogression of older basement units, the lowermost structural unit of the infra-Bucovinian nappe system in the East Carpathians yielded Upper Cretaceous metamorphic ages in apparently monometamorphic medium-grade schists. In the Apuseni Mountains, schists of the Baia de Arieş Unit display an Upper Jurassic age spectrum, corresponding to a clearly prograde medium-grade event. The ages recorded not only question some of the currently accepted correlations among basement units, but urge to reconsideration of the way in which the basement-cover relationships are interpreted and extrapolated.

Metamorphic sole formation reveals plate interface rheology during early subduction

NASA Astrophysics Data System (ADS)

Mathieu, S.; Agard, P.; Dubacq, B.; Plunder, A.; Prigent, C.

2015-12-01

Metamorphic soles are m to ~500m thick tectonic slices welded beneath most large ophiolites. They correspond to highly to mildly deformed portions of oceanic lithosphere metamorphosed at amphibolite to granulite facies peak conditions. Metamorphic soles are interpreted as formed ≤1-2Ma after intraoceanic subduction initiation by heat transfer from the hot, incipient mantle wegde to the underthrusting lower plate. Their early accretion and exhumation together with the future ophiolite implies at least one jump of the subduction plate interface from above to below the metamorphic sole. Metamorphic soles thus represent one of the few remnants of the very early evolution of the subduction plate interface and provide major constraints on the thermal structure and the effective rheology of the crust and mantle along the nascent slab interface.We herein present a structural and petrological detailed description of the Oman and Turkey metamorphic soles. Both soles present a steep inverted metamorphic structure, with isograds subparallel to the peridotite contact, in which the proportion of mafic rocks, pressure and temperature conditions increase upward. They comprise, as most metamorphic soles worldwide, two main units: (1) a high-grade unit adjacent to the overlying peridotite composed of granulitized to amphibolized metabasalts, with rare metasedimentary interlayers (~800±100ºC at 10±2kbar) and (2) a low-grade greenschist facies unit composed of metasedimentary rocks with rare metatuffs (~500±100ºC at 5±2kbar). We provide for the first time refined P-T peak condition estimations by means of pseudosection modelling and maximum temperature constraints for the Oman low-grade sole by RAMAN thermometry. In order to quantify micro-scale deformations trough the sole, we also present EBSD data on the Oman garnet-bearing and garnet-free high-grade sole.With these new constraints, we finally propose a new conceptual mechanical model for metamorphic sole formation. This model excludes the presence of a continuous inverted metamorphic gradient through the sole but implies the stacking of several homogeneous slivers to constitute the present structure of the sole. These successive thrusts are the result of rheological changes as the plate interface progressively cools.

Development of inverted metamorphic isograds in the western metamorphic belt, Juneau, Alaska

USGS Publications Warehouse

Himmelberg, G.R.; Brew, D.A.; Ford, A.B.

1991-01-01

An inverted metamorphic gradient is preserved in the western metamorphic belt near Juneau, Alaska. Detailed mapping of pelitic single-mineral isograds, systematic changes in mineral assemblages, and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km. Silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9-11 kbar. Our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of tonalite sill magma. -from Authors

Design considerations for λ ˜ 3.0- to 3.5-μm-emitting quantum cascade lasers on metamorphic buffer layers

NASA Astrophysics Data System (ADS)

Rajeev, Ayushi; Sigler, Chris; Earles, Tom; Flores, Yuri V.; Mawst, Luke J.; Botez, Dan

2018-01-01

Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40 kA/cm2.

Characterization of Organic Materials in the Xenolithic Clasts in Sharps (H3.4) Meteorite Using Micro-Raman Spectroscopy

NASA Technical Reports Server (NTRS)

Chan, Q. H. S.; Zolensky, M. E.; Bodnar, R. J.; Kebukawa, Y.

2015-01-01

Graphitization of carbon is an irreversible process which alters the structure of graphitic materials in response to the increase in metamorphic grade (temperature and/or pressure). Carbonaceous materials offer a reliable geothermometer as their Raman spectra change systematically with increasing metamorphic grade. In this study, we identified carbonaceous materials in the xenolithic clasts in Sharps and interpreted their metamorphic history by revealing the structural organization (order) of the polyaromatic organic phases using micro-Raman spectroscopy.

Characterization of Organic Materials in the Xenolithic Clasts in Sharps (H3.4) Meteorite Using Microraman Spectroscopy

NASA Technical Reports Server (NTRS)

Chan, Q. H. S.; Zolensky, M. E.; Bodnar, R. J.; Kebukawa, Y.

2015-01-01

Graphitization of carbon is an irreversible process which alters the structure of graphitic materials in response to the increase in metamorphic grade (temperature and/or pressure). Carbonaceous materials offer a reliable geothermometer as their Raman spectra change systematically with increasing metamorphic grade [1-3]. In this study, we identified carbonaceous materials in the xenolithic clasts in Sharps and interpreted their metamorphic history by revealing the structural organization (order) of the polyaromatic organic phases using µ-Raman spectroscopy.

Oman metamorphic sole formation reveals early subduction dynamics

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Plunder, Alexis; Ildefonse, Benoît; Yamato, Philippe; Prigent, Cécile

2016-04-01

Metamorphic soles correspond to m to ~500m thick tectonic slices welded beneath most of the large-scale ophiolites. They typically show a steep inverted metamorphic structure where the pressure and temperature conditions of crystallization increase upward (from 500±100°C at 0.5±0.2 GPa to 800±100°C at 1.0±0.2 GPa), with isograds subparallel to the contact with the overlying ophiolitic peridotite. The proportion of mafic rocks in metamorphic soles also increases from the bottom (meta-sediments rich) to the top (approaching the ophiolite peridotites). These soles are interpreted as the result of heat transfer from the incipient mantle wedge toward the nascent slab (associated with large-scale fluid transfer and possible shear heating) during the first My of intra-oceanic subduction (as indicated by radiometric ages). Metamorphic soles provide therefore major constraints on early subduction dynamics (i.e., thermal structure, fluid migration and rheology along the nascent slab interface). We present a detailed structural and petrological study of the metamorphic sole from 4 major cross-sections along the Oman ophiolite. We show precise pressure-temperature estimates obtained by pseudosection modelling and EBSD measurements performed on both the garnet-bearing and garnet-free high-grade sole. Results allow quantification of the micro-scale deformation and highlight differences in pressure-temperature-deformation conditions between the 4 different locations, showing that the inverted metamorphic gradient through the sole is not continuous in all locations. Based on these new constraints, we suggest a new tectonic-petrological model for the formation of metamorphic soles below ophiolites. This model involves the stacking of several homogeneous slivers of oceanic crust leading to the present-day structure of the sole. In this view, these thrusts are the result of rheological contrasts between the sole and the peridotite as the plate interface progressively cools down. These slivers later underwent several stages of retrogression (partly mediated by ascending fluids from the slab) from amphibolite- to prehnite/pumpellite-facies conditions.

Protracted tectono-metamorphic history of the SE Superior Province : contribution of 40Ar/39Ar thermochronology in the Abitibi-Opatica contact zone, Québec, Canada

NASA Astrophysics Data System (ADS)

Daoudene, Yannick; Tremblay, Alain; Ruffet, Gilles; Leclerc, François; Goutier, Jean

2015-04-01

Archean orogens mainly consist of greenstone belts juxtaposing deeper crustal domains of TTG-type plutonic rocks. The greenstone belts show regional folds, penetrative steeply-dipping fabrics, and localised shear zones, whereas the plutonic belts predominantly display dome structures. Concurrently, rocks in Archean orogens undergone MT/HT-LP/MP metamorphic conditions that vary, from upper to lower crustal domains, between greenschist- and granulite-facies, respectively. These structural and metamorphic variations are well-documented, but modes of deformation related to such orogens is still debated. Some studies suggest that the Archean tectonic processes were comparable to present-day plate tectonics and the Archean greenstone belts were interpreted as tectonic collages commonly documented in Phanerozoic subduction/collision zones. Alternative models propose that the Archean tectonics were different from those predicted by the plate tectonics paradigm, mainly due to the existence of a hotter mantle and a mechanically weak crust. In such models, the burying and exhumation of crustal rocks are attributed to the vertical transfer of material, resulting in the development of pop-down and domes structures. As a contribution of the study of mechanisms that might have operated during the Archean, we present a structural and metamorphic study of the contact zone between the Abitibi subprovince (ASP), which contains greenstone belts, and the Opatica subprovince (OSP), which is dominated by plutonic rocks, of the Superior Province. The 40Ar/39Ar dating of amphiboles and micas is used to constrain the age and duration of regional metamorphism and associated deformations. On the basis of seismic profiling, showing a north-dipping lithospheric-scale reflector, the ASP-OSP contact has been interpreted as the surficial trace of an Archean subduction zone. However, our structural analysis suggest that the ASP overlies the OSP and that the ASP-OSP contact does not show evidences of an important sub-vertical shearing deformation as expected if it was a major upper plate-lower plate boundary. Furthermore, the contact does not present significant metamorphic break between the two domains, but a progressive increasing of metamorphism toward the OSP, from greenschist- to amphibolite-facies conditions. Based on these structural and metamorphic characteristics, we suggest that the OSP exposes the deepest rocks at outcrop of an ASP-OSP crust in the study area. Regionally, the 40Ar/39Ar ages acquired during this study indicate that the ASP-OSP contact records a protracted metamorphic history that started around 2685 Ma. The structural and isotopic age data suggest that, from ~2685 Ma to ~2632 Ma, the deepest level of the ASP and the underlying OSP reached amphibolite-facies metamorphic conditions and that regional deformation was accommodated by an overall horizontal shortening and sub-vertical transfers of crustal material. Subsequently, the cooling of these crustal rocks was accompanied by strain localisation, which led to the development of oblique strike-slip shear zones from ~2600 Ma, when the lateral flowing of crustal material became predominant. Our 40Ar/39Ar data compared with metamorphic ages documented in adjacent areas of the Superior Province suggests that the peak and duration of regional metamorphism might have been coeval over a large region. This rather favours a mode of pervasive deformation as expected in vertical tectonics.

Structural evolution of the Semail Ophiolite metamorphic sole, Wadi Hawasina and Northern Jebel Nakhl Culmination, Oman

NASA Astrophysics Data System (ADS)

Hurtado, C.; Bailey, C.; Visokay, L.; Scharf, A.

2017-12-01

The Semail ophiolite is the world's largest and best-exposed ophiolite sequence, however the processes associated with both oceanic detachment and later emplacement onto the Arabian continental margin remain enigmatic. This study examines the upper mantle section of the ophiolite, its associated metamorphic sole, and the autochthonous strata beneath the ophiolite at two locations in northern Oman. Our purpose is to understand the structural history of ophiolite emplacement and evaluate the deformation kinematics of faulted and sheared rocks in the metamorphic sole. At Wadi Hawasina, the base of the ophiolite is defined by a 5- to 15-m thick zone of penetratively-serpentinized mylonitic peridotite. Kinematic indicators record top-to-the SW (reverse) sense-of-shear with a triclinic deformation asymmetry. An inverted metamorphic grade is preserved in the 300- to 500-m thick metamorphic sole that is thrust over deep-water sedimentary rocks of the Hawasina Group. The study site near Buwah, in the northern Jebel Nakhl culmination, contains a N-to-S progression of mantle peridotite, metamorphic sole, and underlying Jurassic carbonates. Liswanite crops out in NW-SE trending linear ridges in the peridotite. The metamorphic sole includes well-foliated quartzite, metachert, and amphibolite. Kinematic evidence indicates that the liswanite and a serpentinized mélange experienced top to-the north (normal) sense-of-shear. Two generations of E-W striking, N-dipping normal faults separate the autochthonous sequence from the metamorphic sole, and also cut out significant sections of the metamorphic sole. Fabric analysis reveals that the metamorphic sole experienced flattening strain (K<0.2) that accumulated during pure shear-dominated general shear (Wk<0.4). Normal faulting and extension at the Buwah site indicates that post-ophiolite deformation is significant in the Jebel Akhdar and Jebel Nakhl culminations.

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

USGS Publications Warehouse

Dusel-Bacon, C.; Lanphere, M.A.; Sharp, W.D.; Layer, P.W.; Hansen, V.L.

2002-01-01

We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon-Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215-212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ???188-186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ???135-115 Ma, and muscovite and biotite ???110-108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Caledonian evolution of the Moine Supergroup: Prograde garnet growth and context for quartz fabric-based deformation thermometry

NASA Astrophysics Data System (ADS)

Law, Richard; Ashley, Kyle; Thigpen, Ryan

2014-05-01

Despite the detailed Caledonian structural/tectonic framework developed for the Moine Supergroup of northern Scotland, debate continues over the tectonic processes that drove metamorphism. Rapid temporal evolution of the metamorphic sequence has led some geologists to suggest that crustal thickening alone cannot provide sufficient heat flow to reach the metamorphic grades observed. Rather, they postulate that large-scale contact metamorphism or initial heating in an extensional, back-arc setting is required. We present coupled petrographic analyses and forward phase stability modeling for quantifying prograde metamorphic evolution in pelite horizons dispersed across the Caledonian thrust sheets. Results suggest garnet growth was syn-kinematic during prograde decompression. Rutile and ilmenite inclusions in garnet cores and rims, respectively, support this claim, while chemical profiles and crystal morphology argue against a detrital origin for these garnet grains. The observed clockwise P-T path for these garnets is incompatible with extensional or contact metamorphic models (would require counter-clockwise paths). Rather, the P-T data suggests advection of isotherms during thrusting as the dominant mechanism for metamorphism (Thigpen et al., 2013). Recent studies in other orogens (e.g., Spear et al., 2012) suggest that heating over long time scales under mid-crustal conditions may not be needed to reach the metamorphic grades observed. Therefore the structurally higher, more hinterland Caledonian thrust sheets may have reached peak metamorphism in a much shorter time period than previously expected. The paucity of pelitic horizons across the foreland-positioned Moine thrust sheet has previously limited insight into the prograde evolution of these rocks. However, the dominance of quartz-rich units has allowed the thermal structure of the thrust sheet to be evaluated using quartz c-axis fabric opening angle-based deformation thermometry. Microstructures in the pelites sampled indicate that garnet (rim) growth is syn-kinematic with respect to the Scandian (mid-Silurian) deformation fabrics. Deformation temperatures indicated by quartz fabric opening angles are very similar to temperatures of metamorphism constrained using pseudosection and petrographic data from adjacent pelite horizons. This suggests that the deformation- and petrology-based data sets are providing information on the same thermal event. These results support the use of quartz deformation thermometry in obtaining thermal profiles across tectonic units where rock types (usually pelites), with metamorphic mineral assemblages suitable for petrology-based thermometry, are not present. Thigpen, J.R., Law, R.D., Loehn, C.L., Strachan, R.A., Tracy, R.J., Lloyd, G.E., Roth, B.L., and Brown, S.J., 2013, Thermal structure and tectonic evolution of the Scandian orogenic wedge, Scottish Caledonides: integrating geothermometry, deformation temperatures and conceptual kinematic-thermal models, J. Metamorphic Geol., 31, 813-842. Spear, F.S., Ashley, K.T., Webb, L.E., and Thomas, J.B., 2012, Ti diffusion in quartz inclusions: implications for metamorphic time scales, Contrib. Mineral Petrol., 164, 977-986.

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England.

USGS Publications Warehouse

Sutter, J.F.; Ratcliffe, N.M.; Mukasa, S.B.

1985-01-01

40Ar/39Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mt massifs, as well as 40Ar/39Ar and K/Ar mineral and whole-rock ages from Palaeozoic metamorphic rocks, suggest that the thermal peaks for the dominant metamorphic recrystallization in western New England occurred 465 + or - 5 m.y. (Taconian). 40Ar/39Ar age data from a poorly-defined terrain along the eastern strip of the area suggests that the area has been retrograded during a metamorphism that peaked at least 376 + or - 5 m.y. (Acadian). Available age and petrological data from western New England indicate the presence of at least three separate metamorphic-structure domains of Taconic age: 1) a small area of relict high-P and low-T metamorphism, 2) a broad area of normal Barrovian metamorphism from chlorite to garnet grade characterized by a gentle metamorphic gradient and, 3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks. Areas of maximum metamorphic intensity within the last domain coincide with areas of maximum crustal thickening in the later stage of Taconic orogeny. -L.di H

Pseudo- and real-inverted metamorphism caused by the superposition and extrusion of a stack of nappes: a case study of the Southern Brasília Orogen, Brazil

NASA Astrophysics Data System (ADS)

da Motta, Rafael Gonçalves; Moraes, Renato

2017-10-01

The Southern Brasília Orogen is a Neoproterozoic belt that occurs along the southernmost border of the São Francisco Craton where the Andrelândia Nappe System represents the subducted sedimentary domain and is divided into three allochthonous groups, of which the ages and P-T conditions of metamorphism are studied here. The basal unit, the Andrelândia Nappe, exhibits an inverted metamorphic pattern. The base of the structure, composed of staurolite, garnet, biotite, kyanite, quartz, and muscovite, marks the metamorphic peak, whereas at the top, the association of the metamorphic peak does not contain staurolite. The Liberdade Nappe, the middle unit, presents a normal metamorphic pattern; its base, close to the Andrelândia Nappe, shows paragneiss with evidence of in situ partial melting, and towards the top, coarse-grained staurolite schist is found. The staurolite-out and melt-in isograds are coincident and parallel to the main foliation. Thus, the shear zone that limits the nappes is syn-metamorphic, reheating the underlying Andrelândia Nappe and influencing the establishment of metamorphic inversion. This suggestion is supported by the monazite chemical ages, which indicates that the Andrelândia Nappe metamorphic peak (586 ± 15 Ma) is younger than that of the Liberdade Nappe (622.3 ± 7.6 Ma). The upper unit, the Serra da Natureza Klippe, bears a typical high-pressure granulite mineral assemblage that is composed of kyanite, garnet, K-feldspar, rutile, and leucosome, as well as a metamorphic peak at 604.5 ± 6.1 Ma. This tectonic assembly, with inverted and non-inverted metamorphic patterns and generation of klippen structures, is consistent with exhumation models and a strong indentor located in the lower continental crust.

Geochronology of multi-stage metamorphic events: Constraints on episodic zircon growth from the UHP eclogite in the South Altyn, NW China

NASA Astrophysics Data System (ADS)

Liu, Liang; Wang, Chao; Cao, Yu-Ting; Chen, Dan-Ling; Kang, Lei; Yang, Wen-Qiang; Zhu, Xiao-Hui

2012-04-01

Petrography, mineral chemistry and pressure-temperature (P-T) estimates were carried out for the eclogite from the South Altyn in NW China. The results suggest three stages of metamorphism: an ultra-high pressure (UHP) eclogite-facies metamorphism at 717-871 °C and ≥ 2.8 GPa, a high pressure (HP) granulite-facies metamorphism at 624-789 °C and 1.42-1.52 GPa, and an amphibolite-facies metamorphism at 597-728 °C and 0.99-1.17 GPa. Cathodoluminescence investigation revealed that zircons from the retrograde eclogite display a distinct core-rim structure. Cores are grey-white luminescent and contain mineral inclusions of Garnet + Omphacite + Rutile + Quartz, suggesting eclogite-facies metamorphic origin. The rims are dark grey luminescent and contain Garnet + Clinopyroxene + Pagioclase inclusions, forming at HP granulite-facies conditions. A few residual zircon grains with mottled internal structure also occur as the metamorphic cores. LA-ICPMS zircon U-Pb dating yielded three discrete age groups: (1) a Neoproterozoic protolith age of 752 ± 7 Ma for the residual grains, (2) an eclogite-facies metamorphic age of 500 ± 7 Ma for the metamorphic cores, and (3) a HP granulite-facies retrograde age of 455 ± 2 Ma for the rims. These ages indicate that the protolith of the Altyn eclogite probably formed in response to breakup of the Rodinia supercontinent during the Neoproterozoic; it was subjected to continental deep subduction and UHP metamorphism during early Paleozoic (~ 500 Ma) and subsequently underwent two stages of retrograde metamorphism during exhumation. The petrological and geochronological data suggest a clockwise P-T-t path for the UHP eclogite. According to pressures and ages for the peak UHP eclogite-facies and the retrograde HP granulite-facies metamorphism, an exhumation rate of 1.2 mm/yr was estimated for the eclogite, which is considerably slower than that of some UHP rocks from other UHP terranes (> 5 mm/yr). While the peak metamorphic age of 500 Ma is consistent with previous dates of 480-504 Ma, it is 40-60 Myr older than the HP/UHP metamorphic ages of 420-461 Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age is 455 ± 2 Ma for the Altyn eclogite, which is 30-55 Myr older than ~ 400-425 Ma for the North Qaidam eclogites. These age differences suggest that the South Altyn and North Qaidam eclogites do not belong to the same HP/UHP metamorphic zone.

The problem of the age and structural position of the Blyb metamorphic complex (Fore Range zone, Great Caucasus) granitoids.

NASA Astrophysics Data System (ADS)

Kamzolkin, Vladimir; Latyshev, Anton; Ivanov, Stanislav

2016-04-01

The Blyb metamorphic complex (BMC) of the Fore Range zone is one of the most high-grade metamorphosed element of the Great Caucasus fold belt. Determination of the timing and the mechanism of formation of the Fore Range fold-thrust structures are not possible without investigation of the BMC located at the basement of its section. At the same time, the conceptions about its structure and age are outdated and need revision. Somin (2011) determined the age of the protolith and metamorphism of the Blyb complex as the Late Devonian - Early Carboniferous. We have recently shown that the BMC has not the dome, as previously thought, but nappe structure (Vidjapin, Kamzolkin, 2015), and is metamorphically coherent with the peak metamorphism pressures up to 22 kbar (Kamzolkin et al., 2015; Konilov et al., 2013). Considering the age and structure of the Blyb complex it is necessary to revise the age of granitoid intrusions and their relations with gneisses and schists, which constitute the main part of the section of the complex. Most authors (Gamkrelidze, Shengelia, 2007; Lavrischev, 2002; Baranov, 1967) adheres to Early Paleozoic age of intrusives, which is doubtful, considering the younger age of metamorphic rocks. We suppose, that the intrusive bodies broke through a BMC nappe structure during the exhumation of the complex (Perchuk, 1991) at the Devonian - Carboniferous boundary. Seemingly, the massive monzodiorites body (Lavrischev, 2002), intruding garnet-muscovite schists and amphibolite gneisses of the Blyb complex and cut by the Main Caucasian fault (MCF), are younger. Given the timing of termination of the MCF movement activity as the Middle Jurassic (Greater Caucasus..., 2005), their age should be in the Early Carboniferous - Middle Jurassic interval. At the same time, on the modern geological map (Lavrischev, 2002) monzodiorites body is assigned to the Middle Paleozoic. The study of the BMC granitoids and monzodiorites will help in determining of the mechanism and age of exhumation of the Blyb metamorphic complex high-pressure rocks. The reported study was partially supported by RFBR, research projects No. 16-35-00571mol_a; 16-05-01012a

FinalTechRep_DE_EE0007363_15

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahrenkiel, Phil

The primary objective of the project was to develop Ga0.82In0.18As (GaInAs) solar cells grown on epilayers of elemental Al. At this composition, GaInAs has a nearly optimal bandgap (1.16 eV) for a single-junction photovoltaic device. However, GaInAs lacks a convenient, lattice-matched substrate, restricting most investigations to metamorphic structures. The metal Al is, in fact, precisely lattice-matched to GaInAs in the orientation GaInAs (001)[100]||Al(001)[110]. At present, however, epi-ready Al substrates are not readily available commercially and are subject to oxidation. However, epitaxial Al buffer layers could enable control of defect generation, thermal and light management, and rapid epitaxial lift-off for ultrathinmore » devices.« less

The magmatism and metamorphism at the Malayer area, Western Iran

NASA Astrophysics Data System (ADS)

Ahadnejad, V.; Valizadeh, M. V.; Esmaeily, D.

2009-04-01

The Malayer area is located in the NW-SE aligned Sanandaj-Sirjan metamorphic belt, western Iran and consists mainly of Mesozoic schists so-called Hamadan Phyllites, Jurassic to Tertiary intrusive rocks and related contact metamorphic aureoles, aplites and pegmatites. The Sanandj-Sirjan Zone is produced by oblique collisional event between Arabian plate and Central Iran microcontinent. Highest level of regional metamorphism in the area is greenschist facies and injection of felsic magmas is caused contact metamorphism. Magmatism is consist of a general northwest trend large felsic to intermediate intrusive bodies. The main trend of structural features i.e. faults, fractures and other structural features is NW-SE. The Malayer granitoid complex is ellipsoid in shape and has NW-SE foliation especially at the corners of the intrusions. Petrography of the magmatic rocks revealed recrystallization of quartz and feldspars, bending of biotite, and aligment of minerals paralle to the main trend of magmatic and metamorphic country rocks. These indicated that intrusion of felsic magma is coincide to the regional metamorphism and is syn-tectoinc. Non-extensive contact metamorphism aureoles and rareness of pegmatite and aplite in the area are interpreted as injection of felsic magmas into the high-strain metamorphic zone. The regional metamorphic rocks mainly consist of meta-sandstone, slate, phyllite, schist. These gray to dark metasedimentary rocks are consist of quartz, muscovite, turmaline, epidote, biotite and chlorite. Sheeted minerals form extended schistosity and study of porphyroblast-matrix relationships shows that injection of granitic magma into the country rocks is syn to post-tectonic. Syn-tectonic indicating porphyroblast growth synchronous with the development of the external fabric. The thermal contact area of the granite can be observed in the contact margin of granite and regional metamorphic rocks, where it produced hornfelses, andalusit-garnet schists and local feldspatisation. Hornfels has surrounded the Malayer intrusive body in its southern, eastern and to some extent northeastern parts. It shows a rather sharp contact with the granodiorite. According to field and microscopic investigations, an original clay-sandstone has been converted into hornfels due to contact metamorphism. Some small highly altered granitic patches are seen in the hornfels unit, especially close to its contact with the Malayer intrusive body.

Tectono-metamorphic evolution and magmatic processes in the thermo-metamorphic aureole of the Monte Capanne pluton (Elba Island, Northern Tyrrhenian Sea, Italy).

NASA Astrophysics Data System (ADS)

Morelli, M.; Pandeli, E.; Principi, G.

2003-04-01

Introduction In this work we present new structural and petrographic data collected in the thermo-metamorphic aureole of Monte Capanne (western Elba Island) and its metamorphic evolution. In the western Elba Island the Monte Capanne monzogranitic body (ca. 7 Ma) and its thermo-metamorphic aureole crop out. At least two different tectonic units can be distinguished: the Punta Le Tombe Unit, weak re-crystallized, and the Punta Nera Unit. In the latter one the re-crystallization is strong and a pre-intrusion tectono-metamorphic framework is evident (Morelli et al., 2002). The latter is mainly constituted by thermo-metamorphosed meta-ophiolites and meta-sedimentary successions previously correlated by Barberi et al. (1969) with the un-metamorphic ones (Complex IV and V of Trevisan, 1950) cropping out in the central-eastern Elba. According to Perrin (1975) and Reutter &Spohn (1982) a pre-intrusion tectono-metamorphic framework was recognized into such rocks. As suggested by Daniel &Jolivet (1995) complex relationships between metamorphic evolution and magmatic events are also recognizable. Geological Data The Punta Nera Unit crops out all around the Monte Capanne magmatic body and the primary contact with the underlying granitoid is somewhere preserved. This unit, strongly re-crystallized and locally crosscut by aplitic and porphyritic dikes, is represented by (Coli &Pandeli, 1997; Morelli, 2000) tectonized meta-serpentinites, meta-gabbros with rodingitic dikes, rare meta-basalts and meta-ophicalcites, meta-cherts, marbles, cherty meta-limestones, phyllites and meta-limestones with rare meta-arenites intercalations. A "pre-magmatic" tectono-metamorphic framework of this unit is well evident only in its meta-sedimentary portion. The meta-sediments are deformed by syn-metamorphic isoclinal folds caractherized by N-S trending axes, west dipping axial planes and easternward vergence. A later folding and flattening event clearly post-dated the above said folds and associated main foliation. Both at the meso- and micro-scale few generations of syn-/inter-kinematic and static thermo-metamorphic blasteses (e.g. biotite, wollastonite, garnet, epidote etc.) are evident. Finally weak local folding and faulting affected the Monte Capanne thermo-metamorphic aureole during the uplift of the cooled pluton. Along the coast of Spartaia (north-eastern side of Monte Capanne) the meta-sedimentary upper portion of the Punta Nera Unit crops out and it is represented by alternating marbles and phyllites with rare meta-arenites. Here, the relationships between tectono-metamorphic and thermo-metamorphic structures, and intrusive magmatic bodies are particularly evident. D_1M_1: is recognizable at the meso-scale only as rare relics of intrafolial isoclinal rootless hinges and locally as a transposed foliation (S1). At the microscope a relic fine-spaced foliation, locally preserved by the following re-crystallization and parallel to the original lithological partitions, is evident. The original mineral associations is often replaced by HT/LP mimetic and syn-kinematic or static blasteses. D_2M_2 is the most evident structural fabric pre-dating the thermo-metamorphic events. It is represented by tight to isoclinal folds with N-S trending axes and easternward vergence. At the microscope the S2 foliation deformed the previously S1 and at the fold hinges a variously penetrative discrete crenulation cleavage (S2) is recognizable. M_3 is the first thermo-metamorphic event. It is associated to the intrusions of the Capo Bianco, San Martino and Portoferraio laccoliths (between ca. 8 and 7.2 Ma, Rocchi et alii, 2002) along pre-existing tectonic and lithological discontinuity. The laccoliths clearly crosscut the tectono-metamorphic fabric of the Punta Nera Unit. At this first magmatic event is associated a HT/LP blastesis of static garnet, wollastonite and epidote. Close to the contact between the magmatic intrusive bodies the country rocks are strongly re-crystallized and the original structure is oblitered. D_3M_4: is linked to the intrusion of the Monte Capanne pluton (6.9 Ma). Because of the flattening of the country rocks a penetrative foliation, sub-parallel to the surface of the plutonic body, developed during its intrusion and the uplift. Fluidal structures and discharge folding also developed. This structures affected both the country rocks and the laccoliths intruded in. At the meso-scale are also evident fractures, filled by calcite, cutting the garnet/wollastonite aggregate and later flattened and-or partially transposed. Late felsic products associated with the Monte Capanne pluton partly are deformed by the flattening and partly cut the flattening structures. The contact between the pluton and the country rocks itself is locally deformed by flattening structures. At the microscope the growth of HT/LP minerals, mimetic on the previous associations, or syn-kinematic or static, is evident. M_5: finally, during the last intrusive event (Orano porphyries, 6.8 Ma) a static thermo-metamorphic blastesis locally occurs post-dating the structures previously described. Only gentle folding, veins and fractures formation, and faulting processes followed the tectono-metamorphic and thermo-metamorphic deformation events. Conclusions The Punta Nera Unit represents another important element to understand the relationships between Alps and Apennines. In fact, the evidences of a pre-granitoid polyphased tectono-metamorphic framework in this metamorphic oceanic unit allow to correlate it to the Schistes Lustrés of the Alpine Corsica (e.g. Inzecca Massif, Duran Delga, 1984) and of the Northern Tyrrhenian area (e.g. the outcrops of oceanic metamorphic units in the Gorgona Island, Giglio Island and eastern Elba Island: Bortolotti et al., 2001; Pandeli et alii, 2001; Rossetti et al., 2001; Orti et al., 2002). References Barberi et al. (1969). F.126. Isola d'Elba. Note Ill. Carta Geol. It., 1:100.000. Ser. Geol. d'It. Bortolotti V. et al. (2001), Ofioliti, 26, 2001. Coli &Pandeli, (2001), Ofioliti, 26, 2001. Daniel &Jolivet, (1995), Bull. Soc. Geol. France, 166(4), 341--354. Durand-Delga M. (1984), Mem. Soc. Geol. It., 28, 285--329. Morelli M. et alii, (2002), Proceedings of Eurogranites 2002, 15th--21th Sept. 2002. Orti et alii, (2002), Ofioliti, 27, 2002. Pandeli et alii, (2001), Ofioliti, 26, 2001. Perrin M. (1975), Boll. Soc. Geol. It., 94, pp.1929--1955. Reutter K.J. and Spohn A., (1982), Ofioliti, 7, 467--478. Rocchi et alii, (2002), in Eurogranites 2002 Field Meeting Guide. 15th--21th Sept. 2002 Rossetti et al., (2001), Ofioliti, 26, 2001. Trevisan L., (1950), Mem. Ist. Geol. Univ. Padova, 16, 2--39.

Metamorphic Proteins: Emergence of Dual Protein Folds from One Primary Sequence.

PubMed

Lella, Muralikrishna; Mahalakshmi, Radhakrishnan

2017-06-20

Every amino acid exhibits a different propensity for distinct structural conformations. Hence, decoding how the primary amino acid sequence undergoes the transition to a defined secondary structure and its final three-dimensional fold is presently considered predictable with reasonable certainty. However, protein sequences that defy the first principles of secondary structure prediction (they attain two different folds) have recently been discovered. Such proteins, aptly named metamorphic proteins, decrease the conformational constraint by increasing flexibility in the secondary structure and thereby result in efficient functionality. In this review, we discuss the major factors driving the conformational switch related both to protein sequence and to structure using illustrative examples. We discuss the concept of an evolutionary transition in sequence and structure, the functional impact of the tertiary fold, and the pressure of intrinsic and external factors that give rise to metamorphic proteins. We mainly focus on the major components of protein architecture, namely, the α-helix and β-sheet segments, which are involved in conformational switching within the same or highly similar sequences. These chameleonic sequences are widespread in both cytosolic and membrane proteins, and these folds are equally important for protein structure and function. We discuss the implications of metamorphic proteins and chameleonic peptide sequences in de novo peptide design.

High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east- central Alaska

USGS Publications Warehouse

Dusel-Bacon, C.; Hansen, V.L.; Scala, J.A.

1995-01-01

Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

Structure, metamorphism, and geochronology of the Cosmos Hills and Ruby Ridge, Brooks Range schist belt, Alaska

USGS Publications Warehouse

Christiansen, Peter B.; Snee, Lawrence W.

1994-01-01

The boundary of the internal zones of the Brooks Range orogenic belt (the schist belt) is a fault contact that dips toward the hinterland (the Yukon-Koyukuk province). This fault, here referred to as the Cosmos Hills fault zone, juxtaposes oceanic rocks and unmetamorphosed sedimentary rocks structurally above blueschist-to-greenschist facies metamorphic rocks of the schist belt. Near the fault contact, schist belt rocks are increasingly affected by a prominent, subhorizontal transposition foliation that is locally mylonitic in the fault zone. Structural and petrologic observations combined with 40Ar/39Ar incremental-release geochronology give evidence for a polyphase metamorphic and deformational history beginning in the Middle Jurassic and continuing until the Late Cretaceous. Our 40Ar/39Ar cooling age for Jurassic metamorphism is consistent with stratigraphic and other evidence for the onset of Brooks Range orogenesis. Jurassic metamorphism is nearly everywhere overprinted by a regional greenschist-facies event dated at 130–125 Ma. Near the contact with the Cosmos Hills fault zone, the schist belt is increasingly affected by a younger greenschist metamorphism that is texturally related to a prominent foliation that folds and transposes an older fabric. The 40Ar/39Ar results on phengite and fuchsite that define this younger fabric give recrystallization ages ranging from 103 to less than 90 Ma. We conclude that metamorphism that formed the transposition fabric peaked around 100 Ma and may have continued until well after 90 Ma. This age for greenschist metamorphism is broadly synchronous with the depositional age of locally derived, shallow-marine clastic sedimentary strata in the hanging wall of the fault zone and thus substantiates the interpretation that the fault zone accommodated extension in the Late Cretaceous. This extension unroofed and exhumed the schist belt during relative subsidence of the Yukon-Koyukuk province.

Geology of the Sierra de Fiambala, northwestern Argentina: implications for Early Palaeozoic Andean tectonics

USGS Publications Warehouse

Grissom, G.C.; DeBari, S.M.; Snee, L.W.

1998-01-01

This paper is included in the Special Publication entitled 'The proto- Andean margin of Gondwana', edited by R.J. Pankhurst and C.W. Rapela. Field mapping in conjunction with structural, metamorphic, and geochronological data document the tectono-thermal history of exhumed deep crustal rocks in the Sierra de Fiambala, NW Argentina. The range consists of two structural blocks distinguished by different metasedimentary sequences and different grades of metamorphism. Orthogneiss and paragneiss in the northern structural block may have a Precambrian history. Greenschist- to amphibolite-facies metamorphism, intrusion, and injection magmatization affected all rocks at 540-550 Ma. A subsequent event in the Late Cambrian to Ordovician (c.515 to 470 Ma) involved amphibolite- to granulite-facies metamorphism, mafic intrusion, and deformation, followed by cooling through mid-Palaeozoic time. The emplacement of Carboniferous (325-350 Ma) post-tectonic granites caused reheating and retrogression that was strongest toward the northeast part of the range. The Cambrian, Ordovician, and Carboniferous events in the Sierra de Fiambala were of regional extent as indicated by temporal correlations with events reported for other deep crustal rocks of the northern Sierras Pampeanas. Correlations between periods of intrusion and high-grade metamorphism in the northern Sierras Pampeanas and volcanic-sedimentary events in the adjacent supracrustal exposures confirm that rocks in the northern Sierras Pampeanas formed at deep (10-25 km) structural levels in the early Palaeozoic continental margin of Gondwana.

Development, Qualification and Production of Space Solar Cells with 30% EOL Efficiency

NASA Astrophysics Data System (ADS)

Guter, Wolfgang; Ebel, Lars; Fuhrmann, Daniel; Kostler, Wolfgang; Meusel, Matthias

2014-08-01

AZUR SPACE's latest qualified solar cell product 3G30-advanced provides a high end-of-life (EOL) efficiency of 27.8% for 5E14 (1 MeV e-/cm2) at low production costs. In order to further reduce the mass, the 3G30-advanced was thinned down to as thin as 20 μm and tested in space. Next generation solar cells must exceed the EOL efficiency of the 3G30-advanced and therefore will utilize the excess current of the Ge subcell. This can be achieved by a metamorphic cell concept. While average beginning-of-life efficiencies above 31% have already been demonstrated with upright metamorphic triple-junction cells, AZUR's next generation product will comprise a metamorphic 4- junction device targeting 30% EOL.

Archean metamorphic sequence and surfaces, Kangerdlugssuaq Fjord, East Greenland

NASA Technical Reports Server (NTRS)

Kays, M. A.

1986-01-01

The characteristics of Archean metamorphic surfaces and fabrics of a mapped sequence of rocks older than about 3000 Ma provide information basic to an understanding of the structural evolution and metamorphic history in Kangerdlugssuaq Fjord, east Greenland. This information and the additional results of petrologic and geochemical studies have culminated in an extended chronology of Archean plutonic, metamorphic, and tectonic events. The basis for the chronology is considered, especially the nature of the metamorphic fabrics and surfaces in the Archean sequence. The surfaces, which are planar mineral parageneses, may prove to be mappable outside Kangerdlugssuaq Fjord, and if so, will be helpful in extending the events that they represent to other Archean sequences in east Greenland. The surfaces will become especially important reference planes if the absolute ages of their metamorphic assemblages can be determined in at least one location where strain was low subsequent to their recrystallization. Once an isochron is obtained, the dynamothermal age of the regionally identifiable metamorphic surface is determined everywhere it can be mapped.

Tectono-stratigraphy and low-grade metamorphism of Late Permian and Early Jurassic accretionary complexes within the Kurosegawa belt, Southwest Japan: Implications for mechanisms of crustal displacement within active continental margin

NASA Astrophysics Data System (ADS)

Hara, Hidetoshi; Kurihara, Toshiyuki; Mori, Hiroshi

2013-04-01

We characterize the tectono-stratigraphic architecture and low-grade metamorphism of the accretionary complex preserved in the Kurosegawa belt of the Kitagawa district in eastern Shikoku, Southwest Japan, in order to understand its internal structure, tectono-metamorphic evolution, and assessments of displacement of continental fragments within the complex. We report the first ever documented occurrence of an Early Jurassic radiolarian assemblage within the accretionary complex of the Kurosegawa belt that has been previously classified as the Late Permian accretionary complex, thus providing a revised age interpretation for these rocks. The accretionary complex is subdivided into four distinct tectono-stratigraphic units: Late Permian mélange and phyllite units, and Early Jurassic mélange and sandstone units. The stratigraphy of these four units is structurally repeated due to an E-W striking, steeply dipping regional fault. We characterized low-grade metamorphism of the accretionary complex via illite crystallinity and Raman spectroscopy of carbonaceous material. The estimated pattern of low-grade metamorphism showed pronounced variability within the complex and revealed no discernible spatial trends. The primary thermal structure in these rocks was overprinted by later tectonic events. Based on geological and thermal structure, we conclude that continental fragments within the Kurosegawa belt were structurally translated into both the Late Permian and Early Jurassic accretionary complexes, which comprise a highly deformed zone affected by strike-slip tectonics during the Early Cretaceous. Different models have been proposed to explain the initial structural evolution of the Kurosegawa belt (i.e., micro-continent collision and klippe tectonic models). Even if we presuppose either model, the available geological evidence requires a new interpretation, whereby primary geological structures are overprinted and reconfigured by later tectonic events.

Mesozoic to Eocene ductile deformation of western Central Iran: From Cimmerian collisional orogeny to Eocene exhumation

NASA Astrophysics Data System (ADS)

Kargaranbafghi, Fariba; Neubauer, Franz; Genser, Johann; Faghih, Ali; Kusky, Timothy

2012-09-01

To advance our understanding of the Mesozoic to Eocene tectonics and kinematics of basement units exposed in the south-western Central Iran plateau, this paper presents new structural and thermochronological data from the Chapedony metamorphic core complex and hangingwall units, particularly from the Posht-e-Badam complex. The overall Paleogene structural characteristics of the area are related to an oblique convergent zone. The Saghand area represents part of a deformation zone between the Arabian and Eurasian plates, and can be interpreted to result from the Central Iran intracontinental deformation acting as a weak zone during Mesozoic to Paleogene times. Field and microstructural evidence reveal that the metamorphic and igneous rocks suffered a ductile shear deformation including mylonitization at the hangingwall boundary of the Eocene Chapedony metamorphic core complex. Comparison of deformation features in the mylonites and other structural features within the footwall unit leads to the conclusion that the mylonites were formed in a subhorizontal shear zone by NE-SW stretching during Middle to Late Eocene extensional tectonics. The Chapedony metamorphic core complex is characterized by amphibolite-facies metamorphism and development of S and S-L tectonic fabrics. The Posht-e-Badam complex was deformed by two stages during Cimmerian tectonic processes forming the Paleo-Tethyan suture.

Tectono-metamorphic evolution of the Chinese Altai, central Asia: new insights from microstructures

NASA Astrophysics Data System (ADS)

Jiang, Yingde; Zhang, Jian; Schulmann, Karel; Sun, Min; Zhao, Guochun

2013-04-01

The Altai Orogen, extending from Russia, through northeast Kazakhstan and northwest China, to western and southern Mongolia, occupies a pivotal position in understanding the accretionary process of the Central Asian Orogenic Belt and has drawn much attention in recent years. However, its orogenic evolution remains poorly constrained, because previous studies were mainly focused on the geochronological and geochemical signatures and much less work has been done on metamorphic and structural studies. Metamorphic rocks widely occur in the southern Altai Range and have previously been separated into high-T/low-P and medium-P types. Recent studies demonstrated that these two kinds of rocks may have similar protoliths, i.e. early Paleozoic arc-related assemblages, but experienced different metamorphic histories. The development of biotite, garnet, staurolite and kyanite metamorphic zonal sequences in the low- to medium- grade rocks, demonstrate typical medium-pressure metamorphism that has been suggested as a major consequence of the orogenesis. The high-T/low-P metamorphism, represented by the growth of garnet+cordierite+sillimanite+k-feldspar and was accompanied by extensive anatexis, remains its tectonic significance poorly constrained. Field structural investigation in the Chinese Altai reveals that the high-T/low-P metamorphic rocks have major S-L fabrics (defined by the strongly aligned biotite and sillimanite) exactly in the same orientations as those developed in the associated medium-P grade rocks. Geochronological studies constrain the major fabrics in both kinds of rocks developed during mid-Devonian, coeval with the strong magmatism in the region. Micro-structural investigation on both kinds of rocks show similar prograde metamorphic history featured by clockwise P-T path evolution. Phase equilibrium modeling in the MnNCKFMASH system indicates that the development of major fabrics in the medium-P metamorphic rocks mainly recorded the notable increase of pressure and that in the high-T rocks was featured by the significant increase of temperature. The pressure increase could attribute to the progressive crustal thickening that may be correlated to the accretionary regime of the southern Altai in the mid-Devonian and the high temperature conditions most likely imply a significant heat input from the deep depth, consistent with the syn-chronologically emplacement of juvenile magmas on a large scale. Our study indicates the development of high-T metamorphism was genetically linked with that of the medium-P metamorphism and suggests that the crustal thickening during the orogenic process of the Altai region was accompanied by large heat input. This study is supported by Hong Kong Research Grant Council (HKU705311P and HKU704712P), National Science Foundation of China (41273048), IGCP #592 Project "Continental construction in Central Asia" and Research grant of State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLIG-KF-12-06) .

Principles of Metamorphic Petrology

NASA Astrophysics Data System (ADS)

Williams, Michael L.

2009-05-01

The field of metamorphic petrology has seen spectacular advances in the past decade, including new X-ray mapping techniques for characterizing metamorphic rocks and minerals, new internally consistent thermobarometers, new software for constructing and viewing phase diagrams, new methods to date metamorphic processes, and perhaps most significant, revised petrologic databases and the ability to calculate accurate phase diagrams and pseudosections. These tools and techniques provide new power and resolution for constraining pressure-temperature (P-T) histories and tectonic events. Two books have been fundamental for empowering petrologists and structural geologists during the past decade. Frank Spear's Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths, published in 1993, builds on his seminal papers to provide a quantitative framework for P-T path analysis. Spear's book lays the foundation for modern quantitative metamorphic analysis. Cees Passchier and Rudolph Trouw's Microtectonics, published in 2005, with its superb photos and figures, provides the tools and the theory for interpreting deformation textures and inferring deformation processes.

Structural profile reconstructions and thermal metamorphic evolution in the slate belt of southern Hsuehshan Range in the active Taiwan mountain belt

NASA Astrophysics Data System (ADS)

Wu, Yu; Chen, Chih-Tung; Lee, Jian-Cheng; Shyu, J. Bruce H.

2017-04-01

The fate of passive continental margin in collisional orogens is crucial in understanding tectonic evolution of mountain belts. The active arc-continent collision of Taiwan is considered as a model case in studying mountain building processes, and largely consists of deformed margin basement and cover series. Among the whole orogeny belt, the slate belt of the Hsuehshan Range (HR) is a prominent large-scale pop-up structural on the prowedge part of the orogen, and is composed of metamorphosed Eocene to Miocene sediments which experienced only the Neogene Taiwan orogeny after diagenesis in margin graben. Characterizing the metamorphic history of the HR is essential for reconstructing its geological evolution during the mountain building processes. However, previous studies were mostly focused on northern and central HR, structural investigation coupled with metamorphic documentation in the southern part of HR, which is the most active part of the orogeny belt, is therefore targeted in this work. Since carbonaceous material is common in pelitic protolith of HR slates, the Raman spectrum of carbonaceous material (RSCM) measuring the rock peak temperature is chosen for quantitative thermal metamorphic documentation. In this study, we reconstruct a geological structural profile in western central Taiwan across the prowedge part of the mountain belt containing the southern HR by combining the surface geological data, well log records and published seismic reflection profiles. Although most of the existing data are concentrated in the fold-and-thrust belt, they are now reinforced by new field structural measurements and RSCM samples in the southern HR. In total 27 RSCM samples were collected along 2 transects perpendicular to the average strike with a dense interval about 2 km. The results allow us to map peak temperature distribution across southern HR, and provide new constraints for structural profile reconstruction and reappraisal of the structural evolution of the HR and neighboring fold-and-thrust belt. As shown in the previous thermal metamorphic investigation, we expected that southern HR strata acquired highest temperature during its burial stage than the orogenic stage like their central HR counterparts, thus experiencing mostly retrograde metamorphism in the entire mountain building processes.

Tectonic exhumation and boundary structure of the Kokchetav HP - UHP metamorphic belt (Northern Kazakhstan): constraints from 40Ar/39Ar geochronology

NASA Astrophysics Data System (ADS)

Zhimulev, Fedor; de Grave, Johan; Travin, Aleksey; Buslov, Mikhail

2010-05-01

The Kokchetav metamorphic belt (KMB) is part of the Early Paleozoic orogenic belt of Northern Kazakhstan and constitutes one of the most famous, classical ultra-high pressure (UHP) metamorphic terranes. The KMB mainly consists of gneisses, mica schists and eclogites. These were formed by Cambrian continental subduction and associated metamorphism of the Precambrian Kokchetav microcontinent and subsequent exhumation of fragments of this metamorphosed continental crust. Several subterranes can be distinguished in the KMB: Barchi, Kumdi-Kol, Sulu-Tube, Enbek-Berlyk, Kulet and Borovoe. These subterranes differ not only in rock composition or in genetic pT conditions, but also in the age of the individual metamorphic events, including the timing of peak, and regressive stages. Most geochronological data indicate a Cambrian age of UHP and HP metamorphism and subsequent exhumation of the KMB. However, there is no field evidence of Cambrian geodynamic processes in the region: Cambrian sediments, volcanic rocks, or large magmatic bodies are completely absent in the KMB setting. The youngest geochronological information in the KMB was obtained on the garnet-mica schists from the Enbek-Berlyk subterrane. The 40Ar/39Ar ages of the muscovite from these schists lies in the range of 490 to 475 Ma (mainly 480-485 Ma). All 40Ar/39Ar stepwise heating experiments yield well-defined plateau and isochron ages. This age is considered to represent the time of emplacement of various heterogeneous nappes, including nappes that consist of HP - UHP metamorphic rocks, to upper crustal levels. To the north, the Kokchetav HP - UHP metamorphic belt is bounded by the Northern Kokchetav tectonic zone (NKTZ). This zone includes thin nappes of (1) Palaeo-Mesoproterozoic gneiss of the metamorphic basement of the Kokchetav microcontinent and Neoproterozoic meta-sandstones and dolomites of its deformed sedimentary cover, (2) pre-Ordovician volcanic rocks of island-arc affinity, (3) Early Ordovician turbidities with olistostrome lenses, and (4) gneiss with eclogite boudins. These nappes are tectonically juxtaposed distinctly against unmetamorphosed rocks. The fault zones between the different tectonic units are formed by quartz-muscovite schists. The 40Ar/39Ar ages of the muscovite from these schists cluster between 492-476 Ma (mainly about 490 Ma). The NKTZ was formed by the Early Ordovician collision between the Kokchetav microcontinent, including the Kokchetav metamorphic belt, and the Stepnyak island-arc. Late Ordovician and Silurian granites cross-cut the internal structural architecture of both the KMB and NKTZ, providing a relative upper age limit. As a consequence, only the youngest stage of the formation of the metamorphic belt is directly related to a collision event. Comparison of the structural-metamorphic history of the KMB with the regional geological context, indicates that tectonic exhumation of the Kokchetav HP - UHP rocks must be precollisional. In this model, the individual nappes, detached from the underthrusting slab during progressive continental subduction, were exhumated before subduction cessation. The observed imbricated-nappe structure in the field, where UHP and HP rocks are juxtaposed against unmetamorphosed Ordovician sediments and volcanic deposits was formed during subsequent Ordovician collisional deformation events.

Metamorphic and structural evidence for significant vertical displacement along the Ross Lake fault zone, a major orogen-parallel shear zone in the Cordillera of western North America

USGS Publications Warehouse

Baldwin, J.A.; Whitney, D.L.; Hurlow, H.A.

1997-01-01

Results of an investigation of the petrology and structure of the Skymo complex and adjacent terranes constrain the amount, timing, and sense of motion on a segment of the > 600-km-long Late Cretaceous - early Tertiary Ross Lake fault zone (RLFZ), a major orogen-parallel shear zone in the Cordillera of western North America. In the study area in the North Cascades, Washington state, the RLFZ accommodated significant pre-middle Eocene vertical displacement, and it juxtaposes the Skymo complex with upper amphibolite facies (650??-690??C and 6-7 kbar) Skagit Gneiss of the North Cascades crystalline core to the SW and andalusite-bearing phyllite of the Little Jack terrane (Intermontane superterrane) to the NE. The two main lithologic units of the Skymo complex, a primitive mafic intrusion and a fault-bounded block of granulite facies metasedimentary rocks, are unique in the North Cascades. Granulite facies conditions were attained during high-temperature (> 800??C), low pressure (??? 4 kbar) contact metamorphism associated with intrusion of the mafic magma. P-T estimates and reaction textures in garnet-orthopyroxene gneiss suggest that contact metamorphism followed earlier, higher pressure regional metamorphism. There is no evidence that the Skagit Gneiss experienced high-T - low-P contact metamorphism. In the Little Jack terrane, however, texturally late cordierite ?? spinel and partial replacement of andalusite by sillimanite near the terrane's fault contact with Skymo gabbro suggest that the Little Jack terrane experienced high-T (??? 600??C) - low-P (??? 4 kbar) contact metamorphism following earlier low-grade regional metamorphism. Similarities in the protoliths of metasedimentary rocks in the Skymo and Little Jack indicate that they may be part of the same terrane. Differences in pressure estimates for the Little Jack versus Skymo for regional metamorphism that preceded contact metamorphism indicate vertical displacement of ??? 10 km (west side up) on the strand of the RLFZ that now separates the two structural blocks. High-angle faults in the study area are dextral-reverse mylonitic shear zones that experienced later brittle normal slip. Vertical motion on these shear zones before intrusion of Skymo gabbro can account for metamorphic discontinuities indicated by P-T results. The terranes have also been internally deformed by nonintersecting but coeval dextral and sinistral shear zones that formed after the terranes were brought together in the RLFZ and intruded by Eocene dikes. These results show that the RLFZ has accommodated significant vertical displacement but perhaps no more than tens of kilometers of early Tertiary lateral movement. Structural evidence for earlier, large-magnitude strike-slip displacement is not preserved.

Structural and petrographic constraints on the stratigraphy of the Lapataia Formation, with implications for the tectonic evolution of the Fuegian Andes

NASA Astrophysics Data System (ADS)

Cao, Sebastián J.; Torres Carbonell, Pablo J.; Dimieri, Luis V.

2018-07-01

The structure of the Fuegian Andes central belt is characterized by a first phase of peak metamorphism and ductile deformation, followed by a brittle-ductile thrusting phase including juxtaposition of different (first phase) structural levels; both related to the closure and inversion of the Late Jurassic-Early Cretaceous Rocas Verdes basin. The second phase involved thrust sheets of pre-Jurassic basement, as well as Upper Jurassic and Lower Cretaceous units from the volcanic-sedimentary fill of the basin. Rock exposures in the Parque Nacional Tierra del Fuego reveal a diversity of metamorphic mineral assemblages, dynamic recrystallization grades and associated structures, evidencing a variety of protoliths and positions in the crust during their orogenic evolution. Among the units present in this sector, the Lapataia Formation portrays the higher metamorphic grade reported in the Argentine side of the Fuegian Andes, and since no precise radiometric ages have been established to date, its stratigraphic position remains a matter of debate: the discussion being whether it belongs to the pre-Jurassic basement, or the Upper Jurassic volcanic/volcaniclastic initial fill of the Rocas Verdes basin. The mapping and petrographic/microstructural study of the Lapataia Formation and those of undoubtedly Mesozoic age, allow to characterize the former as a group of rocks with great lithological affinity with the Upper Jurassic metamorphic rocks found elsewhere in the central belt of the Fuegian Andes. The main differences in metamorphic grade are indebted to its deformation at deeper crustal levels, but during the same stages than the Mesozoic rocks. Accordingly, we interpret the regional structure to be associated with the stacking of thrust sheets from different structural levels through the emplacement of a duplex system during the growth of the Fuegian Andes.

Structure and thermochronology of the metamorphic core of the Brooks Range, Alaska

NASA Astrophysics Data System (ADS)

Toro, Jaime

1999-11-01

Detailed field studies were undertaken in two key areas of the Central Belt of the Brooks Range: (1) the north flank of Mt. Igikpak in the Survey Pass Quadrangle and (2) in the Shishakshinovik Pass area in the eastern Ambler River Quadrangle. In both areas structural, stratigraphic, petrologic, 40Ar/39Ar, apatite fission-track and U-Pb data were used to constrain the kinematic and thermal history of metamorphic rocks of those areas. North of the Mt. Igikpak massif a crustal section ˜15 km thick is exposed. There are upper greenschist facies rocks in the deeper portions, and very low grade metamorphic rocks at higher structural levels. Two foliations are found: a higher grade relict S1 fabric and a lower grade S 2 fabric that controls the metamorphic layering. 40Ar/ 39Ar analyses from S1 white mica in the low-grade rocks at the northern end of the transect indicate that peak M1 metamorphism occurred before ˜112 Ma. We ascribe M1 to shortening that occurred during collision of an island arc against the Arctic Alaska margin. S 2 involved the retrogression of earlier assemblages. Kinematic indicators on S2 are top-to-the-north. A rapid cooling event from 500 +/- 50°C to 300 +/- 50°C took place between ˜98 and ˜90 Ma. The driving mechanism for ductile deformation during S2, and for rapid cooling documented by our thermochronologic data, was probably the gravitational collapse of the core of the orogen, over-thickened during the preceding collision. At Shishakshinovik Pass there are Mississippian Lisburne Group strata surrounded by metamorphic rocks typical of the Central Belt of the Brooks Range. All the rocks at Shishakshinovik Pass are intensely deformed, so that one cannot distinguish between an autochthonous and an allochthonous sequence. Furthermore the Mississippian rocks, instead of being attached to the underlying basement, are in the hanging wall of a northwest dipping shear zone. Based on the variations in metamorphic grade and the 40Ar/ 39Ar thermochronology, we argue that this shear zone was an extensional structure active during the mid-Cretaceous orogenic collapse of the Brooks Range. A consequence of this structural interpretation is that the Endicott Mountains allochthon need not be restored south of the Shishakshinovik orthogneiss.

Raman spectral characteristics of magmatic-contact metamorphic coals from Huainan Coalfield, China

NASA Astrophysics Data System (ADS)

Chen, Shancheng; Wu, Dun; Liu, Guijian; Sun, Ruoyu

2017-01-01

Normal burial metamorphism of coal superimposed by magmatic-contact metamorphism makes the characteristics of the Raman spectrum of coal changed. Nine coal samples were chosen at a coal transect perpendicular to the intrusive dike, at the No. 3 coal seam, Zhuji Coal Mine, Huainan Coalfield, China, with different distances from dike-coal boundary (DCB). Geochemical (proximate and ultimate) analysis and mean random vitrinite reflectance (R0, %) indicate that there is a significant relationship between the values of volatile matter and R0 in metamorphosed coals. Raman spectra show that the graphite band (G band) becomes the major band but the disordered band (D band) disappears progressively, with the increase of metamorphic temperature in coals, showing that the structural organization in high-rank contact-metamorphosed coals is close to that of well-crystallized graphite. Evident relationships are observed between the calculated Raman spectral parameters and the peak metamorphic temperature, suggesting some spectral parameters have the potentials to be used as geothermometers for contact-metamorphic coals.

Organic matter and metamorphic history of CO chondrites

NASA Astrophysics Data System (ADS)

Bonal, Lydie; Bourot-Denise, Michèle; Quirico, Eric; Montagnac, Gilles; Lewin, Eric

2007-03-01

The metamorphic grades of a series of eight CO3 chondrites (ALHA77307, Colony, Kainsaz, Felix, Lancé, Ornans, Warrenton and Isna) have been quantified. The method used was based on the structural grade of the organic matter trapped in the matrix, which is irreversibly transformed by thermal metamorphism. The maturation of the organic matter is independent with respect to the mineralogical context and aqueous alteration. This metamorphic tracer is thus valid whatever the chemical class of chondrites. Moreover, it is sensitive to the peak metamorphic temperature. The structural grade of the organic matter was used along with other metamorphic tracers such as petrography of opaque minerals, Fa and Fs silicate composition in type I chondrules, presolar grains and noble gas (P3 component) abundance. The deduced metamorphic hierarchy and the attributed petrographic types are the following: ALHA77307 (3.03) < Colony (3.1) < Kainsaz (3.6) < Felix (3.6 (1)) < Ornans (3.6 (2)) < Lancé (3.6 (3)) < Warrenton (3.7 (1)) < Isna (3.7 (2)). For most metamorphosed objects, the peak metamorphic temperature can be estimated using a geothermometer calibrated with terrestrial metasediments [Beyssac O., Goffe B., Chopin C., and Rouzaud J. N. (2002) Raman spectrum of carbonaceous material in metasediments: a new geothermometer. J. Metamorph. Geol., 20, 859-871]. A value of 330 °C was obtained for Allende (CV chondrite), Warrenton and Isna, consistent with temperatures estimated from Fe diffusion [Weinbruch S., Armstrong J., and Palme H. (1994). Constraints on the thermal history of the Allende parent body as derive from olivine-spinel thermometry and Fe/Mg interdiffusion in olivine. Geochim. Cosmochim. Acta58(2), 1019-1030.], from the Ni content in sulfide-metal assemblages [Zanda B., Bourot-Denise M., and Hewins R. (1995) Condensate sulfide and its metamorphic transformations in primitive chondrites. Meteorit. Planet. Sci.30, A605.] and from the d002 interlayer spacing in poorly graphitized carbon [Rietmeijer, F., and MacKinnon, I. (1985) Poorly graphitized carbon as a new cosmothermometer for primitive extraterrestrial materials. Nature, 315, 733-736]. The trapped noble gas and C content appear to be sensitive but not precise metamorphic tracers, indicating that the "Ornans paradox" does not exist. Major problems with the current petrologic types derived from Induced ThermoLuminescence are pointed out.

Metamorphic core complexes: Expression of crustal extension by ductile-brittle shearing of the geologic column

NASA Technical Reports Server (NTRS)

Davis, G. H.

1985-01-01

Metamorphic core complexes and detachment fault terranes in the American Southwest are products of stretching of continental crust in the Tertiary. The physical and geometric properties of the structures, fault rocks, and contact relationships that developed as a consequence of the extension are especially well displayed in southeastern Arizona. The structures and fault rocks, as a system, reflect a ductile-through-brittle continuum of deformation, with individual structures and faults rocks showing remarkably coordinated strain and displacement patterns. Careful mapping and analysis of the structural system has led to the realization that strain and displacement were partitioned across a host of structures, through a spectrum of scales, in rocks of progressively changing rheology. By integrating observations made in different parts of the extensional system, especially at different inferred depth levels, it has been possible to construct a descriptive/kinematic model of the progressive deformation that achieved continental crustal extension in general, and the development of metamorphic core complexes in particular.

Threading Dislocations in InGaAs/GaAs (001) Buffer Layers for Metamorphic High Electron Mobility Transistors

NASA Astrophysics Data System (ADS)

Song, Yifei; Kujofsa, Tedi; Ayers, John E.

2018-07-01

In order to evaluate various buffer layers for metamorphic devices, threading dislocation densities have been calculated for uniform composition In x Ga1- x As device layers deposited on GaAs (001) substrates with an intermediate graded buffer layer using the L MD model, where L MD is the average length of misfit dislocations. On this basis, we compare the relative effectiveness of buffer layers with linear, exponential, and S-graded compositional profiles. In the case of a 2 μm thick buffer layer linear grading results in higher threading dislocation densities in the device layer compared to either exponential or S-grading. When exponential grading is used, lower threading dislocation densities are obtained with a smaller length constant. In the S-graded case, lower threading dislocation densities result when a smaller standard deviation parameter is used. As the buffer layer thickness is decreased from 2 μm to 0.1 μm all of the above effects are diminished, and the absolute threading dislocation densities increase.

Multi-scale characterization of pore evolution in a combustion metamorphic complex, Hatrurim basin, Israel: Combining (ultra) small-angle neutron scattering and image analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Hsiu-Wen; Anovitz, Lawrence; Burg, Avihu

Backscattered scanning electron micrograph and ultra small- and small-angle neutron scattering data have been combined to provide statistically meaningful data on the pore/grain structure and pore evolution of combustion metamorphic complexes from the Hatrurim basin, Israel. Three processes, anti-sintering roughening, alteration of protolith (dehydration, decarbonation, and oxidation) and crystallization of high-temperature minerals, occurred simultaneously, leading to significant changes in observed pore/grain structures. Pore structures in the protoliths, and in lowand high-grade metamorphic rocks show surface (Ds) and mass (Dm) pore fractal geometries with gradual increases in both Ds and Dm values as a function of metamorphic grade. This suggests thatmore » increases in pore volume and formation of less branching pore networks are accompanied by a roughening of pore/grain interfaces. Additionally, pore evolution during combustion metamorphism is also characterized by reduced contributions from small-scale pores to the cumulative porosity in the high-grade rocks. At high temperatures, small-scale pores may be preferentially closed by the formation of high-temperature minerals, producing a rougher morphology with increasing temperature. Alternatively, large-scale pores may develop at the expense of small-scale pores. These observations (pore fractal geometry and cumulative porosity) indicate that the evolution of pore/grain structures is correlated with the growth of high-temperature phases and is a consequence of the energy balance between pore/grain surface energy and energy arising from heterogeneous phase contacts. The apparent pore volume density further suggests that the localized time/temperature development of the high-grade Hatrurim rocks is not simply an extension of that of the low-grade rocks. The former likely represents the "hot spots (burning foci)" in the overall metamorphic terrain while the latter may represent contact aureoles.« less

Developing an inverted Barrovian sequence; insights from monazite petrochronology

NASA Astrophysics Data System (ADS)

Mottram, Catherine M.; Warren, Clare J.; Regis, Daniele; Roberts, Nick M. W.; Harris, Nigel B. W.; Argles, Tom W.; Parrish, Randall R.

2014-10-01

In the Himalayan region of Sikkim, the well-developed inverted metamorphic sequence of the Main Central Thrust (MCT) zone is folded, thus exposing several transects through the structure that reached similar metamorphic grades at different times. In-situ LA-ICP-MS U-Th-Pb monazite ages, linked to pressure-temperature conditions via trace-element reaction fingerprints, allow key aspects of the evolution of the thrust zone to be understood for the first time. The ages show that peak metamorphic conditions were reached earliest in the structurally highest part of the inverted metamorphic sequence, in the Greater Himalayan Sequence (GHS) in the hanging wall of the MCT. Monazite in this unit grew over a prolonged period between ∼37 and 16 Ma in the southerly leading-edge of the thrust zone and between ∼37 and 14.5 Ma in the northern rear-edge of the thrust zone, at peak metamorphic conditions of ∼790 °C and 10 kbar. Monazite ages in Lesser Himalayan Sequence (LHS) footwall rocks show that identical metamorphic conditions were reached ∼4-6 Ma apart along the ∼60 km separating samples along the MCT transport direction. Upper LHS footwall rocks reached peak metamorphic conditions of ∼655 °C and 9 kbar between ∼21 and 16 Ma in the more southerly-exposed transect and ∼14.5-12 Ma in the northern transect. Similarly, lower LHS footwall rocks reached peak metamorphic conditions of ∼580 °C and 8.5 kbar at ∼16 Ma in the south, and 9-10 Ma in the north. In the southern transect, the timing of partial melting in the GHS hanging wall (∼23-19.5 Ma) overlaps with the timing of prograde metamorphism (∼21 Ma) in the LHS footwall, confirming that the hanging wall may have provided the heat necessary for the metamorphism of the footwall. Overall, the data provide robust evidence for progressively downwards-penetrating deformation and accretion of original LHS footwall material to the GHS hanging wall over a period of ∼5 Ma. These processes appear to have occurred several times during the prolonged ductile evolution of the thrust. The preserved inverted metamorphic sequence therefore documents the formation of sequential 'paleo-thrusts' through time, cutting down from the original locus of MCT movement at the LHS-GHS protolith boundary and forming at successively lower pressure and temperature conditions. The petrochronologic methods applied here constrain a complex temporal and thermal deformation history, and demonstrate that inverted metamorphic sequences can preserve a rich record of the duration of progressive ductile thrusting.

Structural and luminescent Properties of Bulk InAsSb

DTIC Science & Technology

2011-12-21

have used compositionally graded metamorphic buffer layers to accommodate the misfit strain between InAsxSb1-x alloys and GaSb and InSb substrates in...wavelength range. The authors have used compositionally graded metamorphic buffer layers to accommodate the misfit strain between InAsxSb1x alloys...long wave IR range. We used compositionally graded GaInSb, AlGaInSb, and InAsxSb1x metamorphic buffer layers to accommodate the misfit strain between

The Cimmerian accretionary wedge of Anarak, Central Iran

NASA Astrophysics Data System (ADS)

Zanchi, Andrea; Malaspina, Nadia; Zanchetta, Stefano; Berra, Fabrizio; Benciolini, Luca; Bergomi, Maria; Cavallo, Alessandro; Javadi, Hamid Reza; Kouhpeyma, Meyssam

2015-04-01

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone. Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered "ophiolites" which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the "ophiolites" within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the "ophiolites" were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290 Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Metamorphic distributed Bragg reflectors for the 1440–1600 nm spectral range: Epitaxy, formation, and regrowth of mesa structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Egorov, A. Yu., E-mail: anton@beam.ioffe.ru; Karachinsky, L. Ya.; Novikov, I. I.

It is shown that metamorphic In{sub 0.3}Ga{sub 0.7}As/In{sub 0.3}Al{sub 0.7}As distributed Bragg reflectors (DBRs) with a reflection band at 1440–1600 nm and a reflectance of no less than 0.999 can be fabricated by molecular beam epitaxy (MBE) on a GaAs substrate. It is demonstrated that mesa structures formed from metamorphic DBRs on a GaAs substrate can be regrown by MBE and microcavities can be locally formed in two separate epitaxial processes. The results obtained can find wide application in the fabrication of vertical-cavity surface-emitting lasers (VCSELs) with a buried tunnel junction.

From an active continental plate margin to continental collision: New constraints from the petrological, structural and geochronological record of the (ultra) high-P metamorphic Rhodope domain (N-Greece)

NASA Astrophysics Data System (ADS)

Mposkos, E.; Krohe, A.; Wawrzenitz, N.; Romer, R. L.

2012-04-01

The Rhodope domain occupies a key area along the suture between the European and the Apulian/Adriatic plate (Schmid et al., 2008), which collided in the early Tertiary (closure of the Vardar/Axios ocean, cf. Mposkos & Krohe, 2006). An integrated study of the geochronological, tectonic and petrological data of the Rhodope domain provides the unique opportunity resolving a 160 my lasting metamorphic evolution (Jurassic to Miocene) of an active plate margin to a high degree. The Greek Rhodope consists of several composite metamorphic complexes bounded by the Nestos thrust and several normal detachment systems. The PT- and structural records of the complexes constrain metamorphic, magmatic and tectonic processes, associated with subduction along a convergent plate margin including UHP metamorphism, MP to HP metamorphism associated with continental collision, and core complex formation linked to Aegean back arc extension. We focus on the Sidironero Complex that shows a polymetamorphic history. This is documented by SHRIMP and LA-ICP-MS U-Pb zircon ages of ca. 150 Ma from garnet-kyanite gneisses that are interpreted to record the HP/UHP metamorphism (Liati, 2005; Krenn et al., 2010). SHRIMP zircon ages of ca. 51 Ma from an amphibolitized eclogite is interpreted by Liati (2005) to record a second Eocene HP metamorphic event. We present new data from an integrated petrological, geochronological and tectonic study. Granulite facies and upper amphibolite facies metamorphic conditions are recorded by the mineral assemblage Grt-Ky-Bt-Pl-Kfs-Qtz-Rt and Grt-Ky-Bt-Ms-Pl-Qtz-Rt, respectively, in deformed migmatitic metapelites. Deformation occurred under granulite facies conditions. Monazites from the matrix, that formed during the granulite facies deformation, lack core/rim structures and are only locally patchy zoned. Monazite chemical compositions are related to varying reaction partners. Single grains and fractions of few grains yield ID-TIMS U-Pb ages that plot along the concordia between 64 to 60 Ma. One date of 55 Ma might represent Pb-loss during later fluid-induced dissolution-reprecipitation. We discuss the following questions: What is the history of the high-P metamorphic rocks in the Sidironero Complex? Were high-P rocks that have been already exhumed again dragged into the subduction channel? Which rocks from the upper plate are affected by high-P metamorphism evincing that subduction erosion is an important mechanism? We reconsider the significance of the P-T-t evolution in the light of the tectonic processes that took place along the depth extension of a convergent plate interface and during subsequent continental collision along the European/Apulian Suture zone. Krenn et al., 2010. Tectonics 29, TC4001. Liati, A., 2005. Contribution to Mineralogy and Petrology 150, 608-630. Mposkos, E. & Krohe, A. 2006. Canadian Journal of Earth Sciences 43, 1755-1776. Schmid S.M., et al. 2008. Swiss Journal of Geoscience 101, 139-183.

Structural and metamorphic evolution of serpentinites and rodingites recycled in the Alpine subduction wedge

NASA Astrophysics Data System (ADS)

Zanoni, D.; Rebay, G.; Spalla, M. I.

2015-12-01

Hydration-dehydration of mantle rocks affects the viscosity of the mantle wedge and plays a prominent role in subduction zone tectonics, facilitating marble cake-type instead of large-slice dynamics. An accurate structural and petrologic investigation of serpentinites from orogenic belts, supported by their long-lived structural memory, can help to recognize pressure-sensitive mineral assemblages for deciphering their P-prograde and -retrograde tectonic trajectories. The European Alps preserve large volumes of the hydrated upper part of the oceanic lithosphere that represents the main water carrier into the Alpine subduction zone. Therefore, it is important to understand what happens during subduction when these rocks reach P-T conditions proximal to those that trigger the break-down of serpentine, formed during oceanic metamorphism, to produce olivine and clinopyroxene. Rodingites associated with serpentinites are usually derived from metasomatic ocean floor processes but rodingitization can also happen in subduction environments. Multiscale structural and petrologic analyses of serpentinites and enclosed rodingites have been combined to define the HP mineral assemblages in the Zermatt-Saas ophiolites. They record 3 syn-metamorphic stages of ductile deformation during the Alpine cycle, following the ocean floor history that is testified by structural and metamorphic relics in both rock types. D1 and D2 developed under HP to UHP conditions and D3 under lower P conditions. Syn-D2 assemblages in serpentinites and rodingites indicate conditions of 2.5 ± 0.3 GPa and 600 ± 20°C. This interdisciplinary approach shows that the dominant structural and metamorphic imprint of the Zermatt-Saas eclogitized serpentinites and rodingites developed during the Alpine subduction and that subduction-related serpentinite de-hydration occurred exclusively at Pmax conditions, during D2 deformation. In contrast, in the favourable rodingite bulk composition (Ca-rich), hydrated minerals such as vesuvianite are stable up to the estimated P-climax conditions.

Stacking and metamorphism of continuous segments of subducted lithosphere in a high-pressure wedge: The example of Alpine Corsica (France)

NASA Astrophysics Data System (ADS)

Vitale Brovarone, Alberto; Beyssac, Olivier; Malavieille, Jacques; Molli, Giancarlo; Beltrando, Marco; Compagnoni, Roberto

2013-01-01

Alpine Corsica consists of a stack of variably metamorphosed units of continental and Tethys-derived rocks. It represents an excellent example of high-pressure (HP) orogenic belt, such as the Western Alps, exposed over a small and accessible area. Compared to the Western Alps, the geology of Alpine Corsica is poorly unraveled. During the 1970s-80s, based on either lithostratigraphic or metamorphic field observations, various classifications of the belt have been proposed, but these classifications have been rarely matched together. Furthermore, through time, the internal complexity of large domains has been progressively left aside in the frame of large-scale geodynamic reconstructions. As a consequence, major open questions on the internal structure of the belt have remained unsolved. Apart from a few local studies, Alpine Corsica has not benefited of modern developments in petrology and basin research. This feature results in several uncertainties when combining lithostratigraphic and metamorphic patterns and, consequently, in the definition of an exhaustive architecture of the belt. In this paper we provide a review on the geology of Alpine Corsica, paying particular attention to the available lithostratigraphic and metamorphic classifications of the metamorphic terranes. These data are completed by a new and exhaustive metamorphic dataset obtained by means of thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM). This technique provides reliable insights on the peak temperature of the metamorphic history for CM-bearing metasediments. A detailed metamorphic characterization of metasediments, which have been previously largely ignored due to retrogression or to the lack of diagnostic mineralogy, is thus obtained and fruitfully coupled with the available lithostratigraphic data. Nine main tectono-metamorphic units are defined, from subgreenschist (ca. 280-300 °C) to the lawsonite-eclogite-facies (ca. 500-550 °C) condition. These units are homogeneous in metamorphism, laterally continuous and have characteristic lithostratigraphic features. This study also suggests a direct link between the pre-orogenic extensional setting and the present-day compressional structure of Alpine Corsica, indicating that large sections of subducted lithosphere were subducted and exhumed as coherent domains. These features provide important insight on the mechanism of stacking and exhumation of HP rocks, and make Alpine Corsica a unique reference for mountain-building processes in Tethyan-type orogens.

Breccia pipes in the Karoo Basin, South Africa, as conduits for metamorphic gases to the Early Jurassic atmosphere

NASA Astrophysics Data System (ADS)

Silkoset, Petter; Svensen, Henrik; Planke, Sverre

2014-05-01

The Toarcian (Early Jurassic) event was manifested by globally elevated temperatures and anoxic ocean conditions that particularly affected shallow marine taxa. The event coincided with the emplacement of the vast Karoo-Ferrar Large Igneous Province. Among the suggestions for trigger mechanisms for the climatic perturbation is metamorphic methane generation from black shale around the sills in the Karoo Basin, South Africa. The sill emplacement provides a mechanism for voluminous in-situ production and emission of greenhouse gases, and establishes a distinct link between basin-trapped and atmospheric carbon. In the lower stratigraphic levels of the Karoo Basin, black shales are metamorphosed around sills and the sediments are cut by a large number of pipe structures with metamorphic haloes. The pipes are vertical, cylindrical structures that contain brecciated and baked sediments with variable input of magmatic material. Here, we present borehole, petrographic, geochemical and field data from breccia pipes and contact aureoles based on field campaigns over a number of years (2004-2014). The metamorphism around the pipes show equivalent metamorphic grade as the sediments around nearby sills, suggesting a more prominent phreatomagmatic component than previously thought. The stratigraphic position of pipes and the breccia characteristics strengthens the hypothesis of a key role in the Toarcian carbon isotope excursion.

Near-isothermal conditions in the middle and lower crust induced by melt migration.

PubMed

Depine, Gabriela V; Andronicos, Christopher L; Phipps-Morgan, Jason

2008-03-06

The thermal structure of the crust strongly influences deformation, metamorphism and plutonism. Models for the geothermal gradient in stable crust predict a steady increase of temperature with depth. This thermal structure, however, is incompatible with observations from high-temperature metamorphic terranes exhumed in orogens. Global compilations of peak conditions in high-temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure, for both isothermal decompression and isobaric cooling paths. Here we develop simple one-dimensional thermal models that include the effects of melt migration. These models show that long-lived plutonism results in a quasi-steady-state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust. The models also predict that the upward advection of heat by melt generates granulite facies metamorphism, and widespread andalusite-sillimanite metamorphism in the upper crust. Once the quasi-steady-state thermal profile is reached, the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions, thus setting the stage for gravitational collapse, exhumation and isothermal decompression after the onset of plutonism. Near-isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that, in part, control the shape of the geotherm.

Age and tectonic implications of some low-grade metamorphic rocks from the Yucatan Channel

USGS Publications Warehouse

Vedder, J.G.; MacLeod, N.S.; Lanphere, M.A.; Dillon, William P.

1973-01-01

Phyllite and marble dredged from the lower part of the continental slope between Cuba and the Yucatan Peninsula seem to support the contention that a pre-early Tertiary metamorphic belt extends from the western Greater Antilles into northern Central America. The minimum K-Ar ages derived from the samples suggest that the metamorphic event was pre-Late Cretaceous, and evaluation of the K-Ar data implies that this metamorphic event is not older than Late Jurassic. Greater antiquity, however, is inferred from structural and stratigraphic relations in British Honduras, where the latest regional metamorphic event was post-Early Permian and pre-Middle Jurassic.  Rifting and extension related to plate motions along the British Honduras Quintana Roo margin through Mesozoic and earliest Cenozoic time presumably would preclude extensive regional metamorphism, permitting only limited development of schistose rocks there during that interval. The timing of metamorphic events in western Cuba is uncertain, but a pre-Middle Jurassic episode possibly is reflected in the phyllite and marble terranes of Isla de Pinos and Sierra de Trinidad. Local incipient metamorphism of Early and Middle Jurassic strata in the Sierra de los Organos may have resulted from severe tectonism that began in Late Cretaceous time and diminished in the Eocene.

Metamorphic InAsSb-based Barrier Photodetectors for the Long Wave Infrared Region

DTIC Science & Technology

2013-08-02

The character of the I–V for structures with AlInSb layer grown undoped reflects the complex nature of the potential profile in the valence band ...Al0.75In0.25Sb-based barrier photodetectors were grown metamorphically on compositionally graded Ga1?xInxSb buffer layers and GaSb substrates by...ABSTRACT InAs0.6Sb0.4/Al0.75In0.25Sb-based barrier photodetectors were grown metamorphically on compositionally graded Ga1?xInxSb buffer layers and GaSb

The lateral boundary of a metamorphic core complex: the Moutsounas shear zone on Naxos, Cyclades, Greece

NASA Astrophysics Data System (ADS)

Cao, S.; Neubauer, F.

2012-04-01

One of the apparently best investigated metamorphic core complexes all over world is that of Naxos in the Aegean Sea and numerous high-quality data on structures and microfabrics have been published. Among these structures is the Naxos-Paros ductile low-angle fault (Gautier et al., 1993), which is located along the northern margin of Naxos and which is part of the North Cycladic Detachment System (Jolivet et al., 2010). There, structural evidence indicates that the hanging wall of the core complex experienced large-scale top-to-the-north (ca. 010°) transport along a low-angle detachment fault. Interestingly no attention has been paid on the well exposed boundary fault on the eastern margin of the Naxos Island, which is even not mentioned in the lierarure. We denote this fault as Moutsounas shear zone, which represents the lateral boundary of the Naxos metamorphic core complex. The Naxos metamorphic core complex is a N-trending elongated dome, which exposes on its eastern side moderately E-dipping micaschists and marbles, which are largely well annealed due to late heating. These annealed rocks grade towards the Moutsounas Peninsula in retrogressed sheared rocks, mostly phyllonitic micaschists and phyllites with an E-dipping foliation and a ca. NNE-trending subhorizontal stretching lineation. Shear bands, asymmetric fringes around rigid clasts and oblique mineralized extension veins consistently indicate top-to-the-NNE shear. The shear zone is structurally overlain by hydrothermally altered Miocene conglomerates, which contain no pebbles from the Naxos metamorphic core complex but exclusively from the ophiolitic hangingwall unit. Miocene rocks are exposed both on the northern and southern edge of the Moutsounas Peninsula. Their bedding is variable but dips generally towards NW, oblique to the detachment fault, which dips with a medium-angle towards east indicating therefore a rollover structure. The Miocene succession is overlain by subhorizontal conglomerates of Pliocene age, which form the main portion of the Moutsounas Peninsula and which contain numerous clasts, mainly marble, of the metamorphic core complex. These sedimentary data indicate that exhumation of the Naxos metamorphic core complex postdate deposition of Miocene successions and predate Pliocene rocks. We interpret the Moutsounas shear zone as a lateral boundary of the Naxos migmatite dome and relate their main activity with top NNE-shear with the main stage of updoming during migmatite formation and granite uplift between ca. 15 and 11 Ma.

Metamorphic sole formation and early plate interface rheology: Insights from Griggs apparatus experiments

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Hirth, Greg; Yamato, Philippe; Ildefonse, Benoît; Prigent, Cécile

2016-04-01

Metamorphic soles correspond to m to ~500 m thick highly strained metamorphic rock units found beneath mylonitic banded peridotites at the base of large-scale ophiolites, as exemplified in Oman. Metamorphic soles are mainly composed of metabasalts deriving from the downgoing oceanic lithosphere and metamorphosed up to granulite-facies conditions by heat transfer from the mantle wedge. Pressure-temperature peak conditions are usually estimated at 1.0±0.2 GPa and 800±100°C. The absence of HP-LT metamorphism overprint implies that metamorphic soles have been formed and exhumed during subduction infancy. In this view, metamorphic soles were strongly deformed during their accretion to the mantle wedge (corresponding, now, to the base of the ophiolite). Therefore, metamorphic soles and banded peridotites are direct witnesses of the dynamics of early subduction zones, in terms of thermal structure, fluid migration and rheology evolution across the nascent slab interface. Based on fieldwork and EBSD analyses, we present a detailed (micro-) structural study performed on samples coming from the Sumeini window, the better-preserved cross-section of the metamorphic sole of Oman. Large differences are found in the deformation (CPO, grain size, aspect ratio) of clinopyroxene, amphibole and plagioclase, related to mineralogical changes linked with the distance to the peridotite contact (e.g., hardening due to the appearance of garnet and clinopyroxene). To model the incipient slab interface in laboratory, we carried out 5 hydrostatic annealing and simple-shear experiments on Griggs solid-medium apparatus. Deformation experiments were conducted at axial strain rates of 10-6 s-1. Fine-grained amphibolite was synthetized by adding 1 wt.% water to a (Mid-Ocean Ridge) basalt powder as a proxy for the metamorphic sole (amphibole + plagioclase + clinopyroxene ± garnet assemblage). To synthetize garnet, 2 experiments were carried out in hydrostatic conditions and with deformation at 800°C with confining pressure of 2 GPa. Another simple-shear experiment has been carried out at 800°C and 1 GPa with fined-grained natural garnet. With the aim of mimicking the early slab interface (between the metamorphic sole and banded peridotites at the base of the ophiolite), 2 simple-shear deformation experiments with 2 layers have been carried out at 800°C and confining pressure of 1 GPa. The bottom layer was made of hydrated basalt powder and the top layer was made of olivine. Fined-grained garnet-free amphibolite is significantly weaker than dunite but the appearance of harder minerals in the amphibolite (garnet and clinopyroxene) has major implications on its rheological evolution. These results allow liking field observations of strain localization at the interface to the metamorphic sole formation.

Deciphering the tectonometamorphis history of the Anarak Metamorphic Complex, Central Iran

NASA Astrophysics Data System (ADS)

Zanchetta, Stefano; Malaspina, Nadia; Zanchi, Andrea; Martin, Silvana; Benciolini, Luca; Berra, Fabrizio; Javadi, Hamid Reza; Koohpeyma, Meysam; Ghasemi, Mohammad R.; Sheikholeslami, Mohammad Reza

2014-05-01

The Cimmerian orogeny shaped the southern margin of Eurasia during the Late Permian and the Triassic. Several microplates, detached from Gondwana in the Early Permian, migrated northward to be accreted to the Eurasia margin. In the reconstruction of such orogenic event Iran is a key area. The occurrence of several "ophiolites" belt of various age, from Paleozoic to Cretaceous, poses several questions on the possibility that a single rather than multiple Paleotethys sutures occur between Eurasia and Iran. In this scenario the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted to be part of microplate of Gondwanan affinity, a relic of an accretionary wedge developed at the Eurasia margin during the Paleothetys subduction or part of the Cimmerian suture zone, occurring in NE Iran, displaced to central Iran by counterclockwise rotation of the central Iranian blocks from the Triassic. Our field structural data, petrographic and geochemical data, carried out in the frame of the DARIUS PROGRAMME, indicate that the AMC is not a single coherent block, but it consists of several units (Morghab, Chah Gorbeh, Patyar, Palhavand Gneiss, Lakh Marble, Doshak and dismembered "ophiolites") which display different tectonometamorphic evolutions. The Morghab and Chah Gorbeh units share a common history and they preserve, as a peculiar feature within metabasites, a prograde metamorphism with sin- to post-deformation growth of blueschists facies assemblages on pre-existing greenschist facies mineralogical associations. LT-HP metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the southern limit of the Chah Gorbeh unit. Finally, evidence of LT-HP metamorphism also occur in the metabasites and possibly also in the serpentinites that form most of the "ophiolites" within the AMC. Structural analyses show that the Chah Gorbeh, Morghab units and the "ophiolites" have been tectonically coupled during at least two deformational phases that occurred at greenschist facies conditions and predate the LT-HP metamorphic overprint. Available geochronological data loosely constraints the subduction event in the Late Permian - Early Triassic times. Subsequent deformation events that occurred during the whole Mesozoic and the Cenozoic up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original tectonic contacts. Therefore, the correlations among deformation structures and metamorphic events in the different units are not straightforward. The other units of the AMC lack evidence of HP metamorphism, especially the Lakh Marble a large thrust sheet that occupies the uppermost structural position in the AMC. The contact with the underlying units is invariably tectonic, thus no original relationships have been preserved. So, if structural and petrographic data point out an accretionary wedge setting for the evolution of the Chah Gorbeh, Morghab and the "ophiolites", geodynamic significance and paleogeographic attribution of other units still remain controversial. In progress U-Pb dating of undeformed intrusive bodies and metamorphic minerals in the LT-HP rocks will soon help to better constrain the evolution of the ACM.

Late-Paleozoic-Mesozoic deformational and deformation related metamorphic structures of Kuznetsk-Altai region

NASA Astrophysics Data System (ADS)

Zinoviev, Sergei

2014-05-01

Kuznetsk-Altai region is a part of the Central Asian Orogenic Belt. The nature and formation mechanisms of the observed structure of Kuznetsk-Altai region are interpreted by the author as the consequence of convergence of Tuva-Mongolian and Junggar lithospheric block structures and energy of collision interaction between the blocks of crust in Late-Paleozoic-Mesozoic period. Tectonic zoning of Kuznetsk-Altai region is based on the principle of adequate description of geological medium (without methods of 'primary' state recovery). The initial indication of this convergence is the crust thickening in the zone of collision. On the surface the mechanisms of lateral compression form a regional elevation; with this elevation growth the 'mountain roots' start growing. With an approach of blocks an interblock elevation is divided into various fragments, and these fragments interact in the manner of collision. The physical expression of collision mechanisms are periodic pulses of seismic activity. The main tectonic consequence of the block convergence and collision of interblock units is formation of an ensemble of regional structures of the deformation type on the basis of previous 'pre-collision' geological substratum [Chikov et al., 2012]. This ensemble includes: 1) allochthonous and autochthonous blocks of weakly deformed substratum; 2) folded (folded-thrust) systems; 3) dynamic metamorphism zones of regional shears and main faults. Characteristic of the main structures includes: the position of sedimentary, magmatic and PT-metamorphic rocks, the degree of rock dynamometamorphism and variety rock body deformation, as well as the styles and concentrations of mechanic deformations. 1) block terranes have weakly elongated or isometric shape in plane, and they are the systems of block structures of pre-collision substratum separated by the younger zones of interblock deformations. They stand out among the main deformation systems, and the smallest are included into the deformation systems. 2) folded (folded-thrust) deformation systems combine deformation zones with relic lenses of Paleozoid substratum, and predominantly conform systems of the main faults. Despite a high degree of regional deformation the sedimentary-stratified and intrusive-contact relations of geological bodies are stored within the deformation systems, and this differs in the main the collision systems from zones of dynamic metamorphism. 3) regional zones of dynamic metamorphism of Kuznetsk-Altai region are the concentration belts of multiple mechanic deformations and contrast dynamometamorphism of complexes. The formational basis of dynamic metamorphism zones is tectonites of the collision stage. Zones of dynamic metamorphism attract special attention in the structural model of Kuznetsk-Altai region. They not only form the typical tectonic framework of collision sutures, but also contain the main part of ore deposits of this region. Pulse mode of structure formation of Kuznetsk-Altai region is detected. Major collision events in Kuznetsk-Altai region were in the late-Carboniferous-Triassic time (307-310, 295-285, 260-250 and 240-220 Ma). This study was supported by a grant of the Russian Foundation for Basic Research (project nos. 14-05-00117).

Effects of Arabia-Eurasia Collision on Strike-slip Faults in Central Anatolia?

NASA Astrophysics Data System (ADS)

Whitney, D. L.; Lefebvre, C.; Thomson, S. N.; Idleman, L.; Cosca, M. A.; Kaymakci, N.; Teyssier, C. P.; Umhoefer, P. J.

2013-12-01

The North and East Anatolian faults accommodate much of the tectonic escape of Anatolia in response to Arabia-Eurasia collision and building of the Turkish-Iranian plateau, but these structures formed <10 m.y. ago, at least 25 m.y. after the onset of collision at ~35 Ma. Some of the major strike-slip fault zones located between the North and East Anatolian faults have had long and complex histories of displacement. These faults have deformed, and in some cases exhumed, metamorphic massifs located between fault strands. One example is the Nigde Massif, which was initially exhumed in the Late Cretaceous, then reburied and reheated, along with its overlying sedimentary basin, to a depth of ~10 km at 30 × 5 Ma. Final exhumation and cooling occurred by ~15-17 Ma (massif margin) to ~12 Ma (structurally deepest levels). This depth-temperature-time-deformation history is tracked by a combination of thermobarometric methods, structural and stratigraphic analysis, and geo/thermochronometry (U-Pb zircon, monazite; 40Ar/39Ar hornblende, muscovite, biotite, K-feldspar; zircon and apatite fission-track in metamorphic rocks and basin deposits; and apatite (U-Th)/He). Recent mapping shows the presence of at least two oblique-thrust slices; the structurally higher one accounts for the resetting of detrital apatite fission track and AHe ages in the basin rocks as well as metamorphic apatite near the margin of the massif. The structurally deeper one cuts through the metamorphic basement and explains why mineral lineations and metamorphic assemblages are different along the eastern margin relative to those in the core of the massif. Although the timing of displacement has not been dated directly, low-T thermochronology age and modeling results document a perturbation at ~30 Ma, consistent with the idea that the Ecemis Fault of the Central Anatolian Fault Zone, and probably other pre-existing strike-slip faults in central Anatolia, experienced Late Eocene-Oligocene displacement in response to Arabia-Eurasia collision to the south and SE.

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction processes and products from post-subduction processes and products. Nevertheless, available results indicate that our definition and understanding of subduction zone processes and products can be advanced by the convergence of observations and interpretations from geochemical, geological, geophysical and geodynamic studies of both oceanic and continental subduction zones. Therefore, insights into subduction zones can be provided by intergration of different approaches from different targets in the near future.

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction processes and products from post-subduction processes and products. Nevertheless, available results indicate that our definition and understanding of subduction zone processes and products can be advanced by the convergence of observations and interpretations from geochemical, geological, geophysical and geodynamic studies of both oceanic and continental subduction zones. Therefore, insights into subduction zones can be provided by integration of different approaches from different targets in the near future.

Anisotropic strain relaxation of Si-doped metamorphic InAlAs graded buffers on InP

NASA Astrophysics Data System (ADS)

Gu, Yi; Zhang, Yonggang; Chen, Xingyou; Ma, Yingjie; Zheng, Yuanliao; Du, Ben; Zhang, Jian

2017-09-01

The effects of Si doping on the strain relaxation of InP-based metamorphic In x Al1-x As graded buffers have been investigated. The highly Si-doped sample shows an increased ridge period along the [1 1 0] direction in the cross-hatch morphology measured by atomic force microscope. X-ray diffraction reciprocal space mapping measurements reveal that the high Si-doping induced incomplete relaxation as well as inhomogeneous residual strain along the [1 -1 0] direction, which was also observed in micro-Raman measurements. The anisotropic strain relaxation is attributed to the Si-doping enhanced anisotropy of misfit dislocations along the orthogonal directions. The α-misfit dislocations along the [1 -1 0] direction are further delayed to generate in highly Si-doped InAlAs buffer, while the β-misfit dislocations along the [1 1 0] direction are not. These results supply useful suggestions on the design and demonstration of semiconductor metamorphic devices.

Constraining metamorphic rates through allanite and monazite petrochronology: a case study from the Miyar Valley (High Himalayan Crystalline of Zanskar, NW India)

NASA Astrophysics Data System (ADS)

Robyr, Martin; Goswami-Banerjee, Sriparna

2014-05-01

Dating metamorphic rocks raises specific issues because metamorphism comprises a complex sequence of structural changes and chemical reactions that can be extended over millions or tens of millions of years so that metamorphic rocks cannot in general be said to have "an age". Therefore, an accurate interpretation of radiometric age data from metamorphic rocks requires first to establish the behavior of the isotopic system used for dating relative to the pressure and temperature (P-T) conditions that a metamorphic rock experienced. As the U-Th-Pb system in LREE-accessory phases like monazite and allanite is not easily reset during subsequent temperature increase, allanite and monazite U-Th-Pb ages are collectively interpreted as reflecting crystallization ages. As a consequence, to correctly interpret allanite and monazite crystallization ages, it is essential to accurately determine the physical conditions of their crystallization. A meticulous account of the chemical and textural evolution of monazite and allanite along a well constrained prograde pelitic sequence of the High Himalayan Crystalline of Zanskar (Miyar Valley; e.g. Robyr et al., 2002; 2006; 2014) reveals that: (1) the occurrence of the first metamorphic allanite coincides with the biotite-in isograd and (2) the formation of the first metamorphic monazite occurs at the staurolite-in isograd. The finding of both monazite and allanite as inclusion in staurolite porphyroblasts indicates that the breakdown of allanite and the formation of monazite occurred during staurolite crystallization. Thermobarometry results show that the metamorphic allanites are appeared in the 400-420 °C, while the signature of the first metamorphic monazite is found at ~ 600 °C with staurolite-in isograd. Allanite and monazite U-Th-Pb ages thus constrain the timing when the rocks reached the ~ 420 °C and ~ 600 °C isotherms respectively. In situ LA-ICPMS dating of coexisting allanite and monazite inclusions in garnet porphyroblasts yield respective ages of 33.6 ± 0.9 Ma and 29.5 ± 0.2 Ma, constraining the time elapsed between allanite crystallization (~ 420 °C) and monazite crystallization (~ 600°C). These data indicate that the rock needed ~ 4 Myr to be subducted from the 420 °C isotherm down to the 600°C isotherm, implying a heating rate of ca. 45°C/m.y. References Robyr, M., Epard, J.-L. & El Korh, A., 2014. Structural, metamorphic and geochronological relations between the Zanskar Shear Zone and the Miyar Shear Zone (NW Indian Himalaya): Evidence for two distinct tectonic structures and implications for the evolution of the High Himalayan Crystalline of Zanskar. Journal of Asian Earth Sciences, 79, 1-15. Robyr, M., Hacker, B. R. & Mattinson, J. M., 2006. Doming in compressional orogenic settings: New geochronological constraints from the NW Himalaya. Tectonics, 25. Robyr, M., Vannay, J. C., Epard, J. L. & Steck, A., 2002. Thrusting, extension, and doming during the polyphase tectonometamorphic evolution of the High Himalayan Crystalline Zone in NW India. Journal of Asian Earth Sciences, 21, 221-239.

Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures

DOE PAGES

Young, James L.; Steiner, Myles A.; Döscher, Henning; ...

2017-03-13

Solar water splitting via multi-junction semiconductor photoelectrochemical cells provides direct conversion of solar energy to stored chemical energy as hydrogen bonds. Economical hydrogen production demands high conversion efficiency to reduce balance-of-systems costs. For sufficient photovoltage, water-splitting efficiency is proportional to the device photocurrent, which can be tuned by judicious selection and integration of optimal semiconductor bandgaps. Here, we demonstrate highly efficient, immersed water-splitting electrodes enabled by inverted metamorphic epitaxy and a transparent graded buffer that allows the bandgap of each junction to be independently varied. Voltage losses at the electrolyte interface are reduced by 0.55 V over traditional, uniformly p-dopedmore » photocathodes by using a buried p-n junction. Lastly, advanced on-sun benchmarking, spectrally corrected and validated with incident photon-to-current efficiency, yields over 16% solar-to-hydrogen efficiency with GaInP/GaInAs tandem absorbers, representing a 60% improvement over the classical, high-efficiency tandem III-V device.« less

Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Young, James L.; Steiner, Myles A.; Döscher, Henning

Solar water splitting via multi-junction semiconductor photoelectrochemical cells provides direct conversion of solar energy to stored chemical energy as hydrogen bonds. Economical hydrogen production demands high conversion efficiency to reduce balance-of-systems costs. For sufficient photovoltage, water-splitting efficiency is proportional to the device photocurrent, which can be tuned by judicious selection and integration of optimal semiconductor bandgaps. Here, we demonstrate highly efficient, immersed water-splitting electrodes enabled by inverted metamorphic epitaxy and a transparent graded buffer that allows the bandgap of each junction to be independently varied. Voltage losses at the electrolyte interface are reduced by 0.55 V over traditional, uniformly p-dopedmore » photocathodes by using a buried p-n junction. Lastly, advanced on-sun benchmarking, spectrally corrected and validated with incident photon-to-current efficiency, yields over 16% solar-to-hydrogen efficiency with GaInP/GaInAs tandem absorbers, representing a 60% improvement over the classical, high-efficiency tandem III-V device.« less

Device characterization for design optimization of 4 junction inverted metamorphic concentrator solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geisz, John F.; France, Ryan M.; Steiner, Myles A.

Quantitative electroluminescence (EL) and luminescent coupling (LC) analysis, along with more conventional characterization techniques, are combined to completely characterize the subcell JV curves within a fourjunction (4J) inverted metamorphic solar cell (IMM). The 4J performance under arbitrary spectral conditions can be predicted from these subcell JV curves. The internal radiative efficiency (IRE) of each junction has been determined as a function of current density from the external radiative efficiency using optical modeling, but this required the accurate determination of the individual junction current densities during the EL measurement as affected by LC. These measurement and analysis techniques can be appliedmore » to any multijunction solar cell. The 4J IMM solar cell used to illustrate these techniques showed excellent junction quality as exhibited by high IRE and a one-sun AM1.5D efficiency of 36.3%. This device operates up to 1000 suns without limitations due to any of the three tunnel junctions.« less

Improved GaSb-based quantum well laser performance through metamorphic growth on GaAs substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Richardson, Christopher J. K., E-mail: richardson@lps.umd.edu; He, Lei; Apiratikul, Paveen

The promise of the metamorphic growth paradigm is to enable design freedom of the substrate selection criteria beyond current choices that are limited by lattice matching requirements. A demonstration of this emerging degree of freedom is reported here by directly comparing identical laser structures grown both pseudomorphically on a GaSb substrate and metamorphically on a GaAs substrate. Improved thermal performance of the metamorphic laser material enables a higher output power before thermal roll-over begins. These performance gains are demonstrated in minimally processed gain-guided broad-area type-I lasers emitting close to 2-μm wavelengths and mounted p-side up. Continuous wave measurements at roommore » temperature yield a T{sub 0} of 145 K and peak output power of 192 mW from metamorphic lasers, compared to a T{sub 0} of 96 K and peak output power of 164 mW from identical lasers grown pseudomorphically on GaSb.« less

Structural and Optical Characteristics of Metamorphic Bulk InAsSb

DTIC Science & Technology

2014-01-01

0.815 0.820 0.825 InAsSb 5 /a ⊥ ( Å ) 3√2/a|| (Å) 0.25 0.40 0.75 1.3 1.5 1.8 2.2 hkl = 335 GaSb Structural and Optical Characteristics of Metamorphic...Conduction- and Valence- Band Energies in Bulk InAs1−xSbx and Type II InAs1−xSbx/InAs Strained-Layer Superlattices”, J. of Electron. Mater., 42, 918...0188 3. DATES COVERED (From - To) - UU UU UU UU Approved for public release; distribution is unlimited. Structural and Optical Characteristics of

Spatiotemporal evolution of magmatic pulses and regional metamorphism during a Cretaceous flare-up event: Constraints from the Ryoke belt (Mikawa area, central Japan)

NASA Astrophysics Data System (ADS)

Takatsuka, Kota; Kawakami, Tetsuo; Skrzypek, Etienne; Sakata, Shuhei; Obayashi, Hideyuki; Hirata, Takafumi

2018-05-01

The spatiotemporal relationship between granitoid intrusions and low-pressure/temperature type regional metamorphism in the Ryoke belt (Mikawa area) is investigated to understand the tectono-thermal evolution of the upper- to middle-crust during a Cretaceous flare-up event at the Eurasian active continental margin. Three plutono-metamorphic stages are recognized; (1) 99-84 Ma: intrusion of granitoids (99-95 Ma pulse) into the upper crust and high-T regional metamorphism reaching sillimanite-grade (97.0 ± 4.4 Ma to 88.5 ± 2.5 Ma) in the middle crust, (2) 81-75 Ma: intrusion of gneissose granitoids (81-75 Ma Ma pulse) into the middle crust at 19-24 km depth, and (3) 75-69 Ma: voluminous intrusions of massive to weakly-foliated granitoids (75-69 Ma pulse) at 9-13 km depth and formation of contact metamorphic aureoles. Cooling of the highest-grade metamorphic zone below the wet solidus of granitic rocks is estimated at 88.5 ± 2.5 Ma. At ca. 75 Ma, the upper-middle crustal section underwent northward tilting, resulting in the exhumation of regional metamorphic zones to 9-13 km depth. Although the highest-grade metamorphic rocks and the 99-95 Ma pulse granitoids preserve similar U-Pb zircon ages, the absence of spatial association suggests that the regional metamorphic zones were mainly produced by a transient thermal anomaly in the mantle and thermal conduction through the crust, supplemented by localized advection due to granitoid intrusions. The successive emplacement of granitoids into shallow, deep and shallow levels of the crust was probably controlled by the combination of change in thermal structure of the crust and tectonics during granitoid intrusions.

Morphological preservation of carbonaceous plant fossils in blueschist metamorphic rocks from New Zealand.

PubMed

Galvez, M E; Beyssac, O; Benzerara, K; Bernard, S; Menguy, N; Cox, S C; Martinez, I; Johnston, M R; Brown, G E

2012-03-01

Morphological and chemical evidence of ancient life is widespread in sedimentary rocks retrieved from shallow depths in the Earth's crust. Metamorphism is highly detrimental to the preservation of biological information in rocks, thus limiting the geological record in which traces of life might be found. Deformation and increasing pressure/temperature during deep burial may alter the morphology as well as the composition and structure of both the organic and mineral constituents of fossils. However, microspore fossils have been previously observed in intensely metamorphosed rocks. It has been suggested that their small size, and/or the nature of the polymer composing their wall, and/or the mineralogy of their surrounding matrix were key parameters explaining their exceptional preservation. Here, we describe the remarkable morphological preservation of plant macrofossils in blueschist metamorphic rocks from New Zealand containing lawsonite. Leaves and stems can be easily identified at the macroscale. At the microscale, polygonal structures with walls mineralized by micas within the leaf midribs and blades may derive from the original cellular ultrastructure or, alternatively, from the shrinkage during burial of the gelified remnants of the leaves in an abiotic process. Processes and important parameters involved in the remarkable preservation of these fossils during metamorphism are discussed. Despite the excellent morphological preservation, the initial biological polymers have been completely transformed to graphitic carbonaceous matter down to the nanometer scale. This occurrence demonstrates that plant macrofossils may experience major geodynamic processes such as metamorphism and exhumation involving deep changes and homogenization of their carbon chemistry and structure but still retain their morphology with remarkable integrity even if they are not shielded by any hard-mineralized concretion. © 2012 Blackwell Publishing Ltd.

Zircon U-Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

NASA Astrophysics Data System (ADS)

Zhai, Qing-guo; Jahn, Bor-ming; Li, Xian-hua; Zhang, Ru-yuan; Li, Qiu-li; Yang, Ya-nan; Wang, Jun; Liu, Tong; Hu, Pei-yuan; Tang, Suo-han

2017-06-01

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U-Pb dating of zircon gave an age of 232-237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu-Hf mineral isochron and phengite Ar-Ar ages. The zircon U-Pb and mineral Lu-Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U-Pb and phengite Ar-Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP-UHP metamorphic rocks of oceanic derivation.

High-Concentration III-V Multijunction Solar Cells | Photovoltaic Research

Science.gov Websites

| NREL High-Concentration III-V Multijunction Solar Cells High-Concentration III-V transfer to the high-efficiency cell industry, and the invention and development of the inverted metamorphic multijunction (IMM) cell technology. PV Research Other Materials & Devices pages: High

Interdisciplinary approach to exploit the tectonic memory in the continental crust of collisional belts.

NASA Astrophysics Data System (ADS)

Gosso, G.; Marotta, A. M.; Rebay, G.; Regorda, A.; Roda, M.; Spalla, M. I.; Zanoni, D.; Zucali, M.

2015-12-01

Collisional belts result by thoroughly competing thermo-mechanical disaggregation and coupling within both continental and oceanic lithospheric slices, during construction of tectono-metamorphic architectures. In multiply reworked metamorphics, tectonic units may be contoured nowadays on the base of coherent thermo-baric and structural time-sequences rather than simply relying on lithologic affinities. Sequences of equilibrium assemblages and related fabric imprints are an approach that appears as a more reliable procedure, that enables to define tectonic units as the volume of crustal slices that underwent corresponding variations during the dynamics of an active margin and takes into account a history of physical imprints. The dimensions of these tectonic units may have varied over time and must be reconstructed combining the tracers of structural and metamorphic changes of basement rocks, since such kind of tectono-metamorphic units (TMUs) is a realistic configuration of the discrete portions of orogenic crust that experienced a coherent sequence of metamorphic and textural variations. Their translational trajectories, and bulk shape changes during deformation, cannot simply be derived from the analysis of the geometries and kinematics of tectonic units, but are to be obtained by adding the reconstruction of quantitative P-T-d-t paths making full use of fossil mineral equilibria. The joint TMU field-and-laboratory definition is an investigation procedure that bears a distinct thermo-tectonic connotation, that, through modelling, offers the opportunity to test the physical compatibilities of plate-scale interconnected variables, such as density, viscosity, and heat transfer, with respect to what current interpretative geologic histories may imply. Comparison between predictions from numerical modelling and natural data obtained by this analytical approach can help to solve ambiguities on geodynamic significance of structural and thermal signatures, also as a function of tectonic rate of simulated convergent or divergent kinematics. In addition the estimate of structurally and mineral-chemically re-equilibrated volumes assists the choice of physical parameters selected to constrain numerical models.

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Golovynskyi, S. L., E-mail: golovynskyi@isp.kiev.ua; Seravalli, L.; Trevisi, G.

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In{sub 0.15}Ga{sub 0.85}As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In{sub 0.15}Ga{sub 0.85}As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV ismore » related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.« less

The Mt. Ochi melange (South Evvia Island, Greece): a case study for HP metamorphism and syn-convergent exhumation.

NASA Astrophysics Data System (ADS)

Moustaka, Eleni; Soukis, Konstantinos; Huet, Benjamin; Lozios, Stylianos; Magganas, Andreas

2014-05-01

The Attic-Cycladic complex (central Aegean Sea, Greece) experienced profound extension since at least the Oligo-Miocene boundary during which the previously thickened crust was reworked by a series of detachments forming the NE directed North Cycladic Detachment System (NCDS) and the SSW directed West Cycladic Detachment System (WCDS). South Evvia Island is located at the northwestern part of the Attic Cycladic complex linking the highly thinned and polymetamorphosed central part of the complex with mainland Greece. Furthermore, greenschists-facies retrograde metamorphism has only partially overprinted the HP mineral assemblages. Consequently, it is an ideal area to study tectonic processes associated with subduction, HP metamorphism and subsequent exhumation from eclogitic depths to the surface. Geological mapping in 1:2:000 scale revealed that the tectonostratigraphy of Mt. Ochi includes three distinct units all metamorphosed in HP conditions followed by greenschist facies overprint. These units are from top to bottom a) the Ochi Unit, a thick metavolcanosedimentary sequence with some intensely folded cipoline marble intercalations and isolated occurrences of metabasic rocks b) the ophiolitic mélange (metagabbros, metawherlites, peridotites, metabasites within a metasedimentary+serpentinite matrix) and c) the lowermost Styra Unit, a cipoline marble-dominated unit with thin mica schists and rare quartzitic layers often boudinaged. The thrust fault that was responsible for the juxtaposition of these three units acted in an early stage during HP metamorphism and it was isoclinally folded and sheared by the following syn-metamorphic deformation events. Detailed structural study in meso- and microscopic scale combined with petrological and geochemical analyses of the Mt Ochi rocks led to the distinction of at least three syn-metamorphic and two post-metamorphic deformation episodes that affected all units. The oldest structure identified is a relic foliation formed by the mineral assemblage Na-amphibole + lawsonite seen as inclusion in epidote porphyroblasts within the melange. It could represent a structure of the prograde path but it could also have formed during the peak HP event. This is followed by successive folding episodes that are related to axial plane foliations and a ~E-W intersection/stretching lineation formed by typical blueschist- to epidote-blueschist facies mineral assemblages. The main foliation that can be observed in all three units is a greenschist-facies axial plane foliation accompanied by a ~ENE-WSW stretching lineation. The shear sense during the prograde path is constantly towards the WSW. In the greenschists-facies an unambiguous top-to ENE can be observed mostly in mylonitic rocks. The following deformation episodes include semi-brittle to brittle structures (shear bands brittle open folds, crenulation cleavage, and faults with increasingly higher-angle) that are not as penetrative and record the passage of the units through the brittle-ductile transitions and to higher structural levels. The kinematics of these late episodes is also towards the NE. Based on the above, the Mt Ochi HP units exhibit a common tectonometamorphic evolution since at least the early stages of the prograde path. The Ochi Unit/Styra Unit contact is a structure that formed prior to or during peak HP metamorphism and therefore it couldn't have served as the normal fault to an extrusion wedge.

Regional fluid and metal mobility in the Dalradian metamorphic belt, Southern Grampian Highlands, Scotland

NASA Astrophysics Data System (ADS)

Craw, D.

1990-10-01

A prominent set of veins was formed during post-metamorphic deformation of the Caledonian Dalradian metamorphic belt. These veins are concentrated in dilational zones in fold hinges, but apophyses follow schistosity and fold axial surface fractures. The veins are most common in the cores of regional structures, especially the Dalradian Downbend and consist of quartz, calcite, chlorite and metallic sulphides and oxides. Metals, including gold, have been concentrated in the veins. The fluid which formed the veins was low salinity (1 5 wt% NaCl and KCl) CO2-bearing (3 16 wt% CO2) water of metamorphic origin. The fluid varies slightly in composition within and between samples, but is essentially uniform in composition over several hundred km2. Vein formation occurred at about 350±50 °C and 200 300 MPa pressure. Further quartz mineralization occurred in some dilational zones at lower temperatures (160 180 °C). This later mineralization was accompanied by CO2 immiscibility. Dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle-ductile transition. A similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby Tyndrum Fault at a later stage in the Dalradian uplift.

The Later Paleozoic granites of the Greater Caucasus Fore Range zone: geochemistry, magnetic properties and the structural and metamorphic evolution.

NASA Astrophysics Data System (ADS)

Kamzolkin, Vladimir; Latyshev, Anton; Ivanov, Stanislav; Vidjapin, Jury

2017-04-01

Clarification of the position of the granitic intrusions associated with the Blyb Metamorphic Complex is the important problem of the reconstruction of the structural evolution of the Greater Caucasus Fore Range zone. Based of the rock geochemistry we found out that the quartz diorites, granodiorites and syeno-granites of the BMC formed in suprasubduction conditions and refer to I-type granites. However, their emplacement was multistage coinciding with the various stages of the BMC evolution. We detected the mineral associations typical for the epidote-amphibolite facies in the Balkan massif, but these metamorphic features are absent in the granodiorite intrusions in the southern part of the Fore Range zone. Thus, quartz diorites of the Balkan intrusion intruded after the high-pressure metamorphism of the host rocks, but before the epidote-amphibolite stage, and the Southern granodiorite intrusions are younger. The measurements of the anisotropy of the magnetic susceptibility (AMS) in the Balkan intrusion indicated the shallow orientation of the minimal (north-eastern strike) and maximal (north-western strike) axes of the AMS ellipsoid. This result is compatible with the idea of the north-east compression fixed in the fold deformation structures of the BMC host rocks (Vidyapin, Kamzolkin, 2015). However, the macroscopic foliation in the granites dips to the east steeply. The discrepancy of the texture orientation of the granites, the host rock structure and the magnetic fabric can be explained as a result of the repeated changes of the stress field during the evolution of the Fore Range nappe structures. The reported study was partially supported by RFBR, research projects No. 16-35-00571mol_a; 16-05-01012a.

The "granite pump": LP/HT metamorphism and exhumation in the Montagne Nore (S-France)

NASA Astrophysics Data System (ADS)

Franke, W.; Doublier, M. P.; Doerr, W.; Stein, E.

2003-04-01

The Montagne Noire at the southern margin of the French Massif Central represents an exceptional case of a hot metamorphic core complex evolved from a thrust stack in a foreland position. The core of the structure (Zone Axiale) exposes granites and LP/HT gneisses up to anatectic grade. The hot core is encased by ENE-trending shear zones, which define a dextral pull-apart structure. Ductile extension is documented by top WSW shearing in the W, and ENE shearing in the E part of the Zone Axiale (eg, MATTE et al., 1998). Extension in ENE and reduction of the metamorphic profile are accompanied by NNW-directed contraction ("pinched pull-apart"). Palaeozoic sediments on the southern flank of the Zone Axiale exhibit only greenschist to diagenetic grades of metamorphism. Conodont alteration index (WIEDERER et al., 2002) and illite crystallinity (Doublier, this meeting) reveal a decrease of metamorphic temperature away from the hot core. Metamorphic isograds cut across the axial planes of D1 nappes. These features suggest that metamorphism was imposed by the rising hot core. Accordingly, the palaeozoic sediments show a tectonic evolution which closely resembles that of the gneissic core (extension top ENE, contraction in NNW). Structures relating to stacking (D1) have survived at the southern margin of the Montagne Noire. U-Pb studies (TIMS on single zircon and monazite) reveal peak metamorphism and magmatism already at c. 315 Ma (KLAMA et al., 2001), i.e., only <10 Ma after the end of flysch deposition in latest Visean/Early Namurian time (<= 323 Ma). The coincidence, within error, of the U-Pb ages and earlier Ar/Ar ages (MALUSKI et al., 1991) suggest rapid cooling. Synchronous granite emplacement and metamorphism is best explained by advective heating. Since granites are not generated in foreland settings, we propose derivation of the melts from areas of thickened crust adjacent to the N. Transport and emplacement of granites was essentially driven by the hydraulic gradient between the locus of melt generation in the orogenic root, and the opening pull-apart window. Such a pumping model may also be applied to other thermal anomalies in the Variscan Belt, e.g., in the SW-Bohemian Transverse Zone (FRANKE 2000), or in the Saxonian Granulites (FRANKE and STEIN 2000). Hydraulic expulsion of hot, low viscously materials has played an important role in the transport of heat for the hot Variscan root to higher and more external parts of the crust. FRANKE (2000); Geol. Soc. Spec. Publ. No. 179, 35-63. FRANKE and STEIN (2000); Geol. Soc. Spec. Publ. No. 179, 337-355. KLAMA et al. (2001); J. Conf. Abs.,6, 235. MALUSKI et al. (1991); Lithos, 26: 287-304. MATTE et al. (1998); Geodynamica Acta: 13-22. WIEDERER et al. (2002); Schweiz. Mineral. Petrogr. Mitt. 82, 393-407.

Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: The consequences of temperature and timescale on 40Ar/39Ar mica geochronology

NASA Astrophysics Data System (ADS)

Mottram, Catherine M.; Warren, Clare J.; Halton, Alison M.; Kelley, Simon P.; Harris, Nigel B. W.

2015-12-01

40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (P-T) conditions. The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span 2-9 Ma within each sample in the structurally lower levels (garnet grade) but only 0-3 Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2 ± 3.9 Ma (2σ) to 13.2 ± 1.3 Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27 ± 0.13 Ma to 12.96 ± 0.05 Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7 ± 11.8 Ma (2σ) at the lowest structural levels to 18.6 ± 4.7 Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed. Temperatures > 600 °C at pressures of 0.4-0.8 GPa sustained over > 5 Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of 50-80 °C Ma- 1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene.

Late Cretaceous to Paleocene metamorphism and magmatism in the Funeral Mountains metamorphic core complex, Death Valley, California

USGS Publications Warehouse

Mattinson, C.G.; Colgan, J.P.; Metcalf, J.R.; Miller, E.L.; Wooden, J.L.

2007-01-01

Amphibolite-facies Proterozoic metasedimentary rocks below the low-angle Ceno-zoic Boundary Canyon Detachment record deep crustal processes related to Meso-zoic crustal thickening and subsequent extension. A 91.5 ?? 1.4 Ma Th-Pb SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) monazite age from garnet-kyanite-staurolite schist constrains the age of prograde metamorphism in the lower plate. Between the Boundary Canyon Detachment and the structurally deeper, subparallel Monarch Spring fault, prograde metamorphic fabrics are overprinted by a pervasive greenschist-facies retrogression, high-strain subhorizontal mylonitic foliation, and a prominent WNW-ESE stretching lineation parallel to corrugations on the Boundary Canyon Detachment. Granitic pegmatite dikes are deformed, rotated into parallelism, and boudinaged within the mylonitic foliation. High-U zircons from one muscovite granite dike yield an 85.8 ?? 1.4 Ma age. Below the Monarch Spring fault, retrogression is minor, and amphibolite-facies mineral elongation lineations plunge gently north to northeast. Multiple generations of variably deformed dikes, sills, and leucosomal segregations indicate a more complex history of partial melting and intrusion compared to that above the Monarch Spring fault, but thermobarometry on garnet amphibolites above and below the Monarch Spring fault record similar peak conditions of 620-680 ??C and 7-9 kbar, indicating minor (<3-5 km) structural omission across the Monarch Spring fault. Discordant SHRIMP-RG U-Pb zircon ages and 75-88 Ma Th-Pb monazite ages from leucosomal segregations in paragneisses suggest that partial melting of Proterozoic sedimentary protoliths was a source for the structurally higher 86 Ma pegmatites. Two weakly deformed two-mica leucogranite dikes that cut the high-grademetamorphic fabrics below the Monarch Spring fault yield 62.3 ?? 2.6 and 61.7 ?? 4.7 Ma U-Pb zircon ages, and contain 1.5-1.7 Ga cores. The similarity of metamorphic, leuco-some, and pegmatite ages to the period of Sevier belt thrusting and the period of most voluminous Sierran arc magmatism suggests that both burial by thrusting and regional magmatic heating contributed to metamorphism and subsequent partial melting. ??2007 Geological Society of America. All rights reserved.

The timing of tertiary metamorphism and deformation in the Albion-Raft River-Grouse Creek metamorphic core complex, Utah and Idaho

USGS Publications Warehouse

Strickland, A.; Miller, E.L.; Wooden, J.L.

2011-01-01

The Albion-Raft River-Grouse Creek metamorphic core complex of southern Idaho and northern Utah exposes 2.56-Ga orthogneisses and Neoproterozoic metasedimentary rocks that were intruded by 32-25-Ma granitic plutons. Pluton emplacement was contemporaneous with peak metamorphism, ductile thinning of the country rocks, and top-to-thewest, normal-sense shear along the Middle Mountain shear zone. Monazite and zircon from an attenuated stratigraphic section in the Middle Mountain were dated with U-Pb, using a SHRIMP-RG (reverse geometry) ion microprobe. Zircons from the deformed Archean gneiss preserve a crystallization age of 2532 ?? 33 Ma, while monazites range from 32.6 ?? 0.6 to 27.1 ?? 0.6 Ma. In the schist of the Upper Narrows, detrital zircons lack metamorphic overgrowths, and monazites produced discordant U-Pb ages that range from 52.8 ?? 0.6 to 37.5 ?? 0.3 Ma. From the structurally and stratigraphically highest unit sampled, the schist of Stevens Spring, narrow metamorphic rims on detrital zircons yield ages from 140-110 Ma, and monazite grains contained cores that yield an age of 141 ??2 Ma, whereas rims and some whole grains ranged from 35.5 ?? 0.5 to 30.0 ?? 0.4 Ma. A boudinaged pegmatite exposed in Basin Creek is deformed by the Middle Mountains shear zone and yields a monazite age of 27.6 ?? 0.2 Ma. We interpret these data to indicate two periods of monazite and metamorphic zircon growth: a poorly preserved Early Cretaceous period (???140 Ma) that is strongly overprinted by Oligocene metamorphism (???32-27 Ma) related to regional plutonism and extension. ?? 2011 by The University of Chicago.

Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell

DOE PAGES

Geisz, John F.; Steiner, Myles A.; Jain, Nikhil; ...

2017-12-20

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm 2 that are transparent to <2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solutionmore » to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with VOC = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.« less

Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geisz, John F.; Steiner, Myles A.; Jain, Nikhil

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm 2 that are transparent to <2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solutionmore » to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with VOC = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.« less

Structure and metamorphism of the Franciscan Complex, Mt. Hamilton area, Northern California

USGS Publications Warehouse

Blake, M.C.; Wentworth, C.M.

1999-01-01

Truncation of metamorphic isograds and fold axes within coherent terranes of Franciscan metagraywacke by intervening zones of melange indicate that the melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-facies rocks of the Franciscan Complex in the Mt. Hamilton area, northern California-the Jurassic Yolla Bolly terrane and the structurally underlying Cretaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates thrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding defines locally coherent structural orientations and folds within the metagraywacke units. These units are crosscut by thin zones of tectonic melange containing blocks of high-grade blueschist, serpentinite, and other exotic rocks, and a broader, but otherwise identical melange zone marks the discordant boundary between the two terranes.

Organic nitrogen chemistry during low-grade metamorphism

USGS Publications Warehouse

Boudou, J.-P.; Schimmelmann, A.; Ader, M.; Mastalerz, Maria; Sebilo, M.; Gengembre, L.

2008-01-01

Most of the organic nitrogen (Norg) on Earth is disseminated in crustal sediments and rocks in the form of fossil nitrogen-containing organic matter. The chemical speciation of fossil Norg within the overall molecular structure of organic matter changes with time and heating during burial. Progressive thermal evolution of organic matter involves phases of enhanced elimination of Norg and ultimately produces graphite containing only traces of nitrogen. Long-term chemical and thermal instability makes the chemical speciation of Norg a valuable tracer to constrain the history of sub-surface metamorphism and to shed light on the subsurface biogeochemical nitrogen cycle and its participating organic and inorganic nitrogen pools. This study documents the evolutionary path of Norg speciation, transformation and elimination before and during metamorphism and advocates the use of X-ray photoelectron spectroscopy (XPS) to monitor changes in Norg speciation as a diagnostic tool for organic metamorphism. Our multidisciplinary evidence from XPS, stable isotopes, traditional quantitative coal analyses, and other analytical approaches shows that at the metamorphic onset Norg is dominantly present as pyrrolic and pyridinic nitrogen. The relative abundance of nitrogen substituting for carbon in condensed, partially aromatic systems (where N is covalently bonded to three C atoms) increases exponentially with increasing metamorphic grade, at the expense of pyridinic and pyrrolic nitrogen. At the same time, much Norg is eliminated without significant nitrogen isotope fractionation. The apparent absence of Rayleigh-type nitrogen isotopic fractionation suggests that direct thermal loss of nitrogen from an organic matrix does not serve as a major pathway for Norg elimination. Instead, we propose that hot H, O-containing fluids or some of their components gradually penetrate into the carbonaceous matrix and eliminate Norg along a progressing reaction front, without causing nitrogen isotope fractionation in the residual Norg in the unreacted core of the carbonaceous matrix. Before the reaction front can reach the core, an increasing part of core Norg chemically stabilizes in the form of nitrogen atoms substituting for carbon in condensed, partially aromatic systems forming graphite-like structural domains with delocalized ??-electron systems (nitrogen atoms substituting for "graphitic" carbon in natural metamorphic organic matter). Thus, this nitrogen species with a conservative isotopic composition is the dominant form of residual nitrogen at higher metamorphic grade. ?? 2007 Elsevier Ltd. All rights reserved.

Relieved kink effects in symmetrically graded In0.45Al0.55As/InxGa1-xAs metamorphic high-electron-mobility transistors

NASA Astrophysics Data System (ADS)

Lee, Ching-Sung; Liao, Chen-Hsian

2007-12-01

Kink effects in an In-rich InxGa1-xAs (x=0.53-0.63) linearly graded channel of an In0.45Al0.55As/InxGa1-xAs metamorphic high-electron-mobility transistor have been effectively relieved by depositing a high-barrier Ni /Au gate with the silicon nitride passivation. Complete physical investigations for the relieved kink effects have been made by comparing identical devices with/without a high-barrier Schottky gate or the surface passivation. After successfully suppressing the kink effects, the proposed device has shown a superior voltage gain of 173.8, low output conductance of 2.09mS/mm, and excellent power-added efficiency of 54.1% with high output power (power gain) of 14.87dBm (14.53dB). Improved linearity and excellent thermal threshold coefficient (∂Vth/∂T) of -0.14mV/K have also been achieved. The proposed design provides good potential for high-gain and high-linearity circuit applications.

Gating of high-mobility InAs metamorphic heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shabani, J.; McFadden, A. P.; Shojaei, B.

We investigate the performance of gate-defined devices fabricated on high mobility InAs metamorphic heterostructures. We find that heterostructures capped with In{sub 0.75}Ga{sub 0.25}As often show signs of parallel conduction due to proximity of their surface Fermi level to the conduction band minimum. Here, we introduce a technique that can be used to estimate the density of this surface charge that involves cool-downs from room temperature under gate bias. We have been able to remove the parallel conduction under high positive bias, but achieving full depletion has proven difficult. We find that by using In{sub 0.75}Al{sub 0.25}As as the barrier withoutmore » an In{sub 0.75}Ga{sub 0.25}As capping, a drastic reduction in parallel conduction can be achieved. Our studies show that this does not change the transport properties of the quantum well significantly. We achieved full depletion in InAlAs capped heterostructures with non-hysteretic gating response suitable for fabrication of gate-defined mesoscopic devices.« less

Deformation sequences of the Day Nui Con Voi metamorphic belt, northern Vietnam

NASA Astrophysics Data System (ADS)

Yeh, M. W.; Lee, T. Y.; Lo, C. H.; Chung, S. L.; Lan, C. Y.; Lee, J. C.; Lin, T. S.; Lin, Y. J.

2003-04-01

The correlation of structure, microstructure and metamorphic assemblages is of fundamental importance to the understanding of the complex tectonic history and kinematics of the Day Nui Con Voi (DNCV) metamorphic belt in Vietnam along the Ailao Shan-Red River (ASRR) shear zone as it provides constraints on the relative timing of the deformation, kinematics and metamorphism. High-grade metamorphic rocks of amphibolite faces showed consistent deformation sequences of three folding events followed by one brittle deformation through all four cross sections from Lao Cai to Viet Tri indicated the DNCV belt experienced similar deformation condition throughout its length. The first deformation event, D1, produced up-right folds (locally preserved) with sub-vertical, NE-SW striking axial planes with dextral sense of shear probably formed during the early phase of the lowermost Triassic Indosinian orogeny. Followed by this compressional event is a gravitational collapsing event, D2, which is the major deformation and metamorphic event characterized by kyanite grade metamorphism and large scale horizontal folds with NW-SE (320) striking sub-horizontal axial pane showing sinsistral sense of shear most likely formed during the Oligocene-Miocene SE extrusion of Indochina peninsula. The 3rd folding event, D3, is a post-metamorphism doming event with NW-SE (310) striking sub-vertical axial plane that folded/tilted the once sub-horizontal D2 axial planes into shallowly (<30 degrees) NE dipping on the NE limb, and SW dipping on the SW limb possibly due to left-lateral movement of the N-S trending Xian Shui He fault system in Mid-Miocene. The outward decreasing of the metamorphic grade from kyanite to garnet then biotite indicated the D3 occurred post metamorphism. Reactivation of the sub-horizontal D2 fold axial planes showed dextral sense of shear possibly due to Late Miocene-Pliocene right-lateral movement of the ASRR shear zone. This right lateral movement continuously deformed the DNCV with brittle fractures such as joints and normal faults (D4) striking NE-SW to E-W and NW-SE.

Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

NASA Astrophysics Data System (ADS)

Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

2006-12-01

Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural data, the rutile age likely represents the prograde-leg of the eclogite-facies P-T path whereas the Lu-Hf garnet age likely represents higher grade metamorphic conditions. The timing of eclogite-facies metamorphism in the MR is within the same time interval as the duration of prograde metamorphism (~55-40) recorded in the structurally overlying Zermatt-Saas ophiolite (ZSO; e.g., Amato et al., 1999; Lapen et al., 2003; Mahlen et al., this meeting). In particular, the Lu-Hf garnet age from the MR is identical within error to a relatively young 40.8 +/- 1.8 Ma Lu-Hf garnet-whole rock-cpx age from a structurally low slice of the ZSO at Saas-Fee, Switzerland (Mahlen et al., this meeting). Not only do the ages of eclogite-facies metamorphism overlap between the MR and ZSO, but so do the P-T conditions (e.g., between 1.6-2.8 GPa; 500-600°C). These data, combined with the relative structural positions of the MR and ZSO in the western Alps, suggest that the MR and ZSO were likely juxtaposed within the subduction channel through underplating of the MR beneath the ZSO. The strong negative buoyancy of the MR has likely aided in the exhumation of sections of the ZSO. Therefore, coupling of continental and oceanic terranes in a subduction channel, perhaps a general feature in the western Alps, may be critical in preventing permanent loss of oceanic crust to the mantle.

Geology of the Biwabik Iron Formation and Duluth Complex.

PubMed

Jirsa, Mark A; Miller, James D; Morey, G B

2008-10-01

The Biwabik Iron Formation is a approximately 1.9 billion year-old sequence of iron-rich sedimentary rocks that was metamorphosed at its eastern-most extent by approximately 1.1 billion year-old intrusions of the Duluth Complex. The metamorphic recrystallization of iron-formation locally produced iron-rich amphiboles and other fibrous iron-silicate minerals. The presence of these minerals in iron-formation along the eastern part of what is known as the Mesabi Iron Range, and their potential liberation by iron mining has raised environmental health concerns. We describe here the geologic setting and mineralogic composition of the Biwabik Iron Formation in and adjacent to the contact metamorphic aureole of the Duluth Complex. The effects of metamorphism are most pronounced within a few kilometers of the contact, and decrease progressively away from it. The contact aureole has been divided into four metamorphic zones-each characterized by the composition and crystal structure of the metamorphic minerals it contains. The recrystallization of iron-formation to iron-rich amphibole minerals (grunerite and cummingtonite) and iron-pyroxene minerals (hedenbergite and ferrohypersthene) is best developed in zones that are most proximal to the Duluth Complex contact.

Geology of the Biwabik Iron Formation and Duluth Complex

USGS Publications Warehouse

Jirsa, M.A.; Miller, J.D.; Morey, G.B.

2008-01-01

The Biwabik Iron Formation is a ???1.9 billion year-old sequence of iron-rich sedimentary rocks that was metamorphosed at its eastern-most extent by ???1.1 billion year-old intrusions of the Duluth Complex. The metamorphic recrystallization of iron-formation locally produced iron-rich amphiboles and other fibrous iron-silicate minerals. The presence of these minerals in iron-formation along the eastern part of what is known as the Mesabi Iron Range, and their potential liberation by iron mining has raised environmental health concerns. We describe here the geologic setting and mineralogic composition of the Biwabik Iron Formation in and adjacent to the contact metamorphic aureole of the Duluth Complex. The effects of metamorphism are most pronounced within a few kilometers of the contact, and decrease progressively away from it. The contact aureole has been divided into four metamorphic zones-each characterized by the composition and crystal structure of the metamorphic minerals it contains. The recrystallization of iron-formation to iron-rich amphibole minerals (grunerite and cummingtonite) and iron-pyroxene minerals (hedenbergite and ferrohypersthene) is best developed in zones that are most proximal to the Duluth Complex contact. ?? 2007 Elsevier Inc. All rights reserved.

CL-imaging and ion microprobe dating of single zircons from a high-grade rock from the Central Zone, Limpopo Belt, South Africa: Evidence for a single metamorphic event at ˜2.0 Ga

NASA Astrophysics Data System (ADS)

Mouri, H.; Brandl, G.; Whitehouse, M.; de Waal, S.; Guiraud, M.

2008-02-01

The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10-30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean 207Pb/ 206Pb age of 2689 ± 15 (2 σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean 207Pb/ 206Pb age of ˜2006.5 ± 8.0 Ma (2 σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ˜2.5 Ga as postulated earlier by some workers.

Metamorphic fluids and uplift-erosion history of a portion of the Kapuskasing structural zone, Ontario, as deduced from fluid inclusions

NASA Technical Reports Server (NTRS)

Rudnick, R. L.; Ashwal, L. D.; Henry, D. J.

1983-01-01

Fluid inclusions can be used to determine the compositional evolution of fluids present in high grade metamorphic rocks (Touret, 1979) along with the general P-T path followed by the rocks during uplift and erosion (Hollister et al., 1979). In this context, samples of high grade gneisses from the Kapuskasing structural zone (KSZ, Fig. 1) of eastern Ontario were studied in an attempt to define the composition of syn- and post-metamorphic fluids and help constrain the uplift and erosion history of the KSZ. Recent work by Percival (1980), Percival and Card (1983) and Percival and Krogh (1983) shows that the KSZ represents lower crustal granulites that form the lower portion of an oblique cross section through the Archean crust, which was up faulted along a northeast striking thrust fault. The present fluid inclusion study places constraints upon the P-T path which the KSZ followed during uplift and erosion.

Development of magnetic and elastic anisotropies in slates during progressive deformation

NASA Astrophysics Data System (ADS)

Hrouda, František; Pros, Zdeněk; Wohlgemuth, Jiří

1993-05-01

Magnetic and elastic anisotropies were investigated in rocks of the Nízký Jeseník Mountains (northeast Bohemian Massif) ranging in lithology from almost unmetamorphosed sediments, through slate, to phyllite, and showing a range of structural styles from sedimentary, through spaced and slaty cleavage, to metamorphic schistosity. In unmetamorphosed and undeformed sedimentary rocks, both the anisotropies display close relationships to the sedimentary fabric. During the development of the spaced and slaty cleavage they are gradually re-oriented into the attitudes of the deformational fabrics, and in the rocks with metamorphic schistosity they are fully related to the deformational fabric elements, which can be oriented in a very different way from the original sedimentary structures. The magnetic anisotropy is mostly due to the preferred orientation of phyllosilicates generated during very weak regional metamorphism, and subordinately due to the preferred orientation of magnetite. The elastic anisotropy is probably controlled by the preferred orientation of phyllosilicates and by the existence of oriented systems of microcracks.

Seismic evidence for multiple-stage exhumation of high/ultrahigh pressure metamorphic rocks in the eastern Dabie orogenic belt

NASA Astrophysics Data System (ADS)

Luo, Yinhe; Zhao, Kaifeng; Tang, Chi-Chia; Xu, Yixian

2018-05-01

The Dabie-Sulu orogenic belt in China contains one of the largest exposures of high and ultrahigh pressure (HP and UHP) metamorphic rocks in the world. The origin of HP/UHP metamorphic rocks and their exhumation to the surface in this belt have attracted great interest in the geologic community because the study of exhumation history of HP/UHP rocks helps to understand the process of continental-continental collision and the tectonic evolution of post-collision. However, the exhumation mechanism of the HP-UHP rocks to the surface is still contentious. In this study, by deploying 28 broadband seismic stations in the eastern Dabie orogenic belt and combining seismic data from 40 stations of the China National Seismic Network (CNSN), we image the high-resolution crustal isotropic shear velocity and radial anisotropy structure using ambient noise tomography. Our high-resolution 3D models provide new information about the exhumation mechanism of HP/UHP rocks and the origin of two dome structures.

From Permo-Triassic lithospheric thinning to Jurassic rifting at the Adriatic margin: Petrological and geochronological record in Valtournenche (Western Italian Alps)

NASA Astrophysics Data System (ADS)

Manzotti, Paola; Rubatto, Daniela; Darling, James; Zucali, Michele; Cenki-Tok, Bénédicte; Engi, Martin

2012-08-01

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic histories, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallisation at HT conditions; ages scatter from 263 to 294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Crustal structure of the Dabie orogenic belt (eastern China) inferred from gravity and magnetic data

NASA Astrophysics Data System (ADS)

Yang, Yu-shan; Li, Yuan-yuan

2018-01-01

In order to better characterize the crustal structure of the Dabie orogen and its tectonic history, we present a crustal structure along a 500 km long profile across the Dabie orogenic belt using various data processing and interpretation of the gravity and magnetic data. Source depth estimations from the spectral analysis by continuous wavelet transform (CWT) provide better constraints for constructing the initial density model. The calculated gravity effects from the initial model show great discrepancy with the observed data, especially at the center of the profile. More practical factors are then incorporated into the gravity modeling. First, we add a high density body right beneath the high pressure metamorphic (HPM) and ultrahigh pressure metamorphic (UHPM) belt considering the exposed HPM and UHPM rocks in the mid of our profile. Then, the anomalous bodies A, B, and C inferred from the CWT-based spectral analysis results are fixed in the model geometry. In the final crustal density structure, two anomalous bodies B and C with high density and low magnetization could possibly be attributed to metasomatised mantle materials by SiO2-rich melt derived from the foundering subducted mafic lower crust. Under the extensional environment in the early Cretaceous, the upwelling metasomatised mantle was partially melted to produce the parental magma of the post-collisional mafic-ultramafic intrusive rocks. As for the low density body A with strong magnetization located in the lower crust right beneath the HP and UHP metamorphic belt, it is more likely to be composed of serpentinized mantle peridotite (SMP). This serpentinized mantle peridotite body (SMPB) represents the emplacement of mantle-derived peridotites in the crust, accompanying the exhumation of the UHP metamorphic rocks.

Over 400 m.y. metamorphic history of the Fennoscandian lithospheric segment in the Proterozoic (the East European Craton)

NASA Astrophysics Data System (ADS)

Skridlaite, G.; Bogdanova, S.; Taran, L.; Baginski, B.; Krzeminska, E.; Wiszniewska, J.; Whitehouse, M.

2009-04-01

Several Palaeoproterozoic terranes in the Fennoscandian lithospheric segment of the East European Craton (EEC) evolved differently prior to their final amalgamation at c. 1.8 Ga. South-westward younging of the major tectono-thermal events characterizes the Baltic -Belarus region between the Baltic and Ukrainian Shields of the EEC. While at c.1.89-1.87 Ga and 1.85-1.84 Ga rocks of some northern and eastern terranes (Estonia, Belarus and eastern Lithuania) experienced syncollisional, moderate P metamorphism, subduction-related volcanic island arc magmatism still dominated southwestern terranes in Lithuania and Poland. The available age determinations of metamorphic zircon (SIMS/NORDSIM and TIMS methods, Stockholm, SHRIMP method, RSES, ANU, Canberra) and metamorphic monazite (TIMS, Stockholm and EPMA method, Warsaw University) allow to distinguish several metamorphic events related to major orogenic processes: - 1.90-1.87 Ga amphibolite-facies H/MP metamorphism occurred along with emplacements of juvenile TTG-type granitoids in the North Estonian and Lithuanian-Belarus terranes. They are coeval with the main accretionary growth of the crust in the Svecofennian Domain in the Baltic Shield (e.g. Lahtinen et al., 2005). - 1.84-1.79 Ga high-grade metamorphism affected sedimentary and igneous rocks in almost all the terranes and is assumed to have been related to the major aggregation of the EEC (Bogdanova et al, 2006, 2008). In the metasedimentary granulites of western Lithuania, a prograde metamorphism commenced with monazite growth prior garnet at 1.84-1.83 Ga. The sediments and mafic igneous rocks in Lithuania, felsic igneous rocks in NE Poland underwent peak metamorphism and deformation at 1.81-1.79 Ga (zircon and monazite ages). The 1.83-1.79 Ga metamorphism has the same age as a metamorphic imprint and strong shearing of the crust in central Sweden (Andersson et al., 2004). The postcollisional granulite metamorphism of mafic intrusions at 1.80-1.79 Ga in Belarus indicates that the NW-SE collision can have triggered the crustal/mantle disturbance along the Fennoscandia-Sarmatia suture zone. - c. 1.7-1.6 Ga moderate PT metamorphic overprint and deformation of 1.83-1.82 Ga magmatic charnockites and c. 1.8 Ga metamorphic granulites in western Lithuania was recorded by the growth of a new garnet, zircon and monazite. The dated charnockites and metasediments contain metamorphic monazite of both 1.60-1.59 Ga and 1.7-1.65 Ga ages. These metamorphic events can reflect a distal influence of the 1.7-1.6 Ga Gothian orogeny in SW Fennoscandia (e.g. Ahall and Connelly, 2008). - 1.55-1.50 and 1.50-1.45 Ga events. In southern Lithuania, the 1.53-1.50 Ga AMCG magmatism was accompanied by high-grade metamorphism. Deformation and amphibolite facies metamorphism are marked by the 1.55-1.45 Ga 40Ar/39Ar ages of hornblende along EW-trending lineaments in central and southeastern Lithuania and Belarus. There are also indications of shearing and low grade, c. 1.50 Ga, metamorphism of metasedimentary rocks and charnockites in NW Lithuania and NE Poland. Altogether, the coeval AMCG magmatism, local high-grade and widespread low-grade metamorphism, and deformation can be manifestations of the Danopolonian orogeny, particularly prominent around the South Baltic Sea. This is a contribution to the project "The Precambrian structure of Baltica as a control of its recent environment and evolution" of the Visby Programme (the Swedish Institute) and SYNTHESYS project SE-TAF-1535. References Ahall, K.I. and Connelly, J.N., 2008. Precambrian Research, 161(3-4): 452-474. Andersson, U.B. et al., 2004, GFF 126, 16-17. Bogdanova, S. et al., 2006, Geological Society, London Memoirs, 32, pp. 599-628 Bogdanova, S. et al., 2008, Precambrian Research 160, 23-45. Lahtinen, R., et al., 2005. In: Precambrian Geology of Finland - Key to the Evolution of the Fennoscandian Shield. Elsevier, Amsterdam, 481-532

Evolution of the Alpine Tethys (Sava) suture zone in Fruška Gora Mountains (N Serbia): from orogenic building to tectonic omissions

NASA Astrophysics Data System (ADS)

Toljić, Marinko; Matenco, Liviu; ĐErić, Nevenka; Milivojević, Jelena; Gerzina, Nataša.; Stojadinović, Uros

2010-05-01

The Fru\\vska Gora Mountains in northern Serbia offers an unique opportunity to study the Cretaceous-Eocene evolution of the NE part of the Dinarides, which is largely covered elsewhere beneath the thick Miocene sediments of the Pannonian basin, deposited during the back-arc collapse associated with the subduction and roll-back recorded in the external Carpathians. The structural grain of the Fru\\vska Gora Mountains is the one of a large scale antiform, exposing a complex puzzle of highly deformed metamorphic rocks in its centre and Triassic-Miocene sequence of non-metamorphosed sediments, ophiolites and volcanics along its flanks. The metamorphic rocks were the target of structural investigations coupled with paleontological dating (conodonts, palynomorphs and radiolarians) in an effort to unravel the geodynamic evolution of an area thought to be located near the suture zone between the Tisza upper plate and the Adriatic lower plate, i.e. the Sava subduction zone of the Dinarides (e.g., Pamic, 2002; Schmid et al., 2008). The existence of this subduction zone was previously inferred here by local observations, such as metamorphosed Mesozoic sediments containing Middle Triassic conodonts (Đurđanović, 1971) or Early Cretaceous blue schists metamorphism (123±5 Ma, Milovanović et al., 1995). The metamorphic sequence is characterized by a Paleozoic age meta-sedimentary basement which contains palynomorphs of Upper Paleozoic - Carboniferous age and a meta-sedimentary and meta-volcanic sequence which contain a succession of contrasting metamorphosed lithologies such sandstones, black limestones, shallow water white limestones, basic volcanic sequences, deep nodular limestiones, radiolarites, meta-ophiolites and turbiditic sequences. The lower part of the sequence is contrastingly similar with the Triassic cover of the Drina-Ivanijca thrust sheet and its metamorphosed equivalent observed in the Kopaonik and Studenica series (Schefer et al., in press). This observation is supported by the newly found micro-fauna of Upper Triassic in age in the meta-sandstones associated with meta-volcanics on the SW slopes of the mountain. The upper part of the sequence display metamorphosed "flysh"-type of sequences and meta-basalts. In these deposits, slightly metamorphosed siliciclastics (lithic sandstones with volcanic-derived clasts) previously interpreted as Upper Jurassic mélange have proved to contain Upper Cretaceous palynomorphs. Among the rocks exposed in the metamorphic core of the mountains, the SW slope of Fru\\vska Gora offers the optimal locality for the study of the kinematic evolution. Here, four phases of folding have been mapped, being associated mainly with large-scale regional contraction. The first phase is characterized by isoclinal folding, with reconstructed SW vergence. The second generation of E-W oriented and coaxial folds is asymmetric and is up to metres in size, displaying a south vergence and has largely refolded the previous generation. The third event was responsible for the formation of upright folds, yet again E-W oriented, re-folding earlier structures. The first two phases of folding are associated with metamorphic conditions, while the third was apparently near the transition with the brittle domain. The relationship with a fourth folding event observed also in the non-metamorphosed clastic-carbonate rocks is rather uncertain, but is apparently associated with the present day antiformal structure of the Fuska Gora Mountains. Interestingly, the metamorphosed Triassic and Upper Cretaceous carbonatic-clastic sequence in the core of the antiform is in structural contact along the antiformal flanks with Lower-Middle Triassic and Upper Cretaceous-Paleogene sediments which display the same facies, but these are not metamorphosed. This demonstrates a large scale tectonic omission along the flanks of the Fru\\vska Gora antiform, 9-10km of rocks being removed by what we speculatively define as an extensional detachment exhuming the metamorphic core. This detachment has been subsequently folded into the present-day antiformal geometry of the Fru\\vska Gora Mountains. These findings demonstrate that the metamorphic and non-metamorphic Upper Cretaceous - Paleogene clastic-carbonate sediments belongs to the main Alpine Tethys (Sava) subduction zone of the Dinarides. The Paleozoic-Triassic metamorphic and non-metamorphic rocks belong to the distal Adriatic lower plate, or more precisely to the Jadar-Kopaonik composite thrust sheet (Schmid et al., 2008), while the layer of serpentinized peridotite found at their contact most probably belongs to the Western Vardar ophiolites obducted over the Adriatic plate during Late Jurassic - Earliest Cretaceous. The distal Jadar-Kopaonik composite unit was partly affected by strong contractional deformation and a Late Eocene greenschist facies metamorphism during the main phase of subduction and collision, similarly to what has been observed elsewhere in the Dinarides (Pamić, 2002; Schefer et al., in press). A Miocene phase of core-complex formation was responsible for the large tectonic omission observed, being probably followed by the formation of a wide open antiformal structure during the Pliocene-Quaternary inversion of the Pannonian basin.

Distributed consensus for metamorphic systems using a gossip algorithm for CAT(0) metric spaces

NASA Astrophysics Data System (ADS)

Bellachehab, Anass; Jakubowicz, Jérémie

2015-01-01

We present an application of distributed consensus algorithms to metamorphic systems. A metamorphic system is a set of identical units that can self-assemble to form a rigid structure. For instance, one can think of a robotic arm composed of multiple links connected by joints. The system can change its shape in order to adapt to different environments via reconfiguration of its constituting units. We assume in this work that several metamorphic systems form a network: two systems are connected whenever they are able to communicate with each other. The aim of this paper is to propose a distributed algorithm that synchronizes all the systems in the network. Synchronizing means that all the systems should end up having the same configuration. This aim is achieved in two steps: (i) we cast the problem as a consensus problem on a metric space and (ii) we use a recent distributed consensus algorithm that only make use of metrical notions.

Full-Wave Tomographic and Moment Tensor Inversion Based on 3D Multigrid Strain Green’s Tensor Databases

DTIC Science & Technology

2014-04-30

grade metamorphic rocks on the southern slope of the Himalaya is imaged as a band of high velocity anomaly...velocity structures closely follow the geological features. As an indication of resolution, the ductile extrusion of high-grade metamorphic rocks on...MATERIEL COMMAND KIRTLAND AIR FORCE BASE, NM 87117-5776 DTIC COPY NOTICE AND SIGNATURE PAGE Using Government drawings, specifications, or other data

The First Evidence of the Precambrian Basement in the Fore Range Zone of the Great Caucasus.

NASA Astrophysics Data System (ADS)

Latyshev, A.; Kamzolkin, V.; Vidjapin, Y.; Somin, M.; Ivanov, S.

2017-12-01

Within the Great Caucasus fold-thrust belt, the Fore Range zone has the most complicated structure, and the highest degree of metamorphism was found there. This zone consists of several salients with the different composition and the structural and metamorphic evolution. The largest Blyb salient includes the metamorphic basement covered by the pack of thrusts. According to the recent isotopic data the upper levels of the Blyb metamorphic complex (BMC) are supposed to be Middle-Paleozoic (Somin, 2011). We studied zircons from the granitic intrusions located in the metamorphic rocks of the BMC. The U-Pb dating (SHRIMP II, VSEGEI, Russia) of zircons from the large Balkan metadiorite massif yielded the ages of 549±7,4, 574,1±6,7, and 567,9±6,9 Ma. All studied zircons show the high Th/U ratios and likely have the magmatic origin. This data is the first confirmation of the presence of the Precambrian basement and Vendian magmatic activity in the Fore Range zone. Zircons from the Unnamed granodiorite massif from the south of the Blyb salient yielded the age of 319±3.8 Ma (the Early Carboniferous). This fact taken together with the low grade of metamorphism in this intrusion reveals the Late Paleozoic magmatic event in the Fore Range zone. We also suggest that the Precambrian basement of the BMC, including the Balkan intrusion, is covered by so-called Armovsky nappe. This is confirmed by the field data, Middle-Paleozoic U-Pb ages and the higher degree of metamorphism of the Armovsky gneisses and schists. Thus, the BMC is not uniform but includes the blocks of the different age and metamorphic grades. Finally, we measured the anisotropy of magnetic susceptibility (AMS) of the Balkan metadiorites. The axes of AMS ellipsoid fix the conditions of the north-east compression, as well as the strain field reconstructed from the macrostructures orientation, which corresponds to the thrusts propagation. Therefore, the emplacement of the Balkan massif happened before the thrust sheets formation. Thus, the first reliable evidence of the Precambrian basement in the Fore Range zone was obtained. Besides, our U-Pb data suggest that in the end of Precambrian the Fore Range zone could be related to Gondwana, where the Vendian granitic magmatism is widely known. This work was funded by RFBR (projects № 16-35-00571, 16-05-01012, 17-05-01121).

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco

NASA Astrophysics Data System (ADS)

Gueydan, Frédéric; Pitra, Pavel; Afiri, Abdelkhaleq; Poujol, Marc; Essaifi, Abderrahim; Paquette, Jean-Louis

2015-06-01

Deciphering Variscan versus Alpine history in the Internal Rif system is a key to constrain the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and laser ablation inductively coupled plasma mass spectrometry U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western Internal Rif. The regional foliation (S2) is characterized by low pressure-high temperature (LP-HT) mineral assemblages and obliterates a former foliation (S1) developed along a Barrovian (medium pressure-medium temperature, MP-MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the Barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP-HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab rollback in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidence for a Tertiary high pressure/low temperature metamorphism has been identified in the studied area.

Microprobe monazite geochronology: new techniques for dating deformation and metamorphism

NASA Astrophysics Data System (ADS)

Williams, M.; Jercinovic, M.; Goncalves, P.; Mahan, K.

2003-04-01

High-resolution compositional mapping, age mapping, and precise dating of monazite on the electron microprobe are powerful additions to microstructural and petrologic analysis and important tools for tectonic studies. The in-situ nature and high spatial resolution of the technique offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on P-T-D paths, constrain the rates of sedimentary, metamorphic, and deformational processes, and provide new links between metamorphism and deformation. New analytical techniques (including background modeling, sample preparation, and interference analysis) have significantly improved the precision and accuracy of the technique and new mapping and image analysis techniques have increased the efficiency and strengthened the correlation with fabrics and textures. Microprobe geochronology is particularly applicable to three persistent microstructural-microtextural problem areas: (1) constraining the chronology of metamorphic assemblages; (2) constraining the timing of deformational fabrics; and (3) interpreting other geochronological results. In addition, authigenic monazite can be used to date sedimentary basins, and detrital monazite can fingerprint sedimentary source areas, both critical for tectonic analysis. Although some monazite generations can be directly tied to metamorphism or deformation, at present, the most common constraints rely on monazite inclusion relations in porphyroblasts that, in turn, can be tied to the deformation and/or metamorphic history. Examples will be presented from deep-crustal rocks of northern Saskatchewan and from mid-crustal rocks from the southwestern USA. Microprobe monazite geochronology has been used in both regions to deconvolute overprinting deformation and metamorphic events and to clarify the interpretation of other geochronologic data. Microprobe mapping and dating are powerful companions to mass spectroscopic dating techniques. They allow geochronology to be incorporated into the microstructural analytical process, resulting in a new level of integration of time (t) into P-T-D histories.

Multiple tectonic mode switches indicate short-duration heat pulses in a Mio-Pliocene metamorphic core complex, West Papua, Indonesia

NASA Astrophysics Data System (ADS)

White, L. T.; Hall, R.; Gunawan, I.

2017-12-01

The Wandaman Peninsula is a narrow (<20 km), but mountainous (>2 km) promontory in remote western New Guinea. The peninsula is almost entirely composed of medium- to high-grade metamorphic rocks considered to be associated with a Mio-Pliocene metamorphic core complex. Previous work has shown that the uplift and exhumation of the core complex has potentially brought some extremely young eclogite to the surface. These might be comparable to the world's youngest (4.3 Ma) eclogites found in the D'Entrecasteaux Islands at the opposite end of New Guinea. We show that tectonic history of this region is complex. This is because the metamorphic sequences in the Wandaman Peninsula record multiple phases of deformation, all within the last few million years. This is demonstrated through methodical collation of cross-cutting relations from field and microstructural studies across the peninsula. The first phase of deformation and metamorphism is associated with crustal extension and partial melting that took place at 5-7 Ma according to new U-Pb data from metamorphic zircons. This extensional phase ceased after a tectonic mode switch and the region was shortened. This is demonstrated by two phases of folding (1. recumbent and 2. open) that overprint the earlier extensional fabrics. All previous structures were later overprinted by brittle extensional faults and uplift. This extensional phase is still taking place today, as is indicated by submerged forests exposed along the coastline associated with recent earthquakes and hot springs. The sequence of metamorphic rocks that are exposed in the Wandaman Peninsula show that stress and thermal conditions can change rapidly. If we consider that the present is a key to the past, then such results can identify the duration of deformation and metamorphic events more accurately than in much older orogenic systems.

Telescoping metamorphic isograds: Evidence from 40Ar/39A dating in the Orange-Milford belt, southern Connecticut

USGS Publications Warehouse

Kunk, Michael J.; Walsh, Gregory J.; Growdon, Martha L.; Wintsch, Robert P.

2013-01-01

New 40Ar/39Ar ages for hornblende and muscovite from the Orange-Milford belt in southern Connecticut reflect cooling from Acadian amphibolite facies metamorphism between ∼380 to 360 Ma followed by retrograde recrystallization of fabric-forming muscovite and chlorite during lower greenschist facies Alleghanian transpression at ∼280 Ma. Reported field temperature and pressure gradients are improbably high for these rocks and a NW metamorphic field gradient climbing from chlorite-grade to staurolite-grade occurs over less than 5 km. Simple tilting cannot account for this compressed isograd spacing given the geothermal gradient of ∼20 °C/km present at the time of regional metamorphism. However, post-metamorphic transpression could effectively telescope the isograds by stretching the belt at an oblique angle to the isograd traces. Textures in the field and in thin section reveal several older prograde schistosities overprinted by lower greenschist facies fabrics. The late cleavages commonly occur at the scale of ∼100 μm and these samples contain multiple age populations of white mica. 40Ar/39Ar analysis of these poly-metamorphic samples with mixed muscovite populations yield climbing or U-shaped age spectra. The ages of the low temperature steps are late Paleozoic, while the ages of the older steps are late Devonian. These results support our petrologic interpretation that the younger cleavage developed under metamorphic conditions below the closure temperature for Ar diffusion in muscovite, that is, in the lower greenschist facies. The correlation of a younger regionally reproducible age population with a pervasive retrograde muscovite ± chlorite cleavage reveals an Alleghanian (∼280 Ma) overprint on the Acadian metamorphic gradient (∼380 Ma). Outcrop-scale structures including drag folds and imbricate boudins suggest that Alleghanian deformation and cleavage development occurred in response to dextral transpression along a northeast striking boundary. Alleghanian oblique collision of accreting terranes from the northeast would have resulted in northeast-southwest dextral transpression against the New York promontory. This deformation was responsible for crystallization of pervasive retrograde muscovite + chlorite cleavages and associated telescoping of the Acadian metamorphic isograds in southern Connecticut at ∼280 Ma.

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

NASA Astrophysics Data System (ADS)

Roda, Manuel; Zucali, Michele; Li, Zheng-Xiang; Spalla, Maria Iole; Yao, Weihua

2018-06-01

In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing amphibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and minor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P = 0.7-1.1 GPa, T = 580-660 °C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P = 0.6-1.0 GPa, T> 780 °C). The two tectono-metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P = 0.4-0.6 GPa, T = 620-750 °C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distinguished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.

Microstructural observation and chemical dating on monazite from the Shilu Group, Hainan Province of South China: Implications for origin and evolution of the Shilu Fe-Co-Cu ore district

NASA Astrophysics Data System (ADS)

Xu, Deru; Kusiak, Monika A.; Wang, Zhilin; Chen, Huayong; Bakun-Czubarow, Nonna; Wu, Chuanjun; Konečný, Patrik; Hollings, Peter

2015-02-01

New monazite chemical U-Th-total-Pb (CHIME) ages, combined with microstructural observations, mineral compositions, and whole-rock geochemistry, indicate that the large-scale, banded iron formation (BIF)-type Shilu Fe-Co-Cu ore district in Hainan Province, South China is a multistage product of sedimentation, metamorphism, and hydrothermal-metasomatic alteration associated with multiple orogenies. Two types of monazite, i.e. "polygenetic" and "metamorphic", were identified. The "polygenetic monazite" comprises a magmatic and/or metamorphic core surrounded by a metamorphic rim, and shows complex zoning. Breakdown corona structure, with a core of monazite surrounded by a mantle of fluorapatite, allanite, and/or epidote as concentric growth rings, is commonly observed. This type of monazite yielded three main CHIME-age peaks at ca. 980 Ma, ca. 880 Ma and ca. 450 Ma. The ages which range up to ca. 880 Ma for detrital cores, record a pre-deformational magmatic and/or metamorphic event(s), and is considered to be the depositional time-interval of the Shilu Group and interbedded BIFs in a marine, back-arc foreland basin likely due to the Grenvillian or South China Sibao orogeny. After deposition, the Shilu district was subjected to an orogenic event, which is recorded by the syndeformational metamorphic monazite with ca. 560-450 Ma population. Probably this event not only caused amphibolite facies metamorphism and associated regional foliation S1 but also enriched the original BIFs, and most likely corresponds to the "Pan-African" and/or the South China Caledonian orogeny. The post-deformational "metamorphic" monazite occurs mostly as inclusions in garnet and shows ca. 260 Ma age. It likely represents the Late Permian post-magmatic hydrothermal and related retrograde event(s) initiated by the Indosinian orogeny due to the closure of the Paleo-Tethys. The breakdown of monazite to secondary coronal mineral phases as well as the Fe-remobilization and associated skarnization of the Shilu BIF ore source rocks might also be induced during this retrograde greenschist-facies metamorphism.

Fore arc tectonothermal evolution of the El Oro metamorphic province (Ecuador) during the Mesozoic

NASA Astrophysics Data System (ADS)

Riel, Nicolas; Martelat, Jean-Emmanuel; Guillot, Stéphane; Jaillard, Etienne; Monié, Patrick; Yuquilema, Jonatan; Duclaux, Guillaume; Mercier, Jonathan

2014-10-01

The El Oro metamorphic province of SW Ecuador is a composite massif made of juxtaposed terranes of both continental and oceanic affinity that has been located in a fore-arc position since Late Paleozoic times. Various geochemical, geochronological, and metamorphic studies have been undertaken on the El Oro metamorphic province, providing an understanding of the origin and age of the distinct units. However, the internal structures and geodynamic evolution of this area remain poorly understood. Our structural analysis and thermal modeling in the El Oro metamorphic province show that this fore-arc zone underwent four main geological events. (1) During Triassic times (230-225 Ma), the emplacement of the Piedras gabbroic unit at crustal-root level ( 9 kbar) triggered partial melting of the metasedimentary sequence under an E-W extensional regime at pressure-temperature conditions ranging from 4.5 to 8.5 kbar and from 650 to 900°C for the migmatitic unit. (2) At 226 Ma, the tectonic underplating of the Arenillas-Panupalí oceanic unit (9 kbar and 300°C) thermally sealed the fore-arc region. (3) Around the Jurassic-Cretaceous boundary, the shift from trench-normal to trench-parallel subduction triggered the exhumation and underplating of the high-pressure, oceanic Raspas Ophiolitic Complex (18 kbar and 600°C) beneath the El Oro Group (130-120 Ma). This was followed by the opening of a NE-SW pull-apart basin, which tilted the massif along an E-W subhorizontal axis (110 Ma). (4) In Late Cretaceous times, an N-S compressional event generated heterogeneous deformation due to the presence of the Cretaceous Celica volcanic arc, which acted as a buttress and predominantly affected the central and eastern part of the massif.

Post-Metamorphic Thermal Anomaly across the Nacimiento Block, Central California: a Hydrothermal Overprint?

NASA Astrophysics Data System (ADS)

Lacroix, B.; Hughes, J.; Lahfid, A.; Delchini, S.

2017-12-01

The thermal history of the Nacimiento block located within the Franciscan Complex (California, USA) has been previously proposed based on both vitrinite reflectance (Rm) and illite cristallinity methods (Underwood et al., 1995). These authors suggest that the Nacimiento block is locally perturbed by a thermal anomaly (up to 300ºC), probably caused by post-metamorphic hydrothermal activity linked to the emplacement of an Au-deposit: the Los Burros Gold deposit. Although both thermal anomaly and deposit seem spatially correlated, their relationship is still poorly constrained. Detailed geological and structural mapping within the Los Burros Mining District (LBMD) coupled with a thermal study was conducted to better understand processes responsible for the anomalous temperatures recorded near the deposit. The regional maximum temperature reached by metasediments from the Nacimiento block have been first investigated using the Raman Spectroscopy of Carbonaceous Materials (RSCM) method. In addition, through careful fluid-inclusion and stable isotopes (O and C) studies on the deposit, the temperature and the potential source of the fluid responsible for the Los Burros Au-deposit emplacement were investigated. RSCM technique confirms the presence of a thermal anomaly in the range 260-320ºC near LBMD. However, our structural and petrographic results suggest that the thermal anomaly is not correlated to a post-metamorphic hydrothermal overprint but rather to a late, transpressive deformation uplifting buried metamorphic rocks.

Early formation of preferential flow in a homogeneous snowpack observed by micro-CT

NASA Astrophysics Data System (ADS)

Avanzi, Francesco; Petrucci, Giacomo; Matzl, Margret; Schneebeli, Martin; De Michele, Carlo

2017-05-01

We performed X-ray microtomographic observations of wet-snow metamorphism during controlled continuous melting and melt-freeze events in the laboratory. Three blocks of snow were sieved into boxes and subjected to cyclic, superficial heating or heating-cooling to reproduce vertical water infiltration patterns in snow similarly to natural conditions. Periodically, samples were taken at different heights and scanned. Results suggest that wet-snow metamorphism dynamics are highly heterogeneous even in an initially homogeneous snowpack. Consistent with previous work, we observed an increase with time in the thickness of the ice structure, which is a measure of grain size. However, this was coupled with large temporal scatter between consecutive measurements of the specific surface area and of the statistical moments of grain thickness distributions. Because of marked differences in the right tail, grain thickness distributions did not show shape invariance with time, contrary to previous analyses. In our experiments, wet-snow metamorphism showed two strikingly different patterns: homogeneous coarsening superimposed by faster heterogeneous coarsening in areas that were affected by preferential percolation of water. Liquid water movement in snow and fast structural evolution may be thus intrinsically coupled by early formation of preferential flow at local scale. These observations suggest that further experiments are highly needed to fully understand wet-snow metamorphism and infiltration patterns in a natural snowpack.

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure

NASA Astrophysics Data System (ADS)

Xu, Xiaoming; Kenkmann, Thomas; Xiao, Zhiyong; Sturm, Sebastian; Metzger, Nicolai; Yang, Yu; Weimer, Daniela; Krietsch, Hannes; Zhu, Meng-Hua

2017-09-01

The Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin-section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic-Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

Records of near-isothermal decompression and clockwise P-T history from the Paleoproterozoic Mahakoshal Belt, Central Indian Tectonic Zone: Constraints from pseudosection modelling and monazite geochronology

NASA Astrophysics Data System (ADS)

Deshmukh, Tanzil; Naraga, Prabhakar; Bhattacharya, Abhijit; Kaliappan, Madhavan

2017-04-01

The Mahakoshal Belt (MB) is regarded as the oldest subunit along the northern collar of the Central Indian Tectonic Zone (CITZ) arguably representing the zone of accretion between the North India Block and the South India Block. The following study focuses on deciphering the structural and metamorphic P-T-t history of the schists/phyllites from the eastern part of the belt, and provides insights into the Paleoproterozoic tectonic development in the CITZ. The schists comprise phengite, quartz, andalusite, biotite, muscovite and margarite, and are associated with veins of rare andalusite + corundum + quartz assemblage. The field relations combined with deformation microtextures in the MB schists suggests three episodes of metamorphism, M1, M2 and M3, corresponding with D1, D2 and D3 deformation events respectively. Inclusion trails (S1) of phengite + biotite + quartz ± chlorite in syn/post-S2 andalusite porphyroblasts constrain the M1 metamorphic event in pelitic schists. The application of pseudosection modelling estimated peak metamorphic conditions at ˜8 kbar and 520 ˚ C. Near isothermal decompression (<4 kbar) resulted in the formation of the andalusite + muscovite bearing retrograde assemblage that stabilized at the expense of phengite-bearing assemblage. Further, andalusite porphyroblasts are replaced by margarite + muscovite + chlorite pseudomorphs (2-3 kbar) during syn/post-S3 fluid-aided metamorphism. Th-U-total Pb dating of monazite grains yield core populations at 1.8-1.9 Ga, and rim populations at 1.7-1.8 Ga and 1.5-1.6 Ga. Thus, the peak metamorphism in MB schists was Paleoproterozoic in age, 1.8-1.9 Ga, and the clockwise P-T path was recorded at 1.7-1.8 Ga, which overlaps with the emplacement of blastoporphyritic granitoids along southern margin of the MB. The results obtained in this study combined with the existing structural-metamorphic-chronological information demonstrate the CITZ to be a composite of desperately-evolved crustal domains. With some major omissions, the tectono-thermal events identified in the CITZ partly overlap with those observed in the Capricorn Orogen (Western Australia) and the Trans North China Orogen. Therefore, these global correlations possibly corroborate new configurations on the assembly and fragmentation of Columbia Supercontinent, but await further studies and robust age determinations in the various parts of CITZ.

Review of metamorphic and kinematic data from Internal Crystalline Massifs (Western Alps): PTt paths and exhumation history

NASA Astrophysics Data System (ADS)

Gasco, Ivano; Gattiglio, Marco; Borghi, Alessandro

2013-01-01

Detailed geological mapping combined with micro-structural and petrological investigation allowed to clarify the tectono-metamorphic relationships between continental and oceanic units transition in the Penninic domain of the Western Alps. The three study areas (Gressoney, Orco and Susa sections) take into consideration the same structural level across the axial metamorphic belt of the Western Italian Alps, i.e., a geological section across the Internal Crystalline Massifs vs Piedmont Zone boundary. The units outcropping in these areas can be grouped into two Tectonic Elements according to their tectono-metamorphic evolution. The Lower Tectonic Element (LTE) consists of the Internal Crystalline Massifs and the Lower Piedmont Zone (Zermatt-Saas like units), both showing well preserved eclogite facies relics. Instead, the Upper Tectonic Element (UTE) consists of the Upper Piedmont Zone (Combin like units) lacking evidence of eclogite facies relics. In the Lower Tectonic Element two main Alpine tectono-metamorphic stages were identified: M1/D1 developed under eclogite facies conditions and M2/D2 is related to the development of the regional foliation under greenschist to epidote-albite amphibolite facies conditions. In the Upper Tectonic Element the metamorphic stage M1/D1 developed under bluschist to greenschist facies conditions and M2/D2 stage under greenschist facies conditions. These two Tectonic Elements are separated by a tectonic contact of regional importance generally developed along the boundary between the Lower and the Upper Piedmont zone under greenschist facies conditions. PT data compared to geochronology indicate that the first exhumation of ICM can be explained by buoyancy forces acting along the subduction channel that occurred during the tectonic coupling between the continental and oceanic eclogite units. These buoyancy forces vanished at the base of the crust where the density difference between the subducted crustal units and the surroundings rocks is too low. A stage where compression prevails on the previous exhumation followed, which leads to the development of the regional foliation under greenschist to amphibolite facies metamorphic conditions. Further exhumation occurred after the M2/D2 stage at shallower crustal levels along conjugated shear zones leading to the development of a composite axial dome consisting of eclogite-bearing continental-oceanic units (ICM and Zermatt-Saas Zones) beneath greenschist ones (Combin Zone).

Geologic Map of the Estes Park 30' x 60' Quadrangle, North-Central Colorado

USGS Publications Warehouse

Cole, James C.; Braddock, William A.

2009-01-01

The rocks and landforms of the Estes Park 30 x 60 minute quadrangle display an exceptionally complete record of geologic history in the northern Front Range of Colorado. The Proterozoic basement rocks exposed in the core of the range preserve evidence of Paleoproterozoic marine sedimentation, volcanism, and regional soft-sediment deformation, followed by regional folding and gradational metamorphism. The metasedimentary rocks of the Estes Park quadrangle are distinct within northern Colorado for preserving the complete metamorphic zonation from low-grade chlorite-muscovite phyllites, through middle greenschist-grade rocks with sequential aluminous porphyroblasts, to partially melted gneisses that contain high-grade cordierite and garnet in the non-melted residues. Regional and textural evidence shows that the widespread metamorphism was essentially concurrent with intrusion of the Boulder Creek Granodiorite and related magmas and with the peak of deformation in the partially melted high-grade rocks. The metamorphic thermal pulse arrived later following the peak of deformation in the physically higher, cooler, low-grade terrane. Mesoproterozoic time was marked by intrusion of biotite granite in the Longs Peak-St Vrain batholith, a complex, irregular body that occupies nearly half of the core of the Front Range in this quadrangle. The magma was dry and viscous as it invaded the metamorphic rocks and caused wholesale plastic folding of the wall rock structure. Steep metamorphic foliation that resulted from the Paleoproterozoic deformations was bowed upward and re-oriented into flat-lying attitudes as the crystal-rich magma rose buoyantly and spread out in the middle crust. Magma invaded the schists and gneisses along weak foliation planes and produced a characteristic sill-upon-sill intrusive fabric, particularly in the higher parts of the batholith. Broad, open arches and swales that are defined by the flow-aligned feldspar foliation of the granite, as well as by compositional banding in the intruded and included metamorphic rocks, formed late during batholith emplacement due to rising, buoyant magma and sinking, dense wall rocks. The Longs Peak-St Vrain batholith was intruded into crust that was structurally neutral or moderately extending in an east-northeast direction. A broad zone of mylonite, the Moose Mountain shear zone, formed within the batholith during the final stages of consolidation as a result of differential buoyancy between the magma and dense wall rock, not as a result of regional tectonic deformation.

20-nm enhancement-mode metamorphic GaAs HEMT with highly doped InGaAs source/drain regions for high-frequency applications

NASA Astrophysics Data System (ADS)

Ajayan, J.; Nirmal, D.

2017-03-01

In this article, the DC and RF performance of a SiN passivated 20-nm gate length metamorphic high electron mobility transistor (MHEMT) on GaAs substrate with highly doped InGaAs source/drain (S/D) regions have investigated using the Synopsys TCAD tool. The 20-nm enhancement-mode (E-mode) MHEMT device also features δ-doped sheets on either side of the In0.53Ga0.47As/InAs/In0.53Ga0.47As channel which exhibits a transconductance of 3100 mS/mm, cut-off frequency (fT) of 740 GHz and a maximum oscillation frequency (fmax) of 1040 GHz. The threshold voltage of the device is found to be 0.07 V. The room temperature Hall mobilities of the 2-dimensional sheet charge density are measured to be over 12,600 cm2/Vs with a sheet charge density larger than 3.6 × 1012 cm-2. These high-performance E-mode MHEMTs are attractive candidates for sub-millimetre wave applications such as high-resolution radars for space research, remote atmospheric sensing, imaging systems and also for low noise wide bandwidth amplifier for future communication systems.

Ostwald ripening of clays and metamorphic minerals

USGS Publications Warehouse

Eberl, D.D.; Srodon, J.; Kralik, M.; Taylor, B.E.; Peterman, Z.E.

1990-01-01

Analyses of particle size distributions indicate that clay minerals and other diagenetic and metamorphic minerals commonly undergo recrystallization by Ostwald ripening. The shapes of their particle size distributions can yield the rate law for this process. One consequence of Ostwald ripening is that a record of the recrystallization process is preserved in the various particle sizes. Therefore, one can determine the detailed geologic history of clays and other recrystallized minerals by separating, from a single sample, the various particle sizes for independent chemical, structural, and isotopic analyses.

Characterization and Analysis of Multi-Quantum Well Solar Cells

NASA Astrophysics Data System (ADS)

Bradshaw, Geoffrey Keith

Multijunction (MJ) photovoltaics are the most efficient solar cells today. Under sufficient solar concentration, these devices can achieve over 44% efficiency, roughly twenty percentage points higher than single crystal silicon based solar cells. Current records for triple junction (3J) multijunction cells are being challenged and broken regularly. However, it is unclear at this time which method of device growth will ultimately produce an efficiency that approaches the Shockley-Queisser limit. Lattice-matched (LM) MJ cells offer benefits over metamorphic and/or inverted metamorphic cells in that the device can be grown continuously, require no extra fabrication steps, and will ultimate produce the highest material quality throughout all junctions. The efficiency of current 3JMJ cells composed of GaInP(1.8eV)/(In)GaAs(1.4eV)/Ge(0.7eV) is limited by the bandgap combination used in the structure. The low energy bandgap bottom Ge cell produces roughly twice as much current as the middle GaAs cell and results in a current mismatch that limits the total current and thus total efficiency. By replacing the middle GaAs subcell with a 1-1.2eV subcell, the current mismatch could be alleviated and the efficiency enhanced. Unfortunately, there are no semiconductors lattice-matched to GaAs/Ge with this bandgap. InGaAs, which has a larger lattice constant than GaAs/Ge, can be grown with the appropriate bandgap, but due to compressive stresses introduced during growth the thickness that can be grown is limited to tens of nanometers, thus limiting absorption and current production. However, by growing layers of tensile strained GaAsP with appropriate thickness and composition, the stresses introduced by the InGaAs can be balanced. By repeating this process and inserting these layers into the intrinsic region of the GaAs middle subcell, a low bandgap material with an effective lattice constant equal to that of GaAs is introduced while maintaining lattice-matching conditions. The InGaAs layers form quantum well capable of absorbing lower energy wavelengths than GaAs which leads to an increase in current. Absorption due to quantum wells is proportional to the number of quantum wells in the intrinsic region. Therefore, in order to grow the maximum number of the absorbing quantum wells within the background doping limited intrinsic region, it is necessary to reduce the width of the non-absorbing GaAsP barriers to as thin as possible. The research presented within shows this concept by exploring the fabrication and electrical characterization of these quantum well devices when balanced with ultra-thin GaAsP layers with very high phosphorus content (˜75-80%). By reducing the width of the barriers to approximately 30 A, tunneling of carriers dominates carrier transport across the structure as opposed to the traditional quantum well approach with very thick, low phosphorus GaAsP barriers that rely on thermionic emission of carriers to escape the InGaAs quantum wells. This research shows the strong effect and sensitivity to not only the thickness the GaAsP barriers, but also to the polarity of the device and the dependence of electric field. As well widths are decreased, quantum confinement of carriers within the InGaAs quantum wells increases. This leads to a blue-shift in the wavelengths of light absorbed and limits the current gain potential of the quantum well structure. To combat this blue-shift, the staggered MQW is introduced. The staggering technique can be use to not only improve wavelength absorption extension, but also lead to an enhancement in the absorption coefficient. These structures were also included into a GaInP/GaAs(MQW) tandem device to see the effects of the structure on the GaInP top cell.

Control of metamorphic buffer structure and device performance of In(x)Ga(1-x)As epitaxial layers fabricated by metal organic chemical vapor deposition.

PubMed

Nguyen, H Q; Yu, H W; Luc, Q H; Tang, Y Z; Phan, V T H; Hsu, C H; Chang, E Y; Tseng, Y C

2014-12-05

Using a step-graded (SG) buffer structure via metal-organic chemical vapor deposition, we demonstrate a high suitability of In0.5Ga0.5As epitaxial layers on a GaAs substrate for electronic device application. Taking advantage of the technique's precise control, we were able to increase the number of SG layers to achieve a fairly low dislocation density (∼10(6) cm(-2)), while keeping each individual SG layer slightly exceeding the critical thickness (∼80 nm) for strain relaxation. This met the demanded but contradictory requirements, and even offered excellent scalability by lowering the whole buffer structure down to 2.3 μm. This scalability overwhelmingly excels the forefront studies. The effects of the SG misfit strain on the crystal quality and surface morphology of In0.5Ga0.5As epitaxial layers were carefully investigated, and were correlated to threading dislocation (TD) blocking mechanisms. From microstructural analyses, TDs can be blocked effectively through self-annihilation reactions, or hindered randomly by misfit dislocation mechanisms. Growth conditions for avoiding phase separation were also explored and identified. The buffer-improved, high-quality In0.5Ga0.5As epitaxial layers enabled a high-performance, metal-oxide-semiconductor capacitor on a GaAs substrate. The devices displayed remarkable capacitance-voltage responses with small frequency dispersion. A promising interface trap density of 3 × 10(12) eV(-1) cm(-2) in a conductance test was also obtained. These electrical performances are competitive to those using lattice-coherent but pricey InGaAs/InP systems.

An inverted metamorphic field gradient in the central Brooks Range, Alaska and implications for exhumation of high-pressure/low-temperature metamorphic rocks

USGS Publications Warehouse

Patrick, B.; Till, A.B.; Dinklage, W.S.

1994-01-01

During exhumation of the Brooks Range internal zone, amphibolite-facies rocks were emplaced atop the blueschist/greenschist facies schist belt. The resultant inverted metamorphic field gradient is mappable as a series of isograds encountered as one traverses up structural section. Amphibolite-facies metamorphism occurred at ??? 110 Ma as determined from 40Ar 39Ar analysis of hornblende. This contrasts with 40Ar 39Ar phengite cooling ages from the uderlying schist belt, which are clearly older (by 17-22 m.y.). Fabrics in both the amphibolite-facies rocks and schist belt are characterized by repeated cycles of N-vergent crenulation and transposition that was likely associated with out-of-sequence ductile thrusting in the internal zone of the Brooks Range orogen. Contractional deformation occurred in an overall environment of foreland-directed tectonic transport, broadly synchronous with exhumation of the internal zone, and shortening within the thin-skinned fold and thrust belt. These data are inconsistent with a recently postulated mid-Cretaceous episode of lithospheric extension in northern Alaska. ?? 1994.

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

NASA Astrophysics Data System (ADS)

Homonnay, Emmanuelle; Corsini, Michel; Lardeaux, Jean-Marc; Romagny, Adrien; Münch, Philippe; Bosch, Delphine; Cenki-Tok, Bénédicte; Ouazzani-Touhami, Mohamed

2018-01-01

In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated. Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with: (1) a syn-metamorphic thrusting event developed under granulite facies conditions (7-10 kbar and 780-820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene. (2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400-550 °C and 1-3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene. We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position. Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20-30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension. 1, BSE image of monazite. 2, CL image of monazite showing a thin rim zonation. 3, BSE image of zircon. 4, CL image of zircon showing zonation.

Structural analysis and deformation characteristics of the Yingba metamorphic core complex, northwestern margin of the North China craton, NE Asia

NASA Astrophysics Data System (ADS)

Yin, Congyuan; Zhang, Bo; Han, Bao-Fu; Zhang, Jinjiang; Wang, Yang; Ai, Sheng

2017-01-01

The presence of the Yingba (Yinggete-Bagemaode) metamorphic core complex (MCC) is confirmed near the Sino-Mongolian border in China. We report its structural evolution and the rheological features of ductile shear zones within this complex. Three deformations (Ds, Dm, and Db) since the Late Jurassic are identified. Ds is characterized by ductile structures that resulted from early NW-oriented, low-angle, extensional ductile shearing. Dm is associated with partial melting and magmatic diapirism, which accelerated the formation of the dome-like geometry of the Yingba MCC. Synchronously with or slightly subsequently to Ds and Dm, the Yingba MCC was subjected to brittle, extensional faulting (Db), which was accompanied by the exhumation of the lower crust and the formation of supracrustal basins. The ductile shearing (Ds) developed under greenschist-to amphibolite-facies metamorphic conditions (400-650 °C), as indicated by microstructures in quartz and feldspar, quartz [c] axis fabrics, and two-feldspar geothermometry. The mean kinematic vorticity estimates of 48-62% show a pure shear-preferred flow during Ds. The Yingba MCC provides an excellent sample that recorded an intermediate to high temperature shearing, which also implies the widely extensional regime in northeastern Asia at that time.

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

NASA Astrophysics Data System (ADS)

Betts, Peter G.; Valenta, Rick K.; Finlay, Jim

2003-05-01

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S 1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north-south shortening (D 2), open to isoclinal south-southeast-oriented F 2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D 2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D 2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.

Structures, microfabrics and textures of the Cordilleran-type Rechnitz metamorphic core complex, Eastern Alps☆

PubMed Central

Cao, Shuyun; Neubauer, Franz; Bernroider, Manfred; Liu, Junlai; Genser, Johann

2013-01-01

Rechnitz window group represents a Cordilleran-style metamorphic core complex, which is almost entirely located within nearly contemporaneous Neogene sediments at the transition zone between the Eastern Alps and the Neogene Pannonian basin. Two tectonic units are distinguished within the Rechnitz metamorphic core complex (RMCC): (1) a lower unit mainly composed of Mesozoic metasediments, and (2) an upper unit mainly composed of ophiolite remnants. Both units are metamorphosed within greenschist facies conditions during earliest Miocene followed by exhumation and cooling. The internal structure of the RMCC is characterized by the following succession of structure-forming events: (1) blueschist relics of Paleocene/Eocene age formed as a result of subduction (D1), (2) ductile nappe stacking (D2) of an ophiolite nappe over a distant passive margin succession (ca. E–W to WNW–ESE oriented stretching lineation), (3) greenschist facies-grade metamorphism annealing dominant in the lower unit, and (4) ductile low-angle normal faulting (D3) (with mainly NE–SW oriented stretching lineation), and (5) ca. E to NE-vergent folding (D4). The microfabrics are related to mostly ductile nappe stacking to ductile low-angle normal faulting. Paleopiezometry in conjunction with P–T estimates yield high strain rates of 10− 11 to 10− 13 s− 1, depending on the temperature (400–350 °C) and choice of piezometer and flow law calibration. Progressive microstructures and texture analysis indicate an overprint of the high-temperature fabrics (D2) by the low-temperature deformation (D3). Phengitic mica from the Paleocene/Eocene high-pressure metamorphism remained stable during D2 ductile deformation as well as preserved within late stages of final sub-greenschist facies shearing. Chlorite geothermometry yields two temperature groups, 376–328 °C, and 306–132 °C. Chlorite is seemingly accessible to late-stage resetting. The RMCC underwent an earlier large-scale coaxial deformation accommodated by a late non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime. The RMCC was rapidly exhumed during ca. 23–18 Ma. PMID:27065502

Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley, Pakistan

NASA Astrophysics Data System (ADS)

Rehman, Hafiz Ur; Yamamoto, Hiroshi; Kaneko, Yoshiyuki; Kausar, Allah Bakhsh; Murata, Mamoru; Ozawa, Hiroaki

2007-02-01

The thermobaric structure of the Himalayan Metamorphic Belt (HMB) has been constructed along the Kaghan Valley transect, Pakistan. The HMB in this valley represents mainly the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystallines (HHC). Mineral parageneses of 474 samples, from an approximately, 80-km traverse from southwest to northeast, were examined. Microprobe analyses were carried out to quantify the mineral composition. To determine the pressure-temperature (P-T) conditions, 65 thin sections (7 pelites from LHS and 25 pelites, 9 mafic rocks/amphibolites and 19 eclogites from HHC) were selected. Based on field observations and mineral paragenesis, low-grade to high-grade metapelites, show Barrovian-type progressive metamorphic sequence, with chlorite, biotite, garnet and staurolite zones in LHS and staurolite, kyanite and sillimanite zones in HHC. By using well-calibrated geothermobarometers, P-T conditions for pelitic and mafic rocks are estimated. P-T estimates for pelitic rocks from the garnet zone indicate a condition of 534 ± 17 °C at 7.6 ± 1.2 kbar. P-T estimates for rocks from the staurolite and kyanite zones indicate average conditions of 526 ± 17 °C at 9.4 ± 1.2 kbar and 657 ± 54 °C at 10 ± 1.6 kbar, respectively. P-T conditions for mafic rocks (amphibolites) and eclogites from HHC are estimated as 645 ± 54 °C at 10.3 ± 2 kbar and 746 ± 59 °C at 15.5 ± 2.1 kbar, respectively. The coesite-bearing ultrahigh-pressure (UHP) eclogites record a peak P-T condition of 757-786 °C at 28.6 ± 0.4 kbar and retrograde P-T conditions of 825 ± 59 °C at 18.1 ± 1.7 kbar. These results suggest that HMB show a gradual increase in metamorphic grade from southwest to northeast. The P-T conditions from Pelitic and adjacent mafic rocks having identical peak conditions in the same metamorphic zone, while the structural middle in HHC reached the highest P-T condition upto the UHP grade.

Geospeedometry in the inverted metamorphic gradient of the Nestos Thrust Zone in central Rhodope (Northern Greece)

NASA Astrophysics Data System (ADS)

Cioldi, Stefania; Moulas, Evangelos; Burg, Jean-Pierre

2015-04-01

Thrust tectonics and inverted metamorphic gradients are major consequences of large and likely fast movements of crustal segments in compressional environments. The purpose of this study is to investigate the tectonic setting and the timescale of inverted metamorphic zonations related to crustal-scale thrusting. The aim is to contribute understanding the link between mechanical and thermal evolution of major thrust zones and to clarify the nature and the origin of orogenic heat. The Rhodope metamorphic complex (Northern Greece) is interpreted as a part of the Alpine-Himalaya orogenic belt and represents a collisional system with an association of both large-scale thrusting and pervasive exhumation tectonics. The Nestos Shear Zone overprints the suture boundary with a NNE-dipping pile of schists displaying inverted isograds. The inverted metamorphic zones start from chlorite-muscovite grade at the bottom and reach kyanite-sillimanite grades with migmatites in the upper structural levels. In order to reconstruct the thermo-tectonic evolution of inverted metamorphic zonation, reliable geochronological data are essential. 40Ar/39Ar geochronology with step-heating technique on white mica from micaschists provided a temporal resolution with the potential to characterize shearing. 40Ar/39Ar dating across the Nestos Shear Zone yields Late Eocene-Early Oligocene (40-30 Ma) cooling (~400-350° C) ages, which correspond to local thermo-deformation episodes linked to late and post-orogenic intrusions. U-Pb Sensitive High Resolution Ion Microprobe (SHRIMP) zircon geochronology on leucosomes from migmatitic orthogneisses were considered to estimate the age of peak metamorphic conditions, contemporaneous with anatexis. U-Pb ages of zircon rims specify regional partial melting during the Early Cretaceous (160-120 Ma). This is in disagreement with previous assertions, which argued that the formation of leucosomes in this region is Late Eocene (42-35 Ma) and implied multiple subductions and multiple metamorphic cycles during orogeny. Garnet geospeedometry considers the kinetic response of minerals and allowed estimating the absolute time-dependent thermal evolution by diffusive element profiles in garnet. Inverse-fitting numerical model considering Fractionation and Diffusion in GarnEt (FRIDGE) calculates garnet composition profiles by introducing P-T-t paths and bulk-rock composition of a specific sample. Preliminary results of Fe-Mg - Ca - Mn garnet fractionation-diffusion modelling indicate very short timescale (between 2 and 5 Ma) for peak metamorphic conditions in the Rhodope collisional system.

Overcoming Limitations in Semiconductor Alloy Design

NASA Astrophysics Data System (ADS)

Christian, Theresa Marie

Inorganic semiconductors provide an astonishingly versatile, robust, and efficient platform for optoelectronic energy conversion devices. However, conventional alloys and growth regimes face materials challenges that restrict the full potential of these devices. Novel alloy designs based on isoelectronic co-doping, metamorphic growth and controllable atomic ordering offer new pathways to practical and ultra-high-efficiency optoelectronic devices including solar cells and light-emitting diodes. Abnormal isoelectronic alloys of GaP1-xBix, GaP 1-x-yBixNy, and GaAs1-xBix with unprecedented bismuth incorporation fractions and crystalline quality are explored in this thesis research. Comparative studies of several GaP1-xBix and GaP1-x-yBixNy alloys demonstrate that the site-specific incorporation of bismuth during epitaxial growth is sensitive to growth temperature and has dramatic effects on carrier transfer processes in these alloys. Additionally, distinctive bismuth-related localized states are spectrally identified for the first time in samples of GaAs1-xBix grown by laser-assisted epitaxial growth. These results address fundamental questions about the nature of bismuth-bismuth inter-impurity interactions. Finally, a metamorphic growth strategy for a novel light-emitting diode (LED) design is also discussed. This work utilized direct-bandgap AlxIn1-xP active layers with atomic ordering-based electron confinement to improve emission in the yellow and green spectral regions, where incumbent technologies are least effective, and demonstrated the feasibility of non-lattice-matched LED active materials for visible light emission.

Optical properties of metamorphic GaAs/InAlGaAs/InGaAs heterostructures with InAs/InGaAs quantum wells, emitting light in the 1250–1400-nm spectral range

DOE Office of Scientific and Technical Information (OSTI.GOV)

Egorov, A. Yu., E-mail: anton@beam.ioffe.ru; Karachinsky, L. Ya.; Novikov, I. I.

It is demonstrated that metamorphic GaAs/InAlGaAs/InGaAs heterostructures with InAs/InGaAs quantum wells, which emit light in the 1250–1400 nm spectral range, can be fabricated by molecular-beam epitaxy. The structural and optical properties of the heterostructures are studied by X-ray diffraction analysis, transmission electron microscopy, and the photoluminescence method. Comparative analysis of the integrated photoluminescence intensity of the heterostructures and a reference sample confirm the high efficiency of radiative recombination in the heterostructures. It is confirmed by transmission electron microscopy that dislocations do not penetrate into the active region of the metamorphic heterostructures, where the radiative recombination of carriers occurs.

Metamorphism in the Potomac composite terrane, Virginia-Maryland

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drake, A.A. Jr.

1985-01-01

Metamorphic rocks in the Potomac Valley occur in three allochthon-precursory melange pairs unconformably overlain by the Popes Head Formation which is at greenschist facies of metamorphism. The highest motif, the Piney Branch Complex and Yorkshire Formation are also in the greenschist facies. The middle motif, consists of the Peters Creek Schist and the Sykesville Formation. Quartzose schists and metagraywacke of the Peters Creek contain serpentinite debris and have had a complex metamorphic history: Barrovian prograde to amphibolite facies (with sillimanite), a localized retrograde event producing chlorite phyllonite, and a later greenschist prograde event. The Sykeville is at biotite +/- garnetmore » grade and contains deformed olistoliths of Peters Creek, including phyllonite, at various grades. The lower motif consists of the Annandale Group (pelitic schists and metasandstone) and Indian Run Formation. The Annandale has experienced two greenschist metamorphisms. The Indian Run is at biotite +/- garnet grade and contains previously metamorphosed and deformed olistoliths of Annandale. The allochthons have had different histories, but after stacking they were metamorphosed with their melanges and the Popes Head to biotite grade. The Popes Head has experienced three phases of folding, the earliest synkinematic with Occoquan emplacement. These fold phases are superposed on earlier structures in the older rocks and are probably of Late Cambrian age (Penobscotian). Earlier deformation is probably of Late Proterozoic age (Cadomian). Neither of these deformations is recognized in North American rocks.« less

Shear heating and metamorphism in subduction zones, 1. Thermal models

NASA Astrophysics Data System (ADS)

Kohn, M. J.; Castro, A. E.; Spear, F. S.

2017-12-01

Popular thermal-mechanical models of modern subduction systems are 100-500 °C colder at c. 50 km depth than pressure-temperature (P-T) conditions determined from exhumed metamorphic rocks. This discrepancy has been ascribed by some to profound bias in the rock record, i.e. metamorphic rocks reflect only anomalously warm subduction, not normal subduction. Accurately inferring subduction zone thermal structure, whether from models or rocks, is crucial for predicting depths of seismicity, fluid release, and sub-arc melting conditions. Here, we show that adding realistic shear stresses to thermal models implies P-T conditions quantitatively consistent with those recorded by exhumed metamorphic rocks, suggesting that metamorphic rock P-T conditions are not anomalously warm. Heat flow measurements from subduction zone fore-arcs typically indicate effective coefficients of friction (µ) ranging from 0.025 to 0.1. We included these coefficients of friction in analytical models of subduction zone interface temperatures. Using global averages of subducting plate age (50 Ma), subduction velocity (6 cm/yr), and subducting plate geometry (central Chile), temperatures at 50 km depth (1.5 GPa) increase by c. 200 °C for µ=0.025 to 700 °C for µ=0.1. However, at high temperatures, thermal softening will reduce frictional heating, and temperatures will not increase as much with depth. Including initial weakening of materials ranging from wet quartz (c. 300 °C) to diabase (c. 600 °C) in the analytical models produces concave-upward P-T distributions on P-T diagrams, with temperatures c. 100 to 500 °C higher than models with no shear heating. The absolute P-T conditions and concave-upward shape of the shear-heating + thermal softening models almost perfectly matches the distribution of P-T conditions derived from a compilation of exhumed metamorphic rocks. Numerical models of modern subduction zones that include shear heating also overlap metamorphic data. Thus, excepting the very hottest examples, exhumed metamorphic rocks represent the products of normal, not anomalous, subduction. Consequently numerous geochemical, petrologic, and geophysical interpretations that have been founded on models that lack shear heating must be re-evaluated.

[The spectrum studies of structure characteristics in magma contact metamorphic coal].

PubMed

Wu, Dun; Sun, Ruo-Yu; Liu, Gui-Jian; Yuan, Zi-Jiao

2013-10-01

The structural parameters evolution of coal due to the influence of intrusions of hot magma was investigated and analyzed. X-ray diffraction and laser confocal microscope Raman spectroscopy were used to test and analyze 4 coal samples undergoing varying contact-metamorphism by igneous magmas in borehole No. 13-4 of Zhuji coal mine, Huainan coalfield. The result showed that coal XRD spectrum showed higher background intensity, with the 26 degrees and 42 degrees nearby apparent graphite diffraction peak. Two significant vibration peaks of coal Raman spectra were observed in the 1 000-2 000 cm(-1) frequency range: broad "D" peak at 1 328-1 369 cm(-1) and sharp "G" peak at 1 564-1 599 cm(-1). With the influence of magma intrusion, the relationship between coal structural parameters and coal ranks was excellent.

Terahertz radiation in In{sub 0.38}Ga{sub 0.62}As grown on a GaAs wafer with a metamorphic buffer layer under femtosecond laser excitation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ponomarev, D. S., E-mail: ponomarev-dmitr@mail.ru; Khabibullin, R. A.; Yachmenev, A. E.

The results of time-domain spectroscopy of the terahertz (THz) generation in a structure with an In{sub 0.38}Ga{sub 0.62}As photoconductive layer are presented. This structure grown by molecular-beam epitaxy on a GaAs substrate using a metamorphic buffer layer allows THz generation with a wide frequency spectrum (to 6 THz). This is due to the additional contribution of the photo-Dember effect to THz generation. The measured optical-to-terahertz conversion efficiency in this structure is 10{sup –5} at a rather low optical fluence of ~40 μJ/cm{sup 2}, which is higher than that in low-temperature grown GaAs by almost two orders of magnitude.

Workshop on Techtonic Evolution of Greenstone Belts

NASA Technical Reports Server (NTRS)

Dewit, M. J. (Editor); Ashwal, Lewis D. (Editor)

1986-01-01

Topics addressed include: greenstone belt externalities; boundaries; rock terranes; synthesis and destiny; tectonic evolution; rock components and structure; sedimentology; stratigraphy; volcanism; metamorphism; and geophysics.

INAA of CAIs from the Maralinga CK4 chondrite: Effects of parent body thermal metamorphism

NASA Technical Reports Server (NTRS)

Lindstrom, D. J.; Keller, L. P.; Martinez, R. R.

1993-01-01

Maralinga is an anomalous CK4 carbonaceous chondrite which contains numerous Ca-, Al-rich inclusions (CAI's) unlike the other members of the CK group. These CAI's are characterized by abundant green hercynitic spinel intergrown with plagioclase and high-Ca clinopyroxene, and a total lack of melilite. Instrumental Neutron Activation Analysis (INAA) was used to further characterize the meteorite, with special focus on the CAI's. High sensitivity INAA was done on eight sample disks about 100-150 microns in diameter obtained from a normal 30 micron thin section with a diamond microcoring device. The CAI's are enriched by 60-70X bulk meteorite values in Zn, suggesting that the substantial exchange of Fe for Mg that made the spinel in the CAI's hercynitic also allowed efficient scavenging of Zn from the rest of the meteorite during parent body thermal metamorphism. Less mobile elements appear to have maintained their initial heterogeneity.

Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed rocks in east- and south-central Alaska

USGS Publications Warehouse

Dusel-Bacon, Cynthia; Csejtey, Bela; Foster, Helen L.; Doyle, Elizabeth O.; Nokleberg, Warren J.; Plafker, George

1993-01-01

Most of the exposed bedrock in east- and south-central Alaska has been regionally metamorphosed and deformed during Mesozoic and early Cenozoic time. All the regionally metamorphosed rocks are assigned to metamorphic-facies units on the basis of their temperature and pressure conditions and metamorphic age. North of the McKinley and Denali faults, the crystalline rocks of the Yukon- Tanana upland and central Alaska Range compose a sequence of dynamothermally metamorphosed Paleozoic and older(?) metasedimentary rocks and metamorphosed products of a Devonian and Mississippian continental-margin magmatic arc. This sequence was extensively intruded by postmetamorphic mid-Cretaceous and younger granitoids. Many metamorphic-unit boundaries in the Yukon-Tanana upland are low-angle faults that juxtapose units of differing metamorphic grade, which indicates that metamorphism predated final emplacement of the fault-bounded units. In some places, the relation of metamorphic grade across a fault is best explained by contractional faulting; in other places, it is suggestive of extensional faulting.Near the United States-Canadian border in the central Yukon- Tanana upland, metamorphism, plutonism, and thrusting occurred during a latest Triassic and Early Jurassic event that presumably resulted from the accretion of a terrane that had affinities to the Stikinia terrane onto the continental margin of North America. Elsewhere in the Yukon-Tanana upland, metamorphic rocks give predominantly late Early Cretaceous isotopic ages. These ages are interpreted to date either the timing of a subsequent Early Cretaceous episode of crustal thickening and metamorphism or, assuming that these other areas were also originally heated during the latest Triassic to Early Jurassic and remained buried, the timing of their uplift and cooling. This uplift and cooling may have resulted from extension.South of the McKinley and Denali faults and north of the Border Ranges fault system, medium-grade metamorphism across much of the southern Peninsular and Wrangellia terranes was early to synkinematic with the intrusion of tonalitic and granodioritic plutons of primarily Early and Middle Jurassic age in the Peninsular terrane and Late Jurassic age in the Wrangellia terrane. Areas metamorphosed during the Jurassic episode that crop out near the Border Ranges fault system were subsequently retrograded and deformed in Cretaceous and early Tertiary time during accretion of younger units to the south. North of the Jurassic metamorphic and plutonic complex, low-grade metamorphism affected the rest of the Wrangellia terrane sometime during Jurassic and (or) Cretaceous time.North of the Wrangellia terrane and immediately south of the McKinley and Denali faults, flyschoid rocks, which were deposited within a basin that separated the Wrangellia terrane from the western margin of North America, form a northeastward-tapering wedge. Within the western half of the wedge, flysch and structurally interleaved tectonic fragments were highly deformed and weakly metamorphosed; much of the metamorphism and deformation probably occurred sometime during mid- to Late Cretaceous time. In the eastern half of the wedge, flyschoid rocks form an intermediate-pressure Barrovian sequence (Maclaren metamorphic belt). Metamorphism of the Maclaren metamorphic belt was synkinematic with the Late Cretaceous to earliest Tertiary intrusion of foliated plutons of intermediate composition. Isotopic data suggest metamorphism extended into the early Tertiary and was accompanied by rapid uplift and cooling. Low- to medium-grade metamorphism throughout the wedge was probably associated with the accretion of the outboard Wrangellia terrane, as has been proposed for the Maclaren metamorphic belt.South of the Border Ranges fault system lie variably metamorphosed sequences of oceanic rocks that comprise the successively accreted Chugach, Yakutat, Ghost Rocks, and Prince William terranes. The Chugach terrane consists of three successively accreted sequences of differing metamorphic histories. Metamorphism in all the sequences was associated with north-directed underthrusting beneath either the combined Peninsular-Wrangellia terrane or the older and inner parts of the Chugach terrane. These sequences, from innermost to outermost are: (1) intermediate- to highpressure, transitional greenschist- to blueschist-facies metabasalt and metasedimentary rocks that were metamorphosed during the Early and Middle Jurassic; (2) prehnite-pumpellyite-facies melange that was metamorphosed sometime during the Jurassic and Cretaceous; and (3) low-pressure prehnite-pumpellyite- or greenschist- facies flysch and metavolcanic rocks that were initially metamorphosed during latest Cretaceous to early Tertiary time and, in the eastern Chugach Mountains, were subsequently overprinted by low-pressure amphibolite-facies metamorphism that accompanied widespread intrusion during Eocene time. A similar low-pressure-facies series also developed within melange and flysch of the Yakutat terrane; these rocks are also intruded by Eocene plutons and are correlated with similar rocks of the Chugach terrane.Seaward of the Chugach terrane are the strongly deformed but weakly metamorphosed (prehnite-pumpellyite-facies) deep-sea metasedimentary rocks and oceanic metavolcanic rocks of the Ghost Rocks and Prince William terranes. Metamorphism and deformation occurred during underthrusting of these terranes beneath the Chugach terrane in early Tertiary time and predated, perhaps by very little, intrusion by early Tertiary granitoids.

Regional metamorphism in the Condrey Mountain Quadrangle, north-central Klamath Mountains, California

USGS Publications Warehouse

Hotz, Preston Enslow

1979-01-01

A subcircular area of about 650 km 2 in northern California and southwestern Oregon is occupied by rocks of the greenschist metamorphic facies called the Condrey Mountain Schist. This greenschist terrane is bordered on the east and west by rocks belonging to the amphibolite metamorphic facies that structurally overlie and are thrust over the Condrey Mountain Schist. The amphibolite facies is succeeded upward by metavolcanic and metasedimentary rocks belonging to the greenschist metamorphic facies. The Condrey Mountain Schist is composed predominantly of quartz-muscovite schist and lesser amounts of actinolite-chlorite schist formed by the metamorphism of graywacke and spilitic volcanic rocks that may have belonged to the Galice Formation of Late Jurassic age. Potassium-argon age determinations of 141?4 m.y. and 155?5 m.y. obtained on these metamorphic rocks seem to be incompatible with the Late Jurassic age usually assigned the Galice. The rocks that border the amphibolite facies are part of an extensive terrane of metavolcanic and metasedimentary rocks belonging to the western Paleozoic and Triassic belt. The metavolcanic rocks include some unmetamorphosed spilite but are mostly of the greenschist metamorphic facies composed of oligoclase (An15-20) and actinolite with subordinate amounts of chlorite and clinozoisiteepidote. The interbedded sedimentary rocks are predominantly argillite and slaty argillite, less commonly siliceous argillite and chert, and a few lenticular beds of marble. On the south, high-angle faults and a tabular granitic pluton separate the greenschist metavolcanic terrane from the amphibolite facies rocks; on the east, nonfoliated amphibolite is succeeded upward, apparently conformably, by metasedimentary rocks belonging to the greenschist metavolcanic terrane. In the southern part of Condrey Mountain quadrangle, an outlier of a thrust plate composed of the Stuart Fork Formation overlies the metavolcanic and metasedimentary rocks. The Stuart Fork in this region is composed of siliceous phyllite and phyllitic quartzite and is believed to be the metamorphosed equivalent of rocks over which it is thrust. In the Yreka-Fort Jones area, potassium-argon determinations on mica from the blueschist facies in the Stuart Fork gave ages of approximately 220 m.y. (Late Triassic) for the age of metamorphism. Rocks of the amphibolite facies structurally overlie the Condrey Mountain Schist along a moderate to steeply dipping thrust fault. The amphibolite terrane is composed of amphibolite and metasedimentary rocks in approximately equal amounts accompanied by many bodies of serpentinite and a number of gabbro and dioritic plutons. Most of the amphibolite is foliated, but some is nonfoliated; the nonfoliated amphibolite has an amphibolite mineralogy and commonly a relict volcanic rock texture. The nonfoliated amphibolite occurs on the southern and eastern borders of the amphibolite terrane between the areas offoliated amphibolite and the overly ing metavolcanic and metasedimentary rocks. Hornblende and plagioclase (An30-35) are the characteristic minerals, indicating that the rocks are of the almandine-amphibolite metamorphic facies. The metasedimentary rocks interbedded with the amphibolites include siliceous schist and phyllite, minor quartzite, and subordinate amounts of marble. Potassium-argon age dates obtained on hornblende from foliated amphibolite yield ages of 146?4 and 148? 4 m.y., suggesting a Late Jurassic metamorphic episode. Mafic and ultramafic rocks are widespread in the amphibolite terrane but are almost entirely absent from the area of greenschist facies metavolcanic and metasedimentary rocks. The ultramafic rocks, predominantly serpentinite, occur as a few large bodies and many small tabular concordant bodies interleaved with the foliated rocks. The ultramafic rocks include harzburgite and d1lIlite and their serpentinized equivalents. In the Condrey Mountain quadrangle, probably more t

Co-axial superposed folding and inverted regional metamorphism in the Tonga Formation: Cretaceous accretionary thrust tectonics in the Cascades crystalline core

NASA Astrophysics Data System (ADS)

Luke, Jensen; Lebit, Hermann; Paterson, Scott; Miller, Robert; Vernon, Ron

2017-04-01

The Cascades crystalline core forms part of the Cretaceous magmatic belt of western North America and exposes a crustal section composed of primarily tonalitic plutons that intruded siliciclastic metasediments of an arc-derived accretional system, and local meta-basalt/chert sequences. This study is the first attempt to correlate the well understood intrusive and P-T-t history of the metasedimentary and plutonic terrane with the kinematics and tectonic boundary conditions by rigorous analysis of structures documented in the Tonga Formation exposed at the western edge of the core. The Tonga Formation comprises pelite-psammite metasediments, which increase from greenschist ( 300-350° C) to amphibolite grade ( 500-600° C) from south to north. This metamorphic gradient is inverted relative to a major westward verging and downward facing fold system that dominates the internal architecture of the formation and implies that the initial regional metamorphic signature was established prior to the early fold generation. Subsequent co-axial fold superposition is seen as a consequence of the persistent accretional west-vergent thrusting in the foreland of the magmatic arc. The central section of the Cascades Range, exposed in western Washington, forms part of the Cretaceous accretional/magmatic arc extending over 4,000 km along western North America from Baja California to British Columbia (Fig. 1a) (e.g. Misch, 1966; Brown, 1987; Tabor et al., 1989). Two models exist for the evolution of the Cascades crystalline core with one invoking magmatic loading (e.g. Brown and Walker, 1993) as the major cause for rapid loading, consequent regional metamorphism and vertical uplift (Evans and Berti, 1986). Conversely, other workers favor a model that suggests loading as a consequence of tectonic, thrust-related thickening, followed by rapid exhumation of the exposed crustal section of 10 to 40 km paleodepth (e.g. Matzel, 2004; Patterson et al., 2004; Stowell et al., 2007). In this context, the Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretaceous plutonism, contact to regional metamorphism, and multiple episodes of folding, evidencing intense, arc-perpendicular contractional deformation, similar to that observed in the neighboring Chiwaukum Schist to the east (Miller and Paterson, 1992; Miller et al., 1993; Paterson and Miller, 1998; Miller et al., 2006). Building on previous extensive mapping and metamorphic and petrologic analysis in the Cascades, we use the Tonga Formation as a means to a comprehensive tectonic synthesis incorporating detailed analysis of the kinematics and timing of structural evolution, magma emplacement, and metamorphism.

Metamorphic density controls on early-stage subduction dynamics

NASA Astrophysics Data System (ADS)

Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

2013-04-01

Subduction is primarily driven by the densification of the downgoing oceanic slab, due to dynamic P-T-fields in subduction zones. It is crucial to unravel slab densification induced by metamorphic reactions to understand the influence on plate dynamics. By analyzing the density and metamorphic structure of subduction zones, we may gain knowledge about the driving, metamorphic processes in a subduction zone like the eclogitization (i.e., the transformation of a MORB to an eclogite), the breakdown of hydrous minerals and the release of fluid or the generation of partial melts. We have therefore developed a 2D subduction zone model down to 250 km that is based on thermodynamic equilibrium assemblage computations. Our model computes the "metamorphic density" of rocks as a function of pressure, temperature and chemical composition using the Theriak-Domino software package at different time stages. We have used this model to investigate how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within subduction systems. These processes are commonly neglected by other approaches (e.g., gravitational or thermomechanical in nature) reproducing the density distribution within this tectonic setting. The process of eclogitization is assumed as being important to subduction dynamics, based on the very high density (3.6 g/cm3) of eclogitic rocks. The eclogitization in a MORB-type crust is possible only if the rock reaches the garnet phase stability field. This process is primarily temperature driven. Our model demonstrates that the initiation of eclogitization of the slab is not the only significant process that makes the descending slab denser and is responsible for the slab pull force. Indeed, our results show that the densification of the downgoing lithospheric mantle (due to an increase of pressure) starts in the early subduction stage and makes a significant contribution to the slab pull, where eclogitization does not occur. Thus, the lithospheric mantle acts as additional ballast below the sinking slab shortly after the initiation of subduction. Our calculation shows that the dogma of eclogitized basaltic, oceanic crust as the driving force of slab pull is overestimated during the early stage of subduction. These results improve our understanding of the force budget for slab pull during the intial and early stage of subduction. Therefore, the complex metamorphic structure of a slab and mantle wedge has an important impact on the development and dynamics of subduction zones. Further Reading: Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth and Planetary Science Letters

Stratigraphy, structure and regional correlation of eastern Blue Ridge sequences in southern Virginia and northwestern North Carolina: an interim report from new USGS mapping

USGS Publications Warehouse

Carter, Mark W.; Merschat, Arthur J.

2014-01-01

The contact between eastern Blue Ridge stratified rocks above Mesoproterozoic basement rocks is mostly faulted (Gossan Lead and Red Valley). The Callaway fault juxtaposes Ashe and Lynchburg rocks above Wills Ridge Formation. Alligator Back Formation rocks overlie Ashe and Lynchburg rocks along the Rock Castle Creek fault, which juxtaposes rocks of different metamorphism. The fault separates major structural domains: rocks with one penetrative foliation in the footwall, and pin-striped recrystallized compositional layering, superposed penetrative foliations, and cleavage characterize the hanging wall. These relationships are ambiguous along strike to the southwest, where the Ashe and Alligator Back formations are recrystallized at higher metamorphic grades.

Hybrid channel flow-type mechanisms in the Greater Himalayan Sequence (West Nepal): new constraints from vorticity of flow and quartz petrofabric analyses.

NASA Astrophysics Data System (ADS)

Frassi, Chiara

2016-04-01

Three main tectono-metamorphic units are classically recognized along the Himalayan belt: the Lesser Himalayan (LH), the Greater Himalayan sequence (GHS) and the Tibetan Sedimentary sequence (TSS). The GHS may be interpreted as a low-viscosity tabular body of mid-crustal rocks extruded southward in Miocene times beneath the Tibetan plateau between two parallel and opposite-sense crustal-scale shear zones: the Main Central thrust at the base, and the South Tibetan Detachment system at the top. The pre-/syn-shearing mineral assemblage documented within these crustal-scale shear zones indicates that the metamorphic grade increases toward the core of the GHS producing an inverted and a normal thermal gradient respectively on the top and on the bottom of the slab. In addition, thermal profiles estimated using both petrology- and microstructures/fabrics-based thermometers indicate that the metamorphic isograds are condensed. Although horizontal extension and vorticity estimates collected across the GHS could be strongly biased by the criteria used to define the map position of the MCT, published vorticity data document general shear flow (1>Wk>0) within the slab with a pure-shear component of flow slightly predominant within the core of the GHS whereas the simple-shear component seems to dominate at the top of the slab. The lower boundary of the GHS records a general shear flow with a comparable contribution of simple and pure shearing. The associated crustal extrusion is compatible with Couette - Poiseuille velocity flow profile as assumed in crustal-scale channel flow-type models In this study, the quartz c-axis petrofabrics, vorticity and deformation-temperature studies are integrated with microstructures and metamorphic studies to individuate the location of the MCT and to document the spatial distribution of ductile deformation patterns across the lower portion of the GHS exposed in the Chaudabise river valley in western Nepal. My results indicate that the Main Central Thrust is located ˜5 km structurally below the previous mapped locations. Deformation temperature increases up structural section from ˜450°C to ˜650°C and overlaps with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at "close" to peak metamorphic conditions. I interpreted the telescoping and the inversion of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm = 0.88-1) pervasively distributed through the lower portion of the GHS. The results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectono-metamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth.

Disjunctive Grade Variation from Greenschist to Granulite Facies, Siyom Valley, Eastern Arunachal Pradesh, India

NASA Astrophysics Data System (ADS)

Clarke, G. L.; Bhowmik, S. K.; Aitchison, J. C.; Ireland, T. R.

2014-12-01

The Siyom Valley section in eastern Arunachal Pradesh exposes an inverted metamorphic succession (Nandini & Thakur, 2011), metapelitic assemblages increasing in grade northwards from chlorite, through biotite, garnet-staurolite and kyanite-bearing schist to kyanite-sillimanite migmatite. Grade changes are mostly controlled by shallowly north, and northwest-dipping fault structures. Two textural stages of garnet growth can be identified in the ilmenite-bearing amphibolite facies rocks, staurolite having formed late in, or after, deformation responsible for the main penetrative foliation (S2). Kyanite and rutile inclusions in garnet indicate that their growth in migmatite preceded that of matrix sillimanite, ilmenite and cordierite, though unrecrystallized kyanite is also common in the feldspathic matrix. Preliminary data indicate the pronounced tectonic thinning of metasedimentary protoliths during exhumation, and the probability of a pronounced step in grade in the middle part of the river section. Similarities with sections in the Sikkim (Dasgupta et al., 2004) and western Arunachal Pradesh (Goswami et al., 2009) Himalaya reflect the lateral continuity of the south-vergent thrusts that controlled the exhumation of the high-grade rocks, with debate concerning the location and significance of the Main Central Thrust zone begging protolith and metamorphic age data. Dasgupta, S.,Ganguly, J. & Neogi, S., 2004. Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P-T gradient and implications. Journal of Metamorphic Geology, 22, 395-412. Goswami, S., Bhowmik, S.K. & Dasgupta, S., 2009. Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India. Journal of Asian Earth Sciences, 36, 390-406. Nandini, P. & Thakur, S.S., 2011. Metamorphic evolution of the Lesser Himalayan Crystalline Sequence, Siyom Valley, NE Himalaya, India. Journal of Asian Earth Sciences, 40, 1089-1100

Structural analysis and tectonic evolution of the eastern Binalud Mountains, NE Iran

NASA Astrophysics Data System (ADS)

Sheikholeslami, M. R.; Kouhpeyma, M.

2012-10-01

The Binalud Mountains are situated in the south of the Kopeh Dagh as a transitional zone between the Alborz and Central Iran zones. The Palaeotethys suture of the north Iran is located in this area. The Binalud Mountains consists of relatively thick successions of sedimentary, metamorphic and igneous rocks. The earliest deformation, a polyphase synmetamorphic deformation which occurred entirely in ductile conditions, is distinguished in the metamorphic rocks of the eastern part. D1, D2 and D3 deformation phases are related to this deformation. The D4 deformation affected the area after a period of sedimentation and erosion. The thrust faults of the central and southern part of the eastern Binalud were classified as structures related to the D5 tectonic event. From the geodynamic point of view, in Late Palaeozoic times the studied area formed an oceanic trench generated by the subduction of the Palaeotethys oceanic lithosphere beneath the Turan Plate. In the Late Triassic, the Early Cimmerian Event resulted in a collisional type orogeny generating a transpression polyphase deformation and the metamorphism of Permian and older sediments. Following this collision, granite intrusions were emplaced in the area and caused contact metamorphism. The exhumation and erosion of the rocks deformed and metamorphosed during Early Cimmerian Event caused the formation of molassic type sediments in a Rhaetian-Lias back arc basin. The continuation of convergence between the Turan and Iran Plates caused the metamorphism of these sediments and their transformation to phyllite and meta-sandstone. During Late Mesozoic and Early Cenozoic times, the convergence between Central Iran and Turan Plates continued and a NE compression caused folding of the Cretaceous and older rocks in the Kopeh Dagh area. In the Binalud area this deformation caused the generation of several thrust fault systems with S to SW vergence, resulting in a thrusting of Palaeozoic and Mesozoic successions on each other and on the Neogene sediments at the southern border of the Binalud Mountains.

Structural development of an Archean Orogen, Western Point Lake, Northwest Territories

NASA Astrophysics Data System (ADS)

Kusky, Timothy M.

1991-08-01

The Point Lake orogen in the central Archean Slave Province of northwestern Canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites. Fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening. In the western part of the orogen the basal decollement places mafic plutonic/volcanic rocks over an ancient tonalitic gneiss complex. Ten kilometers to the east in the Keskarrah Bay area, slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins. Conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel, levee, and overbank facies of this thrust-related sedimentary fan system. The synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation, while other thrusts related to hinterland-progressing imbrication displace this unconformity. Tightening of synorogenic depositional troughs resulted in the conglomerates' present localization in synclines to the west of associated thrust faults and steepening of structural dips. Eastern parts of the orogen consist of isoclinally folded graywackes composed largely of Mutti and Ricci-Lucchi turbidite facies B, C, and D, interpreted as submarine fan deposits eroded from a distant volcanic arc. Thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter-scale duplex structures, and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances. The sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms. Conditions of deformation along the thrusts parallel the regional metamorphic zonation: amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in Keskarrah Bay. Depth differences can account for only half of the metamorphic gradient; thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients. A tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen.

Geological indicators for impact: The anomalous case of the Vredefort structure, South Africa

NASA Technical Reports Server (NTRS)

Antoine, L. A. G.; Reimold, W. U.

1988-01-01

The Vredefort Dome is located within and almost central to the Witwatersrand basin in its presently known extent. It exposes a central Archean granite core which is surrounded by a collar of supracrustal rocks. These collar rocks outline a strong polygonal geometry. The Archean core is comprised of two concentric zones, the Outer Granite Gneiss (OGG), and the core central Inlandsee Leucogranofels (ILG). The rocks of the inner core display granulite facies metamorphism, while the OGG is in amphibolite facies. The inner core is believed from recent drill hole information to be underlain by mafic and ultramafic gneisses, the extent of which cannot be assessed at present. A fairly broad zone of charnockites separates the OGG and ILG domains. This zone is characterized by a high concentration of pseudotachylite and ductile shearing. Whereas a number of other domical structures are located within or surrounding the Witwatersrand basin, the Vredefort structure is anomalous, in that it has: a partly polygonal geometry; extensive alkali intrusives in the northwestern sector; granophyre dykes (ring-dykes peripheral to the contact collar-basement and NW-SE or NE-SW trending dykes within the Archean basement); contact metamorphism of the collar supracrustal rocks; the overturning of collar supracrustals in the northern sectors; deformation phenomena widely regarded as representing shock metamorphism (pseudotachylite, (sub)planar microdeformation features in quartz, shatter cones and occurrences of high-P quartz polymorphs); a positive 30 mgal gravity anomaly; and high amplitude magnetic anomalies. Recent geophysical, structural and petrological evidence pertinent for the identification of the processes that led to the formation of the Vredefort structure are summarized.

First evidence of the Ellesmerian metamorphism on Svalbard

NASA Astrophysics Data System (ADS)

Kośmińska, Karolina; Majka, Jarosław; Manecki, Maciej; Schneider, David A.

2016-04-01

The Ellesmerian fold-and-thrust belt is exposed in the High Arctic from Ellesmere Island in the east, through North Greenland, to Svalbard in the west (e.g. Piepjohn et al., 2015). It developed during Late Devonian - Early Carboniferous, and overprinted older (mainly Caledonian) structures. It is thought that this fold-and-thrust belt was formed due to collision of the Pearya Terrane and Svalbard with the Franklinian Basin of Laurentia. Traditionally, the Ellesmerian fold-and-thrust belt comprises a passive continental margin affected by foreland deformation processes, but the exact larger scale tectonic context of this belt is disputable. It is partly because the Eocene Eurekan deformation superimposed significantly the Ellesmerian structures, thus making the reconstruction of the pre-Eurekan history very difficult. Here we present for the first time evidence for Ellesmerian metamorphism within the crystalline basement of Svalbard. These rocks are exposed in the Pinkie unit on Prins Karls Forland (W-Svalbard), which exhibits tectonic contacts with the overlying sequences. The Pinkie unit is mainly composed of strongly deformed lithologies such as laminated quartzites, siliciclastic rocks and garnet-bearing mica schists. Detrital zircon dating yielded ages as young as Neoproterozoic (0.95-1.05 Ga), thus the Pinkie unit is considered to be Neoproterozoic (Kośmińska et al., 2015a). The M1 assemblages and D1 structures are affected by D2 mylonitization (cf. Faehnrich et al., 2016, this meeting). Petrological characterization and Th-U-total Pb chemical monazite dating have been performed on the Pinkie metapelites. These rocks exhibit an apparent inverted Barrovian metamorphic sequence, within which three metamorphic zones have been distinguished: garnet+staurolite+muscovite+biotite, garnet+staurolite+kyanite+muscovite+biotite, garnet+kyanite+muscovite+biotite. The P-T estimates using the QuiG barometry coupled with thermodynamic modelling revealed that the metapelites were formed under amphibolite facies conditions at c. 7-9 kbar and 550-650 °C (Kośmińska et al., 2015b). Monazite dating was performed on samples from these three metamorphic zones. The chemical zonation of monazite allows the identification of several monazite populations, which likely developed during different stages of Barrovian metamorphism. The geochronology demonstrate protracted monazite growth from the early prograde stage at c. 370 Ma to the peak conditions at c. 355 Ma. Thus it is evident that the Ellesmerian event was not limited to the relatively cold deformation as previously thought. The amphibolite facies metamorphism of c. 370-355 Ma that was documented in our study sheds new light on understanding of the character of this tectonothermal event. This project is financed by NCN research project No 2013/11/N/ST10/00357 and partially funded by AGH research grant no 11.11.140.319. References: Faehnrich et al., 2016. A tectonic window into the crystalline basement of Prins Karls Forland, Spitsbergen. EGU General Assembly 2016. Kośmińska et al., 2015b. Metamorphic evolution of the Pinkie unit metapelites from Svalbard (High Arctic): P-T-t study including Quartz-in-garnet barometry (QuiG). GSA 2015: Annual Meeting, Baltimore. Kośmińska et al., 2015a. Detrital zircon U-Pb geochronology of metasediments from southwestern Svalbard's Caledonian Province. EGU General Assembly 2015. Piepjohn et al., 2015. Tectonic map of the Ellesmerian and Eurekan deformation belts on Svalbard, North Greenland, and the Queen Elizabeth Islands (Canadian Arctic). Arktos, DOI 10.1007/s41063-015-0015-7.

Preliminary geologic map of the Winchester 7.5' quadrangle, Riverside County, California

USGS Publications Warehouse

Morton, Douglas M.

2003-01-01

The Winchester quadrangle is located in the northern part of the Peninsular Ranges Province within the central part of the Perris block, a relatively stable, rectangular in plan view, area located between the Elsinore and San Jacinto fault zones (see location map). The quadrangle is underlain by Cretaceous and older basement rocks. Cretaceous plutonic rocks are part of the composite Peninsular Ranges batholith, which indicates wide variety of granitic rocks, ranging from granite to gabbro. Parts of three major plutonic complexes are within the quadrangle, the Lakeview Mountains pluton, the Domenigoni Valley pluton and the Paloma Valley ring complex. In the northern part of the quadrangle is the southern part of the Lakeview Mountains pluton, a large composite body, most of which lies in the quadrangle to the north. In the center part of the quadrangle is the eastern part of the Domenigoni Valley pluton, which consists of massive biotite-hornblende granodiorite and tonalite; some tonalite in the southern part of the pluton has a relatively pronounced foliation produced by oriented biotite and hornblende. Common to abundant equant-shaped, mafic inclusions occur through out the pluton except in the outermost part where inclusions are absent. The pluton was passively emplaced by piecemeal stoping of a variety of older rocks and the eastern contact is well exposed in the quadrangle. Associated with the Domenigoni Valley pluton is a swarm of latite dikes; the majority of these dikes occur in the Winchester quadrangle, but they extend into the Romoland quadrangle to the west. The latite dikes intrude both the pluton and adjacent metamorphic rocks, most are foliated, and most have a well developed lineation defined by oriented biotite and/or hornblende crystals. Dikes intruding the pluton were emplaced in northwest striking joints; and dikes intruding the metamorphic rocks were emplaced along foliation planes. In the eastern part of the quadrangle a Cretaceous age suture juxtaposes low-metamorphic grade Mesozoic rocks against high-metamorphic grade gneissic-textured Mesozoic rocks. Juxtaposition occurred when the high-metamorphic grade rocks were at upper amphibolite grade temperatures, and produced a steep thermal gradient in the low-metamorphic grade Mesozoic rocks. Age of suturing and attendant metamorphism, based on metamorphic mineral ages, is about 100 Ma (L. Snee, personal communication, 2002). The suture zone appears to vary in thickness, and includes within it a number of metadunite bodies and related rocks. Prebatholithic rocks of Mesozoic age include a wide variety of sedimentary rocks of greenschist or lower metamorphic grade, in the western and central part of the quadrangle, and upper amphibolite grade near the eastern edge of the quadrangle. The metamorphic grade increases from greenschist to upper amphibolite grade over a distance of less than two miles; andalusite and sillimanite isograds are closely spaced near the suture. Metamorphism was Buchan type of relatively high temperature and relatively low pressure (Schwarcz, 1969). Common lithologies of the low metamorphic grade suite include phyllite, lithic greywacke, impure quartzite, meta-arkose, and interlayered quartzite and phyllite. Most of the layering and foliation in the metamorphic rocks is the result of intense structural transposition. Relic bedding appears to be restricted to very local occurrences in hinges of slip folds. The upper amphibolite grade, gneissic-textured Mesozoic rocks consist of sillimanite-biotite gneiss, black amphibolite, and impure quartzite. Anatectic gneiss containing igneous textured segregations of quartz and feldspar is commonly inter leaved with biotite gneiss.

Prolonged episodic Paleoproterozoic metamorphism in the Thelon Tectonic Zone, Canada: an in-situ SHRIMP/EPMA monazite geochronology study

NASA Astrophysics Data System (ADS)

Mitchell, Rhea; William, Davis; Robert, Berman; Sharon, Carr; Michael, Jercinovic

2017-04-01

The Thelon Tectonic zone (TTZ), Nunavut, Canada, is a >500km long geophysically, lithologically and structurally distinct N-NNE striking Paleoproterozoic boundary zone between the Slave and Rae Archean provinces. The TTZ has been interpreted as a ca. 2.0 Ga continental arc on the western edge of the Rae craton, that was deformed during collision with the Slave craton ca. 1.97 Ga. Alternatively, the Slave-Rae collision is interpreted as occurring during the 2.35 Ga Arrowsmith orogeny while the 1.9-2.0 Ga TTZ represents an intra-continental orogenic belt formed in previously thinned continental crust, postdating the Slave-Rae collision. The central part of the TTZ comprises three >100 km long, 10-20 km wide belts of ca. 2.0 Ga, mainly charnockitic plutonic rocks, and a ca. 1910 Ma garnet-leucogranite belt. Metamorphism throughout these domains is upper-amphibolite to granulite-facies, with metasedimentary rocks occurring as volumetrically minor enclaves and strands of migmatites. The Ellice River domain occurs between the western and central plutonic belts. It contains ca. 1950 Ma ultramafic to dacitic volcanic rocks and foliated Paleoproterozoic psammitic metasedimentary rocks at relatively lower grade with lower to middle amphibolite-facies metamorphic assemblages. In-situ U-Pb analyses of monazite using a combination of Sensitive High-Resolution Ion Microprobe (SHRIMP) and Electron Probe Microanalyzer (EPMA) were carried out on high-grade metasedimentary rocks from seventeen samples representing the eastern margin of the Slave Province and all major lithological domains of the TTZ. 207Pb/206Pb monazite ages from SHRIMP analysis form the foundation of this dataset, while EPMA ages are supplementary. The smaller <6µm spot size of EPMA allowed for further constraint on ages of micro-scale intra-crystalline domains in some samples. Monazite ages define four distinct Paleoproterozoic metamorphic events and one Archean metamorphic event at ca. 2580 Ma. The latter is recorded exclusively along the eastern margin of the Slave Province. Metamorphism ca. 1996 Ma, recorded in one high-grade gneiss from the central plutonic belt appears to reflect a regional contact metamorphism associated with intrusion of 2000 Ma plutons. Throughout the TTZ, a selection of monazite grains included in garnet porphyroblasts define a metamorphic event ca. 1962 Ma. One sample from the eastern margin of the Slave Province similarly records metamorphism at 1961 Ma in monazite grains in the matrix. This sample interestingly does not record the ca. 2580 Ma metamorphism typical of the Slave Province. The longest lived and most wide spread metamorphic event in the TTZ occurred ca. 1922 to 1883 Ma. This event is interpreted as the main compressional/collisional and anatectic event, with partial melting forming the extensive ca. 1910 Ma garnet-leucogranite belts. Three samples, located in the eastern margin of the Slave province, the Ellice River domain and the eastern plutonic belt, record younger metamorphism at ca. 1814 Ma. These events may represent post-collisional transpression coeval with movement along nearby regional-scale faults.

Crustal shortening and thickening in Neoarchean granite-greenstone belts: A case study from the link between the ∼2.7 Ga Elu and Hope Bay belts, northeast Slave craton, Canada

NASA Astrophysics Data System (ADS)

Mvondo, Hubert; Lentz, Dave; Bardoux, Marc

2017-11-01

The Elu Link between the ∼2.7 Ga Hope Bay and Elu belts in the northeast Bathurst Block of the Slave craton comprises supracrustal and intrusive rocks variably deformed by three tectono-metamorphic events (D1-D3). The geometry of D1 structures formed during prograde metamorphism is uncertain, because of subsequent overprint. D2 occurred in two stages predating (D2a) and postdating (D2b) peak metamorphism. D1 and D2a were thrusting events inferred from peak metamorphic pressures of ∼6.7 kbar (670 MPa) retained by a garnet orthogneiss. The latter is diagnostic of thrust tectonism in Archean granite-greenstone belts with no characteristic thrust faults. Unlike D2a, D2b was a vertical general flattening event prevailing during the formation of magmatic domes and interdomal folds that form the main strain patterns of the belts. This was followed by the formation of buckled F3 folds associated with D3 vertical constriction. The switch from thrust to vertical tectonics during peak metamorphism and subsequent deformation resulted in intense recrystallization that explains the poor preservation and scarcity of early-formed shears, including thrust zones. A tectonic process, combining D1+D2a thrust stacking, sagduction, and vertical stretching during D2b and D3, is suggested to explain crustal thickening in the Elu Link and terrains of similar ages.

Carbonate-orthopyroxenite lenses from the Neoproterozoic Gerf ophiolite, South Eastern Desert, Egypt: The first record in the Arabian Nubian Shield ophiolites

NASA Astrophysics Data System (ADS)

Gahlan, Hisham A.; Arai, Shoji

2009-01-01

Carbonate-orthopyroxenites (classic sagvandites) are reported in the Gerf ophiolite, South Eastern Desert, Egypt: the first finding from the Arabian Nubian Shield (ANS) ophiolites. They form massive lenses at the southern tip of the Gerf ophiolite, along the contact between the Shinai granite and Gerf serpentinized peridotites. The lenses show structural concordance with the neighboring country rocks and the granite contact. They consist mainly of metamorphic orthopyroxene + magnesite, among other metamorphic, relict primary and retrograde secondary minerals. Based only on chemistry, two types of carbonate-orthopyroxenites can be recognized, Types I (higher-Mg) and II (lower-Mg and higher-Fe). Field constraints, petrography and mineral chemistry indicate a metamorphic origin for the Gerf carbonate-orthopyroxenites. The euhedral form of relict primary chromian spinels combined with their high Cr#/low-TiO 2 character, and absence of clinopyroxene suggest that the protolith for the Gerf carbonate-orthopyroxenites is a highly depleted mantle peridotite derived from a sub-arc setting. Contact metamorphism accompanied by CO 2-metasomatism resulted in formation of the Gerf carbonate-orthopyroxenites during intrusion of the Shinai granite. The source of CO 2-rich fluids is most likely the neighboring impure carbonate layers. Correlation of the carbonate-orthopyroxenite mineral assemblages with experimental data for the system MgO-SiO 2-H 2O-CO 2 suggests metamorphic/metasomatic conditions of 520-560 °C, Pfluid = 2 kbar and extremely high X values (0.87-1).

Indo-Burma Range: a belt of accreted microcontinents, ophiolites and Mesozoic-Paleogene flyschoid sediments

NASA Astrophysics Data System (ADS)

Acharyya, S. K.

2015-07-01

This study provides an insight into the lithotectonic evolution of the N-S trending Indo-Burma Range (IBR), constituting the southern flank of the Himalayan syntaxis. Paleogene flyschoid sediments (Disang-Barail) that represent a shallow marine to deltaic environment mainly comprise the west-central sector of IBR, possibly resting upon a continental base. On the east, these sequences are tectonically flanked by the Eocene olistostromal facies of the Disang, which developed through accretion of trench sediments during the subduction. The shelf and trench facies sequences of the Disang underwent overthrusting from the east, giving rise to two ophiolite suites ( Naga Hills Lower Ophiolite ( NHLO) and Victoria Hills Upper Ophiolite ( VHUO), but with different accretion history. The ophiolite and ophiolite cover rock package were subsequently overthrusted by the Proterozoic metamorphic sequence, originated from the Burmese continent. The NHLO suite of Late Jurassic to Early Eocene age is unconformably overlain by mid-Eocene shallow marine ophiolite-derived clastics. On the south, the VHUO of Mesozoic age is structurally underlain by continental metamorphic rocks. The entire package in Victoria Hills is unconformably overlain by shallow marine Late Albian sediments. Both the ophiolite suites and the sandwiched continental metamorphic rocks are thrust westward over the Paleogene shelf sediments. These dismembered ophiolites and continental metamorphic rocks suggest thin-skinned tectonic detachment processes in IBR, as reflected from the presence of klippe of continental metamorphic rocks over the NHLO and the flyschoid Disang floor sediments and half windows exposing the Disang beneath the NHLO.

Structural evolution of the La Paya pluton (Sierra de Cachi, NW Argentina): insights from the study of its magnetic fabric

NASA Astrophysics Data System (ADS)

Aranguren, Aitor; Hongn, Fernando; María Tubía, José; Vegas, Néstor

2017-04-01

The La Paya granite is a pluton elongated in a northerly direction that spreads over 10 km2 in the Sierra de Cachi (Eastern Cordillera of Salta, Andean belt, NW Argentina). This pluton intrudes into high-temperature and low-pressure migmatites and schists. The metamorphic evolution of the country rocks and the emplacement of the La Paya pluton took place in an extensional tectonic setting during Ordovician times (Hongn et al., 2014). Subsequently, a compressional tectonic event led to the formation of large E-verging folds with a penetrative axial planar foliation. Due to the folding event, the current metamorphic zonation is inverted and the metamorphic degree rises towards the west. The granite is concordant with the metamorphic zonation and with the country rocks foliation. Schists with biotite and andalucite (low to medium metamorphic grade) crop out in the eastern border of the pluton and schists with cordierite (high grade) in the western border. The pluton is characterized by the coexistence of magmatic and solid-state structures. The deformation is localized in shear bands concentrated in the pluton roof. Wide sectors of the pluton core preserve medium- to coarse-grained equigranular textures of magmatic origin on which we have collected samples from 28 sites in order to perform an AMS study. Susceptibility values are very low, with k ranging between 19 and 67 x 10-6 SI. The obtained anisotropy values are in agreement with the magmatic character of the analysed samples (Pj between 1,03-1,07). The magnetic foliations are concordant with the contacts of the pluton. They show northwards trends and dips to the west that are steeper close to the eastern pluton border. Magnetic lineations are concentrated in two main maxima that define a great circle parallel to the pluton elongation. The parallelism between the magnetic fabrics from areas with magmatic structures and the structures observed in domains with solid-state deformation points to a syn-kinematic emplacement of the pluton during the development of the roof shear zone. The cartographic data and the distribution of the magnetic lineations of the La Paya granite allows to recognize an east-vergent antiform that folds the roof shear zone and the overall laminar pluton. Hongn, F.D., Tubía, J.M., Esteban, J.J., Aranguren, A., Vegas, N., Sergeev, S., Larionov, A. and Basei, M. (2014). Journal of Iberian Geology 40 (2), 225-240. DOI: 10.5209/rev_JIGE.2014.v40.n2.45303

Thermobarometric constraints on mid-Cretaceous to late Cretaceous metamorphic events in the western metamorphic belt of the Coast Mountains complex near Petersburg, southeastern Alaska

USGS Publications Warehouse

Himmelberg, Glen R.; Brew, David A.

2005-01-01

The western metamorphic belt is part of the Coast Mountains Complex of southeastern Alaska and western Canada. This complex formed as a result of mid-Cretaceous through middle Eocene crustal shortening between the previously amalgamated Wrangellia and Alexander terranes (Insular superterrane) and previously accreted terranes of the North American continental margin (Intermontane superterrane). The western metamorphic belt, which ranges from a few kilometers to several tens of kilometers in width, records a complex sequence of contact-metamorphic and regional metamorphic events, the most significant of which are designated M1R, M2C-R, and M3R. The M1R regional metamorphic event ranged in grade from subgreenschist to greenschist facies and was overprinted by the M2C-R and M3R metamorphic events. The M2C-R metamorphic event is recorded in discrete contact-metamorphic aureoles and regional metamorphic-mineral assemblages related to tonalite-granodiorite plutons of the Admiralty-Revillagigedo plutonic belt. The M3R metamorphic belt, which is adjacent to the M2C-R belt, is characterized by regional Barrovian isograds of garnet, staurolite, kyanite, and sillimanite. Using the THERMOCALC program, pressure-temperature (P-T) conditions for the M2C-R metamorphic event are estimated to be in the ranges 5.3-7.5 kbars and 525-640 deg.C and for the M3R metamorphic event in the ranges 9.4-12.6 kbars and 730-895 deg.C. The M2C-R metamorphic event occurred at approximately 90 Ma, but the timing of the M3R metamorphic event is poorly documented and uncertain. On the basis of an 40Ar/39Ar age on actinolitic amphibole and a Sm-Nd age on garnet core, the timing of metamorphism might be constrained between 90+/-1 and 80+/-9 Ma, although the Sm-Nd age of 80+/-9 m.y. possibly reflects postpeak growth. Thermobarometric data suggest that the two events occurred at different crustal levels and followed different P-T paths. No evidence exists that M2C-R metamorphic-mineral assemblages were overprinted by the M3R metamorphic event, as proposed by some workers. Juxtaposition of the two belts of rocks probably occurred along the Coast shear zone during uplift and exhumation of the Coast Mountains.

Joint Inversion of Geochemical Data and Geophysical Logs for Lithology Identification in CCSD Main Hole

NASA Astrophysics Data System (ADS)

Deng, Chengxiang; Pan, Heping; Luo, Miao

2017-12-01

The Chinese Continental Scientific Drilling (CCSD) main hole is located in the Sulu ultrahigh-pressure metamorphic (UHPM) belt, providing significant opportunities for studying the metamorphic strata structure, kinetics process and tectonic evolution. Lithology identification is the primary and crucial stage for above geoscientific researches. To release the burden of log analyst and improve the efficiency of lithology interpretation, many algorithms have been developed to automate the process of lithology prediction. While traditional statistical techniques, such as discriminant analysis and K-nearest neighbors classifier, are incompetent in extracting nonlinear features of metamorphic rocks from complex geophysical log data; artificial intelligence algorithms are capable of solving nonlinear problems, but most of the algorithms suffer from tuning parameters to be global optimum to establish model rather than local optimum, and also encounter challenges in making the balance between training accuracy and generalization ability. Optimization methods have been applied extensively in the inversion of reservoir parameters of sedimentary formations using well logs. However, it is difficult to obtain accurate solution from the logging response equations of optimization method because of the strong overlapping of nonstationary log signals when applied in metamorphic formations. As oxide contents of each kinds of metamorphic rocks are relatively less overlapping, this study explores an approach, set in a metamorphic formation model and using the Broyden Fletcher Goldfarb Shanno (BFGS) optimization algorithm to identify lithology from oxide data. We first incorporate 11 geophysical logs and lab-collected geochemical data of 47 core samples to construct oxide profile of CCSD main hole by using backwards stepwise multiple regression method, which eliminates irrelevant input logs step by step for higher statistical significance and accuracy. Then we establish oxide response equations in accordance with the metamorphic formation model and employ BFGS algorithm to minimize the objective function. Finally, we identify lithology according to the composition content which accounts for the largest proportion. The results show that lithology identified by the method of this paper is consistent with core description. Moreover, this method demonstrates the benefits of using oxide content as an adhesive to connect logging data with lithology, can make the metamorphic formation model more understandable and accurate, and avoid selecting complex formation model and building nonlinear logging response equations.

A mineralogical instrument for planetary applications

NASA Technical Reports Server (NTRS)

Blake, David F.; Vaniman, David T.; Bish, David L.

1994-01-01

The mineralogy of a planetary surface can be used to identify the provenance of soil or sediment and reveal the volcanic, metamorphic and/or sedimentological history of a particular region. We have discussed elsewhere the applications and the instrument design of possible X-ray diffraction and X-ray fluorescence (XRD/XRF) devices for the mineralogical characterization of planetary surfaces. In this abstract we evaluate some aspects of sample-detector geometry and sample collection strategies.

Novel Design of Type I High Power Mid-IR Diode Lasers for Spectral Region 3 - 4.2 Microns

DTIC Science & Technology

2014-09-25

multifold improvement of the device characteristics. Cascade pumping was achieved utilizing efficient interband tunneling through "leaky" window in band...Initially cascade pumping scheme was applied to laser heterostructures utilizing gain sections based on either intersubband [1] or type-II interband ...active regions, metamorphic virtual substrate and cascade pumping scheme. Cascade pumping of type-I quantum well gain section opened the whole new

Signatures of Pacific-type orogeny in Lleyn and Anglesey areas, northwest Wales

NASA Astrophysics Data System (ADS)

Asanuma, H.; Okada, Y.; Sawaki, Y.; Yamamoto, S.; Hirata, T.; Maruyama, S.

2014-12-01

The orogeny is a fundamental process of plate tectonics, and its record is useful for understanding of ancient plate motion. Geotectonic history of British isles has been explained by collision-type orogeny accompanied by closure of Iapetus ocean. High pressure metamorphic rocks such as blueschist and eclogite characterizing Pacific-type orogeny occur in some places, but have not attracted much interests because of their smallness. The subduction-related (Pacific-type) orogeny is characterized by contemporaneous formation of a batholith belt, a regional metamorphic belt (high P/T type) and an accretionary complex. Late Proterozoic-Cambrian (677-498 Ma) calc-alkaline volcano-plutonic complexes crop out in Lleyn and Anglesey areas, northwest Wales. The metamorphic age of high-P/T metamorphic belt in eastern Anglesey was constrained by Ar-Ar isochron age of 560-550 Ma. However, depositional age of the rocks composing accretionary complex wasn't fully constrained due to the limited zircon U-Pb age data and vague microfossil records. Monian Supergroup at Lleyn and Anglesey areas includes three groups; South Stack Group (Gp), New Harbour Gp and Gwna Gp. The Gwna Gp is located at the structural top and includes typical rocks of an ocean plate stratigraphy (OPS), a fundamental unit composing of an accretionary complex. We described detailed geological map and reconstructed the OPSs at some localities with careful attention to layer-parallel thrust. In order to constrain the sedimentary ages of each OPS, we collected sandstones from individual OPSs. We determined U-Pb ages of detrital zircons from the sandstones with LA-ICP-MS at Kyoto University. We adopted the youngest age of the detrital zircons as a constraint of sedimentary age. The results indicate that sediments in Gwna Gp deposited from 623 ± 17 Ma to 535 ± 14 Ma. These are contemporary with the ages of both batholith belt and regional metamorphic belt. In addition, it became evident that structurally upper level is older than lower level. This structurally downward-younging polarity is one of the characteristics of accretionary complex. Therefore, we concluded that the accretionary complex at northwestern Wales was formed between 623 ± 17 Ma and 535 ± 14 Ma, and the subduction-related Pacific-type orogeny had formed a part of British Isles.

Timing and Nature of Events Leading to the Formation of the Albion-Raft River-Grouse Creek (ARG) Metamorphic Core complex, Northern Great Basin, W. U.S.

NASA Astrophysics Data System (ADS)

Miller, E. L.; Konstantinou, A.; Sheu, D.; Strickland, A.; Grove, M.

2016-12-01

Interpretations of the geodynamic significance of metamorphic core complexes in the northern Basin and Range are intimately tied to a combination of P-T data, geochronology and mica thermochronology used to infer episodes of deformation and uplift related to syn-shortening gravitational collapse of the crust in the latest Cretaceous-early Cenozoic. The ARG is no exception and we bring new geologic mapping, microstructural analysis, geochronology and 40Ar/39Ar thermochronology to bear on these questions. The petrogenesis of Eocene-Miocene magmas, the structural fabrics and metamorphism developed in wall rocks of plutons and the history of flanking basins outline a three-part Cenozoic story of this complex: Part 1: Mantle-derived heat input into the crust in the Eocene (42-36 Ma), related to Farallon slab removal, produced volcanism, plutonism, but little regional extension. Part 2: Heat input led to increased crustal melting as surface volcanism ceased. Diapiric rise of granite-cored gneiss domes sheathed by high grade, high strain metamorphic fabrics and mylonites took place over a protracted time, 32-25 Ma, stalling at depths > 10 km. Transitions upward from penetrative stretching fabrics to brittle crust were complex damage zones of multiply deformed and faulted Paleozoic strata overlain by a more intact 7-8 km thick section of Late Paleozoic and Triassic. Extension was localized and no sedimentary basins formed during this time. Part 3: Metamorphic and igneous rocks were brought to near surface conditions during Miocene extension, between 14-8 Ma ago. Structures accommodating E-W extension are high-angle, rotational normal faults that currently bound both sides of the ARG complex with linked sedimentary basins in their hanging wall. New 40Ar/39Ar data show that country rocks near the Oligocene Almo pluton share the pluton's cooling history. Further from the pluton, where pre-Oligocene fabrics are variably preserved, white mica total gas and plateau ages increase up structural section. Ages in the 40-50 Ma range are likely the result of incomplete argon loss from Mesozoic (Jurassic and or Cretaceous?) micas in the Oligocene and/or partial retention zone residence prior to final Miocene uplift rather than the result of proposed latest Cretaceous-early Cenozoic tectonic or deformational events.

Metamorphic P-T conditions across the Chugach Metamorphic Complex (Alaska)—A record of focussed exhumation during transpression

NASA Astrophysics Data System (ADS)

Bruand, Emilie; Gasser, Deta; Stüwe, Kurt

2014-03-01

The Chugach Metamorphic Complex (CMC) is a large high-grade metamorphic complex that developed in the Eocene within the Chugach accretionary complex along the margin of Alaska where subduction is still ongoing. The CMC has a conspicuous asymmetric structure with a migmatitic zone flanked in the north and west by amphibolite facies schists and in the south by a metabasite belt. To the north and south, major, crustal-scale fault zones juxtapose the Chugach terrane against much lower-grade and less-deformed sequences belonging to different terranes. Curiously these crustal-scale structures are known to have largely strike slip motion posing the question as to the nature of the exhumation of the high-grade complex between them. However, P-T conditions which would allow an estimation of the amount of exhumation were lacking for large parts of the complex. This paper presents petrographic descriptions, biotite-garnet thermometry, RSCM thermometry, average P-T calculations and pseudosection modelling from three major across-strike transects covering the complex from west to south-east. Our results reveal that, both temperature and pressure vary substantially across the complex. More specifically, peak metamorphic conditions evolve from 4-7 kbar and ~ 550-650 °C in the northern schist zone to 5-11 kbar and ~ 650-750 °C in the migmatite zone in the south of the complex. The higher pressure estimates in the south of the complex indicate that focussed exhumation must have occurred in this area and was probably initiated by the subduction of a high topographic relief (intra-oceanic arc or ridge subduction) and the accretion of the metabasite belt in the south. Exhumation of the CMC occurred in an overall transpressive strain regime, with strike-slip deformation concentrated along the northern Border Range fault zone and thrusting and exhumation focussed within the southern migmatite zone and splay faults of the Contact fault zone. The T/P ratios in the southern migmatite zone indicate that the thermal perturbation of the migmatites is less than previously inferred. These new results, associated with the structural data and the accretion of a metabasite belt in the south of the complex, seem incompatible with the existing ridge-subduction models.

Origin of retrograde fluid in ultrahigh-pressure metamorphic rocks: Constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the Sulu orogen

NASA Astrophysics Data System (ADS)

Chen, Ren-Xu; Zheng, Yong-Fei; Gong, Bing; Zhao, Zi-Fu; Gao, Tian-Shan; Chen, Bin; Wu, Yuan-Bao

2007-05-01

By taking advantage of having depth profiles between contrasting lithologies from core samples of the Chinese Continental Scientific Drilling (CCSD) project, a combined study was carried out to examine changes in mineral H isotope, total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh-pressure (UHP) eclogite and gneiss in the Sulu orogen, east-central China. The samples of interest were from two continuous core segments from the CCSD main hole at depths of 734.21-737.16 and 929.67-932.86 m, respectively. The results show δD values of -116‰ to - 64‰ for garnet and -104‰ to -82‰ for omphacite, consistent with incorporation of meteoric water into protoliths of UHP metamorphic rocks by high-T alteration. Both equilibrium and disequilibrium H isotope fractionations were observed between garnet and omphacite, suggesting fluid-assisted H isotope exchange at local scales during amphibolite-facies retrogression. While bulk water analysis gave total H 2O concentrations of 522-1584 ppm for garnet and 1170-20745 ppm for omphacite, structural hydroxyl analysis yielded H 2O contents of 80-413 ppm for garnet and 228-412 ppm for omphacite. It appears that significant amounts of molecular H 2O are present in the minerals, pointing to enhanced capacity of water storage in the UHP eclogite minerals. Hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents. Petrographically, the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite-gneiss boundary. Thus, retrograde metamorphism results in mineral reactions and H isotope variation. Because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure, significant amounts of water are expected to be released from the minerals during decompression exhumation. Decompression exsolution of structural hydroxyl from 1 m 3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3.07-3.44 kg water that can form 140-156 kg amphibole during exhumation. Therefore, it is concluded that fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular H 2O in nominally anhydrous minerals. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water. Consequently, both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic UHP rocks.

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz, NW Iran

NASA Astrophysics Data System (ADS)

Moosavi, E.; Rasouli-Jamadi, F.

2018-03-01

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.

The Maritsa strike-slip shear zone between Kostenets and Krichim towns, South Bulgaria — Structural, petrographic and isotope geochronology study

NASA Astrophysics Data System (ADS)

Naydenov, Kalin; Peytcheva, Irena; von Quadt, Albrecht; Sarov, Stoyan; Kolcheva, Krastina; Dimov, Dimo

2013-06-01

The present study describes the characteristics of the Maritsa Shear Zone (MSZ), a major tectonic element in the Balkanides in South Central Bulgaria. Metamorphic rocks of four lithotectonic units — Madan, Chepinska, Asenitsa and Thrace units crop out in the study area. Strike-slip ductile deformation in MSZ affects the Thrace Lithotectonic Unit (TLU) for up to 15 km. The stratigraphy of this unit is divided in two: Parvenets succession and variegated succession. U-Pb zircon dating reveals Late Jurassic protolith age for metagranitoids and metagabbros of the variegated succession. For its metasedimentary part Triassic to Upper Jurassic age is suggested based on the strontium isotope signature of the marbles. The Parvenets succession affiliates to the Variscan metamorphic basement of Europe. The metamorphic evolution of the zone is subdivided into synmetamorphic strike-slip deformations and annealing stages. The ductile shearing occurred in greenschist to lower amphibolite facies between 130 Ma (discordant U-Pb ages) and 82-78 Ma (late-syntectonic granites). This stage is connected with the oblique collision of the Rhodope Late Jurassic arc with the European platform. With the docking of the arc and the triggering of the strike-slip movements, MSZ represents an orogen-scale border between the Rhodope south-vergent thrust complex and the north-vergent deformations in the Srednogorie and Sakar-Strandzha zones. During the Late Cretaceous MSZ is the contact between the Srednogorie magmatic arc (part of the Apuseni-Banat-Timok-Srednogorie Belt) and the Rhodopean metamorphic core complexes. NW-SE dextral faulting characterized the brittle tectonics along the zone. Strike-slip faults of the southern border of the TLU are transferred into reverse faults, along which the TLU overthrusted Oligocene sediments. MSZ is an orogen-scale transpressional shear zone and an important border in the structure of the Balkanides. This multidisciplinary research emphasizes its role as a major tectonic element by presenting new structural, petrographic and isotope geochronology data.

Mayer Kangri metamorphic complexes in Central Qiangtang (Tibet, western China): implications for the Triassic-early Jurassic tectonics associated with the Paleo-Tethys Ocean

NASA Astrophysics Data System (ADS)

Wang, Yixuan; Liang, Xiao; Wang, Genhou; Yuan, Guoli; Bons, Paul D.

2018-03-01

The Mesozoic orogeny in Central Qiangtang Metamorphic Belt, northern Tibet, provides important insights into the geological evolution of the Paleo-Tethys Ocean. However, the Triassic-early Jurassic tectonics, particularly those associated with the continental collisionstage, remains poorly constrained. Here we present results from geological mapping, structural analysis, P-T data, and Ar-Ar geochronology of the Mayer Kangri metamorphic complex. Our data reveal an E-W-trending, 2 km wide dome-like structure associated with four successive tectonic events during the Middle Triassic and Early Jurassic. Field observations indicate that amphibolite and phengite schist complexes in this complex are separated from the overlying lower greenschist mélange by normal faulting with an evident dextral shearing component. Open antiform-like S2 foliation of the footwall phengite schist truncates the approximately north-dipping structures of the overlying mélange. Microtextures and mineral chemistry of amphibole reveal three stages of growth: Geothermobarometric estimates yield temperatures and pressures of 524 °C and 0.88 GPa for pargasite cores, 386 °C and 0.34 GPa for actinolite mantles, and 404 °C and 0.76 GPa for winchite rims. Peak blueschist metamorphism in the phengite schist occurred at 0.7-1.1 GPa and 400 °C. Our Ar-Ar dating of amphibole reveals rim-ward decreasing in age bands, including 242.4-241.2 Ma, ≥202.6-196.8, and 192.9-189.8 Ma. The results provide evidence for four distinct phases of Mesozoic tectonic evolution in Central Qiangtang: (1) northward oceanic subduction beneath North Qiangtang ( 244-220 Ma); (2) syn-collisional slab-break off (223-202 Ma); (3) early collisional extension driven by buoyant extrusion flow from depth ( 202.6-197 Ma); and (4) post-collision contraction and reburial (195.6-188.7 Ma).

Distribution and characteristics of metamorphic belts in the south- eastern Alaska part of the North American Cordillera

USGS Publications Warehouse

Brew, D.A.; Himmelberg, G.R.; Loney, R.A.; Ford, A.B.

1992-01-01

The Cordilleran orogen in south-eastern Alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes, from east to west: the Coast plutonic-metamorphic complex; the Glacier Bay-Chichagof plutonic-metamorphic complex; and the Chugach plutonic-metamorphic complex. Each of these complexes is related to a major subduction event. The metamorphic history of the Coast complex is lengthy and is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Stikine terrane to the east. The metamorphic history of the Glacier Bay-Chichagof complex is relatively simple and is related to the roots of a Late Jurassic to late Early Cretaceous island arc. The metamorphic history of the Chugach is complicated and developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes. -from Authors

Postmylonitic deformation in the Raft River metamorphic core complex, northwestern Utah: Evidence of a rolling hinge

NASA Astrophysics Data System (ADS)

Manning, Andrew H.; Bartley, John M.

1994-06-01

Much of the recent debate over low-angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model. The model predicts tilting of seismogenic high-angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation. This shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex. The predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms. Two end-members have been proposed: subvertical simple shear and flexural failure. Each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction. Subvertical simple shear (SVSS) should generate subvertical faults and kink bands with a shear sense antithetic to the detachment. For an SVSS hinge, the hinge-related strain magnitude should depend only on initial fault dip; rolling hinge structures should shorten the mylonitic foliation by >13% for an initial fault dip of >30°. In flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses. Resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation. Extensional sets could predominate as a result of superposition of far-field and flexural stresses. Strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam. Postmylonitic structures were examined in the footwall of the Raft River metamorphic core complex in northwestern Utah to test these predictions. Observed structures strike perpendicular to the regional extension direction and include joints, normal faults, tension-gash arrays, and both extensional and contractional kink bands. Aside from the subvertical joints, the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets. Range-wide, the extensional structures accomplish about 4% elongation of the mylonitic foliation. Contractional structures dip steeply, mainly record shear antithetic to the detachment, and accomplish <1% contraction of the foliation. These observations are consistent with the presence of a rolling hinge in the Raft River Mountains, but a rolling hinge that reoriented a high-angle normal fault by SVSS is excluded. The pattern and magnitudes of strain favor hinge-related deformation mainly by flexural failure with a subordinate component of SVSS.

KRISTINA: Kinematic rib-based structural system for innovative adaptive trailing edge

NASA Astrophysics Data System (ADS)

Pecora, R.; Amoroso, F.; Magnifico, M.; Dimino, I.; Concilio, A.

2016-04-01

Nature teaches that the flight of the birds succeeds perfectly since they are able to change the shape of their wings in a continuous manner. The careful observation of this phenomenon has re-introduced in the recent research topics the study of "metamorphic" wing structures; these innovative architectures allow for the controlled wing shape adaptation to different flight conditions with the ultimate goal of getting desirable improvements such as the increase of aerodynamic efficiency or load control effectiveness. In this framework, the European research project SARISTU aimed at combining morphing and smart ideas to the leading edge, the trailing edge and the winglet of a large commercial airplane (EASA CS25 category) while assessing integrated technologies validation through high-speed wind tunnel test on a true scale outer wing segment. The design process of the adaptive trailing edge (ATED) addressed by SARISTU is here outlined, from the conceptual definition of the camber-morphing architecture up to the assessment of the device executive layout. Rational design criteria were implemented in order to preliminarily define ATED structural layout and the general configuration of the embedded mechanisms enabling morphing under the action of aerodynamic loads. Advanced FE analyses were then carried out and the robustness of adopted structural arrangements was proven in compliance with applicable airworthiness requirements.

Extensional unroofing of the metamorphic core of the southern Appalachian orogen prior to the breakup of Pangea: Insights from 40Ar/39Ar thermochronology

NASA Astrophysics Data System (ADS)

Ma, C.; Foster, D. A.; Hames, W. E.; Mueller, P. A.

2017-12-01

Orogenic collapse commonly occurs following the collisional phase of an orogeny and often leads to exhumation of deep crustal metamorphic rocks. The Alleghanian orogeny in the southern Appalachian orogen (SAO) occurred during final assembly of Pangea. 40Ar/39Ar data of hornblende, muscovite, and biotite from Alleghanian granitic plutons in Georgia, Alabama, and Florida of the SAO give cooling ages that progressively young toward the south-southeast prior to ca. 280 Ma and young locally toward the north-northwest after ca. 280 Ma. These cooling-age gradients, along with geometry of the Suwannee suture zone and timing/structures of the South Georgia basin, suggest that metamorphic rocks north of the Suwannee suture in the study area formed the lower plate of a metamorphic core complex. The faults of the Suwannee suture zone were reactivated to form a master extensional detachment fault with the Suwannee terrane comprising the upper plate. Thermochronologic data show that rapid extension of the metamorphic core complex footwall started at ca. 300-295 Ma and the extension continued to at least ca. 240 Ma. The maximum average extension rate is estimated to be 10.3 km/m.y. during ca. 300-280 Ma along the master detachment fault and 2.4 km/m.y. during ca. 280-240 Ma along a secondary detachment fault, reflecting differential extension over time. Main cooling rates of 10‒85˚C/m.y. and exhumation rates of 0.3‒2.8 km/m.y. are calculated for the Alleghanian granitic plutons studied. This work shows that, in the southernmost Appalachians, orogenic collapse resulted in metamorphic core complex-style extension between about 300 and 240 Ma. The horst-and-graben systems of the South Georgia basin formed within the upper plate in this tectonic setting. Metamorphic core complex-style extension, therefore, played a critical role in initial rifting that led to the eventual breakup of Pangea and formation of the Atlantic Ocean and the Gulf of Mexico.

Pyrite deformation and connections to gold mobility: Insight from micro-structural analysis and trace element mapping

NASA Astrophysics Data System (ADS)

Dubosq, R.; Lawley, C. J. M.; Rogowitz, A.; Schneider, D. A.; Jackson, S.

2018-06-01

The metamorphic transition of pyrite to pyrrhotite results in the liberation of lattice-bound and nano-particulate metals initially hosted within early sulphide minerals. This process forms the basis for the metamorphic-driven Au-upgrading model applied to many orogenic Au deposits, however the role of syn-metamorphic pyrite deformation in controlling the retention and release of Au and related pathfinder elements is poorly understood. The lower amphibolite facies metamorphic mineral assemblage (Act-Bt-Pl-Ep-Alm ± Cal ± Qz ± Ilm; 550 °C) of Canada's giant Detour Lake deposit falls within the range of pressure-temperature conditions (450 °C) for crystal plastic deformation of pyrite. We have applied a complementary approach of electron backscatter diffraction (EBSD) mapping and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) 2D element mapping on pyrite from the Detour Lake deposit. Chemical element maps document an early generation of Au-rich sieve textured pyrite domains and a later stage of syn-metamorphic oscillatory-zoned Au-poor pyrite. Both pyrite types are cut by Au-rich fractures as a consequence of remobilization of Au with trace element enrichment of first-row transition elements, post-transition metals, chalcogens and metalloids during a late brittle deformation stage. However, similar enrichment in trace elements and Au can be observed along low-angle grain boundaries within otherwise Au-poor pyrite, indicating that heterogeneous microstructural misorientation patterns and higher strain domains are also relatively Au-rich. We therefore propose that the close spatial relationship between pyrite and Au at the microscale, features typical of orogenic Au deposits, reflects the entrapment of Au within deformation-induced microstructures in pyrite rather than the release of Au during the metamorphic transition from pyrite to pyrrhotite. Moreover, mass balance calculations at the deposit scale suggest that only a small percentage of Au could have been sourced from pyrite and instead point to the role of substructures in pyrite as depositional traps for Au during syn-metamorphic deformation- and fluid-assisted diffusion Au-upgrading.

EBSD characterization of pre-Cambrian deformations in conglomerate pebbles (Sierra de la Demanda, Northern Spain)

NASA Astrophysics Data System (ADS)

Puelles, Pablo; Ábalos, Benito; Fernández-Armas, Sergio

2010-05-01

Pre-Cambrian and unconformable earliest Cambrian rocks from the Sierra de la Demanda (N Spain) exhibit field and microstructural relationships that attest to orogenic events recorded by concealed basement rocks. Neoproterozoic foliated slates ("Anguiano Schists") crop out under up to 300 m thick, unfoliated quartz-rich conglomerates ("Anguiano Conglomerates") and quartzites which are stratigraphically ca. 600 m below the oldest, paleontologically dated, pre-trilobitic Cambrian layers (likely older than 520 Ma). The Anguiano Conglomerates contain mm to cm grainsized well-rounded pebbles of various types including monocrystalline quartz, detrital zircon and tourmaline-bearing sandstones, black cherts and metamorphic poly-crystalline quartz aggregates. The undeformed matrix is made of much smaller (diagenetically overgrown) monocrystaline quartz grains and minor amounts of accesory zircon, tourmaline and mica. Black chert pebbles exhibit microstructural evidence of brittle deformation (microfaults and thin veins of syntaxial fibrous quartz). These and the fine-grained sandstone pebbles can also exhibit ductile deformations (microfolds with thickened hinges and axial planar continuous foliations), too. Polycrystalline quartz pebbles exhibit a variety of microstructures that resulted from syn-metamorphic ductile deformations. These are recognisable under the petrographic microscope and include continuous foliations, quartz shape fabrics, various types of subgrain or recrystallized new grain microtextures, and lattice preferred orientations (LPOs). Conventional characterization of quartz fabrics (after oriented structural sections) is challenged in conglomerate pebble thin sections by the difficulty of unraveling in them the complete structural reference framework provided by foliation (whose trace can be unraveled) and lineation orientation (which cannot be directly identified). Quartz in various metamorphic polycrystalline pebbles was studied with the Electron Back-Scatter Diffraction (EBSD) technique. The identification of quartz c-axis point maxima or girdles and their geometrical relationships with respect to -axis arrangements and pebble foliation traces enabled us to identify the operation of basal and prism- and occasionally prism-[c] intracrystalline slip systems. This points to upper-greenschists and amphibolite facies syn-metamorphic deformations. By contrast, black chert and sandstone pebbles and matrix quartz aggregates lack any LPO. The source area of the conglomerates was likely a pre-Cambrian basement that contained penetratively deformed low- to medium-grade metamorphic rocks. Radiometric dating of this metamorphism has not been accomplished so far though it is known that inherited Precambrian sources in the Iberian Peninsula relate notably to Neoproterozoic (Pan-African and Cadomian) orogens, and to a lesser extent to Paleoproterozoic (1.8-2.1 Ga) or Neoarchean (2.4-2.8 Ga) ones. Neoproterozoic (Cadomian) metamorphism of this grade has only been recognized in SW Iberia. If the fabrics here studied were Cadomian, they might be related to the arc-related igneous suites that have been detected or inferred in other realms of the northern Iberian Massif.

Eocene to post-Miocene kinematic evolution of the central Cyclades (Greece)

NASA Astrophysics Data System (ADS)

Draganits, E.; Huet, B.; Grasemann, B.; Schneider, D.; Ertl, A.

2012-04-01

Due to the extraordinary geotectonic location of the Aegean above an active subduction zone and an exceptional high seismicity, this area and especially the Cyclades have been in the focus of structural investigations for several decades. The present deformation is the result of ongoing plate tectonic movements in this area since at least the Miocene. The ductile structures of the Miocene extension and related metamorphic core type deformation are quite well studied and understood. Equally well investigated are the active tectonic deformation and associated brittle structures through several decades of seismic records. However, due to the difficulties of dating brittle faults, the kinematic evolution from the early to middle Miocene ductile structures, to later Miocene brittle-ductile and brittle faults is much less understood. For these reasons detailed structural fieldwork, combined with Ar-Ar geochronology and P-T studies, have been carried out on the uninhabited island of Despotiko, SW of Antiparos, which is situated virtually in the center of the Cycladic islands. This island has been selected because the existence of metamorphic rocks penetrated by Messinian rhyolite pipes and Pleistocene eolianites provide exceptional age constraints for Eocene to post-Miocene deformation structures. Despotiko is part of lower structural levels of the polymetamorphic Blueschist Unit of the Attic-Cycladic Metamorphic Belt and correlated lithologically with the Parikia gneisses and Marathi unit of Paros. Foliation is shallowly dipping towards the SSW. The main lithologies of the island, from the footwall to the hanging wall, consist of dark to pale grey, strongly foliated, mylonitic granite gneiss with abundant pegmatite dikes. The gneiss is overlain by prominent white, strongly foliated, mylonitic gneiss. Above are medium-grained, white calcite marble followed by greenish-white, mylonitic gneiss and an alternation of mica schist, greenschist, thin marble layers and some small serpentinite lenses. The structurally highest levels, in the south and southwest of the island, comprise several tens of meters of dolomite marble. This metamorphic succession has been cut by six Messinian rhyolitic volcanic vents and all crystalline rocks have been covered by late Pleistocene eolianites. The kinematic evolution of the investigation area can be divided based on the deformation style and age. (1) The ductile deformation results in NE-SW trending stretching lineation and shear senses both top-to NE and top-to SW. Ar-Ar white mica cooling ages indicate an early Miocene age for this ductile deformation. (2) The brittle/ductile structures, which gradually advance from the previous ductile deformation, start with small but pervasive flanking folds, followed by larger shear bands and finally faults with tourmaline slickenlines. The shear sense is consistently top-to SW with middle to late Miocene age constrained by Ar-Ar white mica cooling ages and zircon fission-track data from Paros. (3a) Large, subvertical, sinistral strike-slip faults cross-cut the metamorphic rocks and show up to hundreds of meters displacement. Late Miocene age is constrained by apatite fission-track data from Paros and the observation that these faults are sealed by Messinian rhyolites. (3b) The Messinian volcanic rocks are almost exclusively deformed by E-W striking conjugate brittle normal faults, which started already during the formation of the volcanic rocks. No unequivocal tectonic deformation structures have been observed in the Pleistocene eolianites.

Plate interface rheological switches during subduction infancy: Control on slab penetration and metamorphic sole formation

NASA Astrophysics Data System (ADS)

Agard, P.; Yamato, P.; Soret, M.; Prigent, C.; Guillot, S.; Plunder, A.; Dubacq, B.; Chauvet, A.; Monié, P.

2016-10-01

Subduction infancy corresponds to the first few million years following subduction initiation, when slabs start their descent into the mantle. It coincides with the transient (yet systematic) transfer of material from the top of the slab to the upper plate, as witnessed by metamorphic soles welded beneath obducted ophiolites. Combining structure-lithology-pressure-temperature-time data from metamorphic soles with flow laws derived from experimental rock mechanics, this study highlights two main successive rheological switches across the subduction interface (mantle wedge vs. basalts, then mantle wedge vs. sediments; at ∼800 °C and ∼600 °C, respectively), during which interplate mechanical coupling is maximized by the existence of transiently similar rheologies across the plate contact. We propose that these rheological switches hinder slab penetration and are responsible for slicing the top of the slab and welding crustal pieces (high- then low-temperature metamorphic soles) to the base of the mantle wedge during subduction infancy. This mechanism has implications for the rheological properties of the crust and mantle (and for transient episodes of accretion/exhumation of HP-LT rocks in mature subduction systems) and highlights the role of fluids in enabling subduction to overcome the early resistance to slab penetration.

The Hardwood Gneiss: Evidence for high P-T Archean metamorphism in the southern province of the Lake Superior region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peterson, J.W.; Geiger, C.A.

1990-03-01

The Hardwood Gneiss is an areally small unit of Precambrian granulite-grade rocks exposed in the Archean gneiss terrane of the southern Lake Superior region. The rocks are located in the southwestern portion of the Upper Peninsula of Michigan and consist of a structurally conformable package of quartzitic, metapelitic, amphibolitic, and metabasic units. Three texturally distinct garnet types are present in the metabasites and are interpreted to represent two metamorphic events. Geothermobarometry indicates conditions of {approximately}8.2-11.6 kbar and {approximately}770C for M1, and conditions of {approximately}6.0-10.1 kbar and {approximately}610-740C for M2. It is proposed that M1 was Archean and contemporaneous with amore » high-grade metamorphic event recorded in the Minnesota River Valley. The M2 event was probably Early Proterozoic and pre-Penokean, with metamorphic conditions more intense than those generally ascribed to the Penokean Orogeny in Michigan, but similar to the conditions reported for the Kapuskasing zone of Ontario. The high paleopressures and temperatures of the M1 event make the Hardwood Gneiss distinct from any rocks previously described in the southern Lake Superior region, and suggest intense tectonic activity during the Archean.« less

Myofiber turnover is used to retrofit frog jaw muscles during metamorphosis.

PubMed

Alley, K E

1989-01-01

Metamorphic reorganization of the head in anuran amphibians entails abrupt restructuring of the jaw complex as larval feeding structures are transformed into their adult configurations. In this morphometric study, light microscopy wa used to analyze the larval maturation and metamorphic transfiguration of the adductor jaw muscles in the leopard frog (Rana pipiens). Larval jaw muscles, first established during embryogenesis, continue to grow by fiber addition until prometamorphosis, stage XII. Thereafter, fiber number remains stable but additional muscle growth continues by hypertrophy of the individual fibers until metamorphic climax. During metamorphic stages XIX-XXIII, a complete involution of all larval myofibers occurs. Simultaneously, within the same muscle beds, a second wave of myogenesis produces myoblasts which are the precursors of adult jaw myofibers. New muscle fibers continue to be added to these muscles well after the completion of metamorphosis; however, the total duration of the postmetamorphic myogenic period has not been defined. These observations provide clear evidence that the entir population of primary myofibers used in larval oral activity disappears from the adductor muscle beds and is replaced by a second wave of myogenesis commencing during climax. These findings indicate that the adductor jaw muscles are prepared for adult feeding by a complicated cellular process that retrofits existing muscle beds with a completely new complement of myofibers.

Evaluating the importance of metamorphism in the foundering of continental crust.

PubMed

Chapman, Timothy; Clarke, Geoffrey L; Piazolo, Sandra; Daczko, Nathan R

2017-10-12

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet-clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

Numerical model of the transition from continental rifting to oceanization: the case study of the Ligure-Piemontese ocean.

NASA Astrophysics Data System (ADS)

Roda, M.; Marotta, A. M.; Conte, K.; Spalla, M. I.

2015-12-01

The transition from continental rifting to oceanization has been investigated by mean of a 2D thermo-mechanical numerical model in which the formation of oceanic crust by mantle serpentinization, due to the hydration of the uprising peridotite, as been implemented. Model predictions have been compared with natural data related to the Permian-Triassic thinning affecting the continental lithosphere of the Alpine domain, in order to identify which portions of the present Alpine-Apennine system, preserving the imprints of Permian-Triassic high temperature (HT) metamorphism, is compatible, in terms of lithostratigraphy and tectono-metamorphic evolution, with a lithospheric extension preceding the opening of the Ligure-Piemontese oceanic basin. At this purpose age, petrological and structural data from the Alpine and Apennine ophiolite complexes are compared with model predictions from the oceanization stage. Our comparative analysis supports the thesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start on a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture. The HT Permian-Triassic metamorphic re-equilibration overprints an inherited tectonic and metamorphic setting consequent to the Variscan subduction and collision, making the Alps a key case history to explore mechanisms responsible for the re-activation of orogenic scars.

Partial eclogitization of the Ambolten gabbro-norite, north-east Greenland Caledonides

USGS Publications Warehouse

Gilotti, J.A.; Elvevold, S.

1998-01-01

Partially eclogitized igneous bodies composed of gabbro, leucogabbro, anorthosite and cross-cutting diabase dikes are well represented in the North-East Greenland Eclogite Province. A 200 x 100 meter intrusive body on Ambolten Island (78?? 20' N, 19?? 15' W) records a prograde transition from gabbro-norite to eclogite facies coronitic metagabro-norite surrounded by hydrated margins of undeformed to strongly foliated amphibolite. Igneous plagioclase + olivine + enstatite + augite + oxides convert to eclogite facies assemblages consisting of garnet, omphacite, diopside, enstatite, kyanite, zoisite, rutile and pargasitic amphibole through several coronitic reactions. Relict cumulus plagioclase laths are replaced by an outer corona of garnet, an inner corona of omphacite and an internal region of sodic plagioclase, garnet, kyanite, omphacite and zoisite. Olivine and intercumulus pyroxene are partly replaced by metamorphic pyroxenes and amphibole. The corona structures, zoning patterns, diversity of mineral compositions in a single thin section, and preservation of metastable asemblages are characteristic of diffusion-controlled metamorphism. The most extreme disequilibrium is found in static amphibolites, where igneous pyroxenes, plagioclase domains with eclogite facies, assemblages, and matrix amphibole coexist. Complete eclogitization was not attained at Ambolten due to a lack of fluids needed to drive diffusion during prograde and retrograde metamorphism. The P-T conditions of the high-pressure metamorphism are estimated at ??? 750??C and > 18 kbar. Well-equilibrated, foliated amphibolites from the margin of the gabbro-norite supports our contention that the entire North-East Greenland Eclogite Province experienced Caledonian high-pressure metamorphism, even though no eclogite facies assemblages have been found in the quartzofeldspathic host gneisses to date.

Nature and probable age of metamorphism in northern New Mexico

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grambling, J.A.; Daniel, C.D.; Dallmeyer, R.D.

1993-02-01

Metamorphic conditions near the Al[sub 2]SiO[sub 5] triple point are unusually common in northern New Mexico. This observation is supported by mineralogy (Ky + And + Sil, Cld + Sil, Sil + Pg + Qz) and Grt-Bt-Pl-Ms thermobarometry (4--5 kb and 500--550 C). Isograds cut across tight folds (overturned to the north) in the Pecos, Rio Mora, Truchas and Picuris areas. Some deformation also accompanied or preceded metamorphism in the Rincon and Cimarron ranges. P-T paths derived from zoning in Grt and Pl, in Mn-andalusite, and the textural transition Ky to Sil to And reflect up to 2 kb ofmore » decompression, at constant temperature in the more southerly ranges but during cooling toward the north. These 500--550 C rocks are in direct contact with gneisses in the Rincon and Cimarron Ranges. Metaplutonic gneisses record hornblende pressures of 6--8 kb. Metasedimentary gneisses are migmatitic. Assemblages include Sil + Kfs, Hc + Qz and Alm + Bt + Sil, whereas Grt-Sil-Pl-Bt yields 6.5--7 kb and 700--725 C. Pressures increase northward from the Cimarrron Mountains. The gneisses display retrograde P-T paths with 2.5--3 kb of decompression and cooling through the Al[sub 2]SiO[sub 5] triple point. Geometric relationships between gneisses and 500--550 C rocks are best constrained in the Cimarron Mountains, where a folded but initially low-angle contact separates the two metamorphic grades. Gneisses are structurally beneath this contact. Other regions may display a similar geometry. The structurally-highest gneisses are locally mylonitic, suggesting that contacts between gneisses and 500--550 C rocks are ductile shear zones. Monazite U-Pb ages from gneisses of the Cimarron Range are 1420-1425 Ma, whereas hornblende argon ages are 1,395--1,397 Ma.« less

Chronology of paleozoic metamorphism and deformation in the Blue Ridge thrust complex, North Carolina and Tennessee

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goldberg, S.A.; Dallmeyer, R.D.

1997-05-01

The Blue Ridge province in northwestern North Carolina and northeastern Tennessee records a multiphase collisional and accretionary history from the Mesoproterozoic through the Paleozoic. To constrain the tectonothermal evolution in this region, radiometric ages have been determined for 23 regionally metamorphosed amphibolites, granitic gneisses, and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone. The garnet ages from the Pumpkin Patch a thrust sheet (458, 455, and 451 Ma) are similar to those from the structurally overlying Spruce Pine thrust sheet (460, 456, 455, and 450 Ma). Both thrust sheets exhibitmore » similar upper amphibolite-facies conditions. Because of the high closure temperature for garnet, the garnet ages are interpreted to date growth at or near the peak of Taconic metamorphism. Devonian metamorphic ages are recognized in the Spruce Pine thrust sheet, where Sm-Nd and Rb-Sr garnet ages of 386 and 393 Ma and mineral isochron ages of 397 {+-} 14 and 375 {+-} 27 Ma are preserved. Hornblendes record similar {sup 40}Ar/{sup 39}Ar, Sm-Nd, and Rb-Sr ages of 398 to 379 Ma. Devonian {sup 40}Ar/{sup 39}Ar hornblende ages are also recorded in the structurally lower Pumpkin Patch thrust sheet. The Devonian mineral ages are interpreted to date a discrete tectonothermal event, as opposed to uplift and slow cooling from an Ordovician metamorphic event. The Mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the Alleghanian orogeny. The composite thrust stack of the Blue Ridge complex was subsequently thrust northwestward along the Linville Falls fault during middle Alleghanian orogeny (about 300 Ma).« less

Pressure Variations in Metamorphic Rocks: Implications for the Interpretation of Petrographic Observations

NASA Astrophysics Data System (ADS)

Tajčmanová, Lucie

2014-05-01

Metamorphic petrologists and structural geologists, using direct measurements, bring the only direct observational constrains for validating geodynamic models. Therefore, petrological and structural geological observations are essential for the quality and reproducibility of geodynamic reconstructions and models. One of the important assumptions for geodynamic reconstructions arises from the pressure and temperature estimates in the petrology analysis. Pressure is commonly converted to depth through the equation for lithostatic pressure and so the original position of the rock sample within the Earth's interior can be constrained. The current assumption that the studied sample corresponds to uniform pressure may not be correct, and if so, it has serious implications. Increasing evidence from analytical data shows that pressure is not constant even on a grain scale, posing new challenges because, if ignored, it leads to an incorrect use of petrology data in constraining geodynamic models. Well known examples of the preservation of coesite and diamond in a host mineral like garnet show that high pressure inclusions are preserved during decompression. Tajčmanová et al. (2014) has shown that grain-scale pressure variations can develop and that these pressure variations allow compositional zoning in minerals preserved over geological time scales. A new unconventional barometric method based on equilibrium under pressure variations has been developed . Such pressure variations are also connected with differences in fluid pressure in open systems and can be thus observed at all scales. Tajčmanová L., Podladchikov Y., Powell R., Moulas E., Vrijmoed J. and Connolly J. (2014). Grain scale pressure variations and chemical equilibrium in high-grade metamorphic rocks.Journal of Metamorphic Geology, doi:10.1111/jmg.12066 This work was supported by ERC starting grant 335577 to Lucie Tajcmanova

The Cooling History and Structure of the Ordinary Chondrite Parent Bodies

NASA Technical Reports Server (NTRS)

Benoit, P. H.; Sears, D. W. G.

1996-01-01

Most major meteorite classes exhibit significant ranges of metamorphism. The effects of metamorphism have been extensively characterized, but the heat source(s) and the metamorphic environment are unknown. Proposed beat sources include Al-26, Fe-60, electromagnetic induction, and impact. It is typically assumed that metamorphism occurred in parent bodies of some sort, but it uncertain whether these bodies were highly structured ("onion skins") or were chaotic mixes of material ("rubble piles"). The lack of simple trends of metallographic cooling rates with petrologic type has been considered supportive of both concepts. In this study, we use induced thermoluminescence (TL) as an indicator of thermal history. The TL of ordinary chondrites is produced by sodic feldspar, and the induced TL peak temperature is related to its crystallographic order/disorder. Ordered feldspar has TL peak temperatures of approx. 120 C, and disordered feldspar has TL peak temperatures of approx. 220 C. While ordered feldspar can be easily disordered in the laboratory by heating above 650 C and is easily quenched in the disordered form, producing ordered feldspar requires cooling at geologic cooling rates. We have measured the induced TL properties of 101 equilibrated ordinary chondrites, including 49 H, 29 L, and 23 LL chondrites. For the H chondrites there is an apparent trend of decreasing induced TL peak temperature with increasing petrologic type. H4 chondrites exhibit a tight range of TL peak temperatures, 190 C - 200 C, while H6 chondrites exhibit TL peak temperatures between 180 C and 190 C. H5 chondrites cover the range between H4 and H6, and also extend up to 210 C. Similar results are obtained for LL chondfiles and most L6 chondrites have lower induced TL peak temperatures than L5 chondrites.

Petrologic evidence for collisional heating of chondritic asteroids

NASA Technical Reports Server (NTRS)

Rubin, Alan E.

1995-01-01

The identification of the mechanism(s) responsible for heating asteroids is among the major problems in planetary science. Because of difficulties with models of electromagnetic induction and the decay of short-lived radionuclides, it is worthwhile to evaluate the evidence for collisional heating. New evidence for localized impact heating comes from the high proportion of relict type-6 material among impact-melt-bearing ordinary chondrites (OC). This relict material was probably metamorphosed by residual heat within large craters. Olivine aggregates composed of faceted crystals with 120 deg triple junctions occur within the melted regions of the Chico and Rose City OC melt rocks; the olivine aggregates formed from shocked, mosaicized olivine grains that underwent contact metamorphism. Large-scale collisional heating is supoorted by the correlation in OC between petrologic type and shock stage; no other heating mechanism can readily account for this correlation. The occurrence of impact-melt-rock clasts in OC that have been metamorphosed along with their whole rocks indicates that some impact events preceded or accompanied thermal metamorphism. Such impacts events, occurring during or shortly after accretion, are probably responsible for substantially melting approximately 0.5% of OC. These events must have heated a larger percentage of OC to subsolidus temperatures sufficient to have caused significant metamorphism. If collisional heating is viable, then OC parent asteroids must have been large; large OC asteroids in the main belt may include those of the S(IV) spectral subtype. Collisional heating is inconsistent with layered ('onion-shell') structures in OC asteroids (wherein the degree of metamorphism increases with depth), but the evidence for such structures is weak. It seems likely that collisional heating played an important role in metamorphosing chondritic asteroids.

Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada

NASA Astrophysics Data System (ADS)

Walton, E. L.; Sharp, T. G.; Hu, J.; Tschauner, O.

2018-01-01

Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm-3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.

Dynamics of Metamorphic Core Complexes Inferred From Modeling and Metamorphic Petrology

NASA Astrophysics Data System (ADS)

Whitney, D. L.; Rey, P.; Teyssier, C.

2008-12-01

Orogenic collapse involves extension and thinning of thick, hot, and in some cases partially molten, crust, leading to the formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. 2D numerical modeling predicts that the geometry and P-T-t history of MCC varies as a function of the presence/absence of a partially molten layer in the deep crust; the nature of heterogeneities that localize the MCC (e.g. normal fault in upper crust vs. point-like anomaly in the deep crust); and extensional strain rate. The presence of melt in particular has a significant effect on the thermal and structural history of MCC because the presence of partially molten crust or magma bodies at depth enhances upward advection of material and heat. At high extension rate (cm/year in the region of the MCC), partially molten crust crystallizes as migmatite and cools along a high geothermal gradient (35-65 C/km); material remains partially molten during ascent, forming a migmatite dome when it crystallizes at shallower crustal levels (e.g. cordierite/sillimanite stability field). At low strain rate (mm/yr in the MCC region), the partially molten crust crystallizes at high pressure (e.g. kyanite zone); this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20-35 C/km) during ascent. MCC that develop during extension of partially molten crust may therefore record distinct crystallization versus exhumation histories as a function of extensional strain rate. The mineral assemblages, metamorphic reaction histories, and structures of migmatite-cored (Mc) MCC can therefore be used to interpret the dynamics of MCC formation, e.g. "fast" McMCC in the northern N American Cordillera and Aegean regions.

­Oligo-Miocene Monazite Ages in the Lesser Himalaya Sequence, Arunachal Pradesh, India; Geological Content of Age Variations

NASA Astrophysics Data System (ADS)

Clarke, G. L.; Bhowmik, S. K.; Ireland, T. R.; Aitchison, J. C.; Chapman, S. L.; Kent, L.

2016-12-01

A telescoped and inverted greenschist-upper amphibolite facies sequence in the in the Siyom Valley of eastern Arunachal Pradesh is tectonically overlain by an upright (grade decreasing upward) granulite to lower amphibolite facies sequence. Such grade relationships would normally attribute the boundary to a Main Central Thrust (MCT) structure, and predict a change from underlying Lesser Himalaya Sequence (LHS) to Greater Himalaya Sequence rocks across the boundary. However, all pelitic and psammitic samples have similar detrital zircon age spectra, involving c. 2500, 1750-1500, 1200 and 1000 Ma Gondwanan populations correlated with the LHS. Isograds are broadly parallel to a penetrative NW-dipping S2 foliation, developed contemporaneously with the inversion. Garnet growth in garnet, staurolite and kyanite zone schists beneath the thrust commenced at P>8 kbar and T≈550°C, before syn- to post-S2 heating of staurolite and kyanite zone rocks to T≈640°C at P≈8.5 kbar, most probably at c. 18.5 Ma. Kyanite-rutile-garnet migmatite immediately above the thrust records peak conditions of P≈10 kbar and T≈750°C and c. 21.5 Ma monazite ages. Complexity in c. 21-1000 Ma monazite ages in overlying amphibolite facies schists reflects the patchy recrystallization of detrital grains, intra-grain complexity being dependent on whole rock composition, metamorphic grade and evolition. Slip on a SE-propagating thrust was likely contemporaneous with early Miocene metamorphism, based on the distribution of structure, metamorphic textures, and overlap of age relationships. It is inferred to have initially controlled the uplift of granulite to mid-crustal levels between 22 and 19 Ma, thermal relaxation within a disrupted LHS metamorphic profile inducing a post-S2 thermal peak in lower grade footwall rocks.

Metamorphosis increases biotransformation of polychlorinated biphenyls: a comparative study of polychlorinated biphenyl metabolism in green frogs (Rana clamitans) and leopard frogs (Rana pipiens) at various life stages.

PubMed

Leney, Jocelyn L; Drouillard, Ken G; Haffner, G Douglas

2006-11-01

Polychlorinated biphenyl (PCB) elimination rates were used to determine whether green frogs (Rana clamitans) and leopard frogs (Rana pipiens) at three life stages (tadpole, metamorph, and adult) were capable of metabolic biotransformation of PCBs. Polychlorinated biphenyls were grouped according to their structural properties to determine whether congeners with specific structures were eliminated more rapidly than others, which would suggest the occurrence of metabolic biotransformation. For both species and at all life stages studied, PCBs with adjacent unsubstituted meta-para sites (group-2 PCBs) were eliminated significantly faster than PCBs with adjacent unsubstituted ortho-meta sites (group-3 PCBs). Because these groups of PCBs span similar hydrophobicity ranges, it was concluded that green and leopard frogs can selectively metabolize group-2 PCBs. Metabolic activity was more pronounced in the metamorph stage than in any other life stage. In green frog metamorphs, the average elimination rate of the group-2 PCBs was 4.9-fold greater than that of the group-3 PCBs. By contrast, in green frog tadpoles, group-2 PCBs were eliminated 1.9-fold faster than group-3 PCBs, and in green frog adults, group-2 PCBs were eliminated 1.5-fold faster than group-3 PDBs.

The post collisional metamorphic evolution from Ultra High Temperature to Amphibolite facies metamorphism in the Odesan area during the Triassic collision between the North and South China cratons.

NASA Astrophysics Data System (ADS)

Lee, Byung Choon; Oh, Chang Whan; Kim, Tae Sung; Yi, Kee Wook

2015-04-01

The Odaesan Gneiss Complex (OGC) is the eastern end of the Hongseong-Odesan collision belt in Korean Peninsula which is the extension of the Dabie-Sulu collision belt between the North and South China blocks. The OGC mainly consists of banded and migmatitic gneiss with porphyritic granitoid and amphibolite. The banded gneiss can be subdivided into garnet-biotite and garnet-orthopyroxene banded gneisses. The highest metamorphic P/T conditions of the migmatitic and garnet-biotite banded gneiss were 760-820°C/6.3-7.2kbar and 810-840°C/7.2-7.8kbar respectively. On the other hand, the garnet-orthopyroxene banded gneiss records 940-950°C/10.5-10.7kbar that is corresponded to UHT metamorphic condition. These data indicate that the peak UHT metamorphic condition of the study area was preserved only within the garnet-orthopyroxene banded gneiss because its lower water content than other gneisses and UHT metamorphic mineral assemblage was completely replaced by the granulite facies metamorphism in other gneisses due to their higher water content than the garnet-orthopyroxene banded gneiss. Finally all gneisses experienced amphibolite facies retrograde metamorphism which is observed locally within rocks, such as garnet rim and surrounding area. The peak UHT metamorphism is estimated to occur at ca. 250-230 Ma using SHRIMP zircon U-Pb age dating and was caused by the heat supplied from asthenospheric mantle through the opening formed by slab break-off during early post collision stage. The calculated metamorphic conditions represent that geothermal gradient of the study area during the post collision stage was 86°C/kbar indicating the regional low-P/T metamorphic event. Besides the Triassic metamorphic age, two Paleoproterozoic metamorphic ages of ca. 1930 and 1886 Ma are also recognized by the SHRIMP age dating from the banded gneisses and Paleoproterozoic emplacement age of ca. 1847 Ma is identified from the porphyritic granitoid which formed in the within plate tectonic setting. These ages are well matched with 1880Ma-1885Ma regional post-collision igneous and metamorphic activities in other areas of the GM indicating that the OGC had undergone Paleoproterozoic metamorphic and igneous activities before the Triassic metamorphism. However it is difficult to confirm the Paleoproterozoic activities due to the strong Triassic metamorphism.

Early Paleozoic tectonic reactivation of the Shaoxing-Jiangshan fault zone: Structural and geochronological constraints from the Chencai domain, South China

NASA Astrophysics Data System (ADS)

Sun, Hanshen; Li, Jianhua; Zhang, Yueqiao; Dong, Shuwen; Xin, Yujia; Yu, Yingqi

2018-05-01

The Shaoxing-Jiangshan fault zone (SJFZ), as a fundamental Neoproterozoic block boundary that separates the Yangtze Block from the Cathaysia Block, is the key to understanding the evolution of South China from Neoproterozoic block amalgamation to early Paleozoic crustal reworking. New structural observations coupled with geochronological ages from the Chencai domain indicate that intense ductile deformation and metamorphism along the SJFZ occurred at ∼460-420 Ma, in response to the early Paleozoic orogeny in South China. To the east of the SJFZ, the deformation involves widespread generations of NE-striking foliation, intrafolial folds, and local development of sinistral-oblique shear zones. The shearing deformation occurred under amphibolite facies conditions at temperatures of >550 °C (locally even >650 °C). To the west of the SJFZ, the deformation corresponds to sinistral-oblique shearing along NE-striking, steep-dipping zones under greenschist facies conditions at temperatures of 400-500 °C. These deformation styles, as typical mid-crustal expressions of continental reworking, reflect tectonic reactivation of the pre-existing, deeply rooted Neoproterozoic block boundary in the early Paleozoic. We infer that the tectonic reactivation, possibly induced by oblique underthrusting of north Cathaysia, facilitated ductile shearing and burial metamorphic reactions, giving rise to the high-strain zones and high-grade metamorphic rocks. With respect to pre-existing mechanical weakness, our work highlights the role of tectonic reactivation of early structures in localizing later deformation before it propagates into yet undeformed domains.

Metamorphic InAsSb/AlInAsSb Heterostructures for Optoelectronic Applications

DTIC Science & Technology

2013-03-20

respectively. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4796181] The development of highly efficient light emitting devi...C12 elastic constants 14 and the standard relationship between strains in perpendicular directions (e ¼ 2 c12c11 ejj) 15 we deter- mined the residual...line as the cloud of reflexes from the pseudomorphic part of the compositionally graded buffers. To obtain information about electroluminescence char

Improving Bone Formation in a Rat Femur Segmental Defect by Controlling Bone Morphogenetic Protein-2 Release

DTIC Science & Technology

2011-04-01

tissue and polymer: mineralized tissue stained dark green, osteoid and collagen bright red, soft tissue pink to light green, and erythrocytes bright...of bone, soft tissue , and polymer, high-resolution digital images were acquired at 1.25 · or 20 · . The area of interest comprising the bone defect...bone, soft tissue , and polymer (when present) within the defect were quantified using Metamorph software (Molecular Devices, Inc.) and were calculated

Exhumation rates of high pressure metamorphic rocks in subduction channels: The effect of Rheology

NASA Astrophysics Data System (ADS)

Gerya, T. V.; Stöckhert, B.

2002-04-01

Exhumation of high-pressure metamorphic rocks can take place with typical plate velocities of cm/year. This is consistent with a model of forced flow in a subduction channel. The (micro)structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep, according to a power-law rheology. Instead deformation appears to be localized in low-strength shear zones, and is dominated by dissolution precipitation creep or fluid assisted granular flow, implying a Newtonian rheology. 1D modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation. When the subduction flux either equals or exceeds the return flux, the maximum exhumation rate for Newtonian behavior of the material is at least twice as high (~1/3 of the subduction burial rate) compared to that for power-law creep (~1/6 of the subduction burial rate).

Anatomy of a metamorphic core complex: seismic refraction/wide-angle reflection profiling in southeastern California and western Arizona

USGS Publications Warehouse

McCarthy, J.; Larkin, S.P.; Fuis, G.S.; Simpson, R.W.; Howard, K.A.

1991-01-01

The metamorphic core complex belt in southeastern California and western Arizona is a NW-SE trending zone of unusually large Tertiary extension and uplift. Midcrustal rocks exposed in this belt raise questions about the crustal thickness, crustal structure, and the tectonic evolution of the region. Three seismic refraction/wide-angle reflection profiles were collected to address these issues. The results presented here, which focus on the Whipple and Buckskin-Rawhide mountains, yield a consistent three-dimensiional image of this part of the metamorphic core complex belt. The final model consists of a thin veneer (<2 km) of upper plate and fractured lower plate rocks (1.5-5.5 km s-1) overlying a fairly homogeneous basement (~6.0 km s-1) and a localized high-velocity (6.4 km s -1) body situated beneath the western Whipple Mountains. A prominent midcrustal reflection is identified beneath the Whipple and Buckskin Rawhide mountains between 10 and 20km depth. -from Authors

Chemical variations among L-chondrites. IV - Analyses, with petrographic notes, of 13 L-group and 3 LL-group chondrites

NASA Astrophysics Data System (ADS)

Jarosewich, E.; Dodd, R. T.

1985-03-01

Procedures are reviewed for selecting, preparing and analyzing meteorite samples, present new analyses of 16 ordinary chondrites, and discuss variations of Fe, S and Si in the L-group. A tendency for Fe/Mg, S/Mg and Si/Mg to be low in L chondrites of facies d to f testifies that post-metamorphic shock melting played a significant role in the chemical diversification of the L-group. However, these ratios also vary widely and sympathetically in melt-free chondrites, indicating that much of the L-group's chemical variation arose prior to thermal metamorphism and is in that sense primary. If all L chondrites come from one parent body, type-correlated chemical trends suggest: (1) that the body had a tradiational 'onion skin' structure, with metamorphic intensity increasing with depth; and (2) that it formed from material that became more homogeneous, slightly poorer in iron, and significantly richer in sulfur as accretion proceeded.

Chemical Variations Among L-Chondrites--IV. Analyses, with Petrographic Notes, of 13 L-group and 3 LL-group Chondrites

NASA Astrophysics Data System (ADS)

Jarosewich, E.; Dodd, R. T.

1985-03-01

We review our procedures for selecting, preparing and analyzing meteorite samples, present new analyses of 16 ordinary chondrites, and discuss variations of Fe, S and Si in the L-group. A tendency for Fe/Mg, S/Mg and Si/Mg to be low in L chondrites of facies d to f testifies that post-metamorphic shock melting played a significant role in the chemical diversification of the L-group. However, these ratios also vary widely and sympathetically in melt-free chondrites, indicating that much of the L-group's chemical variation arose prior to thermal metamorphism and is in that sense primary. If all L chondrites come from one parent body, type-correlated chemical trends suggest: 1) that the body had a traditional "onion skin" structure, with metamorphic intensity increasing with depth; and 2) that it formed from material that became more homogeneous, slightly poorer in iron, and significantly richer in sulfur as accretion proceeded.

Metamorphic and tectonic evolution of the Greater Himalayan Crystalline Complex in Nyalam region, south Tibet

NASA Astrophysics Data System (ADS)

Wang, Jia-Min; Zhang, Jin-Jiang; Rubatto, Daniela

2016-04-01

Recent studies evoke dispute whether the Himalayan metamorphic core - Greater Himalayan Crystalline Complex (GHC) - was exhumed as a lateral crustal flow or a critical taper wedge during the India-Asia collision. This contribution investigated the evolution of the GHC in the Nyalam region, south Tibet, with comprehensive studies on structural kinematics, metamorphic petrology and geochronology. The GHC in the Nyalam region can be divided into the lower and upper GHC. Phase equilibria modelling and conventional thermobarometric results show that peak temperature conditions are lower in the lower GHC (~660-700°C) and higher in the upper GHC (~740-780°C), whereas corresponding pressure conditions at peak-T decrease from ~9-13 kbar to ~4 kbar northward. Monazite, zircon and rutile U-Pb dating results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC underwent higher degree of partial melting (15-25%, via muscovite dehydration melting) that initiated at ~32 Ma, peaked at ~29 Ma to 25 Ma, possibly ended at ~20 Ma. The lower GHC underwent lower degree of melting (0-10%) that lasted from 19 to 16 Ma, which was produced mainly via H2O-saturated melting. At different times, both the upper and lower blocks underwent initial slow cooling (35 ± 8 and 10 ± 5°C/Myr, respectively) and subsequent rapid cooling (120 ± 40°C/Myr). The established timescale of metamorphism suggests that high-temperature metamorphism within the GHC lasted a long duration (~15 Myr), whereas duration of partial melting lasted for ~3 Myr in the lower GHC and lasted for 7-12 Myr in the upper GHC. The documented diachronous metamorphism and discontinuity of peak P-T conditions implies the presence of the Nyalam Thrust in the study area. This thrust is probably connected to the other thrusts in Nepal and Sikkim Himalaya, which extends over ~800 km and is named the "High Himalayan Thrust". Timing of activity along this thrust is at ~25-16 Ma, which is coeval with active timing along the South Tibetan detachment (27-16 Ma) but precedes that along the MCT (16-10 Ma). Comparison between the obtained P-T-t data and model predictions implies that a lateral crustal flow process dominated the exhumation of the high-grade upper GHC migmitites during 25-16 Ma, whereas a critical taper thrusting process dominated the exhumation of the MCT zone nonmigmatites and cooled migmatites in the lower GHC at 16-10 Ma. In other words, at different temporal and spatial scale, both propagating thrusting along large tectonic boundaries and a low-viscosity melting crust could contribute to the exhumation of high-grade metamorphic rocks in Himalaya-like large hot collisional orogens. KEY WORDS: Greater Himalayan Crystalline Complex; P-T path; U-Pb geochronology; channel flow; tectonic discontinuity References: Wang, J.M., Rubatto, D., Zhang, J.J., 2015a. Timing of partial melting and cooling across the Greater Himalayan Crystalline Complex (Nyalam, central Himalaya): in-sequence thrusting and its implications. Journal of Petrology, 56, 1677-1702. Wang, J.M., Zhang, J.J., Wei, C.J., Rai, S.M., Wang, M., Qian, J.H., 2015b. Characterizing the metamorphic discontinuity across the Main Central Thrust Zone of eastern-central Nepal. Journal of Asian Earth Sciences 101, 83-100. Wang, J.M., Zhang, J.J., Wang, X.X., 2013. Structural kinematics, metamorphic P-T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south-central Tibet: implication for a revised channel flow. Journal of Metamorphic Geology 31, 607-628.

The Cycladic Blueschist Belt in the Central Aegean Sea: Resolving the Interplay between Alpine Orogeny and Back-arc Extension

NASA Astrophysics Data System (ADS)

Avigad, D.

2007-12-01

The Aegean Sea, formed via extensional tectonics and floored by an attenuated continental crust, overprinted and dissected a once-continuous Alpine orogenic belt that stretched from mainland Greece to Anatolia. The Cycladic islands, in the central Aegean region, mainly comprise HP-LT metamorphic rocks (and their greenschist-facies derivatives) whose P-T conditions range at 12-15 kbars and 450-500 °C, straddling the blueschist-eclogite facies boundary. The protoliths are supracrustals metavolcanics and volcanoclastics alongside thick marble units that were deposited on the Pindos basin margin. Locally, such as on Syros and Sifnos, kilometer-thick, blueschist and eclogite-facies rocks are preserved intact allowing to explore the bottom of the orogenic edifice. 40Ar/39Ar ages of ~45Ma have been repeatedly obtained on Si rich phengites assessing the Eocene timing of the high-pressure metamorphism and crustal thickening. Upon decompression, the high- pressure rocks were overprinted in the greenschist-facies but locally as on Naxos migmatites were formed on the expense of eclogites at mid-crustal depth, at ~20 Ma. A series of granitoids penetrated the exhumed rock units during the Middle Miocene (until ~10Ma) in relation to whole-lithosphere back-arc extension.//The Cycladic blueschist belt, in the core of the extending Aegean region, is a suitable site to analyze the interplay between Mediterranean-type back-arc extension and the exhumation of the high-pressure metamorphic rocks. The Cycladic blueschist unit is sandwiched between lower pressure rocks: it is topped by greenschist- and amphibolite facies metamorphic rocks comprising metavolcanics interleaved with metamorphosed ultrabasic slices. The tectonic contact is a low-angle extensional detachment of significant lateral dimension and kinematic markers usually portray top-to-the-North sense of motion. Being stitched by mid-Miocene granitoids this is the oldest extensional discontinuity observed in the central Aegean. Where the original architecture of the Alpine orogenic belt was not severely obliterated, such as on Evia, a basal unit (Almyropotamos window) is exposed below the Cycladic blueschists unit. Within the basal unit, the presence of relict glaucophane and Si-rich phengite attest for a LT-HP metamorphism, but carbonates still preserve Lutetian nummullites indicating the basal unit metamorphism outlasted the Middle Eocene as well as cooling of the overlying Cycladic blueschists. The Cycladic blueschist unit is thus allochtonous on a regional scale: it was accreted into the orogenic wedge sometimes after the mid-Eocene. The time interval between the Eocene peak of eclogite metamorphism and the onset of back-arc extension in the Oligo-Miocene involved thrusting and contraction. In the central Aegean, the entire inventory of extensional structures operated subsequently to the emplacement of the Cycladic blueschist unit onto lower pressure sequences implying whole-lithosphere back-arc extension overprinted an Alpine orogen containing eclogites at relatively shallow structural levels. This resembles the mode of occurrence of eclogites in other mountain belts where back-arc extension played no role. Remarkably, despite significant crustal stretching only minor lateral metamorphic breaks can be identified in the Cyclades and the 12-15 kbar level of the former orogen are pervasively exposed over much of the archipelago.

Microstructural evidence for dissolution precipitation creep in high pressure metamorphic serpentinites from subduction zones

NASA Astrophysics Data System (ADS)

Wassmann, S.; Trepmann, C.; Krohe, A.; Stoeckhert, B.

2009-12-01

Serpentinite is generally believed to constitute weak material in subduction zones and to play an essential role for the development of a subduction channel. Information on deformation mechanisms and appropriate rheological models to describe these large scale flow processes is obtained from natural serpentinites exhumed from ancient subduction zones. In this study, we investigate the microstructural record of serpentinites exposed in the Zermatt-Saas-Zone, Western Alps. The metamorphic mineral assemblage comprises antigorite, forsterite, diopside, tremolite, chlorite, oxide phases, and in places titanclinohumite. Original mantle minerals are only locally preserved as relics. The conditions of Alpine metamorphism can be estimated from associated eclogites; the pressures are mostly between 1.5 and 2.5 GPa at temperatures of c. 500-600°C. The serpentinites show a complex structure with several generations of folds and foliations. An early foliation is defined by the combined shape and crystallographic preferred orientation (SPO and CPO) of antigorite and metamorphic diopside. These patterns are undistinguishable from the SPO and CPO of the same minerals in strain shadows, where the CPO must be developed by oriented growth from an aqueous solution. Therefore we suspect that the SPO and CPO in the polyphase matrix layers also result from oriented growth. Also, there is no microstructural evidence for any crystal plastic deformation of diopside. In places, antigorite flakes with SPO and CPO are overgrown by single crystals or aggregates of metamorphic forsterite, locally titanoclinohumite, and tremolite. The aggregates of forsterite exhibit a foam structure without CPO. All microfabrics indicate that dissolution precipitation creep was predominant in the investigated serpentinites, and most finite strain was accumulated by this mechanism. We see no evidence for a significant contribution of dislocation creep, both based on microstructure and on the CPO patterns. This does not preclude dislocation creep and a power law rheology to hold for higher stress levels, as expected for short episodes of postseismic creep. For the long term flow of serpentinites in subduction zones, however, Newtonian behaviour and a low viscosity are indicated.

Tectonic evolution of kid metamorphic complex and the recognition of Najd fault system in South East Sinai, Egypt

NASA Astrophysics Data System (ADS)

Sultan, Yasser M.; El-Shafei, Mohamed K.; Arnous, Mohamed O.

2017-03-01

A low-to medium-grade metamorphic belt of a volcano-sedimentary succession occurs in the eastern side of South Sinai as a part of the northernmost extension of the Arabian-Nubian Shield in Egypt. The belt is known as the Kid metamorphic complex. It is considered as one of the major belt among the other exposed metamorphic belts in South Sinai. Here, we detect and investigate the signature of the Najd Fault system in South Sinai based on detailed structural analysis in field and digital image processing. The enhanced satellite image and the geo-spatial distributions confirm that the Kid belt is essentially composed of nine Precambrian units. Field relations and geometrical analysis of the measured structural data revealed that the study area underwent four successive deformational phases (D1-D4). D1 is an upright tight to isoclinal large-scale folds that caused few F1 small-scale folds and a steeply dipping S1 axial plane foliation. The second deformational event D2 produced dominant of sub-horizontal S2 foliation planes accompanied with recumbent isoclinal folds and NW-SE trending L2 lineations. The main sense during D2 was top-to-the-NW with local reversals to the SE. The third folding generations F3 is recorded as axial plane S3-surfaces and is characterized by open concentric folding that overprinting both F1 and F2 folds and has a flexural-slip mechanism. F3 fold hinges plunge to the west-northwest or east-southeast indicate north-northeast-south-southwest shortening during D3. The fourth deformational event D4 is characterized by NE plunging open concentric folding overprint the pre-existing fold generations and formed under flexural slip mechanism reflecting coaxial deformation and indicating change in the stress regime as a result of the change in shortening from NE-SW to NW-SE. This phase is probably accompanied with the final assembly of east and west Gondwana. The dextral NW-SE shear zone that bounded the southwestern portion of the metamorphic belt is probably related to reactivation of the Najd fault system during Oligo-Miocene in South Sinai.

Isotopic perspectives on the western Himalayan syntaxis

NASA Astrophysics Data System (ADS)

Argles, T. W.; Foster, G. L.; Whittington, A. G.; George, M. T.

2003-04-01

The western syntaxis has been characterised as a structural and metamorphic anomaly within the Himalaya, resulting from extreme Neogene exhumation and associated partial melting. However, an integration of detailed fieldwork with whole-rock isotopic data indicates that all the major tectonic units observed along the arc of the orogen also occur in the syntaxis. Most of the rocks exposed by the extreme exhumation have very different characteristics to their correlatives in the rest of the Himalayan mountain belt, because they represent very different crustal levels. The generally higher metamorphic grade of most syntaxial units obscures their affinities, while high strain throughout the syntaxis also conspires to mask the major tectonic faults that form boundaries to the units in the rest of the orogen. The Lesser Himalayan affinity of the gneissic core of the Nanga Parbat massif has been revealed previously using Nd isotopes. This study confirms the distinction between Lesser (E(Nd) = -20 to -29) and High (E(Nd) = -12 to -19) Himalayan rocks, but further subdivides those units with a High Himalayan Nd signature using Sr isotopic data. Some low-grade schists within the syntaxis have a relatively low 87Sr/86Sr ratio (<0.720) that distinguishes them from the High Himalayan rocks, and suggests they are metamorphic equivalents of the Tethyan sediments exposed in the main Himalayan orogen. The tectonic contact between the Lesser and High Himalayan units in the central Himalaya is the Main Central Thrust, a zone characterised by inverted metamorphism and high strain, but in the uniformly high-strain syntaxis this thrust is difficult to locate except by isotopic signatures. Extensive thermobarometric studies in the syntaxis, however, show two things. The first is the varying intensity of Neogene metamorphic overprint, whose strength is closely related to the degree of deformation (and rheology). The second is a zone of distinctly lower temperature mineral assemblages related to extensional (top-to-the-north) fabrics that straddles the boundary between the High Himalayan gneisses and the Tethyan metasediments. This extensional zone occupies the same structural position in the syntaxis as the South Tibetan Detachment System does in the central Himalaya.

Fi Investigations On Hp-Rocks From The Lower Engadine Window New Insights On Its Late Tectono-Metamorphic Evolution

NASA Astrophysics Data System (ADS)

Bertle, R. J.; Götzinger, M. A.; Koller, F.

2003-04-01

Fluid inclusions studies in metamorphic rocks allow to reconstruct not only the chemistry of the fluids enabling and/or supporting metamorphic reactions but also the late metamorphic evolution of orogenesis. Therefore late, discordant quarz-calcite veins were investigated using FI-techniques. The Engadine Window which is exposed at the Swiss-Austrian-border exposes the penninic units of the Western Alps as a tectonic window within the Austroalpine nappes of the Eastern Alps. The nappes of the Engadine window underwent metamorphism and deformation during Tertiary times (THÖNI 1981, BERTLE 2000). The highest unit (Fimber unit) and the core of the window (= Zone of Pfunds) suffered HP-LT-metamorphism. P-T-conditions for parts of the Zone of Pfunds at the region of Piz Mundin are at 13-15 kbar at 380^oC (BOUSQUET et al. 2002) indicated by the occurrence of carpholite and glaucophane. The late metamorphic history is not very well constrained. There exist only a few FI-data published in an abstract by STÖCKHERT et al. 1990 and some unpublished data in RING 1989. During the ongoing mapping campaign of the first author samples from the Fimber unit and the Zone of Pfunds were collected and investigated using a LINKHAM freezing-cooling-stage. The investigated veins are discordant in respect to the main-foliation of the rocks and show nice cristalls of quarz, calcite and sometimes feldspar (adularia). Structural data implie that the investigated veins correspond to a set of ac-joints that correlate to the late updoming of the large "Engadiner Gewölbe" (Engadin anticlinal structure, MATTMÜLLER 1996). All investigated veins (from all tectonic units) show the same relationship to the anticlinal structure. FI-investigations show, that a large amount of the primary FI are decrepitated, however it was possible to find enough to provide a serious statistical data set. FI from Piz Mundin in the core of the Engadine window exhibit at the base of the vein quarz at the contact to the host rock (blueschist) epidote-clinozoisite cristalls. Futheron amphibole is visible. It is common at the base of the vein quarz and decreases towards the middle of the vein. FI are H2O-rich and indicate high pressure of trapping. Quarzes from the upper most part of the Zone of Pfunds from S of Zeblasjoch (W of Samnaun Dorf) show two main groups of primary FI which could be differentiated at room temperature: homogenous FI and such with a bubble. All FI were frozen at max. temperatures of ca. -56 ^oC. Bigger FI show cracking due to cristallisation pressure (build up of "wings"), the cracks however closed again during heating, so that the FI remained closed. Initial melting started between -20 ^oC (first recristallisation signs) and -9 ^oC, final melting was observable at -1 ^oC to 0 ^oC. Then the FI was a.) homogenous or b.) showed a bubble. Homogenisation Temp. of the inclusions with bubble were in the range of 70 to 150 ^oC , most of them between 70 and 80 ^oC and 110 - 125 ^oC. The data indicate a more or less pure H_2O-system for the FI under high pressure. Assuming a cristallisation temperature of the cristalls of about 200 to 250 ^oC and a density of the FI between 0,97 and 1,0 g/cm^3 pressures of 2,5 to 4,5 Kbar are indicated. The same P-T-conditions (same chemistry and melting & homog. Temp.) could be derived from FI in quarz from the Salaaser Kopf (Idalpe) for the late metamorphic evolution of the Fimber unit, indicating that the late metamorphic history of both units is the same. It is concluded that opening of the veins and first cristallisation of vein quarz corresponds to the first signs of updoming of the Engadine anticlinal structure. Updoming of the anticline started when the whole nappe stack was covered by the Austroalpine nappes. Therefore FI show such high pressures for trapping of the fluid. Acknowledgements: Data partly result from FWF-project P. 15278 "Bündnerschieferakkretion in the westlichen Ostalpen". Financal support is greatly acknowledged. References: BERTLE, R. J. 2000: Zur Geologie und Geochronologie um Alp Trida (Samnaun/Schweiz) einschließlich ingenieurgeologischer Fragen der Gebirgsauflösung und des Permafrosts. - Unpubl. Msc-Thesis. Univ. Wien, 395 S. BOUSQUET, R., GOFFÉ, B., VIDAL, O., OBERHÄNSLI, R. & PATRIAT, M. 2002: The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: new constraints on the evolution of the Alps. - Geol. Soc. America Bull., 114/2, S. 207-225. KÜSTER, M. &STÖCKHERT, B. 1997: Density changes of fluid inclusions in high-pressure low-temperature metamorphic rocks from Crete: A thermobarometric approach based on the creep strength of the host minerals. Lithos, 41, S. 151-167. MATTMÜLLER, C. R. 1996: Geometrische Untersuchung des Inntalgewölbes. - Jahrb. Geol. B.-A., 139/1, S. 45-69, Wien 1996. RING, U. 1989: Tectonogenesis of the Penninic/Austroalpine Boundary Zone: The Arosa Zone (Grisons Rätikon area, Swiss-Austrian Alps). - Unpubl. Phd.-Thesis., 177 p., Tübingen. STOECKHERT, B., RÖSSNER, G., KÜSTER, M., HEIDER, M., GUNDLACH, K. &RICHTER, D.K. 1990: High-Pressure Metamorphism of the Mesozoic Sediments in the Lower Engadine Window, Eastern Alps. - Terra Abstracts, 2, S. 34, 1990. THÖNI, M. 1981: Degree and Evolution of the Alpine Metamorphism in the Austroalpine Unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr Age Determinations on Micas. - Jahrb. Geol. B.-A., 124/1, S. 111-174, Wien 1981.

Ediacaran ( 620 Ma) high grade regional metamorphism in the northern Arabian Nubian Shield: U/Th-Pb monazite ages of the Elat schist

NASA Astrophysics Data System (ADS)

Elisha, Bar; Katzir, Yaron; Kylander-Clark, Andrew

2017-04-01

Ediacaran times witnessed a hemisphere-scale orogenesis forming the extensive Pan-African mountain ranges and resulting in the final assembly of Gondwana supercontinent. The Elat metamorphic basement (S Israel) located at the northernmost tip of a major Pan-African orogenic suture, the Arabian Nubian Shield (ANS), comprises amphibolite facies schists and gneisses and was most likely shaped by this major continental collision. However the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are non-conclusive and a major emphasis was given to pre-Ediacaran island-arc related tectonics. This is mostly because U-Pb dating of zircon, widely used in Elat and elsewhere, is very successful in constraining the ages of the igneous and sedimentary protoliths, but is 'blind' to metamorphism at grades lower than granulite. Here U/Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat basement. Previous U-Pb dating of detrital zircon has shown that the sedimentary protoliths of the Elat schist are the oldest basement components (≥800 Ma), and detailed structural observations of the schists portrayed a complex deformation history including four successive phases (Shimron, 1972). The earliest three phases were defined as ductile and penetrative, but some of the available geochronological data apparently contradict field relations. In-situ analysis of metamorphic monazites by LASS (Laser Ablation Split Stream) involves simultaneous measurement of U/Th-Pb isotope ratios and REE contents in a single 10 μm sized grain or domain, thus allowing determining the age of specific texture and metamorphic assemblage. Monazite dating of the Elat schist yielded two concordant age clusters at 712±6 and 613±5 Ma. The corresponding REE patterns of the dated monazite grains indicate that porphyroblast growth, either garnet or staurolite, took place only during the younger event (M2). Likewise the regional south dipping penetrative foliation, common to the Elat schist and to all of the rocks of the Elat association, formed during the Ediacaran event (M2). This profound event started at 630 Ma and reached peak conditions of mid amphibolite facies at 620 Ma. Retrogression and stress relaxation shortly followed, involving overprint of staurolite schists by a cordierite-bearing assemblage at 613 Ma (M3), and was contemporaneous with the intrusion of andesitic dykes that were immediately metamorphosed to low-amphibolite. This metamorphic P-T-t path corresponds to the collision of East- and West-Gondwana as constrained by large goechronological database of post collision batholiths from all around the Arabian-Nubian Shield.

Thermal durations and heating behaviour for the Barrovian metamorphism, Scotland

NASA Astrophysics Data System (ADS)

Viete, D. R.; Lister, G. S.; Hermann, J.; Forster, M. A.; Oliver, G. J.

2008-12-01

Published U/Pb ages for the syn-metamorphic gabbros and granites of the Grampian Terrane, Scotland, that provided heat for the classical Barrovian metamorphism, suggests that they were emplaced between 473.5 and 470 Ma. New SHRIMP U/Pb ages of 472.2 ± 5.8 Ma and 470.4 ± 6.1 Ma for peak metamorphism in the highest-grade units of the Barrovian metamorphic series are consistent with a 473.5 to 470 Ma heating episode in the highest-grade units. U/Pb-calibrated 40Ar/39Ar ages for white mica from the Barrovian metamorphic series vary from c. 465 Ma for the biotite zone to c. 461 Ma for the sillimanite zone and suggest that the Barrovian thermal episode lasted less than 8.5 million years in the biotite zone and less than 12.5 million years in the sillimanite zone. The lowest-grade units of the Barrovian metamorphic series retain detrital ages in white mica 40Ar/39Ar step-heating spectra, while units metamorphosed to temperatures of 475°C or more yield Grampian 40Ar/39Ar plateau ages. Forward modelling of Ar diffusion from white mica grains was carried out for different grain sizes and thermal histories to match the position of the across-metamorphic-grade transition from detrital 40Ar/39Ar patterns to Grampian 40Ar/39Ar plateau ages. The results of Ar diffusion modelling are consistent with thermal durations of between one and 4.5 million years for the Barrovian metamorphism of the biotite zone. Microstructural observations suggest that peak metamorphism and cooling occurred earliest in the lowest-grade units of the Barrovian metamorphic series and metamorphism in the higher-grade units continued for longer. We propose metamorphic durations of between 3.5 and eight million years for the Barrovian metamorphism of the sillimanite zone. Geochemical textures preserved within high-grade garnets from the Barrovian metamorphic series record evidence of Mn diffusion over c. 1000 μm lengthscales during the Barrovian metamorphism. In addition, sillimanite-grade garnets from the Barrovian metamorphic series preserve c. 100 μm diffusion textures between sillimanite-grade rim domains and lower-grade cores. Timescales for Fickian diffusion processes increase with the square of the diffusion lengthscale. Lengthscales of diffusion are considered within the context of 3.5- to eight-million-year duration for the Barrovian thermal event. Heat associated with regional metamorphism appears to have accumulated within the metamorphosed units following numerous, short- timescale (tens of thousands of year) heating events. Shear zones that occur in the highest-grade parts of the Barrovian metamorphic series provide a suitably narrow heating region for regional metamorphism over a several million years and, with episodic movement histories, can account for self-similar heating behaviour (by mechanical work and/or the introduction of magmas and hot fluids).

Kamenetsk—A new impact structure in the Ukrainian Shield

NASA Astrophysics Data System (ADS)

Gurov, Eugene; Nikolaenko, Nikolay; Shevchuk, Helena; Yamnichenko, Anatoly

2017-12-01

The Kamenetsk impact structure is a deeply eroded simple crater that formed in crystalline rocks of the Ukrainian Shield. This study presents structural, lithologic, and shock metamorphic evidence for an impact origin of the Kamenetsk structure, which was previously described as a paleovolcano. The Kamenetsk structure is an oval depression that is 1.0-1.2 km in diameter and 130 m deep. The structure is deeply eroded, and only the lower part of the sequence of lithic breccia has been preserved in the deepest part of the crater to recent time, while the predominant part of impact rocks and postimpact sediments was eroded. Manifestations of shock metamorphism of minerals, especially planar deformation features in quartz and feldspars, were determined by petrographic investigations of lithic breccia that allowed us to determine the impact origin of the Kamenetsk structure. The erosion of the crater and surrounding target to a minimal depth of 220 m preceded the deposition of the postimpact sediments. The time of the formation of the Kamenetsk structure is bracketed within a wide interval from 2.0 to 2.1 Ga, the age of the crystalline target rocks, to the Late Miocene age of the sediments overlaying the crater. The deep erosion of the structure suggests it is probably Paleozoic in age.

Modélisation magnétotellurique de la structure géologique profonde de l’unité granulitique de l’In Ouzzal (Hoggar occidental)

NASA Astrophysics Data System (ADS)

Bouzid, Abderrezak; Akacem, Nouredine; Hamoudi, Mohamed; Ouzegane, Khadidja; Abtout, Abdeslam; Kienast, Jean-Robert

2008-11-01

Magnetotelluric modeling of the deep geologic structure of In Ouzzal Granulitic Unit (western Hoggar). The In Ouzzal Granulitic Unit (IOGU) or In Ouzzal Terrane (IOT) is an Archaean block belonging to the Hoggar terrane mosaic. It has been reworked during the Eburnean and is characterized by ultrahigh temperature metamorphism of the structures, which are likely to be old dome and basin structures. The aim of this study, based on a survey of 12 magnetotelluric (MT) soundings, was to characterize the IOGU deep lateral boundaries and to see if it is possible to reconstruct some of these old dome and basin structures after their transformation by metamorphism and deformation. MT data analysis and modeling show that IOGU boundaries extend downwards, at least down to the crust's basement, and may represent suture zones. Inside the terrane, the MT observations do not allow separation between dome and basin structures, because these features are severely stretched. However, the main MT transverse response feature is a deeply rooted great accident, which may be interpreted as a major fault that separates IOGU into two compartments.

Inherent flexibility of CLIC6 revealed by crystallographic and solution studies.

PubMed

Ferofontov, Alisa; Strulovich, Roi; Marom, Milit; Giladi, Moshe; Haitin, Yoni

2018-05-02

Chloride intracellular channels (CLICs) are a family of unique proteins, that were suggested to adopt both soluble and membrane-associated forms. Moreover, following this unusual metamorphic change, CLICs were shown to incorporate into membranes and mediate ion conduction in vitro, suggesting multimerization upon membrane insertion. Here, we present a 1.8 Å resolution crystal structure of the CLIC domain of mouse CLIC6 (mCLIC6). The structure reveals a monomeric arrangement and shows a high degree of structural conservation with other CLICs. Small-angle X-ray scattering (SAXS) analysis of mCLIC6 demonstrated that the overall solution structure is similar to the crystallographic conformation. Strikingly, further analysis of the SAXS data using ensemble optimization method unveiled additional elongated conformations, elucidating high structural plasticity as an inherent property of the protein. Moreover, structure-guided perturbation of the inter-domain interface by mutagenesis resulted in a population shift towards elongated conformations of mCLIC6. Additionally, we demonstrate that oxidative conditions induce an increase in mCLIC6 hydrophobicity along with mild oligomerization, which was enhanced by the presence of membrane mimetics. Together, these results provide mechanistic insights into the metamorphic nature of mCLIC6.

Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Chen, Ren-Xu

2017-09-01

Regional metamorphism at extreme conditions refers either to Alpine-type metamorphism at low geothermal gradients of <10 °C/km, or to Buchan-type metamorphism at high geothermal gradients of >30 °C/km. Extreme pressures refer to those above the polymorphic transition of quartz to coesite, so that ultrahigh-pressure (UHP) eclogite-facies metamorphism occurs at mantle depths of >80 km. Extreme temperatures refer to those higher than 900 °C at crustal depths of ≤80 km, so that ultrahigh-temperature (UHT) granulite-facies metamorphism occurs at medium to high pressures. While crustal subduction at the low geothermal gradients results in blueschist-eclogite facies series without arc volcanism, heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite-granulite facies series with abundant magmatism. Therefore, UHP metamorphic rocks result from cold lithospheric subduction to the mantle depths, whereas UHT metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths. Active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling, and this tectonism is suggested as a feasible mechanism for regional granulite-facies metamorphism, with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting. While lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens, the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen-parallel rifting metamorphism and magmatism. Thus, the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere. Because of the partial melting and melt extraction of the lower continental crust, contemporaneous granite-migmatite-granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens. The UHT metamorphic rocks have occurred since the Archean, suggesting that the hot underplating has operated very early in the Earth's history. In contrast, the UHP metamorphic rocks primarily occur in the Phanerozoic, indicating that the thermal regime of many subduction zones has changed since the Neoproterozoic for the cold subduction.

Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south-central Ohio: Confirmation of an origin by impact

USGS Publications Warehouse

Carlton, R.W.; Koeberl, C.; Baranoski, M.T.; SchuMacHer, G.A.

1998-01-01

The origin of the Serpent Mound structure in south-central Ohio has been disputed for many years. Clearly, more evidence was needed to resolve the confusion concerning the origin of the Serpent Mound feature either by endogenic processes or by hypervelocity impact. A petrographic study of 21 samples taken from a core 903 m long drilled in the central uplift of the structure provides evidence of shock metamorphism in the form of multiple sets of planar deformation features in quartz grains, as well as the presence of clasts of altered impact-melt rock. Crystallographic orientations of the planar deformation features show maxima at the shock-characteristic planes of {101??3} and {101??2} and additional maxima at {101??1}, {213??1}, and {516??1}. Geochemical analyses of impact breccias show minor enrichments in the abundances of the siderophile elements Cr, Co, Ni, and Ir, indicating the presence of a minor meteoritic component.

Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site

USGS Publications Warehouse

James, O.B.

1969-01-01

Olivine trachybasalt metamorphosed by nuclear explosion is classified into categories of progressive metamorphism: (i) Weak. Plagioclase is microfractured, and augite cotainis fine twin lamellae. (ii) Moderate. Plagioclase is converted to glass, and mafic minerals show intragranular deformation (undulatory extinction, twin lamellae, and, possibly, deformation lamellae), but rock texture is preserved. (iii) Moderately strong. Plagioclase glass shows small-scale flow, mafic minerals are fractured and show intragranular deformation, and rocks contain tension fractures. (iv) Strong. Plagioclase glass is vesicular, augite is minutely fractured, and olivine is coarsely fragmented, shows mosaic extinction, distinctive lamellar structures, and is locally recrystallized. (v) Intense. Rocks are converted to inhomogeneous basaltic glass.

Modelling Metamorphism by Abstract Interpretation

NASA Astrophysics Data System (ADS)

Dalla Preda, Mila; Giacobazzi, Roberto; Debray, Saumya; Coogan, Kevin; Townsend, Gregg M.

Metamorphic malware apply semantics-preserving transformations to their own code in order to foil detection systems based on signature matching. In this paper we consider the problem of automatically extract metamorphic signatures from these malware. We introduce a semantics for self-modifying code, later called phase semantics, and prove its correctness by showing that it is an abstract interpretation of the standard trace semantics. Phase semantics precisely models the metamorphic code behavior by providing a set of traces of programs which correspond to the possible evolutions of the metamorphic code during execution. We show that metamorphic signatures can be automatically extracted by abstract interpretation of the phase semantics, and that regular metamorphism can be modelled as finite state automata abstraction of the phase semantics.

The Tectonometamorphic Evolution of a 3.2 Ga Tectonic Mélange at the Base of the Barberton Greenstone Belt, South Africa: P-T-t Constraints From the Theespruit Formation

NASA Astrophysics Data System (ADS)

Diener, J.; Stevens, G.; Kisters, A.; Poujol, M.

2004-05-01

The Paleo- to Mesoarchaean Barberton granite-greenstone terrain in South Africa consists of two main components: a southern high-grade metamorphic granite-gneiss terrain and the low-grade metamorphic supracrustal sequence of the Barberton greenstone belt in the north. The gneiss terrain records peak metamorphic conditions of 8 - 11 kbar and 650 - 700 °C, attained at 3229 ± 9 Ma (Dziggel et al., 2002), coinciding with the main phase of collisional tectonics in the greenstone belt (De Ronde and De Wit, 1994). Conversely, estimates of 2.6 ± 0.6 kbar and 360 ± 50 °C reflect metamorphic conditions in the low-grade supracrustal at this time (Cloete, 1999). The boundary of the two different domains corresponds to the Theespruit Formation, a highly tectonized mélange of metabasites, felsic volcanics and rare, aluminous clastic sediments. The metamorphic and structural evolution of the Theespruit Formation was investigated in the Tjakastad Schist Belt in order to constrain the tectonometamorphic history and the significance of this basal mélange for the juxtaposition of these two crustal domains. The pretectonic peak metamorphic assemblage Ky-St-Bt-Ms-Pl-Q quartz occurs in aluminous horizons within the metasediments. These horizons are intercalated with more Fe-Mg-rich sediments that record the peak metamorphic assemblage Grt-St-Bt-Chl-Pl-Q. THERMOCALC estimates from the garnet-bearing metasediments constrain peak P-T conditions at 7.7 ± 0.9 kbar and 560 ± 15 °C. Associated metabasites display peak assemblages consisting of garnet + hornblende + epidote + plagioclase + quartz, which yields a P-T estimate of 7.0 ± 1.6 kbar and 560 ± 60 °C. Retrograde estimates of 3.8 ± 1.3 kbar and 543 ± 20 °C, as well as sillimanite overgrowths on kyanite, indicate that retrogression involved close to isothermal decompression of ca. 4 kbar prior to cooling into the greenschist facies. The age of amphibolite facies metamorphism has been determined by in situ LA-ICP-MS analysis of sphene in metabasite and is constrained at 3229 ± 25 Ma. The timing of peak metamorphic conditions in the Theespruit Formation and granite-gneiss terrain are identical and both localities exhibit a similar style of high-P low-T metamorphism with remarkably low apparent geothermal gradients of 18 - 20 °C/km. Thus, rocks of both the Theespruit Formation and the gneiss terrain record burial metamorphism to mid- and lower crustal levels during the main collisional event at 3230 Ma. P-T estimates from the Onverwacht Group some 4 km north of the study area indicate that a metamorphic break of at least 5 kbar, corresponding to a crustal column of ~18 km, exists between these two localities. This investigation has revealed that the metamorphic history of the Theespruit Formation is very similar to that experienced by the granite-gneiss terrain and differs radically from that experienced by the greenstone sequence. This high-grade terrain is allochtonous to the greenstone belt and represents a tectonically exhumed and underplated terrain. The depths of burial and extremely low geothermal gradients recorded in this terrain are only possible in cool and rigid crustal environments, indicating that a modern continental crustal thermal regime must have been established in the Barberton terrain as early as 3230 Ma. Cloete, M. (1999), Mem. Geol. Survey S. Africa, 84, 232pp De Ronde, C.E.J., De Wit, M.J. (1994), Tectonics, 13, 983 - 1005 Dziggel, A., Stevens, G., Poujol, M., Anhaeusser, C.R., Armstrong, R.A. (2002), Precambrian Res., 114, 221 - 247

Thermal Evolution of Juvenile Subduction Zones ' New Constraints from Lu-Hf Geochronology on HP oceanic rocks (Halilbaǧi, Central Anatolia)

NASA Astrophysics Data System (ADS)

Pourteau, Amaury; Scherer, Erik; Schmidt, Alexander; Bast, Rebecca

2015-04-01

The thermal structure of subduction zones plays a key role on mechanical and chemical processes taking place along the slab-mantle interface. Until now, changes through time of this thermal structure have been explored mostly by the means of numerical simulations. However, both "warm" (i.e., epidote-bearing), and "cold" (i.e., lawsonite-bearing) HP oceanic rocks have been reported in some fossil subduction complexes exposed at the Earth's surface (e.g., Franciscan Complex, California; Rio San Juan Complex, Hispañola; Halilbağı Unit, Central Anatolia). These a-priori "incompatible" rocks witness different thermal stages of ancient subduction zones and their study might provide complementary constraints to numerical models. To decipher the meaning of these contrasting metamorphic rocks in the Halilbağı Unit, we are carrying out Lu-Hf geochronology on garnet (grt) and lws from a variety of HP oceanic rocks, as well as the metamorphic sole of the overlying ophiolite. We selected five samples that are representative of the variety of metamorphic evolutions (i.e. peak conditions and P-T paths) encountered in this area. Preliminary analyses yielded 110 Ma (grt-hbl isochron) for a sub-ophiolitic grt amphibolite; 92 Ma (grt-omp) for an eclogite with prograde and retrograde ep; 90 Ma (grt-omp) for an eclogitic metabasite with prograde ep and retrograde ep+lws; 87 Ma (grt-gln) for a lws eclogite with prograde ep; and 86 Ma (grt-gln) for a blueschist with prograde and retrograde lws. These ages are mainly two-point isochrons. Further-refined data will be presented at the EGU General Assembly 2015, in Vienna. The consistent younging trend from "warm" to "cold" metamorphic rocks revealed by these first-order results points to metamorphic-sole formation during the initiation of intra-oceanic subduction at ~110 Ma, and subsequent cooling of the slab-mantle interface between 92 and 86 Ma. Therefore, the contrasting metamorphic evolutions encountered in the Halilbağı Unit record the progressive thermal maturation of the juvenile Neotethyan subduction zone. This period of ~23 myr between subduction initiation and thermal "steady state" is significantly shorter than that obtained for the Rio San Juan Complex (~60 myr; Krebs et al. 2008, Lithos, 103, 106-137), but compares well with that for the Franciscan Complex (~22 myr; Anczkiewicz et al. 2004, EPSL, 225, 147-161) and falls in the range predicted in numerical simulations (e.g., Gerya et al. 2002, Tectonics, 21/6, 1056).

Towards an integrated magmatic, structural and metamorphic model for the 1.1-0.9 Ga Sveconorwegian orogeny

NASA Astrophysics Data System (ADS)

Slagstad, Trond; Roberts, Nick M. W.; Røhr, Torkil S.; Marker, Mogens K.

2013-04-01

Orogeny involves magmatic, metamorphic, deformational and erosional processes that are caused by or lead to crustal thickening and the development of high topography. In general, these processes operate along the margins of continental plates, either as a result of subduction of oceanic crust (accretionary) or collision between two or more continental plates (collisional). Many of these processes are common to accretionary and collisional orogeny, and do not uniquely discriminate between the two. With only a fragmented geological record, unravelling the style of orogenesis in ancient orogens may, therefore, be far from straightforward. Adding to the complexity, modern continental margins, e.g., the southern Asian margin, display significant variation in orogenic style along strike, rendering along-strike comparisons and correlations unreliable. The late Mesoproterozoic Sveconorwegian province in SW Baltica is traditionally interpreted as the eastward continuation of the Grenville province in Canada, resulting from collision with Amazonia and forming a central part in the assembly of the Rodinia supercontinent. We recently proposed that the Sveconorwegian segment of this orogen formed as a result of accretionary processes rather than collision. This hypothesis was based mainly on considerations of the Sveconorwegian magmatic evolution. Here, we show how the metamorphic/structural record supports (or at least may be integrated in) our model as well. The key elements in our accretionary model are: 1) formation of the Sirdal Magmatic Belt (SMB) between 1070 and 1020 Ma, most likely representing a continental arc batholith. Coeval deformation and high-grade metamorphism farther east in the orogen could represent deformation in the retroarc. 2) cessation of SMB magmatism at 1020 Ma followed by UHT conditions at 1010-1005 Ma, with temperatures in excess of 1000°C at 7.5 kbar. Subduction of a spreading ridge at ca. 1020 Ma would result in an end to arc magmatism and juxtaposition of hot asthenosphere and lower crust. This is a plausible explanation for the UTH event, in contrast to simple crustal thickening and radiogenic self-heating that are generally considered unable to produce such PT conditions. 3) long-lived (990-920 Ma) ferroan magmatism, temporally associated with high-grade metamorphism and large-scale deformation, probably reflecting formation inboard of an alternating compressional/extensional continental margin. We have no known record of events after ca. 920 Ma, but it is conceivable that the active margin persisted well into the Neoproterozoic, possibly indicated by metamorphic and magmatic activity recorded in Grenville/Sveconorwegian orogen-derived sedimentary rocks.

Metamorphic facies map of Southeastern Alaska; distribution, facies, and ages of regionally metamorphosed rocks

USGS Publications Warehouse

Dusel-Bacon, Cynthia; Brew, D.A.; Douglass, S.L.

1996-01-01

Nearly all of the bedrock in Southeastern Alaska has been metamorphosed, much of it under medium-grade conditions during metamorphic episodes that were associated with widespread plutonism. The oldest metamorphisms affected probable arc rocks near southern Prince of Wales Island and occurred during early and middle Paleozoic orogenies. The predominant period of metamorphism and associated plutonism occurred during Early Cretaceous to early Tertiary time and resulted in the development of the Coast plutonic-metamorphic complex that extends along the inboard half of Southeastern Alaska. Middle Tertiary regional thermal metamorphism affected a large part of Baranof Island.

Release behavior and formation mechanism of polycyclic aromatic hydrocarbons during coal pyrolysis.

PubMed

Gao, Meiqi; Wang, Yulong; Dong, Jie; Li, Fan; Xie, Kechang

2016-09-01

Polycyclic aromatic hydrocarbons (PAHs) are major environmental pollutants. They have attracted considerable attention due to their severe potential carcinogenic, mutagenic and genotoxic effects on human health. In this study, five different rank coals from China were pyrolyzed using pyro-probe CDS 5250 and the release behavior of 16 PAHs under different pyrolysis conditions were studied by Gas Chromatography-Mass Spectrometer (GC-MS). The structural characteristics of the five coals were determined by Cross-Polarization/Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS (13)C NMR) spectroscopy, and then the factors influencing the formation of PAHs during coal pyrolysis were discussed together with the coal structural data. It was shown that the amount of PAHs generated during coal pyrolysis was largely related to coal rank and followed the order of medium metamorphic coal > low metamorphic coal > high metamorphic coal. The amount of total PAHs varied as the temperature was increased from 400 °C to 1200 °C, which showed a trend of first increasing and then decreasing, with the maximum value at 800 °C. Moreover, the species of PAHs released varied with pyrolysis temperatures. When the temperature was lower than 800 °C, the small ring PAHs were the most abundant, while the proportion of heavy rings increased at higher temperature. The results indicate that the formation of PAHs during coal pyrolysis depends on the structure of the coal. The species and amounts of PAHs generated during coal pyrolysis are closely related to the contents of protonated aromatic carbons and bridging ring junction aromatic carbons present in the coal structure. Copyright © 2016 Elsevier Ltd. All rights reserved.

Variability in Rock Thermal Properties in the Late Archean Crust of the Kapuskasing Structural Zone and Implications for its Thermal Structure and Metamorphic History.

NASA Astrophysics Data System (ADS)

Merriman, J. D.; Whittington, A. G.; Hofmeister, A. M.

2017-12-01

The thermal properties of rocks such as internal heat production and thermal diffusivity (α) play a key role in determining the thermal structure of the lithosphere, and consequently, the rates and styles of metamorphism within the crust. Over the last decade, measurements of α using the method laser flash analysis have shown the ability of a rock to conduct heat can vary by as much as a factor of 5 between common rock types, and decrease by up to a factor of 10 for the same rock between 25-1000°C. Here we present a preliminary model for the variability in rock throughout the crust based on measurements of the α of a suite of 100 samples from late Archean crust exposed in and around the Kapuskasing Structural Zone in Ontario, Canada. Preliminary results suggest that α is controlled primarily by mineralogy, and can vary not only between different rock types as described above, but also within the same rock by a factor of 1.5 (or more). Thermal diffusivity results were combined with heat producing element concentrations measured with ICP-MS to create a thermal model of the Kapuskasing Structural Zone prior its uplift and exposure. To provide additional constraints for P-T conditions within the pre-uplift KSZ crust, a combination of trace-element and pseudosection thermobarometry was used to estimate metamorphic temperatures during an extended period of crustal stability at the end of the Archean. Preliminary results were compared to finite-difference numerical models of the steady-state geothermal gradient using heat production back-calculated to 2.6 Ga. Results suggest a minimum thickness of the continental lithosphere during the late Archean of at least 150 km. To test the response of the crust to the effects of large thermal events such as pluton emplacement, we also performed time-dependent models of the thermal structure of the pre-uplift KSZ crust. These models suggest that heat from thermal events in the upper and middle crust result in a more insulating crust, which causes heat to be retained in the lower crust for 10s of millions of years after the thermal event has ceased. Thus, metamorphic temperatures preserved in granulites are likely higher than steady-state, suggesting that lithospheric thickness at the end of the Archean for this region was considerably more than 150 km.

Timing and heat sources for the Barrovian metamorphism, Scotland

NASA Astrophysics Data System (ADS)

Viete, Daniel R.; Oliver, Grahame J. H.; Fraser, Geoff L.; Forster, Marnie A.; Lister, Gordon S.

2013-09-01

New SHRIMP U/Pb zircon ages of 472.2 ± 5.8 Ma and 471.2 ± 5.9 Ma are presented for the age of peak metamorphism of Barrovian migmatites. 40Ar/39Ar ages for white mica from the Barrovian metamorphic series are presented, and are recalculated using recently-proposed revisions to the 40K decay constants to allow more precise and accurate comparison with U/Pb ages. The 40Ar/39Ar ages are found to vary systematically with increasing metamorphic grade, between c. 465 Ma for the biotite zone and c. 461 Ma for the sillimanite zone. There is no evidence for any significant metamorphic heating during the first 15 Myr of the Grampian Orogeny (before c. 473 Ma) or the final 4 Myr (after c. 465 Ma). The Barrovian metamorphism occurred over a period of ~ 8 Myr within the ~ 27-Myr Grampian Orogeny. The Barrovian metamorphism records punctuated heating, was temporally and spatially associated with large-scale bimodal magmatism, and developed within crust that was not overthickened. The temporally distinct nature of the Barrovian metamorphic episode within the Grampian Orogeny, and its heating pattern and tectonic context, are not consistent with significant heat contribution from thermal equilibration of overthickened crust. Rather, the Barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the Grampian Orogeny. Temporal and spatial association with Grampian bimodal magmatism is consistent with production of the Barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and/or mantle. The Barrovian metamorphic series - the classic example of ‘orogenic regional metamorphism’ - did not form in response to crustal thickening and thermal relaxation, but appears to record large-scale contact metamorphism.

C-O-H-N fluids circulations and graphite precipitation in reactivated Hudsonian shear zones during basement uplift of the Wollaston-Mudjatik Transition Zone: Example of the Cigar Lake U deposit

NASA Astrophysics Data System (ADS)

Martz, Pierre; Cathelineau, Michel; Mercadier, Julien; Boiron, Marie-Christine; Jaguin, Justine; Tarantola, Alexandre; Demacon, Mickael; Gerbeaud, Olivier; Quirt, David; Doney, Amber; Ledru, Patrick

2017-12-01

Graphitic shear zones are spatially associated with unconformity-related uranium deposits that are located around the unconformity between the strata of the Paleo- to Mesoproterozoic Athabasca Basin (Saskatchewan, Canada) and its underlying Archean to Paleoproterozoic basement. The present study focuses on basement-hosted ductile-brittle graphitic shear zones near the Cigar Lake U deposit, one of the largest unconformity-related U deposits. The goal of the study is to decipher the pre-Athabasca Basin fluid migration history recorded within such structures and its potential role on the formation of such exceptional deposit. Dominantly C-O-H(-N) metamorphic fluids have been trapped in Fluid Inclusion Planes (FIPs) in magmatic quartz within ductile-brittle graphitic shear zones active during retrograde metamorphism associated with the formation of the Wollaston-Mudjatik Transition Zone (WMTZ) between ca. 1805 and 1720 Ma. Such fluids show a compositional evolution along the retrograde path, from a dense and pure CO2 fluid during the earliest stages, through a lower density CO2 ± CH4-N2 (± H2O) fluid and, finally, to a very low density CH4-N2 fluid. Statistical study of the orientation, distribution, proportion, and chemical characterization of the FIPs shows that: i) CO2 (δ13CCO2 around - 9‰ PDB) from decarbonation reactions and/or partial water-metamorphic graphite equilibrium initially migrated regionally and pervasively under lithostatic conditions at about 500 to 800 °C and 150 to 300 MPa. Such P-T conditions attest to a high geothermal gradient of around 60 to 90 °C/km, probably related to rapid exhumation of the basement or a large-scale heat source. ii) Later brittle reactivation of the shear zone at around 450 °C and 25-50 MPa favored circulation of CO2-CH4-N2(± H2O) fluids in equilibrium with metamorphic graphite (δ13CCO2 around - 14‰) under hydrostatic conditions and only within the shear zones. Cooling of these fluids and the water uptake linked to fluid-basement rock reactions led to the precipitation at around 450 °C of poorly-crystallized hydrothermal graphite. This graphite presents isotopic (δ13C - 30 to - 26‰ PDB) and morphological differences from the high-T metamorphic graphite (> 600 °C, - 29 to - 20‰ δ13C) derived from metamorphism of C-rich sedimentary material. The brittle structural reactivation and the related fluid migration and graphite precipitation were specifically focused within the shear zones and related damage zones. The brittle reactivation produced major changes in the petro-physical, mineralogical, and chemical characteristics of the structures and their damage zones. It especially increased the fracture paleoporosity and rock weakness toward the fault cores. These major late metamorphic modifications of the graphitic shear zones were likely key parameters favoring the enhanced reactivity of these basement zones under tectonic stress following deposition of the Athabasca Basin, and so controlled basinal brine movement at the basin/basement interface related to the formation of the unconformity-related uranium deposits. This relationship consequently readily explains the specific spatial relationships between unconformity-related U deposits and the ductile-brittle graphitic shear zones.

Cenozoic oblique collision of South American and Caribbean plates: New evidence in the Coastal Cordillera of Venezuela and Trinidad

DOE Office of Scientific and Technical Information (OSTI.GOV)

Speed, R.C.; Russo, R.M.; Foland, K.A.

The hinterland of the Caribbean Mts. orogen in Trinidad and Venezuela contains schist and gneiss whole protoliths are wholly or partly of continental provenance. The hinterland lies between the foreland thrust belt and terranes. The terranes are alien to continental South America (SA) and may have proto-Caribbean or Caribbean plate origins. The hinterland rocks were widely thought to come from sediments and granitoids of Mesozoic protolithic ages and to be of Cretaceous metamorphic age. Such rocks are now know to be of at least two or more types, as follows: (1) low grade, protoliths of pre-Mesozoic basement and shelfal covermore » of uncertain age range, inboard locus, Oligocene to mid-Miocene metamorphic ages younging eastward (Caracas, Paria, and Northern Range belts), and (2) higher grade including high P/T, varies protoliths of uncertain age range, Cretaceous and ( )early Paleogene metamorphic ages (Tacagua, Araya, Margarita). The geometry, protoliths, structures, and metamorphic ages of type 1 parautochthoneity and an origin as a thickened wedge of crust-cored passive margin cover. The wedge grew by accretion between about 35 and 20 Ma during oblique transport toward the foreland. The diachroneity of metamorphism implies, as does the timing of foreland deformation, that the wedge evolved in a right-oblique collision between northern SA and terranes moving wholly or partly with the Caribbean plate since the Eocene. Type 2 rocks probably came with the terranes and are products of convergent zone tectonics, either in the proto-Caribbean plate. The hinterland boundaries are brittle thrusts that are out of sequence and imply progressive contraction from mid-Cenozoic to the present.« less

Using U-Th-Pb petrochronology to determine rates of ductile thrusting: Time windows into the Main Central Thrust, Sikkim Himalaya

NASA Astrophysics Data System (ADS)

Mottram, Catherine M.; Parrish, Randall R.; Regis, Daniele; Warren, Clare J.; Argles, Tom W.; Harris, Nigel B. W.; Roberts, Nick M. W.

2015-07-01

Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains. In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology. U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from 50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times. In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of 550°C and 0.8 GPa between 21 and 18 Ma along the prograde path. Peak metamorphic conditions of 650°C and 0.8-1.0 GPa were subsequently reached as this footwall material was underplated to the hanging wall at 17-14 Ma. This same process occurred at analogous metamorphic conditions between 18-16 Ma and 14.5-13 Ma in the midsection of the thrust zone and between 13 Ma and 12 Ma in the northern, rear edge of the thrust zone. Northward younging muscovite 40Ar/39Ar ages are consistently 4 Ma younger than the youngest monazite ages for equivalent samples. By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr-1 can be calculated. This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.

Tectono-metamorphic evolution of the Jomolhari massif: Variations in timing of syn-collisional metamorphism across western Bhutan

NASA Astrophysics Data System (ADS)

Regis, Daniele; Warren, Clare J.; Young, David; Roberts, Nick M. W.

2014-03-01

Our current understanding of the rates and timescales of mountain-building processes is largely based on information recorded in U-bearing accessory minerals such as monazite, which is found in low abundance but which hosts the majority of the trace element budget. Monazite petrochronology was used to investigate the timing of crustal melting in migmatitic metasedimentary rocks from the Jomolhari massif (NW Bhutan). The samples were metamorphosed at upper amphibolite to granulite facies conditions (~ 0.85 GPa, ~ 800 °C), after an earlier High-Pressure stage (P > 1.4 GPa), and underwent partial melting through dehydration melting reactions involving muscovite and biotite. In order to link the timing of monazite growth/dissolution to the pressure-temperature (P-T) evolution of the samples, we identified 'chemical fingerprints' in major and accessory phases that were used to back-trace specific metamorphic reactions. Variations in Eu anomaly and Ti in garnet were linked to the growth and dissolution of major phases (e.g. growth of K-feldspar and dehydration melting of muscovite/biotite). Differences in M/HREE and Y from garnet core to rim were instead related to apatite breakdown and monazite-forming reactions. Chemically zoned monazite crystals reacted multiple times during the metamorphic evolution suggesting that the Jomolhari massif experienced a prolonged high-temperature metamorphic evolution from 36 Ma to 18 Ma, significantly different from the P-T-time path recorded in other portions of the Greater Himalayan Sequence (GHS) in Bhutan. Our data demonstrate unequivocally that the GHS in Bhutan consists of units that experienced independent high-grade histories and that were juxtaposed across different tectonic structures during exhumation. The GHS may have been exhumed in response to (pulsed) mid-crustal flow but cannot be considered a coherent block.

Stepwise exhumation of the Triassic Lanling high-pressure metamorphic belt in Central Qiangtang, Tibet: Insights from a coupled study of metamorphism, deformation, and geochronology

NASA Astrophysics Data System (ADS)

Liang, Xiao; Wang, Genhou; Yang, Bo; Ran, Hao; Zheng, Yilong; Du, Jinxue; Li, Lingui

2017-04-01

The E-W trending Central Qiangtang metamorphic belt (CQMB) is correlated to the Triassic orogeny of the Paleo-Tethys Ocean prior to Cenozoic growth of the Tibetan Plateau. The well-exposed Lanling high-pressure, low-temperature (HP-LT) metamorphic complex was chosen to decipher the process by which it was exhumed, which thereby provides insights into the origin of the CQMB and Qiangtang terrane. After a detailed petrological and structural mapping, three distinct N-S-trending metamorphic domains were distinguished. Microscopic observations show that core domain garnet (Grt)-bearing blueschist was exhumed in a heating plus depressurization trajectory after peak eclogitic conditions, which is more evident in syntectonic vein form porphyroblastic garnets with zoning typical of a prograde path. Grt-free blueschist of the mantle domain probably underwent an exhumation path of temperature increasing and dehydration, as evidenced by pervasive epidote veins. The compilation of radiometric results of high-pressure mineral separates in Lanling and Central Qiantang, and reassessments on the published phengite data sets of Lanling using Arrhenius plots allow a two-step exhumation model to be formulated. It is suggested that core domain eclogitic rocks were brought onto mantle domain blueschist facies level starting at 244-230 Ma, with exhumation continuing to 227-223.4 Ma, and subsequently were exhumed together starting at 223-220 Ma, reaching lower greenschist facies conditions generally after 222-217 Ma. These new observations indicate that the CQMB formed as a Triassic autochthonous accretionary complex resulting from the northward subdcution of the Paleo-Tethys Ocean and that HP-LT rocks therein were very probably exhumed in an extensional regime.

The direction of fluid flow during contact metamorphism of siliceous carbonate rocks: new data for the Monzoni and Predazzo aureoles, northern Italy, and a global review

NASA Astrophysics Data System (ADS)

Ferry, John M.; Wing, Boswell A.; Penniston-Dorland, Sarah C.; Rumble, Douglas

2002-03-01

Periclase formed in siliceous dolomitic marbles during contact metamorphism in the Monzoni and Predazzo aureoles, the Dolomites, northern Italy, by infiltration of the carbonate rocks by chemically reactive, H2O-rich fluids at 500 bar and 565-710 °C. The spatial distribution of periclase and oxygen isotope compositions is consistent with reactive fluid flow that was primarily vertical and upward in both aureoles with time-integrated flux ~5,000 and ~300 mol fluid/cm2 rock in the Monzoni and Predazzo aureoles, respectively. The new results for Monzoni and Predazzo are considered along with published studies of 13 other aureoles to draw general conclusions about the direction, amount, and controls on the geometry of reactive fluid flow during contact metamorphism of siliceous carbonate rocks. Flow in 12 aureoles was primarily vertically upward with and without a horizontal component directed away from the pluton. Fluid flow in two of the other three was primarily horizontal, directed from the pluton into the aureole. The direction of flow in the remaining aureole is uncertain. Earlier suggestions that fluid flow is often horizontal, directed toward the pluton, are likely explained by an erroneous assumption that widespread coexisting mineral reactants and products represent arrested prograde decarbonation reactions. With the exception of three samples from one aureole, time-integrated fluid flux was in the range 102-104 mol/cm2. Both the amount and direction of fluid flow are consistent with hydrodynamic models of contact metamorphism. The orientation of bedding and lithologic contacts appears to be the principal control over whether fluid flow was either primarily vertical or horizontal. Other pre-metamorphic structures, including dikes, faults, fold hinges, and fracture zones, served to channel fluid flow as well.

The direction of fluid flow during contact metamorphism of siliceous carbonate rocks: new data for the Monzoni and Predazzo aureoles, northern Italy, and a global review

NASA Astrophysics Data System (ADS)

Ferry, John; Wing, Boswell; Penniston-Dorland, Sarah; Rumble, Douglas

2001-11-01

Periclase formed in siliceous dolomitic marbles during contact metamorphism in the Monzoni and Predazzo aureoles, the Dolomites, northern Italy, by infiltration of the carbonate rocks by chemically reactive, H2O-rich fluids at 500 bar and 565-710 °C. The spatial distribution of periclase and oxygen isotope compositions is consistent with reactive fluid flow that was primarily vertical and upward in both aureoles with time-integrated flux 5,000 and 300 mol fluid/cm2 rock in the Monzoni and Predazzo aureoles, respectively. The new results for Monzoni and Predazzo are considered along with published studies of 13 other aureoles to draw general conclusions about the direction, amount, and controls on the geometry of reactive fluid flow during contact metamorphism of siliceous carbonate rocks. Flow in 12 aureoles was primarily vertically upward with and without a horizontal component directed away from the pluton. Fluid flow in two of the other three was primarily horizontal, directed from the pluton into the aureole. The direction of flow in the remaining aureole is uncertain. Earlier suggestions that fluid flow is often horizontal, directed toward the pluton, are likely explained by an erroneous assumption that widespread coexisting mineral reactants and products represent arrested prograde decarbonation reactions. With the exception of three samples from one aureole, time-integrated fluid flux was in the range 102-104 mol/cm2. Both the amount and direction of fluid flow are consistent with hydrodynamic models of contact metamorphism. The orientation of bedding and lithologic contacts appears to be the principal control over whether fluid flow was either primarily vertical or horizontal. Other pre-metamorphic structures, including dikes, faults, fold hinges, and fracture zones, served to channel fluid flow as well.

Different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks

NASA Astrophysics Data System (ADS)

Xia, Qiong-Xia; Zhou, Li-Gang

2017-09-01

Garnet in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions, which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction-zone conditions. It provides an important target to retrieve physicochemical changes in subduction-zone processes, including those not only in pressure and temperature but also in the durations of metamorphism and anatexis. Garnet from different compositions of HP to UHP metamorphic rocks may show different types of major and trace element zonation, as well as mineral inclusions. Discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the HP to UHP metamorphic rocks. Magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction-zone conditions, with spessartine-increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim. Metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation, with spessartine-decreasing from core to rim if the intracrystalline diffusion is not too fast. The compositional profiles of metamorphic garnet in the abundances of grossular, almandine and pyrope are variable depending on the composition of host rocks and co-existing minerals. Peritectic garnet grows through peritectic reactions during partial melting of HP to UHP rocks, with the composition of major elements to be controlled by anatectic P-T conditions and the compositions of parental rocks and anatectic melts. Trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages, the garnet-forming reactions and the property of metamorphic fluids or anatectic melts. Mineral inclusions not only present key clues to identify the different origins of garnet, but also serve as sound candidates for the temporal constraint on garnet growth.

Progressive Extensional Exhumation of the Ultrahigh-Pressure Tso Morari Terrain, NW Indian Himalaya

NASA Astrophysics Data System (ADS)

Hodges, K.; Clark, R.; Monteleone, B.; Sachan, H.; Mukherjee, B. K.; Ahmad, T.

2011-12-01

The core of the Tso Morari dome in the Ladakh region of NW India (roughly 33 °10'N; 78°10'E) is one of only two known ultrahigh-pressure (UHP) terrains in the Himalayan-Tibetan orogenic system. The quartzofeldspathic Puga Orthogneiss from the structurally deepest portions of the terrain does not contain UHP mineralogy but surrounds dismembered lenses of mafic eclogite with accessory coesite, confirming that at least the eclogite lenses experienced UHP metamorphic conditions (Mukherjee et al., 2003, International Geology Review; Sachan et al., 2004, European Journal of Mineralogy). U-Pb zircon dates from the Puga orthogneiss (53.3 ± 0.7 Ma: Leech et al., 2007, International Geology Review) provide what appear to be the most precise available constraints on the age of UHP metamorphism at Tso Morari provided we presume that the UHP assemblages in the eclogite lenses developed at the same time as the 53.3 ± 0.7 Ma metamorphic zircon in the orthogneiss. However, other components of the zircon population studied by Leech and co-workers, as well as the results obtained using other thermochronometers and geochronometers (de Sigoyer et al., 2004, Tectonics), demonstrate that a series of lower pressure metamorphic events also affected the Tso Morari terrain between ca. 53 Ma and ca. 45 Ma, implying rapid decompression at elevated temperatures (ca. 800 - 350°C). Our 1:50000-scale geologic mapping at Tso Morari provides evidence that this exhumation was largely accommodated by two previously unrecognized low-angle ductile detachments that separate the terrain into three tectonostratigraphic units with distinctive metamorphic histories. The structurally lowest shear zone (Karla detachment) separates the Puga Orthogneiss from overlying lower amphibolite facies metasedimentary rocks of the Zoboshisha Unit, which contains no UHP assemblages. Structurally higher and demonstrably younger detachments separate the Zoboshisha Unit and the Puga Orthogneiss from greenschist to epidote-amphibolite facies metasedimentary (and possibly metavolcanic) units of the Taglang La formation. The role of nested detachments in the early, rapid exhumation of UHP rocks at Tso Morari support a geodynamic model such as that of Chemenda and co-workers (1996, Earth and Planetary Science Letters) that involve extrusion of UHP terrains beneath rooted detachment systems. We suggest that synconvergence detachment systems can involve multiple generations of detachments that result in the progressive exhumation of UHP rocks over a brief period of time, a concept similar to that promoted by Johnston et al. (2007, Tectonics) for much slower, postconvergence exhumation of Norwegian UHP rocks.

Tectono-metamorphic evolution of the Kibuye-Gitarama-Gatumba area (Rwanda): using petrochronology to unravel the geodynamic framework of the Karagwe-Ankole Belt

NASA Astrophysics Data System (ADS)

Van Daele, Johanna; Jacques, Dominique; Hulsbosch, Niels; Dewaele, Stijn; Muchez, Philippe

2017-04-01

The Mesoproterozoic Karagwe-Ankole Belt (KAB) extends from Burundi over Rwanda and NW-Tanzania to S-Uganda (Central Africa). The integration of the metamorphic and magmatic evolution of this orogenic belt in a consistent geodynamic framework is still controversial. Additionally, geochronological information on the deformation phases is limited. This tectono-metamorphic model is, however, a crucial component in the understanding of the Meso- to Early Neoproterozoic mineralization processes. A detailed structural mapping of road and river transects was performed in the Kibuye-Gitarama-Gatumba area (West Rwanda) to determine the deformation history of the KAB. Structural analyses and petrographic studies identified two main compressive deformation phases. A locally observed foliation with a N45W-N50W orientation is interpreted as the consequence of a first compressional phase (D1, shortening direction N40E-N45E). Additionally, a well-developed crenulation cleavage and a regionally pervasive foliation were found. The cleavage and foliation have an orientation of N20W-N30W (exceptionally N20E) and are indicative of a second compressional phase (D2) with an EW shortening direction. Final extension (D3) along a N30W-N10E direction resulted in boudinage and joint development. Fieldwork observations combined with known ages of the granites in the KAB indicate that D1 and D2 took place prior to 986 Ma while D3 is younger than 986 Ma. Based on thin section petrography, a petrochronological strategy was outlined to fill in the gaps of the currently broadly defined timeframe. The regional metamorphic grade of the study area is upper greenschist, with the formation of muscovite, biotite, chlorite, garnet, staurolite and cordierite. The main penetrative tectonic foliations (D1 and D2) are expressed by the preferential orientation of muscovite or biotite. In some cases, muscovite growing along the crenulation cleavage (syn-D2) was observed. Furthermore, pre- and syn-D2 garnets were identified. Different generations of quartz veins (post-dating D3) contain muscovite, garnet and/or biotite. These minerals provide excellent dating possibilities (Ar-Ar, in-situ Rb-Sr and Sm-Nd). The obtained results will be used to refine the chronological aspects of the deformation history of the KAB. Furthermore, these data will be combined with regional structural data, petrographic and geothermobarometric analyses to reconstruct the tectono-metamorphic evolution of the Karagwe-Ankole Belt, which will allow to obtain a better insight in the geodynamic evolution and the ore-forming processes.

Diverging Histories of the Liberty Creek and Iceberg Lake Blueschist Bodies, south central Alaska

NASA Astrophysics Data System (ADS)

Day, E. M.; Pavlis, T. L.; Amato, J. M.

2011-12-01

New studies of the Liberty Creek and Iceberg Lake blueschist bodies of south central Alaska indicate that despite structural similarities, these blueschist bodies are derived from a different protolith and were metamorphosed to blueschist facies at distinctly different times. Both blueschists are located just south of the Border Ranges Fault (BRF) within outcrop belts of the McHugh Complex, a low-grade mélange assemblage that is now known from detrital zircon studies to consist of two distinct assemblages: a Jurassic to Earliest Cretaceous assemblage and a Late Cretaceous assemblage. The BRF is a megathrust system that represents the Late Triassic-Early Jurassic initiation of southern Alaskan subduction. Large scale (1:24,000) mapping revealed similar fabric overprint histories, epitomized by a previously undescribed youngest vertical N-S trending crenulation cleavage in both blueschist bodies which implies a structural correlation despite their separation of ~100 kilometers along strike. Despite structural similarities detrital zircon studies show that the Liberty Creek and Iceberg Lake blueschists do not have a similar maximum age of deposition. Thirteen samples from the Iceberg Lake blueschist were processed, none of which produced detrital zircons. Samples from the McHugh Complex greenschists that surround the Iceberg Lake blueschist produced numerous zircons indicating a Late Jurassic (~160 Ma) maximum age of deposition. Three out of sixteen samples from the Liberty creek blueschist produced detrital zircons indicating maximum depositional ages ranging from Late Jurassic (~160.1 Ma, n=64 grains; ~152.25 Ma, n=68 grains) to Early Cretaceous (~137.1 Ma, n=95 grains). The Late Jurassic dates are consistent with maximum depositional ages determined by Amato and Pavlis (2010) for McHugh Complex rocks along Turnagain Arm near Anchorage, AK. Sisson and Onstott (1986) reported a metamorphic cooling age of 185 Ma for the Iceberg Lake blueschist, thus, although no depostitional age constraints were obtained for the Iceberg Lake body, its metamorphic cooling age is far older than the younger depositional ages of the Liberty Creek blueschists indicating these areas record two different blueschist facies metamorphic assemblages. Work in progress on cooling ages from the Liberty Creek rocks should clarify the age of this younger metamorphism. Although these assemblages record different metamorphic events, the similar overprint history may indicate that the Liberty Creek and Iceberg Lake blueschists were subjected to the same, younger deformation series, possibly Cenozoic strike-slip related deformation.

Magnetic structure of the crust

NASA Technical Reports Server (NTRS)

Wasilewski, P.

1985-01-01

The bibuniqueness aspect of geophysical interpretation must be constrained by geological insight to limit the range of theoretically possible models. An additional step in depth understanding of the relationship between rock magnetization and geological circumstances on a grand scale is required. Views about crustal structure and the distribution of lithologies suggests a complex situation with lateral and vertical variability at all levels in the crust. Volcanic, plutonic, and metamorphic processes together with each of the observed anomalies. Important questions are addressed: (1) the location of the magnetic bottom; (2) whether the source is a discrete one or are certain parts of the crust cumulatively contributing to the overall magnetization; (3) if the anomaly to some recognizable surface expression is localized, how to arrive at a geologically realistic model incorporating magnetization contrasts which are realistic; (3) in the way the primary mineralogies are altered by metamorphism and the resulting magnetic contracts; (4) the effects of temperature and pressure on magnetization.

Structural Maturation of HIV-1 Reverse Transcriptase—A Metamorphic Solution to Genomic Instability

PubMed Central

London, Robert E.

2016-01-01

Human immunodeficiency virus 1 (HIV-1) reverse transcriptase (RT)—a critical enzyme of the viral life cycle—undergoes a complex maturation process, required so that a pair of p66 precursor proteins can develop conformationally along different pathways, one evolving to form active polymerase and ribonuclease H (RH) domains, while the second forms a non-functional polymerase and a proteolyzed RH domain. These parallel maturation pathways rely on the structural ambiguity of a metamorphic polymerase domain, for which the sequence–structure relationship is not unique. Recent nuclear magnetic resonance (NMR) studies utilizing selective labeling techniques, and structural characterization of the p66 monomer precursor have provided important insights into the details of this maturation pathway, revealing many aspects of the three major steps involved: (1) domain rearrangement; (2) dimerization; and (3) subunit-selective RH domain proteolysis. This review summarizes the major structural changes that occur during the maturation process. We also highlight how mutations, often viewed within the context of the mature RT heterodimer, can exert a major influence on maturation and dimerization. It is further suggested that several steps in the RT maturation pathway may provide attractive targets for drug development. PMID:27690082

Regional and contact metamorphism within the Moy Intrusive Complex, Grampian Highlands, Scotland

NASA Astrophysics Data System (ADS)

Zaleski, E.

1985-04-01

In central Scotland, the Moy Intrusive Complex consists of (1) the Main Phase — syntectonic peraluminous granodiorite to granite emplaced at c. 455 Ma, intruded by (2) the Finglack Alaskite — post-tectonic leucocratic granite emplaced at 407+/-5 Ma. The Main Phase was emplaced into country rocks at amphibolite facies temperatures. Rb-Sr dates and a compositional spectrum of decreasing celadonite content in Main Phase muscovite suggest the persistence of c. 550° C temperatures for c. 30 Ma but with a declining pressure regime, i.e. isothermal uplift. The Finglack Alaskite was intruded at high structural level, leading to the development of a contact metamorphic aureole in the Main Phase. The thermal effects of contact metamorphism include intergrowths of andalusite, biotite and feldspar in pseudomorphs after muscovite. This is associated with recrystallized granoblastic quartz. Muscovite breakdown and reaction with adjacent biotite, quartz and feldspar, i.e. a function of local mineral assemblage rather than bulk rock composition, is postulated to explain the occurrence of metamorphic andalusite in a granitoid rock. The Main Phase pluton of the Moy Intrusive Complex lies within a NNE trending belt of c. 450 Ma Caledonian tectonic and magmatic activity paralleling the Moine Thrust, and extending from northern Scotland to the Highland Boundary Fault. Syntectonic ‘S-type’ magmatism with upper crustal source areas implies crustal thickening and suggests an intracratonic orogeny.

Linking microstructures, petrology and in situ U-(Th)-Pb geochronology to constrain P-T-t-D evolution of the Greather Himalyan Sequences in Western Nepal (Central Himalaya)

NASA Astrophysics Data System (ADS)

Iaccarino, Salvatore; Montomoli, Chiara; Carosi, Rodolfo; Langone, Antonio

2013-04-01

Last advances in forward modelling of metamorphic rocks and into the understanding of accessories minerals behaviour, suitable for geochronology (e.g. zircon and monazite), during metamorphism, bring new insights for understanding the evolution of metamorphic tectonites during orogenic cycles (Williams and Jercinovic, 2012 and reference therein). One of the best exposure of high- to medium grade- metamorphic rocks, is represented by the Greater Himalayan Sequence (GHS) in the Himalayan Belt, one of the most classic example of collisional orogen. Recent field work in Mugu Karnali valley, Western Nepal (Central Himalaya), identified a compressional top to the South ductile shear zone within the core of the GHS, named Magri Shear Zone (MSZ), developed in a high temperature regime as testified by quartz microstructures and syn-kinematic growth of sillimanite. In order to infer the tectono-metamorphic meaning of MSZ, a microstructural study coupled with pseudosection modelling and in situ U-(Th)-Pb monazite geochronology was performed on selected samples from different structural positions. Footwall sample constituted by (Grt + St ± Ky) micaschist shows a prograde garnet growth (cores to inner rims zoning), from ~500°C, ~0.60GPa (close to garnet-in curve) to ~580°C, ~1.2 GPa temporal constrained between 21-18 Ma, by medium Y cores to very low Y mantles monazite micro-chemical/ages domain . In this sample garnet was still growing during decompression and heating at ~640°C, ~0.75 GPa (rims), and later starts to be consumed, in conjunction with staurolite growth at 15-13 Ma, as revealed by high Y rims monazite micro-chemical/ages domain. Hanging-wall mylonitic samples have a porphyroclastic texture, with garnet preserve little memory of prograde path. Garnet near rim isoplets and matrix minerals intersect at ~700°C and ~0.70 GPa. A previous higher P stage, at ~1.10 GPa ~600°C, is testified by cores of larger white mica porhyroclasts. Prograde zoned allanite (Janots et al., 2008) is rarely found within garnet crystal, while monazite found only along mylonitic foliation helps to constrain the age of shearing and hanging-wall rocks exhumation, between 25 Ma (low Y cores interpretd as Aln out product, close to P peak) and 18 Ma (high Y rims interpreted as Grt breakdown/melt crystallization product during decompression). The present results point out the occurence of a high-temperature shear zone, in the core of the GHS, active before the onset of the Main Central Thrust, responsible of at least a part of the exhumation of the metamorphic rocks. References Janots, E., Engi, M., Berger, J., Allaz, J., Schwarz, O., Spandler, C., (2008): Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite monazite-xenotime phase relations from 250 to 610°C. Journal of Metamorphic Geology 26, 509-526. Williams, M.L., Jercinovic, M.J., (2012): Tectonic interpretation of metamorphic tectonites: integrating compositional mapping, microstructrual analyses and in situ monazite dating. Journal of Metamorphic Geology 30, 739-752.

CrossTalk: The Journal of Defense Software Engineering. Volume 28, Number 2, March/April 2015

DTIC Science & Technology

2015-04-01

that would not be detected using only the metamorphic properties of the full application. Second, the metamorphic properties of individual functions...the so-called “oracle problem” has focused on the use of metamorphic testing [3]. In metamorphic testing changes are made to existing test inputs in...one specified metamorphic property, we can now check two properties and run a total of three tests. This combined approach also allows us to reveal

Post-metamorphic brecciation in type 3 ordinary chondrites

NASA Technical Reports Server (NTRS)

Scott, E. R. D.; Mccoy, T. J.; Keil, K.

1993-01-01

Type 3.1-3.9 ordinary chondrites can be divided into two kinds: those in which the compositions of chondrule silicates are entirely consistent with metamorphism of type 3.0 material, and those in which the computational heterogeneity appears to be too extreme for in situ metamorphism. We present petrologic data for three LL3 chondrites of the second kind--Ngawi, ALH A77278 (both type 3.6), and Hamlet (type 3.9)--and compare these data with results for the first kind of LL3-4 chondrites. Given that chondrules form in the nebula and that metamorphic equilibration occurs in asteroids, our new data imply that Ngawi, A77278, Hamlet, and many other type 3 ordinary chondrites are post-metamorphic breccias containing materials with diverse metamorphic histories; they are not metamorphic rocks or special kinds of 'primitive breccias.' We infer also that metamorphism to type 3.1-3.9 levels produces very friable material that is easily remixed into breccias and lithified by mild shock. Thus, petrologic types and subtypes of chondrites indicate the mean metamorphic history of the ingredients, not the thermal history of the rock. The metamorphic history of individual type 1 or 2 porphyritic chondrules in type 3 breccias is best derived from olivine and pyroxene analyses and the data of McCoy et al. for unbrecciated chondrites. The new chondrule classification schemes of Sears, DeHart et al., appears to provide less information about the original state and metamorphic history of individual porphyritic chondrules and should not replace existing classification schemes.

Post-metamorphic brecciation in type 3 ordinary chondrites

NASA Astrophysics Data System (ADS)

Scott, E. R. D.; McCoy, T. J.; Keil, K.

1993-03-01

Type 3.1-3.9 ordinary chondrites can be divided into two kinds: those in which the compositions of chondrule silicates are entirely consistent with metamorphism of type 3.0 material, and those in which the computational heterogeneity appears to be too extreme for in situ metamorphism. We present petrologic data for three LL3 chondrites of the second kind--Ngawi, ALH A77278 (both type 3.6), and Hamlet (type 3.9)--and compare these data with results for the first kind of LL3-4 chondrites. Given that chondrules form in the nebula and that metamorphic equilibration occurs in asteroids, our new data imply that Ngawi, A77278, Hamlet, and many other type 3 ordinary chondrites are post-metamorphic breccias containing materials with diverse metamorphic histories; they are not metamorphic rocks or special kinds of 'primitive breccias.' We infer also that metamorphism to type 3.1-3.9 levels produces very friable material that is easily remixed into breccias and lithified by mild shock. Thus, petrologic types and subtypes of chondrites indicate the mean metamorphic history of the ingredients, not the thermal history of the rock. The metamorphic history of individual type 1 or 2 porphyritic chondrules in type 3 breccias is best derived from olivine and pyroxene analyses and the data of McCoy et al. for unbrecciated chondrites. The new chondrule classification schemes of Sears, DeHart et al., appears to provide less information about the original state and metamorphic history of individual porphyritic chondrules and should not replace existing classification schemes.

Pluton emplacement within an extensional transfer zone during dextral strike-slip faulting: an example from the late Archaean Abitibi Greenstone Belt

NASA Astrophysics Data System (ADS)

Lacroix, S.; Sawyer, E. W.; Chown, E. H.

1998-01-01

The Lake Abitibi area within the late Archaean Abitibi Greenstone Belt exhibits an interlinked plutonic, structural and metamorphic evolution that may characterize segmented strike-slip faults at upper-to-mid-crustal levels. Along the major, southeastward propagating Macamic D2 dextral strike-slip fault, Theological and preexisting D1 structural heterogeneities induced the development of NNW-trending dextral-oblique splays which evolved into an extensional trailing fan and created an extensional, NNW-dipping stepover. Magma flowing upwards from deeper parts of the Macamic Fault spread towards the southeast at upper crustal levels along both the oblique-slip and extensional D2 splays, and built several plutons in a pull-apart domain between 2696 and 2690 Ma. Different emplacement and material transfer mechanisms operated simultaneously in different parts of the system, including fault dilation and wedging, lateral expansion, wall-rock ductile flow and stoping. Transfer of movement between D2 splays occurred under ductile conditions during syn-emplacement, amphibolite-grade metamorphism (500-700 °C). During cooling (< 2690 Ma), narrower brittle-ductile zones of greenschist-grade shearing were concentrated along the pluton-wall rock contacts, but the extensional stepover locked since both normal and reverse movements occurred along NNW-dipping faults. Pluton emplacement, contact metamorphism and propagation of D2 faults appear to have been closely linked during the Superior Province-wide late transpressional event.

Structure characterization of MHEMT heterostructure elements with In0.4Ga0.6As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

NASA Astrophysics Data System (ADS)

Aleshin, A. N.; Bugaev, A. S.; Ermakova, M. A.; Ruban, O. A.

2016-03-01

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In0.4Ga0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In x Ga1- x As ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.

Extensional faulting in the southern Klamath Mountains, California

USGS Publications Warehouse

Schweickert, R.A.; Irwin, W.P.

1989-01-01

Large northeast striking normal faults in the southern Klamath Mountains may indicate that substantial crustal extension occurred during Tertiary time. Some of these faults form grabens in the Jurassic and older bedrock of the province. The grabens contain continental Oligocene or Miocene deposits (Weaverville Formation), and in two of them the Oligocene or Miocene is underlain by Lower Cretaceous marine formations (Great Valley sequence). At the La Grange gold placer mine the Oligocene or Miocene strata dip northwest into the gently southeast dipping mylonitic footwall surface of the La Grange fault. The large normal displacement required by the relations at the La Grange mine is also suggested by omission of several kilometers of structural thickness of bedrock units across the northeast continuation of the La Grange fault, as well as by significant changes in bedrock across some northeast striking faults elsewhere in the Central Metamorphic and Eastern Klamath belts. The Trinity ultramafic sheet crops out in the Eastern Klamath terrane as part of a broad northeast trending arch that may be structurally analogous to the domed lower plate of metamorphic core complexes found in eastern parts of the Cordillera. The northeast continuation of the La Grange fault bounds the southeastern side of the Trinity arch in the Eastern Klamath terrane and locally cuts out substantial lower parts of adjacent Paleozoic strata of the Redding section. Faults bounding the northwestem side of the Trinity arch generally trend northeast and juxtapose stacked thrust sheets of lower Paleozoic strata of the Yreka terrane against the Trinity ultramafic sheet. Geometric relations suggest that the Tertiary extension of the southern Klamath Mountains was in NW-SE directions and that the Redding section and the southern part of the Central Metamorphic terrane may be a large Tertiary allochthon detached from the Trinity ultramafic sheet. Paleomagnetic data indicate a lack of rotation about a vertical axis during the extension. We propose that the Trinity ultramafic sheet is structurally analogous to a metamorphic core complex; if so, it is the first core complex to be described that involves ultramafic rocks. We infer that Mesozoic terrane accretion produced a large gravitational instability in the crust that spread laterally during Tertiary extension

Multi-stage metamorphism in the South Armenian Block during the Late Jurassic to Early Cretaceous: Tectonics over south-dipping subduction of Northern branch of Neotethys

NASA Astrophysics Data System (ADS)

Hässig, M.; Rolland, Y.; Sahakyan, L.; Sosson, M.; Galoyan, G.; Avagyan, A.; Bosch, D.; Müller, C.

2015-04-01

The geologic evolution of the South Armenian Block (SAB) in the Mesozoic is reconstructed from a structural, metamorphic, and geochronologic study including U-Pb and 40Ar/39Ar dating. The South Armenian Block Crystalline Basement (SABCB) outcrops solely in a narrow tectonic window, NW of Yerevan. The study of this zone provides key and unprecedented information concerning closing of the Northern Neotethys oceanic domain north of the Taurides-Anatolides platform from the Middle Jurassic to the Early Cretaceous. The basement comprises of presumed Proterozoic orthogneiss overlain by metamorphosed pelites as well as intrusions of granodiorite and leucogranite during the Late Jurassic and Early Cretaceous. Structural, geochronological and petrological observations show a multiphased evolution of the northern margin of the SAB during the Late Jurassic and Early Cretaceous. A south-dipping subduction under the East Anatolian Platform-South Armenian Block (EAP-SAB) is proposed in order to suit recent findings pertaining emplacement of relatively hot subduction related granodiorite as well as the metamorphic evolution of the crystalline basement in the Lesser Caucasus area. The metamorphism is interpreted as evidencing: (1) M1 Barrovian MP-MT conditions (staurolite-kyanite) at c. 157-160 Ma and intrusion of dioritic magmas at c. 150-156 Ma, (2) near-adiabatic decompression is featured by partial melting and production of leucogranites at c. 153 Ma, followed by M2 HT-LP conditions (andalusite-K-feldspar). A phase of shearing and recrystallization is ascribed to doming at c. 130-150 Ma and cooling at 400 °C by c. 123 Ma (M3). Structural observations show (1) top to the north shearing during M1 and (2) radial extension during M2. The extensional event ends by emplacement of a thick detrital series along radial S, E and W-dipping normal faults. Further, the crystalline basement is unconformably covered by Upper Cretaceous-Paleocene series dated by nannofossils, evolving from Maastrichtian marly sandstones to Paleocene limestones.

The gabbros and associated hornblende rocks occurring in the neighborhood of Baltimore, Maryland

USGS Publications Warehouse

Williams, George Huntington

1886-01-01

Considerable attention has been devoted during late years to the metamorphism of igneous rocks, and it can now be regarded as placed beyond reasonable doubt that such rocks may be changed to more or less schistose masses, which often closely resemble crystallized sediments. This possibility has heretofore been largely ignored, owing doubtless to the extensive obliteration of those characteristics which are generally regarded as most typical of eruptive rocks. Schistose or banded structure, however, can now hardly be considered as necessarily an indication of sedimentary origin. The minerals which are most characteristic of the so-called crystalline schists have been repeatedly shown to be derived from the alteration of igneous as well as of aqueous formations. These minerals only represent the final and most stable combination of certain elements under certain conditions, and are quite independent of the earlier combinations in which these elements may have existed. A lava bed and a clay bank, if the two may be supposed to have originally had the same chemical composition, might, under the influence of the same metamorphic agencies, ultimately give rise to the same rock in spite of original differences in structure or mineralogical composition. Stratification may be obliterated by metamorphism, while foliation, or even a banded structure, may, by the same means, be secondarily induced. Neither structure nor mineral composition can be taken as an infallible guide in determining the origin or the age of rocks.The present paper is intended as a contribution to our knowledge of a particular phase of metamorphism in eruptive rocks, i. e., that one which is dependent on the secondary development of hornblende by the paramorphism or pseudomorphism of pyroxene. This is a change the frequency of which renders it of fundamental geological importance. It has already received considerable attention from many eminent geologists, but no locality heretofore studied seems to have afforded opportunities for tracing out every stage in the process of alteration superior to those offered by the area of massive rocks west and northwest of the city of Baltimore. Here, covering a district of over fifty square miles, the unchanged pyroxene rock and its resultant hornblendic equivalent occur in the most intimate relations. Exposures of both rocks in situ are numerous, and the opportunity of following out the gradual transition of one into the other is proportionately great.

Intermediate P/T-type regional metamorphism of the Isua Supracrustal Belt, southern west Greenland: The oldest Pacific-type orogenic belt?

NASA Astrophysics Data System (ADS)

Arai, Tatsuyuki; Omori, Soichi; Komiya, Tsuyoshi; Maruyama, Shigenori

2015-11-01

The 3.7-3.8 Ga Isua Supracrustal Belt (ISB), southwest Greenland, might be the oldest accretionary complex on Earth. Regional metamorphism of the ISB has a potential to constrain the tectonothermal history of the Earth during the Eoarchean. Chemical and modal analyses of metabasite in the study area (i.e., the northeast part of the ISB) show that the metamorphic grade increases from greenschist facies in the northern part of the study area to amphibolite facies in the southern part. To determine the precise metamorphic P-T ranges, isochemical phase diagrams of minerals of metabasite were made using Perple_X. A synthesis of the estimated metamorphic P-T ranges of the ISB indicates that both the metamorphic pressure and temperature increase systematically to the south in the study area from 3 kbar and 380 °C to 6 kbar and 560 °C. The monotonous metamorphic P-T change suggests that the northeast part of the ISB preserves regional metamorphism resulting from the subduction of an accretionary complex although the ISB experienced metamorphic overprints during the Neoarchean. Both the presence of the regional metamorphism and an accretionary complex having originating at subduction zone suggest that the ISB may be the oldest Pacific-type orogenic belt. The progressive metamorphism can be considered as a record of intermediate-P/T type geothermal gradient at the subduction zone in the Eoarchean. Intermediate-P/T type geothermal gradient is typical at the current zones of subducting young oceanic crust, such as in the case of the Philippine Sea Plate in the southwest part of Japan. Considering the fact that almost all metamorphisms in the Archean are greenschist-amphibolite facies, the intermediate-P/T type geothermal gradient at the ISB might have been worldwide in the Archean. This would indicate that the subduction of young micro-plates was common because of the vigorous convection of hot mantle in the Archean.

Metamorphic P-T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex, NE China

NASA Astrophysics Data System (ADS)

Kabir, Md. Fazle; Takasu, Akira; Li, Weimin

2018-05-01

In the Gotsu area of the c. 200 Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic schists. Prograde, peak, and retrograde stages are distinguished in the blueschists, and the prograde and the peak metamorphic conditions are determined using pseudosection modelling in the NCKFMASHO system. The prograde metamorphic stage is defined by inclusions in porphyroblastic epidote and glaucophane, such as phengite, chlorite, albite, epidote and glaucophane/winchite, and the estimated metamorphic conditions are <325 °C and < 4-5 kbar at the boundary between the glaucophane schist facies and the greenschist facies. The peak metamorphic stage is well-defined by the schistosity-forming minerals, i.e. epidote, glaucophanic amphibole, phengite, and chlorite, suggesting the glaucophane schist facies conditions of 475-500 °C and 14-16 kbar. Actinolite/magnesiohornblende, chlorite, and albite replacing the peak stage minerals suggest the retrograde metamorphism into the greenschist facies. The metamorphic facies series of the Suo belt is defined by pumpellyite-actinolite facies to epidote-blueschist facies, and it has a relatively lower-P/T compared with the c. 300 Ma Renge belt in the Inner Zone of southwest Japan, which is defined by a sequence of lawsonite-blueschist facies to glaucophane-eclogite facies. The P- {M}_{{H}_2O} pseudosection and water isopleth show that the rocks were dehydrated during the initial stage of the exhumation and remained in water-saturated conditions. Similarities of the detrital zircon and peak metamorphic ages of the blueschists from the Suo metamorphic belt in southwest Japan and the Heilongjiang Complex in northeast China suggest that both metamorphic belts were probably formed in the same Paleo-Pacific subduction system in the Late Triassic to Jurassic period.

On the preservation mechanism of intragranular coesite in the Yangkou, Sulu UHP eclogite

NASA Astrophysics Data System (ADS)

Wang, L.; Zhang, J.; Wang, S.; Shi, F.; Cen, Y.

2012-12-01

Yangkou Bay, in the Sulu ultrahigh-pressure (UHP) belt of eastern China is the only known locality in the world in which UHP eclogite contains intragranular coesite. The question remains then, how is the extremely rare occurrence of intragranular coesite preserved in the Sulu belt, and if we can identify the reasons for its preservation, might it be found in other UHP belts? Preservation of coesite inclusions or intragranular coesite has been interpreted to result from multiple reasons, but lack of fluid availability is a critical factor, and the survival is not only because of their incorporation in a strong host phase but because of the ability of the host to prevent fluid infiltration until fracturing occurs at low temperatures. High-precision field structural geology mapping in the Yangkou area has revealed the complex deformation history including multi-stage folding events. The earliest folding event occurred before the eclogite reached peak metamorphism, and is preserved as rootless F1 isoclines that preserve the earliest evidence for UHP metamorphism, including intragranular coesite. We report the structural and petrological phenomenon that the intragranular coesite is uniquely preserved within the hinge zones of F1 rootless eclogite folds that have a mineral assemblage of Grt+Omp+Rt+Cs. However, the limbs of F1 folds or overprinted F1+F2 folds have a mineral assemblage of Grt+Omp+Rt+Qtz+Phg, and experienced different degrees of retrogression. The peak metamorphic P-T condition for coesite-bearing eclogite is P=4.0-4.5GPa, 745-909°C. however, the peak metamorphic P-T condition for phengite-quartz bearing eclogite is 3.8-4.1GPa, 733-840°C. The hydrogen concentration was investigated by FTIR (Fourier Transform Infrared Spectroscopy) in these two samples. In the intragranular coesite eclogite, coesite is basically free of water (<10 ppm), very low in garnet (8-50 ppm) and average hydrogen concentration of omphacite is 106-200 ppm; however, with the appearance of phengite and coesite's transformation into quartz, the water content within phengite bearing eclogite is raised. The average hydrogen concentration of garnet is 26-157 ppm , that of omphacite is 270-405 ppm. This indicates that with the appearance of phengite and retrogression, the water content of eclogite is raised up gradually, but at this stage, the eclogite is still within a localized "dry" environment compared to most water-rich (1000-2000ppm) eclogite in the Dabie-Sulu orogen. Through the structural and petrological analysis, we find that the intragranular coesite is more likely preserved within the F1 rootless isoclinal folds which formed prior to the peak metamorphism. This suggests that F1 rootless folds act as rigid and impervious shells under ultrahigh-pressure conditions, and shelter the eclogite from the surrounding fluid, which is helpful to maintain a relatively dry environment during rapid exhumation of UHP rocks, allowing the intergranular coesite to be preserved. This mechanism may have important implications for understanding the prograde path of continental-continental collision, and understanding the structural setting of the preserved intergranular coesite, which has important implications for the search for the early fabrics and metamorphic assemblages in other UHP-terrains around the world.

Origin and geodynamic significance of the early Mesozoic Weiya LP and HT granulites from the Chinese Eastern Tianshan

NASA Astrophysics Data System (ADS)

Mao, Ling-Juan; He, Zhen-Yu; Zhang, Ze-Ming; Klemd, Reiner; Xiang, Hua; Tian, Zuo-Lin; Zong, Ke-Qing

2015-12-01

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U-Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic-pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet-cordierite-sillimanite-bearing granulites, cordierite-sillimanite-bearing granulites, cordierite-orthopyroxene-bearing granulites, and orthopyroxene-clinopyroxene-bearing granulites. Average P-T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600-650 °C at 3.2-4.2 kbar and peak metamorphism of 750-840 °C at 2.9-6.3 kbar, indicating a rather high geothermal gradient of ca. 60 °C/km. Zircon U-Pb LA-ICP-MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380 Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution.

The Grenville orogeny in the Llano Uplift, Texas: A record of collision and contraction along the southern margin of North America

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reese, J.F.

1993-02-01

Precambrian metamorphic rocks in the SE Llano Uplift record NE-directed ductile thrusting and regional-scale polyphase folding. This deformation is in response to Grenville-age shortening and crustal thickening associated with the collision of a south-lying tectonic block with the southern margin of North America. In the SE Llano Uplift, the most intense and pervasive deformational event, D2, is characterized in the Packsaddle Schist (PS) and Valley Spring Gneiss (VSG) by SSE-plunging, NE-verging isoclinal folds (F2) with an associated SW-dipping axial planar metamorphic layering (S2), and SW-dipping mylonite zones with kinematic indicators showing top-to-the-NE motion. In the Red Mountain and augen-bearing Bigmore » Branch gneisses, D2 structures are SW-dipping mylonite zones parallel to S2, and a SW-plunging stretching lineation. Taken together, this suite of structures indicates tectonic transport was to the NE, perpendicular to the NW trending regional structural and metamorphic grain. D2 structures were reoriented by at least two later phases of folding. Timing of all ductile deformation in the SE Llano Uplift is constrained from post-1,215 Ma (deformed PS) to pre-1,098 Ma (undeformed melarhyolite dike). From south to north, metatonalitic, arc-derived Big Branch Gneiss ([approximately] 1,303 Ma) and older mafic schist country rock, previously interpreted as possible ophiolitic melange, structurally overlie much younger, lithologically heterogeneous PS units (1,248-1,215 Ma), previously considered as arc flank deposits. In turn, the PS has been tectonically emplaced above the predominantly felsic VSG (1,270-1,232 Ma). The presence of older zircons in the VSG, of similar age ([approximately]1360 Ma) to Western Granite-Rhyolite Terrane rocks to the north, suggests that the VSG formed in a settling proximal to North America.« less

Zircon growth in shear zones

NASA Astrophysics Data System (ADS)

Kaulina, Tatiana

2013-04-01

The possibility of direct dating of the deformation process is critical for understanding of orogenic belts evolution. Establishing the age of deformation by isotopic methods is indispensable in the case of uneven deformation overlapping, when later deformation inherits the structural plan of the early strains, and to distinguish them on the basis of the structural data only is impossible. A good example of zircon from the shear zones is zircon formed under the eclogite facies conditions. On the one hand, the composition of zircon speaks about its formation simultaneously to eclogitic paragenesis (Rubatto, Herman, 1999; Rubatto et al., 2003). On the other hand, geological studies show that mineral reactions of eclogitization are often held only in areas of shear deformations, which provides access of fluid to the rocks (Austrheim, 1987; Jamtveit et al., 2000; Bingen et al., 2004). Zircons from mafic and ultramafic rocks of the Tanaelv and Kolvitsa belts (Kola Peninsula, the Baltic Shield) have showed that the metamorphic zircon growth is probably controlled by the metamorphic fluid regime, as evidenced by an increase of zircon quantity with the degree of shearing. The internal structure of zircon crystals can provide an evidence of zircon growth synchronous with shearing. The studied crystals have a sector zoning and often specific "patchy" zoning (Fig. 1), which speaks about rapid change of growth conditions. Such internal structure can be compared with the "snowball" garnet structure reflecting the rotation of crystals during their growth under a shift. Rapidly changing crystallization conditions can also be associated with a small amount of fluid, where supersaturation is changing even at a constant temperature. Thus, the growth of metamorphic zircon in shear zones is more likely to occur in the fluid flow synchronous with deformation. A distinctive feature of zircons in these conditions is isometric shape and sector "patchy" zoning. The work was supported by Russian Foundation of Basic Research (project: 13-05-00035.) and the DES-6 program.

Geologic Map of the Upper Wolf Island Creek Watershed, Reidsville Area, Rockingham County, North Carolina

USGS Publications Warehouse

Horton, J. Wright; Geddes, Donald J.

2006-01-01

This geologic map provides a foundation for hydrogeologic investigations in the Reidsville area of Rockingham County, north-central North Carolina. The 16-mi2 area within the Southeast Eden and Reidsville 7.5-min quadrangles includes the watershed of Wolf Island Creek and its tributary, Carroll Creek, upstream of their confluence. Layered metamorphic rocks in this area of the Milton terrane, here informally named the Chinqua-Penn metamorphic suite, include a heterogeneous mica gneiss and schist unit that contains interlayers and lenses of white-mica schist, felsic gneiss, amphibolite, and ultramafic rock; a felsic gneiss that contains interlayers of amphibolite, white-mica schist, and minor ultramafic lenses; and a migmatitic biotite gneiss. Crushed stone is produced from an active quarry in the felsic gneiss. Igneous intrusive rocks include a mafic-ultramafic assemblage that may have originated as mafic intrusive bodies containing ultramafic cumulates, a foliated two-mica granite informally named the granite of Reidsville, and unmetamorphosed Jurassic diabase dikes. The newly recognized Carroll Creek shear zone strikes roughly east-west and separates heterogeneous mica gneiss and schist to the north from structurally overlying felsic gneiss to the south. Regional amphibolite-facies metamorphism accompanied polyphase ductile deformation in the metamorphic rocks. Two phases of isoclinal to tight folding and related penetrative deformation, described as D1 and D2, were followed by phases of high-strain mylonitic deformation in shear zones and late gentle to open folding. Later brittle deformation produced minor faults, steep joints, foliation-parallel parting, and sheeting joints. The metamorphic and igneous rocks are mantled by saprolite and residual soil derived from weathering of the underlying bedrock, and unconsolidated Quaternary alluvium occupies the flood plains of Wolf Island Creek and its tributaries. The geologic map delineates lithologic and structural features that may act as barriers or conduits for ground-water flow. It provides a hydrogeologic framework for the upper Wolf Island Creek drainage basin, including coreholes and ground-water monitoring wells along two transects. Collaborative hydrogeologic investigations by the North Carolina Department of Environment and Natural Resources and the U.S. Geological Survey are in progress to increase understanding of the influence of geological features on ground-water quality, availability, and transport in an area representative of large areas in the west-central Piedmont.

Evolution of the Specific Surface Area of Snow in a High Temperature Gradient Metamorphism

NASA Astrophysics Data System (ADS)

Wang, X.; Baker, I.

2014-12-01

The structural evolution of low-density snow under a high temperature gradient over a short period usually takes place in the surface layers during diurnal recrystallization or on a clear, cold night. To relate snow microstructures with their thermal properties, we combined X-ray computed microtomography (micro-CT) observations with numerical simulations. Different types of snow were tested over a large range of TGs (100 K m-1- 500 K m-1). The Specific Surface Area (SSA) was used to characterize the temperature gradient metamorphism (TGM). The magnitude of the temperature gradient and the initial snow type both influence the evolution of SSA. The SSA evolution under TGM was dominated by grain growth and the formation of complex surfaces. Fresh snow experienced a logarithmic decrease of SSA with time, a feature been observed previously by others [Calonne et al., 2014; Schneebeli and Sokratov, 2004; Taillandier et al., 2007]. However, for initial rounded and connected snow structures, the SSA will increase during TGM. Understanding the SSA increase is important in order to predict the enhanced uptake of chemical species by snow or increase in snow albedo. Calonne, N., F. Flin, C. Geindreau, B. Lesaffre, and S. Rolland du Roscoat (2014), Study of a temperature gradient metamorphism of snow from 3-D images: time evolution of microstructures, physical properties and their associated anisotropy, The Cryosphere Discussions, 8, 1407-1451, doi:10.5194/tcd-8-1407-2014. Schneebeli, M., and S. A. Sokratov (2004), Tomography of temperature gradient metamorphism of snow and associated changes in heat conductivity, Hydrological Processes, 18(18), 3655-3665, doi:10.1002/hyp.5800. Taillandier, A. S., F. Domine, W. R. Simpson, M. Sturm, and T. A. Douglas (2007), Rate of decrease of the specific surface area of dry snow: Isothermal and temperature gradient conditions, Journal of Geophysical Research: Earth Surface (2003-2012), 112(F3), doi: 10.1029/2006JF000514.

Problems with the concept of deformation phases as illustrated for the Goantagab Domain, NW Namibia

NASA Astrophysics Data System (ADS)

Passchier, C. W.

2010-12-01

The concept of deformation phases is one of the corner stones of structural geology and is used to reconstruct tectonic history in all metamorphic rocks. Despite its simplicity, however, there are situations where the concept breaks down. The junction of the Neoproterozoic-Cambrian Kaoko and Damara Belts in the well-exposed desert of Namibia is ideally suited for a critical assessment of our use of the deformation phase concept. Metaturbidites and granite intrusions in the Goantagab Domain at the junction of the belts record the amalgamation of the Congo, Kalahari and Rio de la Plata Cratons. The local structure is complicated, with km-scale sheath folds, and despite perfectly exposed geology over a large area, could only be reconstructed by detailed structural mapping. Structures can be subdivided into at least four sets, attributed to four deformation phases on the basis of overprinting relations. Three of these sets of structures, however, formed during the same tectonic event under similar metamorphic conditions but slightly different flow regime. These sets show unusual gradational “ring” transitions in space, where older DA structures are reoriented and overprinted by new structures DA+1 that have similar orientation, and seem to grade into DA structures outside the overprinted area. In the core of the Goantagab Domain, D2 is thus reoriented and overprinted by local D2b folds and foliations that have the same orientation and style as D2 structures outside the domain core. This kind of behaviour may be common in inhomogeneous non-coaxial flow in other, less well exposed terrains and would go there unnoticed, leading to erroneous interpretations. An additional general problem is that the geometry of critical structures is laterally highly variable because of changes in (1) lithology; (2) previous structure; (3) metamorphic conditions (4) orientation and geometry of stress and flow tensors and (5) finite strain magnitude. Of these, only (2) and (4) are relevant to understand local tectonics, while the other effects have to be filtered out. Work in the Goantagab Domain shows how such “expressions” of deformation can be organised. Foliation traces in metaturbidites of the Goantagab Domain, central Namibia. S2 and S2b show partially overlapping "ring" transitions

Processes in continental collision zones: Preface

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhang, Lifei; McClelland, William C.; Cuthbert, Simon

2012-04-01

Formation and exhumation of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in continental subduction zones are the two fundamental geodynamic aspects of collisional orogensis. This volume is based on the Session 08c titled "Geochemical processes in continental collision zones" at Goldschmidt 2010 in Knoxville, USA. It focuses on micro- to macro-scale processes that are temporally and spatially linked to different depths of crustal subduction/exhumation and associated mineralogical changes. They are a key to understanding a wide spectrum of phenomena, involving HP/UHP metamorphism and syn-/post-collisional magmatism. Papers in this volume report progresses in petrological, geochronological and geochemical studies of UHP metamorphic rocks and their derivatives in China, with tectonic settings varying from arc-continent collision to continent-continent collision. Microbeam in-situ analyses of metamorphic and magmatic minerals are successfully utilized to solve various problems in the study of continental deep subduction and UHP metamorphism. In addition to their geochronological applications to dating of HP to UHP metamorphic events during continental collision, microbeam techniques have also served as an efficient means to recognize different generations of mineral growth during continental subduction-zone metamorphism. Furthermore, metamorphic dehydration and partial melting of UHP metamorphic rocks during subduction and exhumation are highlighted with respect to their effects on fluid action and element mobilization. These have provided new insights into chemical geodynamics in continental subduction zones.

Low-Ca pyroxenes from LL group chondritic meteorites: Crystal structural studies and implications for their thermal histories

NASA Astrophysics Data System (ADS)

Artioli, G.; Davoli, G.

1994-12-01

Crystal structural refinements of one orthorhombic (Pbca) and two monoclinic (P21/c) single crystals, from chondrules of low-Ca pyroxenes from unequilibrated chondritic meteorites of the LL group, were carried out. The intracrystalline Fe-Mg distribution between the M1 and M2 crystallographic sites of the Parnallee (LL-3) orthoenstatite is suggestive of very rapid cooling, whereas both the structural state and intracrystalline Fe-Mg distribution in the Soko Banja (LL-4) and Jolomba (LL-6) clinoenstaties indicate rapid cooling from the high temperature polymorphs, with no significant re-equilibration at lower temperatures. These results imply that thermal metamorphism in the parent body, if present, was insufficient to allow re-equilibration of the pyroxene minerals to low temperature, ordered crystal structures. The data also indicate that, assuming low or mild pressure and shock effects, there is no well defined correlation between equilibrium temperature of the mineral phases and the alleged petrologic type of the meteorites. This evidence is consistent with a rubble pile model for the parent body accretional history, or with an onion shell model with very low thermal peak metamorphism, as is assumed for a very small object.

Protracted or multiple subduction of metapelites (Rhodope UHP domain, Greece)?

NASA Astrophysics Data System (ADS)

Krohe, A.; Wawrzenitz, N. H.; Mposkos, E.; Romer, R. L.

2012-12-01

The Rhodope domain formed along the suture between the European and the Apulian/Adriatic plate, which collided in the early Tertiary (closure of the Vardar/Axios ocean). Its metamorphic history includes UHP metamorphism documented by diamond inclusions in garnet (Mposkos & Kostopoulos 2001, Perraki et al. 2006, Schmidt et al. 2010), presumably of Jurassic age, and Eocene stages of MP and HP metamorphism. The age of UHPM is still a matter of debate: U-Pb SHRIMP ages extend from 184-172 Ma (monazite in metapelites) to ca. 42 Ma with clusters at 170-160, 150-140, 80-60, 50, 42 Ma, (U-Pb SHRIMP dating of zircon from amphibolitized eclogites and metapelites). These ages are interpreted to date subsequent stages of (U)HP metamorphism and decompression (Liati et al., 2005, Hoinkes et al. 2008, Bauer et al. 2006, Krenn et al 2010). However, these ages are obviously difficult to link with the metamorphic reactions. The metamorphic history has been interpreted in different ways, reflecting: (i) successive accretion of small terranes with rapid subduction and uplift histories (e.g. Liati et al. 2005); (ii) a composite of different tectonic units varying in earlier P-T histories, assembled by shear zones that reflect tectonic erosion and differential exhumation along the plate interface and that are now erased and overprinted (Krohe and Mposkos, 2002, Mposkos et al., 2010). These interpretations imply a different kinematics of the tectonic movements at depths, mechanical processes and process rates. Additionally, a protracted polymetamorphic history of larger volumes of the Rhodope UHP domain may be considered; e.g. the Kimi complex stayed in the lower crust for ca. 50-60 Ma after exhumation of the UHP rocks to this lower crustal level (Mposkos and Krohe, 2006). To constrain a precise age of the HP granulite facies and a minimum age of UHP metamorphism, we conduct an integrated structural, petrologic and geochronological study in a metapelite from the Sidronero Complex. The mineral assemblages Grt-Ky-Bt-Pl-Kfs-Qtz-Rt and Grt-Ky-Bt-Ms-Pl-Qtz-Rt, record a HP granulite facies metamorphism followed by upper amphibolite facies. The rock is particularly well suited for studying the granulite facies metamorphism, as it contains domains that are only weakly overprinted by later metamorphic episodes. ID-TIMS U-Pb ages of single monazite grains and fractions of few grains, that are only locally patchy-zoned and associated with garnet and kyanite, plot along the concordia between 64 to 60 Ma. One date of 55 Ma might represent Pb-loss during later fluid-induced dissolution-reprecipitation, probably related to biotite growth during the amphibolite facies overprint. On the base of these data, a model is discussed, in which rocks from the upper plate and HP-rocks that have been already exhumed, were dragged again into the subduction channel by subduction erosion Bauer et al. 2006, Lithos, 29, 207-228; Hoinkes et al. 2008, 3rd IGC Oslo, UHP-4; Krenn et al 2010, Tectonics, 29, TC4001; Krohe & Mposkos, 2002, Geol. Soc. Sp. Pub. 204, 151-178; Liati, A., 2005, Contrib. Mineral. Petrol., 150, 608-630; Mposkos, & Kostopoulos, 2001, EPSL, 192, 497-506; Mposkos & Krohe, 2006. Can. J. Earth Sci., 43, 1755-1776; Mposkos et al., 2010 Proc. XIX CBGA Congress, 100, 173-178; Perraki et al., 2006, EPSL, 241, 672-685; Schmidt et al., 2010, EJM, 22, 189-198.

Characterising the metamorphic discontinuity across the Main Central Thrust Zone of eastern-central Nepal

NASA Astrophysics Data System (ADS)

Wang, Jiamin; Zhang, Jinjiang; Wei, Chunjing; Rai, SantaMan; Wang, Meng; Qian, Jiahui

2015-04-01

The Main Central Thrust Zone (MCTZ) is a top-to-south shear zone that has exhumed the high-grade Himalayan metamorphic core during the orogeny. Identifying the location of the MCTZ is a major challenge and the characteristics of the metamorphic discontinuity remain under debate. To clarify this issue, petrologic and thermobarometric studies were carried out on metapelites and metapsammites that were collected from the basal Nyalam transect in eastern-central Nepal. Results reveal that the metamorphic discontinuity across the MCTZ is characterised by a continuous increase in peak P-T conditions toward higher structural levels, a relatively high field temperature gradient (25-50 °C km-1) and different types of P-T paths. Specifically, representative rocks in the MCTZ record sub-solidus peak conditions (637 ± 16 °C and 9.2 ± 1.0 kbar) and a hairpin-type P-T path. The lower GHC rocks record supra-solidus peak conditions (690 ± 32 °C and 10.3 + 1.1/-1.4 kbar) and a prograde loading path with a small segment of decompression. The presence of a high field pressure gradient across the MCTZ is debatable in the Nyalam transect due to the large uncertainties in pressure estimates. Comparison between obtained P-T results and model predictions indicates that a multiple thrusting process dominated exhumation of the MCTZ and lower GHC rocks, while crustal flow contributed partly to exhumation of the lower GHC rocks.

Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California

USGS Publications Warehouse

Barth, A.P.; Wooden, J.L.; Coleman, D.S.; Vogel, M.B.

2009-01-01

The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time. ?? 2009 by The University of Chicago.

Metamorphism within the Chugach accretionary complex on southern Baranof Island, southeastern Alaska

USGS Publications Warehouse

Zumsteg, Cathy L.; Himmelberg, Glen R.; Karl, Susan M.; Haeussler, Peter J.

2003-01-01

On Baranof Island, southeastern Alaska, we identify four metamorphic events that affect rocks associated with the Chugach accretionary complex. This study focuses on the M1 and M4 metamorphic events. Mesozoic schists, gneisses, and migmatitic gneisses exposed near the Kasnyku pluton on central Baranof Island represent the M1 metamorphic rocks. These rocks underwent amphibolite facies metamorphism. Calculated temperatures and pressures range from about 620 to 780 ºC and 5.5 to 6.6 kbar and are compatible with the observed metamorphic mineral assemblages.The M4 metamorphism affected rocks of the Sitka Graywacke on southern Baranof Island, producing extensive biotite and garnet zones as well as andalusite and sillimanite zones at the contacts of the Crawfish Inlet and Redfish Bay plutons. Calculated M4 temperatures and pressures from the andalusite and sillimanite zones range from 575 to 755 ºC and 3.4 to 6.9 kbar. These results fall within the sillimanite stability field, at pressures higher than andalusite stability. These results may indicate the M4 metamorphic event occurred along a P-T path along which the equilibration of aluminosilicate-garnet-plagioclase-quartz did not occur or was not maintained. This interpretation is supported by the occurrence of andalusite and sillimanite within the same sample. We propose the data reflect a clockwise P-T path with peak M4 metamorphism of the sillimanite-bearing samples adjacent to the intrusions at an approximate depth of 15 to 20 km, followed by rapid uplift without reequilibration of garnet-plagioclase-aluminosilicate-quartz.The large extent of the biotite zone, and possibly the garnet zone, suggests that an additional heat source must have existed to regionally metamorphose these rocks during the M4 event. We suggest the M4 regional thermal metamorphism and intrusion of the Crawfish Inlet and Redfish Bay plutons were synchronous and the result of heat flux from a slab window beneath the accretionary complex at that time. If our conclusions regarding the effect of the slab window are correct, the style of metamorphism is different from the Chugach metamorphic complex, which is clearly linked to a slab window. Therefore, our findings would suggest that there is no distinct metamorphic signature for slab window effects.

Dry Snow Metamorphism

DTIC Science & Technology

2012-09-19

behavior of snow during metamorphism and grain sintering using mathematical models. 2 Approach Our approach involved the collection and...examination of both types of specimens at various stages of metamorphism using the SEM and micro-CT. More specifically, the above approach involved...than 10ºC·m-1). 5. High-resolution images and X-ray spectra of snow specimens at various metamorphism stages were obtained using an SEM and EDS. 6

Late Cretaceous tectonothermal evolution of the southern Lhasa terrane, South Tibet: Consequence of a Mesozoic Andean-type orogeny

NASA Astrophysics Data System (ADS)

Dong, Xin; Zhang, Ze-ming; Klemd, Reiner; He, Zhen-yu; Tian, Zuo-lin

2018-04-01

The Lhasa terrane of the southern Tibetan Plateau participated in a Mesozoic Andean-type orogeny caused by the northward subduction of the Neo-Tethyan oceanic lithosphere. However, metamorphic rocks, which can unravel details of the geodynamic evolution, are rare and only exposed in the south-eastern part of the Lhasa terrane. Therefore, we conducted a detailed petrological, geochemical and U-Pb zircon geochronological study of the late Cretaceous metamorphic rocks and associated gabbros from the Nyemo inlier of the southern Lhasa terrane. The Nyemo metamorphic rocks including gneisses, schists, marbles and calc-silicate rocks, experienced peak amphibolite-facies contact metamorphism under P-T conditions of 3.5-4.0 kbar and 642-657 °C with a very high geothermal gradient of 45-50 °C/km, revealing a distinct deflection from the steady-state geotherm during low-pressure metamorphism. Inherited magmatic zircon cores from the metamorphic rocks yielded protolith ages of 197-194 Ma, while overgrowth zircon rims yielded metamorphic ages of ca. 86 Ma. Whole-rock chemistry and zircon Hf isotopes suggest that the protoliths of the gneisses and schists are andesites and tuffs of the early Jurassic Sangri Group, which were derived from a depleted mantle source of a continental arc affinity. The coeval intimately-associated gabbro (ca. 86 Ma) crystallized under P-T conditions of 3.5-5.3 kbar and 914-970 °C, supplying the heat flux high enough to cause the contact metamorphism of the Sangri Group rock types. We propose that the intrusion of the gabbro and a simultaneous pressure increase of up to 4.0 kbar, which is related to crustal thickening due to crustal overthrusting and the intrusion of mafic material, resulted in the late Cretaceous metamorphism of the early Jurassic Sangri Group during an Andean-type orogeny. Furthermore the Nyemo metamorphic rocks, which have previously been considered to represent slivers of the Precambrian metamorphic basement of the Lhasa terrane, are late Cretaceous metamorphic supracrustal rocks.

Growth and characterization of metamorphic InAs/GaSb tunnel heterojunction on GaAs by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Jheng-Sin; Clavel, Michael B.; Hudait, Mantu K., E-mail: mantu.hudait@vt.edu

The structural, morphological, optical, and electrical transport characteristics of a metamorphic, broken-gap InAs/GaSb p-i-n tunnel diode structure, grown by molecular beam epitaxy on GaAs, were demonstrated. Precise shutter sequences were implemented for the strain-balanced InAs/GaSb active layer growth on GaAs, as corroborated by high-resolution X-ray analysis. Cross-sectional transmission electron microscopy and detailed micrograph analysis demonstrated strain relaxation primarily via the formation of 90° Lomer misfit dislocations (MDs) exhibiting a 5.6 nm spacing and intermittent 60° MDs at the GaSb/GaAs heterointerface, which was further supported by a minimal lattice tilt of 180 arc sec observed during X-ray analysis. Selective area diffraction and Fastmore » Fourier Transform patterns confirmed the full relaxation of the GaSb buffer layer and quasi-ideal, strain-balanced InAs/GaSb heteroepitaxy. Temperature-dependent photoluminescence measurements demonstrated the optical band gap of the GaSb layer. Strong optical signal at room temperature from this structure supports a high-quality material synthesis. Current–voltage characteristics of fabricated InAs/GaSb p-i-n tunnel diodes measured at 77 K and 290 K demonstrated two bias-dependent transport mechanisms. The Shockley–Read–Hall generation–recombination mechanism at low bias and band-to-band tunneling transport at high bias confirmed the p-i-n tunnel diode operation. This elucidated the importance of defect control in metamorphic InAs/GaSb tunnel diodes for the implementation of low-voltage and high-performance tunnel field effect transistor applications.« less

Metamorphic P-T-t path retrieved from metapelites in the southeastern Taihua metamorphic complex, and the Paleoproterozoic tectonic evolution of the southern North China Craton

NASA Astrophysics Data System (ADS)

Lu, Jun-Sheng; Zhai, Ming-Guo; Lu, Lin-Sheng; Wang, Hao Y. C.; Chen, Hong-Xu; Peng, Tao; Wu, Chun-Ming; Zhao, Tai-Ping

2017-02-01

The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9 Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite + plagioclase + quartz + ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite + biotite + plagioclase + quartz + K-feldspar + ilmenite + rutile + pyrite. The retrograde mineral assemblage (M3), biotite + plagioclase + quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P-T conditions of 12.2 kbar and 830 °C, whereas garnet rims record P-T conditions of 10.2 kbar and 840 °C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P-T path can be retrieved. High resolution SIMS U-Pb dating and LA-ICP-MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92 Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83 Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous model in which the Tran-North China Craton (TNCO) was formed through the collision between the East and West blocks.

Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Williams, Ian S.; Buick, Ian S.

2001-01-01

We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U-Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade, at ˜600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557-1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750-800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U-Pb system in relatively large monazite crystals can exceed 750-800 °C.

Mercury contamination in agricultural soils from abandoned metal mines classified by geology and mineralization.

PubMed

Kim, Han Sik; Jung, Myung Chae

2012-01-01

This survey aimed to compare mercury concentrations in soils related to geology and mineralization types of mines. A total of 16,386 surface soils (0~15 cm in depth) were taken from agricultural lands near 343 abandoned mines (within 2 km from each mine) and analyzed for Hg by AAS with a hydride-generation device. To meaningfully compare mercury levels in soils with geology and mineralization types, three subclassification criteria were adapted: (1) five mineralization types, (2) four valuable ore mineral types, and (3) four parent rock types. The average concentration of Hg in all soils was 0.204 mg kg(-1) with a range of 0.002-24.07 mg kg(-1). Based on the mineralization types, average Hg concentrations (mg kg(-1)) in the soils decreased in the order of pegmatite (0.250) > hydrothermal vein (0.208) > hydrothermal replacement (0.166) > skarn (0.121) > sedimentary deposits (0.045). In terms of the valuable ore mineral types, the concentrations decreased in the order of Au-Ag-base metal mines ≈ base metal mines > Au-Ag mines > Sn-W-Mo-Fe-Mn mines. For parent rock types, similar concentrations were found in the soils derived from sedimentary rocks and metamorphic rocks followed by heterogeneous rocks with igneous and metamorphic processes. Furthermore, farmland soils contained relatively higher Hg levels than paddy soils. Therefore, it can be concluded that soils in Au, Ag, and base metal mines derived from a hydrothermal vein type of metamorphic rocks and pegmatite deposits contained relatively higher concentrations of mercury in the surface environment.

Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska

USGS Publications Warehouse

Bleick, Heather A.; Till, Alison B.; Bradley, Dwight C.; O’Sullivan, Paul; Wooden, Joe L.; Bradley, Dan B.; Taylor, Theresa A.; Friedman, Sam B.; Hults, Chad P.

2012-01-01

New geochronologic and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed ~60 Ma metamorphic rocks with the base of the Arkose Ridge Formation. U-Pb (analyzed by Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG)) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3±0.3 (previously considered a Jurassic unit), 79.1±1.0, 76.1±0.9, 75.8±0.7, 72.5±0.4, 71.9±0.3, 70.5±0.2, and 67.3±0.2 Ma. The cooling of these plutons occurred between 72 and 66 Ma (zircon fission track (FT) closure ~225°C). 40Ar/39Ar analyses of hornblende, white mica, and biotite fall into this range (Harlan and others, 2003). New apatite FT data collected on a west-to-east transect reveal sequential exhumation of fault blocks at 62.8±2.9, 54±2.5, 52.6±2.8, and 44.4±2.2 Ma. Plutonic clasts accumulated in the Paleocene Arkose Ridge Formation to the south. Detrital zircon (DZ) ages from the formation reflect this provenance: a new sample yielded one grain at 61 Ma, a dominant peak at 76 Ma, and minor peaks at 70, 80, 88, and 92 Ma. The oldest zircon is 181 Ma. Our apatite FT ages range from 35.1 to 50.9 Ma. Greenschist facies rocks now sit structurally between the plutonic rocks and the Arkose Ridge Formation. They are separated from plutonic rocks by the vertical Hatcher Pass fault and from the sedimentary rocks by a detachment fault. Ar cooling ages (Harlan and others, 2003) and new zircon FT ages for these rocks are concordant at 61-57 Ma, synchronous with deposition of the Arkose Ridge Formation. A cooling age of ~46 Ma came from one apatite FT sample. The metamorphic protolith (previously considered Jurassic) was deposited at or after 75 Ma based on new DZ data. The probability curve has a major peak from 76 to 102 Ma, minor peaks at 186, 197, 213, 303, 346, and 1,828, and two discordant grains at ~2,700 Ma. This is similar to DZ populations in the Valdez Group. The short period of time between deposition, metamorphism, and exhumation are consistent with metamorphism in a subduction-zone setting. Ductile and brittle structures in the metamorphic rocks are consistent with exhumation in a transtensional setting.

Tectonic and metamorphic discontinuities in the Greater Himalayan Sequence in Central Himalaya: in-sequence shearing by accretion from the Indian plate

NASA Astrophysics Data System (ADS)

Carosi, Rodolfo

2016-04-01

The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayas, stretching for over 2400 km, bounded to the South by the Main Central Thrust (MCT) and to the North by the South Tibetan Detachment (STD) whose contemporanous activity controlled its exhumation between 23 and 17 Ma (Godin et al., 2006). Several shear zones and/or faults have been recognized within the GHS, usually regarded as out of sequence thrusts. Recent investigations, using a multitechnique approach, allowed to recognize a tectonic and metamorphic discontinuity, localized in the mid GHS, with a top-to-the SW sense of shear (Higher Himalayan Discontinuity: HHD) (Carosi et al., 2010; Montomoli et al., 2013). U-(Th)-Pb in situ monazite ages provide temporal constraint of the acitivity of the HHD from ~ 27-25 Ma to 18-17 Ma. Data on the P and T evolution testify that this shear zone affected the tectono-metamorphic evolution of the belt and different P and T conditions have been recorded in the hanging-wall and footwall of the HHD. The HHD is a regional tectonic feature running for more than 700 km, dividing the GHS in two different portions (Iaccarino et al., 2015; Montomoli et al., 2015). The occurrence of even more structurally higher contractional shear zone in the GHS (above the HHD): the Kalopani shear zone (Kali Gandaki valley, Central Nepal), active from ~ 41 to 30 Ma (U-Th-Pb on monazite) points out to a more complex deformation pattern in the GHS characterized by in sequence shearing. The actual proposed models of exhumation of the GHS, based exclusively on the MCT and STD activities, are not able to explain the occurrence of the HHD and other in-sequence shear zones. Any model of the tectonic and metamorphic evolution of the GHS should account for the occurrence of the tectonic and metamorphic discontinuities within the GHS and its consequences on the metamorphic paths and on the assembly of Himalayan belt. References Godin L., Grujic D., Law, R. D. & Searle, M. P. 2006. Geol. Soc. London Sp. Publ., 268, 1-23. Carosi R., Montomoli C., Rubatto D. & Visonà D. 2010. Tectonics, 29, TC4029. Iaccarino S., Montomoli C., Carosi R., Massonne H-J., Langone A., Visonà D. 2015. Lithos, 231, 103-121. Montomoli C., Iaccarino S., Carosi R., Langone A. & Visonà D. 2013. Tectonophysics 608, 1349-1370, doi:10.1016/j.tecto.2013.06.006. Montomoli C., Carosi R., Iaccarino S. 2015. Geol. Soc. London Sp. Publ., 412, 25-41.

Beyond the Kubler index

USGS Publications Warehouse

Eberl, D.D.; Velde, B.

1989-01-01

The value of peak width at half-height for the illite 001 XRD reflection is known as the Kubler index or the illite 'crystallinity' index. This measurement, which has been related to the degree of metamorphism of very low-grade, pelitic rocks, is a function of at least two crystal-chemical factors: (1) illite X-ray scattering domain size; and (2) illite structural distortions (especially swelling). Reynolds' NEWMOD computer program is used to construct a grid with which these two contributions to illite peak width can be determined independently from measurements of the 001 peak width at half-height and the Srodofi intensity ratio. This method yields more information about changes undergone by illite during metamorphism than application of the Kubler index method alone.

Tectonic significance of Kibaran structures in Central and Eastern Africa

NASA Astrophysics Data System (ADS)

Rumvegeri, B. T.

Tectonical movements of the Kibaran belt (1400-950 Ma) can be subdivided into two major deformation events, corresponding to tight, upright or recumbent folds, thrust faults, nappes and stretching lineation with a general plunging southwards. At the regional scale, the stretching lineation, associated with thrust faults and nappes is interpreted as an indication of a northwards moving direction. The shear zone with mafic-ultramafic rocks across Burundi, MW-Tanzania, SW-Uganda and NE-Zaïre is the suture zone of the Kibaran belt. Kibaran metamorphism is plurifacial and has four epizodes. The second, syn-D2, is the most important and constitutes the climax; it reached the granulite facies. The succession of tectonic, metamorphic and magmatic features suggests geotectonic evolution by subduction-collision.

Growth and characterization of (110) InAs quantum well metamorphic heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Podpirka, Adrian A., E-mail: adrian.podpirka.ctr@nrl.navy.mil; Katz, Michael B.; Twigg, Mark E.

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000–16 000 cm{sup 2}/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in themore » quantum Hall regime.« less

The next chapter in experimental petrology: Metamorphic dehydration of polycrystalline gypsum captured in 3D microtomographic time series datasets

NASA Astrophysics Data System (ADS)

Bedford, John; Fusseis, Florian; Leclere, Henry; Wheeler, John; Faulkner, Dan

2016-04-01

Nucleation and growth of new minerals in response to disequilibrium is the most fundamental metamorphic process. However, our current kinetic models of metamorphic reactions are largely based on inference from fossil mineral assemblages, rather than from direct observation. The experimental investigation of metamorphism has also been limited, typically to concealed vessels that restrict the possibility of direct microstructural monitoring. Here we present one of the first time series datasets that captures a metamorphic reaction, dehydration of polycrystalline gypsum to form hemihydrate, in a series of three dimensional x-ray microtomographic datasets. We achieved this by installing an x-ray transparent hydrothermal cell (Fusseis et al., 2014, J. Synchrotron Rad. 21, 251-253) in the microtomography beamline 2BM at the Advanced Photon Source (USA). In the cell, we heated a millimetre-sized sample of Volterra Alabaster to 388 K while applying an effective pressure of 5 MPa. Using hard x-rays that penetrate the pressure vessel, we imaged the specimen 40 times while it reacted for approximately 10 hours. Each microtomographic dataset was acquired in 300 seconds without interrupting the reaction. Our absorption microtomographic data have a voxel size of 1.3 μm, which suffices to analyse the reaction progress in 4D. Gypsum can clearly be distinguished from hemihydrate and pores, which form due to the large negative solid volume change. On the resolved scale, the first hemihydrate needles appear after about 2 hours. Our data allow tracking of individual needles throughout the entire experiment. We quantified their growth rates by measuring their circumference. While individual grains grow at different rates, they all start slowly during the initial nucleation stage, then accelerate and grow steadily between about 200 and 400 minutes before reaction rate decelerates again. Hemihydrate needles are surrounded by porous haloes, which grow with the needles, link up and eventually encapsulate the remaining gypsum crystals. The reaction appears to be homogenously distributed throughout the sample and we find no evidence for metamorphic overpressure. We used an advanced machine learning algorithm (http://fiji.sc/Trainable_Weka_Segmentation) to segment the porosity from the microtomographic data and quantify it in Fiji (Schindelin et al., 2012, Nature Methods 9, 676-682). The porosity evolution follows the grain growth curves and reaches 23%, which indicates that the dehydration reaction is 80% complete. Our 4D data provide a unique opportunity not only to explore evolving reaction microtextures in spectacular visualisations but also to test general metamorphic theory. Our data, which track the entire reaction history from nucleation through to interaction with surrounding grains raise several questions. While individual grains grow quicker than others, why do, when grain growth is normalised against final grain size, all grains have a very similar growth history? This is not currently explained by an Avrami type model, and an alternative model for metamorphic reaction kinetics may be based on our data. Do metamorphic transport distances change during the reaction as the pore structure evolves? What controls the orientation of hemihydrate needles? In this presentation we present not only the images and data highlighting these questions, but also explore possible answers.

U-Th-PbT Monazite Gechronology in the South Carpathian Basement: Variscan Syn-Metamorphic Tectonic Stacking and Long-Lasting Post-Peak Decompressional Overprints

NASA Astrophysics Data System (ADS)

Săbău, G.; Negulescu, E.

2012-12-01

Dating metamorphic events appears to be unsatisfactorily addressed by most of the widely-employed and otherwise accurate and productive isotopic techniques, because the phases and systems investigated do not directly relate to the metamorphic events themselves. An adequate answer to this challenge is instead provided by microprobe-assisted chemical U-Th-PbT monazite geochronology, by its spatial resolution, truly in situ character and the possibility to reference analyses against well-defined textural environments and features, as well as a qualitative timeframe derived therefrom. Though chemical U-Th-PbT monazite geochronology is increasingly applied to seek answers ranging form a general characterization to fine details of the thermotectonic evolution of magmatic and metamorphic rocks, there are so far, unlike in the case of isotopic geochronological methods, no clearly defined standard analytical and data processing protocols. Two main reasons for this have to be mentioned, namely that chemical U-Th-PbT chronology is actually a proxy for isotopic geochronology, and the quantification of the errors and their propagation cannot be directly assessed because apparent ages are related to the measured element concentrations by an implicit function, the law of radioactive decay. Current approaches rely on treating calculated individual age values as primary data, a priori grouping of analyses supposed (and subsequently tested) to be coeval, and their statistical processing in order to obtain age values. An alternative approach we applied in basement units of the South Carpathians consists in an explicit approximation of the age formula and associated errors propagated from element concentrations to age values, and individual treatment of each age datum. The separation of the age clusters from the overall age spectrum of each sample was operated by tracing the variations of the normalized age gradient on the age spectrum sorted by increasing age values, and fine-tuned by comparison with the general probability function calculated from all individual age and error values. Monazite chemical compositions and variations, in connection with the textural and zonal setting of the analyzed spots, were used to estimate the geological relevance of the derived age clusters, along with inter-sample comparisons anchored on granitoid samples displaying well-expressed age plateaus, conspicuously related to consolidation and emplacement ages. The resultant ages are consistent with sandwiching of juvenile Variscan metamorphic units and slivers of reworked older basement fragments in structurally coherent sequences, formed by syn/late-metamorphic tectonic stacking. Differential exhumation and ensuing areal or local lower pressure overprinting initiated in early Permian lasted up until the Late Jurassic. The age distribution of the pervasive metamorphic overprints, in consistency with the variation of the metamorphic conditions recorded, requires a partial revision and an iterative adjustment between determined ages and metamorphic features, and the lithostratigraphic separations operated in several basement units of the South Carpathians. Acknowledgements Grant PN-II-ID-PCE-2011-3-0030 by the Romanian Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI)

Detailed thermal fingerprinting of obduction-related processes: insights from Northern New Caledonia

NASA Astrophysics Data System (ADS)

Vitale Brovarone, A.; Agard, P.; Monié, P.; Chauvet, A.

2012-04-01

Northern New Caledonia comprises large and continuous units of high-pressure metamorphic rocks formed in response to the Eocene subduction of a continental margin, classically viewed as a northern extension of the Norfolk ridge, below an oceanic island-arc system (well-exposed in southern New Caledonia) [1, 2]. Metamorphic conditions increase gradually towards the east, providing a continuous window on prograde metamorphism from low-grade, lawsonite-bearing assemblages to epidote-bearing eclogite [3, 4, 5]. Unfortunately, available P-T constraints are mostly restricted to metamafics, but are almost completely lacking in metasediments, which represent the dominant lithology in these mainly continental-derived units. This is due both to the lack of diagnostic mineral assemblages in the metasedimentary lower grade units and to the intense late retrogression affecting the higher-grade metasediments. We herein present an extensive thermometric dataset (encompassing the area from Hienghene to the south and from Koumac to Pouébo) obtained via Raman Spectroscopy of Carbonaceous Material (RSCM), which provides quantitative estimates of the peak metamorphic temperature of CM-bearing metasediments in the range ~200-650°C [6, 7]. Metamorphic conditions vary from about 200 °C in the lower-grade units and progressively increase toward the east to about 550 °C in the eclogite facies unit. Sharp metamorphic gaps are nevertheless found across some major tectonic boundaries (such as the Gendarmerie fault zone). Importantly, the main metamorphic units defined by means of our dataset do not always match with previous studies. This dataset also provides useful insights on the architecture of the high-pressure belt in Northern New Caledonia, where structures are poorly exposed due to thick vegetation. [1] Cluzel, D., Aitchison, J.C., Picard, C., 2001. Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic forearc of New Caledonia (Southwest Pacific): geodynamic implications. Tectonophysics 340 (1-2), 23-59. [2] Ulrich, M., Picard C., Guillot S., Chauvel C., Cluzel D., Meffre S. (2010) The New Caledonia Ophiolite : multiple Stage of melting and refertilisation process as indicators of ridge to subduction formation. Lithos. doi 10.1016/j.lithos.2009.12.011. [3] Brothers, R. N. & Blake, M. C., 1972. Tertiary plate tectonics and high-pressure metamorphism in New Caledonia. Tectonophysics, 17, 359-391. [4] Fitzherbert, J. A., Clarke, G. L. & Powell, R., 2003. Lawsonite- omphacite bearing metabasites of the Pam Peninsula, NE New Caledonia: Evidence for disrupted blueschist to eclogite facies conditions. Journal of Petrology, 44, 1805-1831. [5] Spandler, C., & Hermann, J., 2006. High-pressure veins in eclogite from New Caledonia and their significance for fluid migration in subduction zones. Lithos, 89 (1-2). pp. 135-153. ISSN 1872-6143 [6] Beyssac, O., Goffé, B., Chopin, C. & Rouzaud, J.N., 2002. Raman spectra of carbonaceous material in metasediments: a new geothermometer. J. Metamorph. Geol., 20, 859-871. [7] Lahfid, A., Beyssac, O., Deville, E., Negro, F., Chopin, C. & Goffé, B., 2010. Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland). Terra Nova, 22: 354-360. doi: 10.1111/j.1365-3121.2010.00956.x

Structure characterization of MHEMT heterostructure elements with In{sub 0.4}Ga{sub 0.6}As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aleshin, A. N., E-mail: a.n.aleshin@mail.ru; Bugaev, A. S.; Ermakova, M. A.

2016-03-15

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In{sub 0.4}Ga{sub 0.6}As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for themore » 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In{sub x}Ga{sub 1–x}As ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.« less

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating.

PubMed

Burda, Jolanta; Klötzli, Urs

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20-1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG-type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

Metamorphic geology: Why should we care?

NASA Astrophysics Data System (ADS)

Tajcmanova, Lucie; Moulas, Evangelos; Vrijmoed, Johannes

2016-04-01

Estimation of pressure-temperature (P-T) from petrographic observations in metamorphic rocks has become a common practice in petrology studies during the last 50 years. This data then often serves as a key input in geodynamic reconstructions and thus directly influences our understanding of lithospheric processes. Such an approach might have led the metamorphic geology field to a certain level of quiescence. Obtaining high-quality analytical data from metamorphic rocks has become a standard part of geology studies. The numerical tools for geodynamic reconstructions have evolved to a great extend as well. Furthermore, the increasing demand on using the Earth's interior for sustainable energy or nuclear waste disposal requires a better understanding of the physical processes involved in fluid-rock interaction. However, nowadays, metamorphic data have apparently lost their importance in the "bigger picture" of the Earth sciences. Interestingly, the suppression of the metamorphic geology discipline limits the potential for understanding the aforementioned physical processes that could have been exploited. In fact, those phenomena must be considered in the development of new generations of fully coupled numerical codes that involve reacting materials with changing porosity while obeying conservation of mass, momentum and energy. In our contribution, we would like to discuss the current role of metamorphic geology. We will bring food for thoughts and specifically touch upon the following questions: How can we revitalize metamorphic geology? How can we increase the importance of it? How can metamorphic geology contribute to societal issues?

High-P metamorphic rocks from the Himalaya and their tectonic implication ? a review

NASA Astrophysics Data System (ADS)

Jan, M. Qasim

The suture zones bordering the Indian subcontinent on the E, N and W are characterized in several places by the occurrence of ophiolitic complexes and tectonic melanges. High-P metamorphic rocks have recently been discovered in the melanges in Burma, Naga Hills, southern Tibet, eastern and western Ladakh, Kohistan (Jijal, Allai, Shangla) and Khost (Afghanistan). The development of these rocks has an important bearing on the plate tectonics of the Himalaya. The High-P metamorphic rocks belong to prehnite-pumpellyite, blueschist and high-P greenschist facies but extensive garnet-granulites have developed at 35 km depth in Jijal. In the Indus-Zangbo suture zone (IZS) the high-P metamorphism is complemented to the N by low- or medium-P metamorphism and calc-alkaline magmatism in Tibet, Ladakh as well as Kohistan. High-P metamorphism in Jijal has been dated at 104 Ma, in Shangla at 70-100 Ma and in western Ladakh during mid-Cretaceous. Elsewhere, the timing of the high-P metamorphism is not known but a Cretaceous age is inferred. Since collision along the IZS occurred during Eocene, the high-P metamorphism is therefore related to the northwards subduction of the neo-Tethyan lithosphere under Tibet or late Mesozoic magmatic arcs. The timing of high-P metamorphism coincides with the breakup of India from Gondwanaland and its rapid northwards movement, whereas the tectonic melanges may principally have formed during Eocene collision and obduction.

Strain relaxation in convex-graded InxAl1-xAs (x = 0.05-0.79) metamorphic buffer layers grown by molecular beam epitaxy on GaAs(001)

NASA Astrophysics Data System (ADS)

Solov'ev, V. A.; Chernov, M. Yu; Baidakova, M. V.; Kirilenko, D. A.; Yagovkina, M. A.; Sitnikova, A. A.; Komissarova, T. A.; Kop'ev, P. S.; Ivanov, S. V.

2018-01-01

This paper presents a study of structural properties of InGaAs/InAlAs quantum well (QW) heterostructures with convex-graded InxAl1-xAs (x = 0.05-0.79) metamorphic buffer layers (MBLs) grown by molecular beam epitaxy on GaAs substrates. Mechanisms of elastic strain relaxation in the convex-graded MBLs were studied by the X-ray reciprocal space mapping combined with the data of spatially-resolved selected area electron diffraction implemented in a transmission electron microscope. The strain relaxation degree was approximated for the structures with different values of an In step-back. Strong contribution of the strain relaxation via lattice tilt in addition to the formation of the misfit dislocations has been observed for the convex-graded InAlAs MBL, which results in a reduced threading dislocation density in the QW region as compared to a linear-graded MBL.

Geology and neotectonism in the epicentral area of the 2011 M5.8 Mineral, Virginia, earthquake

USGS Publications Warehouse

Burton, William C.; Spears, David B.; Harrison, Richard W.; Evans, Nicholas H.; Schindler, J. Stephen; Counts, Ronald C.

2015-01-01

arc (Ordovician Chopawamsic Formation) to Laurentia, intrusion of a granodiorite pluton (Ordovician Ellisville pluton), and formation of a post-Chopawamsic successor basin (Ordovician Quantico Formation), all accompanied by early Paleozoic regional deformation and metamorphism. Local transpressional faulting and retrograde metamorphism occurred in the late Paleozoic, followed by diabase dike intrusion and possible local normal faulting in the early Mesozoic. The overall goal of the bedrock mapping is to determine what existing geologic structures might have been reactivated during the 2011 seismic event, and surfi cial deposits along the South Anna River are being mapped in order to determine possible neotectonic uplift. In addition to bedrock and surfi cial studies, we have excavated trenches in an area that contains two late Paleozoic faults and represents the updip projection of the causative fault for the 2011 quake. The trenches reveal faulting that has offset surfi cial deposits dated as Quaternary in age, as well as numerous other brittle structures that suggest a geologically recent history of neotectonic activity.

Structures, microfabrics, fractal analysis and temperature-pressure estimation of the Mesozoic Xingcheng-Taili ductile shear zone in the North China craton

NASA Astrophysics Data System (ADS)

Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Jin, Wei; Zeng, Zuoxun; Bernroider, Manfred; Li, Weimin; Wen, Quanbo; Han, Guoqing; Zhao, Yingli

2014-05-01

The ductile shear zone in Xingcheng-Taili area (western Liaoning Province in China) is tectonically located in the eastern section of the northern margin of the North China craton, and dominantly comprises deformed granitic rocks of Neoarchean and Triassic to Late Jurassic age, which were affected by shearing within middle- to low-grade metamorphic conditions. Because a high-temperature metamorphic overprint is lacking, microstructures attesting to low-temperature ductile deformation are well preserved. However, the rocks and its structures have not been previously analyzed in detail except by U-Pb zircon dating and some geochemistry. Here, we describe the deformation characteristics and tectonic evolution of the Xingcheng-Taili ductile shear zone, in order to understand the mode of lithosphericscale reactivation, extension and thinning of the North China craton. The ductile deformation history comprises four successive deformation phases: (1) In the Neoarchean granitic rocks, a steep gneissosity and banded structures trend nearly E-W (D1). (2) A NE-striking sinistral structure of Upper Triassic rocks may indicate a deformation event (D2) in Late Triassic times, which ductile deformation structures superimposed on Neoarchean granitic rocks. (3) A gneissose structure with S-C fabrics as well as an ENE-trending sinistral strike-slip characteristic (D3) developed in Upper Jurassic biotite adamellite and show the deformation characteristics of a shallow crustal level and generated mylonitic fabrics superimposed on previous structures. (4) Late granitic dykes show different deformational behavior, and shortening with D4 folds. The attitude of the foliation S and mineral stretching lineation of three main types of rocks shows remarkable differences in orientation. The shapes of recrystallized quartz grains from three main types of granitic rocks with their jagged and indented boundaries were natural records of deformation conditions (D1to D3). Crystal preferred orientation of quartz determined by electron back scatter diffraction (EBSD) suggest sinistral strike-slip displacement within a temperature at about 400 to 500° C. Quartz mainly shows low-temperature fabrics with dominant {0001}-slip system. As the deformed rocks show obvious deformation overprint, we have estimated flow stresses from dynamically recrystallized grain sizes of quartz separately. But coincident fractal analysis showed that the boundaries of recrystallized grains had statistically self similarities with the numbers of fractal dimension from 1.153 to 1.196 with the range of deformation temperatures from 500 to 600° C, which is corresponding to upper greenschist to lower amphibolite facies conditions. Together with published flow laws to estimated deformation rates between the region of 10-11 - 10-13 S-1depending on the temperature 500 ° C, and the paleo-stress was calculated with grain size of recrystallized quartz to be at 5.0 to 32.3 MPa. Even though the deformation history and kinematics are different, progressive microstructures and texture analysis indicate an overprint by the low-temperature deformation (D3). Typical regional-dynamic metamorphic conditions ere deduced by mineral pair hornblende-plagioclase and phengite barometry identified within the ductile shear zone. The hornblende-plagioclase pair of porphyritic granitic gneiss gives metamorphic conditions of T =450-500 ° C and p=0.39 GPa, which indicate a metamorphic grade of lower-amphibolite facies conditions and a depth of around 13 km estimated following a normal lithostatic pressure. All of the structural characteristics indicate that the Xingcheng-Taili ductile shear zone represents a mainly ENE-striking sinistral ductile strike-slip zone, which formed after intrusion of the Upper Jurassic biotite adamellite and transformed and superimposed previous deformation structures. This deformation event might have occurred in Early Cretaceous times and was related to the lithospheric thinning and extension, due to roll-back of the Pacific plate beneath the eastern North China craton.

Earthquakes, fluid pressures and rapid subduction zone metamorphism

NASA Astrophysics Data System (ADS)

Viete, D. R.

2013-12-01

High-pressure/low-temperature (HP/LT) metamorphism is commonly incomplete, meaning that large tracts of rock can remain metastable at blueschist- and eclogite-facies conditions for timescales up to millions of years [1]. When HP/LT metamorphism does take place, it can occur over extremely short durations (<<1 Myr) [1-2]. HP/LT metamorphism must be associated with processes that allow large volumes of rock to remain unaffected over long periods of time, but then suddenly undergo localized metamorphism. Existing models for HP/LT metamorphism have focussed on the role of fluids in providing heat for metamorphism [2] or catalyzing metamorphic reactions [1]. Earthquakes in subduction zone settings can occur to depths of 100s of km. Metamorphic dehydration and the associated development of elevated pore pressures in HP/LT metamorphic rocks has been identified as a cause of earthquake activity at such great depths [3-4]. The process of fracturing/faulting significantly increases rock permeability, causing channelized fluid flow and dissipation of pore pressures [3-4]. Thus, deep subduction zone earthquakes are thought to reflect an evolution in fluid pressure, involving: (1) an initial increase in pore pressure by heating-related dehydration of subduction zone rocks, and (2) rapid relief of pore pressures by faulting and channelized flow. Models for earthquakes at depth in subduction zones have focussed on the in situ effects of dehydration and then sudden escape of fluids from the rock mass following fracturing [3-4]. On the other hand, existing models for rapid and incomplete metamorphism in subduction zones have focussed only on the effects of heating and/or hydration with the arrival of external fluids [1-2]. Significant changes in pressure over very short timescales should result in rapid mineral growth and/or disequilibrium texture development in response to overstepping of mineral reaction boundaries. The repeated process of dehydration-pore pressure development-earthquake-pore pressure relief could conceivably produce a record of episodic HP/LT metamorphism driven by rapid pressure pulses. A new hypothesis is presented for the origins of HP/LT metamorphism: that HP/LT metamorphism is driven by effective pressure pulses caused by localized, earthquake-related modifications to fluid pressures in the subducted slab. In other words, HP/LT metamorphism marks abrupt changes in stress state within the subducted slab, driven by earthquake rupture and fluid flow, and involving a rapid return toward lithostatic pressure from effective pressures well below lithostatic. References: 1. Bjørnerud, MG, Austrheim, H & Lund, MG, 2002. Processes leading to eclogitization (densification) of subducted and tectonically buried crust. Journal of Geophysical Research 107, 2252. 2. Camacho, A, Lee, JKW, Hensen, BJ & Braun, J, 2005. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids. Nature 435, 1191-1196. 3. Green, HW & Houston, H, 1995. The mechanics of deep earthquakes. Annual Reviews of Earth and Planetary Sciences 23, 169-213. 4. Hacker, BR, Peacock, SM, Abers, GA & Holloway, SD, 2003. Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?. Journal of Geophysical Research 108, 2030.

Preliminary report on the geology and deposits of monazite, thorite and niobium-bearing rutile of the Mineral Hill district, Lemhi County, Idaho

USGS Publications Warehouse

Kaiser, Edward Peck

1956-01-01

Deposits of minerals containing niobium (columbium), thorium, and rare earths occur in the Mineral Hill district, 30 miles northwest of Salmon, Lemhi County, Idaho. Monazite, thorite, allanite, and niobium-bearing rutile form deposits in metamorphic limestone layers less than 8 feet thick. The known deposits are small, irregular, and typically located in or near small folds. Minor faults are common. Monazite generally is coarsely crystalline and contains less than one percent thorium. Rutile forms massive lumps up to 3 inches across; it contains between 5 and 10 percent niobium. Rutile occurs in the northwestern half of the district, thorite in the central and southeastern parts. Monazite occurs in all deposits. Allanite is locally abundant and contains several percent thorium. Magnetite and ilmenite are also locally abundant. A major thrust fault trending northwest across the map-area separates moderately folded quartzite and phyllitic rocks of Belt age, on the northeast, from more intensely metamorphosed and folded rocks on the southwest. The more metamorphosed rocks include amphibolite, porphyroblastic feldspar gneiss, quartzite, and limestone, all probably of sedimentary origin, and probably also of Belt (late Precambrian) age. The only rocks of definite igneous origin are rhyolite dikes of probable Tertiary age. The more metamorphosed rocks were formed by metasomatic metamorphism acting on clastic sediments, probably of Belt age, although they may be older than Belt. Metamorphism doubtless was part of the episode of emplacement of the Idaho batholith, but the history of that episode is not well understood. The rare-element deposits show no evidence of fracture-controlled hydrothermal introduction, such as special fracture systems, veining, and gangue material. They may, however, be of hydrothermal type. More likely they are metamorphic segregations or secretions, deposited in favorable stratigraphic and structural positions during regional metamorphism.

Thermal history of type-3 chondrites in the NASA antarctic collection

NASA Astrophysics Data System (ADS)

Bonal, L.; Quirico, E.; Montagnac, G.

2014-07-01

Chondrites are the most primitive meteorites. However, they were all modified in some ways by post-accretion geological processes operating on their asteroidal parent bodies. Hence, to decipher the formation(s) and origin(s) of their components, we must first understand how chondritic materials were modified in their asteroidal parent bodies. The modifications induced by secondary processes should not be underestimated and have to be precisely estimated before any interpretation of chondrite properties in terms of cosmochemistry. In particular, all chondrites contain some organic components that were potentially chemically and physically modified through post-accretion processes. A thin understanding of the induced evolution is required to allow for pertinent comparisons with other primitive extraterrestrial materials, such as cometary grains, to finally address questions such as the origin of organics in the Solar System. Type 3 chondrites experienced thermal metamorphism on their asteroidal parent body due to the radioactive decay of elements such as ^{26}Al. Temperatures higher than 300 °C were experienced on timescales of several thousands of years. Still, type 3 chondrites remain as unequilibrated rocks and common mineralogical thermometers cannot be applied. The polyaromatic carbonaceous matter is sensitive to thermal episodes (of long and short duration) experienced by the host meteorite. In particular, its structural order directly reflects the thermal history experienced on their parent bodies. The structural modification of the aromatic carbonaceous matter towards a higher order is irreversible, and independent of the mineralogy and degree of aqueous alteration. It is mainly controlled by the peak metamorphic temperature. Moreover, under the assumption of fairly similar organic precursors among chondrites of distinct groups, the structural order of polyaromatic organic matter allows for a direct comparison of their metamorphic grades. It is then possible to evaluate the metamorphic grade of the objects and to assign a petrologic type along a unique petrologic scale [1-4]. This technique has been successfully applied to type 3 Unequilibrated Ordinary Chondrites [1], carbonaceous CV chondrites [2], and CO chondrites [3]. The interpretation of the structural order of the polyaromatic carbonaceous matter in terms of thermal history is thus reliable. Raman spectroscopy enables the determination of the degree of structural order of the polyaromatic organic matter present in the matrix of chondrites. Both falls and finds, from Antarctica [4] and elsewhere, have been analyzed. It does not require a large amount of samples and is relatively easy to implement. Raman spectroscopy is particularly sensitive to the lowest petrologic types (3.0-3.2). The present NASA collection of Antarctic meteorites represents an incredible source of precious samples for our community. The present work finely characterizes the thermal history of most of the type 3 chondrites (UOCs, CVs, and COs) from that collection. At the present time, the objectives are threefold: (i) determination of reliable petrologic types indispensable for our community; (ii) identification of the most primitive type 3 chondrites (petrologic type ≤ 3.1); and (iii) identification of potential ''anomalous'' samples having experienced a slightly different thermal history. The JSC Meteorite Working Group generously allocated us with more than 150 chondrites (UOCs, CVs, and COs). The following points summarize the main results. (i) At the present time, the thermal histories of more than 100 samples have been characterized. (ii) The terrestrial weathering experienced by several chondrites (˜25 chondrites) has been too pervasive for the method to be applied. For these meteorites, as signatures of oxide minerals dominate Raman spectra of the matrix, the organic matter might have been significantly altered through oxidation. (iii) Real discrepancies with the preliminary JSC petrologic type attributions were found for several chondrites with mostly underestimations of the metamorphic grades. (iv) The structural grade of the polyaromatic carbonaceous matter is fairly homogeneous in most of the considered chondrites with a few exceptions, interpreted in terms of shock events. (v) Recently, there were some promising advances (e.g. [5,6]) in terms of interpretation of the structural order of the polyaromatic carbonaceous matter as a geothermometer for terrestrial rocks of low maturity grades. The used spectral tracers will be considered and the thermometry potentially applied to infer new constraints on the metamorphic temperature experienced by these type 3 chondrites.

Proterozoic polymetamorphism in the Quanji Block, northwestern China: Evidence from microtextures, garnet compositions and monazite CHIME ages

NASA Astrophysics Data System (ADS)

Wang, Qinyan; Pan, Yuanming; Chen, Nengsong; Li, Xiaoyan; Chen, Haihong

2009-05-01

The Quanji Block, situated close to the triple junction of three major Precambrian terranes in China (i.e., the North China Craton, the Yangtze Block and the Tarim Block), is composed of Precambrian metamorphic crystalline basement and an unmetamorphosed Mesozoic-Paleozoic sedimentary cover; it has been interpreted as a remnant continental fragment. Microtextural relationships, garnet trace element compositions, and monazite CHIME ages in paragneisses, schists and granitic leucosomes show two episodes of regional metamorphism in the Quanji Block basement. The first regional metamorphism and accompaning anatexis took place at ˜1.93 Ga; the second regional metamorphism occurred between ˜1.75 and ˜1.71 Ga. Mineral compositions of the first metamorphism, including those of monazite, were significantly disturbed by the second event. These two regional metamorphic episodes were most likely linked to assembly and breakup of the supercontinent Columbia, respectively.

Timing and conditions of regional metamorphism and crustal shearing in the granulite facies basement of south Namibia: Implications for the crustal evolution of the Namaqualand metamorphic basement in the Mesoproterozoic

NASA Astrophysics Data System (ADS)

Bial, Julia; Büttner, Steffen; Appel, Peter

2016-11-01

Granulite facies basement gneisses from the Grünau area in the Kakamas Domain of the Namaqua-Natal Metamorphic Province in south Namibia show high-grade mineral assemblages, most commonly consisting of garnet, cordierite, sillimanite, alkali feldspar and quartz. Cordierite + hercynitic spinel, and in some places quartz + hercynitic spinel, indicate granulite facies P-T conditions. The peak assemblage equilibrated at 800-850 °C at 4.0-4.5 kbar. Sillimanite pseudomorphs after kyanite1 and late-stage staurolite and kyanite2 indicate that the metamorphic record started and ended within the stability field of kyanite. Monazite in the metamorphic basement gneisses shows a single-phase growth history dated as 1210-1180 Ma, which we interpret as the most likely age of the regional metamorphic peak. This time coincides with the emplacement of granitic plutons in the Grünau region. The ∼10 km wide, NW-SE striking Grünau shear zone crosscuts the metamorphic basement and overprints high-temperature fabrics. In sheared metapelites, the regional metamorphic peak assemblage is largely obliterated, and is replaced by synkinematic biotite2, quartz, alkali feldspar, sillimanite and cordierite or muscovite. In places, gedrite, staurolite, sillimanite and green biotite3 may have formed late- or post-kinematically. The mylonitic mineral assemblage equilibrated at 590-650 °C at 3.5-5.0 kbar, which is similar to a retrograde metamorphic stage in the basement away from the shear zone. Monazite cores in two mylonite samples are similar in texture and age (∼1200 Ma) to monazite in metapelites away from the shear zone. Chemically distinct monazite rims indicate a second growth episode at ∼1130-1120 Ma. This age is interpreted to date the main deformation episode along the Grünau shear zone and the retrograde metamorphic stage seen in the basement. The main episode of ductile shearing along the Grünau shear zone took place 70-80 million years after the thermal peak metamorphism and granite emplacement, and after substantial isobaric cooling of the basement. Metamorphism and regional shearing in the Grünau area can be correlated with the crustal evolution in the Kakamas Domain in South Africa, but not with the timing of metamorphism in the Aus area, 230 km to the NW of Grünau, which is significantly younger.

The Pinkie Unit of the Southwestern Svalbard Caledonian Province and its bearing on distribution of the Torellian-Timanian basement in the High Arctic

NASA Astrophysics Data System (ADS)

Kośmińska, Karolina; Majka, Jarosław; Manecki, Maciej

2015-04-01

Here we present for the first time petrological characteristics of metapelites from the Pinkie Unit (Prins Karls Forland, western Svalbard). Rocks belonging to the Pinkie Unit are represented mainly by laminated quartzites, siliciclastic rocks (sometimes with garnet) and garnet-bearing mica schists. They are overthrust by the lower grade lithologies of the Grampian Group. The mineral composition confirmed by preliminary microscopic observations suggests that the Pinkie rocks were subjected to at least amphibolite facies metamorphism. The metamorphic zoning from chloritoid through staurolite, up to kyanite zone is apparent. This indicates Barrovian type metamorphism. The rocks are strongly deformed, M1 assemblages and D1 structures are overprinted by pervasive D2 mylonitic pattern. This study is focused on kyanite-bearing schists. They consist mainly of garnet, plagioclase, biotite, muscovite, kyanite, quartz and turmaline. Garnet forms anhedral crystals. Its chemical composition is characterized by Alm79-84Sps1-5Prp5-11Grs5-10. The profiles through the garnets are almost flat and they seem to be homogenized most probably during peak temperature at an early stage of retrogression. The Si content in muscovite varies from 3.06 to 3.13. Biotite is characterized by XFe in the range of 0.53 - 0.66. The garnet-biotite-muscovite-plagioclase (GBPM) geothermobarometer (Holdaway, 2001; Wu, 2014) has been used for estimation of pressure - temperature conditions. Preliminary calculations indicate peak metamorphic conditions at 8 - 9 kbar and 650 - 700°C. Our P-T calculations provide further evidence for the Barrovian type of metamorphism along the western coast of Svalbard. The correlation of the Pinkie Unit with other higher grade complexes within Southwestern Svalbard Caledonian Province is still difficult, but the studied rocks resemble the Isbjørnhamna Group of Wedel Jarlsberg Land. This is based on the metamorphic grade, mineral assemblage and probable protolith age (post-950Ma; Kośmińska, unpublished data). If this is a case, the Pinkie Unit will provide another evidence of the Torellian-Timanian (late Neoproterozoic, e.g. Majka et al. 2008) tectonothermal event within the Svalbard's Caledonides. In turn, it can bear important implications for Arctic tectonic reconstructions. This project is financed by NCN research project No 2013/11/N/ST10/00357. References: Holdaway M.J., 2001. Recalibration of the GASP geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet-biotite geothermometer. American Mineralogist, 86, 1117-1129. Majka J., Mazur S., Czerny J., Manecki M., Holm D.K., 2008. Late Neoproterozoic amphibolite facies metamorphism of a pre-Caledonian basement block in southwest Wedel Jarlsberg Land, Spitsbergen: new evidence from U-Th-Pb dating of monazite. Geological Magazine, 145, 822-830. Wu C. M., 2014. Revised empirical garnet-biotite-muscovite-plagioclase (GBMP) geobarometer in metapelites. Journal of Metamorphic Geology. doi: 10.1111/jmg.12115

Microdeformation in Vredefort rocks; evidence for shock metamorphism

NASA Technical Reports Server (NTRS)

Reimold, W. U.; Andreoli, M. A. G.; Hart, R. J.

1988-01-01

Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been cited for years as the main microtextural evidence for shock metamorphism in this structure. In addition, Schreyer describes feldspar recrystallization in rocks from the center of the Dome as the result of transformation of diaplectic glass, and Lilly reported the sighting of mosaicism in quartz. These textural observations are widely believed to indicate either an impact or an internally produced shock origin for the Vredefort Dome. Two types of (mostly sub) planar microdeformations are displayed in quartz grains from Vredefort rocks: (1) fluid inclusion trails, and (2) straight optical discontinuities that sometimes resemble lamellae. Both types occur as single features or as single or multiple sets in quartz grains. Besides qualitative descriptions of cleavage and recrystallization in feldspar and kinkbands in mica, no further microtextural evidence for shock metamorphism at Vredefort has been reported to date. Some 150 thin sections of Vredefort basement rocks were re-examined for potential shock and other deformation effects in all rock-forming minerals. This included petrographic study of two drill cores from the immediate vicinity of the center of the Dome. Observations recorded throughout the granitic core are given along with conclusions.

Model for threading dislocations in metamorphic tandem solar cells on GaAs (001) substrates

NASA Astrophysics Data System (ADS)

Song, Yifei; Kujofsa, Tedi; Ayers, John E.

2018-02-01

We present an approximate model for the threading dislocations in III-V heterostructures and have applied this model to study the defect behavior in metamorphic triple-junction solar cells. This model represents a new approach in which the coefficient for second-order threading dislocation annihilation and coalescence reactions is considered to be determined by the length of misfit dislocations, LMD, in the structure, and we therefore refer to it as the LMD model. On the basis of this model we have compared the average threading dislocation densities in the active layers of triple junction solar cells using linearly-graded buffers of varying thicknesses as well as S-graded (complementary error function) buffers with varying thicknesses and standard deviation parameters. We have shown that the threading dislocation densities in the active regions of metamorphic tandem solar cells depend not only on the thicknesses of the buffer layers but on their compositional grading profiles. The use of S-graded buffer layers instead of linear buffers resulted in lower threading dislocation densities. Moreover, the threading dislocation densities depended strongly on the standard deviation parameters used in the S-graded buffers, with smaller values providing lower threading dislocation densities.

New Mapping in the Sand Springs Range of Western Nevada Clarifies and Constrains Regional Deformation Sequences of the Luning-Fencemaker Thrust Belt

NASA Astrophysics Data System (ADS)

Czarnecki, S.; Jarvis, J.; Satterfield, J. I.

2016-12-01

The Sand Springs Range in western Nevada exposes Mesozoic through Cenozoic structures of the eastern Sierra Nevada, Luning-Fencemaker Thrust Belt (LFTB), Basin and Range province, and Walker Lane. A recent undergraduate geologic mapping project in the northern Sand Springs Range (nSSR) set out to map igneous intrusions in detail, specifically smaller intrusions which had not been a focus in previous work. This was accomplished using different techniques including mapping at a smaller scale (1:8000 vs. 1:24000), locating contacts and faults using handheld GPS, and focusing on relationships between metamorphic tectonites and igneous units. This revealed key cross-cutting relations between structures and diverse Triassic through Tertiary igneous rocks as well as distinctions between the nSSR and the surrounding LFTB assemblages. During our mapping we identified four metamorphic tectonite map units, Cretaceous granitoid and diorite plutons and sills, Tertiary rhyolite sills and dikes, and interbedded Tertiary basalt and ash flow tuff. The cross-cutting relations of these units overturn previously published sequences of events and constrain the timing of a deformation sequence which differs from the surrounding LFTB assemblages. We found that the nSSR contains three phases of deformation: a pre-LFTB syn-metamorphic event which achieved amphibolite facies that is not described elsewhere in the LFTB (D1), followed by two non-metamorphic folding and thrusting phases characteristic of the LFTB (D2 and D3). Our mapping provided four key timing constraints. First, D1 axial-planar cleavage (S1) deformed Triassic intrusions. Second, Cretaceous granitoid and diorite units cross-cut S1 foliation, D1 folds, and low-angle faults. Third, Cretaceous and Tertiary sills that locally terminate at a low-angle fault actually post-dated faulting. Fourth, cross-cutting relations showed a basaltic lava previously mapped as Jurassic is actually Tertiary. The large Sand Springs Pluton was the only intrusion mapped in detail during previous studies; but our mapping has demonstrated the importance of both small and large intrusions in understanding the overall structural history of a complex area. This project was supported by research grants from Angelo State University and the Southwest Section AAPG.

Paleozoic and Mesozoic deformations in the central Sierra Nevada, California

USGS Publications Warehouse

Nokleberg, Warren J.; Kistler, Ronald Wayne

1980-01-01

Analysis of structural and stratigraphic data indicates that several periods of regional deformation, consisting of combined folding, faulting, cataclasis, and regional metamorphism, occurred throughout the central Sierra Nevada during Paleozoic and Mesozoic time. The oldest regional deformation occurred alono northward trends during the Devonian and Mississippian periods in most roof pendants containing lower Paleozoic metasedimentary rocks at the center and along the crest of the range. This deformation is expressed in some roof pendants by an angular unconformity separating older thrice-deformed from younger twice-deformed Paleozoic metasedimentary rocks. The first Mesozoic deformation, which consisted of uplift and erosion and was accompanied by the onset of Andean-type volcanism during the Permian and Triassic, is expressed by an angular unconformity in several roof pendants from the Saddlebag Lake to the Mount Morrison areas. This unconformity is defined by Permian and Triassic andesitic to rhyolitic metavolcanic rocks unconformably overlying more intensely deformed Pennsylvanian, Permian(?), and older metasedimentary rocks. A later regional deformation occurred during the Triassic along N. 20?_30? W. trends in Permian and Triassic metavolcanic rocks of the Saddlebag Lake and Mount Dana roof pendants, in upper Paleozoic rocks of the Pine Creek roof pendant, and in the Calaveras Formation of the western metamorphic belt; the roof pendants are crosscut by Upper Triassic granitic rocks of the Lee Vining intrusive epoch. A still later period of Early and Middle Jurassic regional deformation occurred along N. 30?-60? E. trends in upper Paleozoic rocks of the Calaveras Formation of the western metamorphic belt. A further period of deformation was the Late Jurassic Nevadan orogeny, which occurred along N. 20?_40? W. trends in Upper Jurassic rocks of the western metamorphic belt that are crosscut by Upper Jurassic granitic rocks of the Yosemite intrusive epoch. Structures of similar age occur in intensely deformed oceanic-lithospheric and syntectonic plutonic rocks of the lower Kings River area, in Jurassic metavolcanic rocks of the Ritter Range roof pendant, and in Triassic metasedimentary rocks of the Mineral King roof pendant. The final Mesozoic deformation occurred along N. 50?-80? W. trends in both high-country roof pendants and the lower Kings River area; structures of this generation are crosscut by relatively undeformed Upper Cretaceous granitic rocks of the Cathedral Range intrusive epoch.

Proterozoic orogens in southern Peninsular India: Contiguities and complexities

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Santosh, M.

2013-12-01

The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics. Our synthesis of the spatial distribution, geometry, kinematics and the transpressional strain of the shear zone systems provides insights into the tectono-metamorphic history of the Proterozoic orogens of southern India and their contiguity and complexities. Recent understanding of subduction, accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in Earth history, although the major and common structural architecture was built during the final assembly of the Gondwana supercontinent.

Removing the effects of metamorphism from the Neoproterozoic carbon isotope record: a case study on Islay, western Scotland

NASA Astrophysics Data System (ADS)

Skelton, Alasdair

2016-04-01

The Port Askaig Formation on Islay, western Scotland is the first discovered tillite (glacial sediment) of Neoproterozoic age. This formation is sandwiched between carbonate rocks which preserve an extreme negative carbon isotope excursion. This so called "Islay anomaly" has been correlated with other such anomalies worldwide and together with the tillites has been cited as evidence of major (worldwide) glaciation events. During subsequent mountain building, this carbonate-tillite- carbonate sequence has been folded, producing a major en-echelon anticlinal fold system. Folding was accompanied by metamorphism at greenschist facies conditions which was, in turn, accompanied by metamorphic fluid flow. Mapping of the δ18O and δ13C values of these carbonate rocks reveals that metamorphic fluids were channelled through the axial region of the anticlinal fold. The metamorphic fluid was found to have a highly negative δ13C value, which was found to be in equilibrium with metamorphosed graphitic mudstones beneath the carbonate-tillite-carbonate sequence. Devolatilisation of these mudstones is therefore a likely source of this metamorphic fluid. Removal of the effects of metamorphic fluid flow on δ13C values recorded by metamorphosed carbonate rocks on Islay allows us to re-evaluate the isotopic evidence used to reconstruct Neoproterozoic climate. We are able to show that extreme negative δ13C values can partly be attributed to metamorphic fluid flow.

Histamine is a modulator of metamorphic competence in Strongylocentrotus purpuratus (Echinodermata: Echinoidea)

PubMed Central

2012-01-01

Background A metamorphic life-history is present in the majority of animal phyla. This developmental mode is particularly prominent among marine invertebrates with a bentho-planktonic life cycle, where a pelagic larval form transforms into a benthic adult. Metamorphic competence (the stage at which a larva is capable to undergo the metamorphic transformation and settlement) is an important adaptation both ecologically and physiologically. The competence period maintains the larval state until suitable settlement sites are encountered, at which point the larvae settle in response to settlement cues. The mechanistic basis for metamorphosis (the morphogenetic transition from a larva to a juvenile including settlement), i.e. the molecular and cellular processes underlying metamorphosis in marine invertebrate species, is poorly understood. Histamine (HA), a neurotransmitter used for various physiological and developmental functions among animals, has a critical role in sea urchin fertilization and in the induction of metamorphosis. Here we test the premise that HA functions as a developmental modulator of metamorphic competence in the sea urchin Strongylocentrotus purpuratus. Results Our results provide strong evidence that HA leads to the acquisition of metamorphic competence in S. purpuratus larvae. Pharmacological analysis of several HA receptor antagonists and an inhibitor of HA synthesis indicates a function of HA in metamorphic competence as well as programmed cell death (PCD) during arm retraction. Furthermore we identified an extensive network of histaminergic neurons in pre-metamorphic and metamorphically competent larvae. Analysis of this network throughout larval development indicates that the maturation of specific neuronal clusters correlates with the acquisition of metamorphic competence. Moreover, histamine receptor antagonist treatment leads to the induction of caspase mediated apoptosis in competent larvae. Conclusions We conclude that HA is a modulator of metamorphic competence in S. purpuratus development and hypothesize that HA may have played an important role in the evolution of settlement strategies in echinoids. Our findings provide novel insights into the evolution of HA signalling and its function in one of the most important and widespread life history transitions in the animal kingdom - metamorphosis. PMID:22541006

Geology and Mineral Deposits of the Snow Camp-Saxapahaw Area, Central North Carolina

USGS Publications Warehouse

Schmidt, Robert G.; Gumiel, Pablo; Payas, Alba

2006-01-01

The Snow Camp-Saxapahaw study area, in the Carolina slate belt in the Southeastern United States, is notable for large zones of high-sulfidation alteration in arc-related metavolcanic rocks. The area has potential for additional significant pyrophyllite and related aluminosilicate refractory mineral deposits and may have potential for small- to medium-size gold deposits also associated with the high-sulfidation hydrothermal systems. The Carolina slate belt is an elongate zone of mostly low-grade metamorphic rocks of Neoproterozoic to early Paleozoic age that extends from northeastern Georgia to southern Virginia. It is dominated by volcanic rocks but locally consists of fine-grained epiclastic sedimentary rocks. Plutons and subvolcanic bodies have intruded the rocks of the Carolina slate belt in many places and have been important in controlling the metamorphism and in localizing hydrothermal alteration. The Snow Camp-Saxapahaw area is mostly underlain by volcanic and volcaniclastic rocks and lesser amounts of intrusive shallow plutons. The volcanic rocks range in composition from basalt to rhyolite; however andesites, dacites, and rhyodacites are the most abundant. The intrusive bodies are largely granite and quartz monzonite; gabbroic bodies also are common. It was possible to establish the relative ages of only part of these rocks. Two northeast-trending fault zones and fractures divide the map area into three structural blocks; the central block was tilted down to the southwest to form a grabenlike structure. Most of the hydrothermally altered rocks and all of the intensely altered zones are confined to the downdropped block, which we think may have been calderalike in origin. A major volcanic unit, the Reedy Branch Tuff, is limited to the southwestern part of the graben and may be the youngest volcanic rock in the area. Layered rocks record one or more strong folding events, but the diversity of rock types, lack of recognizable stratigraphic markers, and uneven distribution of outcrops prevented comprehensive structural studies. Except for a few late plutons and dikes, all of the rocks of the area have been metamorphosed in middle to upper greenschist facies, and contact aureoles were recognized around some of the plutons. Several relatively small bodies of granitic rock contain plagioclase grains in which primary oscillatory zoning was unaffected by metamorphism. These were interpreted to be post-metamorphic. We think that there were three separate stages of hydrothermal alteration in the complex volcanic terrane in the area. The oldest, an area of at least 8.5 square miles (22 square kilometers), was subjected to an intense hydrothermal alteration, ranging from peripheral zones of quartz-sericite-paragonite through a patchy marginal zone of pyrophyllite, andalusite, and other high-alumina minerals, to almost totally silicified core zones. The second event resulted in large areas of weak to moderate sericitic and propylitic alteration recognizable only in the Reedy Branch Tuff. The last event was related to post-metamorphic plutons. All of the pyrophyllite-andalusite deposits and perhaps most of the gold and silver mineralization can be related to the first period of hydrothermal alteration. The subsequent metamorphism did not produce significant changes in mineral species in the zones of most intense hydrothermal alteration. Gold- and silver-bearing sulfide minerals in fracture zones along the southeastern margin of the graben may also have been deposited during this earliest alteration stage. No metallic mineralization appears to have occurred during the second event. A group of molybdenum-bearing greisenlike bodies formed during the emplacement of the youngest plutons during the post-metamorphic event. One gold-bearing sulfide zone occurs in the exocontact of one such porphyritic stock. Significant amounts of pyrophyllite-andalusite-bearing rock have been extracted from the Snow Camp Mine and from

Structural analysis of the Elbow-Cranberry-Iskwasum lakes area: A multiply reactivated deformation corridor in the trans-Hudson orogen of Manitoba

NASA Astrophysics Data System (ADS)

Ryan, James Joseph

The Elbow-Cranberry-Iskwasum lakes area comprises a large portion of the eastern Amisk collage in the Palaeoproterozoic Flin Flon Belt (southern Trans-Hudson Orogen) of Manitoba, Canada. Deformation episodes recorded in the Flin Flon Belt are divided into pre-, early, late and post-Hudsonian orogeny, and are distinguished by the orientation of structures and changes in metamorphic conditions. Detailed structural analysis, petrography, geochemistry and U-Pb geochronology indicate a structural history spanning 180 m.y. in the Amisk collage. Accretion of the 1.92--1.88 Ga tectonostratigraphic assemblages that constitute the Amisk collage began prior to 1.868 Ga, the age of the oldest dyke to cross-cut the earliest mylonitic fabrics. The deformational history has been discerned, in which six generations of ductile structures F1 - F6 were followed by development of brittle-ductile and brittle structures F7 . Movements along the late structures may have continued until 1.690 Ga, during exhumation of the collage. The macroscopic structural grain in the central Flin Flon Belt is steeply dipping, generally trends north to north-northeast, and is dominated by two regionally pervasive foliations ( S2 and S5 ). Its grain contrasts strongly with the shallowly-dipping, east--west-trending grain in the adjacent Kisseynew domain. Foliations of different generations have been distinguished by their age relative to regional metamorphic mineral growth. Regional metamorphism in the Flin Flon Belt is interpreted as having culminated at moderate pressure and temperature, between 1.820 and 1.805 Ga. The development of S2 between 1.868 and 1.845 Ga was associated with east--west shortening of the successor magmatic arc that overprinted the Amisk collage. S3 and S4 were associated with shear zones, and are not regionally widespread. The S5 regional-scale Elbow Lake shear zone, and a pervasive crenulation cleavage in the wall rocks, developed during an episode of sinistral transpression that postdated regional metamorphism. The Elbow Lake shear zone appears to have triclinic symmetry. Most of the tectonostratigraphic assemblages, and subordinate formations, in the study area are structurally bound; the boundaries vary in age from early accretionary to post-collisional. Maximum displacements between assemblages occurred along the early shear zones. A high-strain corridor south of Elbow Lake, with four generations of near-parallel foliations ( S1,S 2,S3 and S5 ), records multiple reactivations. Vertical extension was important in post- S1 deformations, even in the later stages. Post-orogenic, low-angle extensional features, common in many mountain belts, appear to be absent in the southern portion of the Trans-Hudson Orogen. This may indicate that erosion was the dominant unroofing mechanism. The regional-scale Berry Creek shear zone transects the southern portion of the field area. Though covered by Ordovician limestone for most of its length, the Berry Creek shear zone is well imaged in regional geophysical maps. The latest portion of the brittle history on the Berry Creek shear zone probably controlled the sharp truncation of the geophysically imaged anomalies.

The nature of the Ailao Shan-Red River (ASRR) shear zone: Constraints from structural, microstructural and fabric analyses of metamorphic rocks from the Diancang Shan, Ailao Shan and Day Nui Con Voi massifs

NASA Astrophysics Data System (ADS)

Liu, Junlai; Tang, Yuan; Tran, My-Dung; Cao, Shuyun; Zhao, Li; Zhang, Zhaochong; Zhao, Zhidan; Chen, Wen

2012-03-01

The structural geology, timing of shearing, and tectonic implications of the ASRR shear zone, one of the most striking lineaments in Southeast Asia, have been the topics of extensive studies over the past few decades. The Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution. Our field structural analysis, detailed microstructural and fabric analysis, as well as the quartz, sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during Cenozoic tectonic evolution in the shear zone. Among the contrasting structural and microstructural associations in the shear zone, D2 structures, which were formed at the brittle to ductile transition during large-scale left-lateral shearing in the second deformation episode, predominate over the structural styles of the other two deformation episodes. Discrete micro-shear zones with intensive grain size reduction compose the characteristic structural style of D2 deformation. In addition, several types of folds (early shearing folds, F21, and late-shearing folds, F22) were formed in the sheared rocks, including discrete to distributed mylonitic foliation, stretching lineation and shear fabrics (e.g., mica fish, domino structures, as well as sigma and delta fabrics). A sequence of microstructures from syn-kinematic magmatic flow, high-temperature solid-state deformation, to brittle-ductile shearing is well-preserved in the syn-kinematic leucocratic intrusions. Deformation structures from the first episode (D1) are characterized by F1 folds and distributed foliations (S1) in rocks due to pure shearing at high temperatures. They are preserved in weakly sheared (D2) rocks along the eastern margin of the ALS belt or in certain low-strain tectonic enclaves within the shear zone. Furthermore, semi-brittle deformation structures, such as hot striae and discrete retrogression zones, are attributed to normal-slip shearing in the third deformation episode (D3), which was probably locally active, along the eastern flank of the DCS range, for example. There are four quartz c-axis fabric patterns in the mylonitic rocks, including type A point maxima, type B Y point maxima with crossed girdles superimposition, type C quadrant maxima, as well as type D point and quadrant maxima combination. They are consistent with microscopic observations of microstructures of high-temperature pure shearing, low-temperature simple shearing and their superimposition. Integrated microstructural analysis and fabric thermometer studies provide information on both high temperature (up to 750 °C) and dominant low-temperature (300-600 °C) deformations of quartz grains in different rock types. Sillimanite and garnet fabrics, especially the latter, were primarily formed at the peak metamorphism during high-temperature pure shearing. The above structural, microstructural and fabric associations were generated in the tectonic framework of the Indian-Eurasian collision. The low-temperature microstructures and fabrics are attributed to left-lateral shearing along the ASRR shear zone from 27 to 21 Ma during the southeastward extrusion of the Indochina block, which postdated high-temperature deformation at the peak metamorphism during the collision.

In silico prediction of the G-protein coupled receptors expressed during the metamorphic molt of Sagmariasus verreauxi (Crustacea: Decapoda) by mining transcriptomic data: RNA-seq to repertoire.

PubMed

Buckley, Sean J; Fitzgibbon, Quinn P; Smith, Gregory G; Ventura, Tomer

2016-03-01

Against a backdrop of food insecurity, the farming of decapod crustaceans is a rapidly expanding and globally significant source of food protein. Sagmariasus verreauxi spiny lobster, the subject of this study, are decapods of underdeveloped aquaculture potential. Crustacean neuropeptide G-protein coupled receptors (GPCRs) mediate endocrine pathways that are integral to animal fecundity, growth and survival. The potential use of novel biotechnologies to enhance GPCR-mediated physiology may assist in improving the health and productivity of farmed decapod populations. This study catalogues the GPCRs expressed in the early developmental stages, as well as adult tissues, with a view to illuminating key neuropeptide receptors. De novo assembled contiguous sequences generated from transcriptomic reads of metamorphic and post metamorphic S. verreauxi were filtered for seven transmembrane domains, and used as a reference for iterative re-mapping. Subsequent putative GPCR open reading frames (ORFs) were BLAST annotated, categorised, and compared to published orthologues based on phylogenetic analysis. A total of 85 GPCRs were digitally predicted, that represented each of the four arthropod subfamilies. They generally displayed low-level and non-differential metamorphic expression with few exceptions that we examined using RT-PCR and qPCR. Two putative CHH-like neuropeptide receptors were annotated. Three dimensional structural modelling suggests that these receptors exhibit a conserved extracellular ligand binding pocket, providing support to the notion that these receptors co-evolved with their ligands across Decapoda. This perhaps narrows the search for means to increase productivity of farmed decapod populations. Copyright © 2016 Elsevier Inc. All rights reserved.

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

NASA Astrophysics Data System (ADS)

Kunz, Barbara E.; Regis, Daniele; Engi, Martin

2018-03-01

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.

The Effects of Thermal Metamorphism on the Amino Acid Content of the CI-Like Chondrite Y-86029

NASA Technical Reports Server (NTRS)

Burton, A. S.; Grunsfeld, S.; Glavin, D. P.; Dworkin, J. P.

2014-01-01

Carbonaceous chondrites con-tain a diverse suite of amino acids that varies in abundance and structural diversity depending on the degree of aqueous alteration and thermal histo-ry that the parent body experienced [1 - 3]. We recently determined the amino acid contents of several fragments of the Sutter's Mill CM2 chon-drite [4]. In contrast with most other CM2 chon-drites, the Sutter's Mill meteorites showed minimal evidence for the presence of indigenous amino acids. A notable difference between the Sutter's Mill meteorites and other CM2 chondrites are that the Sutter's Mill stones were heated to tempera-tures of 150 - 400 C [4], whereas most other CM2 chondrites do not show evidence for thermal met-amorphism [5]. Because empirical studies have shown that amino acids rapidly degrade in aqueous solutions above 150 C and the presence of miner-als accelerates this degradation [6], a plausible explanation for the lack of amino acids observed in the Sutter's Mill meteorites is that they were destroyed during metamorphic alteration. Fewer CI chondrites have been analyzed for amino acids because only a small number of these meteorites have been recovered. Nevertheless, indigenous amino acids have been reported in the CI chondrites Ivuna and Orgueil [7]. Here we report on the amino acid analysis of the CI-like chondrite, Yamato 86029 (Y-86029; sample size of 110 mg). Just as the Sutter's Mill meteorites were thermally metamporphosed CM2 chondrites, Y-86029 has experienced thermal metamorphism at higher temperatures than Orgueil and Ivuna (normal CI chondrites) experienced, possibly up to 600 C [8].

Structural development of the Red Hill portion of the Feather River ultramafic complex, Pulmas County, California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weisenberg, C.W.

1979-01-01

The Feather River Ultramafic Complex is a partially serpentinized body of metamorphosed alpine peridotite and gabbro that lies along the northern part of the Melones fault zone, a NNW trending belt in the Northern Sierra Nevada. The complex was studied in the area of Red Hill, near the canyon of the North Fork, Feather River. The complex is separated from the Calaveras Terrain and Arlington Formation country rocks by steep faults; the Melones Fault on the east and the Rich Bar Fault on the west. Units recognized within the complex include Rich Bar metamorphic rocks, peridotite, metaperidotite, tremolite-olivine schist, hornblendemore » schist, and layered metagabbro. The Rich Bar metamorphic rocks are tectonic slices of amphibolite grade hornblende schist, mica schist, and quartzite found along the Rich Bar Fault. The complex shows evidence of 4 major events. E-1 (Pennsylvania-Permian) was formation of the peridotite-gabbro complex. E-2 (Permo-Triassic) consisted of pervasive shearing parallel to the Rich Bar Fault associated with initial emplacement within the Sierra Nevada. E-3 is believed to be compression and metamorphism (serpentinization) associated with the Nevadan orogeny. E-4 was associated with intrusion of nearby plutons. The regional association of the complex with late paleozoic arc volcanics of the Taylorsville area suggest formation near or under an island arc. Metamorphism during emplacement indicates association with the arc at that time. Left-lateral shear during emplacement along the Rich Bar Fault indicates NW directed thrusting when the layering in metagabbro is rotated to horizontal.« less

Cenozoic geodynamic evolution of the Aegean

NASA Astrophysics Data System (ADS)

Jolivet, Laurent; Brun, Jean-Pierre

2010-01-01

The Aegean region is a concentrate of the main geodynamic processes that shaped the Mediterranean region: oceanic and continental subduction, mountain building, high-pressure and low-temperature metamorphism, backarc extension, post-orogenic collapse, metamorphic core complexes, gneiss domes are the ingredients of a complex evolution that started at the end of the Cretaceous with the closure of the Tethyan ocean along the Vardar suture zone. Using available plate kinematic, geophysical, petrological and structural data, we present a synthetic tectonic map of the whole region encompassing the Balkans, Western Turkey, the Aegean Sea, the Hellenic Arc, the Mediterranean Ridge and continental Greece and we build a lithospheric-scale N-S cross-section from Crete to the Rhodope massif. We then describe the tectonic evolution of this cross-section with a series of reconstructions from ~70 Ma to the Present. We follow on the hypothesis that a single subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and we show that the geological record is compatible with this hypothesis. The reconstructions show that continental subduction (Apulian and Pelagonian continental blocks) did not induce slab break-off in this case. Using this evolution, we discuss the mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the backarc region during slab retreat. The tectonic histories of the two regions showing large-scale extension, the Rhodope and the Cyclades are then compared. The respective contributions to slab retreat, post-orogenic extension and lower crust partial melting of changes in kinematic boundary conditions and in nature of subducting material, from continental to oceanic, are discussed.

Very low-grade metamorphic rocks in some representative districts in Tibet

NASA Astrophysics Data System (ADS)

Bi, X.; Mo, X.

2011-12-01

*Response author: Bi,Xianmei,bixm10@sina.com Very low grade metamorphic rocks are widely distributed in Tibet, providing an insight into deformation and metamorphism during the evolution of the Tibetan Plateau. Eighty five Samples of clay mineral-bearing rocks has been collected from various strata including D, P1, T1, T2, T3, J1, J3, K1, K2 and N strata in the Qiangtang terrane, the Gangdese, the Yarlung Zangbo suture and the Tethyan Himalaya. Analyses and refining of clay minerals in samples have been conducted in the Laboratory of X-ray Diffraction, Institute of Petroleum Exploration. Index of illite crystallinity (Ic) along with average thickness of crystal layers of illite, reflectivity of vitrinite and of clay mineral association have been employed as indicators of degree of very low-grade metamorphism. The scheme of classification[1,2] of very-low grade metamorphism based on clay mineral indexes ( mainly index of illite crystallinity) has been used in the present work, that is, low metamorphism (Ic<0.25), higher very-low grade metamorphism (Ic = 0.25-0.30), lower very-low grade metamorphism (Ic = 0.30-0.42) and diagenesis (Ic>0.42). The analytical results show interesting information. In the Qiangtang terrane, clay minerals in the Jurassic strata have indexes of illite crystalinity (Ic) 0.47-0.70, indicating higher diagenesis and in favor of petroleum-generation. However, index of illite crystalinity (Ic) for the Devonian is 0.23, indicating low metamorphism. Indexes of illite crystalinity (Ic) for the J-K strata in middle Gangdese are mostly 0.37-0.25 (very-low grade metamorphism) and a few 0.78-0.48 (diagenesis). Indexes of illite crystalinity (Ic) for the C-P strata in eastern Gangdese are mostly 0.25-0.42 (very-low grade metamorphism) and a few 0.20-0.25 (low metamorphism). The Mesozoic and Cenozoic magmatism and related mineralization are very strong in the Gangdese, which may affect in some extent on indexes of illite crystalinity. In Tethyan Himalaya, clay mineral-bearing rocks from P1, T1, T2, T3, J1 and J3 strata underwent low-very low grade metamorphism, having indexes of illite crystalinity (Ic) 0.12 for P1 stratum, 0.21 for T1 stratum and 0.22-0.33 for the strata from T2 to J3, whereas K1 and N strata underwent diagenesis, having Ic = 0.52 and 1.61, respectively. Metamorphic degree generally reduced from older strata to younger strata according to clay mineral indexes. The rocks affected by magmatism or by major faulting, however, were out of the general trend and increased their metamorphic degree by 0.1-0.3 units reduction of index of illite crystallinity. Rocks within Yarlung Zangbo suture show higher metamorphic degree than those in Tethyan Himalaya. For instance, while the late Triassic has 0.19 of index of illite crystallinity in the former (the Yarlung Zangbo suture), 0.27-0.33 of indexes of illite crystallinity in the latter. The early Cretaceous has 0.28 of index of illite crystallinity in the former, whereas 0.52 in the latter. However, the late Cretaceous Xigaze Group formed in a fore-arc environment was weakly metamorphosed, having Ic 0.7-1.6 and all falling into diagenesis field.

Trace-element record in zircons during exhumation from UHP conditions, North-East Greenland Caledonides

USGS Publications Warehouse

McClelland, W.C.; Gilotti, J.A.; Mazdab, F.K.; Wooden, J.L.

2009-01-01

Coesite-bearing zircon formed at ultrahigh-pressure (UHP) conditions share general characteristics of eclogite-facies zircon with trace-element signatures characterized by depleted heavy rare earth elements (HREE), lack of an Eu anomaly, and low Th/ U ratios. Trace-element signatures of zircons from the Caledonian UHP terrane in North-East Greenland were used to examine the possible changes in signature with age during exhumation. Collection and interpretation of age and trace-element analyses of zircon from three samples of quartzofeldspathic gneiss and two leucocratic intrusions were guided by core vs. rim zoning patterns as imaged by cathodoluminesence. Change from igneous to eclogite-facies metamorphic trace-element signature in protolith zircon is characterized by gradual depletion of HREE, whereas newly formed metamorphic rims have flat HREE patterns and REE concentrations that are distinct from the recrystallized inherited cores. The signature associated with eclogite-facies metamorphic zircon is observed in coesite-bearing zircon formed at 358 ?? 4 Ma, metamorphic rims formed at 348 ?? 5 Ma during the initial stages of exhumation, and metamorphic rims formed at 337 ?? 5 Ma. Zircons from a garnet-bearing granite emplaced in the neck of an eclogite boudin and a leucocratic dike that cross-cuts amphibolite-facies structural fabrics have steeply sloping HREE patterns, variably developed negative Eu anomalies, and low Th/U ratios. The granite records initial decompression melting and exhumation at 347 ?? 2 Ma and later zircon rim growth at 329 ?? 5. The leucocratic dike was likely emplaced at amphibolite-facies conditions at 330 ?? 2 Ma, but records additional growth of compositionally similar zircon at 321 ??2 Ma. The difference between the trace-element signature of metamorphic zircon in the gneisses and in part coeval leucocratic intrusions indicates that the zircon signature varies as a function of lithology and context, thus enhancing its ability to aid in the interpretation of U-Pb data and track the exhumation history of UHP terranes. The differences may reflect variation in elemental availability through breakdown reactions in quartzofeldpathic gneiss vs. availability during melt production and/or crystallization. UHP rocks in North-East Greenland began exhumation by 347 ?? 2 Ma, were still at HP eclogite-facies conditions at 337 ?? 5 Ma and were at amphibolite-facies conditions by 330 ?? 2 Ma. ?? 2009 E. Schweizerbart'sche Verlagsbuchhandlung.

Fluid-absent metamorphism in the Adirondacks

NASA Technical Reports Server (NTRS)

Valley, J. W.

1986-01-01

Results on late Proterozoic metamorphism of granulite in the Adirondacks are presented. There more than 20,000 sq km of rock are at granulite facies. Low water fugacites are implied by orthopyroxene bearing assemblages and by stability of k'spar-plag-quartz assemblages. After mentioning the popular concept of infiltration of carbon dioxide into Precambrian rocks and attendent generation of granulite facies assemblages, several features of Adirondack rocks pertinent to carbon dioxide and water during their metamorphism are summarized: wollastonite occurs in the western lowlands; contact metamorphism by anorthosite preceeding granulite metamorphism is indicated by oxygen isotopes. Oxygen fugacity lies below that of the QFM buffer; total P sub water + P sub carbon dioxide determined from monticellite bearing assemblages are much less than P sub total (7 to 7.6 kb). These and other features indicate close spatial association of high- and low-P sub carbon dioxide assemblages and that a vapor phase was not present during metamorphism. Thus Adirondack rocks were not infiltrated by carbon dioxide vapor. Their metamorphism, at 625 to 775 C, occurred either when the protoliths were relatively dry or after dessication occurred by removal of a partial melt phase.

Pyrite deformation and connections to gold mobility: insight from micro-structural analysis and trace element mapping

NASA Astrophysics Data System (ADS)

Dubosq, Renelle; Rogowitz, Anna; Lawley, Christopher; Schneider, David; Jackson, Simon

2017-04-01

Pyrite is an important and ubiquitous gold-bearing phase in many orogenic gold deposits making the study of its deformation behaviour under metamorphic conditions crucial to the understanding of gold (re)mobilization. However, pyrite deformation mechanisms and their influence on the retention or release of trace elements during deformation and metamorphism remain poorly understood. We propose a syn- to post-peak metamorphic and deformation driven gold upgrading model where gold is remobilized through deformation-induced diffusion pathways in the form of substructures in pyrite. The middle amphibolite facies assemblage (actinolite-biotite-plagioclase-almandine) of the Detour Lake deposit (Canada) makes it an ideal study area due to maximum temperatures reaching 550°C, exceeding the conditions for plastic deformation in pyrite (450°C). The world-class Detour Lake deposit, containing 16.4 Moz of Au at 1 g/t, is a Neoarchean orogenic gold ore body located in the northern Abitibi district within the Superior Province. The mine is situated along the high strain, sub-vertical ductile-brittle Sunday Lake Deformation Zone (SLDZ) parallel to the broadly E-W trending Abitibi greenstone belt. Herein we combine orientation contrast (OC) forescatter imaging, electron backscatter diffraction (EBSD) and 2D laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element pyrite mapping to evaluate the influence of pyrite brittle and plastic deformation on the release of trace elements during syn-metamorphic gold remobilization. Local misorientation patterns in pyrite exhibit parallel bands that can be described by continuous rotation around one of the <100> axes, whereas higher strain areas reveal more heterogeneous misorientation patterns and the development of low-angle grain boundaries with late fractures indicative of dislocation creep and strain hardening. These late fractures are an important micro-structural setting for gold and clusters of precious-metal mineral inclusions (telluride minerals). Minor recrystallization processes can also be observed along phase boundaries between pyrite and more competent amphibole crystals. LA-ICP-MS trace element maps document primary, syn-metamorphic oscillatory zoning of some chalcophile and siderophile elements during crystallization of pyrite porphyroblasts. These primary pyrite features are cut by late metal-rich fractures suggesting that remobilization of gold occurred with trace element enrichment of other chalcophile and siderophile elements (Cu, Pb, Zn, Ag, Bi, Te), which post-dates the main period of syn-metamorphic pyrite crystallization at the margins of pre- to syn-deformation, high-grade gold veins. Pyrite grain boundaries and subgrains are also base and precious metal rich, suggesting that late gold remobilization also occurred during pyrite recrystallization. Additional trace element mapping will help determine to what extent pyrite plastic deformation facilitates the diffusion of gold and other trace elements during gold precipitation and remobilization, which, in turn, will inform the source to sink pathways of ore deposition.

What can blueschists tell about the Deep? High Pressure in the Anatolide - Taurid Belt

NASA Astrophysics Data System (ADS)

Oberhaensli, R.

2011-12-01

High-pressure metamorphic terranes in the Anatolide - Taurid belt document the complex distribution of paleo-sutures in the Tethyan realm. Field based petrologic studies of metapelites in the Anatolide-Taurid realm allow to trace HP-LT metamorphism not only in the well known ophiolitic Tavsanli Zone (2.4 GPa/500 °C) but also in the Afyon Zone (0.9 GPa/350 °C), the Menderes Massif (1.2 Gpa/500 °C;) and in the Lycian Nappes (1.0 Gpa/400 °C) - all situated north of the so called Taurid Platform. While the HP metamorphism is dated to 90-80 Ma (Rb/Sr; Ar/Ar) in the Tavsanli Zone, it ranges from 60-70 Ma (Ar/Ar) in the Afyon Zone and its tectonic equivalent, the Lycian Nappes. The Afyon Zone s.l. is closely related to the glaucophane- lawsonite-bearing rocks of the Tavsanli Zone and its eastward extension. Blueschist-facies metamorphism is documented by Fe,Mg-carpholite in regionally distributed metapelites and glaucophane in sparse mafic rocks (Afyon, Menderes, Lycia). Since observations of HP are based on Fe,Mg-carpholite bearing metasediments and not on mafic blueschists new thermodynamic data and petrologic modelling was elaborated to match P-T data and field-based observations. Moreover, newly formed phengitic mica allows precise dating. Both, Tavsanli and Afyon Zones can be followed along strike over more than 600 km and around the southern edge of the Central Anatolian Crystalline Complex. The two zones are situated north of the Taurid Platform and correlate with the Amasia Zone in Armenia. To the extreme East the Bitlis Complex underwent a LT - HP metamorphic blueschist evolution (1,1 GPa/ 350 °C; glaucophane, Fe,Mg-carpholite) in its sedimentary cover while the basement is eclogitic. Depending on the structural position and mineral association of phengitic mica metamorphic ages of the Bitlis blueschists scatter around 70-80 Ma. Eclogites from the basement are slightly older. These LT-HP units cannot be correlated with the Tavsanli - Afyon blueschist belts since they occur south of the Taurid Platform. Thus the Bitlis Complex represents a terrane detached from the Arabian Platform that subsequently collided with the Taurus Platform during closure of the Neo-Tethys. In SW Anatolia, south of the Taurus Platform, the Alanya Zone documents a Late Cretaceous HP evolution with blueschists and eclogites. Together with the Bitlis Complex the two Late Cretaceous HP-LT regions represent a suture south of the Taurid Platform but still north of the Hatay - Güleyman - Zagros ophiolites separating the Arabian Platform from the Anatolide-Taurid realm. The dissection of the Anatolide-Taurid realm into several paleo-subduction zones of Late Cretaceous age impacts on the lithospheric structure and has consequences for the Tertiary plateau formation in Central and Eastern Anatolia. Geophysical data and observations from the East Anatolian Plateau can be explained with petrologic modelling when hydration of the lithospheric lids above subduction zones is considered.

Rethinking how Undergraduate ``Hard Rock'' Petrology is Taught

NASA Astrophysics Data System (ADS)

Reid, M. R.

2010-12-01

A course in "hard rock" petrology forms a core component of undergraduate training in the geosciences. In most cases, the subjects of igneous and metamorphic petrology are combined in a single course and the course is traditionally structured so that the two subjects are covered in series. This approach enables students to focus on each subject separately, with knowledge of igneous rocks helping students to understand metamorphic rock protoliths. Student assessment shows, however, that this approach tends to compartmentalize learning and the two main subjects might just as well be taught in separate courses. In practical applications such as fieldwork, students must be able to access their understanding of igneous and metamorphic rocks virtually simultaneously. To better integrate student learning, I developed a spiral learning approach to teaching petrology (e.g., Bruner, 1990; Dyar et al., 2004) so that commonalities could be revisited several times over the course of a semester and, in so doing, students' grasp of the fundamental insights provided by igneous and metamorphic rocks could be scaffolded into greater understanding. The course initially focuses on the dynamics of the environments in which igneous and metamorphic rocks form: heat flow, fluid flow, and plate tectonics. Several subsequent weeks explore topics relevant to identifying and understanding igneous and metamorphic rocks in the field: crystal nucleation and growth, the roles of pressure and heat, and field classification. Laboratory exercises parallel this structure, also emphasizing observations that are valuable in the field: the relationship between minerals and rocks, textural observations, and general rock classification. The final portion of the course explores “hard rocks” in more detail with a greater emphasis on the interplay between chemistry and mineralogy. A variety of learner-centered activities in the course help students bridge the gap between novice and expert and include more explicit emphasis on visualization and on helping students become comfortable with interpreting data numerically and graphically. Pen tablet computers are used extensively in the laboratory for visualization, photomicrograph capture, and annotation. Cooperative learning activities developed for this course make use of learning methods such as pair share, round-robin, small group explorations case studies, and jigsaw exercises (sometimes as introduction to, sometimes as review of material), and Jeopardy-style review sessions. On an assessment questionnaire at the end of the semester students ranked the in-class cooperative learning activities as on par with lectures and homework exercises in facilitating their learning. Students reported satisfactory attainment of three major goals identified for the course even though they were not explicitly reminded of these goals at the time of assessment. References cited: Bruner, J., 1990. Acts of Meaning. Harvard University Press.; Dyar, M.D., Gunter, M.E., Davis, J.C., and Odell, M.R., 2004. Integration of new methods into teaching mineralogy; Huba, M.E. and Freed, J.E., 2000. Learner-centered Assessment on College Campus: Shifting the Focus from Teaching to Learning. Allyn and Bacon.

Tectonic Uplift of the Danba Area in the Eastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chang, C. P.; Ho, H. P.; Horng, C. S.; Hsu, Y. C.; Tan, X. B.

2017-12-01

The Danba anticline in the eastern Tibetan Plateau is located between the Longmenshan orogen to the east and the Xianshuihe sinistral fault zone to the west. This anticline has been recognized as an area with extreme exhumation by previous studies. The Tibetan plateau was built by the convergence between Indo-Australian plate and Eurasian plate since early Cenozoic. The eastward lower crustal flow under the plateau obstructed by the Yangtze craton soon after this convergence generated a very complex structural situation in the southeastern side of the Tibetan plateau. In this study, in order to understand the processes and mechanisms of the structural complexity of the Danba area, we apply two methods: stress analysis and magnetic measurement. By measuring the brittle deformation recorded in the strata, we carry out a series of stress analysis to demonstrate the stress field of this area. In addition, due to comprehend the magnetic characteristics of low-grade metamorphic rocks and volcanic rocks in this area, we process the rock magnetic measurement of hysteresis loop and X-ray diffraction analysis. The occurrence of pyrrhotite can be taken as an important isograd in low-grade metamorphic rocks, which is helpful for stratigraphic and structural studies. Based on our results, we try to explain the mechanism of this rapid uplift, which involves material, structural, and kinematic interaction.

The role of serpentinite melanges in the unroofing of UHPM rocks: An example from the Western Alps of Italy: Chapter 6

USGS Publications Warehouse

Blake, M. Clark; Moore, D.E.; Jayko, A.S.; Coleman, Robert G.

1995-01-01

The ultrahigh pressure metamorphic rocks (UHPM) of the Dora-Maira continental massif are overlain by a stack of oceanic nappes. Metamorphic grade appears to increase downward but with marked discontinuities between each of the nappes, suggesting that section has been removed along the bounding faults. This apparent omission of section is greatest in the lowest oceanic unit where a serpentinite melange containing blocks and slabs of eclogite, metamorphosed at 12–19 kbar, lies on the UHPM rocks. We suggest that this serpentinite melange represents a highly attenuated upper mantle section that structurally overlay the UHP rocks during subduction. Similar serpentinite melanges are known from other high-pressure (HP) and UHPM areas and may have a similar origin.

Palaeomagnetic, rock-magnetic and mineralogical investigations of metadolerites from Western Svalbard : A preliminary report

NASA Astrophysics Data System (ADS)

Michalski, Krzysztof; Nejbert, Krzysztof; Domańska-Siuda, Justyna; Manby, Geoffrey

2014-05-01

A group of 42 independently oriented palaeomagnetic samples from 7 sites located in central part of the West Spitsbergen Thrust and Fault Belt has been investigated. The samples were collected from 5 distinct metadolerite sheets intruded into the Proterozoic - Lower Paleozoic metamorphic complex of Western Oscar II Land (Western Svalbard Caledonian Terrane - Harland, 1997 division). All analyzed metadolerite samples were metamorphosed under greenschist facies metamorphism. The metamorphic assemblage consist of hornblende, biotite, actinolite, chlorite, epidote, stilpnomelane, titanite, albite, and quartz. Calcite, associated with pyrrhotite, pyrite chalcopyrite, sphalerite, and covellite, that occurs as irregular intergrowths or thin veins, document high activity of H2O-CO2-rich fluids during metamorphism. Primary magmatic phases represented by clinopyroxene occur rarely, and only in thick metadolerite dykes. Accessory oxides change their mineralogical and chemical composition during metamorphism. In all examined samples primary Ti-magnetite and oxy-exsolved hematite break-down completely into titanite or have been dissolved. The ilmenite are also replaced by titanite, but in metadolerites at contact with host metapelites, slightly altered ilmenite grains with preserved hematite exsolution were documented. Basing on mineralogical observations it should be expected that metamorphic processes have almost completely reset the paleomagnetic data record from the time of dolerite crystallization. This stage can document only rare hematite oxy-exsolution preserved within ilmenite, and presumably small inclusion of magnetite still preserved within unaltered clinopyroxene. The paleomagnetic record of metamorphic stage is probably recorded by pyrrhotite, hematite, goethite, and late Ti-free magnetite that can grow during breakdown of pyrrhotite to pyrite (Ramdohr. 1980). The NRM (Natural Remanent Magnetisation) intensities of the palaeomagnetic samples exceed the minimum 10 mA/m. The first AF/thermal demagnetizations have revealed a stable NRM structure. ChRM (Characteristic Remanent Magnetisation) components can be extracted precisely from Zijderveld diagrams (precision parameter - ASD max. 10º). The following magnetic procedures have been applied to identify the ferromagnetic carriers of the samples: SIRM (saturation isothermal remanent magnetization) decay curves (procedure after Kadziałko-Hofmokl & Kruczyk, 1976) and the three-component IRM (isothermal remanent magnetization) procedure described by Lowrie (1990). Experimental work has revealed the dominance of pyrrhotite and magnetite phases as carriers with soft-medium coercivity (samples are saturated in 0.2-0.4 T) and distinct unblocking temperatures around 320-350 ºC and 575-600 ºC respectively. Presented study is the part of PALMAG project 2012-2015: "Integration of palaeomagnetic, isotopic and structural data to understand Svalbard Caledonian Terranes assemblage" (see also Michalski et al. 2012), funded by Polish National Science Centre. References: HARLAND,W.B. 1997. The Geology of Svalbard. Geological Society of London, Memoir 17, 521 pp. KĄDZIOŁKO-HOFMOKL,M. & KRUCZYK,J. 1976. Complete and partial self-reversal of natural remanent magnetization of basaltic rocks from Lower Silesia, Poland. Pure and Applied Geophysics 110, 2031-40. LOWRIE,W. 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophysical Research Letters 17, 159-62. MICHALSKI,K., LEWANDOWSKI,M., MANBY,G.M. 2012. New palaeomagnetic, petrographic and 40Ar/39Ar data to test palaeogeographic reconstructions of Caledonide Svalbard. Cambridge University Press. Geological Magazine 149 (4), 696-721. RAMDOHR,P. 1980. The ore minerals and their intergrowths. Pergamon Press, Oxford.

The metamorphic record of subduction-accretion processes in the Neoarchaean: the Nuuk region, southern West Greenland.

NASA Astrophysics Data System (ADS)

Dziggel, Annika; Kolb, Jochen

2013-04-01

The Nuuk region of southern West Greenland exposes an exceptionally well preserved section through Archaean mid- to lower continental crust, and therefore provides a natural laboratory to study the tectonic processes in the Archaean. The area mainly consists of amphibolite to granulite facies TTG gneisses, narrow supracrustal belts, and minor late-tectonic granites. It is made up of several distinct terranes, including, from NW to SE, the Færingehavn, Tre Brødre, and Tasiusarsuaq terranes. Extensive high-grade metamorphism and a clockwise PT evolution of the Færingehavn terrane in the Neoarchaean (2.72-2.71 Ga) have been interpreted as a result of crustal thickening and thrusting of the Tasiusarsuaq terrane on top of the Tre Brødre and Færingehavn terranes (Nutman and Friend, 2007). Prior to final collision, the Tasiusarsuaq terrane (the upper plate in a plate tectonic model) underwent a prolonged period of compressive deformation between 2.8 and 2.72 Ga (Kolb et al., 2012). The structural evolution was associated with near-isobaric cooling from medium-pressure granulite facies conditions of ca. 850°C and 7.5 kbar to amphibolite facies conditions of ca. 700°C and 6.5-7 kbar (Dziggel et al., 2012). Despite this long period of crustal convergence, there is no evidence for exhumation and/or loading, pointing to a rheologically weak and unstable Archaean crust perhaps due to low density differences and ongoing melt extraction. Rocks of the structurally underlying Færingehavn terrane record a distinctly different metamorphic evolution. Although generally more strongly retrogressed, relict higher-pressure mineral assemblages in mafic granulites and felsic gneisses record conditions of > 8-9 kbar and >= 750°C, indicating burial to depths of at least 30 km along an apparent geothermal gradient of 20-25°C/km. The peak of metamorphism was followed by isothermal decompression at ca. 2.715 Ga (Nutman and Friend, 2007), indicating rapid exhumation of lower crustal rocks during, or shortly after, the main accretionary event. The existence of dual thermal regimes with contrasting PT paths, as well as the good correlation between the timing of collision, high-pressure metamorphism and rapid exhumation are all consistent with plate-tectonic processes operating in the Neoarchaean. However, the crustal convergence in the Nuuk region was not associated with the extreme crustal thickening observed in many younger orogenic belts, and this likely reflects the generally higher mantle temperatures in the Neoarchaean. The prolonged period of crustal convergence prior to final collision may further indicate that the convergence rates in the Archaean were rather low. Dziggel, A., Diener, J.F.A., Stoltz, N.B., Kolb, J., 2012. Role of H2O in the formation of garnet coronas during near-isobaric cooling of mafic granulites: the Tasiusarsuaq terrane, southern West Greenland. Journal of Metamorphic Geology, 30, 957-972. Kolb, J., Kokfelt, T.F., Dziggel, A., 2012. Geodynamic setting and deformation history of an Archaean terrane at mid-crustal level: the Tasiusarsuaq terrane of southern West Greenland. Precambrian Research, 212-213, 34-56. Nutman, A.P. & Friend, C.R.L., 2007. Adjacent terranes with ca. 2715 and 2650 Ma high-pressure metamorphic assemblages in the Nuuk region of the North Atlantic Craton, southern West Greenland: Complexities of Neoarchaean collisional orogeny. Precambrian Research, 155, 159-203.

Metamorphic records for subduction erosion and subsequent underplating processes revealed by garnet-staurolite-muscovite schists in central Qiangtang, Tibet

NASA Astrophysics Data System (ADS)

Zhang, Xiu-Zheng; Dong, Yong-Sheng; Wang, Qiang; Dan, Wei; Zhang, Chunfu; Xu, Wang; Huang, Ming-Liang

2017-01-01

Subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological, geochemical, and geophysical observations at modern convergent plate margins. So far, not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion. Here we first revealed metamorphic records for a subduction erosion process based on our study of the Late Paleozoic garnet-staurolite-muscovite schists in the central Qiangtang block, Tibet. Provenance analyses suggest that the protoliths of garnet-staurolite-muscovite schists have the Northern Qiangtang-affinity and were deposited in an active continental margin setting. Mineral inclusion data show that the early metamorphic stage (M1) recorded blueschist facies pressure-temperature (P-T) conditions of 0.8-1.1 GPa and 402-441°C, indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high-pressure/low-temperature metamorphism. The peak metamorphic stage (M2) recorded amphibolite facies P-T conditions of 0.3-0.5 GPa and 470-520°C. The 40Ar/39Ar cooling ages (263-259 Ma) yielded from muscovite suggest the amphibolite facies metamorphism (>263 Ma) occurred at oceanic subduction stage. The distinctly staged metamorphism defines a clockwise and warming decompression P-T-t path which reveals an underplating process following the early subduction erosion. During the tectonic process, the eroded low-density material escaped from the cold subduction channel and rise upward into the warm middle-lower crust of the upper plate, undergoing amphibolite facies metamorphism. Our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the Paleo-Tethys Ocean.

Metabasalts as sources of metals in orogenic gold deposits

NASA Astrophysics Data System (ADS)

Pitcairn, Iain K.; Craw, Dave; Teagle, Damon A. H.

2015-03-01

Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

Strain partitioning in the Belledonne and Pelvoux massifs. Some clues to understand the Variscan tectono-thermal evolution.

NASA Astrophysics Data System (ADS)

Fréville, Kévin; Trap, Pierre; Faure, Michel; Melleton, Jeremie; Blein, Olivier

2016-04-01

This contribution presents new structural, petrological, geochemical and geochronological data obtained in the Variscan basement of the Alpine Belledonne and Pelvoux External Crystalline massifs. The Belledonne-Pelvoux area is a stack of four litho-tectonic units. The uppermost unit is the early Ordovician Chamrousse ophiolite. It overthrusts a volcanic-sedimentary unit (VSU) made of an alternation of volcanoclastic rocks, plagiogranites and metapelites. The VSU crops out in the eastern Belledonne and western Pelvoux massifs. It is unconformably overlain by a Visean sandstone-conglomerate series with felsic lava (keratophyres). The lowermost litho-tectonic unit is made of felsic and mafic migmatites and granitoids that form the major part of the Pelvoux massif. The western boundary of this tectonic pile is the "synclinal median" strike-slip fault, on the western side of which crops out the Belledonne micaschist unit made of Cambro-ordovician turbiditic series. The structural analysis revealed four main tectono-thermal events: Dx, D1, D2, and D3. Dx is only recorded in relictual metamorphic assemblage in Ky-Grt-Ab bearing micaschist from the VSU holding an obduction metamorphic gradient (3kbar, 370°C up to 7kbar, 430°C). The age of the Dx event remains unknown. The D1 event, characterized by westward low-angle dipping foliation (S1) and a NE-SW striking stretching lineation (L1), is responsible for the crustal thickening resulting of the Eastward emplacement of the Chamrousse ophiolite upon the VSU. D1 is coeval with a barrovian metamorphism with P-T conditions of 6kbar, 600°C recorded in metapelites, and partial melting developed at the base of the VSU. Monazite LA-ICP-MS U-Pb dating revealed that D1 crustal thickening occurred at 337±7 Ma. D2 is a sinistral transpressional deformation responsible for the folding of S1 and L1, and the development of a NE-SW trending pervasive sub-vertical foliation S2. In the lower structural domain, i.e. the partially molten Pelvoux core, D2 intensifies with the development of C-C'sub-vertical sinistral shear zones. At mid-crustal level, in the western Pelvoux massif, a flat lying S3 foliation transposes the D2 S-C-C' pattern. The D3 event occurs in response to a vertical shortening probably due to the ascent of the partially molten crust beginning during D2. D3 marks a transition zone where the deformation is partitioned between molten and unmolten rocks. In spite of Alpine shear zones, due to the high elevation, the Belledonne-Pelvoux area provides a continuous section of the upper to middle Variscan crust. From the data set presented above, we propose that the Belledonne-Pelvoux area exposes two different tectono-metamorphic expressions of the same geodynamic history, due to their different structural position in the continental crust. This interpretation challenges the classical "tectonic collage" model along the east Variscan shear zone that would have put in contact different tectono-metamorphic realms.

Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

NASA Astrophysics Data System (ADS)

Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

2018-05-01

Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This pressure-temperature estimate indicates that metagranitoids underwent high-pressure metamorphism but equilibrated at significantly lower pressures than associated eclogite-facies rocks, and, therefore, they do not necessarily share a common high-pressure metamorphic evolution. The new data show that the Chuacús complex in the study area represents a Proterozoic (1.1-1.0 Ga) to Late Triassic (220 Ma) continental basement that was subducted, and consequently metamorphosed under high-pressure conditions, during the Cretaceous evolution of the North America-Caribbean plate boundary.

SHRIMP U–Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: evidence for ages of deposition, hydrothermal alteration, and metamorphism

USGS Publications Warehouse

Aleinikoff, John N.; Lund, Karen; Fanning, C. Mark

2015-01-01

The Belt–Purcell Supergroup, northern Idaho, western Montana, and southern British Columbia, is a thick succession of Mesoproterozoic sedimentary rocks with an age range of about 1470–1400 Ma. Stratigraphic layers within several sedimentary units were sampled to apply the new technique of U–Pb dating of xenotime that sometimes forms as rims on detrital zircon during burial diagenesis; xenotime also can form epitaxial overgrowths on zircon during hydrothermal and metamorphic events. Belt Supergroup units sampled are the Prichard and Revett Formations in the lower Belt, and the McNamara and Garnet Range Formations and Pilcher Quartzite in the upper Belt. Additionally, all samples that yielded xenotime were also processed for detrital zircon to provide maximum age constraints for the time of deposition and information about provenances; the sample of Prichard Formation yielded monazite that was also analyzed. Ten xenotime overgrowths from the Prichard Formation yielded a U–Pb age of 1458 ± 4 Ma. However, because scanning electron microscope – backscattered electrons (SEM–BSE) imagery suggests complications due to possible analysis of multiple age zones, we prefer a slightly older age of 1462 ± 6 Ma derived from the three oldest samples, within error of a previous U–Pb zircon age on the syn-sedimentary Plains sill. We interpret the Prichard xenotime as diagenetic in origin. Monazite from the Prichard Formation, originally thought to be detrital, yielded Cretaceous metamorphic ages. Xenotime from the McNamara and Garnet Range Formations and Pilcher Quartzite formed at about 1160– 1050 Ma, several hundred million years after deposition, and probably also experienced Early Cretaceous growth. These xenotime overgrowths are interpreted as metamorphic–diagenetic in origin (i.e., derived during greenschist facies metamorphism elsewhere in the basin, but deposited in sub-greenschist facies rocks). Several xenotime grains are older detrital grains of igneous derivation. A previous study on the Revett Formation at the Spar Lake Ag–Cu deposit provides data for xenotime overgrowths in several ore zones formed by hydrothermal processes; herein, those results are compared with data from newly analyzed diagenetic, metamorphic, and magmatic xenotime overgrowths. The origin of a xenotime overgrowth is reflected in its rareearth element (REE) pattern. Detrital (i.e., igneous) xenotime has a large negative Eu anomaly and is heavy rare-earth element (HREE)-enriched (similar to REE in igneous zircon). Diagenetic xenotime has a small negative Eu anomaly and flat HREE (Tb to Lu). Hydrothermal xenotime is depleted in light rare-earth element (LREE), has a small negative Eu anomaly, and decreasing HREE. Metamorphic xenotime is very LREE-depleted, has a very small negative Eu anomaly, and is strongly depleted in HREE (from Gd to Lu). Because these characteristics seem to be process related, they may be useful for interpretation of xenotime of unknown origin. The occurrence of 1.16–1.05 Ga metamorphic xenotime, in the apparent absence of pervasive deformation structures, suggests that the heating may be related to poorly understood regional heating due to broad regional underplating of mafic magma. These results may be additional evidence (together with published ages from metamorphic titanite, zircon, monazite, and garnet) for an enigmatic, Grenville-age metamorphic event that is more widely recognized in the southwestern and eastern United States

Geochemical, Metamorphic and Geodynamic Evolution implications from subduction-related serpentinites and metarodingites at East Thessaly (Central Greece)

NASA Astrophysics Data System (ADS)

Koutsovitis, Petros

2017-04-01

In Central Greece, the East Thessaly region encompasses ophiolitic and metaophiolitic formations emplaced onto Mesozoic platform series rocks. Metaophiolitic thrust sheets are characterized either by the predominance of serpentinites or metabasites. Serpentinites have been distinguished into three groups, representing distinct metamorphic degrees. Group-1 serpentinites (East Othris region) are characterized by the progressive transformation of lizardite to antigorite, estimated to have been formed under greenschist facies conditions (˜320-340 ˚ C, P≈6-8 kbar) [1]. Group-2 serpentinites (NE Othris and Agia-Agiokampos region) are marked by the further prevalence of antigorite over lizardite, suggesting upper-greenschist to low-blueschist facies metamorphism (˜340-370 ˚ C, P≈9-11 kbar) [1]. Group-3 serpentinites (Agia-Agiokampos region) are characterized by the predominance of antigorite and Cr-magnetite, as well as by their relatively low LOI (10.9-12.6 wt.%), corresponding to blueschist facies metamorphism (˜360-400 ˚ C, P≈12 kbar) [1]. These metamorphic conditions are highly comparable with the P-T estimates from the Easternmost Thessaly metabasic rocks, strongly indicating that the entire metaophiolitic formation (excluding East Othris) underwent blueschist facies metamorphism. Serpentinites from East Thessaly were formed from serpentinization of highly depleted harzburgitic protoliths under extensive partial melting processes (>15%), pointing to a hydrous subduction-related environment. Group-1 serpentinites exhibit higher Mg/Si ratio values and LOI compared to serpentinite Groups-2 and -3. Differences in the trace element behavior amongst the three serpentinite groups are also consistent with increasing metamorphic conditions (e.g. Pb, La enrichments, Ti, Y, Yb depletions) [1]. The East Thessaly serpentinites reflect highly oxidizing conditions (-0.4< FMQ<1.2) [1]. These serpentinites appear to have also been subjected to deserpentinization retrograde metamorphic processes (P<8 kbar and T<350 ˚ C) [1]. Retrograde metamorphism also resulted in the occurrence of late-stage rodingitization and derodingitization processes upon the rodingite intrusions hosted within the serpentinites. Late-stage derodingitization processes (T=250-300 oC) account for the formation of metarodingites (vesuvianite and/or chlorite bearing). Chlorite-serpentinite schists represent a reaction zone between the serpentinites and the hosted metarodingites [1]. Exhumation of the high-pressure serpentinite- and metabasic-bearing metaophiolitic occurrences may have occurred from either one or even from both of the bilateral oceanic basins (Pindos and Vardar) that coexisted besides the elongated Pelagonian zone. The Middle-Late Jurassic Pindos oceanic SSZ model appears to successfully interpret not only the geochemical and structural data recorded in the western Hellenic-Dinaric ophiolitic complexes, but additionally seems to explain the formation and emplacement for many of the East Thessaly metaophiolite occurrences. In this context, the exhumed metaophiolites represent parts either of a serpentinized subduction channel or of the serpentinized wedge, located on the hanging wall side close to the slab in the forearc system of the Pindos Ocean. The Hellenic-Dinaric ophiolitic units, as well as the metaophiolitic occurrences, were likely remobilized during thrusting of the flyschic nappe at the main Alpine orogenetic phase of the Upper Cretaceous-Paleogene period. References. [1] Koutsovitis 2016: Lithos, Special Issue, in Press. DOI: 10.1016/j.lithos.2016.11.008

The "Key" Method of Identifying Igneous and Metamorphic Rocks in Introductory Laboratory.

ERIC Educational Resources Information Center

Eves, Robert Leo; Davis, Larry Eugene

1987-01-01

Proposes that identification keys provide an orderly strategy for the identification of igneous and metamorphic rocks in an introductory geology course. Explains the format employed in the system and includes the actual key guides for both igneous and metamorphic rocks. (ML)

Structure and tectonic evolution of the southwestern Trinidad dome, Escambray complex, Central Cuba: Insights into deformation in an accretionary wedge

NASA Astrophysics Data System (ADS)

Despaigne-Díaz, Ana Ibis; García Casco, Antonio; Cáceres Govea, Dámaso; Wilde, Simon A.; Millán Trujillo, Guillermo

2017-10-01

The Trinidad dome, Escambray complex, Cuba, forms part of an accretionary wedge built during intra-oceanic subduction in the Caribbean from the Late Cretaceous to Cenozoic. The structure reflects syn-subduction exhumation during thickening of the wedge, followed by extension. Field mapping, metamorphic and structural analysis constrain the tectonic evolution into five stages. Three ductile deformation events (D1, D2 and D3) are related to metamorphism in a compressional setting and formation of several nappes. D1 subduction fabrics are only preserved as relict S1 foliation and rootless isoclinal folds strongly overprinted by the main S2 foliation. The S2 foliation is parallel to sheared serpentinised lenses that define tectonic contacts, suggesting thrust stacks and underthrusting at mantle depths. Thrusting caused an inverted metamorphic structure with higher-grade on top of lower-grade nappes. Exhumation started during D2 when the units were incorporated into the growing accretionary wedge along NNE-directed thrust faults and was accompanied by substantial decompression and cooling. Folding and thrusting continued during D3 and marks the transition from ductile to brittle-ductile conditions at shallower crustal levels. The D4-5 events are related to extension and contributed to the final exhumation (likely as a core complex). D4 is associated with a regional spaced S4 cleavage, late open folds, and numerous extension veins, whereas D5 is recorded by normal and strike-slip faults affecting all nappes. The P-t path shows rapid exhumation during D2 and slower rates during D3 when the units were progressively incorporated into the accretionary prism. The domal shape formed in response to tectonic denudation assisted by normal faulting and erosion at the surface during the final stages of structural development. These results support tectonic models of SW subduction of the Proto-Caribbean crust under the Caribbean plate during the latest Cretaceous and provide insights into the tectonic evolution of accretionary wedges in an intra-arc setting.

Indium antimonide quantum well structures for electronic device applications

NASA Astrophysics Data System (ADS)

Edirisooriya, Madhavie

The electron effective mass is smaller in InSb than in any other III-V semiconductor. Since the electron mobility depends inversely on the effective mass, InSb-based devices are attractive for field effect transistors, magnetic field sensors, ballistic transport devices, and other applications where the performance depends on a high mobility or a long mean free path. In addition, electrons in InSb have a large g-factor and strong spin orbit coupling, which makes them well suited for certain spin transport devices. The first n-channel InSb high electron mobility transistor (HEMT) was produced in 2005 with a power-delay product superior to HEMTs with a channel made from any other III-V semiconductor. The high electron mobility in the InSb quantum-well channel increases the switching speed and lowers the required supply voltage. This dissertation focuses on several materials challenges that can further increase the appeal of InSb quantum wells for transistors and other electronic device applications. First, the electron mobility in InSb quantum wells, which is the highest for any semiconductor quantum well, can be further increased by reducing scattering by crystal defects. InSb-based heteroepitaxy is usually performed on semi-insulating GaAs (001) substrates due to the lack of a lattice matched semi-insulating substrate. The 14.6% mismatch between the lattice parameters of GaAs and InSb results in the formation of structural defects such as threading dislocations and microtwins which degrade the electrical and optical properties of InSb-based devices. Chapter 1 reviews the methods and procedures for growing InSb-based heterostructures by molecular beam epitaxy. Chapters 2 and 3 introduce techniques for minimizing the crystalline defects in InSb-based structures grown on GaAs substrates. Chapter 2 discusses a method of reducing threading dislocations by incorporating AlyIn1-ySb interlayers in an AlxIn1-xSb buffer layer and the reduction of microtwin defects by growth on GaAs substrates that are oriented 2° away from the [011] direction. Chapter 3 discusses designing InSb QW layer structures that are strain balanced. By applying these defect-reducing techniques, the electron mobility in InSb quantum wells at room temperature was significantly increased. For complementary logic technology, p-channel transistors with high mobility are equally as important as n-channel transistors. However, achieving a high hole mobility in III-V semiconductors is challenging. A controlled introduction of strain in the quantum-well material is an effective technique for enhancing the hole mobility beyond its value in bulk material. The strain reduces the hole effective mass by splitting the heavy hole and light hole valence bands. Chapter 4 discusses a successful attempt to realize p-type InSb quantum well structures. The biaxial strain applied via a relaxed metamorphic buffer resulted in a significantly higher room-temperature hole mobility and a record high low-temperature hole mobility. To demonstrate the usefulness of high mobility in a device structure, magnetoresistive devices were fabricated from remotely doped InSb QWs. Such devices have numerous practical applications such as position and speed sensors and as read heads in magnetic storage systems. In a magnetoresistive device composed of a series of shorted Hall bars, the magnetoresistance is proportional to the electron mobility squared for small magnetic fields. Hence, the high electron mobility in InSb QWs makes them highly preferable for geometrical magnetoresistors. Chapter 5 reports the fabrication and characterization of InSb quantum-well magnetoresistors. The excellent transport properties of the InSb QWs resulted in high room-temperature sensitivity to applied magnetic fields. Finally, Chapter 6 provides the conclusions obtained during this research effort, and makes suggestions for future work.

Decompressional metamorphic P-T paths from kyanite-sillimanite-andalusite bearing rocks in north-central New Mexico

DOE Office of Scientific and Technical Information (OSTI.GOV)

Daniel, C.G.; Thompson, A.G.; Grambling, J.A.

1992-01-01

Proterozoic rocks in six uplifts in northern and central New mexico display decompressional metamorphic P-T paths, apparently, related to crustal extension at a time near 1,440 Ma. Metamorphic P-T paths from the Picuris, Rincon, Truchas, Rio Mora, Sandia and southern Manzano Mountains are constrained by three independent techniques: (1) Al[sub 2]SiO[sub 5] and other mineral parageneses; (2) the Gibbs method applied to compositional zoning in garnet and plagioclase; and (3) a new technique based upon Fe[sup 3+] and Mn[sup 3+] zoning in andalusite. Aluminum silicate textures suggest that kyanite and subsequent sillimanite (commonly aligned parallel to the regional foliation andmore » lineation) are earlier than andalusite, which overprints foliation. The decompressional P-T paths are interpreted to reflect crustal extension. Contact metamorphic aureoles associated with the Priest pluton in the southern Manzano and the Sandia Granite in the Sandia Mountains place constraints upon the timing of the Ky-Sil-And metamorphism. In the southern Manzano Mountains metamorphic isograds marking the first appearance of staurolite and (closer to the pluton) sillimanite are parallel to the margin of the 1,440 Ma Priest quartz monzonite pluton. Similarly, isograds also surround the NW margin of the 1,420 Ma Sandia pluton. The regional metamorphism appears to have coincided with and to have been enhanced by the emplacement of the plutons. Because plutonism and regional metamorphism appear to have been synchronous, pluton emplacement occurred during decompression.« less

Recombination activity of threading dislocations in GaInP influenced by growth temperature

NASA Astrophysics Data System (ADS)

Mukherjee, K.; Reilly, C. H.; Callahan, P. G.; Seward, G. G. E.

2018-04-01

Room-temperature non-radiative recombination is studied at single dislocations in Ga0.5In0.5P quantum wells grown on metamorphic templates using cathodoluminescence and electron channeling contrast imaging. An analysis of the light emission intensity profiles around single dislocations reveals that the average recombination strength of a dislocation decreases by a factor of four and seven as a result of decreasing growth temperature of the GaInP quantum well from 725 to 675 and 625 °C, respectively. This reduction occurs despite little change in the diffusion length, precluding the prospect of inducing carrier localization by ordering and phase separation in GaInP at lower growth temperatures. These observations are rationalized by the premise that point defects or impurities are largely responsible for the recombination activity of dislocations, and the extent of decoration of the dislocation core decreases with temperature. Preliminary evidence for the impact of the Burgers vector is also presented. The lowest growth temperature, however, negatively impacts light emission away from dislocations. Carrier recombination in the bulk and at dislocations needs to be considered together for metamorphic devices, and this work can lead to new techniques to limit non-radiative recombination.

Modelling metamorphism in the Hoosac Schist, Western Massachusetts: new approaches to a New England problem

NASA Astrophysics Data System (ADS)

Bidgood, Anna; Waters, Dave; Gardiner, Nick

2015-04-01

Along the western margin of the metamorphic Appalachians in New England, Taconic (Ordovician) tectonism and metamorphism are overprinted towards the east by Acadian (Devonian) structures and metamorphism. The Hoosac Schist, a probable correlate of the well-known Gassetts Schist of Vermont, lies in the region of overprinting. It forms a narrow N-S-trending tectonically-bound zone crossing several Barrovian mineral-assemblage zones from garnet to kyanite grade. Highly aluminous units containing cm-sized garnets (Cheney & Brady, 1992) are noted for the occurrence of textural unconformities within the garnets, separating inclusion-rich cores from inclusion-poor rims. Matrix domains contain both paragonite and muscovite. Muscovite is present in at least two compositionally distinct generations, with broad later laths cutting across a microfolded earlier fabric. Rutile is restricted to inclusions in garnet, whereas the matrix Ti- phase is ilmenite. These features suggest a polymetamorphic history, potentially recording the superimposition of Acadian metamorphism on Taconic, but it has not yet proved possible to demonstrate the presence of two metamorphic cycles. This study aims to test and employ the new and revised activity models recently developed for metapelites in the full system MnNCKFMASHTO (White et al, 2014), for use with the Holland & Powell data-set 6. Features that can now be more explicitly modelled include garnet zonation in relation to its inclusion suites and microstructural features, the occurrence, texture and distribution of Ti-bearing accessory minerals, and the assemblages and compositional trends in white micas. Preliminary modelling, correlated with microstructural observation, indicates (1) some confirmation of the concern expressed by White et al (2014) that the stability of margarite-bearing assemblages may be somewhat overestimated, (2) that apart from this, the early growth history of garnet is consistent with its suite of trapped inclusions at 8-9 kbar, 500-540°C, (3) that the progression from rutile to ilmenite via an interval of coexistence may be consistent with a single prograde process, and (4) that the marked transition from phengitic to low-Si, high-Na muscovite probably occurred during the growth of the cross-cutting mica generation, but after the growth of ilmenite micropoikiloblasts at the expense of rutile. Cheney, JT & Brady, JB (1992), Petrology of the high-alumina Hoosac Schist from the chloritoid+garnet through the kyanite+biotite zones in western Massachusetts, in: Robinson, P and Brady, JB, editors, Guidebook for Field Trips in the Connecticut Valley Region of Massachusetts and Adjacent States, NEIGC 84th Meeting, Amherst, MA, 332-357. White, RW, Powell, R & Johnson, TE (2014), The effect of Mn on mineral stability in metapelites revisited: new a-x relations for manganese-bearing minerals, Journal of Metamorphic Geology, 32, 809-828.

Temporal and spatial distribution of Paleozoic metamorphism in the southern Appalachian Blue Ridge and Inner Piedmont delimited by ion microprobe U-Pb ages of metamorphic zircon

USGS Publications Warehouse

Merschat, Arthur J.; Bream, Brendan R.; Huebner, Matthew T.; Hatcher, Robert D.; Miller, Calvin F.

2017-01-01

Ion microprobe U-Pb zircon rim ages from 39 samples from across the accreted terranes of the central Blue Ridge, eastward across the Inner Piedmont, delimit the timing and spatial extent of superposed metamorphism in the southern Appalachian orogen. Metamorphic zircon rims are 10–40 µm wide, mostly unzoned, and dark gray to black or bright white in cathodoluminescence, and truncate and/or embay interior oscillatory zoning. Black unzoned and rounded or ovoid-shaped metamorphic zircon morphologies also occur. Th/U values range from 0.01 to 1.4, with the majority of ratios less than 0.1. Results of 206Pb/238U ages, ±2% discordant, range from 481 to 305 Ma. Clustering within these data reveals that the Blue Ridge and Inner Piedmont terranes were affected by three tectonothermal events: (1) 462–448 Ma (Taconic); (2) 395–340 Ma (Acadian and Neoacadian); and (3) 335–322 Ma, related to the early phase of the Alleghanian orogeny. By combining zircon rim ages with metamorphic isograds and other published isotopic ages, we identify the thermal architecture of the southern Appalachian orogen: juxtaposed and superposed metamorphic domains have younger ages to the east related to the marginward addition of terranes, and these domains can serve as a proxy to delimit terrane accretion. Most 462–448 Ma ages occur in the western and central Blue Ridge and define a continuous progression from greenschist to granulite facies that identifies the intact Taconic core. The extent of 462–448 Ma metamorphism indicates that the central Blue Ridge and Tugaloo terranes were accreted to the western Blue Ridge during the Taconic orogeny. Zircon rim ages in the Inner Piedmont span almost 100 m.y., with peaks at 395–385, 376–340, and 335–322 Ma, and delimit the Acadian-Neoacadian and Alleghanian metamorphic core. The timing and distribution of metamorphism in the Inner Piedmont are consistent with the Devonian to Mississippian oblique collision of the Carolina superterrane, followed by an early phase of Alleghanian metamorphism at 335–322 Ma (temperature >500 °C). The eastern Blue Ridge contains evidence of three possible tectonothermal events: ~460 Ma, 376–340 Ma, and ~335 Ma. All of the crystalline terranes of the Blue Ridge–Piedmont megathrust sheet were affected by Alleghanian metamorphism and deformation.

Lightweight IMM PV Flexible Blanket Assembly

NASA Technical Reports Server (NTRS)

Spence, Brian

2015-01-01

Deployable Space Systems (DSS) has developed an inverted metamorphic multijunction (IMM) photovoltaic (PV) integrated modular blanket assembly (IMBA) that can be rolled or z-folded. This IMM PV IMBA technology enables a revolutionary flexible PV blanket assembly that provides high specific power, exceptional stowed packaging efficiency, and high-voltage operation capability. DSS's technology also accommodates standard third-generation triple junction (ZTJ) PV device technologies to provide significantly improved performance over the current state of the art. This SBIR project demonstrated prototype, flight-like IMM PV IMBA panel assemblies specifically developed, designed, and optimized for NASA's high-voltage solar array missions.

On-Orbit Measurement of Next Generation Space Solar Cell Technology on the International Space Station

NASA Technical Reports Server (NTRS)

Wolford, David S.; Myers, Matthew G.; Prokop, Norman F.; Krasowski, Michael J.; Parker, David S.; Cassidy, Justin C.; Davies, William E.; Vorreiter, Janelle O.; Piszczor, Michael F.; McNatt, Jeremiah S.

2014-01-01

On-orbit measurements of new photovoltaic (PV) technologies for space power are an essential step in the development and qualification of advanced solar cells. NASA Glenn Research Center will fly and measure several solar cells attached to NASA Goddards Robotic Refueling Mission (RRM), expected to be launched in 2014. Industry and government partners have provided advanced PV devices for evaluation of performance and environmental durability. The experiment is completely self-contained, providing its own power and internal data storage. Several new cell technologies including Inverted Metamorphic Multi-junction and four-junction cells will be tested.

Kinematics of Faulting and Structural Evolution of Neogene Supra-detachment Basins on the Menderes Metamorphic Core Complex, Western Anatolia

NASA Astrophysics Data System (ADS)

Dilek, Y.; Oner, Z.; Davis, E. A.

2007-12-01

The Menderes metamorphic massif (MM) in western Anatolia is a classic core complex with exhumed high-grade crustal rocks intruded by granodioritic plutons and overlain by syn-extensional sedimentary rocks. Timing and the mechanism(s) of the initial exhumation of the MM are controversial, and different hypotheses exist in the literature. Major structural grabens (i.e. Alasehir, Buyuk Menderes) within the MM that are bounded by high-angle and seismically active faults are late-stage brittle structures, which characterize the block-faulting phase in the extensional history of the core complex and are filled with Quaternary sediments. On the southern shoulder of the Alasehir graben high-grade metamorphic rocks of the MM are overlain by the Miocene and younger sedimentary rocks above a N-dipping detachment surface. The nearly 100-m-thick cataclastic shear zone beneath this surface contain S-C fabrics, microfaults, Riedel shears, mica-fish structures and shear bands, all consistently indicating top-to-the North shearing. Granodioritic plutons crosscutting the MM and the detachment surface are exposed within this cataclastic zone, displaying extensional ductile and brittle structures. The oldest sedimentary rocks onlapping the cataclastic shear zone of the MM here are the Middle Miocene lacustrine shale and limestone units, unconformably overlain by the Upper Miocene fluvial and alluvial fan deposits. Extensive development of these alluvial fan deposits by the Late Miocene indicates the onset of range-front faulting in the MM by this time, causing a surge of coarse clastic deposition along the northern edge of the core complex. The continued exhumation and uplift of the MM provided the necessary relief and detrital material for the Plio-Pleistocene fluvial systems in the Alasehir supradetachment basin (ASDB). A combination of rotational normal faulting and scissor faulting in the extending ASDB affected the depositional patterns and drainage systems, and produced local unconformities within the basinal stratigraphy. High-angle, oblique-slip scissor faults crosscutting the MM rocks, the detachment surface and the basinal strata offset them for more than few 100 meters and the fault blocks locally show different structural architecture and metamorphic grades, suggesting differential uplift along these scissor faults. This fault kinematics and the distribution of range-parallel and range-perpendicular faults strongly controlled the shape and depth of the accommodation space within the ASDB. At a more regional scale scissor faulting across the MM seems to have controlled the foci of Plio-Pleistocene point-source volcanism in the Aegean extensional province (e.g. Kula area). There are no major interruptions in the syn-extensional depositional history of the ASDB, ruling out the pulsed-extension models suggesting a period of contractional deformation in the late Cenozoic evolution of the MM. The onset of exhumation and extensional tectonics in the MM and western Anatolia was a result of thermal weakening of the orogenic crust, following a widespread episode of post-collisional magmatism in the broader Aegean region during the Eocene through Miocene.

Revealing the significance and polyphase tectonothermal evolution of a major metamorphic unit in an orogen: the central Sanandaj-Sirjan zone, Zagros Mts., Iran

NASA Astrophysics Data System (ADS)

Shakerardakani, Farzaneh; Neubauer, Franz; Genser, Johann; Liu, Xiaoming; Dong, Yunpeng; Monfaredi, Behzad; Benroider, Manfred; Finger, Fritz; Waitzinger, Michael

2016-04-01

The Dorud-Azna region in the central Sanandaj-Sirjan metamorphic belt plays a key role in promoting the tectonic evolution of Zagros orogen, within the frame of the Arabia-Eurasia collision zone. From footwall to hangingwall, structural data combined with the U-Pb zircon and extensive 40Ar-39Ar mineral dating survey demonstrate three metamorphosed tectonic units, which include: (1) The Triassic June complex is metamorphosed within greenschist facies conditions, overlain by (2) the amphibolite-grade metamorphic Galeh-Doz orthogneiss, which is intruded by mafic dykes, and (3) the Amphibolite-Metagabbro unit. To the east, these units were intruded by the Jurassic Darijune gabbro. We present U-Pb detrital zircon ages of a garnet-micaschist from the Amphibolite-Metagabbro unit, which yield six distinctive age groups, including a previously unrecognized Late Grenvillian age population at ~0.93 to 0.99 Ga. We speculate that this unique Late Grenvillian group coupled with biogeographic evidence suggests either relationship with the South China craton or to the "Gondwana superfan". The laser ablation ICP-MS U-Pb zircon ages of 608 ± 18 Ma and 588 ± 41 Ma of the granitic Galeh-Doz orthogneiss reveals a Panafrican basement same as known from the Yazd block of Central Iran. Geochemistry and Sr-Nd isotopes of alkaline and subalkaline mafic dykes within the Galeh-Doz orthogneiss show OIB-type to MORB-type and indicate involvement of both depleted and enriched sources for its genesis. The new 40Ar-39Ar amphibole age of ca. 322.2 ± 3.9 Ma from the alkaline mafic dyke implies Carboniferous cooling age after intrusion. The metagabbros (including the Dare-Hedavand metagabbro with a 206Pb/238U age of 314.6 ± 3.7 Ma) and amphibolites with E-MORB geochemical signature of the Amphibolite-Metagabbro unit represent an Upper Paleozoic rift. The geochemical composition of the Triassic greenschist facies metamorphosed June complex, implying formation in a same, but younger tectonic environment. The Darijune gabbro with the mean U-Pb zircon age of 170.2 ± 3.1 Ma postdates greenschist facies-grade metamorphism. This age marks the beginning of the initial subduction of Neotethyan oceanic in a continental arc setting. The best average P-T estimates for the metamorphic mineral assemblages of the Galeh-Doz orthogneiss range between 600 ± 50 °C at 2 ± 0.8 kbar. The subsequent cooling history after an amphibolite facies-grade metamorphism has been constrained with 40Ar-39Ar amphibole ages of plateau ages between 240-260 Ma. The estimate P-T conditions for the Carboniferous metagabbro from core (580 ± 50 °C, 4.0 ± 0.8 kbar) to rim (700 ± 20 °C, 7.5 ± 0.7 kbar) are in favor of a prograde P-T path. The new 40Ar-39Ar mineral ages integrated with structural field and microfabric studies reveals that the amphibolite-grade Carboniferous metagabbro indicate a Late Carboniferous age of amphibolite-grade metamorphism associated with ductile deformation of the new-detected Galeh-Doz nappe (Galeh-Doz orthogneiss and Amphibolite-Metagabbro unit). In the same unit, two lenses of metapelite including a garnet-muscovite-biotite schist give a P-T estimate of garnet cores from 640 ± 20 °C at 6.2 ± 0.8 kbar and garnet rims from 680 ± 20 °C at 7.2 ± 1.0 kbar, as well as garnet-biotite schist that yield lower P-T conditions, which vary from 620 °C at 5.5 ± 0.5 kbar in garnet cores to 600 ± 30 °C at 4.0 ± 1.0 kbar in garnet rims. Chemical monazite ages from garnet micaschists are at 322 ± 28 Ma. 40Ar-39Ar experiments on white mica in the first and second types yield staircase patterns from ca. 36 to 170 Ma and a plateau age of 137.84 ± 0.65 Ma, respectively. Taking all data together, we suggest that amphibolite-grade metamorphism is Carboniferous and is overprinted by two events: (1) during Late Jurassic- Cretaceous during ductile dextral transpressive nappe emplacement of the Galeh-Doz nappe over the June complex (peak conditions of greenschist facies metamorphism at ca. 107 Ma followed by an overprint at 50 Ma) and (2) in ca. 50-32 Ma during shortening and emplacement of the metamorphic nappe complexes over unmetamorphic units of the Zagros orogen.

Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

NASA Astrophysics Data System (ADS)

Fréville, Kévin; Trap, Pierre; Faure, Michel; Melleton, Jérémie; Li, Xian-Hua; Lin, Wei; Blein, Olivier; Bruguier, Olivier; Poujol, Marc

2018-02-01

A structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature-moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E-W- to NE-SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE-SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny.

Microstructures of the Kirsehir Complex, Central Turkey

NASA Astrophysics Data System (ADS)

ISIK, V.; Caglayan, A.; Uysal, T.; Bolhar, R.

2011-12-01

Turkey is positioned on the boundary between the Eurasian and African/Arabian plates, providing an ideal natural laboratory for learning passive and active earth processes such as deformation, metamorphism, earthquakes and volcanism. Central Turkey historically has played an important role in evolution of the Alpine orogeny. The Kirsehir Complex is one of three Mesozoic-Early Tertiary metamorphic and plutonic mid-crustal basement units exposed in central Turkey. The most common lithology of the metamorphites are the banded gneisses, which are intercalated with layers of schists, amphibolites and quartzite, and marbles representing the structurally the highest metamorphites of the study area. The metamorphites are characterized by multiple folding episodes and overprinting faults (thrust, normal and strike-slip). These metamorphites reached peak metamorphic conditions of upper amphibolite facies, as indicated by local presence of clinopyroxene, sillimanite, hornblende, andalusite and garnet. Later, retrograde greenschist facies conditions were attained characterized by the alteration of feldspar and mafic minerals to muscovite and chlorite/actinolite, respectively. The microstructures of selected minerals can be used to bracket the metamorphic grade during which microstructure formed. Quartz displays undulose extinction, deformation bands, subgrains and deformation lamellae, and recrystallisation. The presence of lobate grain boundaries of quartz indicates that GBM recrystallisation occurred. Undulose extinction and recrystallisation are common in micas. Recrystallisation, core-mantle structures in feldspar, myrmekites in K-feldspars within the gneisses suggest that deformation occurred within the amphibolite facies. Garnet occurs as slightly elliptical porphroclats. Sillimanite is present as fibrolite growing near biotite and microboudinaged. Andalusite porphyroblast/porphroclats are elongate and microboudinaged. Kinematic indicators (asymmetric mantled grains, S-C fabrics and mineral fish) suggest top-to-the-southwest shear sense. Our preliminary field observations and microstructure studies suggest the following deformation history in the study area: (1) S-directed thrusting, folding of bedding and forming of S1 foliation, (2) the forming of S2 foliation/mineral stretching lineation/kinematic indicators, emplacement of plutons and exhumation of metamorphites during regional extension, (3) folding of metamorphites, plutons and Paleogene sedimentary units, NNE-directed thrusting and local S3 foliation.

Trace element partitioning between coexisting biotite and muscovite from metamorphic rocks, western Labrador: Structural, compositional and thermal controls

NASA Astrophysics Data System (ADS)

Yang, Panseok; Rivers, Toby

2000-04-01

Coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western Labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to ICP-MS (LAM-ICP-MS) for selected trace elements - Li, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf and Ta. The samples have experienced a single prograde Grenvillian metamorphism ranging from 490 to 680°C and from 7 to 12 kbar. The trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure, major element composition and temperature on the partitioning of each element between biotite and muscovite. Overall, trace element distributions are systematic across the range of metamorphic grade and bulk composition, suggesting that chemical equilibrium was approached. Most distribution coefficients (biotite/muscovite) show good agreement with published data. However, distribution coefficients for Co and Sr are significantly different from previous determinations, probably because of contamination associated with older data obtained by bulk analysis techniques. The sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas. In particular, distribution coefficients exhibit the sequence Cr 3+ (0.615 Å) > V 3+ (0.64 Å) > Sc 3+ (0.745 Å) in VI-sites, and Ba 2+ (1.61 Å) > Sr 2+ (1.44 Å) and Cs + (1.88 Å) > K + (1.64 Å) > Rb + (1.72 Å) > Na + (1.39 Å) in XII-sites. The distributions of Li, Sc, Sr and Ba appear to be thermally sensitive but are also controlled by major element compositions of micas. V and Zr partitioning is dependent on T and may be used to cross-check thermometry calculations where the latter suffer from retrograde re-equilibration and/or high concentrations of Fe 3+. The ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling.

Late Cretaceous extensional denudation along a marble detachment fault zone in the Kırşehir massif near Kaman, central Turkey

NASA Astrophysics Data System (ADS)

Lefebvre, Côme; Barnhoorn, Auke; van Hinsbergen, Douwe J. J.; Kaymakci, Nuretdin; Vissers, Reinoud L. M.

2011-08-01

In the Central Anatolian Crystalline Complex (CACC), 100 km scale metamorphic domains were exhumed in a context of north-south plate convergence during late Cretaceous to Cenozoic times. The timing, kinematics and mechanisms of exhumation have been the focus of previous studies in the southern Niğde Massif. In this study, we investigate the unexplored northern area regarding the tectonic features preserved on the edges of the Kırşehir Massif, based on detailed field-mapping in the Kaman area where high-grade metasediments, non-metamorphic ophiolites and monzonitic plutons are locally exposed together. Close to the contact with the ophiolites, west-dipping foliated marble-rich rocks display mylonites and discrete protomylonites with normal shear senses indicating a general top-to-the W-NW direction. Both of these structures have been brittlely overprinted into cataclastic corridors parallel to the main foliation. The mylonite series and superimposed brittle structures together define the Kaman fault zone. The study of the evolution of calcite deformation fabrics along an EW section supported by Electron Back Scattered Diffraction measurements (EBSD) on representative fabrics indicates that the Kaman fault zone represents an extensional detachment. In Ömerhacılı, in the vicinity of the Baranadağ quartz-monzonite, the metamorphic sequence shows static annealing of the calcite mylonitic fabrics. This evidence suggests that intrusion took place at shallow depth (˜10 km) into an already exhuming metamorphic sequence. As a consequence for the Kaman area, buried metasediments have been rapidly exhumed between 84 and 74 Ma (˜1 km/Ma) where exhumation along a detachment zone, displaying a top-to-the W-NW shear motion, took place in the mid to upper crust prior to magmatic intrusion in the late Campanian. As the intrusion cut through the detachment fault, the main shearing deformation ceased. Brittle tectonics coupled with erosion likely took over during the final unroofing stages at a slower rate (<0.2 km/Ma), until the pertinent rocks reached the Earth's surface in the late Paleocene.

A Simulated Research Problem for Undergraduate Metamorphic Petrology.

ERIC Educational Resources Information Center

Amenta, Roddy V.

1984-01-01

Presents a laboratory problem in metamorphic petrology designed to simulate a research experience. The problem deals with data on scales ranging from a geologic map to hand specimens to thin sections. Student analysis includes identifying metamorphic index minerals, locating their isograds on the map, and determining the folding sequence. (BC)

Detrital zircon age patterns and provenance of the metamorphic complexes of southern Chile

NASA Astrophysics Data System (ADS)

Hervé, F.; Fanning, C. M.; Pankhurst, R. J.

2003-05-01

Zircon SHRIMP U-Pb age patterns are reported for 13 metasedimentary rocks from the low grade metamorphic complexes of the Patagonian Andes. Combined with four recently published patterns, these provide the first detailed survey of the provenance of these complexes. The youngest dated zircons, corresponding to maximum sedimentation ages, are Devonian-Late Triassic in the eastern Andes metamorphic complex, Carboniferous in the main range metamorphic complex, Permian in the Duque de York complex, and Late Triassic in the Chonos metamorphic complex. In the last two cases, these ages are in agreement with their respective fossil ages. Older components in the eastern Andes metamorphic complex include a large proportion of Proterozoic (predominantly 1000-1200 Ma) zircons, which may indicate distribution, probably by rivers, of detrital material from regions currently in northern South America, Africa, or east Antarctica. The abundance of Proterozoic zircons is very much less in the Duque de York complex, possibly because of the rise of an inferred Permian magmatic arc related to the Gondwanan orogeny and consequent westward migration of the watershed. A Late Triassic magmatic episode is registered in the Chonos metamorphic complex, where reappearance of significant Proterozoic zircons indicates exhumation of the cratonic areas or of recycled sedimentary material.

The case for simultaneous deformation, metamorphism and plutonism: an example from Proterozoic rocks in central Arizona

NASA Astrophysics Data System (ADS)

Karlstrom, K. E.; Williams, M. L.

1995-01-01

The syntectonic 1.70 Ga Crazy Basin Monzogranite provides an example of the complex spatial and temporal interactions between metamorphism, deformation, and plutonism. Synchronous plutonism and deformation is indicated by syn-shortening dikes, sills, and veins; parallel magmatic and solid state fabrics; fabrics in xenoliths; and a foliation triple point. Synchronous plutonism and metamorphism is indicated by a systematic increase from 400 °C to 630 °C towards the pluton at a constant pressure of 300 MPa (3 kb). Temperatures are consistent with a conductive cooling model in which a 700 °C pluton was emplaced into country rocks undergoing greenschist facies regional metamorphism. Synchronous deformation and metamorphism is indicated by porphyroblast inclusion geometries that document the synmetamorphic development of the S2 cleavage. The pluton was emplaced adjacent to the Shylock shear zone during progressive shortening. Emplacement of granite as NE-trending sheets was facilitated by temporal partitioning of transpressional convergence into strike-slip and dip-slip components. At the scale of the pluton's aureole and on the relatively rapid time scale of 10 3-10 6 y, regional deformation and metamorphism were punctuated by thermal softening and increased diffusion rates. Data suggests that accretion of Proterozoic arcs in Arizona involved diachronous pluton-enhanced deformation and associated high temperature-low pressure regional metamorphism.

Bedrock Geology and Asbestos Deposits of the Upper Missisquoi Valley and Vicinity, Vermont

USGS Publications Warehouse

Cady, Wallace Martin; Albee, Arden Leroy; Chidester, A.H.

1963-01-01

The upper Missisquoi Valley and vicinity as described in this report covers an area of about 250 square miles at the headwaters of the Missisquoi River in north-central Vermont. About 90 percent of the area is forested and the remainder is chiefly farm land. The topography reflects the geologic structure and varied resistance of the bedrock to erosion. Most of the area is on the east limb of the Green Mountain anticlinorium, which is the principal structural feature of Vermont. The bedrock is predominantly sedimentary and volcanic rock that has been regionally metamorphosed. It was intruded before metamorphism by mafic and ultramafic igneous rocks, and after metamorphism by felsic and mafic igneous rocks. The metamorphosed sedimentary and volcanic rocks range in age from Cambrian(?) to Middle Silurian, the intrusive igneous rocks from probably Late Ordovician to probably late Permian. Metamorphism and principal folding in the region occurred in Middle Devonian time. The metamorphosed sedimentary and volcanic rocks make up a section at least 25,000 feet thick and can be divided into nine formations. The Hazens Notch formation of Cambrian(?) and Early Cambrian age is characterized by carbonaceous schist. It is succeeded in western parts of the area by the Jay Peak formation of Early Cambrian age, which is chiefly a schist that is distinguished by the general absence of carbonaceous zones; in central parts of the area the Hazens Notch formation is followed by the Belvidere Mountain amphibolite, probably the youngest of the formations of Early Cambrian age. The Ottauquechee formation, composed of carbonaceous phyllite and quartzite, and phyllitic graywacke, is of Middle Cambrian age. The Stowe formation of Late Cambrian(?) and Early(?) Ordovician age overlies the Ottauquechee and is predominantly noncarbonaceous schist, though it also contains greenstone and carbonaceous schist and phyllite. The Umbrella Hill formation of Middle Ordovician age is characteristically a conglomerate in which the mineral chloritoid is common. The overlying Moretown formation, also of Middle Ordovician age, contains granulite and slate, also greenstone and amphibolite of the Coburn Hill volcanic member. The Shaw Mountain formation, made up of conglomerate, phyllite, and limestone, is the oldest Silurian unit. The Shaw Mountain formation is succeeded by the Northfield slate of Middle Silurian age. The igneous rocks of the region include various ultramafic plutonic rocks, such as dunite, peridotite, and serpentinite, probably of Late Ordovician age; sills and nearly concordant dikes of metagabbro of Late Ordovician age; biotite granite plutons or Middle or Late Devonian age, most notably on Eltey Mountain; and hypabyssallamprophyre, probably of late Permian age. Metamorphic zoning is shown by the distribution of rocks of the epidote-amphibolite facies and the greenschist facies in and near the Green Mountains, and near Coburn Hill and Eltey Mountain. Metasomatism related to regional metamorphism has produced porphyroblasts and quartz segregations in the sedimentary and volcanic rocks, and steatitization and carbonatization of serpentinite. Contact metamorphism has formed rocks of the epidote-amphibolite facies near granite plutons, and probably calc-silicate rock at the contacts of ultramafic plutons. The axial anticline of the Green Mountain anticlinorium and other anticlines and synclines to the east are the major longitudinal structural features of the area. These structures are complicated by transverse folds, particularly a syncline in the vicinity of Tillotson Peak. Early minor cross folds that are best developed in the Hazens Notch formation are believed to be genetically related to the transverse folds. The axial planes of the cross folds are folded about the axes of the later longitudinal folds of the Green Mountain anticlinorium. The longitudinal and transverse fold systems probably formed in the same episode of defor

P-T paths of ophiolite-related metamorphic rocks from the Dinaride ophiolite zone in Bosnia

NASA Astrophysics Data System (ADS)

Balen, Dražen; Massonne, Hans-Joachim; Koller, Friedrich; Theye, Thomas; Opitz, Joachim; Hrvatović, Hazim; Premužak, Lucija

2017-04-01

The Dinarides, an Alpine mountain chain in south-eastern Europe, is characterized by complex fold, thrust, and imbricate structures. Partially dismembered ophiolites, a regular part of the Inner Dinarides, are considered as separate ultramafic massifs. The large Krivaja-Konjuh ultramafic massif (KKUM) within the Dinaride Ophiolite Zone (DOZ), composed of tectonic spinel lherzolite, occurs as NE-dipping thrust sheet underlain by gradually decreasing, up to 1200 m thick, high- to medium-grade metamorphic rocks. The metamorphic rocks geochemically resemble MORB-like rocks with tholeiitic signature. Such metamorphic rocks, which originated from cumulate gabbro and/or troctolite, are mainly represented by granulite and amphibolite varieties (subordinate eclogite and epidote-amphibolite facies metamafic rocks are also present) with various proportions of amphibole, plagioclase, pyroxenes (diopside and hypersthene), garnet, corundum, sapphirine, spinel and quartz. These rocks vary in textures (granoblastic, porphyroblastic and nematoblastic) and grain size (coarse- to fine-grain varieties). Conventional thermobarometry of garnet- and clinopyroxene-bearing amphibolites directly beneath the contact to the overlying peridotite resulted in peak pressure (P) - temperature (T) conditions of 10-12 kbar (depth of ca. 35-40 km) and 745-830°C. Those amphibolites without clinopyroxene but with garnet experienced peak conditions of 7 kbar and 630°C. Amphibole + plagioclase amphibolite gave temperatures of 670-730 °C and lowermost-grade amphibolites yielded peak temperatures of 550°C. These estimates are thought to reflect the metamorphic conditions during the Late Jurassic obduction of the hot upper mantle part of the KKUM onto the ophiolite mélange. The hot obducted ultramafic fragments acted as a heat source for metamorphism that transformed cumulate gabbroic protolith into high- to medium-grade amphibolites and granulites. P-T pseudosections constructed for various metamorphic rock types in the MnNCFMASHTO system, contoured by mineral isopleths and modes, combined with chemical zonation of garnet (elucidated by X-ray mapping), succession of accessory Ti-minerals (ilmenite -> rutile -> titanite) and textural features (particularly occurrence of complex kelyphite textures around garnet and clinopyroxene) gave us important clues for P-T paths (re)constructions. The petrographic details and mineral chemistry point to composite clockwise P-T paths characterized by high-temperature high-pressure conditions (ca. 20 kbar, 700 °C for garnet- and amphibole-bearing metaperidotite), followed by significant pressure decrease to medium-pressure values accompanied by temperature increase to > 830 °C. Such a composite P-T path can be interpreted in the frame of Late Jurassic to Early Cretaceous regional geodynamic processes that involve collision at the edge of the Adriatic microplate, intra-oceanic NE-dipping subduction and underplating of mafic cumulate rocks under the hot upper mantle part of the KKUM and subsequent erosional events. Processes of the final emplacement of the KKUM metamorphic rocks must have been terminated in Early Cretaceous times as indicated by amphibolite fragments in the adjacent Pogari Formation overlying the ophiolite mélange. Support by the Croatian Science Foundation (IP-2014-09-9541) is acknowledged.

Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

USGS Publications Warehouse

Nesheim, T.O.; Vervoort, J.D.; McClelland, W.C.; Gilotti, J.A.; Lang, H.M.

2012-01-01

Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu-Hf garnet-whole rock ages of 1085??2. Ma, 1198??79. Ma, 1207??8. Ma, 1255??28. Ma, and 1314??2. Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347??10. Ma and 1102??47. Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu-Hf garnet ages indicate two metamorphic garnet growth events at ~. 1330. Ma (M1) and ~. 1080. Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~. 1080. Ma, which overlaps in time with the Grenville Orogeny. The older ~. 1330. Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia. ?? 2011 Elsevier B.V.

Pressure-temperature evolution of Neoproterozoic metamorphism in the Welayati Formation (Kabul Block), Afghanistan

NASA Astrophysics Data System (ADS)

Collett, Stephen; Faryad, Shah Wali

2015-11-01

The Welayati Formation, consisting of alternating layers of mica-schist and quartzite with lenses of amphibolite, unconformably overlies the Neoarchean Sherdarwaza Formation of the Kabul Block that underwent Paleoproterozoic granulite-facies and Neoproterozoic amphibolite-facies metamorphic events. To analyze metamorphic history of the Welayati Formation and its relations to the underlying Sherdarwaza Formation, petrographic study and pressure-temperature (P-T) pseudosection modeling were applied to staurolite- and kyanite-bearing mica-schists, which crop out to the south of Kabul City. Prograde metamorphism, identified by inclusion trails and chemical zonation in garnet from the micaschists indicates that the rocks underwent burial from around 6.2 kbar at 525 °C to maximum pressure conditions of around 9.5 kbar at temperatures of around 650 °C. Decompression from peak pressures under isothermal or moderate heating conditions are indicated by formation of biotite and plagioclase porphyroblasts which cross-cut and overgrow the dominant foliation. The lack of sillimanite and/or andalusite suggests that cooling and further decompression occurred in the kyanite stability field. The results of this study indicate a single amphibolite-facies metamorphism that based on P-T conditions and age dating correlates well with the Neoproterozoic metamorphism in the underlying Sherdarwaza Formation. The rocks lack any paragenetic evidence for a preceding granulite-facies overprint or subsequent Paleozoic metamorphism. Owing to the position of the Kabul Block, within the India-Eurasia collision zone, partial replacement of the amphibolite-facies minerals in the micaschist could, in addition to retrogression of the Neoproterozoic metamorphism, relate to deformation associated with the Alpine orogeny.

Morphology and metamorphosis of Eupsophus calcaratus tadpoles (Anura: Leptodactylidae).

PubMed

Vera Candioti, M F; Ubeda, C; Lavilla, E O

2005-05-01

Eupsophus calcaratus, a leptodactyloid frog from the austral Andean forests of Argentina and Chile, has endotrophic, nidicolous tadpoles. We studied a metamorphic series from Stages 31 to 46 of Gosner's developmental table (1960). Other than the scarce pigmentation, proportionately large eyes, and massive developing hindlimbs, the remaining external characters are similar to those of generalized, exotrophic larvae. At the same time, internal morphology does not reveal any character state attributable to the endotrophic-nidicolous way of life; conversely, structures such as the hyobranchial skeleton and the mandibular cartilages are similar to those of exotrophic-macrophagous tadpoles. The metamorphic process is characterized by the delayed development of diverse structures (e.g., ethmoid region, palatoquadrate, and hyobranchial apparatus), and the retention of some larval characters (e.g., parietal fenestrae, overall absence of ossification) with the absence of development of some "juvenile" characters (e.g., adult otic process, several bones) in metamorphosed individuals. These heterochronic processes and truncation of larval development are related to a shorter larval life (when compared to other species of the austral Andean region) and to the small size at metamorphosis. 2004 Wiley-Liss, Inc.

Late Vendian Complexes in the Structure of Metamorphic Basement of the Fore Range Zone, Greater Caucasus

NASA Astrophysics Data System (ADS)

Kamzolkin, V. A.; Latyshev, A. V.; Vidyapin, Yu. P.; Somin, M. L.; Smul'skaya, A. I.; Ivanov, S. D.

2018-05-01

The paper presents new data on the composition, age, and relationships (with host and overlying deposits) of intrusive rocks in the basement of the Fore Range zone (Greater Caucasus), in the Malaya Laba River Basin. The evolutionary features of intrusive units located within the Blyb metamorphic complex are described. It is shown for the first time that the lower levels of this complex are, in a structural sense, outcrops of the Late Vendian basement. The basement is composed of the Balkan Formation and a massif of quartz metadiorites that intrudes it; for the rocks of this massif, ages ranging from 549 ± 7.4 to 574.1 ± 6.7 Ma are obtained for three U-Pb datings by the SHRIMP-II method. The Herzyinan magmatic event is represented by a group of granodiorite intrusions penetrating the Blyb complex on a series of faults extending along its boundary with the Main Range zone. The obtained estimate for the U-Pb age of one of the intrusions (319 ± 3.8 Ma) corresponds to the end of the Serpukhovian stage of the Early Carboniferous.

Ternary feldspar thermometry of Paleoproterozoic granulites from In-Ouzzal terrane (Western Hoggar, southern Algeria)

NASA Astrophysics Data System (ADS)

Benbatta, A.; Bendaoud, A.; Cenki-Tok, B.; Adjerid, Z.; Lacène, K.; Ouzegane, K.

2017-03-01

The In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9-12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT). In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin. The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism.

Temperature dependence of ballistic mobility in a metamorphic InGaAs/InAlAs high electron mobility transistor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Jongkyong; Gang, Suhyun; Jo, Yongcheol

We have investigated the temperature dependence of ballistic mobility in a 100 nm-long InGaAs/InAlAs metamorphic high-electron-mobility transistor designed for millimeter-wavelength RF applications. To extract the temperature dependence of quasi-ballistic mobility, our experiment involves measurements of the effective mobility in the low-bias linear region of the transistor and of the collision-dominated Hall mobility using a gated Hall bar of the same epitaxial structure. The data measured from the experiment are consistent with that of modeled ballistic mobility based on ballistic transport theory. These results advance the understanding of ballistic transport in various transistors with a nano-scale channel length that is comparable tomore » the carrier's mean free path in the channel.« less

2.8 {mu}m emission from type-I quantum wells grown on InAs{sub x}P{sub 1-x}/InP metamorphic graded buffers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jung, Daehwan; Song, Yuncheng; Larry Lee, Minjoo

We report 2.8 {mu}m emission from compressively strained type-I quantum wells (QWs) grown on InP-based metamorphic InAs{sub x}P{sub 1-x} step-graded buffers. High quality metamorphic graded buffers showed smooth surface morphology and low threading dislocation densities of approximately 2.5 Multiplication-Sign 10{sup 6} cm{sup -2}. High-resolution x-ray diffraction scans showed strong satellites from multiple quantum wells grown on metamorphic buffers, and cross-sectional transmission electron microscopy revealed smooth and coherent quantum well interfaces. Room-temperature photoluminescence emission at 2.8 {mu}m with a narrow linewidth ({approx}50 meV) shows the promise of metamorphic growth for mid-infrared laser diodes on InP.

About 129Xe ∗ in meteoritic nanodiamonds

NASA Astrophysics Data System (ADS)

Fisenko, A. V.; Semjonova, L. F.

2008-08-01

The analysis of excess 129Xe in meteoritic nanodiamonds and the kinetics of its release during stepwise pyrolysis allow to suggest that (1) in the solar nebula 129I atoms were adsorbed onto nanodiamond grains and (or) chemisorbed by forming covalent bonds with carbon atoms. Most 129I atoms existed in a surface connected state, but a minor amount of them was in nanopores of the grains. At radioactive decay of 129I the formed 129Xe ( 129Xe ∗) was trapped by diamond grains due to nuclear recoil. (2) During thermal metamorphism or aqueous alteration, the surface-sited 129I atoms were basically lost. On the basis of these assumptions and calculated concentrations of 129Xe ∗ in meteoritic nanodiamonds it is shown that the minimum closing time of the I-Xe system for meteorites of different chemical classes and low petrologic types may be about one million years relative to the minimally thermally metamorphized CO3 meteorite ALHA 77307. With increasing metamorphic grade the closing time of the I-Xe system increases and can range up to several ten millions years. This tendency is in agreement with an onion-shell model of structure and cooling history of meteorite parent bodies where the temperature increases in the direction from surface to center of the asteroids.

Microbial Diversity of Impact-Generated Habitats

NASA Astrophysics Data System (ADS)

Pontefract, Alexandra; Osinski, Gordon R.; Cockell, Charles S.; Southam, Gordon; McCausland, Phil J. A.; Umoh, Joseph; Holdsworth, David W.

2016-10-01

Impact-generated lithologies have recently been identified as viable and important microbial habitats, especially within cold and arid regions such as the polar deserts on Earth. These unique habitats provide protection from environmental stressors, such as freeze-thaw events, desiccation, and UV radiation, and act to trap aerially deposited detritus within the fissures and pore spaces, providing necessary nutrients for endoliths. This study provides the first culture-independent analysis of the microbial community structure within impact-generated lithologies in a Mars analog environment, involving the analysis of 44,534 16S rRNA sequences from an assemblage of 21 rock samples that comprises three shock metamorphism categories. We find that species diversity increases (H = 2.4-4.6) with exposure to higher shock pressures, which leads to the development of three distinct populations. In each population, Actinobacteria were the most abundant (41%, 65%, and 59%), and the dominant phototrophic taxa came from the Chloroflexi. Calculated porosity (a function of shock metamorphism) for these samples correlates (R2 = 0.62) with inverse Simpson indices, accounting for overlap in populations in the higher shock levels. The results of our study show that microbial diversity is tied to the amount of porosity in the target substrate (as a function of shock metamorphism), resulting in the formation of distinct microbial populations.

Identifying Early Paleozoic tectonic relations in a region affected by post-Taconian transcurrent faulting, an example from the PA-DE Piedmont

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alcock, J.; Wagner, M.E.; Srogi, L.A.

1993-03-01

Post-Taconian transcurrent faulting in the Appalachian Piedmont presents a significant problem to workers attempting to reconstruct the Early Paleozoic tectonic history. One solution to the problem is to identify blocks that lie between zones of transcurrent faulting and that retain the Early Paleozoic arrangement of litho-tectonic units. The authors propose that a comparison of metamorphic histories of different units can be used to recognize blocks of this type. The Wilmington Complex (WC) arc terrane, the pre-Taconian Laurentian margin rocks (LM) exposed in basement-cored massifs, and the Wissahickon Group metapelites (WS) that lie between them are three litho-tectonic units in themore » PA-DE Piedmont that comprise a block assembled in the Early Paleozoic. Evidence supporting this interpretation includes: (1) Metamorphic and lithologic differences across the WC-WS contact and detailed geologic mapping of the contact that suggest thrusting of the WC onto the WS; (2) A metamorphic gradient in the WS with highest grade, including spinel-cordierite migmatites, adjacent to the WC indicating that peak metamorphism of the WS resulted from heating by the WC; (3) A metamorphic discontinuity at the WS-LM contact, evidence for emplacement of the WS onto the LM after WS peak metamorphism; (4) A correlation of mineral assemblage in the Cockeysville Marble of the LM with distance from the WS indicating that peak metamorphism of the LM occurred after emplacement of the WS; and (5) Early Paleozoic lower intercept zircon ages for the LM that are interpreted to date Taconian regional metamorphism. Analysis of metamorphism and its timing relative to thrusting suggest that the WS was associated with the WC before the WS was emplaced onto the LM during the Taconian. It follows that these units form a block that has not been significantly disrupted by later transcurrent shear.« less

Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Rioux, Matthew; Garber, Joshua; Bauer, Ann; Bowring, Samuel; Searle, Michael; Kelemen, Peter; Hacker, Bradley

2016-10-01

The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

USGS Publications Warehouse

Aleinikoff, J.N.; Schenck, W.S.; Plank, M.O.; Srogi, L.A.; Fanning, C.M.; Kamo, S.L.; Bosbyshell, H.

2006-01-01

High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmingto n Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ?? Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ?? 4 and 432 ?? 6 Ma) and monazite (429 ?? 2 and 426 ?? 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ?? 4, 411 ?? 3, and 398 ?? 3 Ma. ?? 2006 Geological Society of America.

Recycling argon through metamorphic reactions: The record in symplectites

NASA Astrophysics Data System (ADS)

McDonald, Christopher S.; Regis, Daniele; Warren, Clare J.; Kelley, Simon P.; Sherlock, Sarah C.

2018-02-01

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20-25 to c. 8-12 kbar at 700 °C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely 'resets' the Ar clock, or whether there is some inheritance of 40Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of 700 °C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history.

U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand - A new report of Early Paleozoic granite

NASA Astrophysics Data System (ADS)

Kawakami, T.; Nakano, N.; Higashino, F.; Hokada, T.; Osanai, Y.; Yuhara, M.; Charusiri, P.; Kamikubo, H.; Yonemura, K.; Hirata, T.

2014-07-01

In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th-U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas. This suggests that these three areas belong to the Sibumasu block, and the Sibumasu block records similar plutono-metamorphic history from Northern to Peninsular Thailand. Relative abundance of oceanic components in the Khao Chao gneiss, their Late Triassic magmatic ages, and the Early Jurassic metamorphic ages prefer the interpretation that the Khao Chao gneiss belongs to the Sukhothai Arc.

SHRIMP U-Pb evidence for a Late Silurian age of metasedimentary rocks in the Merrimack and Putnam-Nashoba terranes, eastern New England

USGS Publications Warehouse

Wintsch, R.P.; Aleinikoff, J.N.; Walsh, G.J.; Bothner, Wallace A.; Hussey, A.M.; Fanning, C.M.

2007-01-01

U-Pb ages of detrital, metamorphic, and magmatic zircon and metamorphic monazite and titanite provide evidence for the ages of deposition and metamorphism of metasedimentary rocks from the Merrimack and Putnam-Nashoba terranes of eastern New England. Rocks from these terranes are interpreted here as having been deposited in the middle Paleozoic above Neoproterozoic basement of the Gander terrane and juxtaposed by Late Paleozoic thrusting in thin, fault-bounded slices. The correlative Hebron and Berwick formations (Merrimack terrane) and Tatnic Hill Formation (Putnam-Nashoba terrane), contain detrital zircons with Mesoproterozoic, Ordovician, and Silurian age populations. On the basis of the age of the youngest detrital zircon population (???425 Ma), the Hebron, Berwick and Tatnic Hill formations are no older than Late Silurian (Wenlockian). The minimum deposition ages of the Hebron and Berwick are constrained by ages of cross-cutting plutons (414 ?? 3 and 418 ?? 2 Ma, respectively). The Tatnic Hill Formation must be older than the oldest metamorphic monazite and zircon (???407 Ma). Thus, all three of these units were deposited between ???425 and 418 Ma, probably in the Ludlovian. Age populations of detrital zircons suggest Laurentian and Ordovician arc provenance to the west. High grade metamorphism of the Tatnic Hill Formation soon after deposition probably requires that sedimentation and burial occurred in a fore-arc environment, whereas time-equivalent calcareous sediments of the Hebron and Berwick formations probably originated in a back-arc setting. In contrast to age data from the Berwick Formation, the Kittery Formation contains primarily Mesoproterozoic detrital zircons; only 2 younger grains were identified. The absence of a significant Ordovician population, in addition to paleocurrent directions from the east and structural data indicating thrusting, suggest that the Kittery was derived from peri-Gondwanan sources and deposited in the Fredericton Sea. Thus, the Kittery should not be considered part of the Laurentian-derived Merrimack terrane; it more likely correlates with the early Silurian Fredericton terrane of northeastern New England and Maritime Canada.

Age and origin of base and precious metal veins of the Coeur d'Alene mining district, Idaho

USGS Publications Warehouse

Fleck, R.J.; Criss, R.E.; Eaton, G.F.; Cleland, R.W.; Wavra, C.S.; Bond, W.D.

2002-01-01

Ore-bearing quartz-carbonate veins of the Coeur d'Alene mining district yield 87Sr/86Sr ratios of 0.74 to >1.60 for low Rb/Sr, carbonate gangue minerals, similar to current ranges measured in Middle Proterozoic, high Rb/Sr rocks of the Belt Supergroup. Stable isotope and fluid inclusion studies establish a genetic relationship between vein formation and metamorphic-hydrothermal systems of the region. These extraordinary 87Sr/86Sr ratios require accumulation of radiogenic 87Sr in a high Rb/Sr system over an extended period prior to incorporation of Sr into the hydrothermal veins. Evaluation of the age and composition of potential sources of highly radiogenic Sr indicates that the ore-bearing veins of the Coeur d'Alene district formed during the Cretaceous from components scavenged from rocks of the Belt Supergroup, the primary host rocks of the district. Proterozoic Pb isotope ratios observed in galena from many Coeur d'Alene veins were established when Pb separated from uranium during deposition or diagenesis of the Belt Supergroup at 1400 to 1500 Ma, possibly as disseminated syngenetic deposits. K-Ar and Rb-Sr apparent ages and ??18O values of Belt Supergroup rocks decrease from the Coeur d'Alene district toward the Idaho and Kaniksu batholiths, approximately normal to the trends of metamorphic isograds, fold axes, foliation, and the major reverse faults of the district. Isoclinal folding, thrust faulting, high-temperature metamorphism, granitic plutonism, and regional-scale metamorphic-hydrothermal activity is documented in the region between 140 and 45 Ma, representing the only such combination of events in the Coeur d'Alene region subsequent to about 1300 Ma. The Sr and oxygen results and geologic evidence favor formation of the ore-bearing carbonate veins by fluids related to a complex metamorphic-hydrothermal system during the Cretaceous. Pb with Proterozoic isotopic compositions was probably mobilized and incorporated like other metals into the hydrothermal veins during this event. The ore-bearing veins were sheared and displaced during early Tertiary northwest-trending dextral strike-slip faulting along the Osburn fault and related structures of the Lewis and Clark line.

A new parameterization for integrated population models to document amphibian reintroductions

USGS Publications Warehouse

Duarte, Adam; Pearl, Christopher; Adams, Michael J.; Peterson, James T.

2017-01-01

Managers are increasingly implementing reintroduction programs as part of a global effort to alleviate amphibian declines. Given uncertainty in factors affecting populations and a need to make recurring decisions to achieve objectives, adaptive management is a useful component of these efforts. A major impediment to the estimation of demographic rates often used to parameterize and refine decision-support models is that life-stage-specific monitoring data are frequently sparse for amphibians. We developed a new parameterization for integrated population models to match the ecology of amphibians and capitalize on relatively inexpensive monitoring data to document amphibian reintroductions. We evaluate the capability of this model by fitting it to Oregon spotted frog (Rana pretiosa) monitoring data collected from 2007 to 2014 following their reintroduction within the Klamath Basin, Oregon, USA. The number of egg masses encountered and the estimated adult and metamorph abundances generally increased following reintroduction. We found that survival probability from egg to metamorph ranged from 0.01 in 2008 to 0.09 in 2009 and was not related to minimum spring temperatures, metamorph survival probability ranged from 0.13 in 2010–2011 to 0.86 in 2012–2013 and was positively related to mean monthly temperatures (logit-scale slope = 2.37), adult survival probability was lower for founders (0.40) than individuals recruited after reintroduction (0.56), and the mean number of egg masses per adult female was 0.74. Our study is the first to test hypotheses concerning Oregon spotted frog egg-to-metamorph and metamorph-to-adult transition probabilities in the wild and document their response at multiple life stages following reintroduction. Furthermore, we provide an example to illustrate how the structure of our integrated population model serves as a useful foundation for amphibian decision-support models within adaptive management programs. The integration of multiple, but related, data sets has an advantage of being able to estimate complex ecological relationships across multiple life stages, offering a modeling framework that accommodates uncertainty, enforces parsimony, and ensures all model parameters can be confronted with monitoring data.

A new parameterization for integrated population models to document amphibian reintroductions.

PubMed

Duarte, Adam; Pearl, Christopher A; Adams, Michael J; Peterson, James T

2017-09-01

Managers are increasingly implementing reintroduction programs as part of a global effort to alleviate amphibian declines. Given uncertainty in factors affecting populations and a need to make recurring decisions to achieve objectives, adaptive management is a useful component of these efforts. A major impediment to the estimation of demographic rates often used to parameterize and refine decision-support models is that life-stage-specific monitoring data are frequently sparse for amphibians. We developed a new parameterization for integrated population models to match the ecology of amphibians and capitalize on relatively inexpensive monitoring data to document amphibian reintroductions. We evaluate the capability of this model by fitting it to Oregon spotted frog (Rana pretiosa) monitoring data collected from 2007 to 2014 following their reintroduction within the Klamath Basin, Oregon, USA. The number of egg masses encountered and the estimated adult and metamorph abundances generally increased following reintroduction. We found that survival probability from egg to metamorph ranged from 0.01 in 2008 to 0.09 in 2009 and was not related to minimum spring temperatures, metamorph survival probability ranged from 0.13 in 2010-2011 to 0.86 in 2012-2013 and was positively related to mean monthly temperatures (logit-scale slope = 2.37), adult survival probability was lower for founders (0.40) than individuals recruited after reintroduction (0.56), and the mean number of egg masses per adult female was 0.74. Our study is the first to test hypotheses concerning Oregon spotted frog egg-to-metamorph and metamorph-to-adult transition probabilities in the wild and document their response at multiple life stages following reintroduction. Furthermore, we provide an example to illustrate how the structure of our integrated population model serves as a useful foundation for amphibian decision-support models within adaptive management programs. The integration of multiple, but related, data sets has an advantage of being able to estimate complex ecological relationships across multiple life stages, offering a modeling framework that accommodates uncertainty, enforces parsimony, and ensures all model parameters can be confronted with monitoring data. © 2017 by the Ecological Society of America.

Teaching Igneous and Metamorphic Petrology Through Guided Inquiry Projects

NASA Astrophysics Data System (ADS)

McMillan, N. J.

2003-12-01

Undergraduate Petrology at New Mexico State University (GEOL 399) has been taught using three, 5-6 week long projects in place of lectures, lab, and exams for the last six years. Reasons for changing from the traditional format include: 1) to move the focus from identification and memorization to petrologic thinking; 2) the need for undergraduate students to apply basic chemical, structural, and field concepts to igneous and metamorphic rocks; 3) student boredom in the traditional mode by the topic that has captivated my professional life, in spite of my best efforts to offer thrilling lectures, problems, and labs. The course has three guided inquiry projects: volcanic, plutonic, and pelitic dynamothermal. Two of the rock suites are investigated during field trips. Each project provides hand samples and thin sections; the igneous projects also include whole-rock major and trace element data. Students write a scientific paper that classifies and describes the rocks, describes the data (mineralogical and geochemical), and uses data to interpret parameters such as tectonic setting, igneous processes, relationship to phase diagrams, geologic history, metamorphic grade, metamorphic facies, and polymetamorphic history. Students use the text as a major resource for self-learning; mini-lectures on pertinent topics are presented when needed by the majority of students. Project scores include evaluation of small parts of the paper due each Friday and participation in peer review as well as the final report. I have found that petrology is much more fun, although more difficult, to teach using this method. It is challenging to be totally prepared for class because students are working at different speeds on different levels on different aspects of the project. Students enjoy the course, especially the opportunity to engage in scientific investigation and debate. A significant flaw in this course is that students see fewer rocks and have less experience in rock classification. This is partially remedied by four field trips and two supplemental assignments (igneous and metamorphic) in which students identify hand samples of a wide variety of rock types. The project-based approach enhances critical thinking, math, reading, and writing skills at the expense of hand sample identification and the benefits of review of material prior to testing.

The origin of zircon and the significance of U-Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

NASA Astrophysics Data System (ADS)

Skrzypek, E.; Štípská, P.; Cocherie, A.

2012-12-01

U-Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700-500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340-335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8-10 kbar/820-860 °C. In felsic granulite samples, the assemblage K-feldspar-garnet-kyanite-Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P-T decrease to 6.5-8.5 kbar/800-820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P-T paths are used to discuss the significance of the U-Pb ages. In the monotonous unit, inherited zircon ages of 700-500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U-Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

Evolution of the blueschist and greenschist facies rocks of Sifnos, Cyclades, Greece

NASA Astrophysics Data System (ADS)

Matthews, Alan; Schliestedt, Manfred

1984-11-01

The metamorphism on the island of Sifnos is characterized by the Eocene development of a coherent highpressure blueschist terrane and an early Miocene greenschist facies overprint. This study documents the metamorphic evolution of the blueschist assemblages, still preserved in the northern parts of the island, and their subsequent transformation into greenschists in the central and southern parts. The oxygen isotope geothermometry is based on calibrations for quartz, pyroxenes and magnetite (Matthews et al. 1983a) augmented by revised calibrations for the minerals muscovite ( Δ Qz-Mu=1.55×106 T -2), epidote ( δ Qz-Ep= 1.56+1.92 Δ ps)106 T -2), and rutile ( Δ Qz-Ru=4.54×106 T -2). Oxygen isotope analyses of minerals from the Blueschist unit of northern Sifnos give consistent fractionations which are independent of rock type. An average temperature of 455° C was obtained, although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions. Analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred. The minerals and whole rocks of the greenschists of central Sifnos are systematically enriched in 18O relative to the blueschist assemblages. Chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of Ca2+ and CO2. The petrologic, isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18O-CO2 enriched aqueous solutions. It is reasonable to assume that this is connected with the Miocene magmatic activity observed throughout the Cyclades. The marbles separating the Blueschist from the Greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation. The pressure of the blueschist metamorphism is estimated at 14±2 kbar, corresponding to a depth of ca. 50 km. The structural style and stratigraphy of Sifnos are suggestive of the subduction of a continental margin sequence. It is clear that the considerable tectonic depression may be associated with continental collision and underthrusting.

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA

USGS Publications Warehouse

Raymond, Loren A.; Merschat, Arthur J.; Vance, R. Kelly

2016-01-01

Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al2O3 and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.

Geology of Precambrian rocks and isotope geochemistry of shear zones in the Big Narrows area, northern Front Range, Colorado

USGS Publications Warehouse

Abbott, Jeffrey T.

1970-01-01

Rocks within the Big Narrows and Poudre Park quadrangles located in the northern Front Range of Colorado are Precambrian metasedimentary and metaigneous schists and gneisses and plutonic igneous rocks. These are locally mantled by extensive late Tertiary and Quaternary fluvial gravels. The southern boundary of the Log Cabin batholith lies within the area studied. A detailed chronology of polyphase deformation, metamorphism and plutonism has been established. Early isoclinal folding (F1) was followed by a major period of plastic deformation (F2), sillimanite-microcline grade regional metamorphism, migmatization and synkinematic Boulder Creek granodiorite plutonism (1.7 b.y.). Macroscopic doubly plunging antiformal and synformal structures were developed. P-T conditions at the peak of metamorphism were probably about 670?C and 4.5 Kb. Water pressures may locally have differed from load pressures. The 1.4 b.y. Silver Plume granite plutonism was post kinematic and on the basis of petrographic and field criteria can be divided into three facies. Emplacement was by forcible injection and assimilation. Microscopic and mesoscopic folds which postdate the formation of the characteristic mineral phases during the 1.7 b.y. metamorphism are correlated with the emplacement of the Silver Plume Log Cabin batholith. Extensive retrograde metamorphism was associated with this event. A major period of mylonitization postdates Silver Plume plutonism and produced large E-W and NE trending shear zones. A detailed study of the Rb/Sr isotope geochemistry of the layered mylonites demonstrated that the mylonitization and associated re- crystallization homogenized the Rb87/Sr 86 ratios. Whole-rock dating techniques applied to the layered mylonites indicate a probable age of 1.2 b.y. Petrographic studies suggest that the mylonitization-recrystallization process produced hornfels facies assemblages in the adjacent metasediments. Minor Laramide faulting, mineralization and igneous activity occurred within this area. A sinuous band of gravel deposits trending into the Livermore embayment and lying well above the present drainage is believed to represent a late Tertiary course of the Cache La Poudre river.

The Limits of Extrusion in the Western Himalaya

NASA Astrophysics Data System (ADS)

Zhang, K.; Webb, A. G.; Donaldson, D.; Johnson, S.; Elorriaga, T.

2014-12-01

Himalayan orogenesis is commonly explained by 1) extrusion models, involving expulsion of high-grade rocks southwards from beneath Tibet and up towards the High Himalayan orographic front, and/or 2) duplexing models, involving accretion of thrust horses from the downgoing Indian plate to the over-riding orogenic wedge. Most extrusion models predict exhumation and erosion of upper-amphibolite facies metamorphic rocks between the Main Central thrust (MCT) and a structurally higher normal fault, and therefore can be tested by determining if such high grade rocks occur between the MCT and the Indus-Yalu suture to the north. Prior qualitative studies suggest that such rocks are missing across the east Ladakh / Chamba and Kashmir regions of the western Himalaya. Here we present new quantitative and semi-quantitative results that document low peak metamorphic temperatures along a northeast-trending transect across the east Ladakh / Chamba Himalaya. We performed illite crystallinity (IC) and quartz grain boundary analyses to determine metamorphic and deformation temperatures, respectively. Calibrated IC values of structurally high samples range from 0.25 to 0.54, indicating temperatures of ~100 ˚C to ~300 ˚C. In structurally lower, muscovite +/- biotite-bearing meta-pelitic and meta-psammitic rocks, quartz grain boundaries show bulging recrystallization fabrics, corresponding to deformation temperatures of <~450 ˚C. Local exceptions occur along the southeast margin of the study region near a dome, where quartz sub-grain rotation fabrics indicate deformation temperatures between ~450 ˚C and ~550 ˚C. Our results, combined with similar IC values to the north from Girard et al. [2001, Clay Minerals v. 36, p. 237-247], demonstrate that a continuous strip of <~450 ˚C rocks extends from the MCT to the Indus-Yalu suture here. Therefore the predictions of extrusion models are not met in this portion of the Himalaya; we present alternative duplexing models.

Age, temperature and pressure of metamorphism in the Tasriwine Ophiolite Complex, Sirwa, Morocco

NASA Astrophysics Data System (ADS)

Samson, S. D.; Inglis, J.; Hefferan, K. P.; Admou, H.; Saquaque, A.

2013-12-01

Sm-Nd garnet-whole rock geochronology and phase equilbria modeling have been used to determine the age and conditions of regional metamorphism within the Tasriwine ophiolite complex,Sirwa, Morocco. Pressure and temperature estimates obtained using a NaCaKFMASHT phase diagram (pseudosection) and garnet core and rim compositions predict that garnet growth began at ~0.72GPa and ~615°C and ended at ~0.8GPa and ~640°C. A bulk garnet Sm-Nd age of 645.6 × 1.6 Ma, calculated from a four point isochron that combines whole rock, garnet full dissolution and two successively more aggressive partial dissolutions, provides a precise date for garnet formation and regional metamorphism. The age is nearly 20 million years younger than a previous age estimate of regional metamorphism of 663 × 14 Ma based upon a SHRIMP U-Pb date from rims on zircon from the Irri migmatite. The new data provide further constraints on the age and nature of regional metamorphism in the Anti-Atlas mountains and emphasizes that garnet growth during regional metamorphism may not necessarily coincide with magmatism/anatexis which predominate the signature witnessed by previous U-Pb studies. The ability to couple PT estimates for garnet formation with high precision Sm- Nd geochronology highlights the utility of garnet studies for uncovering the detailed metamorphic history of the Anti-Atlas mountain belt.

Ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western Maine, USA

NASA Astrophysics Data System (ADS)

Plessen, Birgit; Harlov, Daniel E.; Henry, Darrell; Guidotti, Charles V.

2010-08-01

Ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic-inorganic interactions during diagenesis as well as of the devolatilization history and fluid/rock interaction during metamorphism. In this study, a collection of geochemically well-characterized biotite separates from a series of graphite-bearing Paleozoic greenschist- to upper amphibolite-facies metapelites, western Maine, USA, were analyzed for ammonium nitrogen ( NH4+-N) contents and isotopic composition (δ 15N NH4) using the HF-digestion distillation technique followed by the EA-IRMS technique. Biotite separates, sampled from 9 individual metamorphic zones, contain 3000 to 100 ppm NH4+-N with a wide range in δ 15N from +1.6‰ to +9.1‰. Average NH4+-N contents in biotite show a distinct decrease from about 2750 ppm for the lowest metamorphic grade (˜500 °C) down to 218 ppm for the highest metamorphic grade (˜685 °C). Decreasing abundances in NH4+ are inversely correlated in a linear fashion with increasing K + in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect. Despite expected increasing δ 15N NH4 values in biotite with nitrogen loss, a significant decrease from the Garnet Zones to the Staurolite Zones was found, followed by an increase to the Sillimanite Zones. This pattern for δ 15N NH4 values in biotite inversely correlates with Mg/(Mg + Fe) ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between NH4+-NH or NH4+-N, reflecting devolatilization history and redox conditions during metamorphism.

40Ar/39Ar thermochronology of mesoproterozoic metamorphism in the Colorado Front Range

USGS Publications Warehouse

Shaw, C.A.; Snee, L.W.; Selverstone, J.; Reed, J.C.

1999-01-01

A low-pressure metamorphic episode in the Colorado Front Range has been identified by the presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages. The overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding ~525??C. Twenty-eight 40Ar/39Ar dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamorphism. Muscovite and biotite 40Ar/39Ar ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature interval of micas (~400??-300??C) after about 1400 Ma. In contrast, hornblende apparent ages (T(c)~500??-550??C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon. Forward modeling of argon diffusion shows that the distribution of hornblende and mica ages is consistent with the partial resetting of argon systematics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 550??C and decaying within <20 m.yr. These temperatures match the conditions inferred from the overprinting assemblage; thus, muscovite and biotite ages are interpreted to date the cooling phase of this metamorphic event. This late metamorphism is broadly coeval with the intrusion of ca. 1400-Ma granitic plutons in the study area and throughout the southwestern United States. However, thermal effects are observed far from pluton margins, suggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and plutonism are manifestations of a regional thermal episode that both partially melted the lower crust and pervasively metamorphosed middle crustal rocks.

Definitive Mineralogical Analysis of Martian Rocks and Soil Using the CheMin XRD/XRF Instrument and the USDC Sampler

NASA Technical Reports Server (NTRS)

Blake, D. F.; Sarrazin, P.; Chipera, S. J.; Bish, D. L.; Vaniman, D. T.; Bar-Cohen, Y.; Sherrit, S.; Collins, S.; Boyer, B.; Bryson, C.

2003-01-01

The search for evidence of extant or extinct life on Mars will initially be a search for evidence of present or past conditions supportive of life (e.g., evidence of water), not for life itself. Definitive evidence of past or present water activity lies in the discovery of: * Hydrated minerals: The "rock type" hosting the hydrated minerals could be igneous, metamorphic, or sedimentary, with only a minor hydrated mineral phase. Therefore, the identification of minor phases is important. * Clastic sediments: Clastic sediments are commonly identified by the fact that they contain minerals of disparate origin that could only have come together as a mechanical mixture. Therefore, the identification of all minerals present in a mixture to ascertain mineralogical source regions is important. * Hydrothermal precipitates and chemical sediments: Some chemical precipitates are uniquely identified only by their structure. For example, Opal A, Opal CT, tridymite, crystobalite, high and low Quartz all have the same composition (SiO2) but different crystal structures indicative of different environments - from hydrothermal hydrothermal formation to low temperature precipitation. Other silica types such as stishovite can provide evidence of shock metamorphism. Therefore, identification of crystal structures and structural polymorphs is important. The elucidation of the nature of the Mars soil will require the identification of mineral components that can unravel its history and the history of the Mars atmosphere.

Knowledge representation of rock plastic deformation

NASA Astrophysics Data System (ADS)

Davarpanah, Armita; Babaie, Hassan

2017-04-01

The first iteration of the Rock Plastic Deformation (RPD) ontology models the semantics of the dynamic physical and chemical processes and mechanisms that occur during the deformation of the generally inhomogeneous polycrystalline rocks. The ontology represents the knowledge about the production, reconfiguration, displacement, and consumption of the structural components that participate in these processes. It also formalizes the properties that are known by the structural geology and metamorphic petrology communities to hold between the instances of the spatial components and the dynamic processes, the state and system variables, the empirical flow laws that relate the variables, and the laboratory testing conditions and procedures. The modeling of some of the complex physio-chemical, mathematical, and informational concepts and relations of the RPD ontology is based on the class and property structure of some well-established top-level ontologies. The flexible and extensible design of the initial version of the RPD ontology allows it to develop into a model that more fully represents the knowledge of plastic deformation of rocks under different spatial and temporal scales in the laboratory and in solid Earth. The ontology will be used to annotate the datasets related to the microstructures and physical-chemical processes that involve them. This will help the autonomous and globally distributed communities of experimental structural geologists and metamorphic petrologists to coherently and uniformly distribute, discover, access, share, and use their data through automated reasoning and enhanced data integration and software interoperability.

Neoproterozoic Evolution and Najd‒Related Transpressive Shear Deformations Along Nugrus Shear Zone, South Eastern Desert, Egypt (Implications from Field‒Structural Data and AMS‒Technique)

NASA Astrophysics Data System (ADS)

Hagag, W.; Moustafa, R.; Hamimi, Z.

2018-01-01

The tectonometamorphic evolution of Nugrus Shear Zone (NSZ) in the south Eastern Desert of Egypt was reevaluated through an integrated study including field-structural work and magnetofabric analysis using Anisotropy of Magnetic Susceptibility (AMS) technique, complemented by detailed microstructural investigation. Several lines of evidence indicate that the Neoproterozoic juvenile crust within this high strain zone suffered an impressive tectonic event of left-lateral transpressional regime, transposed the majority of the earlier formed structures into a NNW to NW-directed wrench corridor depicts the northwestern extension of the Najd Shear System (NSS) along the Eastern Desert of Egypt. The core of the southern Hafafit dome underwent a high metamorphic event ( M 1) developed during the end of the main collisional orogeny in the Arabian-Nubian Shield (ANS). The subsequent M 2 metamorphic event was retrogressive and depicts the tectonic evolution and exhumation of the Nugrus-Hafafit area including the Hafafit gneissic domes, during the origination of the left-lateral transpressive wrench corridor of the NSS. The early tectonic fabric within the NSZ and associated highly deformed rocks was successfully detected by the integration of AMS-technique and microstructural observations. Such fabric grain was checked through a field-structural work. The outcomes of the present contribution advocate a complex tectonic evolution with successive and overlapped deformation events for the NSZ.

Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska

USGS Publications Warehouse

Turner, Donald L.; Forbes, Robert B.; Aleinikoff, John N.; McDougall, Ian; Hedge, Carl E.; Wilson, Frederic H.; Layer, Paul W.; Hults, Chad P.

2009-01-01

The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as having been partially reset during the late Mesozoic thermal event. 40Ar/39Ar incremental heating experiments suggest metamorphism occurred at least 1.2 b.y. ago but do not exhibit high temperature plateau ages significantly older than the 40Ar/39Ar total fusion ages of these samples. The age spectra are much more uniform than expected from a terrane with such a complex thermal history, perhaps caused by the small grain size of the samples which may possibly be less than the effective Ar diffusion radii of the analyzed hornblendes.

Stable Isotopes, Multidisciplinary Studies, and the Leadership of J.G. Liou in UHP Metamorphism

NASA Astrophysics Data System (ADS)

Rumble, D.

2005-12-01

J.G. Liou has played a crucial role in improving knowledge of UHP metamorphism by leading multi-disciplinary, multi-institutional teams of researchers and by encouraging new investigators and providing them access to samples. Stable isotope geochemistry has made important contributions to understanding UHP metamorphism including: (1) The discovery of O- and H-isotope signatures of meteoric water in UHP rocks from China and Kazakhstan demonstrates that their protoliths originated at or near Earth's surface in a cold climate(a); (2) The mapping of contiguous tracts of outcrops extending over distances of 100 km where both eclogites and their wall rocks retain unusually low d18O and dD is consistent with the subduction and exhumation of UHP slabs as coherent structural units(b); (3) Analysis of samples from the Chinese Continental Scientific Drilling project reveals not only that UHP metamorphic rocks have not exchanged O-isotopes with mantle rocks while they were buried in the upper mantle but also that garnet peridotite slabs from the mantle have not exchanged with crustal wall rocks(c). Recent advances have resulted from multidisciplinary geochemical investigations. The analysis of zircons for both d18O and U-Pb established the age of cold climate, meteoric water alteration of protoliths to be Neoproterozoic for UHP rocks from Dabie and Sulu, China(d). Thus, O-isotopes plus age dating raises the possibility that evidence of snowball Earth conditions has been preserved in an unlikely host: UHP metamorphic rocks. A comparison of U-Pb, Sm-Nd, and Rb-Sr isotope data with analyses for d18O in coexisting minerals shows that discordant age dates correlate with mineral pairs that are not in O-isotope exchange equilibrium(e). It may be seen that multidisciplinary geochemical investigations provide mutually reinforcing data that greatly strengthens interpretations. New discoveries of de novo microdiamonds accompanied by multiphase mineral inclusions in UHP metamorphosed crustal rocks raise exciting possibilities for future stable isotope research on their origin(f). Micron-scale analytical techniques including ion microprobe, "Nano-SIMS", and UV-laser ablation, should be applied to the mineral assemblages to determine whether parent fluids were super-critical C-O-H fluids or carbonate-rich melts. (a) Geochim.Cosmochim.Acta (GCA) 59, 2859; Euro.J.Mineral 8, 317; GCA 61, 1658.(b) GCA 62, 3307.(c) Amer.Mineral. 90, 857.(d) GCA 66, 2299; GCA 68, 4145.(e) GCA 66, 625.(f) J.Metamorph.Geol. 21, 425.

Magnetic properties of Apollo 14 breccias and their correlation with metamorphism.

NASA Technical Reports Server (NTRS)

Gose, W. A.; Pearce, G. W.; Strangway, D. W.; Larson, E. E.

1972-01-01

The magnetic properties of Apollo 14 breccias can be explained in terms of the grain size distribution of the interstitial iron which is directly related to the metamorphic grade of the sample. In samples 14049 and 14313 iron grains less than 500 A in diameter are dominant as evidenced by a Richter-type magnetic aftereffect and hysteresis measurements. Both samples are of lowest metamorphic grade. The medium metamorphic-grade sample 14321 and the high-grade sample 14312 both show a logarithmic time-dependence of the magnetization indicative of a wide range of relaxation times and thus grain sizes, but sample 14321 contains a stable remanent magnetization whereas sample 14312 does not. This suggests that small multidomain particles (less than 1 micron) are most abundant in sample 14321 while sample 14312 is magnetically controlled by grains greater than 1 micron. The higher the metamorphic grade, the larger the grain size of the iron controlling the magnetic properties.

Geometric description and analysis of metamorphic tectonites (Pelagonian Zone, Internal Hellenides, Northern Greece)

NASA Astrophysics Data System (ADS)

Diamantopoulos, A.

2009-04-01

An assortment of alpine and pre-Permian metamorphic tectonites, belonging to the Pelagonian Zone of the Internal Hellenides, are analyzed from Askion, Vernon and Vorras mountains. They in fact compose the Upper plate of the Western Macedonia core complex, overlying Late Tertiary high-P rocks through large-scale detachment fautls (Diamantopoulos et al. 2007). This work wants to determine the architecture and the kinematic path of rocks in a 3D assumption. Field analysis concludes: a) Meta-sedimentary lithologies and amphibolites, meta-igneous lithologies, granitoid mylonites composed of augen fieldspar gneisses, Permo-Triassic fossiliferous rocks, meta-carbonates of Triassic-Jurassic age, a Jurassic mélange including meta-sedimentary lithologies, serpentinites and carbonate tectonic blocks, Mesozoic Ophiolites, Cretaceous limestones and conglomerates as well as flysch sediments compose the architecture of the study area, b) Multiple high and low-angle cataclastic zones of intense non-coaxial strain separate distinct pre-Permian lithologies, alpine from pre-alpine rocks, Triassic-Jurassic rocks from Permo-Triassic rocks, Jurassic mélange from flysch sediments, Jurassic mélange from Triassic-Jurassic rocks, Cretaceous rocks from the Jurassic mélange, Cretaceous limestones from flysch lithologies and Cretaceous rocks from serpentinites, c) Geometric analysis and description of asymmetric structures found in fault cores, damage zones and in the footwall-related rocks showed a prominent kinematic direction towards WSW in low-T conditions affected all the rock lithologies, d) Multiple S- and L- shape fabric elements in the pre-Permian and Permo-Triassic rocks appear an intricate orientation, produced by intense non-coaxial syn-metamorphic deformation, e) Sheath and isoclinal folds oriented parallel to the L-shape fabric elements as well as a major S-shape fabric element, producing macroscopic fold-like structures compose the main syn-metamorphic fabric elements in the pre-alpine tectonites, f) Discrete and distributed strain along the former boundaries and within footwall- and hangingwall rocks is connoted to control the bulk kinematic path of the involved sequences, g) Field evaluation of the structural geology and the tectonics connote the conjugate character of the cataclastically-deformed boundaries, causing overprinting of the pre-existed ductile-related geometries, h) For the age of the inferred WSW kinematic direction of the involved rocks we believe that it is closely associated with the tectonic superimposition of the Pelagonian Zone onto the Olympos tectonic window during post-Late Eocene times. Miocene to Quaternary faulting activity in all the scales overprint the above Late Tertiary perturbation, resulting a real complicated structural feature (Diamantopoulos 2006). Diamantopoulos A., 2006. Plio-Quaternary geometry and Kinematics of Ptolemais basin (Northern Greece). Implications for the intra-plate tectonics in Western Macedonia. Geologica Croatica 59/1, pages 85-96. Diamantopoulos A., Krohe A., Mposkos E., 2007. Structural asymmetry and distributed strain of low-T shear planes inducing evidence for orogen-scale kinematic partitioning during denudation of high-P rocks (Pelagonian Zone, Greece). Geophysical Research Abstracts, Vol. 9, 03622.

Oblique Northeastward Lateral Extrusion of a Crustal Block in North-central Taiwan: a Mechanism for Syn-tectonic Extension

NASA Astrophysics Data System (ADS)

Gourley, J. R.; Byrne, T.

2005-12-01

An integrated data set of earthquake locations (Taiwan's Central Weather Bureau), focal mechanisms from the Broadband Array of Taiwan Seismicity (BATS), GPS velocities and geologic data are combined to constrain the geometry and kinematics of a crustal block within the metamorphic basement of Taiwan's northeastern Central Range. The active block is bounded by two parallel seismic zones that accommodate uplift and northeastward oblique lateral extrusion. The western shear zone is a region that dips vertically to steeply west and projects generally to the western boundary between the Slate Belt and pre-Tertiary metamorphic basement. BATS focal mechanisms consistently show east-side-up, left-lateral normal displacements. Late-stage geologic structures published previously show left-lateral faulting followed by east-west extension. The eastern shear zone dips vertically to steeply west and projects to the eastern boundary of the metamorphic basement, which correlates with the eastern mountain front in this area. BATS focal mechanisms show west-side-up reverse displacements. The kinematics of these two zones define a crustal scale block that is interpreted to be moving up and northeast towards the Okinawa Trough. The extrusion of this crustal block may be driven in part by the topographic difference between the Central Range and the Okinawa Trough, as well as by the active collision between the Philippine Sea Plate and the Eurasian basement high. This proposed northeastern lateral extrusion mirrors the active lateral extrusion in southwestern Taiwan which is observed on the southern side of the Eurasian basement high collision. The involvement of the basement high in the collision and adjacent regions appears to be an important factor in understanding local structural variations in the arc-continent collision and should be considered in both forward and reverse modeling of Taiwan deformation.

Petrotectonic framework of granulites from northern part of Chilka Lake area, Eastern Ghats Belt, India: Compressional vis-à-vis transpressional tectonics

NASA Astrophysics Data System (ADS)

Das, Kaushik; Bose, Sankar; Karmakar, Subrata; Chakraborty, Supriya

2012-02-01

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M1-D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite-quartz-plagioclase-biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2-D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet-sillimanite gneiss. Petrological data suggest that the M4-D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M3-D3 event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.

Structural and functional maturation of skin during metamorphosis in the Atlantic halibut (Hippoglossus hippoglossus).

PubMed

Alves, Ricardo N; Sundell, Kristina S; Anjos, Liliana; Sundh, Henrik; Harboe, Torstein; Norberg, Birgitta; Power, Deborah M

2018-06-01

To establish if the developmental changes in the primary barrier and osmoregulatory capacity of Atlantic halibut skin are modified during metamorphosis, histological, histochemical, gene expression and electrophysiological measurements were made. The morphology of the ocular and abocular skin started to diverge during the metamorphic climax and ocular skin appeared thicker and more stratified. Neutral mucins were the main glycoproteins produced by the goblet cells in skin during metamorphosis. Moreover, the number of goblet cells producing neutral mucins increased during metamorphosis and asymmetry in their abundance was observed between ocular and abocular skin. The increase in goblet cell number and their asymmetric abundance in skin was concomitant with the period that thyroid hormones (THs) increase and suggests that they may be under the control of these hormones. Several mucin transcripts were identified in metamorphosing halibut transcriptomes and Muc18 and Muc5AC were characteristic of the body skin. Na + , K + -ATPase positive (NKA) cells were observed in skin of all metamorphic stages but their number significantly decreased with the onset of metamorphosis. No asymmetry was observed between ocular and abocular skin in NKA cells. The morphological changes observed were linked to modified skin barrier function as revealed by modifications in its electrophysiological properties. However, the maturation of the skin functional characteristics preceded structural maturation and occurred at stage 8 prior to the metamorphic climax. Treatment of Atlantic halibut with the THs disrupter methimazole (MMI) affected the number of goblet cells producing neutral mucins and the NKA cells. The present study reveals that the asymmetric development of the skin in Atlantic halibut is TH sensitive and is associated with metamorphosis and that this barrier's functional properties mature earlier and are independent of metamorphosis.

Igneous petrogenesis and tectonic setting of granitic rocks from the eastern Blue Ridge, Alabama Appalachians

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drummond, M.S.; Allison, D.T.; Tull, J.F.

1994-03-01

A span of 150 my of orogenic activity is recorded within the granitic rocks of the eastern Blue Ridge of Alabama (EBR). Four discrete episodes of plutonism can be differentiated, each event exhibiting distinct field relations and geochemical signatures. (1) Penobscotian stage: this initial stage of plutonic activity is represented by the Elkahatchee Quartz Diorite (EQD), a premetamorphic (495 Ma) batholith and the largest intrusive complex (880 km[sup 2]) exposed in the Blue Ridge. Calc-alkaline I-type tonalite-granodiorite are the principal lithologies, with subordinate cumulate hbl-bt diorite, metadacite, granite and trondhjemite. The parental tonalitic magmas are interpreted to have been derivedmore » from a subducted MORB source under eclogite to get amphibolite conditions. (2) Taconic stage: the Kowaliga augen gneiss (KAG) and the Zana granite gneiss (ZG) are 460 Ma granitic bodies that reside in the SE extremity and structurally highest portion of the EBR. Both of these bodies are pre-metamorphic with strongly elongate sill- and pod-like shapes concordant with S[sub 1] foliation. Granite and granodiorite comprise the bulk of the KAG. (3) Acadian stage: Rockford Granite (RG), Bluff springs Granite (BSG, 366 Ma), and Almond Trondhjemite represent a suite of pre- to syn-metamorphic granitic intrusions. (4) late-Acadian stage: The Blakes Ferry pluton (BFP) is a post-kinematic pluton displaying spectacular by schlieren igneous flow structures, but no metamorphic fabric. The pluton's age can be bracketed between a 366 Ma age on the BSG and a 324 Ma K-Ar muscovite age on the BFP. BFP's petrogenesis has involved partial melting a MORB source followed by assimilation of metasedimentary host rock.« less

The extending lithosphere (Arthur Holmes Medal Lecture)

NASA Astrophysics Data System (ADS)

Brun, Jean-Pierre

2017-04-01

Extension of the lithosphere gives birth to a wide range of structures, with characteristic widths between 10 and 1000 km, which includes continental rifts, passive margins, oceanic rifts, core complexes, or back-arc basins. Because the rheology of rocks strongly depends on temperature, this variety of extensional structures falls in two broad categories of extending lithospheres according to the initial Moho temperature TM. "Cold extending systems", with TM < 750°C and mantle-dominated strength, lead to narrow rifts and, if extension is maintained long enough, to passive margins and then mantle core complexes. "Hot extending systems", with TM > 750°C and crustal-dominated strength, lead, depending on strain rate, to either wide rifts or metamorphic core complexes. A much less quoted product of extension is the exhumation of high-pressure (HP ) metamorphic rocks occurring in domains of back-arc extension driven by slab rollback (e.g. Aegean; Appennines-Calabrian) or when the subduction upper plate undergoes extension for plate kinematics reasons (e.g. Norwegian Caledonides; Papua New Guinea). In these tectonic environments, well-documented pressure-temperature-time (P - T - t) paths of HP rocks show a two-stage retrogression path whose the first part corresponds to an isothermal large pressure drop ΔP proportional to the maximum pressure Pmax recorded by the rocks. This linear relation between ΔP and Pmax, which likely results from a stress switch between compression and extension at the onset of exhumation, is in fact observed in all HP metamorphism provinces worldwide, suggesting that the exhumation of HP rocks in extension is a general process rather than an uncommon case. In summary, the modes and products of extension are so diverse that, taken all together, they constitute a very versatile natural laboratory to decipher the rheological complexities of the continental lithosphere and their mechanical implications.

Transdomes: Emplacement of Migmatite Domes in Oblique Tectonic Settings

NASA Astrophysics Data System (ADS)

Teyssier, C. P.; Rey, P. F.; Whitney, D. L.; Mondy, L. S.; Roger, F.

2014-12-01

Many migmatite domes are emplaced within wrench corridors in which a combination of strike-slip and extensional detachment zones (pull-apart, extensional relay, or transfer zones) focus deep-crust exhumation. The Montagne Noire dome (France, Variscan Massif Central) exemplifies wrench-related dome formation and displays the following structural, metamorphic, and geochronologic characteristics of a 'transdome': the dome is elongate in the direction of extension; foliation outlines a double dome separated by a high-strain zone; lineation is shallowly plunging with a fairly uniform trend that parallels the strike of the high-strain zone; subdomes contain recumbent structures overprinted by upright folds that affected upward by flat shear zones associated with detachment tectonics; domes display a large syn-deformation metamorphic gradient from core (upper amphibolite facies migmatite) to margin (down to greenschist facies mylonite); some rocks in the dome core experienced isothermal decompression revealed by disequilibrium reaction textures, particularly in mafic rocks (including eclogite); and results of U-Pb geochrononology indicate a narrow range of metamorphic crystallization from core to mantling schist spanning ~10 Myr. 3D numerical modeling of transdomes show that the dome solicits a larger source region of partially molten lower crust compared to 2D models; this flowing crust creates a double-dome architecture as in 2D models but there are differences in the predicted thermal history and flow paths. In a transtension setting, flow lines converge at depth (radial-centripetal flow) toward the zone of extension and diverge at shallow levels in a more uniform direction that is imposed by upper crust motion and deformation. This evolution produces a characteristic pattern of strain history, progressive fabric overprint, and P-T paths that are comparable to observed dome rocks.

The petrology, structure and geochemistry of an Archean terrane in the North Snowy Block, Beartooth Mountains, Montana

NASA Astrophysics Data System (ADS)

Mogk, D. W.

1984-12-01

Six major rock units in the North Snowy Block in an Archean mobile belt are recognized between all units representing discontinuities in metamorphic grade, structural style, geochemistry, and isotopic ages. Four of the units occur in NE trending linear belts; the Basement Gneiss; the phyllitic Davis Creek Schist; the mount cowen augen gneis; the Paragneiss unit. Overlying the linear units is the 3.2 Ga old Pine Creek Nappe Complex, an isoclinally folded, middle to upper amphibolite facies, thrust nappe consisting of the Barney Creek Amphibolite, George Lake Marble and Jewel Quartzite. The highest structural units, including a thick sequence of upper amphibolite grade supracrustal rocks and a lower section of injected 3.4 Ga old granitic to tonalitic migmatitic rocks were emplaced on the Columbine Thrust. It is shown that there was secular variation in tectonic style in the Archean of southwest Montana. Three stages are recognized: (1) melting of ancient matic crust produced trondhjemitic continental nuclei; (2) numerous ensialic basins were created and destroyed, resulting in high grade metamorphism and mignatization of supracrustal rocks; and (3) contemporary style plate tectonics resulted in generation of large volumes of andesities and calc-alkaline granitic rocks, transcurrent faulting, and thrust faulting.

Cathodoluminescence of diamond as an indicator of its metamorphic history

NASA Astrophysics Data System (ADS)

Kopylova, Maya; Bruce, Loryn; Longo, Micaela; Ryder, John; Dobrzhinetskaya, Larissa

2010-05-01

Diamond displays a supreme resistance to chemical and mechanical weathering, ensuring its survival through complex and prolonged crustal processes, including metamorphism and exhumation. For these reasons, volcanic sources and secondary and tertiary collectors for detrital placer diamonds, like Ural or Bingara diamonds, may be difficult to determine. If metamorphic processes leave their marks on diamond, they can be used to reconstruct crustal geologic processes and ages of primary diamondiferous volcanics. Four diamond suites extracted from metamorphic rocks have been characterized using optical CL, infrared and CL spectroscopy, and photoluminescence at the liquid nitrogen temperature. The studied diamonds are from the ~2.7 Ga sedimentary conglomerate and lamprophyric breccia metamorphosed in the greenschist facies (Wawa, Northern Ontario, Canada) during the 2.67 Ga Kenoran orogeny, and from the ultra-high pressure (UHP) terranes of Kokchetav (Kazakhstan) and Erzgebirge (Germany) exhumated in the Paleozoic. Wawa diamonds (Type IaAB and Type II) displayed green, yellow, orange, and red CL colours controlled by the CL emittance at 520, 576 nm, and between 586 and 664 nm. The UHP terranes diamonds show much weaker CL; few luminescent stones display CL peaks at 395, 498, 528 nm and a broad band at 580-668 nm. In contrast, most common diamonds found in unmetamorphosed rocks, i.e. octahedrally grown Type IaAB stones, luminescence blue emitting light at ~415-440 nm and 480-490 nm. There is a noticeable difference between cathodoluminescence of these diamonds and diamonds in metamorphic rocks. The studied diamonds that experienced metamorphism show a shift of CL emission to longer wavelengths (above 520 nm) and to green, yellow and red CL colours. Photoluminescence has the high resolution necessary to assign luminescence to specific optical centers of diamond. Diamonds in metamorphic rocks contain H3 (pairs of substitutional nitrogen atoms separated by a vacancy) and NVo optical centers (neutrally charged complexes of a vacancy and a single nitrogen). We ascribe the effect of metamorphism on the diamond CL to low-T, low-P deformation that creates lattice dislocations and vacancies. These combine with substitutional N to make and enhance optical centers. The metamorphism-induced CL anneals when diamonds are stored at high-T mantle conditions, as the mobility of dislocations at T>750oC quenches the luminescence. Indeed, all studied diamonds that displayed unusual green, yellow and red CL were found in low and medium grade metamorphic rocks, i.e. Wawa greenschists (T<350oC and P< 3 kb) and Kokchetav and Erzgebirge UHP terranes retrograded in the amphibolite facies (T<750oC, P<14 kb) Our study suggest that a low abundance of octahedrally grown Type IaAB diamonds with blue CL colours among detrital diamonds may indicate that the stones may have once been a part of a low- or medium-grade metamorphic terrane. The CL characteristics superimposed by metamorphism could survive through billions of years of the geological history if not annealed by a high -T process. The discovered record of metamorphism in the diamond crystal lattice provides an opportunity for a better reconstruction of the crustal history and provenance studies of diamond.

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

NASA Astrophysics Data System (ADS)

Alves Ribeiro, J.; Monteiro-Santos, F. A.; Pereira, M. F.; Díez Fernández, R.; Dias da Silva, Í.; Nascimento, C.; Silva, J. B.

2017-12-01

A new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2-D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large-scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite-rich rocks. The lower layer consists of high-grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large-scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike-slip shear zone (i.e., Juzbado-Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

Age of zircons from the xenolith of metapelite in granitoids of the Verkhisetsk massif (Middle Urals): Evidence for granite-related stages of metamorphism

NASA Astrophysics Data System (ADS)

Zinkova, E. A.; Pribavkin, S. V.

2016-02-01

Two age stages in the formation of high-aluminous gneisses related to the major stages of granite formation of the Uralian mobile belt were revealed in this study. The first stage (372 ± 2 Ma) corresponds to the age of metamorphism of the amphibolite facies and is controlled by intrusion of the tonalite-trondhjemite series under the environment of the continental margin. At the second stage (307 ± 3 Ma), gneiss underwent contact metamorphism under the influence of plutons of the adamellite-granite composition formed during the early episodes of collisional metamorphism.

Biochemical effects of chlorpyrifos on two developmental stages of Xenopus laevis.

PubMed

Richards, Sean M; Kendall, Ron J

2002-09-01

Abstract-The effects of a 96-h static exposure to chlorpyrifos were examined in two developmental stages of larval Xenopus laevis (premetamorph and metamorph). Measures of effect included mortality, deformity, cholinesterase (ChE) activity, and DNA and protein concentration. All parameters indicated that metamorphs were more sensitive than were premetamorphs. For larvae exposed as premetamorphs, the median lethal concentration and median effective concentration were 14.6 mg/L and 1.71 mg/L; for those exposed as metamorphs, values were 0.56 mg/L and 0.24 mg/L, respectively. Cholinesterase activity was the most sensitive biochemical parameter. Exposure to chlorpyrifos at 0.01 mg/L caused significant decreases in the ChE activity of metamorphs; 0.1 mg/L significantly decreased premetamorph ChE activity. Metamorph DNA was significantly decreased at 0.1 mg/L; premetamorph DNA was not reduced until exposure to 1.0 mg/L. Whole-body protein was the least sensitive biochemical measure of effect. Premetamorphs did not experience a reduction in protein concentrations. Metamorph protein concentration was significantly decreased at 1.0 mg/L. Based on current surface water data, the most sensitive effect would not have a high probability (< or = 4.2%) of occurring in the environment.

Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

USGS Publications Warehouse

Engvik, A.K.; Elevevold, S.; Jacobs, J.; Tveten, E.; de Azevedo, S.; Njange, F.

2007-01-01

Granulite-facies metamorphism is extensively reported in Late Neoproterozoic/Early Palaeozoic time during formation of the East-African-Antarctic orogen (EAAO). Metamorphic data acquired from the Pan-African orogen of central Dronning Maud Land (cDML) are compared with data from northern Mozambique. The metamorphic rocks of cDML are characterised by Opx±Grt-bearing gneisses and Sil+Kfs-bearing metapelites which indicate medium-P granulite-facies metamorphism. Peak conditions, which are estimated to 800-900ºC at pressures up to 1.0 GPa, were followed by near-isothermal decompression during late Pan-African extension and exhumation. Granulite-facies lithologies are widespread in northern Mozambique, and Grt+Cpx-bearing assemblages show that high-P granulite-facies conditions with PT reaching 1.55 GPa and 900ºC were reached during the Pan-African orogeny. Garnet is replaced by symplectites of Pl+Opx+Mag indicating isothermal decompression, and the subsequent formation of Pl+amphibole-coronas suggests cooling into amphibolite facies. It is concluded that high-T metamorphism was pervasive in EAAO in Late Neoproterozoic/Early Paleozoic time, strongly overprinting evidences of earlier metamorphic assemblages.

Near infrared spectra of muscovite, Tschermak substitution, and metamorphic reaction progress: Implications for remote sensing

NASA Astrophysics Data System (ADS)

Duke, Edward F.

1994-07-01

Near infrared (NIR) spectra of Precambrian metagraywacke in the Black Hills, South Dakota, demonstrate that reflectance spectroscopy can be used to monitor progressive changes in mineral chemistry as a function of metamorphic grade. The wavelength of a combination Al-O-H absorption band in muscovite, measured using both laboratory and field-portable NIR spectrometers, shifts from 2217 nm in the biotite zone to 2199 nm in the sillimanite + K-feldspar zone. The band shift corresponds to an increase in the Alvi content of muscovite, determined by electron microprobe, and is thus a monitor of Al2Si-1(Fe,Mg)-1 (Tschermak) exchange. Spectroscopic measurements such as these are useful in the case of aluminum-deficient rocks, which lack metamorphic index minerals or appropriate assemblages for thermobarometric studies, and in low-grade rocks (subgarnet zone), which lack quantitative indicators of metamorphic grade and are too fine grained for petrographic or microprobe studies. More important, spectroscopic detection of mineral-chemical variations in metamorphic rocks provides petrologists with a tool to recover information on metamorphic reaction histories from high-spectral-resolution aircraft or satellite remote sensing data.

High-temperature metamorphism and its relation with magmatism (Svecofennian Belt, the Ladoga region, Russia)

NASA Astrophysics Data System (ADS)

Baltybaev, Shauket

2010-05-01

The Ladoga region, situated in the south-eastern part of the Fennoscandian shield, is subdivided into the Archean (ARD) and the Proterozoic (PRD) domains. The boundary between them is a wide shear-zone. The ARD consists mostly of AR-PR middle-low temperature gneisses and the PRD consists of turbidites, pelites, volcanics metamorphosed under HT-conditions (granulite facies). Metamorphism within the PRD is culminated at T= 800-900C and P=5-6 kbar. The peak of metamorphism of granulite facies is dated at 1881 Ma by Pb-Pb stepwise leaching method of rock-forming minerals of the granulites. Pb-Pb results are within error limits coeval with the U-Pb ages of metamorphic monazites. The same (1881Ma) age has gabbro-enderbites. Next stage of metamorphism lasts from 1881 to 1860 Ma under conditions of amphibolite facies. It was restricted with U-Pb, Pb-Pb, Sm-Nd data based on the closure temperature of zircon, monazite, garnet, sillimanite from gneisses, leucosomes of migmatites and synmetamorphic diorites and tonalites. The lowermost point of the trend shows P-T: ~3-4 kbar, 600C. By the time 1860 Ma K-rich granites were emplaced and the uppermost limit for granulite metamorphism comes from the ages of the aplitic/pegmatitic veins (1860-1850 Ma), which cut the K-rich granites. Thermal and tectonic settings can be described based on spatial and temporal changes during magma emplacement. The granulites of the PRD were produced by the emplacement of the extensive basic intrusion (gabbro-enderbites) into the lower-middle crust. A prolonged thermal flux over all area was supported by new generated dioritic and tonalitic melts, which were intruded into the middle crust. The final stage of tectono-metamorphic evolution was marked by emplacement of the K-rich granites. Numerical simulation of the process of magma emplacement (sequences: gabbro-enderbites, diorites and tonalites) and related heat production shows good correlation between intrusive activity and metamorphism of the surrounding rocks. Baltybaev Sh. K., Levchenkov O. A., Levsky L. K., Eklund O., Kilpeläinen T. 2006. Two metamorphic stages in the Svecofennian Domain: evidence from the isotopic geochronological study of the Ladoga and Sulkava metamorphic complexes. Petrology, 14(3), 247-261.

Cretaceous crust beneath SW Borneo: U-Pb dating of zircons from metamorphic and granitic rocks

NASA Astrophysics Data System (ADS)

Davies, L.; Hall, R.; Armstrong, R.

2012-12-01

Metamorphic basement rocks from SW Borneo are undated but have been suggested to be Palaeozoic. This study shows they record low pressure 'Buchan-type' metamorphism and U-Pb SHRIMP dating of zircons indicates a mid-Cretaceous (volcaniclastic) protolith. SW Borneo is the southeast promontory of Sundaland, the continental core of SE Asia. It has no sedimentary cover and the exposed basement has been widely assumed to be a crustal fragment from the Indochina-China margin. Metamorphic rocks of the Pinoh Group in Kalimantan (Indonesian Borneo) are intruded by granitoid rocks of Jurassic-Cretaceous age, based on K-Ar dating, suggesting emplacement mainly between 130 and 80 Ma. The Pinoh metamorphic rocks have been described as a suite of pelitic schists, slates, phyllites, and hornfelses, and have not been dated, although they have been correlated with rocks elsewhere in Borneo of supposed Palaeozoic age. Pelitic schists contain biotite, chlorite, cordierite, andalusite, quartz, plagioclase and in some cases high-Mn almandine-rich garnet. Many have a shear fabric associated with biotite and fibrolite intergrowth. Contact metamorphism due to intrusion of the granitoid rocks produced hornfelses with abundant andalusite and cordierite porphyroblasts. Granitoids range from alkali-granite to tonalite and contain abundant hornblende and biotite, with rare white mica. Zircons from granitoid rocks exhibit sector- and concentric- zoning; some have xenocrystic cores mantled by magmatic zircon. There are four important age populations at c. 112, 98, 84 and 84 Ma broadly confirming earlier dating studies. There is a single granite body with a Jurassic age (186 ± 2.3 Ma). Zircons from pelitic metamorphic rocks are typically euhedral, with no evidence of rounding or resorbing of grains; a few preserve volcanic textures. They record older ages than those from igneous rocks; U-Pb ages are Cretaceous with a major population between 134 and 110 Ma. A single sample contains Proterozoic and Phanerozoic zircons. The metamorphic rocks from SW Borneo are not an ancient core to the island as previously assumed. We propose that extensive arc volcanism produced fine grained volcanogenic sediments during the Early Cretaceous deposited on, or reworking, older crust. These sediments were subjected to low pressure 'Buchan-type' metamorphism soon after deposition. Magmatism continued into the Late Cretaceous, resulting in contact metamorphism.

Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera: Ancient crust bounded by juvenile terranes

NASA Astrophysics Data System (ADS)

Samson, Scott D.; Patchett, P. Jonathan; McClelland, William C.; Gehrels, George E.

1991-08-01

Nd isotopic data are reported for 52 samples from the crustal region between the Alexander-Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic-Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, a terrane of probable Early Permian to Late Triassic age, and four assemblages of metamorphic rocks that occur to the west of and within the Coast Mountains batholith. The Gravina belt has ɛNd(T) values that range from -1.1 to +8.3, similar to values of the underlying Alexander terrane, and consistent with the interpretation that it is a juvenile belt that formed in a back-arc or intra-arc basin within the Alexander terrane. Mid-Cretaceous plutons that were emplaced into the Gravina belt have ɛNd(T) values of +4.4 to +5.7 and were probably produced by mantle-derived melts that incorporated some Alexander terrane crust. The Taku terrane has ɛNd(0) values that range from -5.5 to +3.3, with corresponding depleted-mantle model (TDM) ages of 440 to 1430 Ma. A mid-Cretaceous pluton intruding the Taku terrane has an ɛNd(T) value of +5.1, a value indistinguishable from those determined for Cretaceous plutons intruding the Gravina belt. Metamorphic rocks east of and structurally overlying the Taku terrane are divided into the Tracy Arm assemblage, ɛNd(0)=-26 to 0, TDM=800-2450 Ma; the Endicott Arm assemblage, eNd(0)=-10 to -1.3, TDM=950-1500 Ma; the Port Houghton assemblage, ɛNd(0)=-9.4 to +1.1, TDM = 550-1500 Ma; and the Ruth assemblage, ɛNd(0) = -9.4 to +2.0, TDM=650-1300 Ma. These isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. The metamorphic rocks from these assemblages are lithologically very similar to rocks of the Yukon-Tanana (YTT) terrane of eastern Alaska and Yukon Territory and have such similar U-Pb detrital zircon ages and Nd isotopic compositions to YTT rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander-Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander-Wrangellia terrane.

PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous Tertiary continental collision

NASA Astrophysics Data System (ADS)

Azizi, H.; Moinevaziri, H.; Mohajjel, M.; Yagobpoor, A.

2006-06-01

Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian-Paleozoic age to the northeast. The Qom formation (Oligocene-Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet-staurolite schist and garnet-staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 °C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (<2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al 2SiO 5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 °C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post-Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan-Albourz block was the main factor that caused the metamorphism in the studied area.

Petrography of shock features in the 1953 Manson 2-A drill core

NASA Technical Reports Server (NTRS)

Short, N. M.; Gold, D. P.

1993-01-01

Drilling of Nx core in late 1953 into an anomalous zone of disturbed rocks northwest of Manson, Iowa disclosed presence of extensive breccias including crystalline rocks brought to the surface from depths of 4 km or more. Hole 2-A penetrated breccias dominated by leucocratic igneous and metamorphic lithologies, later interpreted to be part of a general ringed peak complex within a 35 km wide impact structure produced about 65 Ma ago. Proof of this origin was given in 1966 by NMS through recognition of shock metamorphic features in 2-A materials during a cursory examination of samples provided by R.A. Hoppin, University of Iowa. A detailed study of this material now underway has revealed that most breccia clasts in 2-A show abundant and varied evidence of shock damage, including extensive planar deformation features (PDF) in quartz, K-feldspar, plagioclase, and a pyroxene and varying degrees of isotropization and incipient melting in feldspars.

Research on Improving Low Rank Coal Caking Ability by Moderate Hydrogenation

NASA Astrophysics Data System (ADS)

Huang, Peng

2017-12-01

The hydrogenation test of low metamorphic coal was carried out by using a continuous hydrogen reactor at the temperature of (350-400)°C and the initial hydrogen pressure of 3 ~ 6Mpa. The purpose of the experiment was to increase the caking property, and the heating time was controlled from 30 to 50min. The test results show that the mild hydrogenation test, no adhesion of low metamorphic coal can be transformed into a product having adhesion, oxygen elements in coal have good removal, the calorific value of the product has been improved significantly and coal particles during pyrolysis, swelling, catalyst, hydrogenation, structural changes and the combined effects of particles a new component formed between financial and is a major cause of coal caking enhancement and lithofacies change, coal blending test showed that the product can be used effectively in the coking industry.

InP-based type-I quantum well lasers up to 2.9 μm at 230 K in pulsed mode on a metamorphic buffer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gu, Y.; Zhang, Y. G., E-mail: ygzhang@mail.sim.ac.cn; Ma, Y. J.

This work reports on up to 2.9 μm lasing at 230 K of InP-based type-I quantum well lasers. This record long wavelength lasing is achieved by applying InP-based Sb-free structures with eight periods of strain-compensated InAs quantum wells grown on metamorphic In{sub 0.8}Al{sub 0.2}As template layers. The continuous-wave threshold current density is 797 A/cm{sup 2} and the idealized extrapolated threshold current density for infinite cavity length is as low as 58 A/cm{sup 2} per quantum well at 120 K. This scheme is a promising pathway for extending the wavelength range of type-I quantum well lasers on InP substrates.

Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

NASA Astrophysics Data System (ADS)

Cao, Dadi; Cheng, Hao; Zhang, Lingmin; Wang, Ke

2017-08-01

A reconstruction of the pressure-temperature-time (P-T-t) path of high-pressure eclogite-facies rocks in subduction zones may reveal important information about the tectono-metamorphic processes that occur at great depths along the plate interface. The majority of studies have focused on prograde to peak metamorphism of these rocks, whereas after-peak metamorphism has received less attention. Herein, we present a detailed petrological, pseudosection modeling and radiometric dating study of a retrograded eclogite sample from the Sumdo ultrahigh pressure belt of the Lhasa terrane, Tibet. Mineral chemical variations, textural discontinuities and thermodynamic modeling suggest that the eclogite underwent an exhumation-heating period. Petrographic observations and phase equilibria modeling suggest that the garnet cores formed at the pressure peak (∼2.5 GPa and ∼520 °C) within the lawsonite eclogite-facies and garnet rims (∼1.5 GPa and <650 °C) grew during post-peak amphibole eclogite-facies metamorphism. The metamorphic evolution of the Sumdo eclogite is characterized by a clockwise P-T path with a heating stage during early exhumation, a finding that conflicts with previously reported heating-compression P-T paths for the Sumdo eclogite. A garnet-whole rock Lu-Hf age of 266.6 ± 0.7 Ma, which is consistent with the loosely constrained zircon U-Pb age of 261 ± 15 Ma within uncertainty, was obtained for the sample. The peak metamorphic temperature of the sample is lower than the Lu-Hf closure temperature of garnet, which combined with the general core-to-rim decrease in the Mn and Lu concentrations and the occurrence of a second maximum Lu peak in the inner rim, is consistent with the Lu-Hf system skewing to the age of the garnet inner rim. Thus the Lu-Hf age likely reflects late eclogite-facies metamorphism. The new U-Pb and Lu-Hf ages, together with previously published radiometric dating results, suggest that the overall growth of garnet spans an interval of ∼7 million years, which is a minimum estimate of the duration of the eclogite-facies metamorphism of the Sumdo eclogite.

Subduction starts by stripping slabs

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Prigent, Cécile; Plunder, Alexis; Yamato, Philippe; Guillot, Stéphane

2017-04-01

Metamorphic soles correspond to tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as having formed during the first My of intra-oceanic subduction from heat transfer from the incipient mantle wedge towards the top of the subducting plate. Our study reappraises the formation of metamorphic sole through detailed field and petrological work on three classical key sections across the Semail ophiolite (Oman and United Arab Emirates). Geothermobarometry and thermodynamic modelling show that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating and proposed by previous studies. The upper, high-temperature metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the downgoing slab to the mylonitic base of the ophiolite. Estimated peak pressure-temperature conditions through the metamorphic sole are, from top to bottom, 850˚C - 1GPa, 725°C - 0.8 GPa and 530°C - 0.5 GPa. These estimates appear constant within each unit but separated by a gap of 100 to 200˚C and 0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ophiolite ridge axis, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow to refine the tectonic-petrological model for the genesis of metamorphic soles, formed through the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles do not so much result from downward heat transfer (ironing effect) but rather from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts are the result of rheological contrasts between the sole (initially at the subducting slab) and the peridotite above as the plate interface progressively cools down. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems.

Geochronology of high-grade metamorphic rocks from the Anjul area, Lut block, eastern Iran

NASA Astrophysics Data System (ADS)

Bröcker, Michael; Fotoohi Rad, Gholamreza; Abbaslu, Fateme; Rodionov, Nikolay

2014-03-01

U-Pb and Rb-Sr geochronology has been used to constrain robust ages for leucosomes and high-grade gneisses from the Anjul area in the eastern part of the Lut block, Iran. The new results do not support the previously suggested Proterozoic age for this occurrence, but instead reveal the importance of Jurassic and Cretaceous magmatic and/or metamorphic processes. Ionprobe U-Pb zircon dating yielded four age groups (>200, ˜168, ˜120 and ˜110 Ma). Textural observations suggest that ages >200 Ma represent inherited zircons. The majority of zircons yielded Jurassic (168 ± 2 and 169 ± 2 Ma) and Cretaceous (120 ± 3, 108 ± 2, 111 ± 3 Ma) intercept ages. Explanations for the two dominant age groups (˜168 and ˜110 Ma) include the following alternatives: (a) the Jurassic ages constrain the protolith age of magmatic precursors that experienced metamorphic overprinting at ˜110 Ma; and (b) both the ˜168 Ma and ˜110 Ma ages indicate the time of metamorphic episodes, e.g. zircon-formation during different anatectic events or migmatization followed by a lower temperature overprint associated with new zircon growth. Multi-point Rb-Sr mineral isochrons of three additional gneisses indicated ages of 102 ± 3 Ma, 102 ± 1 Ma and 97 ± 2 Ma. These ages further document the importance of Cretaceous metamorphism in the Anjul area. The difference compared to the U-Pb ages of zircon overgrowths is interpreted to indicate cooling after a thermal event with or without partial melting. The two major occurrences of metamorphic rocks in the eastern Lut block are exposed in the Deh-Salm and the Anjul region. These occurrences may represent two different segments of a single metamorphic belt that can broadly be related to accretionary and/or collisional processes induced by convergence between the Afro-Arabian and Eurasian plates. Our geochronological study provides a conclusive evidence for Cretaceous metamorphism. We speculate that zircon overgrowths with Cretaceous ages reflect metamorphic processes unrelated to melt formation that overprinted a pre-existing population recording Jurassic anatexis.

Late Carboniferous high-pressure metamorphism of the Kassan Metamorphic Complex (Kyrgyz Tianshan) and assembly of the SW Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Mühlberg, M.; Hegner, E.; Klemd, R.; Pfänder, J. A.; Kaliwoda, M.; Biske, Y. S.

2016-11-01

High-pressure (HP) metamorphism of the Kassan Metamorphic Complex (KMC) in the western Kyrgyz Tianshan has been related to either late Ordovician or late Carboniferous-Permian subduction processes. We report Sm-Nd ages for retrogressed eclogite samples and 40Ar/39Ar cooling ages for enclosing garnet-muscovite samples from the KMC as new age constraints on HP metamorphism and rock exhumation. These data will be used for an upgraded paleogeographic model for late Paleozoic crustal consolidation in the southwestern Central Asian Orogenic Belt. The retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace-element patterns consistent with protoliths derived from garnet-bearing mantle sources at rifting plate margins. Geothermobarometric data for a retrogressed eclogite sample indicate peak-metamorphic conditions of 540 ± 30 °C at 1.6 ± 0.1 GPa. Samples from different lithotectonic units of the KMC provide coherent Sm-Nd garnet-whole rock ages of 317 ± 4 Ma and 316 ± 3 Ma (2σ). The prograde major-element zoning in the mm-sized garnets in combination with the moderate peak-metamorphic temperature, support our interpretation of the Sm-Nd garnet ages as unambiguous evidence for late Carboniferous HP metamorphism. The Sm-Nd garnet growth ages overlap within-error with the 40Ar/39Ar mica cooling ages of 314 ± 2 Ma and 313 ± 2 Ma (2σ) indicating rapid uplift of the subduction complex after peak metamorphism. The ca. 317-313 Ma HP-exhumation event of the KMC is contemporaneous with those of the Atbashi and Akeyazi (ca. 500 km east in NW China) HP complexes and implies similar collision histories at the South Tianshan Suture to the east and west of the Talas-Fergana Fault (TFF). The exhumation of the KMC and Atbashi HP complexes overlaps with the initiation of the TFF (Rolland et al., 2013) suggesting incipient separation of the Chatkal and Atbashi complexes during rock exhumation and early plate collision.

Geophysical Characterization and Structural Model of the Santa ROSALÍA Aquifer, Sonora, MÉXICO

NASA Astrophysics Data System (ADS)

Martínez-Retama, S.; Montaño-Del Cid, M. A.

2017-12-01

The main objective of this work was to determine the morphology and depth of the basement, as well as the elaboration of a structural model for the Santa Rosalía aquifer, from the processing and interpretation of gravimetric and aeromagnetic data and its correlation with the Geology of the area. The study area is located in the central portion of the State of Sonora, Mexico. In general, the geology of the site is characterized by sedimentary, igneous and metamorphic rocks whose ages vary from the Precambrian to Recent. Chronologically, the geology of the study area consists of igneous and metamorphic rocks of Precambrian age, considered as a metamorphic complex. The Paleozoic is represented by a sequence of prebatolytic rocks. This sequence is intruded by rocks of the Upper Cretaceous. The Triassic-Jurassic periods consist of arenaceous units of the Barranca Group. The Cretaceous is constituted by the Tarahumara Formation, as well as granite bodies. The Quaternary is composed of alluvial deposits, which are overlain by sediments of Recent. In this work a gravimetric survey was performed, registering a total of 7 profiles. In addition, measured data from the National Institute of Statistics and Geography (INEGI) were used. The aeromagnetic study was carried out with data from the Mexican Geological Service (SGM). In order to reduce the ambiguity in the modeling process, a rock sampling was taken from the study area and its density and magnetic susceptibility were measured. Finally, two-dimensional models of gravimetric and magnetic profiles were made to obtain the structural model of the study area. The geological-structural models obtained show gravimetric anomalies (low)associated with sedimentary basins with depths of 800 m to 1,500 m., indicating the most susceptible áreas to water storage. The basement is represented by volcanic and granite rocks that are in contact with Paleozoic sedimentary rocks (Limestone) and in some areas with volcanic rocks of the Tarahumara Formation. In these models two types of sliding tectonic events were interpreted. In the first one a system of low-angle normal faulting related to the distensive event Basin and Range was interpreted. In the second, a series of high- angle normal faults, which form Horst and Grabens structures related to the opening of the Gulf of California were modeled.

Limitations of using a thermal imager for snow pit temperatures

NASA Astrophysics Data System (ADS)

Schirmer, M.; Jamieson, B.

2013-10-01

Driven by temperature gradients, kinetic snow metamorphism is important for avalanche formation. Even when gradients appear to be insufficient for kinetic metamorphism, based on temperatures measured 10 cm apart, faceting close to a~crust can still be observed. Recent studies that visualized small scale (< 10 cm) thermal structures in a profile of snow layers with an infrared (IR) camera produced interesting results. The studies found melt-freeze crusts to be warmer or cooler than the surrounding snow depending on the large scale gradient direction. However, an important assumption within the studies was that a thermal photo of a freshly exposed snow pit was similar enough to the internal temperature of the snow. In this study, we tested this assumption by recording thermal videos during the exposure of the snow pit wall. In the first minute, the results showed increasing gradients with time, both at melt-freeze crusts and at artificial surface structures such as shovel scours. Cutting through a crust with a cutting blade or a shovel produced small concavities (holes) even when the objective was to cut a planar surface. Our findings suggest there is a surface structure dependency of the thermal image, which is only observed at times with large temperature differences between air and snow. We were able to reproduce the hot-crust/cold-crust phenomenon and relate it entirely to surface structure in a temperature-controlled cold laboratory. Concave areas cooled or warmed slower compared with convex areas (bumps) when applying temperature differences between snow and air. This can be explained by increased radiative transfer or convection by air at convex areas. Thermal videos suggest that such processes influence the snow temperature within seconds. Our findings show the limitations of the use of a thermal camera for measuring pit-wall temperatures, particularly in scenarios where large gradients exist between air and snow and the interaction of snow pit and atmospheric temperatures are enhanced. At crusts or other heterogeneities, we were unable to create a sufficiently homogenous snow pit surface and non-internal gradients appeared at the exposed surface. The immediate adjustment of snow pit temperature as it reacts with the atmosphere complicates the capture of the internal thermal structure of a snowpack even with thermal videos. Instead, the shown structural dependency of the IR signal may be used to detect structural changes of snow caused by kinetic metamorphism. The IR signal can also be used to measure near surface temperatures in a homogenous new snow layer.

Limitations of using a thermal imager for snow pit temperatures

NASA Astrophysics Data System (ADS)

Schirmer, M.; Jamieson, B.

2014-03-01

Driven by temperature gradients, kinetic snow metamorphism plays an import role in avalanche formation. When gradients based on temperatures measured 10 cm apart appear to be insufficient for kinetic metamorphism, faceting close to a crust can be observed. Recent studies that visualised small-scale (< 10 cm) thermal structures in a profile of snow layers with an infrared (IR) camera produced interesting results. The studies found melt-freeze crusts to be warmer or cooler than the surrounding snow depending on the large-scale gradient direction. However, an important assumption within these studies was that a thermal photo of a freshly exposed snow pit was similar enough to the internal temperature of the snow. In this study, we tested this assumption by recording thermal videos during the exposure of the snow pit wall. In the first minute, the results showed increasing gradients with time, both at melt-freeze crusts and artificial surface structures such as shovel scours. Cutting through a crust with a cutting blade or shovel produced small concavities (holes) even when the objective was to cut a planar surface. Our findings suggest there is a surface structure dependency of the thermal image, which was only observed at times during a strong cooling/warming of the exposed pit wall. We were able to reproduce the hot-crust/cold-crust phenomenon and relate it entirely to surface structure in a temperature-controlled cold laboratory. Concave areas cooled or warmed more slowly compared with convex areas (bumps) when applying temperature differences between snow and air. This can be explained by increased radiative and/or turbulent energy transfer at convex areas. Thermal videos suggest that such processes influence the snow temperature within seconds. Our findings show the limitations of using a thermal camera for measuring pit-wall temperatures, particularly during windy conditions, clear skies and large temperature differences between air and snow. At crusts or other heterogeneities, we were unable to create a sufficiently planar snow pit surface and non-internal gradients appeared at the exposed surface. The immediate adjustment of snow pit temperature as it reacts with the atmosphere complicates the capture of the internal thermal structure of a snowpack with thermal videos. Instead, the shown structural dependency of the IR signal may be used to detect structural changes of snow caused by kinetic metamorphism. The IR signal can also be used to measure near surface temperatures in a homogenous new snow layer.

On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides

NASA Astrophysics Data System (ADS)

Gilio, Mattia; Clos, Frediano; Van Roermund, Herman L. M.

2013-04-01

The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments, called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons, as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions. Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes related to orogeny (Brueckner and Medaris, 2000). The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jämtland/S Västerbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP) metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks (of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or MP granulite "facies" mineral assemblages that can be interpreted, in the absence of retrogression, to have formed in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic "facies" allows for a discrimination (at least theoretically) between "deep" versus "shallow" rooted nappes (in central parts of the Scandinavian Caledonides). Conform this reasoning, this duality should also be present within the Caledonian mineral assemblages (= metamorphic overprint) of orogenic peridotites (in central parts of the orogen), which, at least in the allochtonous nappes, have been interpreted to be "isofacial" with their host country rocks (Bucher, 1991). The latter strongly contrast to the interpretation of their "primary" (="protolith"- related) mineral assemblage(s) which clearly suggest a bimodal origin: here called thick (>80 km) versus thin (< 70 km) rooted lithospheric mantle protoliths. Distinction can be made on the basis of the presence of the stable (minimal Proterozoic) garnet-olivine assemblages in the protolith (i.e. much older than the Scandian collision event (Brueckner et al., 2010). For this reason orogenic garnet peridotite was first called "relict" garnet peridotite (Brueckner and Medaris, 2000), later rephrased into mantle wedge garnet peridotite (MWgp) by Van Roermund (2009). MWgp occurs in the WGC and in the SNC of the Upper Allochthon in central Sweden (Zhang et al., 2009). Most (All?) other protolith assemblages of orogenic peridotite in the CSC belong to the thin-rooted protolith subtype. No examples are known to us in which thin rooted prototypes became overprinted (during the Caledonian orogeny) by (U)HP metamorphic minerals, except for the subduction zone garnet peridotites (SZgp) in the WGC (Van Roermund, 2009). The latter can thus savely be interpreted as being enclosed within normal "MP" (or lower pressure) nappe sequences. As such it will be clear that this duality in protolith (and/or metamorphic) mineral assemblages of orogenic peridotite can be used to identify former, but now strongly retrogressed, (U)HP metamorphic terranes in other parts of the CSC (Gee et al, 2012). For this reason a comparative study has been made concerning field, (micro-)structural, mineral-chemical and/or geochemical aspects of two major orogenic peridotites from the SNC, central Sweden; here called the Friningen Garnet Peridotite (FGP) and the Kittelfjäll Spinel Peridotite (KSP), both exposed within the central belt of the SNC in central Sweden. The ultimate aim was to investigate whether the MWgp sub-type can be extended towards (Al-poor) spinel-bearing protolith assemblages or not. Results, including some hitherto unexpected mechanical effects, will be presented. References: Brueckner, H.K., Carswell, D.A., Griffin, W.L., Medaris, L.G., Van Roermund, H.L.M., Cuthbert, S.J. (2010). The mantle and crustal evolution of two garnet peridotite suites from the Western Gneiss Region, Norwegian Caledonides: An isotopic investigation. Lithos, 117, 1-19. doi:10.1016/j. Lithos.2010.01.011 Brueckner, H.K.and Medaris, L.G. (2000). A general model for the intrusion and evolution of "mantle" garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes. J. Metamorphic Geol., 18, 123-133. Brueckner H.K. and Van Roermund,H.L.M. (2004). Dunk tectonics: A multiple subduction//eduction model for the evolution of the Scandinavian Caledonides. Tectonics, 23, TC2004, doi:10.1029/2003tc001502. Bucher, K. (1991). Mantle fragments in the Scandinavian Caledonides. Tectonophysics, 190, 173-192. Gee, D.G., Fossen, H., Henriksen, N., Higgins, K. (2008). From the Early Paleozoic Platforms of Baltica and Laurentia to the Caledonide Orogen of Scandinavia and Greenland. Episodes, 31, 44-51. Gee, D.G., Janak, M., Majka, J., Robinson, P., Van Roermund, H.L.M (2012). UHP metamorphism along the Baltoscandian outer margin: evidence from the Seve Nappe Complex of the Swedish Caledonides. Lithosphere, in press. Janak, M., Van Roermund, H., Majka, J., Gee, D. (2012). UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides. Gondwana Research, in press. Van Roermund, H.L.M. (2009). Mantle-wedge garnet peridotites from the northernmost ultra-high pressure domain of the Western Gneiss Region, SW Norway. Eur. J. Mineralogy, 21, 1085-1096. Zhang, C., Van Roermund, H.L.M., Zhang, L.F (2011). 16 - Orogenic Garnet Peridotites: Tools to Reconstruct Paleo-Geodynamic Settings of Fossil Continental Collision Zones. In: Ultrahigh Pressure Metamorphism, 25 Years After The Discovery Of Coesite And Diamond. London. Doi:10.1016/B978-0-12-385144-4.00015-1

Garnet zoning and metamorphism of the Barrovian type area, Scotland

NASA Astrophysics Data System (ADS)

Dempster, T. J.

1985-03-01

A microprobe investigation of the high grade metamorphic zones from the Barrovian type area in Angus, Scotland, shows the importance of local zones of retrograde cation exchange between garnet, staurolite and biotite. The interpretation of this zoning, established during a slow cooling history, is critical to any study of metamorphic reactions or conditions. The extent and intensity of these diffusion effects are dependent on a number of parameters including grainsize, fabric orientation, heating and cooling history, and the modal abundance of the phases. Increasing diffusion within garnets with metamorphic grade, and the subsequent retrograde effects are modelled using Temperature-Time-Transformation diagrams and provide information on the activation energy for Fe-Mg diffusion in garnet.

Ion microprobe survey of the grain-scale oxygen isotope geochemistry of minerals in metamorphic rocks

NASA Astrophysics Data System (ADS)

Ferry, John M.; Kitajima, Kouki; Strickland, Ariel; Valley, John W.

2014-11-01

The oxygen isotope compositions of calcite, diopside, dolomite, forsterite, garnet, K-feldspar, kyanite, plagioclase, quartz, and wollastonite were analyzed in suites of contact and regional metamorphic rocks using an ion microprobe. Spatial resolution was ∼10 μm. Precision, measured as the standard deviation of working standards averaged over the entire project, was 0.13-0.18‰ for three carbonate standards and 0.11-0.12‰ for two silicate standards. A total of 1176 analyses (excluding standards) were made of 73 minerals in 23 samples. Both intercrystalline and intracrystalline variability in δ18O is greater in contact than in regional metamorphic rocks. Of 27 minerals analyzed in contact metamorphosed rocks, 70% exhibit statistically significant grain-to-grain variability in δ18O over areas ⩽1.41 cm2 with the largest range in silicates and carbonates in a single sample of 7.4‰ (forsterite) and 10.6‰ (dolomite). Of 88 grains analyzed in two or more places in contact metamorphosed rocks, 32% exhibit statistically significant intracrystalline variability in δ18O with the largest range in a single silicate and carbonate grain of 3.1‰ (forsterite) and 10.1‰ (dolomite). In contrast, 44% of 45 minerals in regional metamorphic rocks exhibit significant grain-to-grain variability in δ18O over areas ⩽1.17 cm2 with the largest range in silicates and carbonates in a single sample of only 1.1‰ (plagioclase) and 0.9‰ (calcite). Only 6% of 144 grains analyzed in two or more places in regional metamorphic rocks exhibit significant intracrystalline variability in δ18O with the largest range in a single silicate and carbonate grain of only 1.5‰ (diopside) and 0.7‰ (calcite). The difference in intercrystalline and intracrystalline variability in δ18O between contact and region metamorphic rocks is explained by the longer duration and slower reaction rates of regional metamorphism rather than to differences in temperature. There is no significant difference in intercrystalline and intracrystalline variability in δ18O in regional metamorphic rocks among samples from the biotite, garnet, and kyanite zones. Calcite inclusions in forsterite, and calcite and quartz inclusions in garnet either have δ18O that is statistically indistinguishable from δ18O of the same mineral occurring as nearby matrix grains or have statistically significant lower δ18O. No reversed isotope fractionations were measured between coexisting mineral pairs. Minerals in individual samples, however, exhibit a wide range in the degree to which they attained and preserve oxygen isotope fractionations consistent with metamorphic temperatures recorded by mineral equilibria. Processes that account for grain-scale departures from isotope exchange equilibrium include: (a) overstepping of prograde mineral reactions, (b) growth zoning in low-diffusivity minerals, (c) interaction of rocks with fluids at the peak of metamorphism and/or during cooling, (d) retrograde mineral reactions, and (e) closed-system isotope exchange between coexisting minerals during cooling. This study provides new information about (1) the degree to which a variety of textural changes experienced by rocks during metamorphism are associated with changes in δ18O, (2) oxygen isotope homogenization at the outcrop scale among contrasting lithologies, (3) changes in δ18O with increasing grade of regional metamorphism, and (4) time scales of metamorphic process.

Composition and formations conditions of andalusite-kyanite-sillimanite pegmatoid segregstions in metamorphic rocks of the Tseel block (Mongolian Altai)

NASA Astrophysics Data System (ADS)

Sukhorukov, Vasiliy

2010-05-01

Quartz veins and pegmatoid segregations containing polymorphous Al2SiO5 modifications often occur in metamorphic complexes. Metapelites abound in various combinations of two Al2SiO5 polymorphs, e.g., andalusite + sillimanite and sillimanite + kyanite (Kerrick, 1990). Rocks with three polymorphs are much scarcer; they result from subsequent crystallization during progressive metamorphism or combined regional and contact metamorphism or from metastable crystallization (Kerrick, 1990). Study of veins containing various Al2SiO5 modifications can give insight into the PT-conditions of metamorphism and their temporal changes. The Tsel block is localized in the basins of the Tseliyn, Hudjertiyn, Sharin, and Deresetuin Rivers and is part of the Hercynides structure on the southern flank of Mongolian Altay. Pegmatoid segregations containing polymorphous Al2SiO5 modifications were discovered in the vicinity of the western contact of one of the largest basic-rock massifs, Buren-Hairhan.They are hosted by schists bearing paragenesis Bt + Ms + St + Grt + Ky + Sill + And + Fibr + Pl + Qtz. Visual examination of the pegmatoid segregations showed the presence of large (up to 4-5 cm) mineral aggregates —kyanite pseudomorphs developed after andalusite. They are prismatic, with rhombic sections, and are composed of chaotically arranged kyanite crystals with irregular-shaped andalusite relics between them. The samples also contain large (1 cm) muscovite plates localized between the pseudomorphs as well as quartz grains. The pegmatoid segregations bear the following mineral assemblage: And + Ky + Sil + Fibr + Ms + Qtz + Pl + St +Grt + Bt. Predominant minerals are Al2SiO5 polymorphs (30-50 vol.%), muscovite (30-60 vol.%), and quartz (up to 20 vol.%). Biotite and plagioclase are present in small amounts; staurolite and garnet are occasional. Based on the observed mineral correlations, the following sequence of formation of Al2SiO5 polymorphs has been established: First, andalusite was replaced by kyanite to form pseudomorphs; then, fibrolite was produced, which, in turn, gave way to prismatic sillimanite. The fact that fibrolite formed after kyanite is proved by its presence in muscovite developed after the latter mineral. Sillimanite was, most likely, produced later than or synchronously with large muscovite replacing andalusite grains and kyanite pseudomorphs developed after andalusite. 2. Though the estimated P and T values are close to those of the triple point (Pattison, 1992), the studied rocks bear evidence for the replacement of andalusite by kyanite and sillimanite. The mineral assemblages pointing to the kyanitesillimanite type of metamorphism of the host rocks and the presence of andalusite relics in the latter suggest that the andalusite formed during earlier metamorphic events. This polymorph formation sequence agrees with Kozakov's (1986) data on change of the regional metamorphism from andalusite-sillimanite to kyanite-sillimanite one. 3. The great amounts of muscovite in veins containing Al2SiO5 polymorphs and its presence at phase contacts suggest a great role of muscovite in the phase transitions between different polymorphs. Since phases other than Al2SiO5 and muscovite are present in negligible amounts, it is not ruled out that the polymorphs were transformed by the ion exchange mechanism (Carmichael, 1969). 1. Carmichael, D.M., 1969. On the mechanism of prograde metamorphic reaction in quartz-bearing pelitic rocks. Contr. Miner. Petrol. 20, 244-267. 2. Kerrick, D.M., 1990. The Al2SiO5 polymorphs. Rev. Miner. 22, p. 406. 3. Kozakov, I.K., 1986. Precambrian infrastructure complexes of Mongolia [in Russian]. Nauka, Leningrad. 4. Pattison, D.R.M., 1992. Stability of andalusite and sillimanite and the Al2SiO5 triple point: constraints from the Ballachulish aureole, Scotland. J. Geol. 100, 423-446.

Nature and origin of fluids in granulite facies metamorphism

NASA Technical Reports Server (NTRS)

Newton, R. C.

1988-01-01

The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed.

Interrelation of the construction of the metamorphic InAlAs/InGaAs nanoheterostructures with the InAs content in the active layer of 76-100% with their surface morphology and electrical properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vasil'evskii, I. S., E-mail: ivasilevskii@mail.ru; Galiev, G. B.; Klimov, E. A.

The influence of the construction of a metamorphic buffer on the surface morphology and electrical properties of InAlAs/InGaAs/InAlAs nanoheterostructures with InAs content in the active layer from 76 to 100% with the use of the GaAs and InP substrates is studied. It is shown that such parameters as the electron mobility and the concentration, as well as the root-mean-square surface roughness, substantially depend on the construction of the metamorphic buffer. It is established experimentally that these parameters largely depend on the maximal local gradient of the lattice constant of the metamorphic buffer in the growth direction of the layers rathermore » than on its average value. It is shown that, with selection of the construction of the metamorphic buffer, it is possible to form nanostructured surfaces with a large-periodic profile.« less

A transect of metamorphic rocks along the Copper River, Cordova and Valdez Quadrangles, Alaska: A section in The United States Geological Survey in Alaska: Accomplishments during 1982

USGS Publications Warehouse

Miller, Marti L.; Dumoulin, Julie A.; Nelson, S.W.

1984-01-01

The lower Tertiary Orca Group is juxtaposed against the Upper Cretaceous Valdez Group along the Contact fault system (Winkler and Plafker, 1974, 198; Plafker and others, 1977)(fig. 33). In both groups, turbidites are the dominant rock type, with lesser mafic volcanic rocks (table 10). The Valdez Group, on the north, has traditionally been considered to be of higher metamorphic grade than the Orca Group (Moffit, 1954; Tysdal and Case, 1979; Winkler and Plafker, 198; Winkler and others, 1981). In 1982, we made a transect across the regional strike of the rocks and the contact between the two groups. The transect area follows the Copper River for 85 km from the Cordova quadrangle north into the Valdez quadrangle and extends for about 25 km on either side of the river (fig. 33). We planned, by systematic sampling of the area, to examine the metamorphic differences between the Orca and Valdez Groups. We found, however, that a strong thermal metamorphic event has overprinted and obscured regional metamorphic relations. We believe intrusion of Tertiary granite (fig. 33) to be responsible for this metamorphism. (Figures 33 and 34 and tables follow this article.)

1.45 Ga granulites in the southwestern Grenville province: Geologic setting, P-T conditions, and U-Pb geochronology

NASA Astrophysics Data System (ADS)

Ketchum, J. W. F.; Jamieson, R. A.; Heaman, L. M.; Culshaw, N. G.; Krogh, T. E.

1994-03-01

In the southwestern Grenville province, the parautochthonous Britt domain includes a variety of pre-Grenvillian metamorphic and plutonic rocks that were reworked at upper amphibolite facies during the Grenvillian orogeny. Near Pointe-au-Baril, Ontario, a crustal block containing pre-Grenvillian granulite facies mineral assemblages and pre-Grenvillian to early Grenvillian tectonic fabrics has been identified. The block is bounded on the northwest and southeast by extensional shear zones that may have isolated it from regional late Gren- villian deformation. Multiequilibria pressure-temperature (P-T) calculations for orthopyroxene-bearing mafic rocks suggest conditions of 625-700 °C and 7.2-8.4 kbar for the pre-Grenvillian metamorphism. The granulite facies assemblages were locally overprinted during higher pressure Grenvillian metamorphism, which peaked at 720-775 °C and 10.8-11.5 kbar. U-Pb zircon data from migmatitic, mafic supracrustal gneiss indicate metamorphism and leucosome development at ca. 1450-1430 Ma, in agreement with other pre-Grenvillian metamorphic ages for the Central gneiss belt and Grenville Front tectonic zone. An expanding data base on pre-Grenvillian events in the southwestern Grenville province indicates that high-grade metamorphism at ca. 1450-1430 Ma affected a large region of crust and was coeval with widespread felsic to intermediate plutonism.

Evolutionary genetics of metamorphic failure using wild-caught vs. laboratory axolotls (Ambystoma mexicanum).

PubMed

Voss, S R; Shaffer, H B

2000-09-01

In many organisms metamorphosis allows for an ecologically important habitat-shift from water to land. However, in some salamanders an adaptive life cycle mode has evolved that is characterized by metamorphic failure (paedomorphosis); these species remain in the aquatic habitat throughout the life cycle. Perhaps the most famous example of metamorphic failure is the Mexican axolotl (Ambystoma mexicanum), which has become a focal species for developmental biology since it was introduced into laboratory culture in the 1800s. Our previous genetic linkage mapping analysis, using an interspecific crossing design, demonstrated that a major gene effect underlies the expression of metamorphic failure in laboratory stocks of the Mexican axolotl. Here, we repeated this experiment using A. mexicanum that were sampled directly from their natural habitat at Lake Xochimilco, Mexico. We found no significant association between the major gene and metamorphic failure when wild-caught axolotls were used in the experimental design, although there is evidence of a smaller genetic effect. Thus, there appears to be genetic variation among Mexican axolotls (and possibly A. tigrinum tigrinum) at loci that contribute to metamorphic failure. This result suggests a role for more than one mutation and possibly artificial selection in the evolution of the major gene effect in the laboratory Mexican axolotl.

Early Metamorphic Insertion Technology for Insect Flight Behavior Monitoring

PubMed Central

Bozkurt, Alper

2014-01-01

Early Metamorphosis Insertion Technology (EMIT) is a novel methodology for integrating microfabricated neuromuscular recording and actuation platforms on insects during their metamorphic development. Here, the implants are fused within the structure and function of the neuromuscular system as a result of metamorphic tissue remaking. The implants emerge with the insect where the development of tissue around the electronics during pupal development results in a bioelectrically and biomechanically enhanced tissue interface. This relatively more reliable and stable interface would be beneficial for many researchers exploring the neural basis of the insect locomotion with alleviated traumatic effects caused during adult stage insertions. In this article, we implant our electrodes into the indirect flight muscles of Manduca sexta. Located in the dorsal-thorax, these main flight powering dorsoventral and dorsolongitudinal muscles actuate the wings and supply the mechanical power for up and down strokes. Relative contraction of these two muscle groups has been under investigation to explore how the yaw maneuver is neurophysiologically coordinated. To characterize the flight dynamics, insects are often tethered with wires and their flight is recorded with digital cameras. We also developed a novel way to tether Manduca sexta on a magnetically levitating frame where the insect is connected to a commercially available wireless neural amplifier. This set up can be used to limit the degree of freedom to yawing “only” while transmitting the related electromyography signals from dorsoventral and dorsolongitudinal muscle groups. PMID:25079130

Geochemical evidence for a brooks range mineral belt, Alaska

USGS Publications Warehouse

Marsh, S.P.; Cathrall, J.B.

1981-01-01

Geochemical studies in the central Brooks Range, Alaska, delineate a regional, structurally controlled mineral belt in east-west-trending metamorphic rocks and adjacent metasedimentary rocks. The mineral belt extends eastward from the Ambler River quadrangle to the Chandalar and Philip Smith quadrangles, Alaska, from 147?? to 156??W. longitude, a distance of more than 375 km, and spans a width from 67?? to 69??N. latitude, a distance of more than 222 km. Within this belt are several occurrences of copper and molybdenum mineralization associated with meta-igneous, metasedimentary, and metavolcanic rocks; the geochemical study delineates target areas for additional occurrences. A total of 4677 stream-sediment and 2286 panned-concentrate samples were collected in the central Brooks Range, Alaska, from 1975 to 1979. The -80 mesh ( 2.86) nonmagnetic fraction of the panned concentrates from stream sediment were analyzed by semiquantitative spectrographic methods. Two geochemical suites were recognized in this investigation; a base-metal suite of copper-lead-zinc and a molybdenum suite of molybdenum-tin-tungsten. These suites suggest several types of mineralization within the metamorphic belt. Anomalies in molybdenum with associated Cu and W suggest a potential porphyry molybdenum system associated with meta-igneous rocks. This regional study indicates that areas of metaigneous rocks in the central metamorphic belt are target areas for potential mineralized porphyry systems and that areas of metavolcanic rocks are target areas for potential massive sulfide mineralization. ?? 1981.

Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories

NASA Astrophysics Data System (ADS)

Williams, Michael L.; Jercinovic, Michael J.; Terry, Michael P.

1999-11-01

High-resolution X-ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural, metamorphic, and tectonic analysis. X-ray maps commonly show complex Th, U, and Pb zoning that reflects monazite growth and overgrowth events. Age maps constructed from the X-ray maps simplify the zoning and highlight age domains. Microprobe dating offers a rapid, in situ method for estimating ages of mapped domains. Application of these techniques has placed new constraints on the tectonic history of three areas. In western Canada, age mapping has revealed multiphase monazite, with older cores and younger rims, included in syntectonic garnet. Microprobe ages show that tectonism occurred ca. 1.9 Ga, 700 m.y. later than mylonitization in the adjacent Snowbird tectonic zone. In New Mexico, age mapping and dating show that the dominant fabric and triple-point metamorphism occurred during a 1.4 Ga reactivation, not during the 1.7 Ga Yavapai-Mazatzal orogeny. In Norway, monazite inclusions in garnet constrain high-pressure metamorphism to ca. 405 Ma, and older cores indicate a previously unrecognized component of ca. 1.0 Ga monazite. In all three areas, microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts.

Combined Detrital U/Pb Zircon and 40Ar/39Ar Mica Geoochronology to Test Structural Models for a Devonian Orogenic Collapse Basin in the Norwegian Caledonides

NASA Astrophysics Data System (ADS)

Templeton, J.; Anders, M.; Fossen, H.

2014-12-01

The Hornelen basin is the largest of the Devonian 'Old Red' sandstone basins in Norway, comprising 25 km of alluvial-fluvial deposits which are organized into basin-wide, coarsening-upward megacycles. Hornelen sits with several smaller basins in the hanging wall a major extensional shear zone along which the ultra-high pressure metamorphic core of subducted Baltican crust was rapidly exhumed during the extensional collapse of the Caledonian orogeny. The timing of orogenic collapse corresponds closely to the timing of the basins, which are loosely constrained by sparse trace-fossil assemblages to a mid-Devonian age. Further, the basins are now in brittle fault contact with the underlying mylonitic shear zone and the metamorphic core, implying that they are the upper-crustal expression of large-scale extension and deep-crustal exhumation. Two distinct structural models have been proposed for Hornelen to account for these observations. The strike-slip model juxtaposes different source terranes across the basin-controlling fault and predicts spatially changing provenance within chronostratigraphic units. The supradetachment model links the filling of the basin directly to unroofing of the metamorphic core on a low-angle detachment fault, and predicts basin-wide changes in provenance through time with progressive exhumation of the metamorphic hinterland. We present an extensive new provenance dataset, spanning the Hornelen basin strata through space and time. Detrital zircon U/Pb ages from 18 new samples comprise three distinct populations (1.6, 1.0, and 0.43 Ga) with the Caledonian-aged zircons (ca 0.43 Ga) present mainly along the northern margin of the basin, representing an Upper Allochthon source not found on the southern or eastern margins of the basin. Juxtaposition of different source terranes across the basin supports the strike-slip model. 40Ar/39Ar detrital white mica from the same sample set documents a younging of the dominant age peak from 432 Ma in the oldest sediments to 401 Ma in the youngest units, but does not document any difference between northern and southern mica sources. This trend supports the supradetachment model, but may also be explained by passive, isostatically-driven erosional unroofing of the overthickened orogenic crust.

Separating Multiple Episodes of Partial Melting in Polyorogenic Crust: AN Example from the Haiyangsuo Complex, Northern Sulu Belt, Eastern China

NASA Astrophysics Data System (ADS)

Feng, P.; Wang, L.; Brown, M.; Wang, S.

2017-12-01

Determining the timing, mechanism and source of partial melts in polyorogenic crust is challenging. In the Sulu belt, the tectonic affinity of the Haiyangsuo (HYS) complex is controversial due to its polyphase metamorphic history. Here we use detailed field mapping, petrology, microstructural analysis and zircon geochronology to study thin stromatic leucosomes in host granite gneiss, and crosscutting leucogranite dykes to decipher the melting history. Zircon grains from both granite gneiss and thin leucosomes exhibit core-mantle-rim structures. Zircon cores yield protolith ages of 2.86-2.81 Ga, whereas the mantles and rims yield younger metamorphic/melt crystallization ages of ca. 1.82-1.80 Ga. The mantles are characterized by gray luminescence, flat HREE distribution patterns and relatively low Th/U ratios, indicating crystallization during granulite-facies metamorphism. Whereas rims show bright luminescence, steep HREE distribution patterns and higher Th/U ratios, suggesting they crystallized from melt. The mantles and rims have ɛHf (t) of -18.2 to -11.0. Using 176Lu/177Hf = 0.001, these data project back to the array of ɛHf (t) values for the zircon cores. This demonstrates that the thin leucosomes were derived from the gneiss without any mass input from a mantle source. These features are consistent with an origin of the HYS as part of the eastern margin of the NCC prior to juxtaposition with the Sulu belt. Zircons from the leucogranite dykes also show core-mantle-rim structure. Inherited cores yield concordant 206Pb/238U ages of 776-701 Ma consistent with the dominant age range for protoliths of the UHP metamorphic rocks in the Sulu belt. Zircon mantle and rim domains, which both contain multiphase solid inclusions (Kfs + Pl + Qz and Hem + Pl + Qz in mantles and Kfs + Pl + Qz + Bt in rims), yield melt crystallization ages of 226-217 and 169-156 Ma, respectively. High Sr, low Y and Yb contents, high Sr/Y ratios, and the range of ɛNd (t) values (-18.2- -15.0) and initial 87Sr/86Sr ratios (0.7106 - 0.7146) for the leucogranites are consistent with melting of thickened lower continental crust of the Sulu belt. We interpret the dykes to have been emplaced during post-collisional collapse of the orogenic root of this belt in the Middle-Upper Jurassic.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

NASA Astrophysics Data System (ADS)

Kaymakci, N.; van Hinsbergen, D. J.; Spakman, W.; Torsvik, T. H.

2010-12-01

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa-Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide-Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~95-90Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavsanli and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~50Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~1400 km of Africa-Europe convergence occurred, of which ~700 km since 50 Ma and ~450 km since 35Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa-Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Uplifted ophiolitic rocks on Isla Gordon, southernmost Chile: implications for the closure history of the Rocas Verdes marginal basin and the tectonic evolution of the Beagle Channel region

NASA Astrophysics Data System (ADS)

Cunningham, W. D.

1994-04-01

A succession of mafic rocks that includes gabbro, sheeted dikes and deformed pillow basalts has been mapped in detail on Isla Gordon, southernmost Chile and is identified as an upper ophiolitic complex representing the uplifted floor of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. The complex was uplifted, deformed, and regionally metamorphosed prior to the intrusion of an undeformed 90 Ma granodiorite that cuts the complex. The complex appears para-autochthonous, is gently tilted to the northeast and is internally sheared by near-vertical foliation zones. No evidence for obduction was observed although the base of the complex is not exposed. The ophiolitic rocks have been regionally metamorphosed to mid-upper greenschist levels. Isla Gordon is bounded by the northwest and southwest arms of the Beagle Channel, two important structural boundaries in the southernmost Andes that are interpreted to have accommodated north-side-up and left-lateral displacements. Directly north of Isla Gordon is the Cordillera Darwin metamorphic complex that exposes the highest grade metamorphic rocks in the Andes south of Peru. On the north coast of Isla Gordon a volcaniclastic turbidite sequence that is interpreted to have been deposited above the mafic floor is metamorphosed to lower greenschist levels in strong metamorphic contrast to amphibolite-grade othogneisses exposed in Cordillera Darwin only 2 km away across the northwest arm of the Beagle Channel. The profound metamorphic break across the northwest arm of the Beagle Channel and the regional northeast tilt of the ophiolitic complex are consistent with the previously proposed hypothesis that Isla Gordon represents the upper plate to an extensional fault that accommodated tectonic unroofing of Cordillera Darwin. However, limited structural evidence for extension was identified in this study to support the model and further work is needed to determine the relative importance of contractional, extensional and strike-slip displacements during the closure of the Rocas Verdes marginal basin and uplift of Cordillera Darwin. The Isla Gordon ophiolitic complex is correlative with other regional occurrences of ophiolitic rocks including the previously studied Tortuga, Sarmiento and Larsen Harbour complexes. The existence of the Isla Gordon ophiolitic complex helps link the known occurrences of the marginal basin floor into a semi-continuous belt that sheds light on the original continuity of the basin.

Geologic map of the Chelan 30-minute by 60-minute quadrangle, Washington

USGS Publications Warehouse

Tabor, R.W.; Frizzell, V.A.; Whetten, J.T.; Waitt, R.B.; Swanson, D.A.; Byerly, G.R.; Booth, D.B.; Hetherington, M.J.; Zartman, R.E.

1987-01-01

Summary -- The Chelan quadrangle hosts a wide variety of rocks and deposits and display a long geologic history ranging from possible Precambrian to Recent. Two major structures, the Leavenworth and Entiat faults divide cross the quadrangle from southeast to northwest and bound the Chiwaukum 'graben', a structural low preserving Tertiary sedimentary rocks between blocks of older, metamorphic and igneous rocks. Pre-Tertiary metamorphic rocks in the quadrangle are subdivided into five major tectonostratigraphic terranes: (1) the Ingalls terrane, equivalent to the Jurassic Ingalls Tectonic Complex of probable mantle and deep oceanic rocks origin, (2) the Nason terrane, composed of the Chiwaukum Schist and related gneiss, (3) the Swakane terrane, made up entirely of the Swakane Biotite Gneiss, a metamorphosed, possibly Precambrian, sedimentary and/or volcanic rock, (4) the Mad River terrane composed mostly of the rocks of the Napeequa River area (Napeequa Schist), a unit of oceanic protolith now considered part of the Chelan Mountains terrane (the Mad River terrane has been abandoned, 2001), and (5) the Chelan Mountains terrane, dominated by the Chelan Complex of Hopson and Mattinson (1971) which is composed of migmatite and gneissic to tonalite of deep-seated igneous and metamorphic origin.During an episode of Late Cretaceous regional metamorphism, all the terranes were intruded by deepseated tonalite to granodiorite plutons, including the Mount Stuart batholith, Ten Peak and Dirty Face plutons, and the Entiat pluton and massive granitoid rocks of the Chelan Complex. The Duncan Hill pluton intruded rocks of the Chelan Mountains terrane in the Middle Eocene. At about the same time fluvial arkosic sediment of the Chumstick Formation was deposited in a depression. The outpouring of basalt lavas to the southeast of the quadrangle during the Miocene built up the Columbia River Basalt Group. These now slightly warped lavas lapped onto the uplifted older rocks. Deformation, uplift, and erosion recorded in the rocks and deposits of the quadrangle continued into post-Miocene time. Quaternary deposits reflect advances of glaciers down the major valleys, a complicated history of catastrophic glacial floods down the Columbia River, the formation of lakes in the Columbia and Wenatchee river valleys by landslides and flood backwaters, and hillslope erosion by large and small landslides and debris flows.

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India-Eurasia collision in Myanmar

NASA Astrophysics Data System (ADS)

Barley, M. E.; Pickard, A. L.; Zaw, Khin; Rak, P.; Doyle, M. G.

2003-06-01

Situated south of the eastern Himalayan syntaxis at the western margin of the Shan-Thai terrane the high-grade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (˜170 Ma) zircons that have partly recrystallized during ˜43 Ma high-grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 ± 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid-Cretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (˜43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina.

High-pressure granulites in the Fuping Complex of the central North China Craton: Metamorphic P-T-t evolution and tectonic implications

NASA Astrophysics Data System (ADS)

Qian, Jiahui; Yin, Changqing; Zhang, Jian; Ma, Li; Wang, Luojuan

2018-04-01

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite-Tonalite-Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ilmenite + quartz within garnet core; an inferred peak assemblage composed of garnet ± hornblende + plagioclase + clinopyroxene + rutile/ilmenite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene ± orthopyroxene + plagioclase, coronae of plagioclase ± clinopyroxene ± hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P-T conditions of 12-13 kbar and 760-800 °C (Pmax) and a post-peak history under P-T conditions of ∼9.0 kbar and 805-835 °C (Tmax), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the Pmax to the Tmax is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz ± diopside = hornblende + plagioclase + liquid and garnet + quartz + hornblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the Tmax is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68-6.88 ppm and crystallization temperatures of 602-712 °C. SIMS zircon U-Pb dating on these zircons yields 207Pb/206Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage of metamorphism. The P-T-t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton.

The effect of thermal resetting and recrystallisation on white mica 40Ar/39Ar ages during retrograde metamorphism on Syros, Greece

NASA Astrophysics Data System (ADS)

Uunk, Bertram; Wijbrans, Jan; Brouwer, Fraukje

2015-04-01

White mica 40Ar/39Ar dating is a proven powerful tool for constraining timing of metamorphism, deformation and exhumation. However, in high-pressure metamorphic rocks, dating often results in wide age ranges which are not in agreement with constraints from other isotopic systems, indicating that geological and chemical processes complicate straightforward 40Ar/39Ar dating. In this research project, white mica ages from rocks of the Cycladic Blueschist Unit on Syros, Greece with contrasting rheology and strain mechanisms are compared, in order to better understand the role of deformation, recrystallization and fluid flow on 40Ar/39Ar ages of white mica during retrograde metamorphism. Resulting ages vary along different sections on the island, inconsistent with other isotopic constraints on eclogite-blueschist metamorphism (55-50 Ma) and greenschist overprinting (41-30 Ma). Two end-member models are possible: 1) Results represent continuous crystallization of white mica while moving from blueschist to greenschist conditions in the metamorphic P-T loop, or 2) white mica equilibrated in eclogite-blueschist conditions and their diffusion systematics were progressively perturbed during greenschist overprinting. The single grain fusion analyses yielded contrasting age distributions, which indicate contrasts in degree of re-equilibration during retrograde metamorphism. Step wise heating of larger grain populations resulted in flat plateau shapes, providing no evidence for partial resetting. Electron microprobe measurements of Si per formula unit, as a proxy for pressure during crystallisation, do not explain age variation within sections or on the island scale. The previously unreported north-south age trend and age ranges per sample, as shown only in the 40Ar/39Ar system of the metapelitic and marble lithologies, contains key information that will allow us to test between different scenarios for age formation. Excess argon infiltration at this stage seems to have been of minor importance. Our new approach should lead to a better understanding of the interplay of these processes during and after HP metamorphism.

The brittle stage of the exhumation of a metamorphic core complex: Naxos, Cyclades, Greece and some general rules

NASA Astrophysics Data System (ADS)

Neubauer, F.; Cao, S.

2012-04-01

Structures of hangingwall units of major detachment systems in extensional settings leading to metamorphic core complexes are equally important to the generally well-studied footwall rocks. Here, we describe hanging-wall structures of the North-Cycladic Detachment System on Naxos Island of the Aegean Sea and found that they well monitor the structural evolution of hanging blocks complementary to the footwall structures, vertical fluid flow as well as late-stage inversion of the whole extensional system. On Naxos, Upper Oligocene-Miocene and Pliocene sedimentary successions are deposited on the hangingwall unit, which is largely an ophiolite. The Upper Oligocene-Miocene and Pliocene sedimentary successions are separated by a hiatus arguing for a two-step evolution. Whereas the first step, Miocene, indicate moderate subsidence and relief, and only denudation of the hangingwall unit, the Pliocene conglomerates indicate a sharply increasing relief and an over-steepened topography. Hydrothermal systems developed in hangingwall rock succession (e.g. Miocene at Steladia) play an important role and resulted in large-scale silica precipitation and associated alteration similar as these found in subvolcanic epithermal systems. This constrains a close link between footwall granodiorite intrusion and near-surface processes. The Pliocene coarse boulder conglomerate with its abundant first appearance of granite/granodiorite, and subsequent marble-rich debris on distant places like Palatia indicate a sudden erosion and high-gradient relief leading to erosion of the mantle of the migmatite dome during Pliocene. On Naxos, we recognize, therefore, a three-stage tectonic evolution in the hangingwall unit: (i) moderate subsidence of an Upper Oligocene-Miocene basin, in part below sea level; (2) a second stage with deposition of Pliocene coarse conglomerates, and (iii) post-Pliocene faulting affecting the conglomerates. During the second stage, surface exposure of the metamorphic core complex was reached resulting in catastrophic alluvial fans. Structural data from the Upper Oligocene-Miocene rocks indicate that NNE-SSW extension still prevailed up to the Miocene/Pliocene boundary. Together with structural data from Pliocene conglomerates, we can distinguish between three major events: The first stage is characterized by mostly NNE-dipping and subordinate SSW-dipping normal faults indicating together ca. NNE-SSW extension. A second palaeostress tensor group (B) mainly comprises ca. NW-trending dextral and WSW-trending sinistral strike-slip faults indicating together ca. E-W strike-slip compression and monitor, therefore, inversion and compression perpendicular to the previous extension direction. The third palaeostress tensor group (C) is characterized by dominating mostly NE-trending subvertical sinistral strike-slip faults and steep NNW-trending dextral strike-slip faults constituting together ca. N-S strike-slip compression. In a few cases, S- to SW-dipping reverse faults also occur. On a general level, our study allows for the following major conclusions: (1) Structures of hangingwall units of major detachments above metamorphic core complexes are equally important compared to the generally well-studied footwall rocks. They allow date several tectonic events not necessarily found in footwall rocks. (2) On Naxos, we can distinguish between three major tectonic events, which are in accordance with large-scale tectonic processes in the Aegean Sea: (a) ca. NNE-SSW extension; (b) ca. E-W strike-slip compression and monitor therefore inversion and compression perpendicular to the previous extension direction, and (c) N-S strike-slip compression.

Geology and geochemistry of the Middle Proterozoic Eastern Ghat mobile belt and its comparison with the lower crust of the Southern Peninsular shield

NASA Technical Reports Server (NTRS)

Rao, M. V. Subba

1988-01-01

Two prominent rock suites constitute the lithology of the Eastern Ghat mobile belt: (1) the khondalite suite - the metapelites, and (2) the charnockite suite. Later intrusives include ultramafic sequences, anorthosites and granitic gneisses. The chief structural element in the rocks of the Eastern Ghats is a planar fabric (gneissosity), defined by the alignment of platy minerals like flattened quartz, garnet, sillimanite, graphite, etc. The parallelism between the foliation and the lithological layering is related to isoclinal folding. The major structural trend (axial plane foliation trend) observed in the belt is NE-SW. Five major tectonic events have been delineated in the belt. A boundary fault along the western margin of the Eastern Ghats, bordering the low grade terrain has been substantiated by recent gravity and the deep seismic sounding studies. Field evidence shows that the pyroxene granulites (basic granulites) post-date the khondalite suite, but are older than the charnockites as well as the granitic gneisses. Polyphase metamorphism, probably correlatable with different periods of deformation is recorded. The field relations in the Eastern Ghats point to the intense deformation of the terrain, apparently both before, during and after metamorphism.

NACSN, note 67--Application for revision of Articles 36 and 37, Lithodemic units of the North American stratigraphic code

USGS Publications Warehouse

Easton, Robert M.; Edwards, Lucy E.; Orndorff, Randall C.; Duguet, Manuel; Ferrusquia-Villafranca, Ismael

2015-01-01

Currently the North American Stratigraphic Code, (NACSN 2005, Article 37) sets restrictions on the use of the term “complex” for lithodemic units. With exceptions for “volcanic complex” and “structural complex,” a complex must consist of more than one genetic class of rock (i.e., sedimentary, igneous or metamorphic). Thus, the use of the term “complex” to describe masses of intrusive rocks is not allowed. Asimilar restriction is also included in a recent British Geological Survey proposal for using lithodemic units to classify igneous rocks (Gillespie et al. 2008).Currently the North American Stratigraphic Code, (NACSN 2005, Article 37) sets restrictions on the use of the term “complex” for lithodemic units. With exceptions for “volcanic complex” and “structural complex,” a complex must consist of more than one genetic class of rock (i.e., sedimentary, igneous or metamorphic). Thus, the use of the term “complex” to describe masses of intrusive rocks is not allowed. Asimilar restriction is also included in a recent British Geological Survey proposal for using lithodemic units to classify igneous rocks (Gillespie et al. 2008).

Clumped isotope thermometry of calcite and dolomite in a contact metamorphic environment

NASA Astrophysics Data System (ADS)

Lloyd, Max K.; Eiler, John M.; Nabelek, Peter I.

2017-01-01

Clumped isotope compositions of slowly-cooled calcite and dolomite marbles record apparent equilibrium temperatures of roughly 150-200 °C and 300-350 °C, respectively. Because clumped isotope compositions are sensitive to the details of T-t path within these intervals, measurements of the Δ47 values of coexisting calcite and dolomite can place new constraints on thermal history of low-grade metamorphic rocks over a large portion of the upper crust (from ∼5 to ∼15 km depth). We studied the clumped isotope geochemistry of coexisting calcite and dolomite in marbles from the Notch Peak contact metamorphic aureole, Utah. Here, flat-lying limestones were intruded by a pluton, producing a regular, zoned metamorphic aureole. Calcite Δ47 temperatures are uniform, 156 ± 12 °C (2σ s.e.), across rocks varying from high-grade marbles that exceeded 500 °C to nominally unmetamorphosed limestones >5 km from the intrusion. This result appears to require that the temperature far from the pluton was close to this value; an ambient temperature just 20 °C lower would not have permitted substantial re-equilibration, and should have preserved depositional or early diagenetic Δ47 values several km from the pluton. Combining this result with depth constraints from overlying strata suggests the country rock here had an average regional geotherm of 22.3-27.4 °C/km from the late Jurassic Period until at least the middle Paleogene Period. Dolomite Δ47 in all samples above the talc + tremolite-in isograd record apparent equilibrium temperatures of 328-12+13 °C (1σ s.e.), consistent with the apparent equilibrium blocking temperature we expect for cooling from peak metamorphic conditions. At greater distances, dolomite Δ47 records temperatures of peak (anchi)metamorphism or pre-metamorphic diagenetic conditions. The interface between these domains is the location of the 330 °C isotherm associated with intrusion. Multiple-phase clumped isotope measurements are complemented by bulk δ13C and δ18O dolomite-calcite thermometry. These isotopic exchange thermometers are largely consistent with peak temperatures in all samples within 4 km of the contact, indicating that metamorphic recrystallization can occur even in samples too low-grade to produce growth of conventional metamorphic index minerals (i.e., talc and tremolite). Altogether, this work demonstrates the potential of these methods to quantify the conditions of metamorphism at sub-greenschist facies.

Fluid evolution during burial and Variscan deformation in the Lower Devonian rocks of the High-Ardenne slate belt (Belgium): sources and causes of high-salinity and C-O-H-N fluids

NASA Astrophysics Data System (ADS)

Kenis, I.; Muchez, Ph.; Verhaert, G.; Boyce, A.; Sintubin, M.

2005-08-01

Fluid inclusions in quartz veins of the High-Ardenne slate belt have preserved remnants of prograde and retrograde metamorphic fluids. These fluids were examined by petrography, microthermometry and Raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the High-Ardenne slate belt. The earliest fluid type was a mixed aqueous/gaseous fluid (H2O-NaCl-CO2-(CH4-N2)) occurring in growth zones and as isolated fluid inclusions in both the epizonal and anchizonal part of the metamorphic area. In the central part of the metamorphic area (epizone), in addition to this mixed aqueous/gaseous fluid, primary and isolated fluid inclusions are also filled with a purely gaseous fluid (CO2-N2-CH4). During the Variscan orogeny, the chemical composition of gaseous fluids circulating through the Lower Devonian rocks in the epizonal part of the slate belt, evolved from an earlier CO2-CH4-N2 composition to a later composition enriched in N2. Finally, a late, Variscan aqueous fluid system with a H2O-NaCl composition migrated through the Lower Devonian rocks. This latest type of fluid can be observed in and outside the epizonal metamorphic part of the High-Ardenne slate belt. The chemical composition of the fluids throughout the metamorphic area, shows a direct correlation with the metamorphic grade of the host rock. In general, the proportion of non-polar species (i.e. CO2, CH4, N2) with respect to water and the proportion of non-polar species other than CO2 increase with increasing metamorphic grade within the slate belt. In addition to this spatial evolution of the fluids, the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin. In addition to the maturity of the metamorphic fluids, the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host-rock. For the earliest and latest fluid inclusions in the anchizonal part of the High-Ardenne slate belt the salinity varies respectively between 0 and 3.5 eq.wt% NaCl and between 0 and 2.7 eq.wt% NaCl, while in the epizonal part the salinity varies between 0.6 and 17 eq.wt% NaCl and between 3 and 10.6 eq.wt% for the earliest and latest aqueous fluid inclusions, respectively. Although high salinity fluids are often attributed to the original sedimentary setting, the increasing salinity of the fluids that circulated through the Lower Devonian rocks in the High-Ardenne slate belt can be directly attributed to regional metamorphism. More specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of Cl-bearing minerals during prograde metamorphism, while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism. The temporal and spatial distribution of the fluids in the High-Ardenne slate belt are indicative for a closed fluid flow system present in the Lower Devonian rocks during burial and Variscan deformation, where fluids were in thermal and chemical equilibrium with the host rock. Such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform δ180 values of both the veins and their host rock demonstrate a rock-buffered fluid flow system.

A Review of Land and Stream Classifications in Support of Developing a National Ordinary High Water Mark (OHWM) Classification

DTIC Science & Technology

2014-08-01

northern Minnesota, Wisconsin, and Michigan. This region is dominated by igneous and metamorphic rock , with some sedimentary units and a generally...faulted igneous and metamorphic rocks and folded sediments in the Appalachians and flat-lying sedimentary rocks in the Plateau and Catskills. Streams...mixture of igneous, metamorphic , and sedimentary rocks . High relief and coarse materials are typical. Riffle and pool development is largely

Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Eastern Mountains and Piedmont Region (Version 2.0)

DTIC Science & Technology

2012-04-01

are the Blue Ridge Province and the Piedmont Plateau, composed mainly of highly eroded Precambrian metamorphic rocks . The Piedmont Plateau extends...older igneous and metamorphic rocks (Atwood 1940; Hunt 1974; USGS 2004). Caverns and karst features are found in marble formations in the Piedmont...colluvium derived from sandstone, shale, limestone, and metamorphic and igneous rocks . Other parent materials include deposits of wind-blown loess

Reptiles and Amphibians of Fairchild Air Force Base, WA

DTIC Science & Technology

2013-05-10

calling, free swimming larvae, and metamorph emer- gence, respectively. General survey methods included the use of timed au- ral surveys, automated...influence metamorph success or use . Although the majority of adult spotted frogs were found in ditches, it is inter- esting to note that a large...number of metamorphs were found within a pond (at UTM 451650 5272683) that had not had any prior evidence of amphibian use . This pond had very little

Crystal Structure Studies of Low-Ca Pyroxenes from LL-Group Chondritic Meteorites

NASA Astrophysics Data System (ADS)

Artioli, G.; Davoli, G.; Sighinolfi, G. P.

1993-07-01

One orthorhombic (Pbca) and two monoclinic (P2(sub)1/c) single crystals of low-Ca pyroxenes were extracted from unequilibrated chondritic meteorites of the LL-group. The results of the crystal structure refinements performed using x-ray diffraction data indicate that: (1) the intracrystalline Fe-Mg distribution over the M1 and M2 crystallographic sites of the Parnallee (LL-3) orthoenstatite is consistent with a temperature of 960 degrees C for the closure of the exchange equilibrium process; and (2) the structural state and intracristalline Fe-Mg order in the Soko Banja (LL-4) and Jolomba (LL-6) clinoenstatites indicate a closing temperature of at least 1000-1100 degrees C, with no significant reequilibration at lower temperatures. The present data represent the first detailed crystallographic investigation of pyroxenes from LL-chondrites and support the hypothesis that the chondrule pyroxenes bear a distinct memory of rapid cooling in the solar nebular and that thermal metamorphism in the parent body, if present, was totally unsufficient to allow reequilibration of the pyroxene minerals to the low-temperature ordered crystal structures. The data also indicate that, assuming low or mild pressure and shock effects, there is no well-defined correlation between equilibrium temperature of the mineral phases and the alleged petrologic type of the meteorites. This evidence is consistent with a rubble-pile model for the parent body accretional history, or with an onion-shell model with very low thermal-peak metamorphism, as it is assumed for a very small object.

Preliminary geologic map of the Murrieta 7.5' quadrangle, Riverside County, California

USGS Publications Warehouse

Kennedy, Michael P.; Morton, Douglas M.

2003-01-01

The Murrieta quadrangle is located in the northern part of the Peninsular Ranges Province and includes parts of two structural blocks, or structural subdivisions of the province. The quadrangle is diagonally crossed by the active Elsinore fault zone, a major fault zone of the San Andreas fault system, and separates the Santa Ana Mountains block to the west from the Perris block to the east. Both blocks are relatively stable internally and within the quadrangle are characterized by the presence of widespread erosional surfaces of low relief. The Santa Ana Mountains block, in the Murrieta quadrangle, is underlain by undifferentiated, thick-layered, granular, impure quartzite and well-layered, fissile, phyllitic metamorphic rock of low metamorphic grade. Both quartzite and phyllitic rocks are Mesozoic. Unconformably overlying the metamorphic rocks are remnants of basalt flows having relatively unmodified flow surfaces. The age of the basalt is about 7-8Ma. Large shallow depressions on the surface of the larger basalt remnants form vernal ponds that contain an endemic flora. Beneath the basalt the upper part of the metamorphic rocks is deeply weathered. The weathering appears to be the same as the regional Paleocene saprolitic weathering in southern California. West of the quadrangle a variable thickness sedimentary rock, physically resembling Paleogene rocks, occurs between the basalt and metamorphic rock. Where not protected by the basalt, the weathered rock has been removed by erosion. The dominant feature on the Perris block in the Murrieta quadrangle is the south half of the Paloma Valley ring complex, part of the composite Peninsular Ranges batholith. The complex is elliptical in plan view and consists of an older ring-dike with two subsidiary short-arced dikes that were emplaced into gabbro by magmatic stoping. Small to large stoped blocks of gabbro are common within the ring-dikes. A younger ring-set of hundreds of thin pegmatite dikes occur largely within the central part of the complex. These pegmatite dikes were emplaced into a domal fracture system, apparently produced by cauldron subsidence, and include in the center of the complex, a number of flat-floored granophyre bodies. The granophyre is interpreted to be the result of pressure quenching of pegmatite magma. Along the eastern edge of the quadrangle is the western part of a large septum of medium metamorphic grade Mesozoic schist. A dissected basalt flow caps the Hogbacks northeast of Temecula, and represents remnants of a channel filling flow. Beneath the basalt is a thin deposit of stream gravel. Having an age of about 10Ma, this basalt is about 2-3Ma older than the basalt flows in the Santa Ana Mountains. The Elsinore fault zone forms a complex of pull-apart basins. The west edge of the fault zone, the Willard Fault, is marked by the high, steep eastern face of the Santa Ana Mountains. The east side of the zone, the Wildomar Fault, forms a less pronounced physiographic step. In the center of the quadrangle a major splay of the fault zone, the Murrieta Hot Springs Fault, strikes east. Branching of the fault zone causes the development of a broad alluvial valley between the Willard Fault and the Murrieta Hot Springs Fault. All but the axial part of the zone between the Willard and Wildomar Faults consist of dissected Pleistocene sedimentary units. The axial part of the zone is underlain by Holocene and latest Pleistocene sedimentary units.

Insights into a fossil plate interface of an erosional subduction zone: a tectono-metamorphic study of the Tianshan metamorphic belt.

NASA Astrophysics Data System (ADS)

Bayet, Lea; Moritz, Lowen; Li, Jilei; Zhou, Tan; Agard, Philippe; John, Timm; Gao, Jun

2016-04-01

Subduction zone seismicity and volcanism are triggered by processes occurring at the slab-wedge interface as a consequence of metamorphic reactions, mass-transfer and deformation. Although the shallow parts of subduction zones (<30-40 km) can be partly accessed by geophysical methods, the resolution of these techniques is insufficient to characterize and image the plate interface at greater depths (>60km). In order to better understand the plate interface dynamics at these greater depths, one has to rely on the rock record from fossil subduction zones. The Chinese Tianshan metamorphic belt (TMB) represents an ideal candidate for such studies, because structures are well exposed with exceptionally fresh high-pressure rocks. Since previous studies from this area focused on fluid-related processes and its metamorphic evolution was assessed on single outcrops, the geodynamic setting of this metamorphic belt is unfortunately heavily debated. Here, we present a new geodynamic concept for the TMB based on detailed structural and petrological investigations on a more regional scale. A ~11km x 13km area was extensively covered, together with E-W and N-S transects, in order to produce a detailed map of the TMB. Overall, the belt is composed of two greenschist-facies units that constitute the northern and southern border of a large high-pressure (HP) to ultra high-pressure (UHP) unit in the center. This HP-UHP unit is mainly composed of metasediments and volcanoclastic rocks, with blueschist, eclogite and carbonate lenses. Only the southern part of the HP-UHP unit is composed of the uppermost part of an oceanic crust (e.g., pillow basalts and deep-sea carbonates). From south to north, the relative abundance and size of blueschist massive boudins and layers (as well as eclogite boudins) decreases and the sequence is increasingly interlayered with metasedimentary and carbonate-rich horizons. This indicates that the subducted material was dominated by trench filling made of sediments and volcanoclastic rocks, with only subordinate pieces of oceanic crust/lithosphere. The whole sequence is cut by km-scale major shear planes orientated WNW-ESE showing consistent top-to-the north shear senses. Lineations marked by glaucophane indicate that most of the deformation occurred during exhumation-related blueschist-facies conditions. Peak pressure and temperatures (P-T) were estimated by Raman spectroscopy, using the degree of organisation of carbonaceous material in metapelites for T and Raman peak shifts of quartz inclusions in garnets for P. In the whole HP-UHP region, consistent and homogeneous peak P-T conditions of 530±30°C and 2.3±3 GPa point to depths around 70 km and HP to UHP conditions, which is further supported by the local presence of coesite. The continuity of the lithological sequence and the lack of significant P/T offsets across the major shear planes indicate that, during exhumation, the HP-UHP unit primarily behaved as a single stack of essentially metasedimentary slices, and was only poorly dismembered on its way to the surface. Our study thus advocates for deep accretion/underplating and stacking of these tectonic slices (dominated by trench infill material) at depths of ~70 km, which has so far rarely been documented.

Precambrian domains in Lithuania: evidence of terrane tectonics

NASA Astrophysics Data System (ADS)

Skridlaite, Grazina; Motuza, Gediminas

2001-09-01

The West Lithuanian Granulite (WLG) and East Lithuanian domains (ELD) form the Proterozoic basement of Lithuania and can be distinguished on the basis of differing structural patterns, lithologies, and evolutionary histories. They are juxtaposed along the Mid-Lithuanian Suture Zone (MLSZ). In the WLG, the main lithotectonic complexes comprise felsic and intermediate, mostly metasedimentary granulites in the south-west and mafic metaigneous granulites in the north-east. The former are interpreted as marine metapelites, while most of the mafic ones have been derived from island-arc tholeiites. These rock complexes trend NW-SE and are marked by contrasting gravity and magnetic anomalies. NE- and E-W-striking faults and shear zones complicate the potential-field patterns. Sets of NW-trending anomalies also extend from Lithuania across the Baltic Sea to south-central Sweden and indicate that the WLG complexes continue into the Baltic/Fennoscandian Shield. Voluminous anatectic granites alternate with the metapelites, whereas the mafic granulites occur together with enderbites and charnockites. In the ELD, the main structures produce strong, NNE-SSW-oriented gravity and magnetic anomalies which trend parallel to the Belarus-Baltic Granulite Belt (BBG) and other terranes situated still farther east. The ELD is composed of metasedimentary rocks interpreted as one-time graywackes, shales and dolomites accumulated in continental-margin arc and shallow-water basinal environments. Amphibolites and gabbros with MORB and IAT characteristics, and voluminous granitoids are also present. The coexistence of juvenile mafic rocks with continental-margin and shelf sediments suggests an oceanic back-arc setting. The two Lithuanian basement domains display contrasting metamorphic histories that suggest separate developments before the eventual amalgamation. In the WLG, the metapelites indicate peak metamorphism at high temperatures (up to 850-900°C) and moderate pressures (8-10 kbar). This was followed by cooling and reheating, and then an uplift event. Repeated magmatic underplating accompanied the metamorphism. In the ELD, in contrast, the rocks have been subjected to comprehensive metamorphism under moderate, amphibolite-facies conditions. That metamorphism, however, was not uniform throughout. The metasediments in the east have recorded pressures similar to those in the neighbouring BBG (7-8 kbar) but lower temperatures (650-680°C), while in the central and western parts of the ELD, metamorphism occurred at ca. 480-580°C with pressures increasing from 3-4 kbar in the centre, to 6 kbar close to the western boundary. Reheating to 700°C due to a ca. 1.5-Ga magmatic event is characteristic. The MLSZ, which separates the two Lithuanian basement domains from each other, is a N-S-oriented, ca. 30-50 km wide, westward-plunging crustal discontinuity marked by magnetic and gravity highs, mafic and felsic intrusions, and sheared rocks. Crustal thicknesses change from 42-44 km in the west to 50 km in the eastern side of the Zone, which also truncates a crustal low-velocity layer characteristic of the WLG. The amalgamation of the WLG and ELD along the MLSZ occurred at ca. 1.71-1.66 Ga, after which time both domains were affected by the same post-kinematic, anorogenic magmatism ca. 1.58-1.45 Ga ago. That event and related shearing were responsible for some ultimate refragmentation of the Lithuanian basement terranes.

The relationship between continental collision process and metamorphic pattern in the Himalayan collision belts

NASA Astrophysics Data System (ADS)

Oh, Chang-Whan

2015-04-01

Both UHP and HP eclogites are reported from the Kaghan Valley and Tso Morari Massif in the western part of the Himalayan collision belt (Ghazanfar and Chaudhry, 1987; Thakur, 1983). UHP eclogites in the Kaghan record peak metamorphic conditions of 770 °C and 30 kbar (O'Brien et al., 2001) and was retrograded into the epidote-amphibolite or blueschist (580-610 °C, 10-13 kbar; Lombardo and Rolfo, 2000). Sensitive high-resolution ion microprobe dating of zircon reveals that the UHP eclogite formed at ca. 46 Ma (Kaneko et al., 2003; Parrish et al., 2006). The Tso Morari UHP eclogite had formed at 750 °C, > 39 kbar (Mukheerjee et al., 2003; Bundy, 1980) and underwent amphibolite facies retro-grade metamorphism (580 °C, 11 kbar) during uplift (Guillot et al., 2008). Peak metamorphism of the Tso Morari Massif was dated at ca. 53-55 Ma (Leech et al., 2005). Only HP eclogites have been reported from the mid-eastern part of the Himalayan collision belt (Lombardo and Rolfo, 2000; Corrie et al., 2010). The HP eclogite in the mid-eastern part may have formed at ca. > 780 °C and 20 kbar and was overprinted by high-pressure granulite facies metamorphism (780-750°C, 12-10 kbar) at ca. 30 Ma (Groppo et al. 2007; Corrie et al., 2010). HP granulite (890 °C, 17-18 kbar) is reported from the NBS, at the eastern terminus of the Himalayan collision belt; the granulite was subjected to retrograde metamorphism to produce lower-pressure granulite (875-850°C, 10-5 kbar), representing near-isothermal decompression (Liu and Zhong, 1997). The HP granulite metamorphism may have occurred at ca. 22-25 Ma. Along the Himalayan collision belt, peak metamorphism changes eastward from UHP eclogite facies through HP eclogite facies to high-pressure granulite facies, indicating a progressive eastwards decrease in the depth of subduction of continental crust and an eastwards increase in the geothermal gradient. The peak metamorphic ages also decrease from 53-46 Ma in the west to 22-25 Ma in the east indicating propagation of collision towards east. The following collision model of the Himalayan collision belt is proposed based on data published in previous studies. Collision between the Indian and Asian blocks started in the west before ca. 55 Ma. In the western part, the amount of oceanic slab subducted prior to continent collision was enough to pull the continental crust down to the depths of UHP metamorphism, as a wide ocean existed between the Asian and Indian blocks prior to collision. Following UHP metamorphism, oceanic slab break-off started at ca. 55~46 Ma in the west due to the very strong buoyancy of the deeply subducted continental block. In contrast, the subduction of continental crust continued at this time in the middle and eastern parts of the belt. The zone of break-off migrated eastward, initiating a change from steep- to low-angle subduction. Final break-off may have occurred in the easternmost part of the belt at ca. 22-25 Ma. The depth of slab break-off decreased toward the east due to the westward decrease of the amount of subducted oceanic crust along the Himalayan collision belt, resulting eastwards decrease of an uplifting rate due to a decrease in buoyancy of the continental slab. The slower uplift resulted in a longer period of thermal relaxation and a higher geothermal gradient. In the west, the high rate of uplift resulted the epidote amphibolite facies (580-610°C) retrograde metamorphic overprint on the UHP eclogites, whereas the relatively slow uplift in the mid-eastern part caused high-grade granulites (850°C) retrograde metamorphic overprint on the HP eclogites. The study indicates that the metamorphic pattern along the collision belt is strongly related to the amount of subducted oceanic crust between continents before collision and the depth of slab break-off. Therefore metamorphic pattern can be used to interpret both the disappeared and ongoing tectonic process during continental collision.

Isotopic and geothermometric constraints on the structural and metamorphic evolution of Homestake gold deposit, Black Hills, South Dakota (USA)

NASA Astrophysics Data System (ADS)

Terry, M.; Dahl, P.; Frei, R.

2003-04-01

The Homestake Deposit, located in the northern Black Hills and host for 40 million ounces of gold, shows evidence for extensive remobilization of gold related to regional metamorphism deformation associated with the Early Proterozoic assembly of supercontinent Laurentia. Field and petrographic evidence for gold remobilization includes the occurrence of abundant quartz veins associated with selvages of chlorite-siderite-ankerite-pyrrhotite-arsenopyrite-gold in the Homestake Fm. The deposit is located on the western limb of a major anticlinorium that coincides with a vertical N-S-striking garnet isograd, and garnet-biotite geothermometry of metapelites sampled across the anticlinorium indicates a steep metamorphic field gradient of 150^oC/km (east side warmer). This gradient is mirrored by a pronounced fractionation of oxygen isotopes observed in the vein quartz, with δ18O ranging from 10 to 18 ppm. The isograd is parallel with a major N-S-striking shear zone, and kinematic indicators predominantly indicate oblique sinistral motion with east-side up. Garnet was separated from a subsurface sample of the Homestake Fm. collected from the nose of the so-called "main ledge" synform and subjected to Pb stepwise leaching (PbSL) to determine the age of garnet growth and thus metamorphism. PbSL analysis revealed a 207Pb/206Pb age of 1746 ± 10 Ma (± 2σ). Recent work in the southern Black Hills indicates that almandine does not contain sufficient Pb to be dated directly by this method; instead, the PbSL result represents the bulk age of abundant allanite inclusions observed in the garnet. Thus, 1746 Ma is interpreted as a maximum age of prograde garnet growth during regional thermotectonism. Mineral assemblages from selvages in Main Ledge indicate that mineralization occurred at or after peak metamorph, which indicates that 1746 Ma also represents a maximum age for gold remobilization. A minimum 1715 Ma age of these events is indicated by published ages of post-tectonic leucogranite in the Black Hills. Regionally, the N-S orientation, 1746--1715 Ma timing, and sinistral-transpressive motion combine to suggest that this major shear zone in the northern Black Hills represents a northerly extension of the Hartville fault, which is exposed in SE Wyoming, ˜200 km SSW. Correlation of these shear zones would have important implications for Proterozoic terrane assembly in this part of Laurentia.

Petrology, geochemistry and zirconology of impure calcite marbles from the Precambrian metamorphic basement at the southeastern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Liu, Yi-Can; Zhang, Pin-Gang; Wang, Cheng-Cheng; Groppo, Chiara; Rolfo, Franco; Yang, Yang; Li, Yuan; Deng, Liang-Peng; Song, Biao

2017-10-01

Impure calcite marbles from the Precambrian metamorphic basement of the Wuhe Complex, southeastern margin of the North China Craton, provide an exceptional opportunity to understand the depositional processes during the Late Archean and the subsequent Palaeoproterozoic metamorphic evolution of one of the oldest cratons in the world. The studied marbles are characterized by the assemblage calcite + clinopyroxene + plagioclase + K-feldspar + quartz + rutile ± biotite ± white mica. Based on petrography and geochemistry, the marbles can be broadly divided into two main types. The first type (type 1) is rich in REE with a negative Eu anomaly, whereas the second type (type 2) is relatively poor in REE with a positive Eu anomaly. Notably, all marbles exhibit remarkably uniform REE patterns with moderate LREE/HREE fractionation, suggesting a close genetic relationship. Cathodoluminescence imaging, trace elements and mineral inclusions reveal that most zircons from two dated samples display distinct core-rim structures. Zircon cores show typical igneous features with oscillatory growth zoning and high Th/U ratios (mostly in the range 0.3-0.7) and give ages of 2.53 - 2.48 Ga, thus dating the maximum age of deposition of the protolith. Zircon rims overgrew during granulite-facies metamorphism, as evidenced by calcite + clinopyroxene + rutile + plagioclase + quartz inclusions, by Ti-in-zircon temperatures in the range 660-743 °C and by the low Th/U (mostly < 0.1) and Lu/Hf (< 0.001) ratios. Zircon rims from two dated samples yield ages of 1839 ± 7 Ma and 1848 ± 23 Ma, respectively, suggesting a Palaeoproterozoic age for the granulite-facies metamorphic event. These ages are consistent with those found in other Precambrian basement rocks and lower-crustal xenoliths in the region, and are critical for the understanding of the tectonic history of the Wuhe Complex. Positive Eu anomalies and high Sr and Ba contents in type 2 marbles are ascribed to syn-depositional felsic hydrothermal activity which occurred at 2.53 - 2.48 Ga. Our results, together with other published data and the inferred tectonic setting, suggest that the marbles' protolith is an impure limestone, rich in detrital silicates of igneous origin, deposited in a back-arc basin within an active continental margin during the late Archean and affected by synchronous high-T hydrothermalism at the southeastern margin of the North China Craton.

Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand

USGS Publications Warehouse

Stowell, H.; Tulloch, A.; Zuluaga, C.; Koenig, A.

2010-01-01

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm-Nd and zircon U-Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm-Nd ages of 126.1??2.0 and 122.6??2.0. Ma date crystal growth. Integration of the garnet ages with U-Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150. Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134. Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126. Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123. Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95. Ma. Phase diagram sections constrain orthopyroxene assemblages to <0.6 GPa @ 650??C, peak garnet granulite facies metamorphic conditions to 680-815??C @ 1.1-1.4. GPa, and a P-T path with a P increase of???0.5. GPa. These sections are compatible with water contents???0.28wt.%, local dehydration during garnet granulite metamorphism, and <0.3. GPa P increases during garnet growth. Results demonstrate the utility of integrated U-Pb zircon and Sm-Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126. Ma, that loading occurred at a rate of ca. 0.06. GPa/m.y., and that garnet granulite metamorphism lasted 3-7m.y. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3m.y. ?? 2010 Elsevier B.V.

Color Me Metamorphic.

ERIC Educational Resources Information Center

Birdd, Donald L.

1990-01-01

Described are five activities using crayons to demonstrate the rock cycle including weathering, erosion and sedimentation, and sedimentary, metamorphic, and igneous rock formation. Discussed are materials, procedures, and probable results. (CW)

Metamorphism, P-T-t Conditions of Formation, and Prospects for the Practical Use of Al2SiO5 Polymorphs, Chloritoid, and Staurolite (Yenisei Ridge)

NASA Astrophysics Data System (ADS)

Kozlov, P. S.

2017-12-01

The Yenisei Ridge is an accretion-collisional orogen located in the southwestern frame of the Siberian Craton in the interfluve between Podkamennaya Tunguska, Angara, Kan, and Yenisei rivers. The Precambrian mono- and polymetamorphic complexes composed predominantly of the Mesoarchean-Neoproterozoic metapelitic rocks have been studied. Based on the typification of metamorphic complexes by pressure, temperature, metamorphic gradient, as well as age of metamorphism, the location scheme of the fields of the Precambrian sedimentary-metamorphic rock which are prospective for searching deposits of high-alumina metamorphic minerals (andalusite, kyanite, and sillimanite, chloritoid, and staurolite) in the Trans-Angara segment of the Yenisei Region, was compiled. The Teya sillimanite and Panimbinsk andalusite deposits, which are confined to the fields of regional metamorphic complexes of iron-alumina metapelites of the And-Sill facies series, are recommended as a priority for the organization of prospecting works and the subsequent involvement to the metallurgical industry. These metapelites are classified as monomineral. Owing to widespread occurrence and abundance of andalusite and sillimanite, the above deposits have significant inferred resources. Stratiform deposits of garnet-staurolite and chloritoid high-alumina rocks are still insufficiently studied and should be investigated further. The prospects for the possible use of high-alumina andalusite and sillimanite together with Middle Tatarka and Kiya nepheline syenite massifs and the bauxites of the Chadobets uplift, already being explored in the region, for production of aluminum oxide, silumin, and aluminum, as well as, the prospects for the expansion of the raw material base of the Boguchansk Electrometallurgical Complex, brought into operation in 2016 in the Lower Angara region, are considered.

Evidence for pre-Taconic metamorphism in the Potomac terrane, Maryland and Virginia: Hornblende and Muscovite [sup 40]Ar/[sup 39]Ar results

DOE Office of Scientific and Technical Information (OSTI.GOV)

Becker, J.L.; Wintsch, R.P.; Kunk, M.J.

1993-03-01

New [sup 40]Ar/[sup 39]Ar age spectra of hornblende and white mica from the Great Falls area of the Potomac terrane of Maryland and Virginia indicate pre-Taconic metamorphism. Age spectra of hornblende samples are interpreted to represent cooling from peak metamorphic conditions through their closure temperatures for argon diffusion ([approximately]500C) at about 490 Ma. These older Ordovician postmetamorphic cooling ages strongly contrast with younger post-Ordovician metamorphic cooling ages now being reported in the Blue Ridge and Goochland terranes to the west and east respectively. A late phyllitic sheen observed on rocks in the field and petrographic observations of undulose plagioclase andmore » amphibole, and older muscovite, and kinked primary muscovite in the Bear Island Granodiorite reflect a younger retrogressive metamorphism involving the growth of secondary muscovite (Fisher's S4 ). [sup 40]Ar/[sup 39]Ar Age spectra of white micas from the Bear Island Granodiorite are complex and probably indicate both primary and secondary white mica, the latter apparently growing below the closure temperature for retention of argon in muscovite ([approximately]350C). The age spectra permit an estimate of a minimum age of 420 Ma for cooling through closure of the older generation of white mica. The above ages of hornblende and muscovite closure imply a minimum cooling rate of [approximately]2C/m.y., and exhumation rate of about 1 mm/yr. The projected time of peak metamorphism at upper amphibolite facies for the Great Falls area clearly predates the Ordovician Taconic orogeny and suggests that these rocks escaped this event and largely escaped younger Paleozoic metamorphic events, which are well documented in adjacent terranes.« less

Mapping age and trace elements using laser ablation split-stream (LASS) ICPMS

NASA Astrophysics Data System (ADS)

Kylander-Clark, A. R.; Hacker, B. R.; Cottle, J. M.

2012-12-01

One of the biggest challenges in the determination of the timing and rates of metamorphic processes is tying the age of a particular mineral to the conditions (i.e., pressure, temperature, fluid composition) at which that phase grew. Conventional microbeam techniques increase our understanding of crustal evolution by enabling this linkage; e.g., x-ray maps of monazite allow us to pinpoint grain segments that grew under a different set of conditions, cathodoluminescence images of zircon reveal zoning patterns and hence targets for dating different metamorphic and/or igneous events, and rare-earth element (REE) transects across garnet reveal the budget of a variety of trace elements during a metamorphic episode, to name but a few. More recent advances in LA-ICPMS and SIMS have allowed the ability to produce age maps or trace element maps—thus further our understanding of crystallization processes—but not both. Here we employ laser ablation split-stream (LASS) to quantitatively image the age, and trace element signature of datable phases such as zircon, monazite, titanite, and rutile in metamorphic rocks on the micron scale. By mapping the age and TE signature of a metamorphic phase, we can better interpret the metamorphic stage at which all portions of that phase grew, and relate it to other phases/portions of phases within that rock, such as garnet. For example, zircons and monazites from from eclogites reveal complex zoning in REEs indicating growth prior to, during, and post eclogite-facies metamorphism; those zones correspond to distinct age domains. Metamorphosed titanite reveals differences in diffusivities of TEs in inherited portions of the grain; e.g., Pb-loss is more prominent than diffusion of REEs, which in turn diffuse faster than higher charged ions, such as Th.

Textural and isotopic development of marble assemblages during the Barrovian-style M2 metamorphic event, Naxos, Greece

NASA Astrophysics Data System (ADS)

Baker, Judy; Matthews, Alan

1994-03-01

A detailed petrological analysis of the marble assemblages observed within the M2 metamorphic complex on Naxos is presented. Two distinct periods of mineral growth are documented; the first is associated with prograde M2 metamorphism and the second with retrograde M2 metamorphism occurring during ductile extensional thinning of the complex. The textural and miner-alogical characteristics and the carbon and oxygen isotope compositions of each generation are described, and the P-T-X CO 2 conditions at which these two mineral generations were stable, and the compositions of the fluids present during metamorphism are characterised. Whereas the low variance and stable isotope compositions of prograde siliceous dolomite assemblages are consistent with internally buffered fluid evolution, the retrograde mineral generation is shown to have grown as a result of the infiltration of a water-rich fluid phase that transported silica, Al2O3, Na2O and FeO into the host rocks. This observation, together with the stable isotope compositions of the retrograde calcite, and the fact that occurrences of veins of this type are limited to marbles in the highest grade areas ( T>600° C) of the metamorphic complex, suggests that the fluids responsible for vein formation were generated during the crystallisation of melts as the metamorphic complex cooled from peak temperatures. The existence of this second generation of minerals has significant implications for previous studies of heat transport by fluid flow on Naxos, because many of the unusually low δ18O compositions of pelites at high grades may be ascribable to the effects of interaction with retrograde M2 fluids, rather than with prograde fluids.

Long distance transport of eclogite and blueschist during early Pacific Ocean subduction rollback

NASA Astrophysics Data System (ADS)

Tamblyn, Renee; Hand, Martin; Kelsey, David; Phillips, Glen; Anczkiewicz, Robert

2017-04-01

The Tasmanides in eastern Australia represent a period of continental crustal growth on the western margin of the Pacific Ocean associated with slab rollback from the Cambrian until the Triassic. During rollback numerical models predict that subduction products can become trapped in the forearc (Geyra et al., 2002), and can migrate with the trench as it retreats. In a long-lived subduction controlled regime such as the Tasmanides, this should result in an accumulation of subduction products with protracted geochronological and metamorphic histories. U-Pb, Lu-Hf, Sm-Nd and Ar-Ar geochronology and phase equilibria modelling of lawsonite-eclogite and garnet blueschist in the Southern New England Fold Belt in Australia demonstrate that high-P low-T rocks remained within a subduction setting for c. 40 Ma, from c. 500 to 460 Ma. High-P metamorphic rocks initially formed close to the Australian cratonic margin during the late Cambrian, and were subsequently transported over 1500 Ma oceanward, during which time subducted material continued to accumulate, resulting in the development of complex mélange which records eclogite and blueschist metamorphism and partial exhumation over 40 Ma. The duration of refrigerated metamorphism approximates the extensional evolution of the upper plate which culminated in the development of the Lachlan Fold Belt. The protracted record of eclogite and blueschist metamorphism indicates that rapid exhumation is not necessarily required for preservation of high-pressure metamorphic rocks from subduction systems. Reference: Gerya, T. V., Stockhert, B., & Perchuk, A. L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation. Tectonics, 21(6), 6-1-6-19. doi:10.1029/2002tc001406

The Age and Geodynamic Evolution of the Metamorphic sole rocks from Izmir-Ankara-Erzıncan suture zone (Northern-Turkey)

NASA Astrophysics Data System (ADS)

Melih Çörtük, Rahmi; Faruk Çelik, Ömer; Özkan, Mutlu; Sherlock, Sarah C.; Marzoli, Andrea; Altıntaş, İsmail Emir; Topuz, Gültekin

2016-04-01

The İzmir-Ankara-Erzincan suture zone in northern Turkey is one of the major tectonic zones separating the Pontides to the North from the Anatolide-Tauride block and Kı rşehir Massif to the South. The accretionary complex of the İzmir-Ankara-Erzincan suture zone, near Artova, is composed mainly of peridotites with varying degree serpentinization, metamorphic rocks, basalt, sandstones, pelagic and neritic limestones. The metamorphic rocks are represented by amphibolite, garnet micaschit, calc-schist and marble. The metamorphic rocks were interpreted as the metamorphic sole rocks. Because; (i) They are tectonically located beneath the serpentinized peridotites. (ii) Foliation planes of both the amphibolites and mantle tectonites are parallel to each other. (iii) The metamorphic rocks are crosscut by non-metamorphic dolerite dikes which exhibite Nb and Ta depletion relative to Th enrichment on the N-MORB normalized multi-element spider diagram. The dolerite dikes display flat REE patterns (LaN/YbN=0.85-1.24). These geochemical signatures of the dolerite dikes are indicative of subduction component during their occurrences. Geochemical observations of the amphibolites suggest E-MORB- and OIB-like signatures (LaN/SmN= 1.39-3.14) and their protoliths are represented by basalt and alkali basaltic rocks. Amphiboles from the amphibolites are represented by calcic amphiboles (magnesio-hornblende, tchermakite and tremolite) and they yielded 40Ar-39Ar ages between 157.8 ± 3.6 Ma and 139 ± 11 Ma. These cooling ages were interpreted to be the intra-oceanic subduction/thrusting time of the İzmir-Ankara-Erzincan oceanic domain. This study was funded by TÜBİTAK (Project no: 112Y123).

Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

USGS Publications Warehouse

Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

2006-01-01

Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ???25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Lu??liang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

Can the Metamorphic Basement of Northwestern Guatemala be Correlated with the Chuacús Complex?

NASA Astrophysics Data System (ADS)

Cacao, N.; Martens, U.

2007-05-01

The Chuacús complex constitutes a northward concave metamorphic belt that stretches ca. 150 km south of the Cuilco-Chixoy-Polochic (CCP) fault system in central and central-eastern Guatemala. It represents the basement of the southern edge of the Maya block, being well exposed in the sierra de Chuacús and the sierra de Las Minas. It is composed of high-Al metapelites, amphibolites, quartzofeldspathic gneisses, and migmatites. In central Guatemala the Chuacús complex contains ubiquitous epidote-amphibolite mineral associations, and local relics of eclogite reveal a previous high-pressure metamorphic event. North of the CCP, in the Sierra de Los Cuchumatanes area of western Guatemala, metamorphic rocks have been considered the equivalent of the Chuacús complex and hence been given the name Western Chuacús group, These rocks, which were intruded by granitic rocks and later mylonitized, include chloritic schist and gneiss, biotite-garnet schist, migmatites, and amphibolites. No eclogitic relics have been found within metamorphic rocks in northwestern Guatemala. Petrographic analyses of garnet-biotite schist reveal abundant retrogression and the formation of abundant zeolite-bearing veins associated with intrusion. Although metamorphic conditions in the greenschist and amphibolite facies are similar to those in the sierra de Chuacús, the association with deformed intrusive granites is unique for western Guatemala. Hence a correlation with metasediments intruded by the Rabinal granite in the San Gabriel area of Baja Verapaz seems more feasible than a correlation with the Chuacús complex. This idea is supported by reintegration of the Cenozoic left-lateral displacement along the CCP, which would place the metamorphic basement of western Guatemala north of Baja Verapaz, adjacent to metasediments intruded by granites in the San Gabriel-Rabinal area.

Sr-isotopic composition of marbles from the Puerto Galera area (Mindoro, Philippines): additional evidence for a Paleozoic age of a metamorphic complex in the Philippine island arc

DOE Office of Scientific and Technical Information (OSTI.GOV)

Knittel, U.; Daniels, U.

1987-02-01

The Sr-isotopic composition of marbles from the Puerto Galera area (Mindoro, Philippines) is compatible with either a Tertiary or a Paleozoic age. The former is considered as unlikely because nonmetamorphic sediments of that age overlie the metamorphic complex. This implies that the metamorphic complex does not represent the basement of the Philippine arc but is an accreted terrane.

Metamorphic waters from the pacific tectonic belt of the west coast of the United States

USGS Publications Warehouse

Barnes, I.

1970-01-01

Waters unusually rich in ammonia, boron, carbon dioxide, hydrogen sulfide, and hydrocarbons are found in more than 100 localities along the Pacific coast of the United States. The waters are believed to be products of low-grade metamorphism of marine sediments. The marine sedimentary rocks would have to be tectonically emplaced below crystalline rocks in many places. Mercury are deposits are probably also products of the low-grade metamorphism.

Evidence for multiple metamorphic events in the Adirondack Mountains, N. Y

DOE Office of Scientific and Technical Information (OSTI.GOV)

McLelland, J.; Lochhead, A.; Vyhnal, C.

1988-05-01

Field evidence consisting of: (1) rotated, foliated xenoliths, (2) country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one, or more, early dynamothermal event(s) of regional scale and high grade. Early metamorphism resulted in pronounced linear and planar fabric throughout the Adirondacks and preceded the emplacement of the anorthosite-mangerite-charnockite-granite-alaskite (AMCA) suite which contains xenoliths of the metamorphosed rocks. Olivine metagabbros, believed to be approximately contemporaneous with the AMCA-suite, also crosscut and contain xenoliths of, strongly foliated metasediments. These intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages. Subsequently,more » this already complex framework underwent three phases of folding, including an early recumbent isoclinical event, and was metamorphosed to granulite facies P,T conditions. The age of the early metamorphism cannot yet be narrowly constrained, but isotopic results suggest that it may be as young as approx. 1200 Ma or older than approx. 1420 Ma. U-Pb zircon ages indicate emplacement of the AMCA-(metagabbro)-suite in the interval 1160-1130 Ma and place the peak of granulite facies metamorphism between 1070-1025 Ma. The anorogenic character of the AMCA-suite, and the occurrence of metadiabase dike swarms within it, are further evidence of the separate nature of the metamorphic events that precede and postdate AMCA emplacement.« less

Oxygen isotope thermometry of quartz-Al2SiO5veins in high-grade metamorphic rocks on Naxos island (Greece)

NASA Astrophysics Data System (ADS)

Putlitz, Benita; Valley, John; Matthews, Alan; Katzir, Yaron

2002-04-01

Diffusion models predict that peak metamorphic temperatures are best recorded by the oxygen isotope fractionation between minerals in a bi-mineralic rock in which a refractory accessory mineral with slow oxygen diffusion rate is modally minor to a mineral with a faster diffusion rate. This premise is demonstrated for high-grade metamorphism on the island of Naxos, Greece, where quartz-kyanite oxygen isotope thermometry from veins in high-grade metamorphic pelites gives temperatures of 635-690 °C. These temperatures are in excellent agreement with independent thermometry for the regional M2 peak metamorphic conditions and show that the vein minerals isotopically equilibrated at the peak of metamorphism. Quartz-sillimanite fractionations in the same veins give similar temperatures (680+/-35 °C) and suggest that the veins grew near to the kyanite-sillimanite boundary, corresponding to pressures of 6.5 to 7.5 kbar for temperatures of 635-685 °C. By contrast, quartz-kyanite and quartz-biotite pairs in the host rocks yield lower temperature estimates than the veins (590-600 and 350-550 °C, respectively). These lower apparent temperatures are also predicted from calculations of diffusional resetting in the polyphase host-rock system. The data demonstrate that bimineralic vein assemblages can be used as accurate thermometers in high-temperature rocks whereas retrograde exchange remains a major problem in many polymineralic rocks.

Effect of food on metamorphic competence in the model system Crepidula fornicata.

PubMed

Padilla, Dianna K; McCann, Michael J; Glenn, Mica McCarty; Hooks, Alexandra P; Shumway, Sandra E

2014-12-01

Food quality and quantity, as well as temperature, are all factors that are expected to affect rates of development, and are likely to be affected by expected climatic change. We tested the effect of a mixed diet versus a single-food diet on metamorphic competence in the emerging model species Crepidula fornicata. We then compared our results with other published studies on this species that examined time to metamorphic competence across a range of food concentrations and rearing temperatures. Ours was the only study to test the effects of single food versus a mixed diet on metamorphic competence for this species. Diet composition did not affect metamorphic competence or survivorship. Comparing results across studies, we found that the shortest time to metamorphic competence was typically found when the food availability per larva was the greatest, independent of rearing temperature. Unfortunately, some published studies did not include important metadata needed for comparison with other studies; these data included larval rearing density, food density, frequency of feeding, and rearing temperature. Mortality rates were not always reported and when reported were often measured in different ways, preventing comparison. Such metadata are essential for comparisons among studies as well as among taxa, and for the determination of generalizable patterns and evolutionary trends. Increased reporting of all such metadata is essential if we are to use scientific studies performed to their fullest potential. © 2014 Marine Biological Laboratory.

Metagabbro associated with the shear zone on Prins Karls Forland (Svalbard, Arctic)

NASA Astrophysics Data System (ADS)

Maraszewska, Maria; Manecki, Maciej; Czerny, Jerzy; Schneider, David; Myhre, Per Inge; Faehnrich, Karol; Barnes, Christopher

2016-04-01

Prins Karls Forland (PKF) is a N-S elongated island situated west of Spitsbergen in the Svalbard archipelago, High Arctic. The northern part of the island is dominated by siliciclastic metasediments regionally metamorphosed to greenshist facies assemblages during one distinct stage of tectonism. Amphibolite facies garnet-mica schists, mica schists, quartzites and carbonate-silicate rocks exhibiting evidence of at least two distinct, strong deformation episodes (including mylonitization) locally outcrop on the east coast of PKF, termed the Pinkie Unit. A ~1 km wide shear zone containing ductile to brittle structures and distinct outcrops of greenstones (metagabbros and greenschists), associated with magnetite ore, separates these two contrasting tectonic units. Ten samples of greenstones were collected on the slopes of Lauratzonfjellet and Boureefjellet for petrologic and geochemical analyses. Despite intense localized shearing, the metagabbros are undeformed and preserve coarse crystalline, magmatic texture, which is locally poikilitic. The primary magmatic assemblage consists of brown hornblende, plagioclase, biotite and opaque minerals, with accessory apatite and titanite. No relicts of pyroxenes are preserved. Formation of secondary uralite, sericite and chlorite is observed. Metamorphic assemblage consists of actinolite pseudomorhs after hornblende, epidote, and second generation biotite. Blue amphibole is observed in one sample from Boureefjellet; greenschists from Boureefjellet also contain fibrous blue amphibole, as well as garnets, actinolite, epidote and biotite. Some rocks sampled on Boureefjellet are more strongly deformed and exhibit probably two stages of metamorphism: amphibolite facies metamorphism resulting in blue amphibole-garnet assemblage followed by greenschist facies metamorphism resulting in actinolite-epidote-biotite paragenesis. Parallel and overlapping patterns on chondrite-normalized REE diagrams and spider diagrams indicate that these metagabbros are comagmatic. Enrichment in incompatible lighter elements and position of projections on discrimination diagrams suggest ocean island basalt (OIB) character of primary magmas. The age of these rocks is unknown and is an objective of ongoing investigation. This work is partially funded by AGH research grant no 11.11.140.319.

Variscan tectonics in Dodecanese, Kalymnos island, Greece

NASA Astrophysics Data System (ADS)

Chatziioannou, Eleftheria; Grasemann, Bernhard; Schneider, David; Hubmann, Bernhard; Soukis, Konstantinos

2015-04-01

Kalymnos island is located in the Dodecanese, southeastern Aegean Sea, and geologically appears to be part of the external Hellenides. Pre-Alpidic basement rocks on the Dodecanese islands have been suggested to record compelling similarities with the basement rocks in Eastern Crete with respect to their lithologies and pre-Alpidic metamorphic evolution. The lithotectonic units experienced greenschist to amphibolite facies conditions during the Variscan orogeny. Whereas the rocks in Eastern Crete reveal Alpine high-pressure overprint, the Variscan basement units in the Dodecanese record no or low-grade Alpine metamorphism. A field study of basement rocks below Mesozoic limestones and dolomites in the NW part of Kalymnos near Emporios uncovered a complex history of metamorphism, folding and faulting. Three different tectonic units can be discriminated from top to bottom: a) a quartz-mica schist, b) a white-grey, fossiliferous coarse grained marble and c) a fine-grained fossiliferous blue-grey marble. In the marbles macrofossils such as brachiopods, ammonoid cephalopods (Goniatids?) and crinoids suggest a Middle-Upper Devonian deposition age (Givetian- Frasnian). Structural mapping the area resolved a dominant W-E shortening event, resulting in an overall inverted metamorphic gradient. The lowermost blue-grey marble unit is folded into large-scale upright folds, which are truncated by top-to-east overthrusting of the white-grey marble unit. Whereas deformation mechanisms in the blue-grey marble unit are dominated by dissolution-precipitation creep, the white-grey marble suffered intense crystal plastic deformation with localized high-strain mylonitic shear zones. The uppermost quartz-mica schist unit is separated from the lower units by a cataclastic phyllonitic shear zone. 40Ar/39Ar geochronological dating on white micas from the quartz-mica schists yielded cooling ages between 240 and 334 Ma indicative of Variscan cooling. Our data suggest that this part of the Dodecanese experienced intense Variscan deformation with no high-temperature Alpine overprint.

Mineral potential tracts for orogenic, Carlin-like, and epithermal gold deposits in the Islamic Republic of Mauritania, (phase V, deliverable 69): Chapter H in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II)

USGS Publications Warehouse

Goldfarb, Richard J.; Marsh, Erin; Anderson, Eric D.; Horton, John D.; Finn, Carol A.; Beaudoin, Georges

2015-01-01

A preliminary evaluation of these gold data can be used to develop broad, firstorder tracts defining favorable and permissive areas for gold resources; detailed metamorphic and structural maps are required for more detailed future tract definition. Such a first-order assessment can, nonetheless, broadly identify four tracts of gold resource potential. Three of these are favorable for discovery of new orogenic gold deposits. One tract, although not favorable, is nevertheless permissive for discovery of epithermal gold deposits. Tract 1 is defined by favorable medium metamorphic grade greenstone belts within vast areas of unfavorable high metamorphic grade, Mesoarchean and Paleoproterozoic granite-gneiss basement of the Rgueïbat Shield. Faults >200 km in length following the general strike of the greenstone belts; lineament intersections with both exposed and buried parts of greenstone belts within 500 m of the surface, as defined by aeromagnetic data (Finn and Anderson, 2015); and areas of banded iron formation (BIF) in the belts are particularly favorable areas for hosting gold resources in orogenic gold deposits within and along the margins of the greenstone belts. Tracts 2 and 3, also for orogenic gold, reflect the favorable Proterozoic-Cambrian metamorphic rocks of the Northern and Southern Mauritanides, with >200-km-long faults following the general strike of the range, and areas underlain by ultramafic and BIF rocks being particularly favorable. Outcrops of Triassic-Jurassic igneous rocks along the margins of the Taoudeni Basin define tract 4, which is permissive for epithermal gold deposits. Although extensive data are lacking for the area, carbonate units along the northern side of the Taoudeni Basin could be considered permissive host rocks for Carlin-type mineralization, but the deep-water carbonate lithologies are typically not favorable for such. 

Early Tertiary Anaconda metamorphic core complex, southwestern Montana

USGS Publications Warehouse

O'Neill, J. M.; Lonn, J.D.; Lageson, D.R.; Kunk, Michael J.

2004-01-01

A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting. ?? 2004 NRC Canada.

Eclogites and garnet clinopyroxenites in the Anrakhai complex, Central Asian Orogenic Belt, Southern Kazakhstan: P-T evolution, protoliths and some geodynamic implications

NASA Astrophysics Data System (ADS)

Pilitsyna, Anfisa V.; Tretyakov, Andrey A.; Degtyarev, Kirill E.; Cuthbert, Simon J.; Batanova, Valentina G.; Kovalchuk, Elena V.

2018-03-01

The Anrakhai Metamorphic Complex (AMC), located in the SE part of the Chu-Ili Mountains of Southern Kazakhstan in the western part of Central Asian Orogenic Belt, exhibits occurrences of HP metamorphic rocks in the form of eclogites and garnet clinopyroxenites with peak metamorphic conditions of 750-850° and 15-19 kbar estimated with both conventional geothermobarometric methods and phase diagram modeling. P-T estimates as well as intimate field relations evidently imply a common metamorphic history for eclogites and garnet clinopyroxenites of the AMC. These high-pressure, medium temperature eclogite facies P-T conditions are indicative of a collision or subduction tectonic setting. Major and trace element geochemistry suggests that they probably had a common magmatic origin as part of a suite of differentiated tholeiitic intrusions. Furthermore, distinctive mineral and chemical compositions of these eclogites and garnet clinopyroxenites correspond to the Fe-Ti type of ultramafic rocks suggesting that they may have been derivatives of intraplate tholeiitic melts, introduced into continental crust before HP metamorphism.

Mineral Reactions Involving Sapphirine and Their Application for Characterization of Metamorphic Conditions

NASA Astrophysics Data System (ADS)

Podlesskii, K. K.

2008-05-01

Assemblages of sapphirine, once considered to be a rare mineral, have in recent years been recognized as important indicators of high-temperature metamorphism. They occur in rocks that have undergone different tectono-metamorphic histories, with the P-T range of formation being estimated from below 700°C and 0.5 GPa to above 1100°C and 1.5 GPa. Sapphirine associated with quartz is attributed exclusively to the highest temperature conditions of crustal metamorphism referred to as ultrahigh-temperatiure metamorphism. Although experimental data involving sapphirine extend over an even wider P-T range, the quantitative interpretation of sapphirine-bearing assemblages remains ambiguous. Thermodynamic properties of magnesian sapphirine end-members have been optimized on the basis of experimentally constrained phase relations with the equation of state developed by Gerya et al., 2004. They differ from the model used in THERMOCALC (Kelsey et al., 2004), and the differences in the estimated stability of assemblages of sapphirine with quartz, kyanite and forsterite, to which a special attention have been paid due to their petrologic importance, look dramatic and may change interpretations of petrogenetic processes.

Uraniferous bitumen nodules in the Talvivaara Ni-Zn-Cu-Co deposit (Finland): influence of metamorphism on uranium mineralization in black shales

NASA Astrophysics Data System (ADS)

Lecomte, Andreï; Cathelineau, Michel; Deloule, Etienne; Brouand, Marc; Peiffert, Chantal; Loukola-Ruskeeniemi, Kirsti; Pohjolainen, Esa; Lahtinen, Hannu

2014-04-01

In the central part of the Fennoscandian Shield, the Talvivaara Ni-Zn-Cu-Co deposit, hosted by Palaeoproterozoic metamorphosed black schists, contains low uranium concentrations ranging from 10 to 30 ppm. The Talvivaara black schists were deposited 2.0-1.9 Ga ago and underwent subsequent metamorphism during the 1.9-1.79 Ga Svecofennian orogeny. Anhedral uraninite crystals rimmed by bitumen constitute the main host of uranium. U-Pb secondary ion mass spectrometry dating indicates that uraninite crystals were formed between 1,878 ± 17 and 1,871 ± 43 Ma, during peak metamorphism. Rare earth element patterns and high Th content (average 6.38 wt%) in disseminated uraninite crystals indicate that U was concentrated during high temperature metamorphism (>400 °C). The formation of bitumen rims around uraninite may be explained by two distinct scenarios: (a) a transport of U coincident with the migration of hydrocarbons or (b) post-metamorphic formation of bitumen rims, through radiolytic polymerization of gaseous hydrocarbons at the contact with uraninite.

Origin of the Lyme Dome and implications for the timing of multiple Alleghanian deformational and intrusive events in southern Connecticut

USGS Publications Warehouse

Walsh, G.J.; Aleinikoff, J.N.; Wintsch, R.P.

2007-01-01

Geologic mapping, structural analysis, and geochronology in the area of the Lyme dome, southern Connecticut provides constraints on the origin of the rocks in the core of the dome, the absolute timing of the principal deformational and thermal events attributed to Alleghanian orogenesis, and the processes that generated the dome. Detrital zircon geochronology in combination with ages on intrusive rocks brackets the deposition of quartzite in the core of the dome sometime between ca. 925 and 620 Ma. Granite and granodiorite intruded the Neoproteorozic metasedimentary rocks in the core of the dome at ca. 620 to 610 Ma. Four major early Permian events associated with the Alleghanian orogeny affected the rocks in the Lyme dome area. Syn-tectonic migmatization and widespread penetrative deformation (D1, ca. 300 - 290 Ma) included emplacement of alaskite at 290 ?? 4 Ma during regional foliation development and aluminosilicate-orthoclase metamorphic conditions. Rocks of the Avalon terrane may have wedged between Gander cover rocks and Gander basement in the core of the Lyme during D1. Limited structural evidence for diapiric uplift of the Lyme dome indicates that diapirism started late in D1 and was completed by D2 (ca. 290 - 280 Ma) when horizontal WNW contractional stresses dominated over vertical stresses. Second sillimanite metamorphism continued and syn-tectonic D2 granite pegmatite (288 ?? 4 Ma) and the Joshua Rock Granite Gniess (284 ?? 3 Ma) intruded at this time. North-northwest extension during D3 (ca. 280 - 275 Ma) led to granitic pegmatite intrusion along S3 cleavage planes and in extensional zones in boudin necks during hydraulic failure and decompression melting. Intrusion of a Westerly Granite dike at 275 ?? 4 Ma suggests that D3 extension was active, and perhaps concluding, by ca. 275 Ma. Late randomly oriented but gently dipping pegmatite dikes record a final stage of intrusion during D4 (ca. 275 - 260 Ma), and a switch from NNW extension to vertical unloading and exhumation. Monazite and metamorphic zircon rim ages record this event at ca. 259 Ma. The evolution of the Lyme dome involved D1 mylonitization, intrusion, and migmatization during north-directed contraction, limited late D1 diapirism, D2 migmatization during WNW contraction with associated flexural flow and fold interference, D3 NNW horizontal extension and decompression melting, and final D4 vertical extension and rapid exhumation. Late regional uplift, extension, and normal faulting at higher crustal levels may have been caused by diapiric rise of the lower crust, below the structural level of the Lyme dome. The rocks record no evidence of Acadian metamorphism or deformation, suggesting that the Gander zone here was not tectonically juxtaposed with Avalon until the Alleghanian orogeny.

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seravalli, L.; Trevisi, G.; Frigeri, P.

We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 10{sup 8} cm{sup -2} dot density and metamorphic In{sub x}Ga{sub 1-x}As (x=0.15,0.30) confining layers result in emission wavelengths at 1.3 {mu}m. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows.

Tectonometamorphic record in a fossilized subduction channel: insights from the Cycladic Blueschist Unit (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Laurent, Valentin; Roche, Vincent; Jolivet, Laurent; Lanari, Pierre; Augier, Romain; Scaillet, Stéphane

2016-04-01

The comprehension of subduction dynamics is partly based on the reconstruction of detailed Pressure-Temperature-time-deformation paths of HP-LT metamorphic rocks, which have undergone a complete burial-exhumation cycle. The Cycladic Blueschist Unit (CBU), located in the Aegean domain (Greece), is one of the best examples of a fossilized subduction channel. The tectonometamorphic history of this domain can be summarized in two successive episodes: (1) From the Paleocene to the Eocene, the formation of the Hellenides-Taurides belt due to the convergence between Eurasia and Africa. During this episode, the entrance of the Apulian crust in the subduction zone led to an episode of crustal thickening and formation-exhumation of HP-LT metamorphic units like the CBU. (2) From the Early Oligocene, consecutively to the retreat of the African slab, back-arc extension affected the previously thickened crust and the Aegean Sea started to form. Syros and Sifnos islands are worldwide known for their excellent preservation of HP-LT parageneses in the CBU, providing one of the best case-studies to understand the tectonometamorphic evolution of a subduction channel. This study aims to decipher the P-T-t-d path of the CBU using for the first time on Syros, Raman spectroscopy of carbonaceous material to constrain metamorphic peak temperature (Beyssac et al., 2002) and a quantitative X-ray micro-mapping approach together with the program XMapTools (Lanari et al., 2014). The micro-mapping tools allowed extracting local chemical compositions observed in zoned garnets to calculate the local effective bulk composition. Forward models are then created to constrain P-T conditions of crystallization of these local assemblages. This study brings new data on the debated metamorphic peak conditions of the CBU, which undoubtedly attained at least 20 ± 2 kbar / 530 ± 50°C. Additionally, the geological and metamorphic maps of Syros and Sifnos have been totally redrawn in order to decipher the structure of a fossilized subduction channel. Based on structural and petrological observations, the CBU has been subdivided into subunits separated by major ductile shear zones. The Vari Detachment, interpreted as the Eocene subduction channel roof, separates these HP subunits from the overlying Vari Unit that has not seen HP-LT conditions. We show that after the prograde top-to-the S/SW shearing deformation, the CBU was exhumed by an overall top-to-the E/NE shearing from the depth of eclogites all the way to the depth of the greenschist-facies. Finally, considering geochronologic data from the literature, we propose a possible P-T-t-d evolution scenario of the CBU in the context of the Hellenic subduction by reconstructing step-by-step north-south cross-sections of the Aegean domain from Late Paleocene (~55 Ma) to the present-day geometry. This tectonometamorphic evolution shows how strain localizes during the history of an accretionary complex, both during the prograde and retrograde paths.

Geological Structure of the Basement of Western and Eastern Parts of the West-Siberian Plain

ERIC Educational Resources Information Center

Ivanov, Kirill S.; Erokhin, Yuriy V.; Ponomarev, Vladimir S.; Pogromskaya, Olga E.; Berzin, Stepan V.

2016-01-01

The U-Pb dating (SHRIMP-II on zircon) was obtained for the first time from the basement of the West Siberian Plain in the Western half of the region. It is established that a large part of the protolith of the metamorphic depth in the Shaim-Kuznetsov meganticlinorium contained sedimentary late- and middle-Devonian rocks (395-358 million years).…

Neogene Basin Evolution Along the Northern Flank of the Papuan Peninsula, Goodenough Bay, Eastern Papua New Guinea

NASA Astrophysics Data System (ADS)

Horton, B. K.; Gillis, R. J.; Mann, P.

2009-12-01

Although large-magnitude extension in the Woodlark Rift of eastern Papua New Guinea (PNG) and the D’Entrecasteaux Islands has been addressed through previous research on the late Cenozoic structure and cooling history of metamorphic domes, few studies have evaluated the exhumational record contained within adjacent sedimentary basins. Onshore exposures of Neogene basin fill in PNG along the northern flank of the Papuan peninsula (east of the Dayman metamorphic dome and west-southwest of the domes of the D’Entrecasteaux Islands) provide a record of basin evolution prior to and during growth of the active spreading center that defines the boundary between the Australian plate and Woodlark microplate. Along the northern margin of the Papuan peninsula, a collection of lithofacies associations consisting of sandstone and subordinate conglomerate and mudstone represent deposition in bottomset, foreset, and topset subenvironments in a series of marine Gilbert-type deltas. Internal angular unconformities within the basin-fill succession indicate slope instability likely related to syndepositional deformation. This deformation is attributed to principally down-to-the north motion along extensional and strike-slip structures bordering the northern margin of Papuan peninsula, notably the ESE-striking Goodenough fault zone. Small-scale folding is interpreted as the product of late Miocene to Quaternary fault-related folding in an extensional setting, although we cannot rule out possible contraction coeval with significant collision-related shortening on the southern flank of the Papuan peninsula within the south-directed Papuan fold-thrust belt. Differences in sandstone petrographic results for the northern margin of the Papuan peninsula and the smaller Vogel peninsula suggest a multiphase history of basin evolution, with early Neogene subsidence of uncertain origin and late Neogene subsidence linked to regional extension. The timing of basin evolution will be assessed through pending chronological analyses based on marine microfossils and 40Ar/39Ar geochronology. These results will define the timing of basin evolution and related exhumation, allowing direct comparison with the structural record of cooling in metamorphic domes of the region.

Late Cenozoic extensional faulting in Central-Western Peloponnesus, Greece

NASA Astrophysics Data System (ADS)

Skourtsos, E.; Fountoulis, I.; Mavroulis, S.; Kranis, H.

2012-04-01

A series of forearc-dipping, orogen-parallel extensional faults are found in the central-western Peloponnesus, (south-western Aegean) which control the western margin of Mt Mainalon. The latter comprises HP/LT rocks of the Phyllites-Quartzites Unit (PQ), overlain by the carbonates and flysch of the Tripolis Unit while the uppermost nappe is the Pindos Unit, a sequence of Mesozoic pelagic sequence, topped by a Paleocene flysch. Most of the extensional structures were previously thought of as the original thrust between the Pindos and Tripolis Units. However, the cross-cutting relationships among these structures indicate that these are forearc (SW-dipping) extensional faults, downthrowing the Pindos thrust by a few tens or hundreds of meters each, rooting onto different levels of the nappe pile. In SW Mainalon the lowermost of the extensional faults is a low-angle normal fault dipping SW juxtaposing the metamorphic rocks of the PQ Unit against the non-metamorphic sequence of the Tripolis Unit. High-angle normal faults, found further to the west, have truncated or even sole onto the low-angle ones and control the eastern margin of the Quaternary Megalopolis basin. All these extensional structures form the eastern boundary of a series of Neogene-Quaternary tectonic depressions, which in turn are separated by E-W horsts. In the NW, these faults are truncated by NE to NNE-striking, NW-dipping faults, which relay the whole fault activity to the eastern margin of the Pyrgos graben. The whole extensional fault architecture has resulted (i) in the Pindos thrust stepping down from altitudes higher than 1000 m in Mainalon in the east, to negative heights in North Messinia and Southern Ilia in the west; and (ii) the gradual disappearance of the Phyllite-Quartzite metamorphics of Mainalon towards the west. The combination of these extensional faults (which may reach down to the Ionian décollement) with the low-angle floor thrusts of the Pindos, Tripolis and Ionian Units leads to additional ENE-WSW shortening, normal to the Hellenic Arc, west of the Peloponnesus.

Neoarchean high-pressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India: Petrology and zircon SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Saitoh, Yohsuke; Tsunogae, Toshiaki; Santosh, M.; Chetty, T. R. K.; Horie, Kenji

2011-08-01

We report the metamorphic pressure-temperature ( P- T) history of mafic granulites from two localities in southern India, one from Kanja Malai in the northern margin and the other from Perundurai in the central domain of the Palghat-Cauvery Suture Zone (PCSZ). The PCSZ is described in recent models as the trace of the suture along which crustal blocks were amalgamated within the Gondwana supercontinent during Late Neoproterozoic-Cambrian. The mafic granulite from Kanja Malai yields P- T conditions of 750-800 °C and 8-12 kbar reflecting the partially retrograded conditions following a peak high-pressure (HP) metamorphic event. The common Grt + Cpx + Qtz assemblage in these rocks and lack of decompression texture suggest that peak metamorphism was probably buffered by Grt + Cpx + Opx + Pl + Qtz assemblage, following which the rocks were exhumed through a gradual P- T decrease. The mafic granulite from Perundurai (Grt + Cpx + Pl) contains Opx + Pl symplectite commonly occurring between garnet and clinopyroxene, suggesting the progress of reaction: Grt + Cpx + Qtz → Opx + Pl, with the Grt + Cpx + Qtz representing the peak metamorphic assemblage. The reaction microstructures and calculated P- T conditions suggest that the mafic granulites from Perundurai underwent peak HP metamorphism at P > 12 kbar and T = 800-900 °C and subsequent isothermal decompression along a clockwise P- T path, in contrast to the P- T path inferred for Kanja Malai. The contrasting P- T paths obtained from the two localities suggest that whereas Perundurai is a part of the metamorphic orogen developed within the PCSZ during Gondwana assembly, the high-pressure granulites of Kanja Malai belong to a different orogenic regime. In order to evaluate this aspect further, we analyzed zircons in a charnockite and garnet-bearing quartzo-feldspathic gneiss associated with the HP granulites from Kanja Malai which yielded mean 207Pb/ 206Pb magmatic protolith emplacement ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530-2540 Ma) and metamorphic (2470-2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P- T data and exhumation path with those reported for Late Neoproterozoic-Cambrian HP-UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.

Metamorphism, Plate Tectonics, and the Supercontinent Cycle

NASA Astrophysics Data System (ADS)

Brown, Michael

Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; G-UHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Medium-temperature eclogite, high-pressure granulite metamorphism (E-HPGM), is also first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E-HPGM belts are complementary to G-UHTM belts and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (high-pressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although perhaps counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a 'Proterozoic plate tectonics regime', although the style of tectonics likely involved differences. The 'Proterozoic plate tectonics regime' evolved during a Neoproterozoic transition to the 'modern plate tectonics regime' characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPM-UHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).

Generalized Optoelectronic Model of Series-Connected Multijunction Solar Cells

DOE PAGES

Geisz, John F.; Steiner, Myles A.; Garcia, Ivan; ...

2015-10-02

The emission of light from each junction in a series-connected multijunction solar cell, we found, both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs ofmore » the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Furthermore, our techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.« less

Faults dominant structure? -Seismic images of the subsurface structure for the Ilan geothermal field in Taiwan.

NASA Astrophysics Data System (ADS)

Chang, Yu-Chun; Shih, Ruey-Chyuan; Wang, Chien-Ying; Kuo, Hsuan-Yu; Chen, Wen-Shan

2016-04-01

A prototype deep geothermal power plant is to be constructed at the Ilan plain in northeastern Taiwan. The site will be chosen from one of the two potential areas, one in the west and the other in the eastern side of the plain. The triangle-shaped Ilan plane is bounded by two mountain ranges at the northwest and the south, with argillite and slate outcrops exposed, respectively. The Ilan plane is believed situating in a structure extending area at the southwestern end of the Okinawa Trough. Many studies about subsurface structure of the plain have been conducted for years. The results showed that the thickest sediments, around 900 m, is located at the eastern coast of the plain, at north of the largest river in the plain, the Lanyang river, and then became shallower to the edges of the plain. Since the plane is covered by thick sediments, formations and structures beneath the sediments are barely known. However, the observed high geothermal gradient and the abundant hot spring in the Ilan area indicate that this area is having a high potential of geothermal energy. In order to build up a conceptual model for tracing the possible paths of geothermal water and search for a suitable site for the geothermal well, we used the seismic reflection method to delineate the subsurface structure. The seismic profiles showed a clear unconformity separating the sediments and the metamorphic bedrock, and some events dipping to the east in the bedrock. Seismic images above the unconformity are clear; however, seismic signals in the metamorphic bedrock are sort of ambiguous. There were two models interpreted by using around 10 seismic images that collected by us in the past 3 years by using two mini-vibrators (EnviroVibe) and a 360-channel seismic data acquisition system. In the first model, seismic signals in the bedrock were interpreted as layer boundaries, and a fractured metamorphic layer down the depth of 1200m was thought as the source of geothermal water reservoir. In the other model, a northwestern dipping normal faults system was interpreted, and the normal faults were the paths for guiding the geothermal energy from the depth. Although both models were possible for obtaining a promising geothermal energy in the study area, a clear conceptual structure model is needed for future development of the geothermal energy in this area. Our interpretation favorites the fault dominant structure model; however, since the bedrock was slate or argillite still needed to be identified, more data from core borings and other geophysical, geologic data are needed. In this paper, we will illustrate a 3 dimensional suburface structure model by using the seismic images and integrate with results obtained from other studies to show the possibility of the proposed fault dominant structure model.

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

NASA Astrophysics Data System (ADS)

Bastida, F.; Brime, C.; García-López, S.; Sarmiento, G. N.

The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism.

Interactions between plutonism and detachments during metamorphic core complex formation, Serifos Island (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Rabillard, Aurélien; Arbaret, Laurent; Jolivet, Laurent; Le Breton, Nicole; Gumiaux, Charles; Augier, Romain; Grasemann, Bernhard

2015-06-01

In order to better understand the interactions between plutonic activity and strain localization during metamorphic core complex formation, the Miocene granodioritic pluton of Serifos (Cyclades, Greece) is studied. This pluton (11.6-9.5 Ma) intruded the Cycladic Blueschists during thinning of the Aegean domain along a system of low-angle normal faults belonging to the south dipping West Cycladic Detachment System (WCDS). Based on structural fieldwork, together with microstructural observations and anisotropy of magnetic susceptibility, we recognize a continuum of deformation from magmatic to brittle conditions within the magmatic body. This succession of deformation events is kinematically compatible with the development of the WCDS. The architecture of the pluton shows a marked asymmetry resulting from its interaction with the detachments. We propose a tectonic scenario for the emplacement of Serifos pluton and its subsequent cooling during the Aegean extension: (1) A first stage corresponds to the metamorphic core complex initiation and associated southwestward shearing along the Meghàlo Livadhi detachment. (2) In the second stage, the Serifos pluton has intruded the dome at shallow crustal level, piercing through the ductile/brittle Meghàlo Livadhi detachment. Southwest directed extensional deformation was contemporaneously transferred upward in the crust along the more localized Kàvos Kiklopas detachment. (3) The third stage was marked by synmagmatic extensional deformation and strain localization at the contact between the pluton and the host rocks resulting in nucleation of narrow shear zones, which (4) continued to develop after the pluton solidification.

Evidence for polymetamorphic garnet growth in the Çine (southern Menderes) Massif, Western Turkey

NASA Astrophysics Data System (ADS)

Baker, C. B.; Catlos, E. J.; Sorensen, S. S.; Çemen, I.; Hancer, M.

2008-07-01

Garnet-based thermobarometry is often used to develop models for the evolution of the Menderes Massif, a key Aegean metamorphic core complex. Here we present X-ray element maps and high-contrast backscattered electron (BSE) and cathodoluminescence (CL) images from a garnet-bearing rock from the Çine (southern Menderes) Massif. The images document a polymetamorphic history as plagioclase and garnet grains show distinct cores and rims. The sample contains matrix monazite in reaction with allanite. The garnet in the sample is likely not in equilibrium with its matrix minerals. This is evidenced by BSE images that document compositional variability in both core and rim zoning and tracks of bright streaks extending from rim to core. We propose that some garnet that is now present in the Menderes Massif formed due to collision during Cambro-Ordovician and may have recrystallized during subsequent collisional and extensional events. These processes led to non-equilibrium compositions and can result in spurious pressure-temperature (P-T) calculations. To establish the feasibility of the P-T estimates of rocks from the Çine Massif for input into tectonic models for the region, more than one sample from single outcrops should be analyzed. Rocks within the Çine Massif have been suggested to display inverted metamorphism, an increase in T towards structurally higher levels. Based on the garnet documented here, we propose that the inverted metamorphism may be a consequence of apparent P-T rather than a real phenomenon.

Submarine hydrothermal metamorphism of the Del Puerto ophiolite, California.

USGS Publications Warehouse

Evarts, R.C.; Schiffman, P.

1983-01-01

Metamorphic zonation overprinted on the volcanic member and overlying volcanogenic sediments of the ophiolite complex increases downward in grade and is characterized by the sequential appearance with depth of zeolites, ferric pumpellyite and pistacitic epidote. Metamorphic assemblages of the plutonic member of the complex are characterized by the presence of calcic amphibole. The overprinting represents the effects of hydrothermal metamorphism resulting from the massive interaction between hot igneous rocks and convecting sea-water in a submarine environment. A thermal gradient of 100oC/km is postulated to account for the zonal recrystallization effects in the volcanic member. The diversity and sporadic distribution of mineral assemblages in the amphibole zone are considered due to the limited availability of H2O in the deeper part of the complex. Details of the zonation and representative microprobe analyses are tabulated.-M.S.

Petrography and zircon U-Pb isotopic study of the Bayanwulashan Complex: Constrains on the Paleoproterozoic evolution of the Alxa Block, westernmost North China Craton

NASA Astrophysics Data System (ADS)

Wu, Sujuan; Hu, Jianmin; Ren, Minghua; Gong, Wangbin; Liu, Yang; Yan, Jiyuan

2014-11-01

The Bayanwulashan Metamorphic Complex (BMC) exposes along the eastern margin of the Alxa Block, the westernmost part of the North China Craton (NCC). BMC is principally composed of metamorphic rocks with amphibole plagiogneiss, biotite plagioclase gneiss and granitic gneiss. Our research has been focused on the petrography and zircon U-Pb geochronology of the BMC to better understand the evolution of the Alxa Block and its relationship with the NCC. Evidences from field geology, petrography, and mineral chemistry indicate that two distinct metamorphic assemblages, the amphibolite and greenschist facies, had overprinted the preexisting granitic gneiss and suggest that the BMC experienced retrograde metamorphic episodes. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of BMC were at ca. 2.30-2.24 Ga and the two metamorphic events were at ca. 1.95-1.91 Ga and ca. 1.88-1.85 Ga respectively. These ages indicate that BMC initially intruded during Paleoproterozoic, not as previously suggested at Archean period. The Early Paleoproterozoic metamorphic records and the magmatic thermochronological data in BMC exhibit different evolution paths between the Alxa Block and the NCC. The Alxa Block was most likely an independent Early Paleoproterozoic terrain. Following different amalgamation processes, The Alxa Block combined with Western Block at ca. 1.95 Ga and then united with NCC at ca. 1.85 Ga.

Burial of thermally perturbed Lesser Himalayan mid-crust: Evidence from petrochemistry and P-T estimation of the western Arunachal Himalaya, India

NASA Astrophysics Data System (ADS)

Goswami-Banerjee, Sriparna; Bhowmik, Santanu Kumar; Dasgupta, Somnath; Pant, Naresh Chandra

2014-11-01

In this work, we establish a dual prograde P-T path of the Lesser Himalayan Sequence (LHS) rocks from the western Arunachal Himalaya (WAH). The investigated metagranites, garnet- and kyanite-zone metapelites of the LHS are part of an inverted metamorphic sequence (IMS) that is exposed on the footwall side of the Main Central Thrust (MCT). Integrated petrographic, mineral chemistry, geothermobarometric (conventional and isopleth intersection methods) and P-T pseudosection modeling studies reveal a near isobaric (at P ~ 8-9 kbar) peak Barrovian metamorphism with increase in TMax from ~ 560 °C in the metagranite through ~ 590-600 °C in the lower and middle garnet-zone to ~ 600-630 °C in the upper garnet- and kyanite-zone rocks. The metamorphic sequence of the LHS additionally records a pre-Barrovian near isobaric thermal gradient in the mid crust (at ~ 6 kbar) from ~ 515 °C (in the middle garnet zone) to ~ 560-580 °C (in the upper garnet- and kyanite zone, adjoining the Main Central Thrust). Further burial (along steep dP/dT gradient) to a uniform depth corresponding to ~ 8-9 kbar and prograde heating of the differentially heated LHS rocks led to the formation of near isobaric metamorphic field gradient in the Barrovian metamorphic zones of the WAH. A combined critical taper and channel flow model is presented to explain the inverted metamorphic zonation of the rocks of the WAH.

New investigations in southwestern Guinea: consequences for the Rokelide belt (West Africa)

NASA Astrophysics Data System (ADS)

Villeneuve, Michel; Bellon, Hervé; Corsini, Michel; Le Metour, Joël; Chatelee, Sébastien

2015-07-01

The southern Guinean terranes belong to the "Rokelide belt" that is located in the southwestern part of the West African craton (Senegal to Liberia). Field investigations and K-Ar and 40Ar-39Ar radiometric analysis performed on samples collected from southern Guinea provide a new interpretation for metamorphic terranes not yet dated. A K-Ar whole-rock age of a gneiss and 40Ar-39Ar plateau ages of amphiboles separated from a mylonitic gneiss of the Ouankifondi formation and a gneiss from the Kissi-Kissi formation yield several Pan-African metamorphic ages at circa 650, 560, and 530 Ma, respectively. Field investigations show that these formations are unconformably capped by the Kolente group. The previous structural framework and the geodynamic evolution of the Rokelide belt based on the coeval evolution of the Ouankifondi and Kissi-Kissi formations and the Kolente group are reassessed. The Rokelide belt is linked to the Bassaride belt. Correlations with adjacent Sierra Leonean terranes and with northern Guinea and Senegal are considered.

Geology of the Byrd Glacier Discontinuity (Ross Orogen): New survey data from the Britannia Range, Antarctica

USGS Publications Warehouse

Carosi, R.; Giacomini, F.; Talarico, F.; Stump, E.

2007-01-01

Field activities in the Britannia Range (Transantarctic Mountains, Antarctica) highlighted new geological features around the so-called Byrd Glacier discontinuity. Recent field surveys revealed the occurrence of significant amounts of medium- to high-grade metamorphic rocks, intruded by abundant coarse-grained porphyritic granitoids. Most of the granitoids are deformed, with foliation parallel to the regional foliation in the metamorphics. Two main episodes of deformation are observed. Tight to isoclinal folds and penetrative axial plane foliation are related to the D1 phase, open folds to the D2. The main foliation (D1) trends nearly E-W in agreement with the trend in the southern portion of the Byrd Glacier. In most outcrops, granitic dykes are folded and stretched by the D2 deformation, which shows similar characteristics with the D2 deformation south of the Byrd Glacier. This suggests the occurrence in the Ross orogen of an orogen-normal structure south and north of the Byrd Glacier.

Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms.

PubMed

Rahman, Imran A; Waters, Johnny A; Sumrall, Colin D; Astolfo, Alberto

2015-10-01

Inferring the development of the earliest echinoderms is critical to uncovering the evolutionary assembly of the phylum-level body plan but has long proven problematic because early ontogenetic stages are rarely preserved as fossils. Here, we use synchrotron tomography to describe a new early post-metamorphic blastoid echinoderm from the Carboniferous (approx. 323 Ma) of China. The resulting three-dimensional reconstruction reveals a U-shaped tubular structure in the fossil interior, which is interpreted as the digestive tract. Comparisons with the developing gut of modern crinoids demonstrate that crinoids are an imperfect analogue for many extinct groups. Furthermore, consideration of our findings in a phylogenetic context allows us to reconstruct the evolution and development of the digestive system in echinoderms more broadly; there was a transition from a straight to a simple curved gut early in the phylum's evolution, but additional loops and coils of the digestive tract (as seen in crinoids) were not acquired until much later. © 2015 The Author(s).

Metamorphozed Hercynian granitoids in the Alpine structures of the Central Rhodope, Bulgaria: geotectonic position and geochemistry

NASA Astrophysics Data System (ADS)

Cherneva, Zlatka; Georgieva, Milena

2005-05-01

Orthogneisses of late-Hercynian protolith age crop out in the Central Rhodope high-grade metamorphic complex, which is part of the Alpine orogen in south-eastern Europe. They compose a tectonic unit bordered by late-Alpine extensional shear zones. These rocks reflect Eocene amphibolite facies migmatization (<750 °C/0.9-0.5 GPa). The low-temperature melting favored zircon inheritance and disturbed mainly the LILE protolith compositions. Despite the intense Alpine metamorphic overprint, the major elements, HFSE and REE reflect the initial composition of the Hercynian protolith. A geochemical data set summarizing 200 whole rock analyses testifies to a calc-alkaline magma differentiation producing a compositional range of tonalite and/or granodiorite to granite and leucocratic granite. Geochemical compositions combined with published isotope and age data suggest dominant I-type protoliths and mixed magma sources including crustal and mantle material, and distinguish between older granitoids of volcanic-arc affinity and probably younger ones of late or post-collision origin.

Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex

USGS Publications Warehouse

ten Brink, Uri S.; Zhang, Jie; Brocher, Thomas M.; Okaya, David A.; Klitgord, Kim D.; Fuis, Gary S.

2000-01-01

We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex, Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

NASA Astrophysics Data System (ADS)

Owona, Sebastien; Schulz, Bernhard; Ratschbacher, Lothar; Mvondo Ondoa, Joseph; Ekodeck, Georges E.; Tchoua, Félix M.; Affaton, Pascal

2011-01-01

Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D 1-D 2, D 3 and D 4. The garnet-bearing assemblages crystallized in course of the deformation stage D 1-D 2 which led to the formation of the regional main foliation S 2. In XCa- XMg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing XMg and variably decreasing XCa signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P- T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P- T trends. The P- T evolution started at ˜450 °C/7 kbar, passed high-pressure conditions at 11-12 kbar at variable temperatures (600-700 °C) and involved a marked decompression toward 6-7 kbar at high temperatures (700-750 °C). Th-U-Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature-intermediate pressure stage at the end of the prograde P- T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P- T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes.

The onset of metamorphism in ordinary and carbonaceous chondrites

USGS Publications Warehouse

Grossman, J.N.; Brearley, A.J.

2005-01-01

Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X-ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO-rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr-rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr-rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr3+. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised. ?? The Meteorological Society, 2005.

Constraining Metamorphic Timing and Processes by Dating Garnet, Zircon, Titanite and Monazite in UHP and HP Rocks from Weihai, Sulu UHP Terrane, Eastern China

NASA Astrophysics Data System (ADS)

Wang, D.; Vervoort, J. D.; Fisher, C. M.; Cao, H.

2016-12-01

The Sulu UHP terrane is the extension of the Dabie orogenic belt to the east, offset 500 km to the northeast by the Tanlu fault [1]. The focus of this study, the Weihai area, is located at the northernmost part of the Sulu UHP terrane, and consists mainly of gneisses overprinted by amphibolite-facies assemblages, in addition to minor eclogite, granulite, and some ultramafic rocks [1]. Time constrains are critical to our understanding of the processes of UHP metamorphism, as well as the tectonic evolution of the region. In the last decade, U-Pb dating of metamorphic domains of zircons has been widely applied to determine the history of the UHP metamorphism (240 - 220 Ma) [1]. Recent garnet Lu-Hf dating from the Dabie terrane (240 - 220Ma) suggests the initiation of prograde metamorphism to be prior to ca. 240 Ma [2]. In-situ U-Pb dating of accessary minerals using LA-ICPMS (i.e. monazite, titanite, rutile, etc.), can provide important information to augment and complement the zircon U-Pb metamorphic dates. In this study, we collected samples throughout the Weihai area. Protolith ages of these samples range from Paleoproterozoic to Neoproterozoic ( 1850 - 700 Ma) as indicated by U-Pb dating of zircon cores. Zircon metamorphic rims yield U-Pb ages of 240 - 220 Ma, likely indicating the UHP stage of the Sulu terrane [3]. Four eclogites yield Lu-Hf garnet isochrons with dates between 239 and 224 Ma, consistent with garnet Lu-Hf dates from Dabie UHP terrane [2]. Sm-Nd isochrons indicate systematic younger dates (220 - 210 Ma) interpreted as cooling ages. Titanites extracted from four samples give U-Pb ages ranging from 220 to 200 Ma, in agreement with the titanite dates from the southern Sulu terrane [4]. Monazites from three samples give precise dates between 214 and 211 Ma. Collectively, monazite and titanite U-Pb ages are broadly consistent with the garnet Sm-Nd isochrons, and thus we interpret these as cooling ages. Based on the dates of different systems/minerals presented above, we suggest the prograde metamorphism of Weihai UHP terrane likely took place prior to 240 Ma, and the peak of the UHP stage is likely between 240 and 220 Ma. [1] Zhang et al., Gondwana Res., 16 (2009) 1-26 [2] Cheng et al., J. Metamorphic Geol., 26 (2008), 741-758 [3] Liou et al., J. Asian Earth Sci., 35 (2009), 199-231 [4] Chen and Zheng, GCA, 150(2015), 53-73

Fluid heterogeneity during granulite facies metamorphism in the Adirondacks: stable isotope evidence

USGS Publications Warehouse

Valley, J.W.; O'Neil, J.R.

1984-01-01

The preservation of premetamorphic, whole-rock oxygen isotope ratios in Adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived, hot CO2-H2O fluids during granulite facies metamorphism. This conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations. The data suggest that these rocks were not an open system during metamorphism; that fluid/rock ratios were in many instances between 0.0 and 0.1; that externally derived fluids, as well as fluids derived by metamorphic volatilization, rose along localized channels and were not pervasive; and thus that no single generalization can be applied to metamorphic fluid conditions in the Adirondacks. Analyses of 3 to 4 coexisting minerals from Adirondack marbles show that isotopic equilibrium was attained at the peak of granulite and upper amphibolite facies metamorphism. Thus the isotopic compositions of metamorphic fluids can be inferred from analyses of carbonates and fluid budgets can be constructed. Carbonates from the granulite facies are on average, isotopically similar to those from lower grade or unmetamorphosed limestones of the same age showing that no large isotopic shifts accompanied high grade metamorphism. Equilibrium calculations indicate that small decreases in ??18O, averaging 1 permil, result from volatilization reactions for Adirondack rock compositions. Additional small differences between amphibolite and granulite facies marbles are due to systematic lithologie differences. The range of Adirondack carbonate ??18O values (12.3 to 27.2) can be explained by the highly variable isotopic compositions of unmetamorphosed limestones in conjunction with minor 18O and 13C depletions caused by metamorphic volatilization suggesting that many (and possibly most) marbles have closely preserved their premetamorphic isotopic compositions. Such preservation is particularly evident in instances of high ??18O calcites (25.0 to 27.2), low ??18O wollastonites (-1.3 to 3.5), and sharp gradients in ??18O (18 permil/15m between marble and anorthosite, 8 permil/25 m in metasediments, and 6 permil/1 m in skarn). Isotopic exchange is seen across marble-anorthosite and marble-granite contacts only at the scale of a few meters. Small (<5 m) marble xenoliths are in approximate exchange equilibrium with their hosts, but for larger xenoliths and layers of marble there is no evidence of exchange at distances greater than 10 m from meta-igneous contacts. ?? 1984 Springer-Verlag.

Metamorphic conditions and CHIME monazite ages of Late Eocene to Late Oligocene high-temperature Mogok metamorphic rocks in central Myanmar

NASA Astrophysics Data System (ADS)

Maw Maw Win; Enami, Masaki; Kato, Takenori

2016-03-01

The high temperature (T)/pressure (P) regional Mogok metamorphic belt is situated in central Myanmar, and is mainly composed of pelitic gneisses, amphibolites, marbles, and calc-silicate rocks. The garnet-biotite-plagioclase-sillimanite-quartz assemblage and its partial system suggest equilibrium P/T conditions of 0.6-1.0 GPa/780-850 °C for the peak metamorphic stage, and 0.3-0.5 GPa/600-680 °C for the exhumation and hydration stage. Monazite grains show complex compositional zoning consisting of three segments-I, II, and III. Taking into consideration the monazite zoning and relative misfit curves, the calculated chemical Th-U-total Pb isochron method (CHIME) monazite age data (284 spot analyses) indicated four age components: 49.3 ± 2.6-49.9 ± 7.9, 37.8 ± 1.0-38.1 ± 1.7, 28.0 ± 0.8-28.8 ± 1.6, and 23.7 ± 1.3 Ma (2σ level). The ages of the Late Eocene and Late Oligocene epochs were interpreted as the peak metamorphic stage of upper-amphibolite and/or granulite facies and the postdated hydration stage, respectively.

The ammonium content in the Malayer igneous and metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)

NASA Astrophysics Data System (ADS)

Ahadnejad, Vahid; Hirt, Ann Marie; Valizadeh, Mohammad-Vali; Bokani, Saeed Jabbari

2011-04-01

The ammonium (NH4+) contents of the Malayer area (Western Iran) have been determined by using the colorimetric method on 26 samples from igneous and metamorphic rocks. This is the first analysis of the ammonium contents of Iranian metamorphic and igneous rocks. The average ammonium content of metamorphic rocks decreases from low-grade to high-grade metamorphic rocks (in ppm): slate 580, phyllite 515, andalusite schist 242. In the case of igneous rocks, it decreases from felsic to mafic igneous types (in ppm): granites 39, monzonite 20, diorite 17, gabbro 10. Altered granitic rocks show enrichment in NH4+ (mean 61 ppm). The high concentration of ammonium in Malayer granites may indicate metasedimentary rocks as protoliths rather than meta-igneous rocks. These granitic rocks (S-types) have high K-bearing rock-forming minerals such as biotite, muscovite and K-feldspar which their potassium could substitute with ammonium. In addition, the high ammonium content of metasediments is probably due to inheritance of nitrogen from organic matter in the original sediments. The hydrothermally altered samples of granitic rocks show highly enrichment of ammonium suggesting external sources which intruded additional content by either interaction with metasedimentary country rocks or meteoritic solutions.

The Interpretation Of Multiple Foliations In Metapelites: An Example From NW-Namibia

NASA Astrophysics Data System (ADS)

Passchier, C. W.

2014-12-01

Foliations in metapelites belong to the most important tools in structural geology to reconstruct deformation history and kinematics. Since foliations are easily developed and are hard to destroy, multiple foliations, associated with other structures such as folds and boudins, serve as a basis in reconstructing the tectonic history of all metamorphic terrains. Traditionally, such reconstructions assume regionally homogeneous tectonic effects producing distinct generations of structures, which are then labelled D1, D2, D3 etc. The Goantagab Domain in NW Namibia consists of Neoproterozoic pelitic and psammitic metaturbidites with only minor changes in facies, exposed over an area of 80x40km. The rocks were deformed in a transpressive Neoproterozoic to Cambrian tectonic event during the amalgamation of Gondwanaland. Five overprinting foliations can be recognised in the area, but only three are recognisable at any location and no foliation extends over the entire area. Apparently, small variations in kinematic vorticity and other kinematic parameters and in orientation of incremental strain axes lead to local foliation development and to gradients in foliation style. The similarity in field and microstructure of the different foliations, and gradations in their development make a classical approach using D1-D2-D3 labelling problematic. Since the fieldwork area is very well exposed, it is possible to determine which factors lead to local development and destruction of foliations during ongoing ductile deformation. Detailed analysis of the local foliation architecture and history is used to explore new methods in structural geology to handle complex multiple foliations in metamorphic terrains

Geochemistry of granitoid rocks from the western Superior Province: Evidence for 2- and 3-stage crustal evolution models

NASA Technical Reports Server (NTRS)

Beakhouse, G. P.; errane) are discussed.

1986-01-01

The Superior Province is divisible into subprovinces that can be classified as greenstone-tonalite, paragneiss, or batholitic terranes and are distinguished by differences in lithologic proportions, metamorphic grade, and structural style. The origin and significance of contrasting geochemical characteristics of plutonic rocks from the Winnipeg River subprovince (a batholithic terrane) and the Wabigoon subprovince (a greenstone-tonalite terrane) are discussed.

Estimation of principal deviatoric stresses imposed on an individual metachert: detailed application of the microboudin palaeopiezometer

NASA Astrophysics Data System (ADS)

Matsumura, T.; Masuda, T.

2017-12-01

The microboudinage structure of columnar mineral grain is an useful marker for the stress imposed on the metamorphic rock. In this presentation, we report a detailed application of the microboudin palaeopiezometer to an individual metachert specimen that includes microboudinaged tourmaline grains. The microboudin palaeostress analysis is conducted to the number of 3621 tourmaline grains divided into every 10° of their long axes on the foliation surface. The analysis revealed that the group of mean orientation ± 15° and perpendicular to the mean orientation ± 15° showed the value of σ1 - σ3 and σ1 - σ2 as 10.2 MPa and 5.3 MPa, respectively. Using both values of σ1 - σ3 and σ1 - σ2, magnitude of principal deviatoric stresses (σ'1, σ'2 and σ'3) are obtained as σ'1 = 5.3 MPa, σ'2 = -0.1 MPa and σ'3 = -5.1 MPa. In this stress state, the stress ratio (σ2 - σ3)/(σ1 - σ3) is 0.48 that indicates typical triaxial compression. As the microboudinage structure is considered to develop immediately before the matrix mineral encountered the cessation of the plastic flow, these values correspond to conditions at ≧ 300 °C on the later stage of the metamorphism.

Bohemian circular structure, Czechoslovakia: Search for the impact evidence

NASA Technical Reports Server (NTRS)

Rajlich, Petr

1992-01-01

Test of the impact hypothesis for the origin of the circular, 260-km-diameter structure of the Bohemian Massif led to the discovery of glasses and breccias in the Upper Proterozoic sequence that can be compared to autogeneous breccias of larger craters. The black recrystallized glass contains small exsolution crystals of albite-oligoclase and biotite, regularly dispersed in the matrix recrystallized to quartz. The occurrence of these rocks is limited to a 1-sq-km area. It is directly underlain by the breccia of the pelitic and silty rocks cemented by the melted matrix, found on several tens of square kilometers. The melt has the same chemistry as rock fragments in major and in trace elements. It is slightly impoverished in water. The proportion of melted rocks to fragments varies from 1:5 to 10:1. The mineralogy of melt viens is the function of later, mostly contact metamorphism. On the contact of granitic plutons it abounds on sillimanite, cordierite, and small bullets of ilmenite. Immediately on the contact with syenodiorites it contains garnets. The metamorphism of the impact rock melt seems the most probable explanation of the mineralogy and the dry total fusion of rocks accompanied by the strong fragmentation. Other aspects of this investigation are discussed.

A histological atlas of the tissues and organs of neotenic and metamorphosed axolotl.

PubMed

Demircan, Turan; İlhan, Ayşe Elif; Aytürk, Nilüfer; Yıldırım, Berna; Öztürk, Gürkan; Keskin, İlknur

2016-09-01

Axolotl (Ambystoma Mexicanum) has been emerging as a promising model in stem cell and regeneration researches due to its exceptional regenerative capacity. Although it represents lifelong lasting neoteny, induction to metamorphosis with thyroid hormones (THs) treatment advances the utilization of Axolotl in various studies. It has been reported that amphibians undergo anatomical and histological remodeling during metamorphosis and this transformation is crucial for adaptation to terrestrial conditions. However, there is no comprehensive histological investigation regarding the morphological alterations of Axolotl organs and tissues throughout the metamorphosis. Here, we reveal the histological differences or resemblances between the neotenic and metamorphic axolotl tissues. In order to examine structural features and cellular organization of Axolotl organs, we performed Hematoxylin & Eosin, Luxol-Fast blue, Masson's trichrome, Alcian blue, Orcein and Weigart's staining. Stained samples from brain, gallbladder, heart, intestine, liver, lung, muscle, skin, spleen, stomach, tail, tongue and vessel were analyzed under the light microscope. Our findings contribute to the validation of the link between newly acquired functions and structural changes of tissues and organs as observed in tail, skin, gallbladder and spleen. We believe that this descriptive work provides new insights for a better histological understanding of both neotenic and metamorphic Axolotl tissues. Copyright © 2016 Elsevier GmbH. All rights reserved.

Protolith relations of the Gravina belt and Yukon-Tanana terrane in central southeastern Alaska

DOE Office of Scientific and Technical Information (OSTI.GOV)

McClelland, W.C.; Gehrels, G.E.; Patchett, P.J.

1992-01-01

Metamorphic rocks west of the Coast Mountains batholith in central southeastern Alaska are divided into the Gravina belt, Taku terrane, and newly defined Ruth assemblage. The Ruth assemblage comprises metapelite, quartzose metaclastic strata, quartzite, marble, felsic metatuff, mafic metavolcanic rocks, and orthogneiss. Depositional and emplacement ages of 367 {plus minus} 10 Ma and 345 {plus minus} 13 Ma inferred from discordant U/Pb zircon analyses on felsic metatuff and granodioritic orthogneiss, respectively, require that at least portions of the Ruth assemblage be Late Devonian and early Mississippian in age. The assemblage is similar in age and protolith to, and thus correlatedmore » with, the Yukon-Tanana terrane. The Gravina belt is characterized by upper Jurassic and lower Cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the Alexander terrane. Metamorphic rocks that structurally underlie the Taku terrane and Rugh assemblage are included in this assemblage. Trace element geochemistry and the abundance of pyroclastic flows associated with tuffaceous turbidites suggest that the Gravina belt evolved in an intra-arc basinal setting. In central southeastern Alaska, the mid-Cretaceous structure that currently separates the Ruth assemblage (Yukon-Tanana correlative) from the Gravina belt marks the fundamental boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes.« less