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Sample records for nogoli metamorphic complex

  1. Alpine Corsica Metamorphic Core Complex

    NASA Astrophysics Data System (ADS)

    Fournier, Marc; Jolivet, Laurent; Goffé, Bruno; Dubois, Roland

    1991-12-01

    Alpine Corsica is an example where superficial nonmetamorphic allochtonous units rest upon a highly strained metamorphic complex. Early ductile deformation under high pressure-low temperature (HP-LT) conditions is due to the westward thrusting of oceanic material onto a continental basement as shown by previous studies. New thermobarometric estimates yield minimal peak HP-LT metamorphism conditions of 11 kbar at 400°C. The early deformation is overprinted by a ductile deformation with an eastward sense of shear postdating or contemporaneous with mineral recrystallizations in the greenschist facies conditions. Early compressive thrust contacts are reworked as east dipping ductile normal faults and the less competent units display only eastward shear criteria. The upper units are affected by an extensional brittle deformation, and east dipping brittle normal faults bound to the west the early to middle Miocene Saint-Florent half-graben. The greenschist metamorphic event lasted until 33 Ma, which is contemporaneous with the beginning of the extension in the Liguro-Provençal basin. We interpret the second deformation stage as the result of a ductile extension following the overthickening of the crust due to the westward thrusting. Extension reduces the thickness of the crust so that upper units free from early P-T conditions are brought into close contact with a HP-LT metamorphic core complex. The geometry of the late extension is controlled by that of the early compressive thrust.

  2. Thermal history of a metamorphic core complex

    NASA Technical Reports Server (NTRS)

    Dokka, R. K.; Mahaffie, M. J.; Snoke, A. W.

    1985-01-01

    Fission track (FT) thermochronology studies of lower plate rocks of the Ruby Mountains-East Humbolt Range metamorphic core complex provide important constraints on the timing an nature of major middle Tertiary extension of northeast Nevada. Rocks analyzed include several varieties of mylonitic orthogneiss as well as amphibolitic orthognesses from the non-mylonitic infrastructural core. Oligocene-age porphyritic biotite granodiorite of the Harrison Pass pluton was also studied. The minerals dated include apatite, zircon, and sphene and were obtained from the same rocks that have been previously studied. FT ages are concordant and range in age from 26.4 Ma to 23.8 Ma, with all showing overlap at 1 sigma between 25.4 to 23.4 Ma. Concordancy of all FT ages from all structural levels indicates that the lower plate cooled rapidly from temperatures above approx. 285 C (assumed sphene closure temperature (2)) to below approx. 150 C (assumed apatite closure temperature) near the beginning of the Miocene. This suggests that the lower plate cooled at a rate of at least approx. 36 deg C/Ma during this event. Rapid cooling of the region is considered to reflect large-scale tectonic denudation (intracrustal thinning), the vertical complement to intense crustal extension. FT data firmly establish the upper limit on the timing of mylonitization during detachment faulting and also coincide with the age of extensive landscape disruption.

  3. Metamorphism of the Chugach Metamorphic Complex, (Alaska). New pressure estimates question the ridge subduction context.

    NASA Astrophysics Data System (ADS)

    Bruand, Emilie; Gasser, Deta; Kurt, Stuewe; Beyssac, Olivier

    2010-05-01

    The Chugach Metamorphic Complex (CMC, Alaska) is a 200 km long and 10-50 km wide complex and is part of an active accretionary prism. According to the sparse existing literature, the complex is believed to be a low-pressure high-temperature terrain (400-650°C and ~3kb) with a migmatitic inner core (~5-10 km) and schist rims surrounded by phyllite (Sisson et al., 1988). Such low pressure conditions are not common in a subduction zone setting and the formation of the complex is thus attributed to the subduction of a ridge during the Eocene (~ 50 my). This contribution presents detailed petrological work from the region to show that the metamorphism occurred at much higher pressures than previously believed. We focus on the petrology of calcareous metapelites from 4 different N-S transects across the complex from west to east (each being 10 to 30 km wide). Several PT thermobarometric tools are used including average PT determination using THERMOCALC, garnet-biotite thermometry and RSCM (graphite) thermometers using Raman spectroscopy. In addition to these methods, several thermodynamic pseudosections were calculated. Our calculations show that the metamorphic conditions vary between 550°C and 3-4 kbar in the north of the complex to >700°C and 7-9 kbar in the south. In the central part of the complex these conditions appear to be attributable to a single metamorphic event that occurred around 50 my. However, in some locations near major granitic intrusions that penetrate the regions two events are observed: 1) a first one characterised by temperatures around 550°C followed by 2) a hotter contact metamorphism (>640°C). Earlier studies have interpreted the supposed low-pressure conditions of the CMC (considered to be no more than 3 kbar) to be connected to a ridge subduction geodynamic context. Within our interpretation, the hypothesis of a ridge subduction context is not needed and indeed appears questionable. In fact, a simple subduction context following by a

  4. Meteoric water in metamorphic core complexes

    NASA Astrophysics Data System (ADS)

    Teyssier, Christian; Mulch, Andreas

    2015-04-01

    The trace of surface water has been found in all detachment shear zones that bound the Cordilleran metamorphic core complexes of North America. DeltaD values of mica fish in detachment mylonites demonstrate that these synkinematic minerals grew in the presence of meteoric water. Typically deltaD values are very negative (-120 to -160 per mil) corresponding to deltaD values of water that are < -100 per mil given the temperature of water-mica isotopic equilibration (300-500C). From British Columbia (Canada) to Nevada (USA) detachment systems bound a series of core complexes: the Thor-Odin, Valhalla, Kettle-Okanogan, Bitterroot -Anaconda, Pioneer, Raft River, Ruby Mountain, and Snake Range. The bounding shear zones range in thickness from ~100 m to ~1 km, and within the shear zones, meteoric water signature is recognized over 10s to 100s of meters beneath the detachment fault. The age of shearing ranges generally from Eocene in the N (~50-45 Ma) to Oligo-Miocene in the S (25-15 Ma). DeltaD water values derived from mica fish in shear zones are consistent with supradetachment basin records of the same age brackets and can be used for paleoaltimetry if coeval isotopic records from near sea level are available. Results show that a wave of topography (typically 4000-5000 m) developed from N to S along the Cordillera belt from Eocene to Miocene, accompanied by the propagation of extensional deformation and volcanic activity. In addition, each detachment system informs a particular extensional detachment process. For example, the thick Thor-Odin detachment shear zone provides sufficient age resolution to indicate the downward propagation of shearing and the progressive incorporation of footwall rocks into the hanging wall. The Kettle detachment provides a clear illustration of the dependence of fluid circulation on dynamic recrystallization processes. The Raft River system consists of a thick Eocene shear zone that was overprinted by Miocene shearing; channels of meteoric

  5. Metamorphic and stratigraphic constraints on the evolution of the Santa Catalina Mountains metamorphic core complex, Arizona

    SciTech Connect

    Palais, D.G.; Peacock, S.M. )

    1990-01-10

    Contact metamorphic mineral assemblages in the Santa Catalina Mountains metamorphic core complex constrain the emplacement depth of the metaluminous Late Cretaceous Leatherwood quartz diorite to between 7.5 and 14 km (2-3.8 kbar). The peraluminous Eocene Wilderness granite is constrained to have been emplaced at depths of less than 14 km. Stratigraphic and tectonic considerations are consistent with emplacement of both the Leatherwood quartz diorite and Wilderness granite at upper crustal levels. Published igneous thermobarometry (Anderson et al., 1988), although in approximate agreement with the metamorphic and tectonostratigraphic estimates for the depth of emplacement of the Wilderness granite, suggests that the Leatherwood quartz diorite was emplaced at depths of {approximately} 21 km (5.6 kbar) or greater. The discrepancy between the different emplacement estimates may reflect ambiguities associated with interpretations of the results from each technique. Igneous barometric assemblages may reflect conditions experienced during a period of crystallization during ascent of the magma through the crust (i.e., ponding of the magma). Alternatively, emplacement of the voluminous Wilderness granite and associated plutons may have entirely overprinted earlier Leatherwood-related contact metamorphism. The proposed upper crustal emplacement of the Leatherwood quartz diorite is consistent with the observation that Laramide-age crustal thickening in southeast Arizona probably did not exceed 8 km.

  6. Metamorphic and geochemical evolution of an amphibolite layer in the Chugach Metamorphic Complex (Alaska)

    NASA Astrophysics Data System (ADS)

    Bruand, E.; Gasser, D.; Stuewe, K.

    2010-12-01

    The Chugach Metamorphic Complex (CMC) is an Eocene high temperature- low to medium pressure complex located in an accretionary prism (Southern Alaska). This complex is more than 300 km long, made up of metasediments associated with an amphibolite layer (~ 10m to several km width) in its southern part. Based on the study of some part of this amphibolite layer, Lull and Plafker (1990) suggested that its geometry was due to the subduction of an intraplate island arc parallel to the Alaskan margin. However following their data, the assumption was highly hypothetic. West of our study area (>150km), another “metabasalt layer” composed by pillow basalts, gabbros and sheeted dikes is described in a southern accreted terrane (Prince William terrane). Some workers suggested a possible link between the origin of these ophiolites and the metabasalt layer extended from Valdez to south of Yakutat (our study area). They argued that these metabasalts or at least the western part (Valdez location) could reflect melt emplaced into the accretionary prism and generated upward to an asthenospheric window link with the subduction of a ridge. Thus the presence of this amphibolite layer in southern Alaska is matter of debate. Therefore, in this contribution, we detailed a petrologic and geochemical study of the amphibolite layer to reconstruct the metamorphic history of this strip using thermobarometry and pseudosection modelling (THERMOCALC). Petrological observations reveal a metamorphic peak assemblage composed by hb-pl±qtz-ilm/hem/mag and PT estimates about 650-750°C and 6-8 kb. Retrograde feature are symbolised by epidote, actinolite, chlorite, albite and sphene and seem to crystallise at temperature below 450°C. The amphibolite show REE and trace elements patterns characteristic for an altered metabasalt (U and Ba positive anomalies) as well as local contaminations by sediments (Pb and Sr positive anomalies). Pressure-temperature results are compared to a recent study on the

  7. Garnet as a reactant during and recorder of mid-crustal metamorphism: Sawtooth Metamorphic Complex, Idaho

    NASA Astrophysics Data System (ADS)

    Dutrow, B. L.; Henry, D.; Fukai, I.; Metz, K.

    2013-12-01

    The Sawtooth Metamorphic Complex (SMC), in central Idaho, is a newly recognized high-grade metamorphic terrain that provides constraints on crustal evolution of a key area in North America. The SMC lies near the southwestern margin of Laurentia, in an area that is largely obscured by the voluminous Idaho Batholith. Originally mapped as undifferentiated Precambrian metamorphic rocks, recent work demonstrates that a range of rock types from aluminous gneisses to calc-silicates record middle-lower crustal conditions. Field mapping, high-resolution sampling, mineral chemical, whole-rock and trace-element characterization define the nature of metamorphism. Multi-equilibria thermobarometry of aluminous gneisses yield the highest pressure (P)-temperature (T) conditions of lower-granulite facies near 765-795 °C and 7 kbar. Peraluminous gneisses passed above the muscovite stability field to produce sillimanite (sil) + Ksp + H2O. Occasionally, fine-grained biotite + sil are included in the cordierite (crd) suggesting the reaction: phlogopite + sil = Mg-crd + garnet + H2O. These dehydration reactions likely enhanced localized melting and migmatite formation. A post peak hydration event produced muscovite and andalusite that cross cuts dominant foliation and indicates a clockwise P-T path. In these rocks, garnet serves as an important phase to react with and to preserve the peak and post-peak phases and conditions. Calc-silicate gneisses record at least two metamorphic (M1, M2) and two deformational (D1, D2) events. M1 reaches upper-amphibolite to lower-granulite facies at temperatures near 800oC at 6 kbar. D1 ductile deformation post-dates peak metamorphism and produces deformation lamellae in clinopyroxene. M2 is characterized by amphibole (magnesiohornblende and tremolite) growth overprinting and retrogressing peak assemblages at Ts from 620-740oC at an assumed P of 6.5 kbar based on the application of the hornblende-plagioclase thermometer. These temperatures are

  8. Early Tertiary Anaconda metamorphic core complex, southwestern Montana

    USGS Publications Warehouse

    O'Neill, J. M.; Lonn, J.D.; Lageson, D.R.; Kunk, M.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

  9. Magmatic and metamorphic belts and plutonic-metamorphic complexes of southeastern Alaska

    SciTech Connect

    Brew, D.A.; Himmelberg, G.R.; Ford, A.B.; Loney, R.A. . Branch of Alaskan Geology Univ. of Missouri, Columbia, MO . Dept. of Geology)

    1993-04-01

    The Cordilleran orogen in southeastern Alaska includes 24 distinct magmatic belts, ranging in age from Cambrian to Holocene, that are defined by map relations, lithology, age, and chemical composition. The youngest magmatic features are Quaternary-age pre- and post-glacial volcanic rocks that occur in three major fields in the region, as well as in isolated locations. Cenozoic magmatic features consist of four major and three minor belts. The major Tkope-Portland Peninsula belt of Oligocene age includes both volcanic and plutonic rocks. The major calcalkalic Coast Mountains belt of early and middle Eocene age is the single largest magmatic feature of the region. Early Tertiary and latest Cretaceous magmatism is represented by the major calcalkalic great tonalite sill belt, a remarkable long and narrow feature along the west side of the Coast Mountains. Cretaceous and Jurassic intrusive rocks occur in five major belts and two minor belts in the region and Paleozoic intrusive rocks occur in four major and two minor belts. The three major plutonic-metamorphic complexes (PMC), from east to west, are: the Coast PMC in the Coast Mountains; the Glacier Bay-Chichag of plutonic complex (Chugach MC) in the northern outer islands. The Coast PMC records dynamothermal and regional contact metamorphic events related to regional plutonism within several juxtaposed terranes; its lengthy and complicated history is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Yukon prong and Stikine terrane to the east. The relatively simple Glacier Bay PC history is recorded as the roots of a Late Jurassic through late Early Cretaceous island arc that probably developed during the early stages of the above tectonic event. The complicated Chugach MC history developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes.

  10. Detrital zircon geochronology of the Tananao Metamorphic Complex, Taiwan

    NASA Astrophysics Data System (ADS)

    Huang, Y. C.; Chen, W. S.; Feng, H. T.; Liu, C. H.; Chung, S. L.; Lee, Y. H.

    2014-12-01

    The oldest rock of the Tananao Metamorphic Complex exposed at the Backbone Range which consists of marble, schist, and gneiss. According to the stratigraphy and structural analysis, the Complex in the ascending order can be divided into Chiuchu Marble, Paiyang Schist with Kuyuan Schist, and Pilu Formation. Based on Sr isotope dating, it is speculated that the formation age of Chiuchu Marble is closed to 250±20 Ma. Using the zircon U-Pb dating, the magnetic age of gneiss (I-type granite) yields 105- 86 Ma. The stratigraphic age of schist is difficult to define because of fossils are poorly-preserved. In our study, we use the detrital zircon U-Pb dating, by LA-ICPMS, to yield the maximum depositional age of the schist formation. We analyze fifteen samples in this study. The depositional age of the Kuyuan and Paiyang Schists would be not older than late Cretaceous, and the depositional age of the Pilu Formation may have formed in late Cretaceous and younger than Kuyuan and Paiyang Schists. Results from the above mentioned, we are interpreted as the tectonic setting of the Tananao Metamorpic Complex. The region was located on a passive continental margin that formed carbonate platform deposits during the late Permian period. The clastic sediments were unconformably deposited on Chiuchu Marble during the early Cretaceous. Then the paleo-Pacific plate subducted to the Cathaysia Block forming the granite at about 105-86 Ma. Afterwards, the clastic sediments were deeply buried caused by continuing subduction formed as schist rock. The metamorphic event is known as the Nanao orogeny.

  11. Age of granites of Wrangel Island metamorphic complex

    NASA Astrophysics Data System (ADS)

    Luchitskaya, Marina; Sergeev, Sergey; Sokolov, Sergey; Tuchkova, Marianna

    2014-05-01

    Within huge arctic shelf of Eastern-Siberian and Chukchi seas the metamorphic basement (Wrangel complex, Berri Formation) is exposed only on the Wrangel Island. There are different points of views on the age of metamorphic rocks of Wrangel complex (Berri Formation): (1) Neoproterozoic (Kameneva, 1970; Ageev, 1979; Kos'ko et al., 1993, 2003), (2) Devonian (Til'man et al., 1964, 1970; Ganelin, 1989). Metamorphic basement is represented by stratified complex, composed of dislocated metavolcanic, metavolcaniclastic and metasedimentary rocks (schists, metasandstones, metaconglomerated) with single lenses and layers of carbonate rocks (Wrangel Island…, 2003). Among basement rocks in the central part of Wrangel Island there are felsic intrusive bodies. They form small tabular bodies from tens centimeters to 70-80 meters in thickness, rarely dikes and small stocks (up to 20 x 30 m) and are composed of granite-porphyres, rarely muscovite porphyr-like granites and granosyenites (Wrangel Island…, 2003). The age of intrusions allow to determine the age of basement formation. Earlier the age of intrusions was determined by different methods and correlated to the boundary between Neoproterozoic and Paleozoic: K-Ar 570-603 Ma, Pb-Pb 590±50 Ma (S.M. Pavlov, Institute of Precambrian Geology and Geochronology, USSR Academy of Sciences), Rb-Sr 475±31 Ma (I.M.Vasil'eva, Institute of Precambrian Geology and Geochronology, USSR Academy of Sciences), U-Pb 609, 633, 677 Ma (Geological Survey of Canada) (Wrangel Island…, 2003; Kos'ko et al., 1993; Cecile et al., 1991). In the lower part of metamorphic rocks of Wrangel complex there are conformable tabular bodies of gneissosed and foliated granitoides. The latter are meramorphosed and transformed in biotite-muscovite-feldspar-quartz-sericite and muscovite-feldspar-quartz-sericite gneisses and schists, where relics of primary minerals (quartz, plagioclase, potassium feldspar, rarely biotite and muscovite) and equigranular granitic

  12. Origin of metamorphic core complexes and detachment faults

    NASA Astrophysics Data System (ADS)

    Wu, G.; Lavier, L. L.

    2013-12-01

    Origin of metamorphic core complexes and detachment faults Guangliang Wu1,2, Luc L. Lavier1,2 1 Institute for Geophysics, University of Texas at Austin, TX 78758, USA 2 Department of Geological Sciences, University of Texas at Austin, TX 78712, USA Metamorphic core complexes (MCCs) and detachment faults are widely observed in collapsing orogens, such as Western US Cordillera, the Aegean and Papua New Guinea. A theory for the origin of MCCs has to provide: i) a viable mechanism to bring deeper crustal material to the surface, ii) a scenario that allows slip on low-angle detachment faults, and iii) a viable mechanism to form a flat Moho at a certain stage of evolution. However, previous models ignored at least one of these three requirements. Using thermo-mechanical models constrained by geological and geophysical observation, we simulated MCCs and detachment faults in the context of collapsing orogens with preexisting shear zones and middle crust of variable strength. We found that MCCs and detachment faults are natural products of gravity driven middle crustal extrusion and exhumation and strong crustal decoupling along the preexisting shear zones in a favorable state of stress in collapsing orogens. Based on previous geological and geophysical observations and our numerical simulations, we categorized MCCs into four types: i) massifs, such as Menderes massif and SW Rhodope massif, ii) single large asymmetric MCC (classic MCC), such as Whipple mountains and Snake Range Mountains, Western US Cordillera and Crete and Cyclades, the Aegean, iii) multiple less evolved MCCs, such as Black Mountains turtlebacks, and iv) subsurface ';MCC', such as interpreted at the Adriatic coast. We also recognized two types of detachment faults: one being listric fault transitioning to a convex upward shear zone at greater depth and the other a shallow exhumed upward convex shear zone. Our new models successfully predict many MCCs and detachment faults known to date.

  13. Displacement calculations across a metamorphic core complex mylonite zone: Pinaleno Mountains, southeastern Arizona

    SciTech Connect

    Naruk, S.J.

    1987-07-01

    Minimum offset of 7 km across the Pinaleno Mountains metamorphic core complex is calculated by integrating the shear strains across the exposed width of the mylonite zone. The calculated displacement equals the offset on the associated detachment fault, estimated from offset marker beds. The method of determining displacement by strain integration may be directly applicable to many other metamorphic core complexes.

  14. The northern coast plutonic-metamorphic complex, southeastern Alaska and northwestern British Columbia

    NASA Astrophysics Data System (ADS)

    Brew, D. A.; Ford, A. B.

    The northern Coast plutonic-metamorphic complex is but one segment of an 8,000-km-long batholithic complex that extends the length of the North American Cordillera from Baja, California to the Aleutian Islands. Different segments of the Cordilleran batholithic complex have significantly different features. This report documents the main characteristics and inferred evolutionary history of the northern Coast plutonic-metamorphic complex that should facilitate comparisons between segments.

  15. The role of boron and fluids in high temperature, shallow level metamorphism of the Chugach Metamorphic Complex, Alaska

    NASA Technical Reports Server (NTRS)

    Sisson, V. B.; Leeman, W. P.

    1988-01-01

    The possible role of boron (B) involvement in granite equilibria and generation of melts during crustal metamorphism has been a focus of speculation in recent literature. Most of the evidence for such involvement derives from experimental data which implies that the addition of B will lower the temperature of the granite solidus. Also the presence of tourmaline has a minor effect on the temperature of the solidus. Further indirect evidence that B may be involved in partial melting processes is the observation that granulites are commonly depleted in B, whereas the B content of low grade metapelites can be high (up to 2000 ppm). Researchers' measurements of the whole-rock B contents of granulites from the Madras region, India are low, ranging from 0.4 to 2.6 ppm. Ahmad and Wilson suggest that B was mobilized in the fluid phase during granulite facies metamorphism of the Broken Hill Complex, Australia. Thus, it appears that during the amphibolite to granulite transition, B is systematically lost from metasediments. The B that is released will probably partition into the vapor phase and/or melt phase. Preliminary measurements imply that the boron content of rocks in the Chugach Metamorphic Complex is not sufficient to influence the processes of partial melting at low pressures.

  16. Deformation and metamorphism of Hamadat complex: Shear zones reshaping the lithosphere

    NASA Astrophysics Data System (ADS)

    Hassan, Mahmoud; Abu-Alam, Tamer; Stüwe, Kurt

    2015-04-01

    In the active tectonic regions, shear zones play an important role to re-configure the structure of the lithosphere. One of the largest pre-Mesozoic shear zones on the Earth is the Najd Fault System of the Arabian-Nubian Shield. Literature data record exhumation of medium-pressure metamorphic complexes due to the activity of the Najd Fault System during the Pan-African Orogeny where the shear zone surrounds the boundaries of metamorphic complexes. The Hamadat complex of Northwestern part of Saudi Arabia shows an exception where the metamorphic rocks appear to occur within the shear zone itself instead of being surrounded by branches of it. The Hamadat complex records peak metamorphic conditions of 505 - 700 °C at two ranges of pressure 8 - 11 and 12.5 - 16.5 kbar. These pressures are higher than those of most other metamorphic complexes associated and exhumed by the Najd Fault system. The higher pressure conditions of the Hamadat complex relatively to other complexes in the shield can be interpreted in two different ways. The documented pressure can be due to lithostatic pressure or shear stress (i.e. in part non-lithostatic). Assuming the lithostatic condition, these pressure ranges indicate that the Hamadat complex consists rocks from two different crustal level (i.e. 28 - 38 km and 43 - 58 km) which shows the importance of the shear zone to reshape the lithosphere by bring rocks from different crustal level into one metamorphic complex.

  17. Syn-extensional plutonism and peak metamorphism in the albion-raft river-grouse creek metamorphic core complex

    USGS Publications Warehouse

    Strickland, A.; Miller, E.L.; Wooden, J.L.; Kozdon, R.; Valley, J.W.

    2011-01-01

    The Cassia plutonic complex (CPC) is a group of variably deformed, Oligocene granitic plutons exposed in the lower plate of the Albion-Raft River- Grouse Creek (ARG) metamorphic core complex of Idaho and Utah. The plutons range from granodiorite to garnet-bearing, leucogranite, and during intrusion, sillimanite- grade peak metamorphism and ductile attenuation occurred in the country rocks and normal-sense, amphibolite-grade deformation took place along the Middle Mountain shear zone. U-Pb zircon geochronology from three variably deformed plutons exposed in the lower plate of the ARG metamorphic core complex revealed that each zircon is comprised of inherited cores (dominantly late Archean) and Oligocene igneous overgrowths. Within each pluton, a spread of concordant ages from the Oligocene zircon overgrowths is interpreted as zircon recycling within a long-lived magmatic system. The plutons of the CPC have very low negative whole rock ??Nd values of -26 to -35, and initial Sr values of 0.714 to 0.718, consistent with an ancient, crustal source. Oxygen isotope ratios of the Oligocene zircon overgrowths from the CPC have an average ??18O value of 5.40 ?? 0.63 permil (2SD, n = 65) with a slight trend towards higher ??18O values through time. The ??18O values of the inherited cores of the zircons are more variable at 5.93 ?? 1.51 permil (2SD, n = 29). Therefore, we interpret the plutons of the CPC as derived, at least in part, from melting Archean crust based on the isotope geochemistry. In situ partial melting of the exposed Archean basement that was intruded by the Oligocene plutons of the CPC is excluded as the source for the CPC based on field relationships, age and geochemistry. Correlations between Ti and Hf concentrations in zircons from the CPC suggest that the magmatic system may have become hotter (higher Ti concentration in zircon) and less evolved (lower Hf in zircon concentration) through time. Therefore, the CPC represents prolonged or episodic magmatism

  18. 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

  19. Koolen metamorphic complex, NE Russia: Implications for the tectonic evolution of the Bering Strait region

    NASA Astrophysics Data System (ADS)

    Akinin, Vyacheslav V.; Gelman, Mikhail L.; Sedov, Boris M.; Amato, Jeffrey M.; Millwer, Elizabeth L.; Toro, Jaime; Calvert, Andrew T.; Fantini, Riccardo M.; Wright, James E.; Natal'in, Boris A.

    Structural culminations of midcrustal metamorphic rocks are found on both sides of the Bering Strait in Alaska and Russia and occur within a magmatic belt of Cretaceous age. Geologic mapping in the Koolen Lake-Lavrentia Bay region of the Chukchi Peninsula, Russia, outlines the basic relations between deformation, metamorphism, and magmatism in one of these structural culminations, the Koolen metamorphic complex. Here, a 10-15 km-thick, southwest dipping structural succession of gneisses and high-grade metamorphic rocks is exposed. The succession consists of a lower sequence of granitic gneisses and an upper sequence of biotite-rich gneisses, quartzofeldspathic gneisses, lesser amphibolite and marble, and gneisses and schists with an increasing abundance of intercalated marble and calc-silicate units toward the top. All rocks are strongly foliated and exhibit north-south trending stretching lineations. Deformation occurred during sillimanite-grade metamorphism concurrent with partial melting of the crust. Metamorphic conditions varied from 7 to 3 kbar and from 700°C-500°C. Three fractions of monazite from a deformed pegmatite yielded ages of 104 Ma. Igneous monazite from undeformed biotite granite yielded a U-Pb age of 94 Ma, indicating peak metamorphism and deformation is Cretaceous. Relations in the Koolen complex are similar to those in the Kigluaik gneiss dome, Seward Peninsula, Alaska, where upper amphibolite to granulite facies metamorphism and deformation occurred between ~105 and 90 Ma. Our findings,together with regional relations, suggest that wholesale crustal extension or extensional collapse of the crust affected this region, perhaps during Pacific-ward migration of subduction. The results do not support large amounts of east-west shortening between North America and Russia predicted by poles of rotation related to opening of the North Atlantic in the Late Cretaceous and Tertiary.

  20. Misoriented faults in exhumed metamorphic complexes: Rule or exception?

    NASA Astrophysics Data System (ADS)

    Massironi, Matteo; Bistacchi, Andrea; Menegon, Luca

    2011-07-01

    Low angle normal faults and other weak faults are common in the metamorphic core of collisional orogens. They frequently show a phyllosilicate-rich mylonitic foliation that was reactivated under brittle conditions. Experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on shear failure and frictional sliding, eventually inhibiting the nucleation and propagation of new Andersonian shear fractures and favoring the localization of brittle failure along the pre-existing foliations. Metamorphic phyllosilicate-rich rocks may show a friction coefficient varying from 0.6, at high angles to the foliation, to 0.2-0.4, for shear along the inherited foliation. To test the influence of mechanical anisotropies on the development of non-Andersonian faults, we have applied a modified slip tendency analysis to three misoriented phyllosilicate-rich faults of the European Alps. The analysis accounts for anisotropy in friction coefficients, and has been named "Anisotropic Slip Tendency analysis". Here we show that brittle deformation along misoriented phyllosilicate-rich foliations is more probable than the development of new Andersonian faults. The presence of a well developed network of weak, phyllosilicate-rich faults may influence the overall structural style and mechanical properties of the brittle lithosphere in collisional orogens.

  1. 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.

  2. 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.

  3. Formation of Neoproterozoic metamorphic complex during oblique convergence (Eastern Desert, Egypt)

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Wallbrecher, E.; Khudeir, A. A.; Abu el Ela, F.; Dallmeyer, D. R.

    1996-10-01

    Major portions of the Pan-African Orogen in the Eastern Desert of Egypt were formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in contrast to the structurally overlying nappes which include ophiolitic melanges and island-arc volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal parts and west to southwest directed imbrication in the external parts of the orogen. Structures related to exhumation of the metamorphic core complexes partition into different displacement paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of the domes. North and south dipping low-angle normal faults developed along the northern and southern margins of the domes and form extensional bridges between them. {40Ar}/{39Ar} ages obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of 595.9±0.5 and 588.2±0.3 Ma. The synchronous activity of strike-slip and normal faults suggests a regional east-west shortening which was accomodated by deep-level basal decollement beneath the metamorphic core complexes and a coeval northwest-southeast, orogen-parallel extension. This extension was accompanied by intramontane molasse sedimentation and emplacement of calc-alkaline plutons. Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening of the crust, the authors favour a model which calls for enhanced heat flow along the Najd fault system which would have enabled the formation of syn-extensional plutonism and triggered the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated within the Najd wrench corridor and

  4. Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

    PubMed

    Abers, Geoffrey A; Ferris, Aaron; Craig, Mitchell; Davies, Hugh; Lerner-Lam, Arthur L; Mutter, John C; Taylor, Brian

    2002-08-22

    In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5-10 km Myr(-1), and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension. PMID:12192406

  5. Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

    USGS Publications Warehouse

    Rye, R.O.; Schuiling, R.D.; Rye, D.M.; Jansen, J.B.H.

    1976-01-01

    At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700??C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The ??18O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated ??18OH2O values of the metamorphic fluids in the schist-rich zones decrease from about 15??? in the lower grades to an average of about 8.5??? in the migmatite. The ??D values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated ??DH2O values for the metamorphic fluid decrease from -5??? in the glaucophane zone to an average of about -70??? in the migmatite. The ??D values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. The??18O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The ??D values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of ??18O values which depend upon metamorphic grade. Below the 540??C isograd very steep ??18O gradients at the margins and large ??18O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540??C isograd lower ??18O values occur in

  6. Reconnaissance and economic geology of Copper Mountain metamorphic complex, Owl Creek Mountains, Wyoming

    SciTech Connect

    Hausel, W.D.

    1983-08-01

    The Copper Mountain metamorphic complex lies within a westerly trending belt of Precambrian exposures known as the Owl Creek Mountains uplift. The metamorphic complex at Copper Mountain is part of a larger complex known as the Owl Creek Mountains greenstone belt. Until more detailed mapping and petrographic studies can be completed, the Copper Mountain area is best referred to as a complex, even though it has some characteristics of a greestone belt. At least three episodes of Precambrian deformation have affected the supracrustals, and two have disturbed the granites. The final Precambrian deformation event was preceded by a weak thermal event expressed by retrogressive metamorphism and restricted metasomatic alteration. During this event, a second phase of pegmatization was accompanied by hydrothermal solutions. During the Laramide orogeny, Copper Mountain was again modified by deformation. Laramide deformation produced complex gravity faults and keystone grabens. Uranium deposits were formed following major Laramide deformation. The genesis of these deposits is attributable to either the leaching of granites or the leaching of overlying tuffaceous sediments during the Tertiary. Production of metals and industrial minerals has been limited, although some gold, copper, silver, tungsten, beryl, feldspar, and lithium ore have been shipped from Copper Mountain. A large amount of uranium was produced from the Copper Mountain district in the 1950s.

  7. Metamorphic and thermal evolution of large contact aureoles - lessons from the Bushveld Igneous Complex

    NASA Astrophysics Data System (ADS)

    Waters, D.

    2012-04-01

    Large igneous intrusions crystallise, cool, and transfer heat out into their host rocks. The thermal structure of the resulting aureole can be mapped as a series of assemblage zones and isograds, and can in principle be modelled on the assumption that heat transfer is dominantly by conduction. The local peak of contact metamorphism occurs later in time with increasing distance from the igneous contact. The importance of fluids as a metamorphic/metasomatic agent or heat transfer mechanism depends on volatile contents of magma and country rock, and on the geometry of the intrusion. Many of these features are spectacularly illustrated by the aureole beneath the mafic Rustenburg Layered Suite of the Bushveld Complex, which was emplaced at ca. 2060 Ma sub-concordantly into the shale-quartzite succession of the Pretoria Group in the Transvaal Basin. The layered suite reaches a thickness of at least 8 km, and the metamorphic aureole extends 4 km or more downwards into the "floor" of the intrusion. The great extent and relative absence of deformation make this a remarkable natural laboratory for studying the fundamental processes of metamorphism. In quantifying the thermal history, however, a number of second-order factors need to be taken into account. The first relates to the markedly different thermal properties of the major quartzite and shale units, and the second to the importance of endothermic metamorphic reactions in shale units relative to the quartzites. Further insights into metamorphic processes arise from the exquisite detail of poikiloblast growth microstructures preserved in graphite-poor metapelites of the Timeball Hill and Silverton Formations, 2.5 to 3.5 km beneath the igneous contact. These allow a detailed reconstruction of the time sequence of mineral growth and replacement, revealing a marked overlap of the growth intervals of porphyroblastic staurolite, cordierite, biotite, garnet and andalusite at the expense of muscovite, chlorite and chloritoid

  8. 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.

  9. 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

  10. 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.

  11. Metamorphic core complex formation by density inversion and lower-crust extrusion.

    PubMed

    Martinez, F; Goodliffe, A M; Taylor, B

    2001-06-21

    Metamorphic core complexes are domal uplifts of metamorphic and plutonic rocks bounded by shear zones that separate them from unmetamorphosed cover rocks. Interpretations of how these features form are varied and controversial, and include models involving extension on low-angle normal faults, plutonic intrusions and flexural rotation of initially high-angle normal faults. The D'Entrecasteaux islands of Papua New Guinea are actively forming metamorphic core complexes located within a continental rift that laterally evolves to sea-floor spreading. The continental rifting is recent (since approximately 6 Myr ago), seismogenic and occurring at a rapid rate ( approximately 25 mm yr-1). Here we present evidence-based on isostatic modelling, geological data and heat-flow measurements-that the D'Entrecasteaux core complexes accommodate extension through the vertical extrusion of ductile lower-crust material, driven by a crustal density inversion. Although buoyant extrusion is accentuated in this region by the geological structure present-which consists of dense ophiolite overlaying less-dense continental crust-this mechanism may be generally applicable to regions where thermal expansion lowers crustal density with depth. PMID:11418853

  12. Sequential Development of Interfering Metamorphic Core Complexes: Numerical Experiments and Comparison to the Cyclades, Greece

    NASA Astrophysics Data System (ADS)

    Tirel, C.; Gautier, P.; van Hinsbergen, D.; Wortel, R.

    2007-12-01

    The Cycladic extensional province (Greece) contains classical examples of metamorphic core complexes (MCCs), where exhumation was accommodated along multiple interfering and/or sequentially developed syn- and antithetic extensional detachment zones. Previous studies on the development of MCCs did not take into account the possible interference between multiple and closely spaced MCCs. In the present study, we have performed new lithosphere-scale experiments in which the deformation is not a priori localized so as to explore the conditions of the development of several MCCs in a direction parallel to extension. In a narrow range of conditions, MCCs are closely spaced, interfere with each other, and develop in sequence. From a comparison between numerical results and geological observations, we find that the Cyclades metamorphic core complexes are in good agreement with the model in terms of Moho geometry and depth, kinematic and structural history, timing and duration of core complex formation and metamorphic history. We infer that, for Cycladic MCC-type to develop, an initial crustal thickness prior to the onset of post-orogenic extension between 40 and 44 km, a boundary velocity close to 2 cm/yr and an initial thermal lithospheric thickness of about 60 km are required. The latter may be explained by a significant heating due to delamination of subducting continental crust or vigorous small-scale thermal convection.

  13. Metamorphic evolution of the Karakaya Complex in northern Turkey based on phyllosilicate mineralogy

    NASA Astrophysics Data System (ADS)

    Tetiker, Sema; Yalçın, Hüseyin; Bozkaya, Ömer; Göncüoğlu, M. Cemal

    2015-04-01

    The Triassic Karakaya Complex (KC) of the Sakarya Composite Terrane in northern Turkey is traditionally subdivided into two units. The Lower Karakaya Complex (LKC) consists of a tectonic mélange with blocks of metabasic rocks, metacarbonates, meta-arenites and metapelites that have been affected by high pressure/low temperature metamorphism. It is followed by a low pressure/low temperature metamorphic overprint; the latter is the only metamorphic event in some tectonic slivers of the LKC. The Upper Karakaya Complex (UKC) units are primarily composed of diagenetic to low-grade metamorphic rocks, comprising Late Permian and Triassic cherts and blocks of OIB-type volcanic rocks interfingering with Anisian limestones. LKC slide-blocks of variable sizes are frequently observed within the UKC. Phyllosilicates of LKC and UKC were examined for their abundance, crystallinity, polytype and b cell dimension. Trioctahedral chamositic chlorites have II b polytype and phengites 2 M 1 polytypes in the LKC units and 2 M 1 + 1 M + 1 M d polytypes of phengitic dioctahedral illites in the UKC units. Kübler index data correspond to the low anchizone and epizone for the LKC units, and to the high diagenesis-low anchizone, and in part to the epizone for the UKC units. The b values of illites are consistent with a high-pressure facies series for the LKC, but only intermediate-pressure facies for the UKC. According to textural features, mineral paragenesis, clay transformations, index minerals, and b values, the lower-middle parts of the LKC represent an accretional tectonic setting, whereas the UKC units reflect pressure temperature conditions of an extensional basin affected by high heat flow.

  14. Subsidence in Gorontalo Bay, Sulawesi (Indonesia) and metamorphic core complex exhumation on land

    NASA Astrophysics Data System (ADS)

    Pezzati, Giovanni; Hennig, Juliane; Advokaat, Eldert; Hall, Robert; Burgess, Peter; Perez-Gussinye, Marta

    2015-04-01

    Gorontalo Bay is a semi-enclosed sea between the North and East Arms of Sulawesi. It is surrounded by land on three sides, separating a northern volcanic province from metamorphic rocks to the south and west, and ophiolites to the southeast. In the western part of Gorontalo Bay there are two sub-basins: the northern Tomini Basin and the southern Poso Basin, which have different histories. This study presents a new geological interpretation of western Gorontalo Bay, based on recent multibeam and 2D seismic data and field observations on land. In Tomini Basin six major seismic sequences (Units A to F) have a total thickness of more than 5 sec TWT. Ages are based on correlation with events on land. A major unconformity above Basement Unit A is interpreted to be the result of an Early Miocene collision. Unit A subsided from the Early Miocene, with deposition of Units B and C largely in a deep marine environment. There was regional uplift in the Middle Miocene followed by renewed subsidence resulting in shallow marine depositional environments in which carbonate platforms developed (Units D-E). Subsidence accelerated during the deposition of Unit E, causing back-stepping of the shelf edge, formation of pinnacle reefs and then drowning of the carbonate platforms, leading to the present depths of 2 km in the basin centre with a thin clastic cover (Unit F). North of Tomini Basin, the Malino Metamorphic Complex exposes strongly deformed mid-crustal rocks which record Middle Miocene extension accommodated by low angle shear zones. A second phase of rapid uplift and extension recorded in these rocks occurred in the Pliocene-Pleistocene, and was accommodated by high angle normal faulting. Poso Basin is younger than Tomini Basin and it occupies the southern part of western Gorontalo Bay. The deeper part of its sedimentary sequence is probably the time equivalent of Unit D in Tomini Basin. Immediately south of Poso Basin, on land, is a large metamorphic core complex. Seismic

  15. 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.; Preface by: 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

  16. From accommodation zones to metamorphic core complexes: Tracking the progressive development of major normal fault systems

    SciTech Connect

    Faulds, J.E. . Dept. of Geology)

    1992-01-01

    The along-strike dimension in rifted continental crust is critical to assessing models of continental extension because individual normal faults or fault systems can potentially be traced from their tips in accommodation zones to their culminations in metamorphic core complexes. Accommodation zones and the linkages between the zones and core complexes have not been thoroughly studied or incorporated extensively into models of continental extension. Regionally extensive, gently dipping normal faults (i.e., detachment faults) that surface in metamorphic core complexes terminate and flip polarity in accommodation zones. Diametrical lateral transport of upper-plate rocks in positively dipping detachment terranes should presumably induce strike-slip faulting on segments of accommodation zones paralleling the extension direction. Most accommodation zones correspond, however, to belts of intermeshing conjugate normal faults with little strike-slip faulting. Normal faults simply terminate along-strike in the zones with little, if any, transfer of slip to strike-slip faults. Decreases in cumulative strain within individual normal fault systems toward some accommodation zones cannot alone account for the lack of strike-slip faulting. These findings pose a serious challenge to generally accepted notions of large-magnitude, lateral motion of parts of detachment terranes. Large-scale lateral translations of rifted continental crust may be governed more by discrete axes of extension than by detachment geometries. The dovetail-like interfingering of conjugate normal fault systems and attendant tilt-block domains observed in some accommodation zones (e.g., Colorado River extensional corridor, US) does suggest, however, that at least some major normal faults projecting into the zones from metamorphic core complexes have listric geometries that flatten out at relatively shallow depths.

  17. Nature and origin of seismic reflection fabric, Ruby-East Humboldt Metamorphic Core Complex, Nevada

    NASA Astrophysics Data System (ADS)

    Valasek, P. A.; Snoke, A. W.; Hurich, C. A.; Smithson, S. B.

    1989-04-01

    Seismic reflection profiling across exposed upper and middle crustal rocks of the Ruby-East Humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during Tertiary extensional deformation. The goals of this study were to trace a Tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex. Reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks. Constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections. East and southwest dipping reflectors in the seismic section appear to correlate with exposed, opposing dipping mylonitic foliation domains. The opposing dips of the mylonitic layering may reflect warping of the normal-sense shear zone during tectonic exhumation coupled with an overall anastomosing character. Deeper in the section, a heterogeneous reflection character correlates with increases in velocity shown from wide-angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad-scale pure shear and localized simple shear. Lower crustal rocks apparently achieved granulite facies metamorphism during extension. A maximum of 6-8 km of mafic material could have been added to the crust during Cenozoic extension. The Moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite. Reflections from the Moho show no significant upwarping of the base of the crust beneath the core complex. This suggests that the lower-crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 Ma which was responsible for the exhumation of the

  18. "High-grade burial metamorphism of sedimentary mélange, Shoo Fly Complex, central Sierra Nevada, California"

    NASA Astrophysics Data System (ADS)

    Mendoza, Y.; Wakabayashi, J.

    2013-12-01

    The Shoo Fly Complex, California is a subduction complex metamorphosed at lower greenschist facies in much of the northern Sierra Nevada. Central Sierra Nevada exposures include higher grade assemblages. Previous studies have interpreted the higher grade rocks as gneissic granitoids representing the roots of a Paleozoic arc. Recent field work in the North Fork Mokelumne River drainage, shows that high-grade and low-grade metamorphic rocks were derived from similar subduction complex protoliths. The Shoo Fly in this region consists of mostly phyllite (metasiltstone, metasandstone, metachert), with some metabasite, and metaultramafic blocks. There is a metamorphic gradient from west to east in the field area, transitioning from sub to lower greenschist facies (white mica only) to middle and upper green schist facies (biotite) within the phyllites to amphibolite/upper amphibolite/granulite grade mica schists, gneisses, and amphibolites This gradient occurs across a zone about 1.5 km wide and this gradient is about 5 km west of the contact between the Shoo Fly Complex and plutons of the Sierra Nevada batholith. The higher-grade rocks do not have an apparent west-east metamorphic gradient. Accordingly the high-grade metamorphism does not appear to be a consequence of either contact metamorphism or raised regional geothermal gradients connected with the batholith. This conclusion is consistent with the fact that published metamorphic ages from probable correlative rocks within the central Sierra are much older than the Sierra Nevada batholith. Protoliths for the higher grade rocks appear identical to the lower grade rocks, for metaclastic rocks dominate with subordinate metacherts, metabasites, and metaultramafic rocks. The latter are represented by tremolite-talc schists. In the lower grade rocks some of the metabasite and metaultramafic blocks exhibit a higher grade of metamorphism than the surrounding metaclastic rocks and metacherts. Amphibolite and tremolite schist

  19. Kinematic interpretation of the 3D shapes of metamorphic core complexes

    NASA Astrophysics Data System (ADS)

    Le Pourhiet, Laetitia; Huet, Benjamin; May, Dave A.; Labrousse, Loic; Jolivet, Laurent

    2012-09-01

    Metamorphic Core Complexes form dome shaped structures in which the ductile crust is exhumed beneath a detachment fault. The 3D dome geometry, inferred by mapping the schistosity in the exhumed crust, can be either elongated normal to the stretching direction or along it. In the first case, the domes are interpreted as having formed during extension. However, in the second case, they are interpreted either as strike-slip, transpressive or constrictive extensional structures, depending on the geodynamic context. Numerical models of metamorphic core complexes published to date are all two-dimensional and therefore, theoretically only apply to domes which are elongated normal to the stretching direction. Here, we explore by means of 3D thermomechanical modeling, the impact of 3D kinematic extensional boundary conditions on the shape of metamorphic core complexes. We examine the impact of a transtensional step over and of horsetail splay fault kinematics on the dynamics of exhumation, finite strain and P-T paths, and compare them to cylindrical 3D models. We show, for the first time, that domes formed in transtensional step over, or at the tip of propagating strike-slip faults, display a finite strain field which can be interpreted as characteristic of a transpressive domes, although no shortening was applied in the far-field. Applying our models to the Cyclades, we propose that the coeval formation of domes elongated normal and parallel to the stretching during the Miocene can be the result of horsetail splay fault kinematics, which could correspond to the formation of a tear in the Aegean slab.

  20. The tectono-metamorphic evolution of gneiss complexes in the Middle Urals, Russia: a reappraisal

    NASA Astrophysics Data System (ADS)

    Echtler, H. P.; Ivanov, K. S.; Ronkin, Y. L.; Karsten, L. A.; Hetzel, R.; Noskov, A. G.

    1997-07-01

    The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted part of the late Palaeozoic collisional belt. This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. The Main Uralian Fault zone (MUF), the east-dipping suture of the orogen containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex in the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed mélange zone. Tectonic and isotopic investigations suggest the following stages for the evolution of the Sysert Complex: (a) pre-orogenic rifting and magmatism during Ordovician and Silurian times; (b) oceanic closure, island arc formation related to convergence and subduction during Devonian times; (c) major ductile deformation under amphibolite facies conditions related to NW-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontinental tectonics during Permian times; and (e) closing of isotope systems related to cooling and the end of orogenic shortening through Triassic times. The Ufaley Complex, in the footwall of the MUF, is interpreted as an east-dipping crustal stack that records an amphibolite facies Uralian metamorphism. Lithologically the complex can be divided into pre-orogenic European basement (West Ufaley) and intensely deformed Palaeozoic metasediments and amphibolites (East Ufaley). High-pressure relics in the East Ufaley Complex are interpreted to be the result of subduction, whereas intense ductile deformation is related to overthrusting onto West Ufaley. The West Ufaley Complex is composed of gneisses, amphibolites, migmatites and granitic intrusions and has been thrust onto Devonian limestones along a major shear zone. In both Sysert and Ufaley Complexes, NW

  1. 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.

  2. Erosion controls on the metamorphic core complex dynamics and its relationship with syn- rift basin evolution

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Zhao, Liang

    2015-04-01

    The wide rifting evolves towards asymmetric extensional thinning of the entire crust and development of different characteristic features such as basins, half-grabens and metamorphic core complexes (MCC). In this context, formation of supra-detachment basins is also a common feature, along with the exhumation of metamorphic rocks and considerable displacements along the hanging wall. Initiation, geometry and mechanisms of metamorphic core complexes have been already largely debated on the basis field observations, analog and numerical models. For example, it has been well demonstrated that strain softening favors asymmetric deformation and accounts for different styles of brittle and ductile strain localization. However, the temporal and spatial relations between the dome formation and basin evolution are still poorly understood. In particular, most of the existing numerical models predict a topographical depression above the metamorphic dome, whereas in nature dome formation often corresponds to a topographical uplift. To explain these phenomena, we have integrated surface erosion, sedimentary processes and strain softening into a state-of-the-art 2-D numerical thermo-mechanical model of MCC development. In the numerical experiments, we first reproduce formation of a univergent MCC by implementing strain softening and testing a large spectrum of lithospheric structures. In the next series of experiments we apply erosion/sedimentation and test model sensitivity to different erosion parameters. The results show two distinctive stages of MCC dynamics and syn-rift basin development. One single broad basin forms above the dome and is divided onto an inactive basin located at the distal detachment and an active supradetachment basin that deepens with further extension, characterized by crustal necking and dome amplificationduring the MCC formation. It is noteworthy that without strain softening, erosion at of the rift flanks mayresult in complete burial of the dome

  3. 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

  4. New U-Pb Age and Trace Element Composition of Young Metamorphic Zircon Rims from the UHP Tso Morari Complex, NW Himalaya, Distinguishes Peak from Retrograde Metamorphism

    NASA Astrophysics Data System (ADS)

    Leech, M. L.; Coble, M. A.; Singh, S.; Guillot, S.; Jain, A. K.

    2014-12-01

    The ultrahigh-pressure (UHP) Tso Morari Complex (TMC) sits in the footwall of the Indus-Yarlung suture zone in the NW Himalaya. The timing of metamorphism during subduction and exhumation in the complex is critical to constraining the age of the India-Asia collision. de Sigoyer et al. (2000) and Leech et al. (2005) reported mean U-Pb ages for thin outer rims of sectioned zircon between 55 ± 6 Ma and 53.3 ± 0.7 Ma, respectively, for the age of peak UHP through retrograde metamorphism, and Leech et al. (2005) used these data to calculate the minimum age for the start of continental subduction at 57 ± 1 Ma. Recently published results for the TMC have reignited debate on the age of metamorphism and thus, the timing of India-Asia collision. We used the same TM38 sample analyzed for the results described in Leech et al. (2005) and performed new SIMS U-Pb depth-profiling analyses to target only the outermost ~1.5 micron rims of zircon. Our results yield a mean age of 44.9 ± 0.7 Ma; adjacent spots for REE analyses yielded positive, enriched HREE profiles with negative Eu anomalies and corresponding Ti-in-zircon temperatures of ~550° to 680° C. Sharp boundaries between zircon domains are clearly resolved with CL and BSE imaging of TM38 zircons, and there is a large age difference between rims and protolith core ages; any mixing during depth-profiling through rims is clear. The positive HREE profiles imply the period of zircon growth in the TMC at c. 45 Ma to be retrograde. We suggest that the 47-43 Ma peak ages and flat heavy REE profiles with no Eu anomaly recently reported by Donaldson et al. (2013) on sectioned zircons, and interpreted as the age of UHP metamorphism of the TMC, may actually represent mixing between zircon rims and cores. The Leech et al. (2005) collision age of 57 ± 1 Ma assumed the TMC represents the leading edge of India. However, numerical modeling of Warren et al. (2008) suggests all exhumed material is derived from the central part of the pro

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

    SciTech Connect

    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 exhibit 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).

  6. Late Cretaceous extensional unroofing in the Funeral Mountains metamorphic core complex, California

    SciTech Connect

    Applegate, J.D.R.; Hodges, K.V. ); Walker, J.D. )

    1992-06-01

    New filed and geochronologic data document the existence of Late Cretaceous extensional structures in the Death Valley region, California-Nevada. The authors have mapped two major, low-angle, ductile shear zones that omit stratigraphy in the footwall of the Funeral Mountains metamorphic core complex. Intervening strata have been strongly attenuated. Although stratigraphic offset across the shear zones is only 1.5 km, the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater. The timing of shear-zone formation, attenuation, and subsequent northwest-vergent folding is constrained by U-Pb geochronology on (1) prekinematic or synkinematic and (2) postkinematic pegmatites. Deformation was taking place by 72 Ma and had ended by 70 Ma. These results support earlier petrologic and geochronologic data that suggested substantial unroofing of the Funeral Mountains in Late Cretaceous time and add to a growing body of evidence for widespread Mesozoic extension in the hinterland of the Sevier thrust belt.

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

    USGS Publications Warehouse

    Blake, M.C., Jr.; 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.

  8. The Ikaria high-temperature Metamorphic Core Complex (Cyclades, Greece): Geometry, kinematics and thermal structure

    NASA Astrophysics Data System (ADS)

    Beaudoin, Alexandre; Augier, Romain; Laurent, Valentin; Jolivet, Laurent; Lahfid, Abdeltif; Bosse, Valérie; Arbaret, Laurent; Rabillard, Aurélien; Menant, Armel

    2015-12-01

    This work attempted at clarifying the structure of Ikaria using primarily intensive geological mapping combined with structural analysis and a geothermometry approach of Raman spectrometry of carbonaceous material. Foliation over the whole island defines a structural dome cored by high-grade to partially molten rocks. Its exhumation was completed by two top-to-the-N ductile extensional shear zones, operating in the ductile and then the brittle fields, through a single extensional event coeval with progressive strain localization. The thermal structure of the dome with regard to position of ductile shear zones was retrieved using the Raman spectroscopy of carbonaceous material. Peak-metamorphic temperatures range from 390 °C in the upper parts of the structure down to 625 °C in the core of the dome in the vicinity of migmatites and S-type granite. Pioneer in situ U-Th-Pb analyses on monazite performed on the leucosome parts of these rock yielded a 15.7 ± 0.2 Ma age. Ikaria Island thus completes the series of Miocene migmatite-cored Metamorphic Core Complex in the central part of the Aegean domain where a genuine high-temperature zone can be defined as the central Aegean HT zone. There, the extreme stretching of the continental crust is associated with dominantly top-to-the-N kinematics.

  9. The Palu Metamorphic Complex, NW Sulawesi, Indonesia: Origin and evolution of a young metamorphic terrane with links to Gondwana and Sundaland

    NASA Astrophysics Data System (ADS)

    van Leeuwen, Theo; Allen, Charlotte M.; Elburg, Marlina; Massonne, Hans-Joachim; Palin, J. Michael; Hennig, Juliane

    2016-01-01

    The Palu Metamorphic Complex (PMC) is exposed in a late Cenozoic orogenic belt in NW Sulawesi, Indonesia. It is a composite terrane comprising a gneiss unit of Gondwana origin, a schist unit composed of meta-sediments deposited along the SE Sundaland margin in the Late Cretaceous and Early Tertiary, and one or more slivers of amphibolite with oceanic crust characteristics. The gneiss unit forms part of the West Sulawesi block underlying the northern and central sections of the Western Sulawesi Province. The presence of Late Triassic granitoids and recycled Proterozoic zircons in this unit combined with its isotopic signature suggests that the West Sulawesi block has its origin in the New Guinea margin from which it rifted in the late Mesozoic. It docked with Sundaland sometime during the Late Cretaceous. U-Th-Pb dating results for monazite suggest that another continental fragment may have collided with the Sundaland margin in the earliest Miocene. High-pressure (HP) and ultrahigh-pressure (UHP) rocks (granulite, peridotite, eclogite) are found as tectonic slices within the PMC, mostly along the Palu-Koro Fault Zone, a major strike-slip fault that cuts the complex. Mineralogical and textural features suggest that some of these rocks resided at depths of 60-120 km during a part of their histories. Thermochronological data (U-Th-Pb zircon and 40Ar/39Ar) from the metamorphic rocks indicate a latest Miocene to mid-Pliocene metamorphic event, which was accompanied by widespread granitoid magmatism and took place in an extensional tectonic setting. It caused recrystallization of, and new overgrowths on, pre-existing zircon crystals, and produced andalusite-cordierite-sillimanite-staurolite assemblages in pelitic protoliths, indicating HT-LP (Buchan-type) metamorphism. The PMC was exhumed as a core complex at moderate rates (c. 0.7-1.0 mm/yr) accompanied by rapid cooling in the Plio-Pleistocene. Some of the UHP rocks were transported to the surface at significantly higher

  10. Time and duration of metamorphism and exhumation of the central Rhodopian core complex, Bulgaria

    NASA Astrophysics Data System (ADS)

    Ovtcharova, M.; von Quadt, A.; Peytcheva, I.; Neubauer, F.; Heinrich, C. A.; Kaiser, M.

    2003-04-01

    The evolution of central Rhodopian dome (Bulgaria) is interpreted in terms of an extensional collapse of thickened crust (Ivanov at al., 2000). U-Pb isotope dating (single Zr and Mnz), Rb-Sr (W.R., Bt and Ap) and Ar-Ar (on Bt) were carried out on different rocks from the central Rhodope, Bulgaria, to constrain the timing and duration of the metamorphism and exhumation of the core complex. The beginning of extensional stage is marked by intrusion of earliest non-penetratively deformed granite bodies at 53Ma (U-Pb on single Zr and Mnz). The late Alpine extensional evolution of the massif is marked by a detachment system connected with exhumation of the migmatites in the core part of the dome (lower plate). U-Pb analyses on Mnz and Zr from mesosome and discordant leucosome yield a Variscan protolith age of the gneiss (311 Ma) and Eocene age (37Ma) of crystallization of the newly formed anatectic melt that corresponds with the peak of the Alpine metamorphic event (P 4.5-6kbar and T 720-750^oC; Georgieva et al., 2002). Rb-Sr mineral system of the weakly deformed gneisses from lower plate of the core complex gives evidence for a cooling age of 34.5±0.34Ma. This result is confirmed by Ar-Ar on Bt from the same rock: 35.5±0.4Ma. Ar-Ar data on biotite from gneisses of the upper plate yield an age of 34.9±0.6Ma. The same age is reflected by an Rb-Sr isochron (W.R., Bt and Ap) of 35.22±0.35Ma. The post-collisional extension was followed by graben depressions filled with sediments of Eocene-Oligocene age and active volcanism and ore mineralization (Zn-Pb and Cu-Pb-Zn ore deposits). Connected with the most intensively "stretched" sections of the extensional system is emplacement of rhyolitic dikes at 32.8±0.41Ma (U-Pb on single Zr, Xe). The available data constrain narrow time bracket between timing of high-grade metamorphism event (37Ma, >600^oC), cooling (35Ma, 300ºC) of the core complex and volcanic activity (32Ma) that corresponds with rapid exhumation tectonic regime

  11. Zircon SHRIMP U-Pb dating of metamorphic complexes in the conjunction of the Greater and Lesser Xing'an ranges, NE China: Timing of formation and metamorphism and tectonic implications

    NASA Astrophysics Data System (ADS)

    Miao, Laicheng; Zhang, Fuqin; Zhu, Mingshuai; Liu, Dunyi

    2015-12-01

    Metamorphic complexes, including the Luomahu, Xinkailing and Fengshuigouhe groups, are scattered in the conjunction of the Greater and the Lesser Xing'an ranges, NE China, and have long been interpreted to represent the Precambrian basement of the so-called "Xing'an" and "Songnen" blocks although reliable evidence is lacking. Thin-section examination and mineral assemblage indicate that the protoliths of these metamorphic rocks are mainly meta-sedimentary and meta-volcanic rocks and have experienced greenschist- to amphibolite-facies metamorphism and strong deformation. Zircon SHRIMP U-Pb dating results of the metamorphic complexes and associated pre- or syn-kinematic veins constrained the formation and metamorphic ages of the Luomahuo Group at 175 ± 3 Ma and 159 ± 3 Ma, the Xingkailing Group at 200 ± 2 Ma and 158 ± 3 Ma, and the Fengshuigouhe Group at between 230-310 Ma and 170 ± 3 Ma, respectively. Additionally, all these rocks contain some Late Paleozoic detrital or inherited zircons with ages between 290-390 Ma, but no Precambrian, even if Early Paleozoic, age information was detected from these metamorphic rocks. These new data suggest that the metamorphic complexes were formed during Mesozoic-Late Paleozoic period, rather than in Precambrian as previously inferred, and that all of them underwent metamorphism and deformation during Middle Jurassic between 170-160 Ma. Consequently, these metamorphic complexes are not basement rocks of the so-called Precambrian continental blocks. Instead, there are likely metamorphosed Late Paleozoic-Mesozoic accretionary complexes and/or arc terranes. These results, in combination with published data, illustrate an overall young trend from north to south in the tectonic evolution of the northern Xing'an region. Significantly, the Jurassic (170-160 Ma) metamorphism and deformation event firstly identified by this study from the metamorphic complexes likely recorded the coinstantaneous Mongol-Okhotsk collisional and

  12. Petrology, geochemistry, and metamorphic evolution of meta-sedimentary rocks in the Diancang Shan-Ailao Shan metamorphic complex, Southeastern Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Wang, Fang; Liu, Fulai; Liu, Pinghua; Shi, Jianrong; Cai, Jia

    2016-07-01

    Meta-sedimentary rocks are widely distributed within the Diancang Shan-Ailao Shan metamorphic complex in the Southeastern Tibetan Plateau. Detailed geochemical analyses show that all of them have similar geochemical features. They are enriched in light rare-earth elements (LREEs) and depleted in heavy rare-earth elements (HREEs), with moderately negative Eu anomalies (Eu/Eu∗ = 0.55-0.75). Major and trace element compositions for the meta-sedimentary rocks suggest that the protoliths were probably claystone, siltstone, and greywacke and deposited in an active continental margin. Garnet porphyroblasts in meta-sedimentary rocks have distinct compositional zonation from core to rim. The zonation of garnet in St-Ky-Grt-Bt-Ms schist indicates an increasing P-T trend during garnet growth. In contrast, garnets from (Sil)-Grt-Bt paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformations and P-T estimates using conventional geothermobarometers and pseudosection calculations, four metamorphic stages have been determined, including an early prograde metamorphic stage (M1), a peak amphibolite-granulite facies metamorphic stage (M2), a near-isothermal decompression stage (M3), and a late amphibolites-facies retrograde stage (M4). The relic assemblage of Ms + St ± Ky ± Bt ± Kfs + Qz preserved as inclusions in garnet porphyroblasts of the meta-sedimentary rocks belongs to prograde (M1) stage and records P-T conditions of 560-590 °C and 5.5-6.3 kb. Matrix mineral assemblages of Grt + Bt + Ky/Sil + Pl + Qz and Grt + Bt ± Sil + Pl ± Kfs + Qz formed at peak (M2) stage yield P-T conditions of 720-760 °C and 8.0-9.3 kb. M3 is characterized by decompression reactions, dehydration melting of assemblages that include hydrous minerals (e.g., biotite), and partial melting of felsic minerals. The retrograde assemblages is Grt + Bt + Sil + Pl + Qz formed at 650-760 °C and 5.0-7.3 kb. At the amphibolites-facies retrograde (M4) stage, fine

  13. The Lopu Kangri High-Pressure Metamorphic Complex: A Tso Morari Analog in Southern Tibet

    NASA Astrophysics Data System (ADS)

    Laskowski, A. K.; Kapp, P. A.

    2015-12-01

    The Lopu Range, located along the Yarlung-Tsangpo suture ~600 km west of Lhasa city in southern Tibet, exposes a high-pressure metamorphic complex composed of Indian passive margin (Tethyan) rocks. An integrated approach involving geologic mapping, kinematic analysis, phengite geobarometry, Zr-in-rutile geothermometry, garnet-phengite Fe-Mg exchange geothermometry and pseudosection modeling reveals that Lopu Range meta-Tethyan rocks reached peak pressures of 20-25 kbar (2.0-2.5 GPa) at temperatures <550-630 ºC along a clockwise P-T path. These data indicate subduction to mantle depths (~75 km) at eclogite facies conditions followed by exhumation to mid-crustal depths and retrogression at upper greenschist to amphibolite facies conditions. The structural geometry and interpreted P-T-t history of Lopu Kangri rocks is similar to the Tso Morari complex, located ~700 km along-strike to the northwest. Therefore, we interpret that these two localities formed in a similar manner following the onset of Tethyan Himalaya—Eurasia collision ca. 58-52 Ma. A previously published Ar-Ar date from Lopu Kangri suggests that exhumation to mid-crustal levels occurred by ~41 Ma. Two key differences exist between the Lopu Kangri and Tso-Morari complexes. 1) the high-grade nappe in the Lopu Kangri complex is composed entirely of Cambrian-Ordovician metasedimentary rocks whereas the high-grade nappe in the Tso Morari complex is composed of the Tso Morari orthogneiss, eclogite boudins (meta-mafic enclaves) and Cambrian-Ordovician metasediments. We interpret that the lack of eclogite boudins at Lopu Kangri resulted from the absence of a basic protolith. 2) Lopu Kangri is located along the Yarlung-Tsangpo segment of the Indus-Yarlung (India-Asia) suture whereas Tso Morari and nearby Kaghan Valley are located along the Indus suture. Prior to this study, no continental high-pressure metamorphic complexes were known along the Yarlung-Tsangpo suture. Previously formulated tectonic models

  14. 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

    SciTech Connect

    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.

  15. New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon

    USGS Publications Warehouse

    Moore, Diane E.; Blake, M.C., Jr.

    1989-01-01

    The early metamorphic history of high-grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high-grade glaucophane schists, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high-grade glaucophane schist and eclogite have replaced an earlier epidote-amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende-rich blocks. At least some of the hornblende-rich blocks may therefore be well-preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partititioning in the mixed-assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende-forming stage. -Authors

  16. 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.

  17. Metamorphic and geochemical signatures within calc-silicate gneisses of the Sawtooth Metamorphic Complex, ID: Implications for western North America crustal evolution

    NASA Astrophysics Data System (ADS)

    Fukai, I.; Dutrow, B. L.; Henry, D.; Mueller, P. A.; Foster, D. A.

    2012-12-01

    High resolution sampling and petrologic/geochemical analyses of calc-silicate metamorphic rocks from the Sawtooth Metamorphic Complex (SMC), ID, constrain the evolution of a key area in North America. The SMC lies within the proposed Paleoproterozoic Selway basement terrane on the southwestern margin of Laurentia. Multi-equilibria thermobarometry calculations on SMC aluminous gneisses yield lower-granulite facies peak metamorphic conditions of 765-795 °C and 7 kbar, suggesting the SMC represents a segment of lower-middle crust. Twenty-five calc-silicate samples were collected 1 km east of the aluminous gneisses along a 132 m transect across the regional N-S strike of the SMC. Samples have a mineral assemblage of clinopyroxene + quartz + tremolite + plagioclase + titanite + apatite ± K-feldspar ± clinozoisite ± biotite ± epidote ± graphite ± calcite, and display a range in whole-rock compositions (wt.%) of SiO2 (53.9-80.9), Al2O3 (3.4-12.4), FeO (1.0-5.9), CaO (5.2-21.5), MgO (3.3-9.5), and K2O (0.04-3.8). Samples exhibit decreasing modal amounts of qz, pl, kf, and bt from west to east, along with an increase in cpx, ttn, czo, ep, gr and cal. Major element geochemical trends are consistent with the observed mineralogical variations, revealing higher weight percentages of SiO2 (62-81) and K2O (0.8-3.8) in western samples, and higher TiO2, Al2O3, FeO, MgO CaO, and LOI in the easternmost samples. Eastern samples have lower compositions of Rb and Ba, and higher Sr and Cr relative to western samples that correspond respectively to decreased modal amounts of fsp and bt, and an increase in cal and cpx. Trace elements Ni, Sc, V, Y, Nb, Ce, Nd, Cu, Pb, Th, and La do not show systematic variations. SMC calc-silicates have an average Th/Sc ratio of 0.89, consistent with Post-Archean values (>0.7), and display high concentrations of Zr relative to Th, and Sc, more characteristic of an evolved, recycled, continental sediment source. Application of the hornblende

  18. Strain and flow in the metamorphic core complex of Ios Island (Cyclades, Greece)

    NASA Astrophysics Data System (ADS)

    Mizera, Marcel; Behrmann, Jan H.

    2015-10-01

    We have analysed strain and flow kinematics in the footwall of the South Cyclades Shear Zone (SCSZ), an important tectonic boundary within the Attic-Cycladic Crystalline Complex exposed on Ios Island, Cyclades, Aegean Sea. Coarse-grained augen gneisses in the basement unit flooring the SCSZ and forming a metamorphic core complex are excellently suited to measure finite strain using the Fry method and estimate the vorticity number (W k) of flow with the "blocked-object" method. The results show that Oligo-Miocene exhumation of the basement unit during extension brought approximately 70 % N-S crustal stretching and up to 40 % subvertical shortening in a plane strain environment (k = 0.99). Linear down-section strain decrease constrains a zone of contact deformation of the SCSZ of about 1.5 km thick. Kinematic vorticity number estimates suggest little deviation from pure shear (W k = 0.26). Finite strain and W k are not correlated, indicating that the Ios basement and the overlying cover units were stretched compatibly. While the SCSZ is a localized zone of high strain, net displacement, however, may be restricted to about ten kilometres. This has important repercussions on large-scale tectonic models for extension in the Aegean.

  19. K-Ar ages of allochthonous mafic and ultramafic complexes and their metamorphic aureoles, Western Brooks Range, Alaska

    SciTech Connect

    Boak, J.L.; Turner, D.L.; Wallace, W.K.; Moore, T.E.

    1985-04-01

    New K-Ar ages from allochthonous mafic and ultramafic complexes of the western Brooks Range (Brooks Range ophiolite) show that igneous rocks yielded ages nearly identical to those of underlying metamorphic aureole rocks. Dated rocks of the Misheguk igneous sequence from Tumit Creek consist of (1) hornblende gabbro with minor greenschist and lower grade alteration, hornblende age 147.2 +/- 4.4 Ma; and (2) hornblende-bearing diorite, also slightly altered, age 155.8 +/- 4.7 Ma. Both samples come from presumed higher levels of the Misheguk sequence. Dated samples of metamorphic aureole rocks come from outcrops near Kismilot Creek and lie structurally beneath the Iyikrok Mountain peridotite body. The rocks consist of amphibolite and garnet-bearing biotite-hornblende gneiss considered to be metamorphosed Copter igneous sequence and related sedimentary rocks. Hornblende ages are 154.2 +/- 4.6 Ma and 153.2 +/- 4.6 Ma. metamorphism is clearly related to the structurally overlying perioditite, as the degree of alteration decreases downward. The authors suggest that the K-Ar ages of these rocks represent the effects of thermal metamorphism post-dating igneous crystallization, and are related to tectonic emplacement of the complex. Earlier K-Ar data on igneous rocks give similar ages and have been interpreted as reflecting tectonothermal events. The age of igneous crystallization of the mafic and ultramafic rocks of the Misheguk igneous sequence remains uncertain.

  20. Opposite P, T, t paths of Hercynian metamorphism between the upper units of the Cabo Ortegal Complex and their substratum (northwest of the Iberian Massif)

    NASA Astrophysics Data System (ADS)

    Arenas, R.

    1991-06-01

    In the Cabo Ortegal region two metamorphic groups can be distinguished, which have followed opposite P-T paths in their Hercynian metamorphism. The upper group comprises the three uppermost units of the allochthonous complex, with mafic and related lithologies, of Cabo Ortegal. The lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the Ollo de Sapo Anticlinorium. The lithologies of the upper group were affected by high- P and medium-high- T eo-Hercynian metamorphism, during a subduction episode related to the onset of the Hercynian Orogenic Cycle. As a result of such metamorphism, the basic rocks were transformed into B-type eclogites and also into high- P mafic granulites. The advance of the Hercynian collision produced the progressive uplift of the subducted lithologies and, finally, their emplacement as large thrust sheets over the lower metamorphic group. This final evolution is clearly reflected in the P, T, t paths of the high- P lithologies, which show pronounced decompressions with only minor cooling. The evolution of the Lower Metamorphic Group does not reflect high- P events. The characteristic regime is a Barrovian-type medium- P metamorphism, showing P, T, t paths with a progressive compression during heating. At least partially, the burial of the rock association was due to the overthrusting of a thick allochthonous pile; this is represented by the Upper Metamorphic Group.

  1. 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

  2. The Silurian to Permian history of a metamorphic core complex in Lofoten, northern Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Steltenpohl, Mark G.; Hames, Willis E.; Andresen, Arild

    2004-02-01

    , Permian extension in Lofoten is largely characterized by brittle structures that formed at conditions substantially less than 300°C. Compared to the exhumation history of the southern Western Gneiss Region, the depth of Caledonian, continental (A-type) subduction and subsequent unroofing of Lofoten are of lesser magnitude, and the present erosional level remained in the middle crust for a much longer interval of time. The Permian 40Ar/39Ar mineral ages documented in this study are the youngest such ages yet identified in Scandinavia. These ages relate to episodes of deformation and cooling in response to extensional tectonic events that occurred roughly 100 m.y. after comparable effects identified on the Caledonian mainland. Our preferred explanation for the Carboniferous-Permian radiometric ages, structural evolution, and stratigraphic data for Lofoten is that they all developed in the context of a long-lived Cordilleran-style metamorphic core complex.

  3. Metamorphism of eclogites from the UHP Maksyutov Complex, south Ural Mountains, Russia

    NASA Astrophysics Data System (ADS)

    Burlick, T. D.; Leech, M. L.

    2013-12-01

    The Maksyutov Complex is a mid- to late Paleozoic ultrahigh-pressure (UHP) subduction terrane in the south Ural Mountains of Russia. Radial fractures around quartz inclusions in garnet, omphacite, and glaucophane interpreted as pre-existing coesite; and microdiamond aggregates in garnet identified by Raman spectroscopy demonstrate Maksyutov rocks were subducted to UHP conditions (>2.8 GPa for coesite and >3.0 GPa for diamond at 600°C). Peak UHP eclogite-facies metamorphism (Grt+Omp+Ph+Coe+Rt ×Ttn) took place at c. 385 M and Maksyutov rocks were exhumed through retrograde blueschist-facies metamorphism (Grt+Gln+Ph+Qz×Chl×Ep) by 360 Ma. Pseudosections were constructed to constrain the P-T conditions recorded by the equilibrium mineral assemblanges in eclogites and their retrograded equivalents using bulk rock XRF analysis in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 and the suite of free energy minimization programs, Perple_X 6.6.8 [Connolly 2009] with the internally consistent end-member thermodynamic database from Holland and Powell [1998] (mod 2004); solution models for omphacite (Holland and Powell, 1996), clinoamphibole (Dale et al. 2005), white mica (Coggon & Holland 2002, Auzanneau et al 2010), chlorite and garnet (Holland and Powell 1998; Powell and Holland 1999), and feldspar (Thompson and Hovis 1979; Newton et al. 1980) were used with H2O as a saturated component. Both conventional thermometry, using microprobe analyses and Grt-Cpx cation exchange as well as pseudosection modeling result in higher peak equilibrium temperatures than has been previously been reported in the Maksyutov. Pseudosection modeling gives minimum P-T conditions of 625°-675°C and 2.8-3.1 GPa for peak assemblages from the least retrogressed eclogites, while Fe-Mg exchange thermometry yields temperatures of 775°C × 25°C for pressures ranging from 2.5 to 3.5 GPa.

  4. The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

    PubMed Central

    Gasser, Deta; Bruand, Emilie; Rubatto, Daniela; Stüwe, Kurt

    2012-01-01

    Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~ 550–650 °C and ≥ 3.4 kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~ 55–50 Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle. PMID:26525358

  5. Dolomitic marbles from the ultrahigh-pressure metamorphic Kimi complex in Rhodope, N.E. Greece

    NASA Astrophysics Data System (ADS)

    Mposkos, E.; Baziotis, I.; Proyer, A.; Hoinkes, G.

    2006-09-01

    Dolomitic marbles from the Organi and Pandrosos areas of the ultrahigh-pressure (UHP) metamorphic Kimi complex in East Rhodope, N.E. Greece have the mineral assemblage: Cal + Dol + Ol + Phl ± Di ± Hbl ± Spl ± Ti Chu + retrograde Srp and Chl. Several generations of calcite and dolomite with variable composition and texture represent different stages of the P T evolution: The first stage is represented by matrix dolomite (X_MgCO_3 = 0.48) and relic domains of homogenous composition in matrix calcite (X_MgCO_3 = 0.11 0.13); the second stage is evident from precipitation of lath-shaped and vermicular dolomite in matrix calcite. The third stage is represented by veinlets of almost pure CaCO3 and domainal replacement of prior calcite by nearly pure CaCO3 + Ca-rich dolomite (X_MgCO_3 = 0.34 0.43). Matrix dolomite adjacent to CaCO3 veinlets also becomes Ca-rich (X_MgCO_3 = 0.42). In fact, Ca-rich dolomites with X_MgCO_3 in the range of 0.40 0.34 are reported for the first time from metamorphic marbles. Coexisting Ca-rich dolomite and Mg-poor calcite cannot be explained by the calcite-dolomite miscibility gap. This assemblage rather suggests that Mg-poor calcite was aragonite originally, which formed together with Ca-rich dolomite according to the reaction Mg Cal → Arg + Dol (1) at ultrahigh pressures and temperatures above at least 850 °C, when dolomite becomes disordered and incorporates more Ca than coexisting aragonite does in terms of Mg. The simplest explanation of these observations probably is to suggest two metamorphic events: The first one represented by relic matrix carbonates at relatively low to moderate pressures and temperatures of ca. 750 °C, and the second one limited by the minimum temperatures for dolomite disorder (ca. 850 °C) and in the aragonite + dolomite stability field, i.e. at a minimum pressure of 3 GPa and, if the presence of diamond-bearing metapelites nearby is considered, at conditions of at least 850 °C and 4.3 GPa in the diamond

  6. Metamorphic evolution of the Rechnitz metamorphic core complex in relation to the Neogene Pannonian basin, Eastern Alps: Constraints from Ar-Ar white mica ages

    NASA Astrophysics Data System (ADS)

    Cao, Shuyun; Neubauer, Franz; Genser, Johann; Bernroider, Manfred; Friedl, Gertrude

    2015-04-01

    The exhumation of Cordilleran-type metamorphic core complexes (MCC) is generally related to largely contemporaneous collapse-type sedimentary basins (mainly halfgrabens). Here, we investigate the example of the Rechnitz MCC, which formed by Miocene orogen-parallel extension within the Neogene Pannonian basin. The Rechnitz MCC is located on the South Burgenland basement High within the western part of the Neogene Pannonian basin, with the Styrian basin in the west and the Danube basin in the east. The Rechnitz MCC is metamorphosed within greenschist facies conditions (maximum temperature of 430 °C) ideal for Ar-Ar white mica dating. For the first time, we undertook an extensive survey of 40Ar/39Ar white mica dating combined with microfabrics and electron microprobe compositional data and we compare the new data with major evolutionary stages of adjacent sedimentary basins, mainly based on re-evaluation of existing reflection seismic lines. The internal structure of the Rechnitz window is characterized by two tectonic cover nappes, a lower nappe with distal continental affinity, and an upper nappe representing the infilling of an oceanic basin. Both within greenschist facies metamorphic conditions and few blueschists were found in the northwestern part of the upper nappe. We found a number of distinct white mica age spectra: (1) A sample from the northwesternmost upper nappe yield a staircase pattern ranging from 14.8 ± 0.9 Ma to 41.5 ± 1.0 Ma. We interpret the older age is minimum age of high-pressure metamorphism and the younger age as age of overprint during extensional exhumation. (2) A number of samples from the western part, independent from positions within the nappes yield plateau ages between 20 and maximum 23 Ma and are variably affected by a younger thermal overprint between 13 and 15 Ma. (3) The eastern and lower units show plateau-like patterns with plateau ages of 17 - 19 Ma with a majority at ca. 18 Ma and a single younger outlier at ca. 16 Ma. Some

  7. Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

    NASA Astrophysics Data System (ADS)

    Scheffer, Christophe; Vanderhaeghe, Olivier; Lanari, Pierre; Tarantola, Alexandre; Ponthus, Léandre; Photiades, Adonis; France, Lydéric

    2016-05-01

    The Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure-temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9-13 kbar) to lower pressure (M2, 6-9 kbar) at a constant temperature of ca. 315 °C. The basal unit has preserved a first set of HP/LT conditions (M1-2, 8-11 kbar, 300 °C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5-8.5 kbar) associated with a slight but

  8. Pseudotachylyte in the Tananao Metamorphic Complex, Taiwan: Occurrence and dynamic phase changes of fossil earthquakes

    NASA Astrophysics Data System (ADS)

    Chu, Hao-Tsu; Hwang, Shyh-Lung; Shen, Pouyan; Yui, Tzen-Fu

    2012-12-01

    Pseudotachylyte veins and cataclasites were studied in the mylonitized granitic gneiss of the Tananao Metamorphic Complex at Hoping, Eastern Taiwan. The aphanitic pseudotachylyte veins vary in thickness, ranging from millimeters to about 1 cm. Field and optical microscopic observations show that such pseudotachylyte veins cut across cataclasites, which, in turn, transect the mylonitized granitic gneiss. Scanning electron microscopic images also show that both the pseudotachylyte veins and the cataclasites have been metasomatized by a K-rich fluid, resulting in the replacement of Na-plagioclase by K-feldspar (veins). Analytical electron microscopic observations reveal further details of physical and chemical changes (mainly fragmentation, dislocations, cleaving-healing with inclusions and relic voids, and retention of high-temperature albite) of quartz and feldspar in crushed grains. Pseudotachylytes occur as dark veins having a higher content of chlorite-biotite, clinozoisite-epidote and titanite fragments than cataclasites. These veins, coupled with hematite/jarosite-Fe-rich amorphous shell/carbonaceous material, indicate that crushing, healing/sintering, and inhomogeneous melt/fluid infiltration involving incipient and intermediate/high temperature melt patches, before and/or contemporaneous with the metasomatic K-rich fluid, prevailed in a coupled or sequential manner in the faulting event to form nonequilibrium phase assemblage. The chlorite-biotite, carbonaceous material and other nanoscale minerals could be vulnerable in future earthquakes under the influence of water. The timing of the formation of these pseudotachylyte veins should be later than the area's age of mylonitization of granitic gneiss of approximately 4.1-3.0 Ma (Wang et al., 1998). The formation of pseudotachylytes registers the fossil earthquakes during early stages in the exhumation history of the uplifting Taiwan Mountain belt since the Plio-Pleistocene Arc-Continent collision.

  9. Metamorphic Core Complex dynamics and structural development: Field evidences from the Liaodong Peninsula (China, East Asia)

    NASA Astrophysics Data System (ADS)

    Charles, Nicolas; Gumiaux, Charles; Augier, Romain; Chen, Yan; Faure, Michel; Lin, Wei; Zhu, Rixiang

    2012-08-01

    Metamorphic Core Complexes (MCC) constitute remarkable features within wide rifts. Based on analogue and numerical modelling, MCC dynamics and structural development are mainly controlled by first geothermal gradient, second the compositional layering, and after the strain rate and partial melting. In the Late Mesozoic, continental extension occurred in East Asia leading to the development of MCC, magmatism and extensional sedimentary basins. Based on an integrated study (i.e. structural and finite strain analysis, petrofabrics, Anisotropy of Magnetic Susceptibility (AMS) and U/Pb on zircon dating), this paper aims at constraining the tectonic evolution and deformation mechanisms in the South Liaodong Peninsula (NE China). The Gudaoling massif is identified as a "migmatitic" MCC developed from Upper Jurassic (ca. 157-154 Ma) to Early Cretaceous (ca. 128-113 Ma). Intrusion of the Yinmawanshan synkinematic pluton (Early Cretaceous) to the south of the dome marks out the final stages of shearing along the Gudaoling detachment zone and of exhumation of the MCC into the upper crust. The Gudaoling MCC and the Yinmawanshan pluton stay in line with the coeval South Liaodong MCC, to the south, all making a ~ 140 × 30 km wide extensional band formed during a regional E-W to NW-SE crustal stretching. The area shows a bi-phased development with a "slow" and a "fast" stage which corresponds to (1) crustal necking and (2) dome amplification/exhumation stages according to published thermo-mechanical modelling results. Finally, the Gudaoling MCC lower unit almost exclusively displays migmatites and anatectic granitoids of Late Jurassic and Early Cretaceous, respectively. Occurrence of partial melting during earlier stages of extension seems controlling the initiation of MCC (as a soft anomaly within the lower crust). In East Asia, a regional-scale thermal event, during Jurassic-Cretaceous times, may have significantly reduced the bulk lithosphere strength and locally induced

  10. Possible giant metamorphic core complex at the center of Artemis Corona, Venus

    USGS Publications Warehouse

    Spencer, J.E.

    2001-01-01

    Hundreds of circular features on Venus known as coronae are characterized by annular fractures and commonly associated radial fractures and lava flows. Coronae are thought to have been produced by buoyant mantle diapirs that flatten and spread at the base of the lithosphere and cause fracturing, uplift, and magmatism. The interior of Artemis Corona, by far the largest corona at 2100 km diameter, is divided in half by a northeast-trending deformation belt that contains numerous rounded ridges resembling antiforms. The largest of these ridges, located at the center of Artemis Corona, is ???5 km high on its steep northwest flank where it is adjacent to a flat-bottomed, 10-km-wide trough interpreted as a rift valley. The 280-km-long antiformal ridge is marked by perpendicular grooves that cross the ???50-km-wide ridge and extend southeastward as far as 120 km across adjacent plains. The grooves abruptly terminate northwestward at the rift trough. The large antiformal ridge terminates southwestward at a transform shear zone that parallels the grooves. These features-rift valley, antiformal uplift, grooves, and transform shear zone-are morphologically and geometrically similar to grooved, elevated, submarine metamorphic core complexes on the inside corners of ridge-transform intersections of slow-spreading ridges on Earth. As with submarine core complexes, the grooved surface on Venus is interpreted as the footwall of a large-displacement normal fault, and the grooves are inferred to be the product of plastic molding of the footwall to irregularities on the underside of the hanging wall followed by tectonic exhumation of the molded grooves and conveyer-belt-like transport up and over the large antiform and across the southeastern plains. According to this interpretation, the trend of the grooves records the direction of extension, which is perpendicular to the thrusts at the leading edge of the annular thrust belt 1000 km to the southeast. Both may have formed at the

  11. A key extensional metamorphic complex reviewed and restored: The Menderes Massif of western Turkey

    NASA Astrophysics Data System (ADS)

    van Hinsbergen, Douwe J. J.

    2010-09-01

    This paper provides a review of the structure and metamorphism of the Menderes Massif in western Turkey, and subsequently a map-view restoration of its Neogene unroofing history. Exhumation of this massif — among the largest continental extensional provinces in the world — is generally considered to have occurred along extensional detachments with a NE-SW stretching direction. Restoration of the early Miocene history, however, shows that these extensional detachments can only explain part of the exhumation history of the Menderes Massif, and that NE-SW stretching can only be held accountable for half, or less, of the exhumation. Restoration back to ˜ 15 Ma is relatively straightforward, and is mainly characterised by a previously reported 25-30° vertical axis rotation difference between the northern Menderes Massif, and the Southern Menderes Massif and overlying HP nappes, Lycian Nappes and Bey Dağları about a pivot point close to Denizli. To the west of this pole, the rotation was accommodated by exhumation of the Central Menderes core complex since middle Miocene times, and to the east probably by shortening. At the end of the early Miocene, the Menderes Massif formed a rectangular, NE-SW trending tectonic window of ˜ 150 × 100 km. Geochronology suggests unroofing between ˜ 25 and 15 Ma. The north-eastern Menderes Massif was exhumed along the early Miocene Simav detachment, over a distance of ≤ 50 km. The accommodation of the remainder of the exhumation is enigmatic, but penetrative NE-SW stretching lineations throughout the Menderes Massif suggest a prominent role of NE-SW extension. This, however, requires that the eastern margin of the Menderes Massif, bordering a region without significant extension, is a transform fault with an offset of ˜ 150 km, cutting through the Lycian Nappes. For this, there is no evidence. The Lycian Nappes — a non-metamorphic stack of sedimentary thrust slices and an overlying ophiolite and ophiolitic mélange

  12. The grand tour of the Ruby-East Humboldt metamorphic core complex, northeastern Nevada: Part 1 - Introduction & road log

    USGS Publications Warehouse

    Snoke, A.W.; Howard, K.A.; McGrew, A.J.; Burton, B.R.; Barnes, C.G.; Peters, M.T.; Wright, J.E.

    1997-01-01

    The purpose of this geological excursion is to provide an overview of the multiphase developmental history of the Ruby Mountains and East Humboldt Range, northeastern Nevada. Although these mountain ranges are commonly cited as a classic example of a Cordilleran metamorphic core complex developed through large-magnitude, mid-Tertiary crustal extension, a preceding polyphase Mesozoic contractional history is also well preserved in the ranges. An early phase of this history involved Late Jurassic two-mica granitic magmatism, high-temperature but relatively low-pressure metamorphism, and polyphase deformation in the central Ruby Mountains. In the northern Ruby Mountains and East Humboldt Range, a Late Cretaceous history of crustal shortening, metamorphism, and magmatism is manifested by fold-nappes (involving Archean basement rocks in the northern East Humboldt Range), widespread migmatization, injection of monzogranitic and leucogranitic magmas, all coupled with sillimanite-grade metamorphism. Following Late Cretaceous contraction, a protracted extensional deformation partially overprinted these areas during the Cenozoic. This extensional history may have begun as early as the Late Cretaceous or as late as the mid-Eocene. Late Eocene and Oligocene magmatism occurred at various levels in the crust yielding mafic to felsic orthogneisses in the deep crust, a composite granitic pluton in the upper crust, and volcanic rocks at the surface. Movement along a west-rooted, extensional shear zone in the Oligocene and early Miocene led to core-complex exhumation. The shear zone produced mylonitic rocks about 1 km thick at deep crustal levels, and an overprint of brittle detachment faulting at shallower levels as unroofing proceeded. Megabreccias and other synextensional sedimentary deposits are locally preserved in a tilted, upper Eocene through Miocene stratigraphic sequence. Neogene magmatism included the emplacement of basalt dikes and eruption of rhyolitic rocks. Subsequent

  13. Metamorphic history of a high-grade blueschist exotic block from the Franciscan complex, California.

    USGS Publications Warehouse

    Moore, Diane E.

    1984-01-01

    A tectonic block (approx 6 m in diameter) in the NE Diablo Range shows evidence of three episodes of retrograde blueschist-facies metamorphism + or - deformation under progressively declining P-T conditions. Retrograde metamorphism began with the tectonic incorporation of fragments of the original metamorphic terrain into serpentinite, which provided Mg-bearing fluids for the reactions of both the first and second retrograde events. There is extensive CaCO3 veining which matches that of the associated metasediments. In the third retrograde event, aragonite crystallized, accompanied by lawsonite, chlorite, jadeitic pyroxene and crossite. Such tectonic blocks may have originally formed in a pre-Franciscan subduction zone setting; fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir and developed alteration zones against the enclosing ultramafic rock. When the serpentinite reached the Earth's surface, such blocks were transferred to the Franciscan as detritus and were then re-subducted and metamorphosed along with their associated sedimentary sequences.-R.A.H.

  14. Dating coeval mafic magmatism and ultrahigh temperature metamorphism in the Anápolis-Itauçu Complex, Central Brazil

    NASA Astrophysics Data System (ADS)

    Giustina, Maria Emilia Schutesky Della; Pimentel, Márcio Martins; Ferreira Filho, Cesar Fonseca; de Hollanda, Maria Helena Bezerra Maia

    2011-05-01

    Dating granulites has always been of great interest because they represent one of the most extreme settings of an orogen. Owing to the resilience of zircon, even in such severe environments, the link between P-T conditions and geological time is possible. However, a challenge to geochronologists is to define whether the growth of new zircon is related to pre- or post-P-T peak conditions and which processes might affect the (re)crystallization. In this context, the Anápolis-Itauçu Complex, a high-grade complex in central Brazil with ultrahigh temperature (UHT) granulites, may provide valuable information within this topic. The Anápolis-Itauçu Complex (AIC) includes ortho- and paragranulites, locally presenting UHT mineral assemblages, with igneous zircon ages varying between 760 and 650 Ma and metamorphic overgrowths dated at around 650-640 Ma. Also common in the Anápolis-Itauçu Complex are layered mafic-ultramafic complexes metamorphosed under high-grade conditions. This article presents the first geological and geochronological constraints of three of these layered complexes within the AIC, the Damolândia, Taquaral and Goianira-Trindade complexes. U-Pb (LA-MC-ICPMS, SHRIMP and ID-TIMS) zircon analyses reveal a spread of concordant ages spanning within an age interval of ~ 80 Ma with an “upper” intercept age of ~ 670 Ma. Under cathodoluminescence imaging, these crystals show partially preserved primary sector zoning, as well as internal textures typical of alteration during high-grade metamorphism, such as inward-moving boundaries. Zircon grains reveal homogeneous initial 176Hf/177Hf values in distinct crystal-scale domains in all samples. Moreover, Hf isotopic ratios show correlation neither with U-Pb ages nor with Th/U ratios, suggesting that zircon grains crystallized during a single growth event. It is suggested, therefore, that the observed spread of concordant U-Pb ages may be related to a memory effect due to coupled dissolution

  15. The co-genetic evolution of metamorphic core complexes and drainage systems

    NASA Astrophysics Data System (ADS)

    Trost, Georg; Neubauer, Franz; Robl, Jörg

    2016-04-01

    Metamorphic core complexes (MCCs) are large scale geological features that globally occur in high strain zones where rocks from lower crustal levels are rapidly exhumed along discrete fault zones, basically ductile-low-angle normal faults recognizable by a metamorphic break between the cool upper plate and hot lower plate. Standard methods, structural analysis and geochronology, are applied to reveal the geodynamic setting of MCCs and to constrain timing and rates of their exhumation. Exhumation is abundantly accompanied by spatially and temporally variable vertical (uplift) and horizontal motions (lateral advection) representing the tectonic driver of topography formation that forces drainage systems and related hillslopes to adjust. The drainage pattern commonly develops in the final stage of exhumation and contributes to the decay of the forming topography. Astonishingly, drainage systems and their characteristic metrics (e.g. normalized steepness index) in regions coined by MCCs have only been sparsely investigated to determine distinctions between different MCC-types (A- and B-type MCCs according to Le Pourhiet et al., 2012). They however, should significantly differ in their topographic expression that evolves by the interplay of tectonic forcing and erosional surface processes. A-type MCCs develop in an overall extensional regime and are bounded partly by strike-slip faults showing transtensional or transpressional components. B-type MCCs are influenced by extensional dynamics only. Here, we introduce C-type MCCs that are updoming along oversteps of crustal-scale, often orogen-parallel strike-slip shear zones. In this study, we analyze drainage systems of several prominent MCCs, and compare their drainage patterns and channel metrics to constrain their geodynamic setting. The Naxos MCC represents an A-type MCC. The Dayman Dome located in Papua New Guinea a B-type MCC, whereas MCCs of the Red River Shear Zone, the Diancang, Ailao-Shan and Day Nui Con Voi

  16. Chemical and mineralogical data and processing methods management system prototype with application to study of the North Caucasus Blybsky Metamorphic Complexes metamorphism PT-condition

    NASA Astrophysics Data System (ADS)

    Ivanov, Stanislav; Kamzolkin, Vladimir; Konilov, Aleksandr; Aleshin, Igor

    2014-05-01

    There are many various methods of assessing the conditions of rocks formation based on determining the composition of the constituent minerals. Our objective was to create a universal tool for processing mineral's chemical analysis results and solving geothermobarometry problems by creating a database of existing sensors and providing a user-friendly standard interface. Similar computer assisted tools are based upon large collection of sensors (geothermometers and geobarometers) are known, for example, the project TPF (Konilov A.N., 1999) - text-based sensor collection tool written in PASCAL. The application contained more than 350 different sensors and has been used widely in petrochemical studies (see A.N. Konilov , A.A. Grafchikov, V.I. Fonarev 2010 for review). Our prototype uses the TPF project concept and is designed with modern application development techniques, which allows better flexibility. Main components of the designed system are 3 connected datasets: sensors collection (geothermometers, geobarometers, oxygen geobarometers, etc.), petrochemical data and modeling results. All data is maintained by special management and visualization tools and resides in sql database. System utilities allow user to import and export data in various file formats, edit records and plot graphs. Sensors database contains up to date collections of known methods. New sensors may be added by user. Measured database should be filled in by researcher. User friendly interface allows access to all available data and sensors, automates routine work, reduces the risk of common user mistakes and simplifies information exchange between research groups. We use prototype to evaluate peak pressure during the formation of garnet-amphibolite apoeclogites, gneisses and schists Blybsky metamorphic complex of the Front Range of the Northern Caucasus. In particular, our estimation of formation pressure range (18 ± 4 kbar) agrees on independent research results. The reported study was

  17. Nappes, tectonics of oblique plate convergence, and metamorphic evolution related to 140 million years of continuous subduction, Franciscan Complex, California

    SciTech Connect

    Wakabayashi, J. )

    1992-01-01

    This paper presents a new synthesis of Franciscan Complex tectonics, with the emphasis on the pre-San Andreas fault history of these rocks. Field relations suggest that the Franciscan is characterized by nappe structures that formed during sequential accretion at the trench. The presence of these structures along with other field relations, including the lack of evidence for large offset of conglomerate suites, indicates that strike-slip fault systems of large displacement ({gt}500 km) did not cut the Franciscan Complex during subduction. Regional geology and comparisons to modern arc-trench systems suggest that strike-slip faulting associated with oblique subduction took place inboard (east) of the Franciscan in the vicinity of the magmatic arc. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism. Franciscan Complex tectonic history is presented in this paper.

  18. Magma interaction processes in syn-extensional granitoids: The Tertiary Menderes Metamorphic Core Complex, western Turkey

    NASA Astrophysics Data System (ADS)

    Erkül, Sibel Tatar; Erkül, Fuat

    2012-06-01

    Western Turkey, which forms the eastern part of the Aegean region, was subjected to continental extension that led to formation of metamorphic core complexes and associated syn-extensional granitoids. This study deals with petrogenesis of the syn-extensional Early Miocene Alaçamdağ (AG) and Middle Miocene Salihli (SG) granitoids and associated mafic microgranular enclaves (MME) in order to better understand the time-progressive evolution of the mantle sources beneath the extended continental crust in western Turkey. AG and SG granitoids consist of undeformed and ductility deformed granitoids together with abundant MMEs. They are calc-alkaline to high-K calc-alkaline rocks that are metaluminous to slightly peraluminous. Mg# of AG host rocks is slightly lower than that of SG host rocks. AG host rocks have higher Na2O, Ba, Rb, Rb/La and lower Al2O3, CaO, MgO, TiO2, Zr/Y values than those of the SG host rocks. AG and SG host rocks differ from those of MMEs, with their lower Al2O3, CaO, MgO, Fe2O3, TiO2, Sr, V, Mg# and higher SiO2 values. MMEs are intermediate, corresponding to monzonite, monzodiorite (in AG) and diorite (in SG) compositions and are more mafic with respect to their host rocks. In the primitive mantle (PM) normalized trace element patterns, host rock and MME samples have similar trace element patterns. All of these rocks are enriched in large ion lithophile elements (LILEs, Cs, Rb, Ba, Th, K and Sr) and strongly depleted high field strength elements (HFSEs, Ta, Nb, P, Ti) compared to the primitive mantle. The isotope ratios of the syn-extensional AG and SG rocks display increasing radiogenic strontium and decreasing radiogenic neodymium. Isotopic values for the AG and SG host rocks and MME samples are 87Sr/86Sr (AG host) = 0.708835-0.710206 and ɛNd(t) (AG host) = (- 5.36 to - 7.36); 87Sr/86Sr (AG MME) = 0.709107-0.709801 and ɛNd(t) (AG MME) = - 5.36 to - 7.36; 87Sr/86Sr (AG MME)=0.709107-0.709801 and ɛNd(t) (AG MME)=-5.55 to -6.51; 87Sr/86Sr (SG

  19. Ortigalita Peak gabbro, Franciscan complex: U-Pb dates of intrusion and high-pressure low-temperature metamorphism

    NASA Astrophysics Data System (ADS)

    Mattinson, James M.; Echeverria, Lina M.

    1980-12-01

    Paleontological and isotopic age data from the Franciscan complex in the Ortigalita Peak quadrangle, Diablo Range, California, provide new insight into the tectonic evolution of at least part of the Franciscan complex. Graywacke, shale, pillowed greenstone, and chert in the quadrangle were deposited in Late Jurassic (Tithonian) time, about 135 to 150 m.y. ago, on the basis of radiolaria. These rocks then were incorporated in an accretionary wedge prior to the intrusion of gabbroic magma 95 m.y. ago (U-Pb dating on zircons). Subduction (metamorphism of gabbro and surrounding sedimentary rocks to blueschist grade) closely followed intrusion at about 92 m.y. ago (U-Pb dating on metamorphic titanite and “plagioclase” = albite + pumpellyite ± quartz). The brief interval between intrusion and subduction confirms the idea that the gabbro was intruded into the accretionary wedge, essentially at the site of plate convergence. The much longer interval (about 40 to 55 m.y.) between deposition and subduction reveals that the Franciscan had a long presubduction history and provides a time frame within which more speculative concepts such as extensive northward translation of the Franciscan must be constrained.

  20. 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

  1. Post-orogenic extension and metamorphic core complexes in a heterogeneous crust: the role of crustal layering inherited from collision. Application to the Cyclades (Aegean domain)

    NASA Astrophysics Data System (ADS)

    Huet, Benjamin; Le Pourhiet, Laetitia; Labrousse, Loic; Burov, Evgenii; Jolivet, Laurent

    2011-02-01

    The development of metamorphic core complexes (MCC) corresponds to a mode of lithospheric continental stretching that follows collision. In most of the models that explain the formation of the MCC, high thermal gradients are necessary to weaken the lower crust and to induce its ascent. Such models fail to explain the exhumation of high pressure-low temperature metamorphic rocks in metamorphic core complex structures as observed in the Cycladic Blueschists in the Aegean domain. Besides, account for the lithological crustal stratification induced from collision has never been tested. In this paper, we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post-orogenic continental lithosphere. The models are designed and validated by petrological, structural and time data from the Cyclades. As a result, high thermal gradients (Moho temperature higher than 800°C) are neither necessary nor always sufficient to induce the development of a metamorphic core complex. At the contrary, the rheological layering of the crust inherited from collision is a first-order parameter controlling the development of extensional structures in post-orogenic settings. `Cold' MCC can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units, as observed in the Cyclades.

  2. Geochemical investigation of Archaean Bimodal and Dwalile metamorphic suites, Ancient Gneiss Complex, Swaziland

    USGS Publications Warehouse

    Hunter, D.R.; Barker, F.; Millard, H.T., Jr.

    1984-01-01

    The bimodal suite (BMS) comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites. Based on geochemical parameters three main groups of siliceous gneiss are recognized: (i) SiO2 14%, and fractionated light rare-earth element (REE) and flat heavy REE patterns; (ii) SiO2 and Al2O3 contents similar to (i) but with strongly fractionated REE patterns with steep heavy REE slopes; (iii) SiO2 > 73%, Al2O3 < 14%, Zr ??? 500 ppm and high contents of total REE having fractionated light REE and flat heavy REE patterns with large negative Eu anomalies. The interlayered amphibolites have major element abundances similar to those of basaltic komatiites, Mg-tholeiites and Fe-rich tholeiites. The former have gently sloping REE patterns, whereas the Mg-tholeiites have non-uniform REE patterns ranging from flat (??? 10 times chondrite) to strongly light REE-enriched. The Fe-rich amphibolites have flat REE patterns at 20-30 times chondrite. The Dwalile metamorphic suite, which is preserved in the keels of synforms within the BMS, includes peridotitic komatiites that have depleted light REE patterns similar to those of compositionally similar volcanics in the Onverwacht Group, Barberton, basaltic komatiites and tholeiites. The basaltic komatiites have REE patterns parallel to those of the BMS basaltic komatiites but with lower total REE contents. The Dwalile tholeiites have flat REE patterns. The basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source. The Fe-rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation. ?? 1984.

  3. Long and complex thermal history of the Song Chay metamorphic dome (Northern Vietnam) by multi-system geochronology

    NASA Astrophysics Data System (ADS)

    Roger, Francoise; Leloup, Philippe Hervé; Jolivet, Marc; Lacassin, Robin; Trinh, Phan Trong; Brunel, Maurice; Seward, Diane

    2000-06-01

    Multi-system geochronology was used to unravel the long and complex thermal history of the Song Chay range (Northern Vietnam), a high-grade granitic and metamorphic dome in the vicinity of the Cenozoic, Ailao Shan-Red River fault zone. It was considered to be Proterozoic South China basement, but its geological history was basically unknown. Scattered field observations suggest three episodes of high-temperature deformation: firstly at the time of granite emplacement, secondly a décollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages. P- T estimates suggest that metamorphism coeval with the second deformation phase culminated at ˜580°C and ˜4.5 kbar (˜16 km depth). Multi-system geochronology is applied to a two-micas granite sample, slightly deformed within the décollement. U/Pb dating of zircon yields an age of 428±5 Ma (±2 σ) interpreted as the time of granite crystallization within the South China 'Caledonian' belt. Rb/Sr on white micas and biotite yields ages of 206±10 Ma and 176±5.3 Ma, respectively (2 σ), whereas 39Ar/ 40Ar ages of the same minerals are 210±9 and 190±8 Ma (2 σ). These ages suggest an Upper Triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the décollement. The K-feldspar irregular 39Ar/ 40Ar age spectrum can, to the first order, be explained by a cooling history with two episodes of rapid cooling: one at ˜140 Ma and a second around 41 to 25 Ma. Apatite fission tracks central age (33.6±3.6 Ma, 1 σ) confirms a Tertiary rapid cooling event interpreted as the final exhumation of the Song Chay dome.

  4. U-Pb geochronological constraints on the timing of episodic regional metamorphism and rapid high-T exhumation of the Grand Forks complex, British Columbia

    NASA Astrophysics Data System (ADS)

    Cubley, J. F.; Pattison, D. R. M.; Tinkham, D. K.; Fanning, C. M.

    2013-01-01

    The Grand Forks complex (GFC) is a fault-bounded metamorphic core complex in the southern Omineca Belt of British Columbia, Canada. It experienced prograde metamorphism ranging from upper-amphibolite to granulite facies conditions during the Mesozoic to early Tertiary compressional stage of the Cordilleran orogeny. Peak metamorphism was followed by multi-stage exhumation in the Early Eocene. This study provides U-Pb monazite and zircon constraints on the timing of metamorphic episodes in the GFC and subsequent high-T, amphibolite facies decompression in the Early Eocene. Monazite LA-ICP-MS ages from metapelitic gneisses record episodic metamorphism from the Late Jurassic to Paleocene, with peak metamorphism occurring between ~ 59 and 50 Ma. Peak metamorphism was followed by rapid, near-isothermal decompression of the GFC between ~ 52 and 50 Ma, and leucosome crystallization at ~ 50 Ma. Thermodynamic modeling of metapelites in the system MnNCKFMASHPYCe predicts that monazite was not stable at peak metamorphic conditions, consistent with the dominant population of ~ 59 Ma ages representing growth along the prograde path, most likely at subsolidus conditions. Growth of widespread high-Y monazite rims (~ 50 Ma) is predicted along suprasolidus decompression and cooling paths. Zircon SHRIMP ages from igneous bodies in the GFC and hanging wall of the bounding Kettle River fault (KRF) suggest ductile deformation related to high-T decompression of the GFC was ongoing at 51 Ma but had ceased by 50 Ma, truncated by post-kinematic granitoids. This high-T deformation predates subsequent greenschist facies extension on the overlying KRF. A pre-KRF, hanging wall ductile shear zone is constrained to ~ 59-51 Ma. It deforms 59 Ma Ladybird suite leucogranites and may be related to high-T exhumation of the core complex. Rapid, > 100 °C/Ma cooling rates are required to accommodate high-T (amphibolite facies) exhumation of the GFC at 52-50 Ma followed by low-T (greenschist facies

  5. Provenance and metamorphic PT conditions of Cryogenian-Ediacaran metasediments from the Kid metamorphic complex, Sinai, NE Arabian-Nubian Shield: Insights from detrital zircon geochemistry and mineral chemistry

    NASA Astrophysics Data System (ADS)

    El-Bialy, Mohammed Z.; Ali, Kamal A.; Abu El-Enen, Mahrous M.; Ahmed, Ahmed H.

    2015-12-01

    The Malhaq and Um Zariq formations occupy the northern part of the Neoproterozoic Kid metamorphic complex of SE Sinai, NE Arabian-Nubian Shield. This study presents new mineral chemistry data and LA-ICP-MS analyses of the trace element concentrations on zircons separated from metapelites from these formations. The detrital zircons of Um Zariq Formation are more enriched in ΣREE, whereas Malhaq Formation zircons are markedly HREE-enriched with strongly fractionated HREE patterns. The quite differences in the overall slope and size of the Eu and Ce anomalies between REE patterns of the two zircon suites provide a robust indication of different sources. The Ti-in-zircon thermometer has revealed that the zircons separated from Malhaq Formation were crystallized within the 916-1018 °C range, while those from Um Zariq Formation exhibit higher range of crystallization temperatures (1084-1154 °C). The detrital zircons of Malhaq Formation were derived mainly from mafic source rocks (basalt and dolerite), whereas Um Zariq Formation zircons have varied and more evolved parent rocks. Most of the investigated zircons from both formations are concluded to be unaltered magmatic that were lately crystallized from a high LREE/HREE melt. All the studied detrital zircon grains show typical trace elements features of crustal-derived zircons. All of the Um Zariq Formation and most of Malhaq Formation detrital zircons are geochemically discriminated as continental zircons. Both formation metapelites record similar, overlapping peak metamorphic temperatures (537-602 °C and 550-579 °C, respectively), and pressures (3.83-4.93 kbar and 3.69-4.07 kbar, respectively). The geothermal gradient, at the peak metamorphic conditions, was quite high (37-41 °C/km) corresponding to metamorphism at burial depth of 14-16 km. The peak regional metamorphism of Um Zariq and Malhaq formations is concluded to be generated during extensional regime and thinning of the lithosphere in an island arc

  6. Processing and Correcting Master Images to Analyze and map Metamorphic Core Complexes in the Southern Basin and Range Province

    NASA Astrophysics Data System (ADS)

    Sanchez, S. O.

    2004-12-01

    Metamorphic core complexes (MCCs) have been of great interest to geologists and geophysicists and our goal is to facilitate integrated studies of these intriguing features. Our specific targets are the exposed Whipple Mountains in Southeastern California and the spectrally similar Mohave Mountains in Western Arizona. These two ranges were selected for study using the MODIS/ASTER airborne sensor also known as MASTER, and NASA/JPL acquired the data for us. These two ranges were chosen because of their close proximity to each other in the imagery. This sensor was chosen because it has a good resolution (15m) and 50 different bands ranging from the visible to thermal infrared. However, because it is flown on a light aircraft its flight line patterns and photogrammetric distortions make it hard to georeference and mosaic with other images from adjacent flight lines. The distortions become misalignments of images during mosaicing. This project involved two efforts: 1) developing a method for correcting and processing MASTER multispectral images; and 2) using those images to analyze and map MCCs in the southern Basin and Range Province. Standard image processing techniques available within the ENVI software package were applied to this imagery to geometrically correct, mosaic, and spectrally process it in order to locate defining characteristics of MCCs that are mappable with the imagery. These techniques include the use of warping, histogram matching, mosaicing, classification, Principal Component Analysis, decorrelation stretching, Minimum Noise Fraction Transformation, Pixel Purity Index, and end member analysis.

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

    PubMed Central

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

    2013-01-01

    We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20–77 MPa and a strain rate of 10−15–10−13 s−1 at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300–360 °C, and 200–250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint. PMID:26523079

  8. Blueschist Blocks in the Shale-matrix Melange of the Franciscan Complex of California: Metamorphic Aureoles and Subduction Channel Upwelling

    NASA Astrophysics Data System (ADS)

    Cloos, Mark; Ukar, Estibalitz

    2014-05-01

    The subduction channel concept postulates that where oceanic lithosphere converges faster than ~2 cm/yr, the plate boundary is best approximated as a viscous shear zone. Channel capacity can abruptly decrease where there is a sharp increase in the pressure gradient along the top of the shear zone. Where this occurs, subducted sediment can upwell and flow back towards the inlet. The Central Belt of the Franciscan Complex is up to tens of km wide and extends from where subduction continues in Oregon southwards to the Nacimiento Block, west of the San Andreas fault. The scaly shale matrix outcrops poorly along the 1500+ km belt because it is weakly metamorphosed (<200°C, pumpellyite±lawsonite-bearing). The best exposures, as pointed out by Ken Hsu in 1969, are found near San Simeon. The Central Belt mélange is renowned for the "high-grade" garnet-bearing blueschists (e.g., Tiburon, Jenner Beach). How these high-T blocks, the oldest metamorphics in the Franciscan and the more voluminous lower-T blueschists became rounded fragments encased in shale-matrix has been the subject of much discussion. Uplift in serpentine diapirs is widely invoked, but the scarcity of these blocks in serpentinite where exposures are good is problematic. Explaining the blocks as sedimentary olistoliths requires both a phantom source terrane and exhumation mechanism. The simplest explanation is that the coarse blueschists are fragments of a metamorphic aureole formed during subduction initiation that were later detached from the base of the ophiolitic leading edge of the North American plate. Later, tectonic melange was generated by subduction-driven shearing that caused upwelling of shale-rich sediment. Slabs of blueschist were boudinaged and mixed with fragments of greenstone and chert detached from seamounts. Blueschist block incorporation into upwelling mélange is a kind of subduction erosion akin to plucking by glaciers. The exposure of the blueschist along the roof of the subduction

  9. Orogenesis at the southern tip of the Americas: the structural evolution of the Cordillera Darwin metamorphic complex, southernmost Chile

    NASA Astrophysics Data System (ADS)

    Cunningham, W. Dickson

    1995-04-01

    New, detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the Cordillera Darwin metamorphic complex of southernmost Chile. Cordillera Darwin is a unique uplift because it exposes the highest grade rocks in the Andes south of Peru and averages 1 km higher in elevation than adjacent areas. The structural data indicate that Cordillera Darwin experienced mid-Late Cretaceous trans-pressional deformation with a partitioned strike-slip component localized along the Beagle Channel that forms the southern boundary to the range. Foliation, lineation and fold axis trends indicate NE-SW-directed contraction and NW-SE strike-slip shearing (present directions) during progressive {D1}/{D2} Andean deformation. D2 deformation is marked by outcrop-to 10 km-scale south-southwest-vergent folds. Late Cretaceous-Tertiary brittle-ductile and brittle left-lateral strike-slip faults and shear zones crosscut all {D1}/{D2} structures. Although limited structural evidence for extensional tectonics was documented in this study, apparent normal offsets across both arms of the Beagle Channel and previously documented field evidence for extension from other areas in Cordillera Darwin suggest that transtensional displacements also may have occurred in southern Cordillera Darwin during the Late Cretaceous-Early Tertiary. Cordillera Darwin's position within the evolving Patagonian Orocline adjacent to an evolving Mesozoic-Cenozoic left-lateral transform boundary between the South American and Antarctic plates, and later the South American and Scotia plates, necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures. Regional counterclockwise rotation of Cordillera Darwin may have controlled the temporal and spatial transition of deformational regimes within Cordillera Darwin. Exhumation of the metamorphic core of Cordillera Darwin during the Late Cretaceous-Early Tertiary is

  10. Tectonic evolution of forearc nappes of the active Banda arc-continent collision: Origin, age, metamorphic history and structure of the Lolotoi Complex, East Timor

    NASA Astrophysics Data System (ADS)

    Standley, Carl E.; Harris, Ron

    2009-12-01

    An integrated multidisciplinary investigation of the Lolotoi Complex of East Timor (Timor Leste) indicates that it is part of the Banda forearc that was metamorphosed and rapidly exhumed during the Eocene and accreted to the NW Australian continental margin during Late Miocene to present arc-continent collision. Greenschist, graphitic phyllite, quartz-mica schist, amphibolite and pelitic schist dominate metamorphic rock types. Mineral, whole rock, and trace element geochemical analyses of metabasites indicate protolith compositions consistent with tholeiitic basalt and basaltic andesite with mixed MORB and oceanic arc affinities. Metapelite schist is mostly composed of metasedimentary units derived from mafic to intermediate rocks with oceanic to continental volcanic arc provenance. Thermobarometric calculations show peak metamorphic conditions of 530 °C to 680 °C for garnet-biotite pairs and amphibole, and peak pressures of 5 to 10 kbar for garnet-aluminosilicate-quartz-plagioclase assemblages. Peak metamorphism occurred at 45.36 ± 0.63 Ma, as indicated by Lu-Hf analyses of garnet. Detrital zircon grains have a U/Pb age distribution with spikes at 663, 120 and 87 Ma, which is typical of detrital zircon ages throughout the Great Indonesian Arc of Asia, but is distinct from Australian affinity units. These data indicate deposition and later metamorphism occurred after 87 Ma. Structural analyses of the metamorphic rocks and their sedimentary and volcanic cover units reveals 5-6 deformational phases of alternating shortening and extension. There is little to no evidence of strike-slip deformation. Phases 1-4 are inferred as pre-Oligocene from age determinations. Phases 5 and 6 are most likely related to latest Miocene to Pliocene nappe emplacement and Pliocene to present collisional deformation. Kinematic indicators show mostly top to the SE directed shortening and top to the south and SE extension. Structural mapping indicates that the Lolotoi Complex and some of

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

    NASA Astrophysics Data System (ADS)

    Wang, Hsiu-Wen; Anovitz, Lawrence M.; Burg, Avihu; Cole, David R.; Allard, Lawrence F.; Jackson, Andrew J.; Stack, Andrew G.; Rother, Gernot

    2013-11-01

    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 low- and 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 that 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.

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

    SciTech Connect

    Wang, Hsiu-Wen; Anovitz, Lawrence {Larry} M; Burg, Avihu; Cole, David; Allard Jr, Lawrence Frederick; Jackson, Andrew J; Stack, Andrew G; Rother, Gernot; Ciarlette, Diane D

    2013-01-01

    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 that 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.

  13. Brittle-ductile deformation and kinematics during exhumation of metamorphic complexes below detachments: examples from Sifnos and Syros Islands (Greece)

    NASA Astrophysics Data System (ADS)

    Cardello, Giovanni Luca; Roche, Vincent; Laurent, Valentin; Jolivet, Laurent

    2014-05-01

    Exhumation of metamorphic core complexes is accompanied by progressive strain localization within large-scale shear zones, which may evolve into long-lived bounding detachments affected by ductile to brittle deformation. Despite the well-studied P-T-t patterns of individual nappes, their relative timing, mode and kinematics of exhumation are debated. In this study, in the frame of the Mediterranean syn- and post-orogenic deformation, examples of shear zone hierarchization and strain localization from Sifnos and Syros islands (Cyclades, Greece) are documented in detail in order to explain 3D-geometries and regional kinematics and are here tentatively related to the Ar/Ar ages available in literature. During the Eocene syn-orogenic uplift, the degree of strain localization increases progressively from blue- to green-schists deformation. Some of these shear zones where then reworked during the Oligo-Miocene post-orogenic deformation in different, usually warmer P-T conditions and a new episode of strain localisation, and an evolution toward brittle faulting, either along the main detachments or along newly created faults (as in Sifnos). Such shear zones demonstrate long-lived efficiency, especially where fluid circulation enhance retrograde metamorphic reactions. During Neogene, the final shape and exhumation of domes is the result of crustal thinning and brittle-ductile deformation in the whole Cycladic region. Although stretching directions along individual kilometric scale shear zones may be complex in the details, a simple general picture is shown for the Oligo-Miocene episode, less so for the Eocene one. Most Cycladic islands show a top-to-the-North sense of ductile shear from the syn-orogenic to the post-orogenic stage, this is the case of Sifnos for instance. The syn-orogenic stretching is however often more E-W trending, as exemplified by Syros and Tinos. The top-North or Top-East sense of shear is attributed to the NCDS for the post-orogenic stage and to a

  14. Tectonic evolution of the Priest River complex, northern Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core complex formation

    NASA Astrophysics Data System (ADS)

    Doughty, P. Ted; Price, Raymond A.

    1999-06-01

    New geologic mapping, 40Ar/39Ar thermochronometry, and geobarometry in the Middle Eocene Priest River metamorphic core complex provide the basis for unraveling the role of en echelon fault systems in core complex formation and for determining the scale of crustal fragments that form during continental extension. Four faults occur in the Priest River complex. The east verging Purcell Trench fault zone on the eastern side consists of two distinct en echelon fault segments separated by an unfaulted homocline. The U-shaped Newport fault system on the northwestern side is a conjugate normal fault set. The west verging eastern Newport fault terminates within the Silver Point Wrencoe pluton, which was intruded syntectonically into the fault zone. The east verging western Newport fault merges with the east verging Spokane dome mylonite zone in the underlying infrastructure. New geobarometric data show that this midcrustal shear zone, which evidently forms part of the regional basal décollement of the Cordilleran fold and thrust belt, also records significant Eocene extensional shearing. Rocks that formed beneath the mylonite zone at a depth of 30-35 km are juxtaposed against rocks that formed at a depth of 10 km above the zone. Eocene 40Ar/39Ar chrontours in the southern part of the infrastructure record progressive exhumation and quenching that becomes younger eastward. In the northern fragment of the infrastructure, alternating domains of progressive westward exhumation/quenching and progressive eastward exhumation/quenching occur beneath the eastern Newport fault and the northern Purcell Trench fault, respectively. These relationships form the basis for a new model of the evolution of the Priest River complex. The southern part of the infrastructure was exhumed by a major east verging detachment system comprising the western Newport fault and the reactivated eastern part of the Spokane dome mylonite zone, into which the western Newport fault merges. This master

  15. Evolution of the Eastern Karakoram Metamorphic Complex, Ladakh, NW India, and its relationship to magmatism and regional tectonics

    NASA Astrophysics Data System (ADS)

    Wallis, David; Phillips, R. J.; Lloyd, G. E.

    2014-06-01

    The eastern Karakoram terrane, NW India, records crustal evolution in the core of the Himalayan-Tibetan orogen. Recent debate has centred on whether prograde metamorphism, anatexis and leucogranite emplacement were the result of localised shear heating and magma advection within the Karakoram Fault Zone (KFZ) or instead predate the KFZ and were the result of regional tectonometamophic events. Inclusions within andalusite porphyroblasts that grew during 15.7 Ma leucogranite emplacement have fabrics that are random or discordant to the KFZ matrix foliation, indicating that the KFZ initiated after this time. Therefore, earlier anatexis and metamorphism are the result of regional metamorphic events. Amphibole-plagioclase thermobarometry on a c. 17 Ma migmatite melanosome, later exhumed within a transpressional zone of the KFZ, shows that melting occurred at 688 °C and 522 MPa. Amphibolites record an older kyanite-grade metamorphic event that occurred at 677-736 °C and 875-1059 MPa. Metapelites also record a kyanite-grade event which is constrained by Ti-in-biotite thermometry to have occurred at 622 °C and > 650 MPa. The tectonometamorphic history of the eastern Karakoram correlates closely with that of the central Karakoram away from the KFZ. This correlation supports the interpretation that metamorphism and anatexis were regional in extent and also indicates a limited offset of < 150 km on the KFZ.

  16. Tectonic stratigraphy near a metamorphic core complex: Lessons from the Castaneda-signal area of west-central Arizona

    SciTech Connect

    Lucchitta, I. ); Suneson, N.H. )

    1993-04-01

    A sequence of latest Oligocene through Quaternary sedimentary and volcanic rocks, when analyzed tectonically and combined with lithologically distinctive source terranes, clarifies the character and timing of Neogene extension just north of the Buckskin-Rawhide metamorphic core complex (BRMCC) in west-central Arizona. The oldest strata (basal arkose of Lucchitta and Suneson) reflect regional stability and a southwesterly paleoslope. In latest Oligocene time, this drainage was ponded by an upwarp (now exposed as the BRMCC) rising to the southwest. The resulting lake beds contain a thin 26.6 MA airfall tuff that marks the beginning of volcanic activity in the region. A widespread breccia records the progressive unroofing of the still-rising CC. Mantle-driven crustal heating probably caused the upwarp and allowed the eruption of voluminous mantle-derived basalt and basaltic andesite about 19 MA (early basalts, Artillery Basalt). The overlying syntectonic conglomerate (arkose of Keenan Camp) was deposited during a period of extreme extension, low-angle detachment faulting, and block rotation, typical of highly extended terranes. The conglomerate is interlayered with widespread silicic volcanic rocks (15--10 MA) derived from the lower crust and large gravity-glide sheets lithologically identical to the breccia and similarly derived from the CC to the south. Unconformably overlying the conglomerate are locally derived fanglomerate and 13--8.5 MA (mesa-forming) basalt that accumulated in present-day basins of classic basin-range type. Untilted and nearly unfaulted 7.7--5.4 MA mantle-derived megacryst-bearing basalt marks the cessation of tectonic activity.

  17. Shallow seismic reflection profiling over a Mylonitic Shear Zone, Ruby Mountains-East Humboldt Range Metamorphic Core Complex, NE Nevada

    NASA Astrophysics Data System (ADS)

    Hawman, Robert B.; Ahmed, Hishameldin O.

    Seismic reflection profiling carried out with a sledgehammer source has imaged Tertiary extensional structures over a depth range of 45-500 m within lower plate rocks of the Ruby Mountains-East Humboldt Range metamorphic core complex. The 400-m CMP profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low-angle ductile-brittle normal faulting. Subhorizontal reflections from layering within the tectonic slices give way at 160 ms (160-220 m depth) to reflections that dip 15-45° to the east, in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up-plunge exposures of a late Proterozoic - early Cambrian sequence of metaquartzites, marbles, schists, and granitic rocks that forms the bulk of the underlying shear zone. Differences with the regional profile are attributed to the higher frequencies (30-100 Hz) generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot-receiver offsets (46-157 m) contributing to the stack for each CMP. The results suggest that the near-surface, east-dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted. Rough estimates of interval velocities (1500-4500 m/s) inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account. Peaks in amplitude spectra of stacked traces suggest long-wavelength components of layering resolved at scales from 5-8 m (depth: 50 m) to 15-25 m (depth: 500 m).

  18. Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Lu, Gang; Mezri, Leila; Zhao, Liang

    2015-12-01

    Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~ 35 km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as the relationships between the underlying mechanisms and the syn-rift basin evolution. In these experiments, we explore the impact of the lithostratigraphic and thermo-rheological structure of the crust. We also examine the lithosphere thickness, strain softening, extension rate, and surface erosion/ sedimentation processes. The experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome, which is geometrically incompatible with the observations of the MCCs in the Liaodong Peninsula. According to our further findings, the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift, while synchronous surface erosion controls the polarity of the syn-rift basin. The synthetic model data are compatible with the geological observations and cooling history based on the thermo-chronology for the eastern part of the East Asia during the late Mesozoic to the early Cenozoic. The model-predicted P-T-t paths are essentially different from those inferred for the other known MCCs, confirming the exceptional character of the MCC formation in the wide rift system of the East Asia.

  19. Evolution of granitoids in the Catalina metamorphic core complex, southeastern Arizona: U-Pb, Nd, and Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Fornash, Katherine F.; Patchett, P. Jonathan; Gehrels, George E.; Spencer, Jon E.

    2013-06-01

    The Santa Catalina Mountains, SE Arizona, was one of the first metamorphic core complexes to be described. Despite its status as a type example, relatively little is known about precise ages and origins of the intrusive rocks that make up most of the crystalline core. U-Pb and Hf isotopic data by laser ablation-inductively coupled plasma-mass spectrometry from zircons and Nd isotopic results from whole rocks were obtained for 12 granitoids ranging from 1,440 to 26 Ma. Results confirm that the 1.44-Ga Oracle Granite extends through the Catalina Range as variably mylonitic granite and banded gneiss. Laramide intrusions (67-73 Ma) display initial ɛNd values -5 to -8 and ɛHf from -7.5 to -9. Magmatic ages for the prominent white granite sills of the Wilderness suite are 46-57 Ma, in agreement with Terrien (2012), and these granites have initial ɛNd values -8 to -10 and ɛHf from -7 to -14. Lastly, the undeformed Catalina Granite has an age of 26 Ma, with an initial ɛNd and ɛHf of -6 and -8, respectively. Our Nd results agree with limited results from Farmer and DePaolo (89:10141-10160, 1984). Although the Catalina Granite seems to have a significant juvenile component based on Nd and Hf, most of the Laramide and Wilderness intrusions contain Nd and Hf compositions lying close to the evolution of 1.44-Ga Oracle Granites, a fact that is confirmed by the U-Pb data, which show both 1.7- and 1.4-Ga zircon cores in these samples, with 1.4 Ga as the dominant core age. In order to become the dominant source of most of the 72-45-Ma magmas, the Oracle pluton must not only extend across the whole Catalina region, but also have abundant deep-seated equivalents to provide magma sources.

  20. Progressive overprinting of normal fault systems and their role in Tertiary exhumation of the East Humboldt-Wood Hills Metamorphic Complex northeast Nevada

    NASA Astrophysics Data System (ADS)

    Mueller, Karl J.; Snoke, Arthur W.

    1993-01-01

    Low- and high-angle, normal-sense faults present along the northern margin of the East Humboldt-Wood Hills metamorphic complex record a protracted history of episodic extensional unroofing. The earliest extension associated with the exhumation of the metamorphic complex occurred in the mid-Eocene (about 50 Ma) during slip along the west-rooted Wells Peak fault. Subsequent extension developed during movement along the east-rooted Black Mountain fault system in the late Eocene (about 35 Ma). Continued exhumation of the high-grade metamorphic rocks, occurred during the late Oligocene to early Miocene (about 29-23 Ma) along a west-rooted, normal-sense, plastic-to-brittle shear zone exposed along the length of the East Humboldt Range and much of the Ruby Mountains. The mylonitic shear zone, Wells Peak fault, and Black Mountain fault system were all overprinted by the west-rooted, low- to high-angle Mary's River fault system which was active from mid-Miocene to Holocene. Broadly coeval extension in the Windermere Hills was also accommodated by the development of high-angle, north and east dipping normal faults between approximately 35-12 Ma and 12-10 Ma.

  1. 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

  2. Significance of orthogonal flow in the Funeral Mountains metamorphic core complex, Death Valley, California: Insights from geochronology and microstructural analysis

    NASA Astrophysics Data System (ADS)

    Sauer, K. M.; Wells, M. L.; Hoisch, T. D.

    2013-12-01

    The Funeral Mountains metamorphic core complex (FMMCC) in Death Valley, California, exposes middle to lower crustal rocks of the Sevier-Laramide orogen in the footwall of the Boundary Canyon detachment (BCD). Monarch Canyon, located in the northwest section of the Funeral Mountains, exposes the structurally deepest rocks in the FMMCC. These Mesoproterozoic to Neoproterozoic metasedimentary rocks record upper amphibolite facies metamorphism with migmatites developed at the deepest levels. The Monarch Spring fault (MSF) juxtaposes migmatitic paragneisses below against pelitic schists, calcsilicate schists, and marbles above, and represents a deformed anatectic front. In the footwall of the BCD above the MSF, distributed ductile deformation and stratigraphically localized high-strain zones, termed intracore shear zones, are responsible for attenuation and local stratigraphic omission during top-northwest non-coaxial deformation. The relative contributions of Late Cretaceous-early Tertiary and Miocene extensional strains which manifest in the top-northwest fabrics remains unclear, and is being addressed by ongoing and combined thermochronologic, microstructural, and EBSD studies. Our working hypothesis is a polystage extensional history in the FMMCC, with Late Cretaceous extensional intracore shear zones locally reactivated during the Miocene. Below the MSF, migmatitic paragneisses lack similar greenschist to lower amphibolite facies top-northwest fabrics. These rocks instead exhibit heterogeneous strain and a weak to moderately developed northeast-trending mineral lineation, and a local, strong fabric asymmetry indicative of a top-southwest sense of shear. We propose that the anatectic front is an apparent zone of structural decoupling between top-southwest shear below and top-northwest shear above the MSF. Structural and geochronologic studies are currently underway to establish whether the orthogonally directed flow above and below the anatectic front were coeval or

  3. Characterization of magmatism and deformation in "Foz do Douro Metamorphic Complex" (N Portugal): insights from AMS studies

    NASA Astrophysics Data System (ADS)

    Sousa, Mónica; Sant'Ovaia, Helena; Noronha, Fernando

    2013-04-01

    The "Foz do Douro Metamorphic Complex" (FDMC) is situated on the shoreline of Porto extending along ~3km series of small beaches. The geology of this zone is marked by magnificent outcrops of a thin band of Precambrian metamorphic rocks intruded by Variscan granites and by the effects of Porto-Tomar-Ferreira do Alentejo, N330° a N340°, dextral, shear zone [1,2]. The metamorphic band is represented by outcrops of metasedimentary rocks, spatially associated to different types of orthogneisses and amphibolites that constitute the FDMC [3]. The granites belong to a late-Variscan granite group (298±11Ma) [4]. Studies of Anisotropy of Magnetic Susceptibility (AMS) were carried out on several types of orthogneisses and amphibolites from the FDMC, as well as on the Variscan granites and tonalites. The results presented here are related to 218 samples collected on 21 sampling sites: 6 sites (n=67) on leucocratic orthogneisses (Group 1), 10 sites (n=99) on biotite orthogneisses (Group 2), 1 site (n=11) on amphibolite (Group 3), 1 site (n=11) on biotite granite (Group 4), 2 sites (n=20) on biotite porphyritic granites (Group 5) and 1 site (n=10) on tonalite (Group 6). Magnetic anisotropy, expressed by the ratio Kmax/Kmin, ranges from 1.045 to 1.144 in Groups 1, 3, 4 and 5 and from 1.139 to 1.297 in Groups 2 and 6. The magnetic fabric is characterized by subvertical magnetic foliations in all lithologies with directions varying from N41° to N86° in Groups 1 and 2, N0° to N10° in Group 3, N80° to N90° in Group 6 and N130° to N140° in Groups 4 and 5, and mainly subvertical magnetic lineations dipping 50° to 86° in Groups 1 and 2, 77 to 80° in Group 3, 75° to 80° in Group 5 and 65° to 70° in Group 6, with the exception of 3 orthogneisses (one belonging to Group 1 and two from Group 2) and two granites (one belonging to Group 4 and one to Group 5) which reveal subhorizontal (04° to 36°) magnetic lineations. Magnetic susceptibility (K) ranges between 20.0 and 74

  4. Metamorphism in the Tlikakila Complex, Lake Clark National Park, Alaska: Does it Record the Collision of the Peninsular Terrane With Alaska?

    NASA Astrophysics Data System (ADS)

    Amato, J. M.; Bogar, M. J.; Calvert, A. T.

    2001-12-01

    The Tlikakila complex is a ~80 km x ~5 km belt of variably metamorphosed and deformed rocks thought to be part of the Peninsular terrane of southern Alaska. This project uses detailed mapping, structural analysis, and thermochronology to address the tectonic evolution of rocks thought to be part of the Peninsular terrane in southern Alaska. Both meta-igneous and metasedimentary rocks of Triassic (?) age are exposed. Meta-igneous protoliths include mafic (gabbro, basalt) and ultramafic rocks. Metasedimentary protoliths include limestone, chert, and other siliceous sediments. Metapelites are rare. Metamorphic rocks in the study area include two distinct occurrences. Smaller outcrops, appear to be roof pendants in Tertiary plutons. At Kasna Creek, near Kontrashibuna Lake, limestone beds were contact metamorphosed with copper sulfide mineralization within a mafic pluton. Larger outcrops in the Tlikakila complex are more continuous, more pervasively deformed, and more recrystallized. A new 40Ar/39Ar analysis of white mica from a metasedimentary rock in the Tlikakila complex located just southwest of Saddle Lake yielded a monotonically increasing age spectrum, with the oldest high-temperature step giving a date of around 160 Ma, and the low-temperature step giving a date of 60.5 Ma. The oldest date could represent the timing of greenschist facies metamorphism of the Tlikakila complex. It is interesting that this 160 Ma date is similar to the youngest of the Middle to Late Jurassic plutons (174-158 Ma) in the Alaska-Aleutian Range batholith, considered to be part of the Peninsular terrane. Metamorphism in the Tlikakila complex could be related to the onset of the collision of the Peninsular terrane with Alaska, which also resulted in the cessation of arc magmatism. The youngest date from this sample overlaps with existing 59-63 Ma K-Ar dates from Tertiary volcanic and plutonic rocks in the area and records new mica growth associated with Tertiary magmatism.

  5. Oxygen and hydrogen isotopic composition of the fluid during formation of anthophyllite metaultramafic rocks in the Sysert metamorphic complex, central Urals

    NASA Astrophysics Data System (ADS)

    Murzin, V. V.

    2014-12-01

    The oxygen (δ18O) and hydrogen (δD) isotopic composition of H2O-bearing minerals was studied for the ore-bearing amphibole metaultramafic rocks, which are the products of the early regional (435 ± 44 Ma) and late local (260 ± 6 Ma) silicic metasomatose in the Sysert metamorphic complex. The gold-sulfide mineralization of the Karas'evogorskoe deposit and anthophyllite-asbestos bodies of the Tersut deposit are related to the regional and local metasomatose combined with plagiogranitization and potassium granitization, respectively. The H2O-bearing minerals of metasomatites (anthophyllite, tremolite, talc) of the Karas'evogorskoe and Tersut deposits are characterized by heavier δ18O (9.8 to 12.2 and 7.6 to 9.4‰, respectively) and lighter ·D (87 to -91 and -56 to -67‰, respectively) values. The calculated isotopic composition of the fluid in equilibrium with these minerals indicates a heterogeneous source of water for the fluids related to the formation of metasomatites and the metamorphic origin of fluids. During the regional metasomatose, this fluid was a result of equilibrium of the deep fluid with volcanosedimentary rocks enriched in the heavy oxygen isotope. At the local metasomatose, the metamorphic fluid was formed by interaction of magmatic water produced by potassium granitization with ultramafic rocks.

  6. Fluid evolution in H2O-CO2-NaCl system and metallogenic analysis of the Surian metamorphic complex, Bavanat Cu deposit, Southwest Iran

    NASA Astrophysics Data System (ADS)

    Asadi, Sina; Moore, Farid

    2016-07-01

    The Bavanat Cu deposit occurs as veins controlled by a NE-trending structure within the Permo-Triassic Surian metamorphic complex (SMC), southwest of Iran. The SMC rocks exposed in the area have undergone greenschist-facies metamorphism. The ore-forming process can be divided into early, middle, and late stages, represented by, respectively, pyrite-quartz, polymetallic sulfide-quartz, and late-stage barren quartz veins. Systematic studies of fluid inclusions (FIs) in the quartz veins found four types: aqueous, mixed aqueous-carbonic, carbonic, and multiphase-bearing inclusions. The FIs of early, middle and late-stages are mainly homogenized at temperatures of 335-417 °C, 230-380 °C, and 190-227 °C, with salinities of 1.1-6.7, 2.9-36.6, and 0.8-2.6 wt.% NaCl equivalent, respectively. The main stage of Cu mineralization is related to the middle-stage, where FIs show evidence of fluid immiscibility. The metal precipitation resulted from a decrease in copper solubility during the fluid immiscibility, cooling, crystallization of multiphase-bearing inclusions, and a small increase in pH. Laser Raman spectroscopy and FIs evidences indicate that the metallogenic system evolved from metamorphic CO2 (+CH4)-rich, relatively high fO2 (10-25 to 10-29 bars) to CO2-poor and relatively low fO2 (10-31 to 10-34 bars). Muscovite from the middle-stage veins yields 40Ar/39Ar plateau age of 195.2 ± 1.0 Ma, suggesting that the Cu mineralization at Bavanat formed in the Early Jurassic coeval with the retrograde metamorphic events during the post-early Cimmerian orogeny.

  7. Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Little, Timothy A.; Baldwin, S. L.; Fitzgerald, P. G.; Monteleone, B.

    2007-02-01

    We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid-ocean ridge (MOR), a >1 km thickness of blueschist-derived mylonites formed in a midcrustal shear zone during the Pliocene at ˜400-500°C. This top-to-the-north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back-tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio-Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling-hinge style MCC, probably since <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ˜30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 0.8) at depth, and provide a sufficient mechanism for activating low-angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1-6 m.y. timescale. Deep-seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB

  8. Two styles of faulting associated with metamorphic core complexes: Importance of initial crustal configuration and mid-crustal flow

    NASA Astrophysics Data System (ADS)

    Wu, G.; Lavier, L. L.; Choi, E.

    2012-12-01

    Two styles of faulting, sequential- and domino-style, have been identified in metamorphic core complexes (MCCs) and discussed by various workers, however, a consensus has not been reached on the exact mechanisms and relationships between the two styles of faulting. We used a three layered model of the crust, i.e., upper, middle and lower crust and used numerical method to study the formation of different types of MCCs. We found that the initial crustal structure and crustal strength of each layer, viscosity contrast between upper and middle crust particularly, are of great importance. An initially symmetric crustal structure, i.e., uniform thickness of each layer throughout the crust, generally favors sequential normal faulting, and leads to MCCs beneath sequentially initiated and then deactivated normal faults. However, an initially asymmetric crustal structure, i.e., variable thickness of each layer of the crust, favors sequential-style migratory low angle master faults to the first order, and domino-style high angle normal faults above their associated low angle master faults to the second order. High angle normal faults in each sequence are active simultaneously with their correlated master low angle normal fault. This correlation implies that the sequential normal faulting is more important in areas where the crust has a relatively uniform thickness, while sequential- and domino-style faulting are both important in provinces where the initial crustal structure is of variable thickness with changing topography and Moho relief, such as central Basin and Range in the Cenozoic for instance. Mid-crustal flow has been proved important in the formation of orogeny. Instead of locally isostatic balance in the crust with initially symmetric crustal structure, we found a long wavelength isostatic balance between the upper and middle crust in the crust with initially asymmetric structure, which is strong evidence that mid-crustal flow plays an important role in extension

  9. Analysis of magnetotelluric profile data from the Ruby Mountains metamorphic core complex and southern Carlin Trend region, Nevada

    USGS Publications Warehouse

    Wannamaker, Philip E.; Doerner, William M.; Stodt, John A.; Sodergen, Timothy L.; Rodriguez, Brian D.

    2002-01-01

    We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings are in three east-west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass and Secret Pass latitudes). Two shorter lines cross a prominent east-west structure to the north of the northern profile. MT impedance tensor and vertical magnetic field rotations imply a N-NNE average regional geoelectric strike, similar to surface geologic trends. Model resistivity cross sections were derived using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure, emphasizing the transverse magnetic (TM) mode and vertical magnetic field data. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity-thickness product) from east to west. These increases in conductance are attributed to graphitization caused by Elko-Sevier era compressional shear deformation and

  10. Contact metamorphism, partial melting and fluid flow in the granitic footwall of the South Kawishiwi Intrusion, Duluth Complex, USA

    NASA Astrophysics Data System (ADS)

    Benko, Z.; Mogessie, A.; Molnar, F.; Severson, M.; Hauck, S.; Lechler, P.; Arehart, G.

    2012-04-01

    The footwall of the South Kawishiwi Intrusion (SKI) a part of the Mesoproterozoic (1.1 Ga) Duluth Complex consists of Archean granite-gneiss, diorite, granodiorite (Giant Range Batholith), thin condensed sequences of Paleoproterozoic shale (Virginia Fm.), as well as banded iron formation (Biwabik Iron Fm). Detailed (re)logging and petrographic analysis of granitic footwall rocks in the NM-57 drillhole from the Dunka Pit area has been performed to understand metamorphic processes, partial melting, deformation and geochemical characteristics of de-volatilization or influx of fluids. In the studied drillhole the footwall consists of foliated metagranite that is intersected by mafic (dioritic) dykes of older age than the SKI. In the proximal contact zones, in the mafic dykes, the orthopyroxene+clinopyroxene+plagioclase+quartz+Fe-Ti-oxide+hornblende±biotite porphyroblasts embedded in a plagioclase+K-feldspar+orthopyroxene+apatite matrix indicate pyroxene-hornfels facies conditions. Migmatitization is revealed by the euhedral crystal faces of plagioclase and pyroxene against anhedral quartz crystals in the in-situ leucosome and by the presence of abundant in-source plagioclase±biotite leucosome veinlets. Amphibole in the melanosome of mafic dykes was formed with breakdown of biotite and implies addition of H2O to the system during partial melting. Towards the deeper zones, the partially melted metatexite-granite can be characterized by K-feldspar+plagioclase+quartz+ortho/clinopyroxene+biotite+Fe-Ti-oxide+apatite mineral assemblage. The felsic veins with either pegmatitic or aplititic textures display sharp contact both to the granite and the mafic veins. They are characterized by K-feldspar+quartz±plagioclase±muscovite mineral assemblage. Sporadic occurrence of muscovite suggest local fluid saturated conditions. Emplacement of gabbroic rocks of the SKI generated intense shear in some zones of the granitic footwall resulting in formation of biotite-rich mylonites with

  11. Metamorphic belts of Anatolia

    NASA Astrophysics Data System (ADS)

    Oberhänsli, Roland; Prouteau, Amaury; Candan, Osman; Bousquet, Romain

    2015-04-01

    Investigating metamorphic rocks from high-pressure/low-temperature (HP/LT) belts that formed during the closure of several oceanic branches, building up the present Anatolia continental micro-plate gives insight to the palaeogeography of the Neotethys Ocean in Anatolia. Two coherent HP/LT metamorphic belts, the Tavşanlı Zone (distal Gondwana margin) and the Ören-Afyon-Bolkardağ Zone (proximal Gondwana margin), parallel their non-metamorphosed equivalent (the Tauride Carbonate Platform) from the Aegean coast in NW Anatolia to southern Central Anatolia. P-T conditions and timing of metamorphism in the Ören-Afyon-Bolkardağ Zone (>70?-65 Ma; 0.8-1.2 GPa/330-420°C) contrast those published for the overlying Tavşanlı Zone (88-78 Ma; 2.4 GPa/500 °C). These belts trace the southern Neotethys suture connecting the Vardar suture in the Hellenides to the Inner Tauride suture along the southern border of the Kirşehir Complex in Central Anatolia. Eastwards, these belts are capped by the Oligo-Miocene Sivas Basin. Another HP/LT metamorphic belt, in the Alanya and Bitlis regions, outlines the southern flank of the Tauride Carbonate Platform. In the Alanya Nappes, south of the Taurides, eclogites and blueschists yielded metamorphic ages around 82-80 Ma (zircon U-Pb and phengite Ar-Ar data). The Alanya-Bitlis HP belt testifies an additional suture not comparable to the northerly Tavşanlı and Ören-Afyon belts, thus implying an additional oceanic branch of the Neotethys. The most likely eastern lateral continuation of this HP belt is the Bitlis Massif, in SE Turkey. There, eclogites (1.9-2.4 GPa/480-540°C) occur within calc-arenitic meta-sediments and in gneisses of the metamorphic (Barrovian-type) basement. Zircon U-Pb ages revealed 84.4-82.4 Ma for peak metamorphism. Carpholite-bearing HP/LT metasediments representing the stratigraphic cover of the Bitlis Massif underwent 0.8-1.2 GPa/340-400°C at 79-74 Ma (Ar-Ar on white mica). These conditions compares to the Tav

  12. The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

    NASA Astrophysics Data System (ADS)

    Lister, Gordon S.; Davis, Gregory A.

    Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, as the result of isostatic rebound due to tectonic denudation, but most likely also as the result of local isostatic adjustments due to granite intrusion in the middle crust. A succession of splays branch off from the master detachment fault at depth, excising substantial portions of the lower portions of the upper plate as successive detachment faults eat upwards through it. At the same time, detachment faults incise into progressively deeper levels of the lower plate, although the amount of incision is limited, because the locus of movement remains at approximately the same level in the lower plate. The detachment faults presently observed in the metamorphic core complexes are relatively young features, formed late in the geological evolution of these bodies, and are only the last in a succession of low-angle normal faults that sliced through the upper crust at the upward terminations of major, shallow-dipping, ductile shear zones in the extending Cordilleran orogen. Excisement of listric fault bottoms can explain some of the enigmatic domino-like fault blocks, and other structural relations observed in these terranes. Evidence in support of this model is illustrated from detachment terranes in the northern Colorado River region of southern Nevada, southeastern California and western Arizona, where multiple generations of detachment faults have produced remarkable

  13. Metamorphic reactions, deformation localization and rheological evolution of gabbro to eclogite transformation: A case study from Lofoten Anorthosite Complex, Norway

    NASA Astrophysics Data System (ADS)

    Nasipuri, P.; Stunitz, H.; Menegon, L.; Ravna, E. J.; Kullerud, K.; Berger, A.

    2011-12-01

    Eclogitization of lower crustal rocks during subduction processes and subsequent retrogression during exhumation will produce a range of mineral assemblages over widely spaced P-T conditions affecting the rheological behavior of the rock significantly. In this contribution, we describe the formation of syn-kinematic mineral assemblages which promotes development of centimeter- to meter-scale shear zones in undeformed leucogabbro from Lofoten, Norway. Plagioclase (Pl0), olivine (Ol0) and orthopyroxene (Opx0) form the igneous texture in the leucogabbro. Opx1 and Grt1 corona indicate the onset of metamorphism (M1) during subduction processes. Cm- wide alternating bands of a mixture of Pl-Amphibole(Amph) ± Spl and GrtI(M1) - Opx -Omph (M2A) phase mixtures characterize prograde metamorphic shear zones in the leucogabbro. Elongated Omph overgrow deformed Opx grains (D1). Relicts of Opx grains are surrounded by recrystallized opx indicating that the Opx grains survived in the Omph stability field. Grt- Cpx - Pl symplectite (M2B) rim the Omph± Opx porphyroblasts. GrtII(M3) occurs along the contact of deformed Opx± Omph and Pl. Amph ± Spl (M4) overgrow the finer grained pyroxene. Conventional thermobarometry indicate that metamorphism (M1) starts at 7000C at 1 GPa pressure. The peak PT-condition (650-7500C, > 1.8 GPa) was attained during the formation of Omph. The retrograde PT-path is represented by Cpx-Pl symplectite (600-6500C, 1 GPa) and Grt corona (6000C, 0.8 GPa). Replacement of fine-grained Cpx by Amph represents the last stage of metamorphism. Crystal orientation of Opx relicts is different from the CPO of the newly formed Opx (10 - 15 μm in size) indicating that Opx did not recrystallize by progressive subgrain rotation. The dramatic grain size reduction without significant change in chemical composition is probably caused by cracking of Opx at the onset of deformation. Small new grains may nucleate from fragments. In the fine grained Pl+ Amph± Spl aggregates

  14. Geochemical features of metabasic rocks from an Early to Middle Jurassic Accretionary Complex (Refahiye metamorphics, Eastern Pontides, NE Turkey): Implications for Late Jurassic-Early Cretaceous magmatic lull

    NASA Astrophysics Data System (ADS)

    Göçmengil, G.; Topuz, G.; Çelik, Ö. F.; Altıntaş, Ä.°. E.; Özkan, M.

    2012-04-01

    The Refahiye metamorphics (Eastern Pontides, NE Turkey) represent a metamorphosed accretionary complex of Early to Middle Jurassic age and occur as an interleave between coeval ophiolite. This Early to Middle Jurassic metamorphics and ophiolites are bound by a Permo-Triassic accretionary complex in the north and a Late Cretaceous accretionary complex in the south. The Refahiye metamorphics are made up of greenschist, marble, serpentine, phyllite and subordinately amphibolite, micaschist, eclogite and metachert knockers. The Jurassic and Late Cretaceous accretionary complexes in Eastern Mediterranean are related to the consumption of a Mesozoic ocean, the so-called Neo-Tethys. Regional geology in the Eastern Pontides indicate that the Early to Middle Jurassic and Late Cretaceous times correspond to volumious igneous activity, while Late Jurassic and Early Cretaceous time to an igneous lull. Here we present whole-rock geochemical data on metabasic rocks from the Refahiye accretionary complex, and discuss these data in terms of accreted material and its implications for the Jurassic evolution of the Eastern Pontides. All the metabasic rocks are well recrystallized, free of any relict texture and are variably hydrated (LOI ~ 1.3-5.1 wt%). Some samples are characterized by the unusually high-Al2O3 contents (up to 20.8 wt%) suggestive of derivation from high-Al basalts. Geochemically three distinct metabasic group are distinguished, on the basis of fluid immobile HFSEs and REEs. Group I is characterized by moderately to strongly fractionated REE patterns [(La/Yb)cn ~8-18], absence of any Nb-Ta anomaly in multi element variation diagrams and high Ti and low Zr/Nb ratios (3.68-5.72), corresponding to unorogenic alkaline basalts (ocean island basalt). Group II characterized by moderately fractionated REE ratios [(La/Yb)cn ~0.6-2.6], absence of any Nb-Ta anomaly, resembling unorogenic tholeiitic basalts (E and N-MORB). Group III on the other hand, displays unfractionated

  15. High-pressure mafic oceanic rocks from the Makbal Complex, Tianshan Mountains (Kazakhstan & Kyrgyzstan): Implications for the metamorphic evolution of a fossil subduction zone

    NASA Astrophysics Data System (ADS)

    Meyer, Melanie; Klemd, Reiner; Konopelko, Dmitry

    2013-09-01

    The Makbal Complex in the western Tianshan Mountains of Kazakhstan and Kyrgyzstan consists of HP/UHP metasedimentary host rocks which enclose various HP mafic blocks or boudins. These mafic rocks comprise rare eclogites (sensu stricto and sensu lato), garnet amphibolites (retrograded eclogites) and a newly discovered glaucophanite (glaucophane-garnet-omphacite bearing rock). So far the Makbal Complex has been interpreted to predominantly consist of continental lithologies and the mafic rocks were considered as dismembered dikes intruding continental metasediments. This interpretation is mainly based on the geological relationship and bulk rock chemistry of the different rock types. It was further suggested that the continental lithologies of the Makbal Complex underwent eclogite-facies metamorphism in a former subduction zone. In the present study we combined conventional geothermometry, P-T pseudosection modeling and major and trace element whole rock geochemistry for different mafic samples (glaucophanite and eclogites (sensu lato)) in order to shed light on both the metamorphic evolution and the protoliths of the mafic HP rocks in the Makbal Complex. Prograde to peak-pressure clockwise P-T paths of glaucophanite and eclogites (sensu lato) were modeled using garnet isopleth thermobarometry. The results show that the glaucophanite and eclogite (sensu lato) samples experienced similar prograde P-T paths and slightly different peak metamorphic conditions at ~ 560 °C at 2.4 GPa for the former and between ~ 520 °C at 2.2 GPa and ~ 555 °C at ~ 2.5 GPa for the latter, corresponding to burial depths between 70 and 85 km. Whole rock major and trace element analyses and petrological evidence imply that the various rock types at the Makbal Complex most likely originated from different precursor rocks. Eclogites (sensu lato) are believed to represent strongly retrogressed former eclogite-facies rocks that had never been eclogites (sensu stricto, i.e. > 70 vol.% garnet and

  16. The transition from large-magnitude extension to distributed dextral faulting in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

    NASA Astrophysics Data System (ADS)

    Singleton, John S.

    2015-08-01

    Brittle fault data from the Buckskin-Rawhide metamorphic core complex, west-central Arizona, document the Miocene transition from large-magnitude, NE directed extension to distributed E-W extension and dextral faulting. The Buckskin-Rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top-NE directed slip to ENE and E directed slip during the last stages of activity. Postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply NE dipping faults. Relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle Miocene detachment fault slip, whereas dextral and oblique dextral faulting postdates detachment fault slip (post ~11 Ma). Based on the persistent SW dip of lower plate fabrics and NE dip of abundant normal faults, the amount of NE-SW brittle extension across the mylonitic lower plate is estimated to be 20-30% in several areas. The end of detachment faulting in the Buckskin-Rawhide metamorphic core complex was marked by a transition to dominantly E-W extension and associated dextral-normal faulting. At least 10 NE dipping postdetachment faults record ~0.1-1 km dextral or oblique dextral displacement. The cumulative amount of dextral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment dextral/transtensional faulting across the core complex reflects the increasing influence of the diffuse Pacific-North America transform plate boundary toward the end of the middle Miocene.

  17. SHRIMP U-Pb age and high temperature conditions of the collisional metamorphism in the Várzea do Capivarita Complex: Implications for the origin of Pelotas Batholith, Dom Feliciano Belt, southern Brazil

    NASA Astrophysics Data System (ADS)

    Philipp, Ruy Paulo; Bom, Francisco Molina; Pimentel, Márcio Martins; Junges, Sérgio Luiz; Zvirtes, Gustavo

    2016-03-01

    The Várzea do Capivarita Complex is composed of pelitic gneisses with subordinate calc-silicate rock, marble and rare quartzite. It is part of the neoproterozoic Dom Feliciano Belt, in southern Brazil. The gneisses are associated to veins and tabular leucogranite bodies, which are the product of anatexis of the pelitic gneiss. The paragneisses of the Várzea do Capivarita Complex are tectonically juxtaposed to orthogneisses of the Arroio dos Ratos Complex. This complex is exposed as megaxenoliths in granites of the Encruzilhada do Sul Suite and as smaller fragments in the Quitéria and Cordilheira Granites, all part of the Pelotas Batholith. The metamorphic foliation is oriented to N30°W, with dips between 35 and 55° to the SW and mineral lineation is suborizontal with rake ranging from 15° to 30° and down dip to NW and SE, suggesting deformation associated with a transpressive system. Based on the paragenesis garnet-cordierite-sillimanite-biotite, metamorphism occurred at 720-820° C and pressure of 8-9 kbar, characterizing it as of intermediate pressure and high temperature series. Zircon grains of one sample of garnet-cordierite-sillimanite-biotite gneiss and one of peraluminous leucogranite was dated by the U-Pb SHRIMP method. The paragneiss metamorphic zircon yielded an age of 619 ± 4.3 Ma interpreted as an age of the main metamorphic event, whereas igneous zircon grains from the leucogranite indicates that magma crystallization took place at 620 ± 6.3 Ma. The main metamorphic foliation (S2) of the paragneiss and the leucogranite magmatic foliation (S0) are parallel to each other, indicating that they were formed during the same event. This suggests that anatexis of the paragneisses of the Várzea do Capivarite Complex generated the peraluminous leucogranites right after the climax of the collisional metamorphism.

  18. The Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean: Further evidence from the Barleik subduction-related metamorphic complex in the West Junggar region, NW China

    NASA Astrophysics Data System (ADS)

    Liu, Bo; Han, Bao-Fu; Xu, Zhao; Ren, Rong; Zhang, Jin-Rui; Zhou, Jing; Su, Li; Li, Qiu-Li

    2016-06-01

    In this study, we present new evidence from the Barleik subduction-related metamorphic complex in the southern West Junggar region, northwestern China, for the Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean. The Barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic mélange and their protoliths are calc-alkaline andesite and alkali and tholeiitic basalts. The calc-alkaline andesite has a zircon U-Pb age of 502 ± 2 Ma, obtained from magmatic cores of zircon grains, and shares geochemical features similar to the 515-485 Ma intra-oceanic arc magmatic rocks in the West Junggar region. By contrast, the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid-ocean ridge basalts, respectively. Rutile and sodic-calcic amphibole from the amphibolite have a U-Pb age of 502 ± 25 Ma and a 40Ar/39Ar age of ∼504 Ma, respectively, which are in good agreement within errors with a 40Ar/39Ar age of 492 ± 4 Ma for phengite from the blueschist. These metamorphic ages of ∼500 Ma are interpreted to represent the timing of Pacific-type subduction-related metamorphism and are also compatible with ages of the oldest supra-subduction zone ophiolites (531-512 Ma) and intra-oceanic arc plutons (515-485 Ma) in the southern West Junggar region. Being one of the oldest subduction-related metamorphic complexes (509-490 Ma) in the southern Central Asian Orogenic Belt, the Barleik metamorphic complex, together with the oldest arc plutons, definitely indicate the initial intra-oceanic subduction in the southern Paleo-Asian Ocean at least in the Early Cambrian.

  19. An enigmatic source of hematitic carbonate beds containing vast amounts of iron oxidizers in a paleozoic metamorphic complex, South Hungary, Geresd-Hills, Ófalu.

    NASA Astrophysics Data System (ADS)

    Jáger, Viktor; Dabi, Gergely; Menyhárt, Adrienn

    2013-04-01

    Near the village of Ófalu, in the Geresd Hills, South Hungary, within the "Mecsekalja tectonic belt", low and intermediate grade paleozoic metamorphic complex (phyllite, gneiss) contains vein-like hematitic carbonate beds, up to 30 cm in thickness. The carbonate mineral is calcite. These hematitic carbonate beds cross-cut the foliation of the phyllite, and show no signs of any metamorphic alteration. In the studied section the red carbonate beds are associated with a vein system filled with multiple generations of vein carbonates(Dabi et al., 2011). The red carbonate beds contain a vaste number of twisted stalks of the iron oxidizing taxon of Gallionella. Rarely in some siliceous parts, Leptothrix-like microbial fossils can be found and these beds also contain numerous unidentifiable, hematitic foraminifers. According to ICP-AES measurements, the hematitic carbonate beds contains 8 % Fe, 0.86 % Mn and 0.12 % Ba. XRD and Raman measurements proved that the iron phase is hematite. The SEM observations revealed that the bacterial microfossils and foraminifers are built up of micron-submicron sized pseudohexagonal platy hematite. The bacterial microfossils of the Gallionella iron oxidizer are very well preserved and reaches about 80 µm length and about 2-3 µm width. The above observations raise the following issues: 1. how did these non metamorphic hematitic-carbonatic beds get inside into the metamorphic complex?, 2. what is the age of the formation of these beds?, and 3. what was the source of the iron? If we consider that the hematitic beds contain foraminifers and iron oxidizing bacteria, and no signs of metamorphic alteration nor foliations can be observed in these beds, the only answer for the first question is that the formations are fractures filled with lime-mud, i.e. neptunian dykes, which penetrated into the cracks of the phyllite. The presence of foraminifers and the geotectonic situation of the unit imply marine origin. Considering that these beds are

  20. Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

    NASA Astrophysics Data System (ADS)

    Martens, Uwe C.; Brueckner, Hannes K.; Mattinson, Christopher G.; Liou, Juhn G.; Wooden, Joseph L.

    2012-08-01

    A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic-Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet-clinopyroxene-phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~ 700 °C and ~ 2.1-2.4 GPa. SHRIMP-RG U-Pb dating of eclogite metamorphic zircon yielded a 75.5 ± 2 Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm-Nd clinopyroxene-two garnet-whole rock isochron from an eclogite band yielded a less precise but consistent age of 77 ± 13 Ma. The above features imply subduction to > 60 km depth of at least a portion of the North American sialic basement during Late Cretaceous collision. The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole + plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5 ± 3.5 Ma SHRIMP-RG U-Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results. Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking

  1. Evidence for Late Cretaceous-early Tertiary lower plate mylonitization and extension in the Harcuvar metamorphic core complex, Arizona: Evidence from U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Wrobel, A.; Wong, M.; Singleton, J. S.

    2014-12-01

    Metamorphic core complexes in the North American Cordillera are interpreted as sites of large-magnitude crustal extension, yet many aspects of their development remain controversial. Most core complexes in the central and southern Basin and Range are thought to have been exhumed by a single fault that evolved from ductile to brittle during the Miocene. However, new field relationships and U-Pb zircon geochronology from the lower plate of the Harcuvar core complex in western Arizona raise questions about the age and tectonic significance of mylonites in the evolution of this core complex. Variably mylonitic plutonic rocks dominate the lower plate. Plutonic units yield three major age populations. A foliated biotite granite yielded a U-Pb zircon age of 152 ± 3 Ma, and a gneiss with an E-W lineation yielded a similar 152 ± 4 Ma age, with some 179 ± 9 Ma cores. Leucogranite plutons and sills dominate much of the footwall and yield 71-72 Ma ages, with some inherited ~152 and ~180 Ma cores. A migmatitic biotite gneiss yielded younger U-Pb zircon ages of ~55 Ma and ~61 Ma with some ca. 1.4-1.5 Ga cores. Pegmatite dikes that cross-cut top-NE-directed mylonitic shear zones at high angles and are only weakly deformed also yield 55-64 Ma ages. These results indicate that a phase of Middle to Late Jurassic magmatism was followed by a major magmatic event at ~72 Ma that emplaced large volumes of leucogranite. Minor melting (mainly pegmatites) continued until ca. 55 Ma. The pegmatites are likely late to post-tectonic and suggest that much of the lower plate mylonitization, including shearing related to top-NE extension, occurred during the Late Cretaceous to early Tertiary (Laramide) rather than the Miocene. Laramide extension may have been driven by Late Cretaceous crustal thickening and partial melting that weakened the crust and caused gravitational collapse. Thus, Miocene extension mainly occurred by brittle fault slip rather than widespread ductile shearing. These

  2. A new model for the Toarcian global climate change: Explosive venting of greenhouse gases from metamorphic aureoles around sill complexes in the Karoo basin

    NASA Astrophysics Data System (ADS)

    Svensen, H.; Planke, S.; Chevallier, L.; Malthe-Sorenssen, A.; Jamtveit, B.; Corfu, F.

    2005-12-01

    Emplacement of voluminous intrusive complexes in organic-rich sedimentary basins may cause global climate change (e.g., intrusive volcanism in the NE Atlantic may have caused the Initial Eocene Thermal Maxima). The Karoo basin in South Africa was intruded by voluminous basaltic melts in the early Jurassic. Black organic-rich shale in the lower parts of the Karoo basin (the Ecca Group) were intruded by up to 130 m thick sub-horizontal magmatic sills, whereas dominantly saucer-shaped sills were emplaced in the overlying fluvial sedimentary sequences. The sills are associated with numerous hydrothermal vent complexes formed in a sub-aerial environment and deep-level breccia pipes. Field relations, borehole data, geochemistry, and geodynamic modeling show that both the deep pipes and the shallow hydrothermal vent complexes were formed as a consequence of pressure build-up in contact aureoles surrounding the sill intrusions. Analyses of shale from the metamorphic aureoles show that most of the organic carbon was lost during the heating event. The formed greenhouse gases (e.g., methane) were explosively released to the atmosphere through the pipes and the vent complexes. Mass balance calculations suggest that the volume of the released greenhouse gases were likely sufficient to initiate the Toarcian greenhouse. The causal relationship between the intrusive event and the Toarcian global warming is corroborated by new U-Pb zircon ages from a dolerite sill associated with breccia pipes in the Ecca Group, both occurring at about 183 Ma.

  3. The timing of eclogite facies metamorphism and migmatization in the Orlica–Śnieżnik complex, Bohemian Massif: Constraints from a multimethod geochronological study

    USGS Publications Warehouse

    Brocker, M.; Klemd, R.; Cosca, M.; Brock, W.; Larionov, A.N.; Rodionov, N.

    2009-01-01

    The Orlica–Śnieżnik complex (OSC) is a key geological element of the eastern Variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks. Sporadic occurrences of eclogites and granulites record high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic conditions. A multimethod geochronological approach (40Ar–39Ar, Rb–Sr, Sm–Nd, U–Pb) has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks. Special attention was given to the unresolved significance of a 370- to 360 Ma age group that was repeatedly described in previous studies. Efforts to verify the accuracy of c.370 Ma K–Ar phengite and biotite dates reported for an eclogite and associated country-rock gneiss from the location Nowa Wieś suggest that these dates are meaningless, due to contamination with extraneous Ar. Extraneous Ar is also considered to be responsible for a significantly older 40Ar–39Ar phengite date of c. 455 Ma for an eclogite from the location Wojtowka. Attempts to further substantiate the importance of 370–360 Ma zircon dates as an indicator for a melt-forming high-temperature (HT) episode did not provide evidence in support of anatectic processes at this time. Instead, SHRIMP U–Pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and (U)HP granulites revealed two age populations (490–450 and 345–330 Ma respectively) that correspond to protolith ages of the magmatic precursors and late Variscan anatexis. The results of this study further underline the importance of Late Carboniferous metamorphic processes for the evolution of the OSC that comprise the waning stages of HP metamorphism and lower pressure HT overprinting with partial melting. Eclogites and their country rocks provided no chronometric evidence for an UHP and ultrahigh-temperature episode at 387–360 Ma, as recently suggested for granulites from the OSC, based on Lu–Hf garnet

  4. The early Cretaceous orogen-scale Dabieshan metamorphic core complex: implications for extensional collapse of the Triassic HP-UHP orogenic belt in east-central China

    NASA Astrophysics Data System (ADS)

    Ji, Wenbin; Lin, Wei; Faure, Michel; Shi, Yonghong; Wang, Qingchen

    2016-03-01

    The Dabieshan massif is famous as a portion of the world's largest HP-UHP metamorphic belt in east-central China that was built by the Triassic North-South China collision. The central domain of the Dabieshan massif is occupied by a huge migmatite-cored dome [i.e., the central Dabieshan dome (CDD)]. Origin of this domal structure remains controversial. Synthesizing previous and our new structural and geochronological data, we define the Cretaceous Dabieshan as an orogen-scale metamorphic core complex (MCC) with a multistage history. Onset of lithospheric extension in the Dabieshan area occurred as early as the commencement of crustal anatexis at the earliest Cretaceous (ca. 145 Ma), which was followed by primary (early-stage) detachment during 142-130 Ma. The central Dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top-to-the-NW shearing. It is thus inferred that the primary detachment was initiated from a flat-lying detachment zone at the middle crust level. Removal of the orogenic root by delamination at ca. 130 Ma came into the extensional climax, and subsequently isostatic rebound resulted in rapid doming. Along with exhumation of the footwall, the mid-crustal detachment zone had been warped as shear zones around the CDD. After 120 Ma, the detachment system probably experienced a migration accommodated to the crustal adjustment, which led to secondary (late-stage) detachment with localized ductile shearing at ca. 110 Ma. The migmatite-gneiss with HP/UHP relicts in the CDD (i.e., the central Dabieshan complex) was product of the Cretaceous crustal anatexis that consumed the deep-seated part of the HP-UHP slices and the underlying para-autochthonous basement. Compared with the contemporaneous MCCs widely developed along the eastern margin of the Eurasian continent, we proposed that occurrence of the Dabieshan MCC shares the same tectonic setting as the "destruction of the North China craton". However, geodynamic trigger

  5. Late Cretaceous Localized Crustal Thickening as a Primary Control on the 3-D Architecture and Exhumation Histories of Cordilleran Metamorphic Core Complexes

    NASA Astrophysics Data System (ADS)

    Gans, P. B.; Wong, M.

    2014-12-01

    The juxtaposition of mylonitic mid-crustal rocks and faulted supracrustal rocks in metamorphic core complexes (MMCs) is usually portrayed in 2 dimensions and attributed to a single event of large-scale slip ± isostatic doming along a low-angle "detachment fault"/ shear zone. This paradigm does not explain dramatic along strike (3-D) variations in slip magnitude, footwall architecture, and burial / exhumation histories of most MMCs. A fundamental question posed by MMCs is how did their earlier thickening and exhumation histories influence the geometric evolution and 3-D slip distribution on the subsequent detachment faults? New geologic mapping and 40Ar/39Ar thermochronology from the Snake Range-Kern Mts-Deep Creek Mts (SKDC) complex in eastern Nevada offer important insights into this question. Crustal shortening and thickening by large-scale non-cylindrical recumbent folds and associated thrust faults during the late Cretaceous (90-80 Ma) resulted in deep burial (650°C, 20-25 km) of the central part of the footwall, but metamorphic grade decreases dramatically to the N and S in concert with decreasing amplitude on the shortening structures. Subsequent Paleogene extensional exhumation by normal faulting and ESE-directed mylonitic shearing is greatest in areas of maximum earlier thickening and brought highest grade rocks back to depths of~10-12 km. After ≥15 Ma of quiescence, rapid E-directed slip initiated along the brittle Miocene Snake Range detachment at 20 Ma and reactivated the Eocene shear zone. The ≥200°C gradient across the footwall at this time implies that the Miocene slip surface originated as a moderately E-dipping normal fault. This Miocene slip surface can be tracked for more than 100 km along strike, but the greatest amount of Miocene slip also coincides with parts of the footwall that were most deeply buried in the Cretaceous. These relations indicate that not only is the SKDC MMC a composite feature, but that the crustal welt created by

  6. Geochemistry of the Neoproterozoic metasediments of Malhaq and Um Zariq formations, Kid Metamorphic Complex, Sinai, Egypt: implications for source-area weathering, provenance, recycling, and depositional tectonic setting

    NASA Astrophysics Data System (ADS)

    El-Bialy, Mohammed Z.

    2013-04-01

    The Kid Metamorphic Complex of SE Sinai represents a thick volcano-sedimentary succession that underwent polyphase deformation and greenschist to upper amphibolite facies metamorphism in the NE part of the Arabian-Nubian Shield (ANS). The Malhaq and Um Zariq Formations, the target of this study, occupy roughly the northern half of this complex. The Malhaq Formation records several phases of Ediacaran sedimentation and volcanic activity (615-607 Ma), whereas Um Zariq Formation metasediments are relicts of an older sedimentary sequence (Cryogenian; 813±6 Ma). The Malhaq Formation comprises a series of dark gray structureless to schistose felsic to intermediate metavolcanics interbedded and intercalated with fine- to medium-grained foliated mica-rich phyllites and schists, while the Um Zariq Formation is a dominantly metasedimentary sequence, mainly represented by well-bedded metapelitic schists. Malhaq metasediments are enriched in SiO2, CaO and K2O and depleted in TiO2, Al2O3 and K2O relative to those of Um Zariq Formation. Aside from the relatively low Ni and Cr concentrations, compatible transition elements of these metasediments are comparable to average crustal contents. Except for marked Sr depletion, LILEs are around average continental crust values. Pronounced negative Nb-Ta anomalies in all samples, and general enrichment of Um Zariq samples in Th, U, Zr, Ti and Y relative to Malhaq ones are the main features of HFSEs. The REE patterns of all samples are parallel to sub-parallel LREE-enriched, with distinct negative Eu anomalies and weakly fractionated HREE segments. Geochemical investigations have revealed that the source rocks of Malhaq Formation metasediments underwent mild to moderate chemical weathering, whereas those of Um Zariq Formation have suffered severe chemical weathering. These metasediments are predominately derived from felsic to intermediate igneous sources, with a particular slight addition from recycled sedimentary source to the Malhaq

  7. What do fault patterns reveal about the latest phase of extension within the Northern Snake Range metamorphic core complex, Nevada, USA?

    NASA Astrophysics Data System (ADS)

    Ismat, Zeshan; Riley, Paul; Lerback, Jory

    2016-08-01

    The Northern Snake Range is a classic example of a metamorphic core complex, Basin-and-Range province, United States. It is composed of a plastically deformed footwall and a brittlely deformed hanging wall, separated by the Northern Snake Range low-angle detachment (NSRD). Brittle deformation, however, is not confined to the hanging wall. This paper focuses on exposures in Cove Canyon, located on the SE flank of the Northern Snake Range, where penetrative, homogeneous faults are well exposed throughout the hanging wall, footwall and NSRD, and overprint early plastic deformation. These late-stage fault sets assisted Eocene-Miocene extension. Detailed analysis of the faults reveals the following: (1) The shortening direction defined by faults is similar to the shortening direction defined by the stretching lineation in the footwall mylonites, indicating that the extensional kinematic history remained unchanged as the rocks were uplifted into the elastico-frictional regime. (2) After ∼17 Ma, extension may have continued entirely within elastic-frictional regime via cataclastic flow. (3) This latest deformation phase may have been accommodated by a single, continuous event. (3) Faults within NSRD boudins indicate that deformation within the detachment zone was non-coaxial during the latest phase of extension.

  8. U-Pb Geochronology of Detrital Zircon in Quartzites of the Sawtooth Metamorphic Complex, Sawtooth Range, Idaho, U.S.A

    NASA Astrophysics Data System (ADS)

    Bergeron, P. G.; Dutrow, B. L.; Mueller, P. A.; Foster, D. A.

    2012-12-01

    Exposed segments of the lower to middle crust provide windows into basement terranes underlying parts of cratonic North America hidden by Phanerozoic cover. In southwest Laurentia, the Selway terrane was proposed to extend from the Wyoming province to the rifted margin of Laurentia. The Sawtooth Metamorphic Complex (SMC), a series of mid-high grade metamorphic rocks that occur within the Sawtooth Range, central Idaho, lies within the proposed boundaries of the Selway, and may be an exposed segment of that terrane. Detrital zircons from five quartzite and quartzofeldspathic samples were analyzed by LA-ICP-MS to determine U/Pb ages that could be used to establish age, provenance and any distinguishing features of their age-distributions. The eastern-most alkali feldspar-biotite quartzite yielded a range of ages from 1070 - 2830 Ma with peak age frequencies from 1050-1100, 1350-1450, and 1750-1800 Ma (n = 52). The western most (> 2 km west), rutile-bearing plagioclase-muscovite-biotite quartzite yielded similar peak age frequencies from 1050-1150, 1400-1450, and 1750-1800 Ma, but contained a larger range of ages from 492 ±13 to 2710 Ma (n = 82). An alkali-feldspar-biotite-muscovite quartzite sampled from a spatially intermediate unit yielded a distinctly older age distribution with ages from 1660-2920 Ma (n=58). Statistically, detrital sources appear to be similar to those of Belt and/or Windermere Supergroups that have provenance in the Southern Canadian Shield and the southwestern United States. Younger zircons in the quartzites are consistent with correlations to the Neoproterozoic Windermere and Cambrian passive margin strata. Abundant Grenville-aged (1150-1000 Ma) grains and ɛHf signatures similar to A-type magmas from the southwestern U.S. support a possible southeastern-eastern provenance for SMC quartzites similar to Windermere - Cordilleran material. These detrital zircon ages suggest that these units of the SMC most likely consist of a series of

  9. P-T evolution of a spinel + quartz bearing khondalite from the Highland Complex, Sri Lanka: Implications for non-UHT metamorphism

    NASA Astrophysics Data System (ADS)

    Dharmapriya, P. L.; Malaviarachchi, Sanjeewa P. K.; Galli, Andrea; Su, Ben-Xun; Subasinghe, N. D.; Dissanayake, C. B.; Nimalsiri, T. B.; Zhu, Bin

    2014-12-01

    Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67-12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03-9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09-4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO = 7.61-7.97 wt% and Cr2O3 = 5.99-6.27 wt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing khondalites and associated intermediate granulites did not exceed T of 900 °C and P of 7.5-8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770 °C and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P < 6.5 kbar and T of 770 °C. We propose that the incorporation of large amount of Zn into spinel from exotic, metasomatic fluids and possibly incorporation of Fe3+ into spinel under high oxidizing conditions may have shifted the stabilization of co-existing spinel + quartz to T < 900 °C. Hence, this study provides insights into the occurrence of spinel + quartz as a non- UHT assemblage suggesting that the coexistence of spinel + quartz should be treated with care and considered only as indicative, but not diagnostic of UHT metamorphism.

  10. Linking deformation structures and low-temperature metamorphic recrystallization: chlorite-illite equilibria in the Nevado-Filabrides complex (Betic Cordillera, Spain)

    NASA Astrophysics Data System (ADS)

    Jentzer, Michael; Verlaguet, Anne; Dubacq, Benoit; Agard, Philippe

    2013-04-01

    Ductile deformation is partly accommodated by mineral recrystallization. Mineral growth is controlled by both thermodynamics and reaction kinetics, where fluid availability and deformation also play a key role, in particular at temperatures below about 400°C. In these temperature conditions, phyllosilicates are ubiquitous and reactive, and often replace the peak metamorphic minerals along the retrograde path. Phyllosilicate assemblages are thus of potential to estimate the pressure-temperature conditions of deformation and provide information on the dynamics of shear zone development. This study focuses on a cross-section in the Nevado-Filabrides complex (Betic cordillera, Spain), which was exhumed along a ductile detachment. We present four outcrops located at different distances from the detachment, for which both the deformation structure densification with detachment proximity and the metamorphic P-T conditions have already been well constrained. The studied unit contains abundant chlorite and illite, often observed seemingly at textural equilibrium. In order to highlight the link between phyllosilicate crystallization and deformation, electron microprobe chemical maps were performed on deformation microstructures (schistosity, shear bands, pressure shadows) where chlorite and illite are present. Using a multi-equilibrium approach with a thermodynamic model for clay minerals, we estimated P-T conditions of crystallization for the successive chlorite-white mica and chlorite-illite assemblages. Obtained P-T paths span the range 250°C - 550°C. These results are in good agreement with calculated pseudo-sections but are inconsistent with parts of the previously published retrograde paths, whose low-pressure high-temperature results are likely offset. Interestingly, chlorite and illite seemingly at textural equilibrium were very rarely computed as such. This might be explained by 1) inadequacy of the thermodynamic models at the latest conditions of re

  11. Crustal structure in the Elko-Carlin Region, Nevada, during Eocene gold mineralization: Ruby-East Humboldt metamorphic core complex as a guide to the deep crust

    USGS Publications Warehouse

    Howard, K.A.

    2003-01-01

    The deep crustal rocks exposed in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada, provide a guide for reconstructing Eocene crustal structure ???50 km to the west near the Carlin trend of gold deposits. The deep crustal rocks, in the footwall of a west-dipping normal-sense shear system, may have underlain the Pin??on and Adobe Ranges about 50 km to the west before Tertiary extension, close to or under part of the Carlin trend. Eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation. The magnitude and timing of Paleogene extension remain indistinct, but dikes and tilt axes in the upper crust indicate that spreading was east-west to northwest-southeast, perpendicular to a Paleozoic and Mesozoic orogen that the spreading overprinted. High geothermal gradients associated with Eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust. Late Eocene eruptions, upper crustal dike intrusion, and gold mineralization approximately coincided temporally with deep intrusion of Eocene sills of granite and quartz diorite and shallower intrusion of the Harrison Pass pluton into the core-complex rocks. Stacked Mesozoic nappes of metamorphosed Paleozoic and Precambrian rocks in the core complex lay at least 13 to 20 km deep in Eocene time, on the basis of geobarometry studies. In the northern part of the complex, the presently exposed rocks had been even deeper in the late Mesozoic, to >30 km depths, before losing part of their cover by Eocene time. Nappes in the core plunge northward beneath the originally thicker Mesozoic tectonic cover in the north part of the core complex. Mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core-complex intrusions and nappes and the overlying upper crust. These structures, as well as the subsequent large

  12. Applicability of the RSCM geothermometry approach in a complex tectono-metamorphic context: The Jebilet massif case study (Variscan Belt, Morocco)

    NASA Astrophysics Data System (ADS)

    Delchini, Sylvain; Lahfid, Abdeltif; Plunder, Alexis; Michard, André

    2016-07-01

    The Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry approach allows determining the peak temperature recorded by metasediments through their metamorphic history. This technique, however, has been calibrated using Meso-Cenozoic metapelitic rocks that underwent a single metamorphic cycle. Until now, the reliability of the RSCM method has never been demonstrated for contexts with superposition of regional and contact metamorphism, such as many Variscan contexts. The present study aims at testing the applicability of the RSCM method to these polyphased metamorphism terrains and at investigating the cumulative molecular transformations of carbonaceous materials related to metamorphic superposition. To address the above issues, samples were collected in the Variscan Jebilet massif of the Moroccan Meseta. This massif was first affected by a regional, greenschist facies metamorphic event (D1 phase), and then by a higher-T, regional and contact metamorphism that reached the hornfels/amphibolite facies conditions (D2 and D2/D3 phases). Mineralogical, thermobarometric and RSCM methods have been used in this study to determine the peak T recorded by the studied rocks. The results obtained for greenschist facies metapelitic rocks show a good agreement between the mineralogical assemblage Chlorite-Phengite-Felspar-Quartz and the Raman temperatures ranging from 330 to 394 ± 50 °C. In the metapelitic rocks that underwent higher metamorphism grades (hornfels/amphibolite facies), four dominant mineral assemblages were observed: (1) Chlorite-Biotite, (2) Cordierite-Biotite, (3) Andalusite-Garnet-Bt, and (4) Andalusite-Cordierite-Biotite. The corresponding Raman temperatures vary respectively between 474 ± 50 °C and 628 ± 50 °C. The pseudo-sections generated for samples from the hornfels/amphibolite facies confirmed the peak temperatures measured by the RSCM method. Our results do not support clear evidence of potential molecular cumulative effect on CM

  13. The Liaonan metamorphic core complex, Southeastern Liaoning Province, North China: A likely contributor to Cretaceous rotation of Eastern Liaoning, Korea and contiguous areas

    NASA Astrophysics Data System (ADS)

    Liu, Junlai; Davis, Gregory A.; Lin, Zhiyong; Wu, Fuyuan

    2005-09-01

    The Mesozoic Liaonan metamorphic core complex (mcc) of the southeastern Liaoning province, North China, is an asymmetric Cordilleran-style complex with a west-rooting master detachment fault, the Jinzhou fault. A thick sequence of lower plate, fault-related mylonitic and gneissic rocks derived from Archean and Early Cretaceous crystalline protoliths has been transported ESE-ward from mid-crustal depths. U-Pb ages of lower plate syntectonic plutons (ca. 130-120 Ma), 40Ar- 39Ar cooling ages in the mylonitic and gneissic sequence (ca. 120-110 Ma), and a Cretaceous supradetachment basin attest to the Early Cretaceous age of this extensional complex. The recent discovery of the coeval and similarly west-rooting Waziyu mcc in western Liaoning [Darby, B.J., Davis, G.A., Zhang, X., Wu, F., Wilde, S., Yang, J., 2004. The newly discovered Waziyu metamorphic core complex, Yiwulushan, western Liaoning Province, North China. Earth Science Frontiers 11, 145-155] indicates that the Gulf of Liaoning, which lies between the two complexes, was the center of a region of major crustal extension. Clockwise crustal rotation of a large region including eastern Liaoning province and the Korean Peninsula with respect to a non-rotated North China block has been conclusively documented by paleomagnetic studies over the past decade. The timing of this rotation and the reasons for it are controversial. Lin et al. [Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implication of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. Journal of Geophysical Research 108 (B-6) (EPM 5-1 to 5-17)] proposed that a clockwise rotation of 22.5° ± 10.2° was largely post-Early Cretaceous in age, and was the consequence of extension within a crustal domain that tapers southwards towards the Bohai Sea (of which the Gulf of Liaoning is the northernmost part). Paleomagnetic studies of Early Cretaceous strata (ca 134-120 Ma) in the Yixian-Fuxin supradetachment basin of

  14. Synextensional magmatism leading to crustal flow in the Albion-Raft River-Grouse Creek metamorphic core complex, northeastern Basin and Range

    NASA Astrophysics Data System (ADS)

    Konstantinou, Alexandros; Strickland, Ariel; Miller, Elizabeth; Vervoort, Jeffrey; Fisher, Christopher M.; Wooden, Joseph; Valley, John

    2013-09-01

    study addresses the origin of granitic magmas in the Albion-Raft River-Grouse Creek (ARG) metamorphic core complex and environs and how these inform us about the deep crustal processes leading to crustal flow and the formation of the ARG. SHRIMP-RG U-Pb zircon ages, whole-rock geochemical data (major and trace element data, as well as Sr and Nd isotopes), and zircon geochemistry (in situ O-isotope, Hf-isotope, and trace element compositions) from Eocene to Oligocene magmas now exposed at three structural levels of the ARG show that the 41-32 Ma Emigrant Pass and the 32-25 Ma Cassia plutonic complexes have a common origin, sharing a deep crustal "hot zone" that remained above solidus temperatures for at least 16 Myr. This magmatism is part of the protracted magmatism that swept southward across the western U.S. between ˜42 and 21 Ma, inferred to be the result of foundering of the shallow Farallon slab. Isotopic modeling of geochemical data from these magmas suggests that between 41 and 32 Ma, the influx of mantle-derived basalt into the lower crust triggered large-scale hybrid magmatism generating calc-alkaline magmas that erupted and intruded the upper crust and significantly weakened the lower and middle crust. Between 32 and 25 Ma, this "hot zone" incorporated large amounts of continental crustal melts, resulting in greater mobility of the lower and middle crust, driving middle crustal flow and the formation of granitic plutons that rose to shallower levels of the crust forming the granite-cored gneiss domes of the ARG.

  15. Protolith age and deformation history of high grade metamorphic rocks from the roots of a continental magmatic arc: the Central Gneiss Complex, British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    MacLeod, D.; Pearson, D. M.

    2014-12-01

    The Central Gneiss Complex, located in western British Columbia, preserves mid and lower crustal igneous and metasedimentary rocks that yield insight into deformational and thermal processes at the roots of a continental magmatic arc. The complex primarily consists of gneiss interpreted as a volcanic sequence with calcareous interlayers and lesser clastic metasedimentary rocks. Cretaceous U-Pb zircon ages from granulite-facies equivalent rocks in the core of the complex hint at rapid burial following deposition. However, a Permian or older crinoid fossil found in one locality (Hill, 1985) requires the presence of some late Paleozoic or early Mesozoic material. A new U-Pb zircon age (313±5 Ma; LA-ICPMS) from ~10 km west of this fossil locality is interpreted to record volcanism. We also conducted focused structural and geochronological analysis at higher structural levels in the northeastern Central Gneiss Complex to positively identify sedimentary lithologies with which to document the early structural history of the complex prior to early Cenozoic rapid exhumation. In this area, a subhorizontal shear zone forms the boundary between mainly clastic metasedimentary rocks and the widespread metavolcanic and carbonate rocks where an important stretched pebble conglomerate has been previously documented. In the footwall of the shear zone, flattening fabrics transition structurally upward into E-W trending stretching lineations, lineation-parallel isoclinal fold axes, and boudinage that record E-W stretching and major shear strain near the contact. S-C fabrics and shear bands yield a top to the east sense of shear. Where observed, a shallowly dipping, ~15 m thick zone of cataclasite forms the lithologic contact and overprints the shear zone. Sills and dikes record ongoing but localized magmatism throughout deformation and steep NE striking brittle normal faults crosscut all features. Ongoing work will further constrain the protolith age of these rocks, the timing of

  16. Low-grade, M1 metamorphism of the western metamorphic belt near Juneau, Alaska

    SciTech Connect

    Himmelberg, G.R. . Dept. of Geology); Brew, D.A.; Ford, A.B. )

    1993-04-01

    The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Sub-greenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska record the earliest metamorphic event (M1). These low-grade rocks are gradational with younger, higher-grade assemblages that define an inverted metamorphic gradient (metamorphic event M5). The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low-variance assemblages can be attributed to minor differences in pressure, temperature, or X[sub CO[sub 2

  17. Surface Uplift and Disequilibrium Fluvial Geomorphology of Metamorphic Core Complexes in the D'Entrecasteaux Islands and Dayman-Suckling Massif, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Miller, S. R.; Baldwin, S. L.; Fitzgerald, P. G.

    2008-12-01

    Structural, thermobarometric, geochronological, and thermochronological evidence suggests that the D'Entrecasteaux Islands (DEI) and the Dayman-Suckling massif (DSM), in southeastern Papua New Guinea, are metamorphic core complexes, which have been exhumed from depths as great as 90 km over the past 2- -8 Ma, possibly because of microplate rotation. The dome-shaped DEI and DSM reach elevations of ~2500 m and ~3500 m, respectively, however little is known about their kinematic histories since their emergence above sea level in the Plio-Pleistocene. Detachment faults are commonly corrugated or dip- slope surfaces occurring in various states of dissection. The question of whether faults bounding the DEI domes are active today remains debated, yet has implications for what mechanisms have exhumed the core complexes as well as where and how active rifting in the Woodlark rift is accommodated. In order to provide a regional overview and direct future tectonic geomorphic studies, we conducted a stream profile analysis of the DEI and DSM using a 3-arc-second Shuttle Radar Topography Mission digital elevation model. Footwall stream profiles in the study area are characterized by large knickpoints across which stream gradients steepen downstream by a factor of 2. These knickpoints are not typically associated with mapped lithologic contacts or faults and occur in all lower plate lithologies. Therefore, we interpret the knickpoints as transients formed as the result of a Quaternary region-wide increase in rock uplift rate or a decrease in stream erodibility. Model profiles extrapolated from relict reaches above knickpoints project to former base levels 40- -1600 m above sea level, indicating similar amounts of incision and rock uplift. Although the timing of knickpoint initiation is not tightly constrained and may vary throughout the region, estimates of profile uplift measured this way correlate linearly with normalized stream steepness index (ksn) below knickpoints

  18. Dynamic coupling among channel flow, plateau growth, foreland shortening, and the formation of metamorphic core complexes: Application to the Tibetan plateau

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

    Gravitational potential energy stored in an orogenic plateau can be sufficiently strong to deform the surrounding region (foreland), hence contributing to both plateau growth and collapse. Gravity-driven channel flow from the plateau lower crust into the foreland lower crust, or channel extrusion, has been proposed as a main contributor to the eastward growth of the Tibetan plateau, possibly driving the lower crust channel as far as 1000 km beneath the foreland (eg. Royden et al., 2008). On the basis of numerical modeling using temperature-dependent viscosities and densities, we show that four processes impose severe limitations to channel extrusion: (1) cooling of the extruded channel, (2) convective motion in the plateau channel, (3) surface extension of the plateau, and (4) erosion of the plateau edge. Model results show that peak velocities in the extrusion channel drop rapidly (in less than a few My) from ca. 5 cm/year to less than 1 cm/year, owing to the rapid cooling in the channel from 750-850°C to 650-550°C as it travels into the foreland region. Channel flow extrusion is further slowed when convective flow initiates in the plateau channel as a result of only a few percent drop in density. This convection inhibits laminar flow in the channel, reduces the peak horizontal velocity in the channel to a few mm, and even drives a counter flow at the base of the channel, preventing its propagation toward the foreland. If the foreland is actively pulled away from the plateau (extending boundaries), the plateau upper crust undergoes extension and the lower crust moves up efficiently into a metamorphic core complex, which inhibits flow of the channel away from the plateau and even generates a counter flow from the foreland to the metamorphic core complex. If the foreland is fixed, the same phenomenon occurs as long as the foreland upper crust undergoes shortening (likely weakened by high pore fluid pressure), which enhances extension of the plateau and upward flow

  19. The UHP metamorphic Seve Nappe Complex of the Swedish Caledonides - a new occurrence of the microdiamond-bearing gneisses and their exhumation

    NASA Astrophysics Data System (ADS)

    Klonowska, Iwona; Janák, Marian; Majka, Jarosław; ‎ Froitzheim, Nikolaus; Gee, David G.

    2015-04-01

    The ultra-high pressure metamorphism (UHPM) in the Seve Nappe Complex of the Swedish Caledonides has been recently recognized within several lithologies including gneisses, eclogites and garnet pyroxenites (e.g. Janák et al. 2013, Klonowska et al. 2014a, Majka et al. 2014). Thermodynamic modelling and thermobarometric calculations indicate peak pressure conditions of >3GPa at c. 800-900°C (reaching the diamond stability field) for eclogites and garnet pyroxenites from northern Jämtland (e.g. Klonowska et al. 2014b). In addition to this, the first microdiamonds were found in paragneisses from the Snasahögarna Mt. in central Jämtland (Majka et al. 2014). Here we report a new discovery of microdiamond together with moissanite (SiC) from one of the world's most famous localities for thrusting, Mount Åreskutan, where long transport distances were recognized already in the 19th century (Törnebohm 1888). Garnet porphyroblasts in gneisses from the Åreskutan Mt. contain abundant mineral inclusions, mainly graphite, carbonates and quartz, together with fluid inclusions of CO2 concentrated in swarms. Among these inclusions three microdiamonds were found in two gneiss samples. In one of the samples moissanite was also discovered. Both minerals were identified by micro-Raman spectroscopy. In addition to these 'swarm' inclusions, biotite, kyanite, rutile, feldspars, zircon, monazite, ±phengite, ±muscovite, ±spinel, ±ilmenite, ±apatite occur in garnets. Phase equilibrium modelling for the phengite-bearing gneiss confirms its UHP history at temperatures of c. 800°C. Recent discoveries of UHP metamorphism within the Seve Nappe Complex derived from the Baltican outer margin (part of the Middle Allochthon) challenged us to present a new tectonic model incorporating exhumation of the deeply subducted continental rocks together with mantle lithosphere peridotites. Majka et al. (2014) introduced a new "under-pressure"-driven exhumation mechanism of rocks buried in

  20. Fluid motion associated with Tertiary mylonitization and detachment faulting: 18O/16O evidence from the Picacho metamorphic core complex, Arizona

    NASA Astrophysics Data System (ADS)

    Kerrich, R.; Rehrig, W.

    1987-01-01

    Major crustal detachment faults of Tertiary age in the Picacho metamorphic core complex of southern Arizona demark three tectonic plates in a structural section characterized by the transition from undeformed granitic basement through a mylonitic carapace to brecciated and hydrothermally altered counterparts. A lower detachment fault defines the top of the lower plate and is coplanar with the overprinting of mylonites to chloritic breccias. Variably altered and fractured granite of an overlying middle plate is bounded by an upper detachment fault on which allochthonous Miocene volcanics have been superposed; the volcanics display intense oxidative potassic alteration. This overall transition is accompanied by a 10‰ increase in δ18O and a 400 °C decrease of temperature. Undeformed Oracle Granite of the lower plate (δ18O ≈7.8) retains near-magmatic mineral fractionations, but these are disturbed in mylonitic equivalents, where ambient temperatures were ˜520 °C, δ18OH2O ≈4‰, and conditions of low water/rock ratios prevailed. Chloritic breccias record a whole-rock shift to +8.5‰, accompanied by diminished temperatures of 300 °C, and the infiltration of fluids where δ18OH2O = 3‰ ± 1‰ under conditions of high water/rock ratios. A pronounced isotopic discontinuity occurs at the upper detachment fault where intensely oxidized and K-metasomatized volcanics of the upper plate are shifted to 18‰ at temperatures that diminished to ˜ 150 °C. Two distinct fluid reservoirs were involved in alteration of the lower two plates and the upper plate, respectively, the tectonic section recording an upward transition from high to low temperature, low to elevated water/rock ratios, ductile creep to brittle fracturing, and an interface of deep “exchanged” fluids with a shallow oxidized surface aqueous reservoir.

  1. Geochemistry of the Neoproterozoic metasediments of Malhaq and Um Zariq formations, Kid metamorphic complex, Sinai, Egypt: Implications for source-area weathering, provenance, recycling, and depositional tectonic setting

    NASA Astrophysics Data System (ADS)

    El-Bialy, Mohammed Zaky

    2013-08-01

    The Um Zariq and Malhaq formations occupy roughly the northern half of the Kid metamorphic complex of SE Sinai, in the NE part of the Arabian-Nubian Shield. The Um Zariq Formation metasediments are relicts of an old sedimentary sequence (Cryogenian; 813 ± 6 Ma), whereas the Malhaq Formation records several phases of Ediacaran sedimentation and volcanic activity (615-607 Ma). The Um Zariq Formation is mainly represented by well-bedded metapelitic schists, while the Malhaq Formation comprises a series of structureless to schistose felsic to intermediate metavolcanics interbedded with mica-rich phyllites and schists. The Um Zariq metasediments are depleted in SiO2, CaO and K2O and enriched in TiO2, Al2O3 and K2O relative to those of the Malhaq Formation. Aside from the relatively low Ni and Cr concentrations, compatible transition elements of these metasediments are comparable to average crustal contents. Except for marked Sr depletion, LILEs are around average continental crust values. Pronounced negative Nb-Ta anomalies and enrichment of Um Zariq samples in Th, U, Zr, Ti and Y relative to Malhaq ones are the main features of HFSEs. The REE patterns of all samples are parallel to sub-parallel LREE-enriched, with distinct negative Eu anomalies and weakly fractionated HREE segments. The source rocks of the Malhaq Formation metasediments underwent mild to moderate chemical weathering, whereas those of the Um Zariq Formation have suffered severe chemical weathering. These metasediments are predominately derived from felsic to intermediate igneous sources, with a particular slight addition from recycled sedimentary source to the Malhaq Formation metasediments. They are collectively geochemically immature and have suffered minor sedimentary recycling, with the experience of the Malhaq Formation metasediments from higher degree of sorting and reworking. The Malhaq and Um Zariq metasediments were originally deposited in a continental arc setting, most probably back

  2. Plate tectonics: Metamorphic myth

    NASA Astrophysics Data System (ADS)

    Korenaga, Jun

    2016-01-01

    Clear evidence for subduction-induced metamorphism, and thus the operation of plate tectonics on the ancient Earth has been lacking. Theoretical calculations indicate that we may have been looking for something that cannot exist.

  3. Metamorphism in mesosiderites

    NASA Technical Reports Server (NTRS)

    Delaney, J. S.; Prinz, M.; Harlow, G. E.; Nehru, C. E.

    1982-01-01

    Previous studies of mesosiderites have identified a metamorphic overprint in these meteorites. However, the effects and implications of this overprint have not yet been explored in detail. The present study documents several important textural and chemical features of the mesosiderites. The components of mesosiderites are examined, taking into account orthopyroxenites, olivine in clasts, mesosiderite mafic clasts, and metal. The characteristics of the silicate matrix of the mesosiderites is explored, and textural and chemical evidence of metamorphism is discussed, giving attention to coronas on olivine clasts, overgrowths on Mg-pyroxene clasts, rims on iron rich pyroxene grains, poikiloblasts of plagioclase, and resorption of clasts. Aspects of redox formation of merrillite are considered along with the causes and the implications of metamorphism. It is found that metamorphism has radically changed the texture of the silicate fraction of the mesosiderites.

  4. 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)

  5. Petrology of metamorphic rocks

    SciTech Connect

    Suk, M.

    1983-01-01

    ''Petrology of Metamorphic Rocks'' reviews Central European opinions about the origin and formation of metamorphic rocks and their genetic systems, confronting the works of such distinguished European scientists as Rosenbusch, Becke, Niggli, Sander, Eskola, Barth and others with present-day knowledge and the results of Soviet and American investigations. The initial chapters discuss the processes that give rise to metamorphic rocks, and the main differences between regional metamorphism and other types of alterations, the emphasis being laid on the material characteristic of the processes of metamorphism, metasomatism and ultrametamorphism. Further chapters give a brief characterization of research methods, together with a detailed genetic classification based on the division of primary rocks into igneous rocks, sediments and ore materials. The effects of metamorphic alterations and those of the properties of the primary rocks are analyzed on the basis of examples taken chiefly from the Bohemian Massif, the West Carpathians, other parts of the European Variscides, from the crystalline Scandinavian Shelf in Norway and Finland, and from the Alps. Typical examples are documented by a number of charts, photographs and petrographical - particularly petrochemical - data.

  6. New U-Pb and Sm-Nd isotope data of the age of formation and metamorphic alteration of the Kandalaksha-Kolvitsa gabbro-anorthosite complex (Baltic Shield)

    NASA Astrophysics Data System (ADS)

    Steshenko, Ekaterina; Bayanova, Tamara; Serov, Pavel; Chashchin, Viktor

    2016-04-01

    The aim of this research was to study the isotope U-Pb age of zircon and rutile and Sm-Nd (rock forming and sulphide minerals) in Kandalaksha-Kolvitsa gabbro-anorthosite complex. Kandalaksha-Kolvitsa gabbro-anorthosite complex is located in the N-E part of Baltic shield and consists of three parts. Marginal zone (mesocratic metanorite) lies at the base of the massif. Main zone is composed of leucocratic metagabbro. The upper zone is alteration of mataanorthosite and leucocratic metagabbro. All rocks were subjected to granulate metamorphism. New U-Pb and Sm-Nd isotopic and geochronological data for the rocks of the Kandalaksha-Kolvitsa Paleoproterozoic gabbro-anorthosite complex is presented. For the first time single zircon grains from metagabbros of Kolvitsa massif were dated 2448±5 Ma, using U-Pb method with an artificial 205Pb tracer. Sm-Nd isotopic age of the metamorphic minerals apatite, garnet and sulphide WR Kolvitsa array is 1985 ± 17 Ma, which is interpreted granulite metamorphism. Two fractions of single zircons from anorthosite of the Kandalaksha massif gave U-Pb age 2450± 3 Ma. Leucocratic gabbro-norite (Kandalaksha massif) were dated by U-Pb on single zircon, with age up to 2230±10 Ma. This age reflects the time of granulite metamorphism according to data of [1]. Two fractions of rutile from anorthosite of the Kandalaksha massif have been analyzed by U-Pb method and reflect age of 1700 ± 10 Ma. It is known that the closure temperature of U-Pb system rutile 400-450 ° C [2], thus cooling of the massif to these temperatures was about 1.7 Ga. These data suggested two stages of metamorphic transformations of the massif. Sm-Nd research Kandalaksha massif reflected the age of the high-temperature metasomatic transformations -1887 ± 37 Ma. Time of regional fluid processing - 1692 ± 71 Ma. A model Sm-Nd age metagabbros Kolvitsa massif is 3.3 Ga with a negative value ɛNd = -4.6, which corresponds to the most likely primary enriched mantle reservoir of

  7. Metamorphic evolution and thermobaric structure of the subduction-related Bacariza high-pressure granulite formation (Cabo Ortegal Complex, NW Spain)

    NASA Astrophysics Data System (ADS)

    Puelles, P.; Ábalos, B.; Gil Ibarguchi, J. I.

    2005-09-01

    The high-pressure Bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses, ultramafic, Mg-rich mafic, intermediate and common mafic granulites, as well as of more exotic intercalations. Mineral assemblages in equilibrium in ultramafic- to intermediate rocks contain garnet, clinopyroxene and plagioclase, with different amounts of zoisite/clinozoisite, kyanite, quartz, scapolite, rutile and ilmenite depending on the granulite lithotype, whereas granulite orthogneisses contain garnet, phengite, biotite, K-feldspar, antiperthitic plagioclase, quartz and rutile as primary phases. Thermobarometry of these rocks supports the existence of a high-pressure metamorphism for which near-peak P- T conditions have been estimated at ca. 790 °C and 1.6 GPa. The preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes. Dynamic retrogression began under still high-pressure granulite facies conditions (1.4 GPa and ca. 740 °C) and is postdated by symplectitization (1.3 GPa and ca. 715 °C). Subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift. Loading/heating and subsequent decompression/cooling are related to a single cycle in a subduction conduit setting. This study suggests that high-pressure granulite metamorphism might not be as uncommon in the high-pressure metamorphic series as previously thought. Moreover, it might constitute a diagnostic feature of convergent lithospheric settings, whether or not associated with eclogite facies metamorphism in adjacent units.

  8. From intra-oceanic subduction to arc accretion and arc-continent collision: Insights from the structural evolution of the Río San Juan metamorphic complex, northern Hispaniola

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Valverde-Vaquero, Pablo; Rojas-Agramonte, Yamirka; Jabites, Janet; Pérez-Estaún, Andrés

    2013-01-01

    The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction-accretionary complex built during Caribbean island arc-North America continental margin convergence. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined detailed mapping, structural and metamorphic analysis, and geochronology show that the deformation can be divided into five main events (D1-D5). An early subduction-related D1 deformation and M1 metamorphism produced greenschist (mafic rocks of the Gaspar Hernández peridotite-tectonite), blueschist and eclogite (metamafic blocks in the Jagua Clara mélange), high-P epidote-amphibolite and eclogite (Cuaba unit), and lower blueschist and greenschist-facies conditions (Morrito unit). This was followed by M2 decompression and cooling in the blueschist, greenschist and low-P amphibolite-facies conditions. The shape of the retrograde P-T path, the age of the exhumation-related D2 structures, and the tectonic significance of D2 deformation are different in each structural unit. Published U-Pb and 40Ar/39Ar plateau ages and T-t/P-t estimations reveal diachronic Turonian-Coniacian to Maastrichtian retrograde M2 metamorphism in the different structural units of the complex, during a consistent D2 top-to-the-NE/ENE tectonic transport. Regionally, a similar top-to-the-ENE tectonic transport also took place in the metasedimentary nappes of the Samaná complex during the Eocene to earliest Miocene. This kinematic compatibility indicates a general northeastward progradation of deformation in the northern Caribbean convergent margin, as the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction-accretionary complex took place. D3-D5 deformations are discontinuous and much less penetrative, recording the evolution from ductile to brittle conditions of deformation in the complex

  9. Constraints on the metamorphic history of a mélange complex within the easternmost Himalayan orogen, northern Indo-Burma Range, based on P-T pseudosection and thermobarometric studies.

    NASA Astrophysics Data System (ADS)

    Haproff, P. J.; Yin, A.

    2015-12-01

    Despite many petrologic and structural studies surrounding the eastern Himalayan syntaxis, P-T histories of metamorphic rocks within the northern Indo-Burman mélange of the easternmost Himalayas remain largely unknown. We present metamorphic P-T conditions of three schists from successive thrust sheets related to generation of the Himalayan orogen, based on mineral assemblage thermobarometry and pseudosection phase diagrams. Use of the garnet-muscovite-biotite-plagioclase thermobarometer and Ti-in-biotite thermometer yield peak conditions of 676 ± 78°C and 10.6 ± 1.3 kbar for schist (PH-1-8-13-26) thrust atop metavolcanics, mafic schist, and ultramafics of the Indus-Ysangpo suture zone (IYSZ). Within this sample, quartz is recrystallized along grain boundaries and garnets contain no significant compositional zoning. Pseudosections constructed from bulk-rock composition and equilibrium mineral assemblages yield a clockwise P-T path with similar peak garnet amphibolite conditions. At structurally lower levels, garnet chlorite schist (PH-1-8-13-8) from a thrust klippe of the IYSZ record peak temperatures below 650°C. Garnets display growth zoning, with increasing Mn and decreasing Fe and Mg from rim to core. Application of the Ti-in-biotite thermometer to a mafic schist (PH-1-3-13-1B) within the klippe near a southwestward-directed thrust yields a peak temperature of 679 ± 24°C. Our study reveals a complex metamorphic history throughout the northern Indo-Burman mélange zone that likely records metamorphism at deep crustal levels during thrust motion and growth of the Himalayan orogen around the northeastern corner of India.

  10. Formation and emplacement of two contrasting late-Mesoproterozoic magma types in the central Namaqua Metamorphic Complex (South Africa, Namibia): Evidence from geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Bial, Julia; Büttner, Steffen H.; Frei, Dirk

    2015-05-01

    The Namaqua Metamorphic Complex is a Mesoproterozoic low-pressure, granulite facies belt along the southern and western margin of the Kaapvaal Craton. The NMC has formed between ~ 1.3 and 1.0 Ga and its central part consists essentially of different types of granitoids intercalated with metapelites and calc-silicate rocks. The granitoids can be subdivided into three major groups: (i) mesocratic granitoids, (ii) leucocratic granitoids and (iii) leucogranites. The high-K, ferroan mesocratic granitoids (54-75 wt% SiO2) have a variable composition ranging from granitic to tonalitic, and contain biotite and/or hornblende or orthopyroxene. They are strongly enriched in REE and LILE, indicating A-type chemical characteristics, and are depleted in Ba, Sr, Eu, Nb, Ta and Ti. The leucocratic granitoids and leucogranites (68-76 wt% SiO2) differ from the other group in having a granitic or slightly syenitic composition containing biotite and/or garnet/sillimanite. They have lower REE and MgO, FeOt, CaO, TiO2, MnO concentrations, but higher Na2O and K2O contents. Compositional variations in mesocratic granitoids indicate their formation by fractional crystallization of a mafic parental magma. Leucocratic granitoids and leucogranites lack such trends, which suggests melting of a felsic crustal source without subsequent further evolution of the generated magmas. The mineralogical and geochemical characteristics of the mesocratic granitoids are consistent magmatic differentiation of a mantle derived, hot (> 900 °C) parental magma. The leucocratic granitoids and leucogranites granites were formed from low-temperature magmas (< 730 °C), generated during fluid-present melting from metasedimentary sources. New U-Pb zircon ages reveal that both magma types were emplaced into the lower crust within a 30-40 million years interval between 1220-1180 Ma. In this time period the crust reached its thermal peak, which led to the formation of the leucocratic granitoids and leucogranites. A

  11. 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

  12. Zircon U-Pb ages, Hf isotopes and geochemistry of the schists, gneisses and granites in Delbar Metamorphic-Igneous Complex, SE of Shahrood (Iran): Implications for Neoproterozoic geodynamic evolutions of Central Iran

    NASA Astrophysics Data System (ADS)

    Balaghi Einalou, Maryam; Sadeghian, Mahmoud; Zhai, Minggou; Ghasemi, Habibollah; Mohajjel, Mohammad

    2014-10-01

    The Delbar Metamorphic-Igneous Complex (DMIC) consists of the medium to high-grade metamorphic rocks and granites - leucogranites is located in the Biarjmand region, in Central Iran. U-Pb dating of the gneisses yielded the ages of 546 ± 3.7-547 ± 6.8 Ma similar to the crystallization ages of leucogranites (541 ± 4.7-547 ± 11 Ma) are consistent to the Late Ediacaran-Early Cambrian ages of Cadomian magmatic arc (∼545 Ma). The 206Pb/238U ages of the detrital zircons from the mica-schists sample are from 551 ± 5.1-549 ± 5.1 Ma. The youngest ages of the meta-pelitic protolith have 10 Ma age intervals compared to the granites emplacement age, which indicates rapid Late Precambrian crustal recycling involving erosion, burial, metamorphism to partial melting of the continental crust in less than ca. 10 Ma. These rapid crustal evolutions were related to the final collision and amalgamation of Gondwana, the Rheic Ocean clouser and coeval paleotethys opening at the end of the Avalonian-Cadomian orogeny. The studied granites are similar to the volcanic arc granitoids and originated from the crustal source in an active continental margin based on the geochemical characteristics and Hf isotopes data. Exhumation of the DMIC Complex, is marked by deposition of Lower Jurassic conglomerates contains pebbles of basement rocks. These rocks were interrupted by mafic swarms dikes that have generated in a back arc extensional setting related to the Neotethys subduction under the Central Iran at the 152 ± 35 Ma (Middle-Late Jurassic) based on the U-Pb apatite dating.

  13. From intra-oceanic subduction to arc accretion and oblique arc-continent collision: Insights from the P-T-D-t path of the southern Río San Juan metamorphic complex, northern Dominican Republic

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, J.; Valverde-Vaquero, P.; Rojas-Agramonte, Y.; Gabites, J.; Pérez-Estaún, A.

    2012-04-01

    An integrated structural, petrological, geochemical and geochronological study was undertaken to constrain the origin, structural development and tectonic control on the exhumation of the high-P Cuaba Unit. Located in the southern part of the Río San Juan Metamorphic Complex, northern Dominican Republic, the Cuaba Unit defines a ~40 km long and 2.5-6 km wide, NW-SE-elongated metamorphic complex. Structurally, it is divided into two zones: an internal antiformal core and an outer shear zone. The core, or lower Guaconejo subunit, consists of retrograded eclogites, garnet-bearing mafic and ultramafic gneisses, metagabbros, metadiorites, subordinated migmatites and mylonitic rocks. The outer shear zone, or upper Jobito subunit, is composed of amphibolites with a strong S-L fabric and metadiorites. The Jobito and Guaconejo subunits are juxtaposed by several rootless bodies of SSZ serpentinized peridotites, sheared Hbl-bearing tonalites and variably retrogressed high-P amphibolites. The unit is also juxtaposed against the underlying Helechal peridotite. Whole-rock geochemical analyses indicate that mafic rocks of the Cuaba Unit originated from low-Ti and low-LREE IAT, IAT, N-MORB and calc-alkaline type protoliths. The evolution of structural fabrics and P-T metamorphic conditions recorded in the Cuaba Unit allows identifying (1) a prograde metamorphism (D1-M1) from amphibolite and high-P epidote-garnet amphibolite to eclogite facies conditions, and (2) a retrogressive event (D2-M2) to mid/low-P amphibolite and greenschist facies conditions. The M2 stage consists in an isothermal decompression and was accompanied by variably partial melting of mafic litologies. U-Pb TIMS dating on zircon grains (89.73±0.15 Ma) coupled with 40Ar-39Ar analyses on pargasite/aluminous tschermakite (~89 Ma to 83 Ma), placed temporal constraints on the exhumation path from the M1 pressure-peak (Turonian-Coniacian boundary) to the M2 stage (Coniacian and Santonian). Zircons in early D2 garnet

  14. Mesozoic and Cenozoic thermal history of the eastern Mojave Desert, California and western Arizona, with emphasis on the Old Woman Mountains area and the Chemehuevi metamorphic core complex

    SciTech Connect

    Foster, D.A.

    1989-01-01

    Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of {sup 40}Ar/{sup 39}Ar thermochronology and petrology/thermobarometry to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman-Piute, Chemehuevi, and Sacramento Mountains attained temperatures > 500C. High grade metamorphism of the Old Woman Mountains area was caused by the intrusion of the Old Woman-Piute batholith at 73 {plus minus} 1 Ma; Cretaceous mineral assemblages in Proterozoic pelites increase in grade from greenschist to upper amphibolite facies, and {sup 40}Ar/{sup 39}Ar hornblende ages from Proterozoic amphibolites decrease in age from {approximately} 1,600 Ma to 73 {plus minus} 1 Ma, in the direction of 73 Ma plutons. Pluton emplacement and metamorphism occurred at 3 to 3.5 kbars and 400 > 600C in the Piute Mountains, and 3.5 to 4.5 kbars and 530 to > 650C in the Old Woman Mountains. Following the Cretaceous, the eastern Mojave Desert underwent a period of cooling at a rate of 2 to 10C/Ma between 65 and 25 Ma. By 30 Ma rocks exposed in the Old Woman-Piute, Marble Ship, Clipper and Turtle Mountains were below {approximately} 100C. {sup 40}/{sup 39}Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 {plus minus} 1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at {approximately} 180C and {approximately} 350C, respectively which suggests that this fault initiated at a dip of 5 to 30{degree}.

  15. Metamorphic facies map of Alaska

    SciTech Connect

    Dusel-Bacon, C.; O-Rourke, E.F.; Reading, K.E.; Fitch, M.R.; Klute, M.A.

    1985-04-01

    A metamorphic-facies of Alaska has been compiled, following the facies-determination scheme of the Working Group for the Cartography of the Metamorphic Belts of the World. Regionally metamorphosed rocks are divided into facies series where P/T gradients are known and into facies groups where only T is known. Metamorphic rock units also are defined by known or bracketed age(s) of metamorphism. Five regional maps have been prepared at a scale of 1:1,000,000; these maps will provide the basis for a final colored version of the map at a scale of 1:2,500,000. The maps are being prepared by the US Geological Survey in cooperation with the Alaska Division of Geological and Geophysical Surveys. Precambrian metamorphism has been documented on the Seward Peninsula, in the Baird Mountains and the northeastern Kuskokwim Mountains, and in southwestern Alaska. Pre-Ordovician metamorphism affected the rocks in central Alaska and on southern Prince of Wales Island. Mid-Paleozoic metamorphism probably affected the rocks in east-central Alaska. Most of the metamorphic belts in Alaska developed during Mesozoic or early Tertiary time in conjuction with accretion of many terranes. Examples are Jurassic metamorphism in east-central Alaska, Early Cretaceous metamorphism in the southern Brooks Range and along the rim of the Yukon-Kovyukuk basin, and late Cretaceous to early Tertiary metamorphism in the central Alaska Range. Regional thermal metamorphism was associated with multiple episodes of Cretaceous plutonism in southeastern Alaska and with early Tertiary plutonism in the Chugach Mountains. Where possible, metamorphism is related to tectonism. Meeting participants are encouraged to comment on the present version of the metamorphic facies map.

  16. The Taili-Yiwulüshan metamorphic core complex corridor: Diachronous exhumation and relationships to the adjacent basins based on new 40Ar/39Ar and (U-Th-Sm)/He mineral ages

    NASA Astrophysics Data System (ADS)

    Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Genser, Johann; Dunkl, István; Heberer, Bianca; Jin, Wei; Zeng, Zuoxun; Li, Weimin; Wen, Quanbo; Li, Jing

    2015-04-01

    The Xingcheng-Taili ductile shear zone (western Liaoning Province in China) formed during latest Jurassic to Early Cretaceous crustal extension of the eastern North China craton, and exhumed low to medium metamorphic grade Archean, Upper Triassic and Upper Jurassic granitic rocks. The Mesozoic Yiwulüshan metamorphic core complex (Yiwulüshan MCC) is dominated by a NNE-SSW elongated dome with a left-lateral shear zone, which is located in the northeastern part of Xingcheng-Taili ductile shear zone, and combine as Taili-Yiwulüshan metamorphic core complex corridor. To the east, it is bounded by the NNE-trending Cretaceous to Eocene Liaohe basin (the northern extension of the Bohai Bay basin), and to the west by the Cretaceous-aged Fuxin-Yixian basin, which could potentially interpreted as supra-detachment basins. Here, we present results from a multi-method thermochronological study and coupled with structural investigations and sections of adjacent supra-detachment basins, which constrain the timing of regional deformation as well as the cooling history and exhumation processes of the low- to middle-grade metamorphic complex in the Taili-Yiwulüshan MCC corridor, in order to understand the mode of lithospheric scale reactivation, extension and thinning of the North China craton. The new40Ar/39Ar muscovite, biotite, K-feldspar and (U-Th)/He apatite ages from granitic rocks help constrain the thermal evolution during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Jurassic to Eocene in Xingcheng-Taili shear zone should be distinguished, e.g., ~ 150-130 Ma, 130-115 Ma and 115-52 Ma, respectively. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history. The Yiwulüshan MCC has a similar exhumation history from 135 to 97 Ma with a similar cooling history. The development of Taili-Yiwulüshan MCC corridor is associated with synkinematic emplacement

  17. The sup 40 Ar/ sup 39 Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

    SciTech Connect

    Foster, D.A.; Harrison, T.M. ); Miller, C.F. ); Howard, K.A. )

    1990-11-10

    The application of {sup 40}Ar/{sup 39}Ar thermochronology provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The {sup 40}Ar/{sup 39}Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The {sup 40}Ar/{sup 39}Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1-5C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman-Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500C which reset the K-Ar systems of minerals from Proterozoic rocks. High-grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman-Piute batholith at 73 {plus minus} 1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were {approximately}100C/m.y. between 73 and 70 Ma and 5-10C/m.y. from 70 to {approximately}30 Ma. By 30 Ma, rocks exposed in the Old Woman-Piute, Marble, Ship, Clipper, and Turtle Mountains were below {approximately}100C. The {sup 49}Ar/{sup 39}Ar ages from the Sacramento Mountains suggest that mylonization caused by the onset of regional extension occurred at 23 {plus minus} 1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at {approximately}180C and {approximately}350C, respectively. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10-50C/m.y. between 22 and 15 Ma.

  18. 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.

  19. 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.

  20. Fluid-driven metamorphism

    NASA Astrophysics Data System (ADS)

    Jamtveit, B.; Ulven, O. I.; Malthe-Sorenssen, A.

    2014-12-01

    Metamorphic processes in the Earth crust are almost invariably associated with fluid migration. Many lines of evidence suggest that fluid migration is intimately coupled both to the metamorphic reactions, and to associated deformation processes. Petrologic arguments suggest that all granulite facies and most amphibolite facies rocks are essentially dry (no free fluid phase) at normal geothermal gradients outside periods of heating-produced fluid generation. In addition, except at high pressure - low temperature condition, fluid-consuming reactions leads to an increase in solid volume and a potential clogging of any initial pore space. Hence, fluid migration in medium and high-grade metamorphic rocks is in general associated with some porosity producing process. Porosity generation may occur by either chemical or mechanical processes. In systems with high fluid fluxes, porosity may be produced by dissolution and transport of mass out of the system. Such fluxes can normally only be sustained over short length scales and limited time scales. In systems where the infiltrating fluid is far from equilibrium with the rock matrix, mechanical porosity generation can arise from local stresses generated by the volume change of volatilization reactions. Furthermore, it has become increasingly clear that crustal rocks may be under significant tectonic stress, even far from plate tectonic boundaries. In situations where the rocks are close to critically stressed, any stress perturbations caused by reaction driven changes in solid volume or fluid pressure gradients may lead to dilatant deformation and porosity production on a scale much larger than the characteristic length scales of the reacting rock units. Field observations, experimental studies and modeling results will be presented that focus on reaction driven porosity generation in systems subject to variable initial differential stresses.

  1. Metamorphic Testing for Cybersecurity

    PubMed Central

    Chen, Tsong Yueh; Kuo, Fei-Ching; Ma, Wenjuan; Susilo, Willy; Towey, Dave; Voas, Jeffrey

    2016-01-01

    Testing is a major approach for the detection of software defects, including vulnerabilities in security features. This article introduces metamorphic testing (MT), a relatively new testing method, and discusses how the new perspective of MT can help to conduct negative testing as well as to alleviate the oracle problem in the testing of security-related functionality and behavior. As demonstrated by the effectiveness of MT in detecting previously unknown bugs in real-world critical applications such as compilers and code obfuscators, we conclude that software testing of security-related features should be conducted from diverse perspectives in order to achieve greater cybersecurity. PMID:27559196

  2. Petrology and U/Pb geochronology of the Santa Maria Ipalapa region in the southeastern part of the Xolapa Complex, Mexico: Constrains of the metamorphic evolution of the Xolapa Terrane

    NASA Astrophysics Data System (ADS)

    Gutiérrez Aguilar, F.; Victoria Morales, A.; Maldonado, R.

    2015-12-01

    The Xolapa Complex is a metamorphic-plutonic basement that forms a large belt with more than 600 km length and 50-100 km wide along the Pacific coast of southeastern Mexico. This Complex is constituted by a high grade sequence of meta-sedimentary and meta-igneous rocks, locally migmatisized, and which are intruded by strongly deformed plutonic rocks. Because of their representative characteristics, two samples were analyzed: 1) Para-schist: this rocks present a mineral assemblage composed of biotite, sillimanite, plagioclase, k-feldspar and garnet, and 2) Amphibolite: which are constituted by amphibole, plagioclase, biotite and garnet. The garnet porphydoblasts in the para-schist are subhedral, presents retrograde compositional zoning, with almandine and pyrope rich core (Alm74-75Sps7.-10.1Pyr12.1-12.5Grs3.8-3.9) and spessartine rich rim (Alm69-71Sps14-19Pyr7.9-9.6Grs3.6-3.7). The garnet in amphibolite, presents a prograde growth zoning with a slight increase in spessartine in the core (Alm59-60Grs24-25Pyr8.0-8.3Sps7.3-7.6), and low content of spessartine component toward the rim (Alm60-62Grs23-24Pyr8.8-9.6Sps5.4-5.5). In order to constrain the P-T evolution of the region, multiequilibria thermobarometry was applied to both samples, the para-schist unit presents P-T data from 706 (ºC) and 7.5 (kbar), in the other hand the garnet amphibolite unit shows P-T data from 734 (ºC) and 7 (kbar). This study provides new geochronological data (U/Pb in zircons) for the amphibolite facies metamorphism and for the migmatitic event in the region that contributes to the understanding of the tectonic evolution of southeastern Mexico.

  3. P-T-t conditions, Nd and Pb isotopic compositions and detrital zircon geochronology of the Massabesic Gneiss Complex, New Hampshire: isotopic and metamorphic evidence for the identification of Gander basement, central New England

    USGS Publications Warehouse

    Dorais, Michael J.; Wintsch, Robert P.; Kunk, Michael J.; Aleinikoff, John; Burton, William; Underdown, Christine; Kerwin, Charles M.

    2012-01-01

    We present new evidence for the assignment of the Neoproterozoic Massabesic Gneiss Complex of New Hampshire to the Gander terrane rather than the Avalon terrane. The majority of Avalonian (sensu stricto) igneous and meta-igneous rocks as defined in Maritime Canada have positive whole-rock ɛNd compared to more negative values for Gander rocks, although there is a region of overlap in ɛNd between the two terranes. Our samples from areas in Connecticut previously thought to be Avalonian and samples from the Willimantic dome have the same isotopic signatures as Maritime Canada Avalon. In contrast, samples from the Clinton dome of southern Connecticut plots exclusively in the Gander field. The majority of the orthogneiss samples from Lyme dome (coastal Connecticut), Pelham dome (central Massachusetts) and Massabesic Gneiss Complex also plot in the Gander field, with a few samples plotting in the overlap zone between Gander and Avalon. U-Pb age distributions of detrital zircon populations from quartzites from the Massabesic Gneiss Complex more closely approximate the data from the Lyme Dome rather than Avalon. Additionally, the similarity of the P-T-t path for the rocks of the Massabesic Gneiss Complex (established by thermobarometry and 40Ar/39Ar dating of amphibole, muscovite, biotite and K-feldspar) with that established in the Ganderian Lyme dome of southern Connecticut strengthens the assignment of these rocks to a single Gander block that docked to Laurentia during the Salinic Orogeny. The identification of Ganderian isotopic signatures for these rocks all of which show evidence for Alleghanian metamorphism, supports the hypothesis that Neoproterozoic Gander lower crustal rocks underlie southern New Hampshire, Massachusetts, and Connecticut, and that all rocks of the overlying Central Maine trough that largely escaped high-grade Alleghanian metamorphism are allochthonous. We suggest that during the Alleghanian, the docking of Gondwana caused Avalon to wedge into

  4. Sm Nd isotope systematics and REE data for leucotroctolites and their amphibolitized equivalents of the Niquelândia Complex upper layered series, central Brazil: further constraints for the timing of magmatism and high-grade metamorphism

    NASA Astrophysics Data System (ADS)

    Ferreira Filho, C. F.; Pimentel, M. M.

    2000-12-01

    The Barro Alto, Niquelândia, and Cana Brava Complexes are major Proterozoic layered intrusions in central Brazil that were affected by high-grade metamorphism with associated ductile deformation during the Neoproterozoic (770-795 Ma). Recent studies recognized that the Niquelândia Complex comprises two petrologically distinct and tectonically juxtaposed magmatic systems: a younger Upper Layered Series to the west and an older Lower Layered Series to the east. Previous geochronological studies on Lower Series rocks suggested a Paleoproterozoic (ca 2.0 Ga) age for the Lower Series magmatic event. New trace element data matched with Sm-Nd isotope data for Upper Series samples yielded well-constrained and original geochronological information. The 1.35 Ga age of the Upper Series magmatism reported in this paper indicates a much younger age of the Upper Series compared with the Lower Series. The tectonic contact between these two distinct magmatic systems is now raised to the category of a major Paleo-Mesoproterozoic crustal discontinuity.

  5. Mesozoic age of the Gilyui Metamorphic Complex in the junction zone of the Selenga-Stanovoi and Dzhugdzhur-Stanovoi superterranes, Central Asian fold belt

    NASA Astrophysics Data System (ADS)

    Velikoslavinskii, S. D.; Kotov, A. B.; Kovach, V. P.; Larin, A. M.; Sorokin, A. A.; Sorokin, A. P.; Tolmacheva, E. V.; Salnikova, E. B.; Wang, K. L.; Jahn, B. M.; Cung, S. L.

    2016-06-01

    The Gilyui Complex includes sedimentary and volcanic rocks metamorphosed to amphibolite and epidote-amphibolite facies, which constitute blocks confined to the main structural sutures of the Dzhugdzhur-Stanovoi superterrane in the Central Asian fold belt. In recent stratigraphic scales, they are considered as being Neoarchean in age with Nd model age values of 1.5-3.0 Ga. The youngest detrital zircons from metamorphosed mudstone of the Gilyui Complex yield a date of 285 ± 4 Ma, which determines the lower age limit for the formation of its protolith. The age of crystallization of rhyolites from the Gilyui Complex is determined to be 231 ± 4 Ma. If the rhyolites form volcanic flow units or sills, the Gilyui Complex is approximately 230 Ma or 231 ± 4 to 285 ± 4 Ma old, respectively.

  6. Fault block kinematics at a releasing stepover of the Eastern California shear zone: Partitioning of rotation style in and around the Coso geothermal area and nascent metamorphic core complex

    NASA Astrophysics Data System (ADS)

    Pluhar, Christopher J.; Coe, Robert S.; Lewis, Jonathan C.; Monastero, Francis C.; Glen, Jonathan M. G.

    2006-10-01

    , K. Richards-Dinger, The Coso geothermal field: a nascent metamorphic core complex, Geol. Soc. Amer. Bull. 117 (2005) 1534-1553.] characterize as a nascent metamorphic core complex. Consistent with upper plate disruption above a detachment, surface rocks (i.e. the upper plate of the detachment system) at the Coso geothermal area are tilted westward. However they appear to exhibit no detectable rotation. Thus, the style of block rotation may be partitioned: with clockwise vertical-axis rotation dominating in the Wild Horse Mesa and horizontal axis rotation (tilting) in the geothermal area.

  7. 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.

  8. Shock metamorphism of Elephant Moraine A79001: Implications for olivine-ringwoodite transformation and the complex thermal history of heavily shocked Martian meteorites

    NASA Astrophysics Data System (ADS)

    Walton, Erin L.

    2013-04-01

    Lithology A of Martian meteorite Elephant Moraine (EET) A79001 contains fragments entrained within a 100 μm-thick shear-induced shock vein. These fragments, the shock vein matrix and walls of olivine along the vein, as well as shock deformation and transformation in rock-forming minerals in the bulk rock, were investigated using scanning electron microscopy, the electron microprobe and Raman spectroscopy. The presence of ringwoodite, the spinel-structured high-pressure (Mg,Fe)2SiO4 polymorph, has been confirmed in EETA79001 for the first time. Ringwoodite occurs within and around the shock vein, exhibiting granular and lamellar textures. In both textures ringwoodite consists of ˜500 nm size distinct grains. Ringwoodite lamellae are 115 nm to 1.3 μm wide. Planar fractures in olivine provided sites for heterogeneous nucleation of ringwoodite. Analyses performed on the largest grains (⩾1 μm) show that ringwoodite is consistently higher in iron (Fa27.4-32.4) relative to surrounding olivine (Fa25.1-267.7), implying that there was Fe-Mg exchange during their transformation, and therefore their growth was diffusion-controlled. In the shock environment, diffusion takes place dynamically, i.e., with concurrent deformation and grain size reduction. This results in enhanced diffusion rates (⩾10-8 m2/s) over nm - μm distances. Shock deformation in host rock minerals including strong mosaicism, pervasive fracturing, polysynthetic twinning (pyroxene only), extensive shock melting, local transformation of olivine to ringwoodite, and complete transformation of plagioclase to maskelynite in the bulk rock, indicate that EETA79001 was strongly shocked. The short shock duration (0.01 s) combined with a complex thermal history, resulted in crystallization of the 100 μm thick shock vein in EETA79001 during the pressure release, and partial back-transformation of ringwoodite to olivine. Based on the pressure stabilities of clinopyroxene + ringwoodite, crystallization at the

  9. Using quantitative phase petrology to understand metamorphism

    NASA Astrophysics Data System (ADS)

    White, Richard

    2015-04-01

    Quantitative phase petrology has become one of the mainstay methods for interpreting metamorphic rocks and processes. Its increased utility has been driven by improvements to end-member thermodynamics, activity-composition relationships and computer programs to undertake calculations. Such improvements now allow us to undertake calculations in increasingly complex chemical systems that more closely reflect those of rocks. Recent progress in activity-composition (a-x) relationships is aimed at developing suites of a-x relationships in large chemical systems that are calibrated together, which will allow a more direct application of the method to metamorphic rocks. In addition, considerable progress has been made in how quantitative phase diagrams can be used to understand features, including chemical potential diagrams for reaction textures, methods for fractionating bulk compositions and methods for modelling open system processes. One feature of calculated phase diagrams is that they present us with a great amount of information, such as mineral assemblages, mineral proportions, phase compositions, volume or density etc. An important aspect to using this information is to understand the potential uncertainties associated with these, which are significant. These uncertainties require that calculated phase diagrams be used with caution to interpret observed features in rocks. Features such as mineral zoning and reaction textures should still be interpreted in a semi-quantitative way, even if based on a fully quantitative diagram. Exercises such as the interpretation of reaction overstepping based on relating phase diagrams to observed mineral core compositions are likely to give spurious results given the infelicities in existing a-x models. Despite these limitations, quantitative phase petrology remains the most useful approach to interpreting the metamorphic history of rocks in that it provides a theoretical framework in which to interpret observed features rather

  10. 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

  11. Permo-Triassic and Paleoproterozoic metamorphism related to continental collision in Yangpyeong, South Korea

    NASA Astrophysics Data System (ADS)

    Oh, Chang Whan; Imayama, Takeshi; Lee, Seung Yeol; Yi, Sang-Bong; Yi, Keewook; Lee, Byung Choon

    2015-02-01

    Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet-biotite gneiss and sillimanite-garnet-biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888-1871 Ma) at P-T conditions of 760-820 °C and 8-10 kbar and 710-750 °C and 5-7 kbar, respectively. These rocks were overprinted by low-P/T type metamorphism (590-650 °C, 3-4 kbar) during the Triassic (ca. 237 Ma). In contrast, a cordierite-rich migmatite near the post-collisional Triassic igneous complex in the eastern part of the area was strongly metamorphosed during the Triassic (ca. 235 Ma) at 750-790 °C and 7-8 kbar. The similar Triassic ages in the western and eastern areas suggest that low-P/T type metamorphism occurred as a second stage of regional metamorphism, which is characterized by the production of cordierite with an irregularly shaped garnet. The metamorphic grade of the Triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions, and the Paleoproterozoic metamorphism is well preserved in the western part with low grade Triassic metamorphism. The new discovery of the Triassic metamorphic event in the Yangpyeong area, in addition to the previously reported Triassic post-collision igneous event, supports the idea that the continental collision belt between the North and South China blocks extends from the Hongseong area into the Odesan area through the Yangpyeong area in South Korea.

  12. 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

  13. Shock metamorphic effects in lunar microcraters

    NASA Technical Reports Server (NTRS)

    Schaal, R. B.; Hoerz, F.; Gibbons, R. V.

    1976-01-01

    Detailed petrographic descriptions and results of electron microprobe analyses are presented for impact glasses as well as shocked and unshocked minerals associated with individual lunar microcraters (diameters of 0.4 to 4.4 mm). Rocks of four typical lunar lithologies are studied: anorthosite, anorthositic norite, ophitic basalt, and polymict breccia. Textures, mineralogies, and chemical compositions are examined along a radial traverse through each microcrater; i.e., across the impact glasses lining the crater wall, the shock-metamorphosed zone immediately underlying the glass liner, and the unshocked host rock. The microcraters are discussed in a sequence of increasing mineralogical complexity of the host rock (from anorthosite to polymict breccia) in order to distinguish shock effects among mineral types. The shock metamorphic features observed are found to be comparable to those reported in shocked basalt from Lonar Crater, India, and are categorized into five shock-intensity classes with pressures experimentally calibrated.

  14. A first find of retrogressed eclogites in the Odenwald Crystalline Complex, Mid-German Crystalline Rise, Germany: evidence for a so far unrecognised high-pressure metamorphism in the Central Variscides

    NASA Astrophysics Data System (ADS)

    Will, Thomas M.; Schmädicke, Esther

    2001-11-01

    Metabasic rocks were recently found in the Böllsteiner Odenwald, being part of the Variscan Mid-German Crystalline Rise (MGCR), that give evidence of a so far unrecognised eclogite-facies metamorphic event and testify, for the first time, to high-pressure metamorphism in the MGCR, the assumed suture zone of the European Variscides. Eclogite-facies metamorphism is indicated by both widespread clinopyroxene-plagioclase symplectites—interpreted as breakdown products of omphacite—and the composition of symplectitic clinopyroxene with measured jadeite contents of up to 27 mol%, extending into the omphacite field. Reintegration of numerous clinopyroxene-plagioclase symplectites implies minimum jadeite contents of the former omphacite of at least 38 mol%. For the eclogite stage, the four-phase assemblage omphacite-garnet-quartz-rutile can be reconstructed. A post-eclogitic overprint led to the formation of symplectitic intergrowths of clinopyroxene and plagioclase, amphibole-plagioclase coronas around garnet and domains with recrystallised amphibole and plagioclase. Preliminary P- T estimates for the eclogite-facies metamorphism indicate minimum pressures of some 16-17 kbar and temperatures of approximately 700±50 °C. Geothermobarometry for the subsequent symplectitic breakdown of omphacite yields some 14 kbar and 700 °C. P- T estimates on retrograde amphibolite-facies domains and on prograde mineral assemblages preserved in garnet cores point to a clockwise P- T path experienced by these rocks. The eclogites formed from a tholeiitic protolith, that may have been genetically linked to a continental extension zone or a young oceanic ridge or back-arc environment.

  15. Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles

    NASA Astrophysics Data System (ADS)

    González-Jiménez, José M.; Reich, Martin; Camprubí, Antoni; Gervilla, Fernando; Griffin, William L.; Colás, Vanessa; O'Reilly, Suzanne Y.; Proenza, Joaquín A.; Pearson, Norman J.; Centeno-García, Elena

    2015-08-01

    The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite-olivine assemblage was replaced by a secondary, porous intergrowth of Fe2+-rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS2, releasing minute particles of Ru-Os-Ir alloys <50 nm in diameter. The following short-lived thermal event that promoted dehydration in the chromitite had the opposite effect on nanoparticle stability, producing a significant coarsening of metal nanoparticles to dimensions larger than a micron. Based on such observations, we argue that IPGE nanoparticles can be exsolved and grown (or coarsen) from sulphide matrices during prograde metamorphism or heating and not exclusively upon cooling under magmatic conditions as it has been previously suggested. These results provide new insights on the relevant role of

  16. Oxidation during metamorphism of the ordinary chondrites

    NASA Technical Reports Server (NTRS)

    Mcsween, Harry Y., Jr.; Labotka, Theodore C.

    1993-01-01

    It is suggested that some current concepts about the conditions of metamorphism in ordinary chondrites may be flawed. These meteorites display small systematic variations in the oxidation state of Fe. Evidence is presented that oxidation of Fe is linked to metamorphic grade in types 4-6 ordinary chondrites. This conclusion is at variance with a commonly accepted model for chondrite metamorphism that assumes Fe reduction by graphite.

  17. Geochemical, geochronological characterization and tectonic setting of the metamorphic rocks from the Biga Peninsula, NW Turkey

    NASA Astrophysics Data System (ADS)

    Şengün, F.; Tunç, Ä.°. O.; Yiǧitbaş, E.

    2012-04-01

    The Biga Peninsula in the northwest Turkey is one of the world's important natural laboratories to study geochronology due to having complex geology. The Biga Peninsula has different metamorphic basements including Kazdağ Massif, Çamlıca metamorphics, Kemer metamorphics and Karadağ Massif under cover of the Cenozoic volcano-sedimentary association. The Çamlıca metamorphic assemblage are one of the most critical regions for understanding of the geology of northwestern Turkey. The Çamlıca metamorphic association located on the westernmost part of Turkey is mainly composed of the Andıktası formation, the Dedetepe formation and the Salihler formation, from bottom to top. Metasedimentary rocks of the Çamlıca metamorphics have high SiO2 and medium Al2O3 and TiO2 values. The protolith of these metasediments is arkose-subarkose and greywacke. However, whole-rock geochemistry for the HP eclogite/blueschist within the Çamlıca metamorphics suggests that their protolith was basalt with high TiO2 and K2O-Na2O content and Nb/Y ratios. REE pattern and trace element contents of the HP eclogite/blueschist similar to typical MORB based on tectonic discrimination diagrams. The metavolcanic rocks occurring on the lowest part of the Çamlıca metamorphicassociation has andesitic composition with calc-alkaline character. All metavolcanic rocks in this unit cluster within the volcanic arc field. Zircon grains from metavolcanic rocks and HP eclogite/blueschists were dated by LA-ICPMS. Zircon ages of two metavolcanic samples yielded 328.6 ± 3.5 Ma and 343.2 ± 2.6 Ma, respectively. These ages are interpreted as the time of protolith crystallization of metavolcanic rocks. Moreover, zircon ages from HP eclogite/blueschist yielded 338 ± 1.8 Ma (Early Carboniferous) which is interpreted as the age of protolith crystallization of HP eclogite/blueschist. Geochemical and isotopic data indicate that Early Carboniferous Variscan ages within the Sakarya Zone may form the eastern

  18. Origin and significance of the ophiolitic high-P mélanges in the northern Caribbean convergent margin: Insights from the geochemistry and large-scale structure of the Río San Juan metamorphic complex

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Friedman, Richard; Castillo-Carrión, Mercedes; Jabites, Janet; Pérez-Estaún, Andrés

    2011-12-01

    The Río San Juan metamorphic complex exposes a segment of a high-pressure accretionary wedge, built during Late Cretaceous intra-oceanic subduction below the Caribbean island-arc. Detailed mapping, large-scale structural analysis, U-Pb/Ar-Ar geochronology, in situ trace element analyses of clinopyroxene, and bulk-rock geochemical data show that the northern half of the complex is composed of arc- and oceanic-derived units, whose large-scale internal structure consists of an imbricate stack of mainly high-P rocks. In the lower structural levels, the Gaspar Hernández serpentinized peridotite-tectonite is composed of massive, serpentinized harzburgite that grades into surrounding sheared serpentinite or gouge. Gabbroic rocks and dolerite sills with N-MORB geochemical signatures are sparse and altered to greenschists facies. U-Pb data indicate a Lower Cretaceous magmatic age (136.40 ± 0.32 Ma). These rocks are interpreted as ophiolitic fragments of the proto-Caribbean Ocean. In the intermediate Jagua Clara serpentinite-matrix mélange, the massive serpentinite with relict peridotite textures was overprinted by schistose and sheared serpentinite, and serpentinite gouge, recording various degrees and conditions of internal deformation. Serpentinite schistosity surfaces warp around mélange hard blocks (< 1 m to 2.5 km). Blocks are massive serpentinized peridotite and exotic high-P crustal rocks. The protoliths of the mafic metaigneous rocks originated from arc-like and non-arc-like magmas. The Jagua Clara serpentinite-matrix mélange represents the deep subduction channel, formed during intra-oceanic subduction. In the upper structural levels, the Morrito unit is composed of high-P Puerca Gorda and Guineal Schists. Protoliths are basic-intermediate volcanic rocks with IAT, low-Ti IAT and boninitic geochemical signatures, similar in composition to the Lower Cretaceous Caribbean island-arc. The Morrito basal fault-zone thrusted the Puerca Gorda Schists onto the Jagua

  19. Lunar anorthosite 15415: Texture, mineralogy, and metamorphic history

    USGS Publications Warehouse

    James, O.B.

    1972-01-01

    Lunar anorthosite 15415 consists almost entirely of anorthite (homogeneous anorthite 96.6 molecule percent), with accessory diopsidic augite and traces of hypersthene, ilmenite, and a silica mineral. The rock has had a complex metamorphic history. The texture reflects at least two episodes of shearing (followed by intense and partial recrystallization, respectively), one episode of cataclastic deformation, and one or more episodes of shattering and fragmentation.

  20. Lunar anorthosite 15415 - Texture, mineralogy, and metamorphic history.

    NASA Technical Reports Server (NTRS)

    James, O. B.

    1972-01-01

    Lunar anorthosite 15415 consists almost entirely of anorthite (homogeneous anorthite 96.6 molecule percent), with accessory diopsidic augite and traces of hypersthene, ilmenite, and a silica mineral. The rock has had a complex metamorphic history. The texture reflects at least two episodes of shearing (followed by intense and partial recrystallization, respectively), one episode of cataclastic deformation, and one or more episodes of shattering and fragmentation.

  1. Lunar anorthosite 15415: texture, mineralogy, and metamorphic history.

    PubMed

    James, O B

    1972-01-28

    Lunar anorthosite 15415 consists almost entirely of anorthite (homogeneous anorthite 96.6 molecule percent), with accessory diopsidic augite and traces of hypersthene, ilmenite, and a silica mineral. The rock has had a complex metamorphic history. The texture reflects at least two episodes of shearing (followed by intense and partial recrystallization, respectively), one episode of cataclastic deformation, and one or more episodes of shattering and fragmentation. PMID:17731367

  2. 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?

  3. 3D geometrical modelling of post-foliation deformations in metamorphic terrains (Syros, Cyclades, Greece)

    NASA Astrophysics Data System (ADS)

    Philippon, Mélody; Le Carlier de Veslud, Christian; Gueydan, Frédéric; Brun, Jean-Pierre; Caumon, Guillaume

    2015-09-01

    Superposed to ductile syn-metamorphic deformations, post-foliation deformations affect metamorphic units during their exhumation. Understanding the role of such deformations in the structuration of metamorphic units is key for understanding the tectonic evolution of convergence zones. We characterize post-foliations deformations using 3D modelling which is a first-order tool to describe complex geological structures, but a challenging task where based only on surface data. We propose a modelling procedure that combines fast draft models (interpolation of orientation data), with more complex ones where the structural context is better understood (implicit modelling), allowing us to build a 3D geometrical model of Syros Island blueschists (Cyclades), based on field data. With our approach, the 3D model is able to capture the complex present-day geometry of the island, mainly controlled by the superposition of three types of post-metamorphic deformations affecting the original metamorphic pile: i) a top-to-South ramp-flat extensional system that dominates the overall island structure, ii) large-scale folding of the metamorphic units associated with ramp-flat extensional system, and iii) steeply-dipping normal faults trending dominantly NNW-SSE and EW. The 3D surfaces produced by this method match outcrop data, are geologically consistent, and provide reasonable estimates of geological structures in poorly constrained areas.

  4. The eastern limit of Acadian high grade metamorphism in northern New England: Implications for the location of the Acadian Suture''

    SciTech Connect

    West, D.P. Jr. . Dept. of Geological Sciences)

    1993-03-01

    Identifying the eastern limit of Acadian high grade metamorphism in New England is complicated by the presence of pre-Devonian high grade relics, locally intense Late Paleozoic thermal overprints, and post-metamorphic faults. New [sup 40]Ar/[sup 39]Ar mineral ages from along the eastern margin of high grade metamorphism in Maine and New Hampshire help delineate the eastern limit of Devonian amphibolite facies metamorphism thereby placing constraints on the location of the Acadian suture. In New Hampshire, Acadian high grade metamorphism extends southeast at least as far as the Campbell Hill fault and perhaps as far as the Flint Hill fault. New [sup 40]Ar/[sup 39]Ar hornblende ages and previously published U-Pb monazite ages from the Massabesic Gneiss Complex are Permian indicating high grade Alleghanian metamorphism. New [sup 40]Ar/[sup 39]Ar hornblende ages from the Rye Formation, although complicated by excess argon, are considerably older, indicating an earlier Pre-Silurian amphibolite facies metamorphism affected these rocks. North of Portland, [sup 40]Ar/[sup 39]Ar hornblende ages east of the Norumbega Fault Zone from high grade rocks of the Casco Bay Group have ages that range from Middle Devonian to Early Carboniferous, consistent with diachronous cooling following Acadian metamorphism. Further northeast, in upper amphibolite facies rocks of the Passagassawakeag Gneiss, new [sup 40]Ar/[sup 39]Ar hornblende ages range from 385--395 Ma suggesting that these rocks were also affected by high grade Acadian metamorphism.

  5. Shock metamorphism of carbonaceous chondrites

    NASA Technical Reports Server (NTRS)

    Scott, Edward R. D.; Keil, Klaus; Stoeffler, Dieter

    1992-01-01

    Shock effects were studied in 69 carbonaceous chondrites, including CM2, CO3, CV3, ungrouped C2-C4, and CK4-6 chondrites, using optical microscopy of thin sections. It is shown that the classification scheme of Stoeffler et al. (1991) for the progressive stages of shock metamorphism in ordinary chondrites is also applicable to carbonaceous chondrites. On the basis of shock effects in olivine, the 69 carbonaceous chondrites could be assigned to four shock stage, S1 to S4. The CM2 and CO3 groups were found to be the least shocked chondrite groups, whereas the CK4-6 and CV3 were the most strongly shocked groups.

  6. UHP metamorphism in Greece: Petrologic data from the Rhodope Mountains

    NASA Astrophysics Data System (ADS)

    Baziotis, I. P.; Mposkos, E.; Krohe, A.; Wawrzenitz, N. H.; Liu, Y.; Taylor, L. A.

    2012-12-01

    Metamorphic rocks contain invaluable information for understanding the orogenic mechanisms of a tectonic regime. It is now well recorded and recognized that subduction of oceanic lithosphere and collision of continental blocks can result in sinking of subducted rocks to deeper levels than normal (>100 km). Further, the discovery of coesite and diamond in apparently regionally metamorphosed rocks provoked issues, for returning these rocks to the surface relatively fast, thereby preserving the UHP conditions. These UHPM terrains have been identified in more than twenty provinces worldwide. In Greece, UHPM rocks occur in the Rhodope area, one of the major tectono-metamorphic units located in NE Greece. This region consists of different metamorphic complexes involved in the Alpine collisional history between the Eurasian and African plates (e.g., Krohe & Mposkos, 2002-Geol Soc London Spec Pub, 204, 151). In Rhodope, a Jurassic UHP metamorphism is confirmed in the uppermost Kimi and the underlying Sidironero complexes (Mposkos & Kostopoulos, 2001- EPSL, 192, 497; Perraki et al., 2004-5th ISEMG, T2-35, 2006- EPSL, 241, 672; Liati, 2005- Con Min Pet, 150, 608; Bauer et al., 2007- Lithos, 99, 207). UHP metamorphism is evidenced by the presence of octahedral microdiamond inclusions (3 to 10 μm) in protective garnets, within the metapelitic gneisses. Microdiamonds probably formed from a supercritical fluid under extreme P-T conditions. The latter is strengthened by the presence of composite inclusions consisting of CO2, calcite, and microdiamonds. Other UHP indicators include: 1) quartz rods and rutile needle exsolutions in metapelitic garnet, suggesting a former titaniferous super-silicic (majoritic) garnet formed at P >4GPa; 2) oriented quartz lamellae in eclogitic clinopyroxene having been exsolved from a former super-silicic UHP precursor; and 3) coesite pseudomorphs in garnet, where radial cracks around multi-crystalline-quartz aggregates are indicative of the former

  7. Thermal metamorphism. [of chondrite parent bodies

    NASA Technical Reports Server (NTRS)

    Mcsween, Harry Y., Jr.; Sears, Derek W. G.; Dodd, Robert T.

    1988-01-01

    Most chondrites have experienced thermal metamorphism, resulting in changes in texture, mineralogy and possibly chemical composition. The physical conditions for metamorphism range from approximately 400 to 1000 C at low lithostatic pressure. Metamorphism may have resulted from decay of short-lived radionuclides, electromagnetic induction or accretion of hot materials. Several thermal models for chondrite parent bodies have been proposed. The least metamorphosed type-3 chondrites probably carry the most information about the early solar system, but even these have been affected to some degree by thermal processing.

  8. Metamorphic density controls on early-stage subduction dynamics

    NASA Astrophysics Data System (ADS)

    Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

    2013-04-01

    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

  9. 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

  10. Tectono-metamorphic processes beneath an obducted ophiolite: evidence from metamorphic soles and accreted units from western Turkey

    NASA Astrophysics Data System (ADS)

    Plunder, A.; Agard, P.; Chopin, C.; Okay, A. I.; Whitechurch, H.

    2015-12-01

    The convergence between the Anatolide Tauride block and Eurasia during Cretaceous times lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. After reconstruction, obducted ophiolite and their sub-ophiolitic units can be found along a 200 kilometre-long north to south transect in western Anatolia. If related to a single event, the dimension of this ophiolite makes it an exceptional object of interest to study obduction and early subduction dynamics. This contribution aim to: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia; (2) reconstruct the Anatolian ophiolite and (3) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part of is this sole is made of garnet ± clinopyroxene amphibolites. In the northern part the sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis whereas it is lacking in the south. PT conditions were refined at 10.5 kbar and 780°C for the south and at 11 kbar and 725°C using pseudosection modelling. Field and petrological observations recognize three principal units in the accretionary complex (from top to bottom, OC1, 2 and 3) with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows pristine oceanic rocks to very low grade metamorphics rocks (lawsonite - pumpelltyite facies). Blueschist facies rocks including Fe-Mg carpholite-bearing layers were found in OC2 and attest high-pressure and low-temperature conditions (~10 kbar - 350°C). OC3 exhibit a clear blueschist facies metamorphism, but higher PT conditions (17 kbar - 450°C). Both OC2 and 3 were only found in the northern area close to the suture zone. Combining these data and recent advances

  11. Shock metamorphism of ordinary chondrites

    NASA Technical Reports Server (NTRS)

    Stoeffler, Dieter; Keil, Klaus; Scott, Edward R. D.

    1991-01-01

    This study proposes a revised petrographic classification of progressive stages of shock metamorphism of 26 ordinary chondrites. Six stages of shock (S1 to S6) are defined on the basis of shock effects in olivine and plagioclase as recognized by thin section microscopy, and the characteristic shock effects of each shock stage are described. It is concluded that shock effects and the sequence of progressively increasing degrees of shock metamorphosis are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor. It is suggested that the collisional histories of the H, L, and LL parent bodies were similar. Petrologic type-3 chondrites are deficient in stages S4 and S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. It is suggested that the more porous and volatile-rich Type-3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Stage S3 is the most abundant in nearly all petrologic types.

  12. Metamorphic probing of subduction dynamics and rheology

    NASA Astrophysics Data System (ADS)

    Agard, Philippe

    2015-04-01

    seismogenesis, fluid migration (and its duration), fluid fluxes and will help improve our general understanding of the earthquake 'factory' (at ~2.6 GPa, 550°C). (3) Amphibolite to granulite-facies metamorphic soles (i.e., ~500m thick tectonic slices welded to the base of ophiolites) provide specific insights into the rheology of nascent subduction, as their accretion is restricted to a transient, optimal P-T-t window (at 1±0.2 GPa, 750-850°C, after < 1-2 My) during which fluid release and infiltration lead to similar effective rheology on both sides of the plate interface (i.e., downgoing crust and mantle wedge). This transient though universal episode maximizes interplate mechanical coupling and ultimately promotes the detachment of the sole from the sinking slab. For all three examples above, one should emphasize the need for a better assessment of the P-T stability of (the complex solid solutions of) amphiboles, which would represent a major breakthrough for our further understanding of subduction dynamics and rheology.

  13. Fluid-absent metamorphism in the Adirondacks

    NASA Astrophysics Data System (ADS)

    Valley, J. W.

    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.

  14. 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.

  15. Seismicity, metamorphism and rheology of the lower continental crust

    NASA Astrophysics Data System (ADS)

    Austrheim, Håkon

    2014-05-01

    Seismological data document that both normal earthquakes and tremors occur in the lower continental crust. Pseudotachylytes (frictional melts and ultracommunited rocks) have been described from several high grade metamorphic terrains and may be the geological manifestation of this seismicity. The Grenville (c. 930Ma) granulite facies complex (T: 800 °C; P: ≤10kbar) of the Lindås Nappe in the Bergen Arcs, W-Norway underwent a fluid induced partial eclogite (T: 600-650 °C; P: 15-20 kbar) and amphibolite facies metamorphism during the Caledonian (c.400-430 Ma) continent collision. Pseudotachylyte fault and injection veins formed in the dry granulites at or close to the reaction fronts both in the eclogitized (western parts) and the amphibolitized (eastern parts) of the Nappe. They are locally recrystalized with the development of amphibolite and eclogite facies assemblages demonstrating that they formed pre or syn the Caledonian metamorphism. The pseudotachylytes transect lithologies ranging from peridotite to anorthosite and consequently the influence of the seismic energy release on a range of granulite facies minerals including garnet, pyroxenes, olivine, plagioclase, hornblende and scapolite can be observed. The seismic energy released promotes the Caledonian metamorphism and change the petrophysical properties of the lower crust in the following ways: The melting and the ultracommunition of the granulite facies minerals increased the reactive surface area and produce local pathways for fluid. S-rich scapolite, a common mineral in granulities play a key role in this process by releasing S and C to form sulfides and carbonates. Small sulfide grains impregnate the pseudotachylyte veins which may lead to an increased electrical conductivity of the deep crust. The pseudotachylyte veins impose inhomogeneities in the massive rocks through grain size reduction and lead to strain localization with development of amphibolite and eclogite facies shear zones. Formation

  16. Resolving the Youngest Episode of Zircon Rim Growth with High-Spatial Resolution SIMS: U-Pb Ages and Trace Element Analyses from <1 um Thick Metamorphic Zircon Rims from the Zanskar Shear Zone and Tso Morari UHP Complex, NW Himalaya

    NASA Astrophysics Data System (ADS)

    Coble, M. A.; Leech, M. L.

    2014-12-01

    Dating the youngest phase of metamorphic or magmatic zircon growth by in-situ techniques on sectioned zircons can be challenging when rims are only a few microns thick; in the worst case, geologically meaningless measurements result from mixing between different age and/or compositional growth domains. This is especially true for zircons from the Zanskar Shear Zone and Tso Morari UHP Complex, NW Himalaya, which contain Eocene to Miocene rims that grew over Paleozoic and Proterozoic protolith cores during peak and retrograde metamorphism. These metamorphic rims are typically only 0.5 to 3.0 microns thick and require high-spatial resolution to resolve. We used the SHRIMP-RG ion-microprobe to perform new U-Pb depth-profiling analyses on zircon surfaces (non-polished) pressed into indium metal. Zircons from a leucogranite dike from Malung Tokpo along the Zanskar Shear Zone, which have 2000-5000 ppm U rims, were selected for detailed depth-profiling to evaluate the depth-resolution by SIMS for U-Pb and trace element analyses. Due to the high U concentration, we were able to decrease the primary beam intensity (sputter rate) and the count times for U and Pb isotopes, and increase the number of cycles through the run-table (45 peak-hopping scans). As a result, each cycle yielded a 0.04 micron depth-resolved zircon age and trace element composition. The youngest 13 scans yielded a U-Pb age of 21.3 ± 0.5 Ma, representing a 0.6 micron rim with uniform U, Th, and Hf concentrations. At 1.15 microns, the primary beam sputtered into a ~600 Ma core; this older age is complicated by the fact that it reflects mixing between the bottom of the sputter volume and the Miocene ages of the pit margins and rim, because the diameter of the spot tends to increase with time. Trace element analyses on zircon surfaces from the Tso Morari UHP Complex are highly reproducible, showing enriched HREE profiles with negative Eu anomalies - a result that is difficult to reproduce by analyses of

  17. Metamorphism of basic and pelitic rocks at Sulitjelma, Norway

    NASA Astrophysics Data System (ADS)

    Boyle, Alan P.

    1986-06-01

    The Sulitjelma area of the Scandinavian Caledonides consists of a variety of metasedimantary units with small basic intrusions, and a large ophiolitic complex of predominantly basic composition. All units underwent prograde greenschist facies to amphibolite facies regional metamorphism during the Scandinavian phase of the Caledonian orogeny. The resultant mineral assemblages and relationships are consistent with the presence of a miscibility gap in the actinolite-hornblende series under greenschist facies conditions; a garnet forming reaction in pelites involving the consumption of carbonate in order to produe the observed grossular content; some conflict between low-baric and medium-baric pressure estimates from equilibria involving Ca amphibole bearing assemblages in metabasites.

  18. Metamorphic Sole and Accreted Units Along a subduction Interface: form Birth to Steady State (the Case of Western Turkey)

    NASA Astrophysics Data System (ADS)

    Plunder, A.; Agard, P.; Chopin, C.; Soret, M.

    2014-12-01

    In Western Turkey, obducted ophiolite, metamorphic sole and oceanic accretionnary complex units linked with the closure of the Neotethys are found along a 400 kilometre-long section (from north to south). We herein reappraise the metamorphic evolution of the sub-ophiolitic metamorphic units (both the metamorphic sole and the accretionnary units) of Western Turkey (i) to better characterize rock units exhumed along a cooling subduction interface, from birth to steady state (ii) to constrain the formation of metamorphic sole during the initiation of subduction (iii) and to track record of obducted ophiolite. On the basis of field and petrological observation three differents accretionnary units are reccognized with pressure-temperature estimates ranging from incipient metamorphism to blueschist-facies conditions providing information on plate coupling at different depths along the subduction interface. The upper part of the metamorphic sole was form in an amphibolite facies (garnet amphibolite - garnet clinopyroxene amphibolite). Different slices of metamorphic sole with different pressure-temperature conditions might be observed probably showing discrete timing of accretion to the upper plate. Part of the samples are characterized by a late blueschsit developpement. Both the blueschist overprint in the metamorphic sole and the high-pressure in oceanic unit were found only in the northern part of the field investigation. On the basis of the presented data, available radiometric and palaeogeographic data as well as recent themomechanical moddeling a tentative reconstruction of the subduction-zone evolution through time and the emplacement of a large-scale ophiolite is presented. Finally a comparison with ongoing work on the metamorphic sole of the Semail ophiolite of Oman is proposed with special highlights on the retrograde evolution in both settings.

  19. High grade metamorphism in the Bundelkhand massif and its implications on Mesoarchean crustal evolution in central India

    NASA Astrophysics Data System (ADS)

    Singh, S. P.; Dwivedi, S. B.

    2015-02-01

    The Bundelkhand Gneissic Complex (BnGC) in the central part of the Bundelkhand massif preserves a supracrustal unit which includes pelitic (garnet-cordierite-sillimanite gneiss, garnet-sillimanite gneiss, biotite gneiss and garnet-biotite gneiss) and mafic (hornblende-biotite gneiss and garnetiferous amphibolite) rocks. Granulite facies metamorphism of the complex initiated with breaking down of biotite to produce garnet and cordierite in the pelitic gneisses. Geothermobarometric calculations indicate metamorphic conditions of 720°C/6.2 kbar, followed by a retrograde (687°C/4.9 kbar) to very late retrograde stages of metamorphism (579°C/4.4 kbar) which is supported by the formation of late cordierite around garnet. The P-T conditions and textural relations of the garnet-cordierite-bearing gneiss suggest a retrograde cooling path of metamorphism.

  20. The Mythology of Metamorphic Fluid Expulsion

    NASA Astrophysics Data System (ADS)

    Connolly, James

    2010-05-01

    Regional metamorphism occurs in an ambiguous rheological regime between the brittle upper crust and ductile sub-lithospheric mantle. This ambiguous position has allowed two schools of thought to develop concerning the nature of metamorphic fluid flow. The classical school holds that metamorphic rocks are inviscid and that any fluid generated by devolatilization is squeezed out of rocks as rapidly as it is produced. According to this school permeability is a dynamic property and fluid flow is upward. In contrast the modern school, selectively uses concepts from upper crustal hydrology that presume implicitly, if not explicitly, that rocks are rigid or, at most, brittle. For the modern school, the details of crustal permeability determine fluid flow and as these details are poorly known almost anything is possible. Reality, to the extent that is reflected by field studies, offers some support to both schools. In particular, evidence of significant lateral and chanellized fluid flow are consistent with flow in rigid media, while evidence for short (104 - 105 y) grain-scale fluid rock interaction during much longer metamorphic events, suggests that reaction-generated grain-scale permeability is sealed rapidly by compaction; a phenomenon that is also essential to prevent extensive retrograde metamorphism. These observations provide a compelling argument for recognizing in conceptual models for metamorphic fluid flow that rocks are neither inviscid nor rigid, but have finite strength. The surprising result of this strength is that the steady state solutions for fluid flow in porous compacting media require that fluid expulsion is channeled into waves of fluid-filled porosity. The waves develop on a characteristic length scale known as the viscous compaction length, δ, that is also the length scale for lateral fluid flow. In this context, porosity refers to any hydraulically connected void space present on spatial scales

  1. 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.

  2. Ultrahigh Pressure Metamorphism (UHPM): Quo Vadimus?

    NASA Astrophysics Data System (ADS)

    Brown, M.

    2007-12-01

    UHPM is the petrologic record (min. P-T equivalent to Coe stability) of transport of continental lithosphere to asthenospheric depth, and return to crustal depth and incorporation into continents. The record of UHPM is scale- independent, but the issue of unit size (boudin/slice/terrane) and whether UHPM is recorded are important. Although the requirement for external hydration may limit equilibration in protoliths, cofacial compatible, indicative and diagnostic assemblages and lack of confidence in thermobarometry obfuscate the record of UHPM. Polymetamorphism and/or overprinting by multiple UHPM events may be possible through successive Wilson cycles and may be cryptic to add complexity. Our ability to image and study inclusion assemblages in zircons has been a major breakthrough, but are diamonds anybody's best friend? Is the max. depth from which continental crust may be retrieved equivalent to P = 10GPa or >10GPa? Will evidence remain to infer such depth? Commonly we tie dates to P-T, which yields rates that constrain mechanisms; advances in numerical modeling will enable progress in testing models of exhumation. UHPM is registered in the rock record since the Cryogenian-Ediacaran; it is inferred to record deep subduction. UHP rocks decorate sutures in Phanerozoic subduction-to-collision orogens. During the Mesoarchean-Tonian, subduction-to-collision orogenesis was marked by eclogite - high-pressure granulite metamorphism (E-HPGM). The implied change in geodynamics was a legacy of Rodinia, and related to the birth of the Pacific during the formation of Gondwana. Hoffman-type supercontinent breakup (turns inside out by subduction of complementary superocean) was the process by which segments of Rodinia were reassembled to form Gondwana by suturing of Braziliano - Pan-African belts, leaving orphaned Laurasian segments to combine with each other and Gondwana to form Pangea. In contrast, Wilson cycles (continental lithosphere rearranged by formation- destruction

  3. Evidence for multiple metamorphic events in the Adirondack Mountains, N. Y

    SciTech Connect

    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, 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.

  4. Metamorphic fluid flow - a question of scale, crustal depth and bulk rock composition

    SciTech Connect

    Tracy, R.J.; Rye, D.M.

    1985-01-01

    Recent studies have indicated that certain metamorphic rocks interacted with significant volumes of aqueous fluid during their time-integrated mineral reaction history. Rather than demonstrating that pervasive fluid flow is general in metamorphic rocks, these documented cases instead suggest the likelihood of pronounced to extreme channelization of through-going in fluids in deep-seated metamorphic terranes (P>3 kbar). In rocks more shallowly buried, and therefore under low lithostatic stress, pervasive flow along grain boundaries and open microfractures probably occurred, as at Skye and the Skaergaard Complex. In higher pressure metamorphic environments, documented cases of high fluid/rock ratio make a strong case for flow channelized in veins or in impure marble aquifers where pore space and permeability were created by decarbonation reactions driven by infiltration of aqueous fluid. The source of this fluid may commonly be traced to a nearby wet granitic intrusion or quartz vein. As long as the pressurized source of aqueous fluid continued, outward flow was possible as fluid held open the intergranular pore space which was created only at the infiltration/reaction front where a reduction in solid volume accompanied reaction. Cessation or interruption of fluid flow would allow the pore space to close due to porous-rock strength being exceeded by lithostatic stress. Pervasive flow or aqueous fluid in deepseated metamorphic terranes is therefore probably limited to carbonate-bearing lithologies adjacent to sources of major volumes of fluid; otherwise, fluid flow is likely to be localized in fractures or veins.

  5. Platy Hematite and Metamorphism on Mars

    NASA Technical Reports Server (NTRS)

    Lane, M. D.; Morris, R. V.; Hartmann, W. K.; Christensen, P. R.; Mertzman, S. A.

    2002-01-01

    Emissivity spectra of Sinus Meridiani, Mars suggest that the hematite consists of platy particles that occur as consolidated, schistose lenses or loose, platy particles. This platy hematite may have originated as a result of burial metamorphism. Additional information is contained in the original extended abstract.

  6. Chemical demineralization of different metamorphic grade coals

    SciTech Connect

    Yusupov, T.S.; Shumskaya, L.G.; Burdukov, A.P.

    2009-07-15

    The paper analyzes a process of deep mineralization of various metamorphic grade coals pre-ground in different destructive units, namely, in centrifugal-planetary mill and disintegrator. Coal dispergation in higher energy intensive mills greatly enhances inorganic component extraction to acidic solutions. This is explained by distortion of crystal structure and amorphization of minerals under various kinds and different intensity mechanical actions.

  7. Metamorphic conditions in the Ashe Metamorphic Suite, North Carolina Blue Ridge

    SciTech Connect

    McSween, H.Y. Jr. ); Abbott, R.N.; Raymond, L.A. )

    1989-12-01

    Taconian metamorphism of mafic rocks in the Ashe Metamorphic Suite can be characterized by reference to an isograd corresponding to the reaction bio + epi = hbl + gar, which separates rocks into two zones of low-variance assemblages. Temperatures and pressures estimated from mineral exchange geothermometers and a barometer suggest that this reaction occurred at approximately 600-650C and 7.5 kbar. Phase equilibria between biotite and hornblende, as well as the sharpness of the mapped isograd, indicate that the reaction is discontinuous. Inferred differences in metamorphic grade between Ashe amphibolites and mafic dikes in the underlying basement suggest that these units are in faulted contact. Isograd patterns in pelitic rocks suggest an elongated domal uplift that developed after metamorphism and thrusting, the core of which is exposed in the adjacent Grandfather Mountain window.

  8. Timing of Proterozoic deformation, plutonism, and metamorphism in the Los Pinos Mountains, Central New Mexico

    SciTech Connect

    Shastri, L.L. . Dept. of Geology); Bowring, S.A. )

    1992-01-01

    Geochronologic, structural, and metamorphic studies within the Los Pinos Mountains (LPM), central NM provide new insights into the Proterozoic geologic history of this area. The LPM consist of a NE-trending, NW-dipping sequence of complexly deformed amphibolites and felsic schists. These have been intruded by a pervasively deformed granitic pluton. Two predominant deformational fabrics exist in the LPM. S1 is an early northwest-trending foliation, commonly parallel to compositional layering, which is folded about S2. S2 is axial planar foliation to tight to isoclinal folds and is the regional NE-trending fabric. Other fabrics and complex fold interference patterns may be related to localized strain partitioning around granitic bodies. A network of granitic dikes associated with the pluton crosscuts S2 but contains a weak foliation parallel to S2, suggesting synkinematic intrusion of the dikes. Regional metamorphism in the LPM took place at upper greenschist to lower amphibolite facies. Electron microprobe traverses of garnets show compositional variation indicative of growth zoning. No abrupt changes in composition representative of multiple metamorphic events are observed. Garnet-biotite geothermometry yields average rim temperatures of 454 [+-] 50 C. U-Pb geochronology of zircons from amphibolite, granite, and a granite dike indicates essentially the same age for all three units (1.66 Ga). The amphibolite contains abundant zircons which have complex morphologies typical of metamorphic growth; however, an igneous origin cannot yet be precluded. Spheres from the same amphibolite yield a near concordant age of 1.62 Ga. Thus, deformation, plutonism, and possibly the peak of metamorphism, were coeval at ca. 1.66 Ga, with metamorphism cooling through the blocking temperature of sphene at 1.62 Ga. The LPM are similar to other mountain ranges in south-central New Mexico where 1.66 Ga ages have been reported.

  9. Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology

    NASA Astrophysics Data System (ADS)

    Mulcahy, S. R.; Vervoort, J. D.

    2013-12-01

    Lawsonite [CaAl2Si2O7(OH)2 H2O] is a critical index mineral for high- to ultrahigh-pressure metamorphism associated with subduction. Lawsonite is an important carrier of water into the mantle, a likely contributor to subduction zone seismicity, and a bearer of trace elements that link metamorphism to arc magmatism. Due to its limited pressure-temperature stability, lawsonite can serve as a powerful petrogenetic indicator of specific metamorphic events. Lu-Hf dating of lawsonite, therefore provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. Broad application of lawsonite Lu-Hf geochronology requires constraining the role of pressure-temperature path, lawsonite forming reactions, and the Lu and Hf systematics within lawsonite and other blueschist facies minerals. We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in melange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet. We sampled exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist from the Berkeley Hills and Tiburon Peninsula of California. We collected four samples from coherent lawsonite blueschist across the lawsonite-pumpellyite-epidote isograds in Ward Creek, near Cazadero California. High-grade blocks give ages similar to existing Franciscan geochronology: multi-stage garnet in hornblendite gives the following ages: 171×1.3 Ma (MSWD 2.8) for the core and 159.4×0.9 Ma (MSWD 2.0) for the corresponding rim; 166

  10. Late Paleozoic onset of subduction and exhumation at the western margin of Gondwana (Chilenia Terrane): Counterclockwise P-T paths and timing of metamorphism of deep-seated garnet-mica schist and amphibolite of Punta Sirena, Coastal Accretionary Complex, central Chile (34° S)

    NASA Astrophysics Data System (ADS)

    Hyppolito, T.; García-Casco, A.; Juliani, C.; Meira, V. T.; Hall, C.

    2014-10-01

    In this study, the Paleozoic albite-epidote-amphibolite occurring as meter-sized intercalations within garnet-mica schist at Punta Sirena beach (Pichilemu region, central Chile) is characterized for the first time. These rocks constitute an unusual exposure of subduction-related rocks within the Paleozoic Coastal Accretionary Complex of central Chile. Whereas high pressure (HP) greenschist and cofacial metasediments are the predominant rocks forming the regional metamorphic basement, the garnet-mica schist and amphibolite yield higher P-T conditions (albite-epidote amphibolite facies) and an older metamorphic age. Combining detailed mineral chemistry and textural information, P-T calculations and Ar-Ar ages, including previously published material from the Paleozoic Accretionary Complex of central Chile, we show that the garnet-mica schist and associated amphibolite (locally retrograded to greenschist) are vestiges of the earliest subducted material now forming exotic bodies within the younger HP units of the paleo-accretionary wedge. These rocks are interpreted as having been formed during the onset of subduction at the southwestern margin of Gondwana. However, we show that the garnet-mica schist formed at a slightly greater depth (ca. 40 km) than the amphibolite (ca. 30 km) along the same hot-subduction gradient developed during the onset of subduction. Both lithotypes reached their peak-P conditions at ca. 335-330 Ma and underwent near-isobaric cooling followed by cooling and decompression (i.e., counterclockwise P-T paths). The forced return flow of the garnet-mica schist from the subduction channel started at ca. 320 Ma and triggered the exhumation of fragments of shallower accreted oceanic crust (amphibolite). Cores of phengite (garnet-mica schist) and amphibole (amphibolite) grains have similar chemical compositions in both the S1 and S2 domains, indicating rotation of these grains during the transposition of the burial-related (prograde peak-T) foliation S1

  11. Chlorine Behavior in Metasedimentary Rocks during Subduction Zone Metamorphism

    NASA Astrophysics Data System (ADS)

    Barnes, J.; Selverstone, J.; Bebout, G. E.; Penniston-Dorland, S.

    2014-12-01

    Chlorine concentrations and isotopic compositions were measured in two well-characterized metasedimentary suites from the Catalina Schist and the Western Alps to determine Cl behavior during prograde metamorphism. The Catalina Schist is a subduction zone metamorphic complex in California, USA containing lawsonite-albite (LA) to amphibolite grade rocks recording temperatures of 350 to 750°C and depths of 15 to 45 km. Previous work has shown a decrease in N, B, Cs, As, and Sb concentrations from the LA to the epidote-blueschist facies, with relatively little loss at higher metametamorphic grade [1], and limited Li loss across all grades [2]. Metapelitic rocks from the Western Alps (Schistes Lustres and Lago di Cignana) record temperatures of 350 to 550°C and depths up to 90 km. In contrast to Catalina, N, B, Cs, Ba, and Rb concentrations are relatively uniform across grade [3]. In the Catalina Schist, Cl concentration shows a pattern of loss similar to B and N, from ~100-500 ppm Cl in the LA facies to ~100 ppm in the lawsonite-blueschist facies to relatively uniform concentrations of ~10-25 ppm at higher grades. This loss is likely not due to the breakdown of apatite as P2O5 concentrations remain constant across grade. In the Alps, Cl concentrations are overall lower and show moderate loss from ~10 ppm in the lowest grade to <5 ppm in the highest grade. δ37Cl values range from -1 to +1.6‰ and -1.7 to -0.7‰ in Catalina and the Alps, respectively. Both suites show significant isotopic heterogeneities within a single metamorphic grade and no systematic change in δ37Cl value with increasing grade. We interpret these heterogeneities to be inherited from the protolith. Despite large Cl losses, limited Cl isotope fractionation at high temperatures minimizes variations in δ37Cl value with increasing metamorphic grade. [1] Bebout et al, 1999, EPSL, 171, 53-81 [2] Penniston-Dorland et al, 2012, GCA, 77, 530-545 [3] Bebout et al, 2013, Chem Geol, 342, 1-20

  12. Metamorphism and aqueous alteration in low petrographic type ordinary chondrites

    NASA Technical Reports Server (NTRS)

    Xie, T.; Lipschutz, M. E.; Sears, D. W. G.; Guimon, R. K.; Jie, Lu; Benoit, P. H.; O'D. Alexander, C. M.; Wright, Ian; Pillinger, C.; Morse, A. D.; Hutchison, Robert

    1995-01-01

    evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process.

  13. Phase-field modeling of dry snow metamorphism.

    PubMed

    Kaempfer, Thomas U; Plapp, Mathis

    2009-03-01

    Snow on the ground is a complex three-dimensional porous medium consisting of an ice matrix formed by sintered snow crystals and a pore space filled with air and water vapor. If a temperature gradient is imposed on the snow, a water vapor gradient in the pore space is induced and the snow microstructure changes due to diffusion, sublimation, and resublimation: the snow metamorphoses. The snow microstructure, in turn, determines macroscopic snow properties such as the thermal conductivity of a snowpack. We develop a phase-field model for snow metamorphism that operates on natural snow microstructures as observed by computed x-ray microtomography. The model takes into account heat and mass diffusion within the ice matrix and pore space, as well as phase changes at the ice-air interfaces. Its construction is inspired by phase-field models for alloy solidification, which allows us to relate the phase-field to a sharp-interface formulation of the problem without performing formal matched asymptotics. To overcome the computational difficulties created by the large difference between diffusional and interface-migration time scales, we introduce a method for accelerating the numerical simulations that formally amounts to reducing the heat- and mass-diffusion coefficients while maintaining the correct interface velocities. The model is validated by simulations for simple one- and two-dimensional test cases. Furthermore, we perform qualitative metamorphism simulations on natural snow structures to demonstrate the potential of the approach. PMID:19391945

  14. Upper Cretaceous exhumation of the western Rhodope Metamorphic Province (Chalkidiki Peninsula, northern Greece)

    NASA Astrophysics Data System (ADS)

    Kydonakis, Konstantinos; Gallagher, Kerry; Brun, Jean-Pierre; Jolivet, Marc; Gueydan, Frédéric; Kostopoulos, Dimitrios

    2014-06-01

    The Vertiskos Unit of northern Greece is an elongated basement belt with a complex poly-metamorphic history. It extends from Greece (Chalkidiki peninsula), to the south, up to Serbia, in the north, and arguably represents the westernmost part of the Rhodope Metamorphic Province (northern Greece to southern Bulgaria). The Vertiskos Unit experienced a medium pressure lower amphibolite-facies metamorphic overprint during the Alpine Orogeny. The available medium-temperature geochronology implies that it remained at temperature of approximately 300°C (or slightly higher) during Lower Cretaceous. In order to constrain its post-Lower Cretaceous thermal history, until near-surface exposure, we applied apatite fission track analysis. The central ages obtained range from 68.5 ± 3.8 to 46.6 ± 3.6 Ma (uppermost Cretaceous to Middle Eocene) and mean track lengths between 13 and 13.5 µm. We applied two inverse thermal modeling approaches using either each sample independently (high degree of freedom in the thermal history, better data fit) or all samples together interpreting them as a vertical profile (simpler thermal history, worse data fit). Irrespective of the modeling approach, we conclude that the bulk thermal history of the Vertiskos Unit crosses the high-temperature limit of the apatite partial annealing zone by the uppermost Cretaceous and reaches near-surface conditions as early as lower/middle Eocene. These results contrast with the thermal history of the other domains of the Rhodope Metamorphic Province further east (namely the Southern Rhodope Core Complex and the Northern Rhodope Complex) and establish the Vertiskos basement complex as the oldest exhumed coherent basement fragment of the Rhodope Metamorphic Province and Greece.

  15. 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

  16. {sup 40}Ar/{sup 39}Ar thermochronology and thermobarometry of metamorphism, plutonism, and tectonic denudation in the Old Woman Mountains area, California

    SciTech Connect

    Foster, D.A.; Miller, C.F.; Harrison, T.M.; Hoisch, T.D.

    1992-02-01

    Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of {sup 40}Ar/{sup 39} Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, southeastern California. Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and {approximately}700 {degrees}C. {sup 40}Ar/{sup 39}Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. {sup 40}Ar/{sup 39}Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Later Cretaceous Plutons of the 73 Ma Old Woman-Piute batholith. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. 68 refs., 11 figs., 3 tabs.

  17. Carbonic metamorphism, granulites and crustal growth

    NASA Technical Reports Server (NTRS)

    Newton, R. C.; Smith, J. V.; Windley, B. F.

    1980-01-01

    Stabilization of early crust against melting by high radioactivity and against resorption into the mantle by fast convective overturn requires that water and heat producers were flushed upwards within 50 Myr of accretion. Creation of a refractory base of granulite by metamorphism associated with CO2 vapour explains CO2-rich fluid inclusions in ancient high-grade rocks, minor-element depletions and local phenomena of arrested development of charnockite in Precambrian terrains. The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots. New tectonic concepts may be needed to explain carbonic metamorphism and other features of early crustal evolution.

  18. Heat transfer by fluids in granulite metamorphism

    NASA Technical Reports Server (NTRS)

    Morgan, Paul; Ashwal, Lewis D.

    1988-01-01

    The thermal role of fluids in granulite metamorphism was presented. It was shown that for granulites to be formed in the middle crust, heat must be advected by either magma or by volatile fluids, such as water or CO2. Models of channelized fluid flow indicate that there is little thermal difference between channelized and pervasive fluid flow, for the same total fluid flux, unless the channel spacing is of the same order or greater than the thickness of the layer through which the fluids flow. The volumes of volatile fluids required are very large and are only likely to be found associated with dehydration of a subducting slab, if volatile fluids are the sole heat source for granulite metamorphism.

  19. Shape Metamorphism Using p-Laplacian Equation

    SciTech Connect

    Cong, Ge; Esser, Mehmet; Parvin, Bahram; Bebis, George

    2004-05-19

    We present a new approach for shape metamorphism, which is a process of gradually changing a source shape (known) through intermediate shapes (unknown) into a target shape (known). The problem, when represented with implicit scalar function, is under-constrained, and regularization is needed. Using the p-Laplacian equation (PLE), we generalize a series of regularization terms based on the gradient of the implicit function, and we show that the present methods lack additional constraints for a more stable solution. The novelty of our approach is in the deployment of a new regularization term when p --> infinity which leads to the infinite Laplacian equation (ILE). We show that ILE minimizes the supremum of the gradient and prove that it is optimal for metamorphism since intermediate solutions are equally distributed along their normal direction. Applications of the proposed algorithm for 2D and 3D objects are demonstrated.

  20. Effect of metamorphism on isolated olivine grains in CO3 chondrites

    NASA Technical Reports Server (NTRS)

    Jones, Rhian H.

    1993-01-01

    The presence of a metamorphic sequence in the CO3 chondrite group has been shown previously to result in changes in properties of chondrule silicates. However, the role of isolated olivine grains during metamorphism of these chondrites has not been addressed. Isolated olivine grains in two metamorphosed CO3 chondrites, Lance and Isna, have been investigated in this study in order to assess the compositional properties of isolated olivine grains that may be attributable to metamorphism. Compositional changes in isolated olivines with increasing petrologic subtype are very similar to changes in chondrule olivines in the same chondrites. Olivine compositions from all occurrences (chondrules, isolated grains, and matrix) converge with increasing petrologic subtype. The degree of equilibration of minor elements is qualitatively related to the diffusion rate of each element in olivine, suggesting that diffusion-controlled processes are the most important processes responsible for compositional changes within the metamorphic sequence. The data are consistent with metamorphism taking place in a closed system on the CO3 chondrite parent body. Fe-poor olivine grains in metamorphosed chondrites are characterized by an Fe-rich rim, which is the result of diffusion of Fe into the grains from Fe-rich matrix. In some instances, 'complex', Fe-rich rims have been identified, which appear to have originated as igneous overgrowths and subsequently to have been overprinted by diffusion processes during metamorphism. Processes experienced by CO3 chondrites are more similar to those experienced by the ordinary chondrites than to those encountered by other carbonaceous chondrites, such as the CV3 group.

  1. Experimental Study on Fluid Distribution at Ultra-High Metamorphic Conditions

    NASA Astrophysics Data System (ADS)

    Mönicke, K.; Burchard, M.; Duyster, J.; Maresch, W. V.; Röller, K.; Stöckhert, B.

    2001-12-01

    Ultra-high pressure (UHP) metamorphic rocks record deep subduction of continental crust. Insight into their rheological behavior at UHP metamorphic conditions is important for the understanding of the mechanical state and the kinematics within subduction zones. Amazingly, many exhumed UHP metamorphic rocks do not show evidence of significant deformation. Thus, it has been proposed that deformation is localized in low-strength zones controlled by partially wetting interstitial fluids [1]. Experimental results [2] show that at UHP metamorphic conditions only one homogenous fluid phase with variable composition exists, whose density and viscosity should be intermediate between those of conventional aqueous solutions and hydrous melts. Inclusions of such supercritical fluid have been recently described from a natural UHP metamorphic rock [3]. Motivated by these findings, experiments using a piston-cylinder apparatus were performed to study the fluid distribution in various rock types at pressures of 3.5 GPa and temperatures between 900 ° C and 600 ° C. Starting materials were natural UHP metamorphic specimens of (1) S-type granitic biotite-phengite-gneiss and (2) pyrope-quartzite, both from the Dora Maira Massif (Western Alps, Italy) and (3) a diamond-bearing garnet-mica-gneiss with granodioritic bulk composition from the Saxonian Erzgebirge (Germany), all with 2 wt.% water added. The supercritical fluids formed in these experiments can be quenched to form a silicic glass with demixing of an aqueous solution without changing the UHP fluid topology significantly. The shape of the fluid-filled interstices is irregular and complex, resulting in a low volume/interface area ratio and a potential of high stress concentration at the edges of wedge-shaped offshoots. We propose that the distribution of supercritical fluids has a pronounced effect on the strength of cool subducted crust, allowing deformation by grain boundary sliding and dissolution precipitation creep, or

  2. Prograde Metamorphism recorded in Antarctic Granulite

    NASA Astrophysics Data System (ADS)

    Marschall, H.; Pauly, J.; Chatterjee, N.; Monteleone, B.; Meyer, H. P.

    2015-12-01

    High-grade metamorphic rocks provide an archive of tectonic processes and record conditions in the deep roots of orogenic belts. Granulites typically preserve chemical and mineralogical evidence of the peak temperature to which crustal rocks may be subjected, and they commonly preserve a multitude of textural and chemical features that allow a rather detailed reconstruction of their cooling and exhumation. More rarely, however, is it possible to reconstruct parts of the prograde path, which would allow a reconstruction of the loading and heating of the rocks. Access to the prograde P-T path and the rates and durations involved in granulite formation would provide important constraints on the convergence part of orogenic processes. Here we investigated a sample of felsic granulite from the H.U. Sverdrupfjella, which is part of the high-grade Maud Belt (East Antarctica). Peak-metamorphic conditions of approximately 925 °C and 1.45 GPa persisted for a maximum of circa 14 million years and were attained shortly after 570 Ma. In addition to the short-lived temperature peak, zircon preserved evidence for protracted granulite facies conditions with temperatures above 800 °C persisting for approximately 40 million years. Constraints on prograde metamorphism are recorded by garnet that preserved pre-peak metamorphic growth zones, by Ti zonation in zircon and by rutile inclusions in garnet. Zr-in-rutile thermometry using rutile included in different generations of garnet is used to reconstruct the prograde P-T path documenting burial followed by heating to ultra-high temperatures at peak pressures. Complementary, Ti zonation in prograde cores of zircon grains document and date heating, whereas younger zircon rims show again lower Ti-in-zircon temperatures and date the retrograde stages of metamorphism. The highest T is recorded in rutile, but not in zircon, consistent with the dissolution rather than growth of zircon at ultra-high temperatures. The clockwise loading

  3. Unraveling P-T-Time-Fluids Paths: in-situ Geochronology Combined with Oxygen Isotopes in Metamorphic Rocks

    NASA Astrophysics Data System (ADS)

    Rubatto, D.; Angiboust, S.

    2014-12-01

    Geochronology of metamorphic rocks aims to determine pressure-temperature-time paths. The development of in-situ oxygen isotope analysis by ion microprobe permits to add fluid evolution to metamorphic histories. Accurate oxygen isotope analysis requires matrix-matched standards and, for chemically complex minerals, also appropriate matrix corrections. We will report on recent progresses on the analysis of oxygen isotopes in monazite, allanite and garnet using the SHRIMP ion microprobe. Zircon is a prime candidate for retrieving the oxygen composition of the pre-metamorphic protolith, as inherited zircons are preserved in many high-grade rocks. Metamorphic zircon rims that differ from the cores in δ18O offer insight into the origin of the fluid and the timing as well as degree of fluid-rock interaction. Garnet is another exceptional recorder of multiple metamorphic stages. Highly zoned garnets are found in a variety of eclogite-facies rocks and they can preserve dramatic oxygen isotopic variations (differences of a few to 10 δ18O ‰ within single crystal). Garnet and zircons from eclogite-facies metamorphic veins and shear zones within the metamorphosed Lago Superiore Unit from the Monviso ophiolite (W. Alps) have distinct growth zones with different oxygen isotopes composition. The shift in oxygen isotopes between the magmatic zircon core and the metamorphic zircon rim testifies to metasomatism by fluids equilibrated with serpentinites. The oxygen isotopes zoning in garnet confirms large scale fluid metasomatism concentrated in shear zones. It also indicates that some major shear zones from the Monviso ophiolitic sequence possibly underwent two stages of metasomatism, an event before or during early subduction and a later stage at around 550°C/2.5 GPa at peak pressure metamorphism.

  4. 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

  5. Metamorphic reactions in mesosiderites - Origin of abundant phosphate and silica

    NASA Technical Reports Server (NTRS)

    Harlow, G. E.; Delaney, J. S.; Prinz, M.; Nehru, C. E.

    1982-01-01

    In light of a study of the Emery mesosiderite, it is determined that the high modal abundances of merrillite and tridymite in most mesosiderites are attributable to redox reactions between silicates and P-bearing Fe-Ni metal within a limited T-fO2 range at low pressure. The recalculated amounts of dissolved P and S in the metallic portion of Emery reduce the metal liquidus temperature to less than 1350 C, and the solidus to less than 800 C, so that the mixing of liquid metal with cold silicates would have resulted in silicate metamorphism rather than melting. This redox reaction and redistribution of components between metal and silicates illuminates the complexities of mesosiderite processing, with a view to the recalculation of their original components.

  6. Telemagmatic metamorphism superimposed on regional metamorphism: Evidence from coals in central China

    SciTech Connect

    Tang, Y. . Dept. of Geological Sciences)

    1993-02-01

    Coal (Lower Permian No. 1) in north-central Henan province, central China, exhibits a zoned rank distribution. The rank varies between high-volatile bituminous and anthracite. Highest rank coal occurs in a northwest-southeast trending zone that cuts across the center of the study area. Coal rank decreases from this central zone towards both the northeast and southwest. Core data indicate that the anthracite is currently overlain by over 4,600 m of sedimentary cover, which represents more or less continuous sedimentation during the Permian and Triassic. In the lower rank area to the southwest, erosion has removed all but approximately 1,000 m of strata. The rank distribution in this area has been attributed to regional metamorphism by previous workers as the higher coal rank coincides with the thicker strata. However, this study reveals that anthracite in the area has a much higher vitrinite reflectance, between 3--6% (Ro max, in oil), with some locations exhibiting reflectances greater than 6%. Petrographically, the anthracite is characterized by well developed pores (5--10 [mu]m in diameter) and mosaic structure. It is suggested that the higher heat flow is due to the presence of deep-seated plutons. It is proposed that coal metamorphism in this area involved three stages: (1) Pre-orogenic (early Permian-late Triassic). Regional metamorphism produced coals of subbituminous to high-volatile bituminous rank; (2) Orogenic (early Jurassic-late Cretaceous). Telemagmatic metamorphism resulted in zones of higher rank coal (medium volatile through anthracite rank); (3) Post-orogenic (Tertiary-Quaternary). Shallow burial depth due to the tectonic uplift followed by erosion had a negligible effect on coal rank. It is suggested, therefore, that coalification in this area is the result of regional metamorphism overprinted by telemagmatic metamorphism.

  7. Petrochronological investigations to unravel the tectono-metamorphic history of Alpine subduction (Briançonnais, Queyras, Western Alps)

    NASA Astrophysics Data System (ADS)

    Lanari, Pierre

    2015-04-01

    The study of the tectonic and metamorphic history of the Alpine metamorphic belt involving oceanic and continental subduction processes requires knowledge of detailed Pressure-Temperature-time-deformation (P-T-t-ɛ) paths recorded by different tectono-metamorphic units across the belt. This task is particularly challenging in low-grade rocks, e.g. metapelites, (a) for thermobarometry, and (b) for geochronology. Metapelites at greenschist facies metamorphic conditions show a narrow spectrum of metamorphic minerals, notably quartz, chlorite and K-white mica, in addition to commonly detrital relics inherited from previous metamorphic rocks. To obtain reliable P-T estimates, a multi-method approach is required, which usefully combines Raman study of Carbonaceous Material (RSCM), chemical analysis in standardized X-ray maps, and multi-equilibrium inverse thermodynamic modelling of chlorite and white mica. In order to be able to link each assemblage to a specific metamorphic stage and determine the time scales and rates of metamorphism, it is critical to use in situ dating techniques. In this study, high-resolution geochronology was conducted including single-grain 40Ar/39Ar dating (step heating), where possible in combination with U-Th-Pb age-dating of allanite by LA-ICP-MS analysis. In the French western Alps, the Briançonnais zone is a remnant of the continental subduction wedge, while the Liguro-Piedmontais zone is a fossil oceanic subduction wedge. Metapelites from these two complexes were investigated to constrain the individual P-T-t paths recorded in each tectonic slice. This study focussed on deciphering four tectono-metamorphic units in the Briançonnais zone: (B1) The Internal basement; (B2) the Lower and (B3) Upper Zone Houillère; (B4) the Mesozoic Nappe Stack. These continental units recorded green-schist Alpine metamorphic conditions. In the Liguro-Piedmontais zone, five tectono-metamorphic units are identified: (LP1) the Péouvou; (LP2) Saint-Véran; (LP

  8. Making MetPetDB a tool for reconnaissance studies of metamorphism and metamorphic rocks

    NASA Astrophysics Data System (ADS)

    Hallett, B. W.; Spear, F. S.; Horkley, L. K.; Adali, S.; Fox, P. A.

    2012-12-01

    Recent data mining efforts have significantly increased the coverage and quantity of published data that form the foundation of MetPetDB: the Database for Metamorphic Petrology. Mineral assemblage, metamorphic grade, geochemical mineral and whole rock analyses, and image data from over 600 published papers have been compiled and uploaded, with focus on a number of particularly well-studied metamorphic belts of regional extent. As a result of data mining efforts in the past several years, MetPetDB now contains data for over 9,000 samples, over 10,000 mineral and whole rock (major or trace element) analyses, and over 20,000 images including maps, thin section scans, photomicrographs, SE and BSE images, and X-ray maps. These data are available for searching and download, exportable in spreadsheets and/or as placemark layers in a Google Earth .kml file. Each Google Earth placemark contains a link to the full data available through MetPetDB's web interface. The improved spatial coverage provides a starting point for a geoscientist to rapidly gather sample and geochemical data for a growing inventory of distinct metamorphic belts. Regional searches can be performed by choosing a user-defined bounding box, or any of a number of bounding polygons that delineate distinct metamorphic belts, such as the Greenland Caledonides, or the Bohemian Massif. MetPetDB is a tool for researchers to share, compile, and organize sample information, both published and unpublished, enabling production of a dynamic GIS to aid in planning field work, producing geologic maps, or making inventory of geochemical data for metamorphic rocks. In addition to regional queries, published metamorphic rock samples with non-spatial commonalities may be queried and compiled using MetPetDB. For example, a petrologist with an interest in the equilibrium exchange of yttrium between garnet and monazite at mid-crustal conditions could easily find garnet with a certain range of yttrium content in amphibolite

  9. Strain-collapsed metamorphic isograds in a sillimanite gneiss dome, Seward Peninsula, Alaska

    SciTech Connect

    Miller, E.L.; Calvert, A.T.; Little, T.A. )

    1992-06-01

    Unusually closely spaced Barrovian series isograds have been described along the flanks of the Kigluaik Mountains, Seward Peninsula, Alaska, where they separate a high-grade gneiss complex intruded by granites of Cretaceous age from surrounding, regionally developed, blueschist to greenschist facies rocks. Structural mapping of the transition zone between the two metamorphic types indicates that their juxtaposition was aided by significant syn- to late-metamorphic solid-state flow that served to attenuate the overlying rock column and thus collapse the field metamorphic gradient. On the basis of field relations, structural data, petrography, and geochronologic data, strain appears to have accompanied the rapid (adiabatic) rise of high-temperature rocks from several tens of kilometers to less than 10 km depth during the Cretaceous, in an event younger than the unrelated to high-P metamorphism. Granite-cored gneiss domes on the Seward Peninsula may have formed during extension of previously thickened continental crust, resulting in the {approximately}35-km-thick crust and near-sea-level elevations of the region today.

  10. Zircon response to high-grade metamorphism as revealed by U-Pb and cathodoluminescence studies

    NASA Astrophysics Data System (ADS)

    Siebel, W.; Shang, C. K.; Thern, E.; Danišík, M.; Rohrmüller, J.

    2012-11-01

    Correct interpretation of zircon ages from high-grade metamorphic terrains poses a major challenge because of the differential response of the U-Pb system to metamorphism, and many aspects like pressure-temperature conditions, metamorphic mineral transformations and textural properties of the zircon crystals have to be explored. A large (c. 450 km2) coherent migmatite complex was recently discovered in the Bohemian Massif, Central European Variscides. Rocks from this complex are characterized by granulite- and amphibolite-facies mineral assemblages and, based on compositional and isotopic trends, are identified as the remnants of a magma body derived from mixing between tonalite and supracrustal rocks. Zircon crystals from the migmatites are exclusively large (200-400 μm) and yield 207Pb/206Pb evaporation ages between 342-328 Ma and single-grain zircon fractions analysed by U-Pb ID-TIMS method plot along the concordia curve between 342 and 325 Ma. High-resolution U-Pb SHRIMP analyses substantiate the existence of a resolvable age variability and yield older 206Pb/238U ages (342-330 Ma, weighted mean age = 333.6 ± 3.1 Ma) for inner zone domains without relict cores and younger 206Pb/238U ages (333-320 Ma, weighted mean age = 326.0 ± 2.8 Ma) for rim domains. Pre-metamorphic cores were identified only in one sample (206Pb/238U ages at 375.0 ± 3.9, 420.3 ± 4.4 and 426.2 ± 4.4 Ma). Most zircon ages bracket the time span between granulite-facies metamorphism in the Bohemian Massif (~345 Ma) and the late-Variscan anatectic overprint (Bavarian phase, ~325 Ma). It is argued that pre-existing zircon was variously affected by these metamorphic events and that primary magmatic growth zones were replaced by secondary textures as a result of diffusion reaction processes and replacement of zircon by dissolution and recrystallization followed by new zircon rim growth. Collectively, the results show that the zircons equilibrated during high-grade metamorphism and record

  11. 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.

  12. Oxyvanite, V3O5, a new mineral species and the oxyvanite-berdesinskiite V2TiO5 series from metamorphic rocks of the Slyudyanka Complex, southern Baikal region

    NASA Astrophysics Data System (ADS)

    Reznitsky, L. Z.; Sklyarov, E. V.; Armbruster, T.; Ushchapovskaya, Z. F.; Galuskin, E. V.; Polekhovsky, Yu. S.; Barash, I. G.

    2010-12-01

    Oxyvanite has been identified as an accessory mineral in Cr-V-bearing quartz-diopside meta- morphic rocks of the Slyudyanka Complex in the southern Baikal region, Russia. The new mineral was named after constituents of its ideal formula (oxygen and vanadium). Quartz, Cr-V-bearing tremolite and micas, calcite, clinopyroxenes of the diopside-kosmochlor-natalyite series, Cr-bearing goldmanite, eskolaite-karelianite dravite-vanadiumdravite, V-bearing titanite, ilmenite, and rutile, berdesinskiite, schreyerite, plagioclase, scapolite, barite, zircon, and unnamed U-Ti-V-Cr phases are associated minerals. Oxyvanite occurs as anhedral grains up to 0.1-0.15 mm in size, without visible cleavage and parting. The new mineral is brittle, with conchoidal fracture. Observed by the naked eye, the mineral is black, with black streak and resinous luster. The microhardness (VHN) is 1064-1266 kg/mm2 (load 30 g), and the mean value is 1180 kg/mm2. The Mohs hardness is about 7.0-7.5. The calculated density is 4.66(2) g/cm3. The color of oxyvanite is pale cream in reflected light, without internal reflections. The measured reflectance in air is as follows (λ, nm- R, %): 440-17.8; 460-18; 480-18.2; 520-18.6; 520-18.6; 540-18.8; 560-18.9; 580-19; 600-19.1; 620-19.2; 640-19.3; 660-19.4; 680-19.5; 700-19.7. Oxyvanite is monoclinic, space group C2/ c; the unit-cell dimensions are a = 10.03(2), b = 5.050(1), c = 7.000(1) Å, β = 111.14(1)°, V = 330.76(5)Å3, Z = 4. The strongest reflections in the X-ray powder pattern [ d, Å, ( I in 5-number scale)( hkl)] are 3.28 (5) (20 bar 2 ); 2.88 (5) (11 bar 2 ); 2.65, (5) (310); 2.44 (5) (112); 1.717 (5) (42 bar 2 ); 1.633 (5) (31 bar 4 ); 1.446 (4) (33 bar 2 ); 1.379 (5) (422). The chemical composition (electron microprobe, average of six point analyses, wt %): 14.04 TiO2, 73.13 V2O3 (53.97 V2O3calc, 21.25 VO2calc), 10.76 Cr2O3, 0.04 Fe2O3, 0.01 Al2O3, 0.02 MgO, total is 100.03. The empirical formula is (V{1.70/3+} Cr0.30)2.0(V{0.59/4+} Ti0.41)1.0O

  13. Anatexis of garnet amphibolites from a subduction zone metamorphic terrane

    SciTech Connect

    Sorensen, S.S.; Barton, M.D.; Ernst, W.G.

    1985-01-01

    Concomitant rehydration, metasomatism and amphibolitization of eclogite blocks from a mafic/ultramafic complex of the Catalina Schist terrane, southern California, at estimated metamorphic P approx. 8-12 kb, T approx. 600/sup 0/-700/sup 0/C was apparently accompanied by partial melting of some blocks. Mobilizates of An approx./sub 10-20/ plagioclase (PL) +/- zoisite (ZO) + quartz (QZ) + celadonitic (Si approx. 3.3 p.f.u.) white mica (WM) +/- tourmaline range from stringers and dikelets (approx. 1 cm-0.5 m) in migmatitic amphibolite blocks to dikes approx. 30 m x 3 m which intrude the surrounding, locally enstatite + chlorite +/- talc +/- aluminous actinolite +/- anthophyllite-bearing ultramafic matrix. The uniform phase proportions and the coarse-grained (PL to approx. 20 cm) pegmatitic, graphic, and myrmekitic textures displayed by the dikes and dikelets suggest that they crystallized from silicate melts. WM and ZO appear to be magmatic phases. Fe-rich GT is migmatitic portions of blocks exhibits higher Mg/(Mg + Ca) p.f.u. than GT in restitic portions of blocks; rims are richer in Mg than cores. Field relations, microprobe mineral chemistry, and bulk compositions suggest the pegmatites are low fractions of amphibolite-derived partial melt. Abundant fluid inclusions occur in GT, QZ, PL and clinopyroxene. T/sub h/ for primary H/sub 2/O-rich, low salinity L + V inclusions in GT and QZ from a migmatite range from 136-169/sup 0/C; estimates of T limits for entrapment are 530-640/sup 0/C at 8 kb, 650-780/sup 0/C at 10 kb. H/sub 2/O-rich fluids evidently enabled metasomatism, amphibolitization, and anatexis of (originally) eclogitic rocks at the P-T conditions reflected by the metamorphic mineral assemblages.

  14. 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.

  15. 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.

  16. Metamorphism of eucrite meteorites studied quantitatively using induced thermoluminescence

    NASA Technical Reports Server (NTRS)

    Batchelor, J. David; Sears, Derek W. G.

    1991-01-01

    Induced thermoluminescence studies provide a new and quantitative means of determining relative metamorphic intensities for eucrite meteorites, the simplest and most ancient products of basaltic volcanism. Using this technique, it is shown that the eucrites constitute a continuous metamorphic series and not, as commonly assumed, two groups of metamorphosed and nonmetamorphosed meteorites. It is suggested that the method may have applications to other basalts.

  17. 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)

  18. Pb and Zn release from crustal sediments during metamorphism

    NASA Astrophysics Data System (ADS)

    Hammerli, Johannes; Spandler, Carl; Oliver, Nick; Sossi, Paolo; Dipple, Greg

    2015-04-01

    Our understanding of the genesis of Pb-Zn ore systems is currently limited by a lack of knowledge about from where these metals are sourced. While it is generally agreed that the temperature and salinity of fluids play a crucial role in mobilizing metals, the importance of the composition of the source lithologies is poorly constrained. Furthermore, fluid-rock processes that lead to element mobility are yet to be fully understood, especially in metamorphic environments. To address these gaps in our knowledge we examine metal distribution and mobility in rocks from a prograde metamorphic sequence of the Mt Lofty Ranges, South Australia, where temperatures range from ~300 to 700˚C at ~3.5 to 5 kbar. This regional metamorphic belt underwent regional up-temperature fluid flow, which allowed for favourable conditions for enhanced element mobility. By studying the distribution and concentration of Pb and Zn on both bulk-rock and mineral scales, in combination with bulk rock Zn-isotope data, we can monitor the behaviour of these elements during pressure and temperature changes. Our results show that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80% of the bulk rock Zn, as well as a considerable proportion of the total Pb, irrespective of the metamorphic grade. Pervasive fluid flow during metamorphism through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk-rock scale, resulting in a mobilization of ~80% of the bulk-rock Zn and ~50% of the bulk-rock Pb, mainly through reactions involving biotite. We calculated that a minimum of 2.7Mt of Pb and 27Mt of Zn was mobilized/lost in the high-grade metamorphic zone (200 km2). Halogen contents of apatite and biotite and bulk-rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying metamorphism of typical sedimentary rocks can mobilize base metals to the degree required to form

  19. Fluid flow and chemical reaction kinetics in metamorphic systems

    SciTech Connect

    Lasaga, A.C.; Rye, D.M. )

    1993-05-01

    The treatment and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow, diffusion, and thermal evolution. The interplay of fluid flow and surface reaction rates, the distinction between steady state and equilibrium, and the possible overstepping of metamorphic reactions are discussed using a simple analytic model. This model serves as an introduction to the second part of the paper, which develops a reaction model that solves the coupled temperature-fluid flow-chemical composition differential equations relevant to metamorphic processes. Consideration of stable isotopic evidence requires that such a kinetic model be considered for the chemical evolution of a metamorphic aureole. A general numerical scheme is discussed to handle the solution of the model. The results of this kinetic model allow us to reach several important conclusions regarding the factors controlling the chemical evolution of mineral assemblages during a metamorphic event. 41 refs., 19 figs., 5 tabs.

  20. Isothermal densification and metamorphism of new snow

    NASA Astrophysics Data System (ADS)

    Schleef, S.; Loewe, H.; Schneebeli, M.

    2012-12-01

    The interplay between overburden stress and surface energy induced growth and coarsening is relevant for the densification of snow and porous ice at all densities. The densification of new snow is amenable to high precision experiments on short time scales. To this end we investigate the coupling of densification and metamorphism of new snow via time-lapse tomography experiments in the laboratory. We compare the evolution of density, strain, and specific surface area to previous long-time metamorphism experiments of snow and creep of polycrystalline ice. Experimental conditions are tailored to the requirements of time-lapse tomography and the measurements are conducted under nearly isothermal conditions at -20°C with a duration of two days. Images were taken with temporal resolution of a few hours which reveal precise details of the microstructure evolution due to sintering and compaction. We used different crystal shapes of natural new snow and snow samples obtained by sieving crystals grown in a snowmaker in the laboratory. To simulate the effect of overburden stress due to an overlying snowpack additional weights were applied to the sample. As expected we find an influence of the densification rate on initial density and overburden stress. We calculated strain rates and identified a transient creep behavior with a similar power law for all crystal types which substantially differs from the Andrade creep of polycrystalline ice. As a main result we found that the evolution of the specific surface area is independent of the density and follows a unique decay form for all measurements of each crystal type. The accuracy of the measurements allows to obtain a decay exponent for the SSA which is the same as previously obtained from the long-time regime during isothermal metamorphism after several months. Our preliminary results for all available types of new snow suggest a correlation between the initial density and SSA. We also find snow samples which coincide in

  1. 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.

  2. 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.

  3. Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events

    NASA Astrophysics Data System (ADS)

    Rolland, Y.; Carrio-Schaffhauser, E.; Sheppard, S. M. F.; Pêcher, A.; Esclauze, L.

    2006-04-01

    The Karakorum Range comprises a crustal section of marbles and metapelites providing an opportunity to study the extent of high-temperature metamorphic reequilibration in an active orogen. Metamorphism culminated during the Mio-Pliocene, at 6 7 Ma. Peak metamorphic conditions increased from south to north, i.e. from (1) the Upper Anchizone grade (lawsonite, chlorite smectite) to (2) lower granulite migmatite grade (HT˜800°C) conditions along strike of a 30-km section perpendicular to the structural fabric of the rocks. The metamorphic section can be separated into two domains: 1. A domain with low to transitional metamorphic conditions, with respect to the HT zone, where initial bedding is preserved. These moderate PT conditions prevailed during the main tectonic stacking event (50 37 Ma), prior to the Mio-Pliocene event. In this domain, metamorphism is governed by fluid-assisted grain-scale diffusion, as suggested by the progressive coarsening of minerals with increasing metamorphic grade and the preservation of sedimentary δ13C signatures in carbonates. A low thermal gradient (17°C/km) is derived from P-T estimates of the prograde metamorphic sequences.

  4. Early planetary metamorphism in chondritic meteorites

    NASA Astrophysics Data System (ADS)

    Hanan, B. B.; Tilton, G. R.

    1985-07-01

    The record of early events in the solar system is presently sought, together with information on the isotopic composition of primordial lead, in the lead isotope relations of whole rock and separated phases of Mezo-Madaras (L3) and Sharps (H3) chondrites; the respective ages of 4.48 and 4.47 billion years are not significantly changed when Canyon Diablo troilite lead is included in the data sets, suggesting that the initial Pb isotopic composition in both meteorites was the same as that in the troilite. The 4.48 billion year age, which is younger than the well established 4.54-4.56 billion years of the Allende chondrite and Angra dos Reis achondrite, appears to date an early metamorphic event rather than the formation of the chondrites.

  5. Redistribution of volatiles during lunar metamorphism

    NASA Technical Reports Server (NTRS)

    Cirlin, E. H.; Housley, R. M.

    1980-01-01

    Thermal release profiles of Pb, Zn, and Cd in sample 66095 (highly shocked breccia with melt rock matrix) showed that these volatiles were mostly present on the surface of the grains. Zn in rusty grains from 66095 was also mostly surface Zn, probably from sphalerite in grain boundaries and cracks. Simulation experiments of volatile transfer showed that Fe, FeCl2, iron phosphide, and troilite (FeS) can be produced and transported during subsolidus reactions. These results suggest that volatiles, rust, schreibersite, and possible siderophiles which are observed in lunar highland samples might have been redistributed during disequilibrium thermal metamorphism in hot ejecta blankets, and were not necessarily introduced by volcanic activity or meteoritic addition.

  6. Shock metamorphism of lunar and terrestrial basalts

    NASA Technical Reports Server (NTRS)

    Schaal, R. B.; Hoerz, F.

    1977-01-01

    Lonar Crater (India) basalt and lunar basalt 75035 were shock loaded under controlled laboratory conditions up to 1000 kbar, generally in a CO/CO2 (1:1) environment evacuated to 10 to the minus seventh power torr. The Kieffer et al. (1976) classification scheme of progressive shock metamorphism is found to apply to lunar basalts. The major shock features of the five classes that span the range 0 to 1000 kbar are described. Only three out of 152 basalt specimens show shock effects in their natural state as severe as Class 2 features. The scarcity of shocked basalt hand samples in contrast to the abundance of shock-produced agglutinates and homogeneous glass spheres in the lunar regolith indicates the dominant role of micrometeorite impact in the evolution of the lunar regolith. The overall glass content in asteroidal and Mercurian regoliths is considered.

  7. Metamorphic Perspectives of Subduction Zone Volatiles Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.

    2008-12-01

    Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites

  8. Conodont color and textural alteration: an index to regional metamorphism, contact metamorphism, and hydrothermal alteration.

    USGS Publications Warehouse

    Rejebian, V.A.; Harris, A.G.; Huebner, J.S.

    1987-01-01

    Experimental and field data are used to extend the utility of conodonts as semi-quantitative thermal indices into the regimes of regional and contact metamorphism, as well as hydrothermal alteration. These experiments approximate the type of Colour Alteration Indices mixture characteristically found in conodonts recovered from hydrothermally altered rocks. These data indicate that CAI values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization. - from Authors

  9. Thrust involvement of metamorphic rocks, southwestern Brooks Range, Alaska

    SciTech Connect

    Till, A.B.; Schmidt, J.M.; Nelson, S.W. )

    1988-10-01

    Most models for the tectonic history of the western Brooks Range treat Proterozoic and lower Paleozoic metamorphic rocks exposed in the southern part of the range as passive structural basement vertically uplifted late in the Mesozoic orogenic episode. Mapping in the metamorphic rocks shows that they can de divided into two structurally and metamorphically distinct belts, both of which were folded and thrust during the orogeny. Recognition of these belts and the nature of the contact separating them is critical to construction of accurate tectonic models of the Brooks Range fold and thrust belt.

  10. Early Proterozoic ultrahigh pressure metamorphism: evidence from microdiamonds.

    PubMed

    Cartigny, Pierre; Chinn, Ingrid; Viljoen, K S; Robinson, Derek

    2004-05-01

    Microdiamonds from the Akluilâk minette dykes (Nunavut, Canada) are similar to diamonds formed in subducted metamorphic rocks. High concentrations of unaggregated nitrogen and positive delta(15)N suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1.8 billion years ago. This ultrahigh pressure metamorphism in North America, probably related to the Trans-Hudson orogen (about 2 billion years ago), extends the occurrence of ultrahigh pressure metamorphism from 0.6 billion years to before 1.8 billion years ago and suggests that Phanerozoic-type subductions were active by the Early Proterozoic. PMID:15131301