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Sample records for fazenda brasileiro gabbro

  1. Drilling to gabbro in intact ocean crust.

    PubMed

    Wilson, Douglas S; Teagle, Damon A H; Alt, Jeffrey C; Banerjee, Neil R; Umino, Susumu; Miyashita, Sumio; Acton, Gary D; Anma, Ryo; Barr, Samantha R; Belghoul, Akram; Carlut, Julie; Christie, David M; Coggon, Rosalind M; Cooper, Kari M; Cordier, Carole; Crispini, Laura; Durand, Sedelia Rodriguez; Einaudi, Florence; Galli, Laura; Gao, Yongjun; Geldmacher, Jörg; Gilbert, Lisa A; Hayman, Nicholas W; Herrero-Bervera, Emilio; Hirano, Nobuo; Holter, Sara; Ingle, Stephanie; Jiang, Shijun; Kalberkamp, Ulrich; Kerneklian, Marcie; Koepke, Jürgen; Laverne, Christine; Vasquez, Haroldo L Lledo; Maclennan, John; Morgan, Sally; Neo, Natsuki; Nichols, Holly J; Park, Sung-Hyun; Reichow, Marc K; Sakuyama, Tetsuya; Sano, Takashi; Sandwell, Rachel; Scheibner, Birgit; Smith-Duque, Chris E; Swift, Stephen A; Tartarotti, Paola; Tikku, Anahita A; Tominaga, Masako; Veloso, Eugenio A; Yamasaki, Toru; Yamazaki, Shusaku; Ziegler, Christa

    2006-05-19

    Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D. PMID:16627698

  2. [The archeology of slavery on Jesuit fazendas: first research notes].

    PubMed

    Symanski, Luís Cláudio P; Gomes, Flávio

    2012-12-01

    These preliminary research notes present theoretical and methodological questions regarding a recently inaugurated investigation in historical archeology that intends to analyze daily life under slavery, demographic regimes, cultural practices, and so on. A survey of archeological sites on former 'senzalas' (slave quarters) and slave-owning fazendas in the Paraíba Valley and northern part of the state of Rio de Janeiro is currently in progress. With the cooperation of historians, archeologists, and anthropologists, records of the material culture of slave populations, which originally comprised indigenes and later Africans, are being located at excavations underway on the fazenda that is part of the Jesuit school in Campos dos Goytacazes, Rio de Janeiro, first run by the clergy and later by members of the laity in the seventeenth, eighteenth, and nineteenth centuries. PMID:23370111

  3. Shock wave properties of anorthosite and gabbro

    NASA Technical Reports Server (NTRS)

    Boslough, M. B.; Ahrens, T. J.

    1984-01-01

    Hugoniot data on San Gabriel anorthosite and San Marcos gabbro to 11 GPA are presented. Release paths in the stress-density plane and sound velocities are reported as determined from particl velocity data. Electrical interference effects precluded the determination of accurate release paths for the gabbro. Because of the loss of shear strength in the shocked state, the plastic behavior exhibited by anorthosite indicates that calculations of energy partitioning due to impact onto planetary surfaces based on elastic-plastic models may underestimate the amount of internal energy deposited in the impacted surface material.

  4. The Amphiolite Layers In The Cumulate Gabbros, (Northern-Turkey)

    NASA Astrophysics Data System (ADS)

    Özkan, Mutlu; Faruk Çelik, Ömer; Altıntaş, İsmail Emir; Sherlock, Sarah; Chelle-Michou, Cyril; Marzoli, Andrea; Ulianov, Alexey; Melih Çörtük, Rahmi; Topuz, Gültekin

    2016-04-01

    The Early-Middle Jurassic SSZ type dismembered ophiolite sequence, which is remnants of the Tethyan oceanic lithosphere, crop out in the accretionary complex around Tokat-Çamlıbel region (Northern Turkey). The main lithology of the ophiolite sequence are cumulate gabbros, isotropic gabbros and basalts. The amphibolite layers, which their thickness are up to 2 m, are observed in the cumulate gabbros. In this study, we aim to discuss a possible formation mechanism of the amphibolitic rocks in the cumulate gabbros, based on the field, mineralogical, geochemical and geochronological data. The cumulate gabbros (olivine-gabbro, gabbro-norite and gabbro) have generally well developed magmatic layers and they show cumulate texture. They are cross cut by pegmatite gabbros, dolerites and plagiogranite dikes. In terms of the mechanism of formation, the amphibolite layers in the cumulate gabbros are different from dolerite, pegmatite gabbro and plagiogranite dikes crosscutting the cumulate gabbros. Although the cumulate gabbros, the mafic and felsic dikes have not undergone any metamorphism (except the hydrothermal metamorphism), the amphibolite layers show well developed foliation and banded structure. Moreover, field and petrographic observations showed that the amphibolitic rocks were highly subjected to shearing. The amphibolitic rocks are mainly composed of magnesio-hornblende + plagioclase (andesine), ± biotite and opaque minerals and they exhibit nematoblastic texture. The amphibolite layers in the cumulate gabbros are crosscut by the plagiogranite dikes. The plagiogranites consist mainly of quartz, plagioclase, biotite and opaque minerals and they show granular texture. Undulose extinction and sub-grain formation in quartz minerals indicate to the presence of deformation phase affecting the plagiogranite dikes. LA-ICP-MS dating on zircon from plagiogranite dikes which is cross-cutting of the amphibolite layers, yielded Middle Jurassic ages. 40Ar/39Ar dating of

  5. Particle velocity experiments in anorthosite and gabbro

    NASA Technical Reports Server (NTRS)

    Boslough, M. B.; Ahrens, T. J.

    1984-01-01

    Shock wave experiments were conducted in San Gabriel anorthosite and San Marcos gabbra 10 11 GPa using a 40 mm-borne propellant gun. Particle velocities were measured directly at several points in each target by means of electromagnetic gauges. Hugoniot states were calculated by determining shock-transit time from the gauge records. Sound speeds indicate a loss of shear strength upon sock compression for both rocks, with the strength loss persisting upon release to zero stress om the anorthosite. Stress-density release paths in the anorthosite indicate possible transformation of albite to jadeite + (quartz or coesite), with the amount of material transformed increasing as a function of shock stress. Electrical interferene effects in the gabbro precluded the determination of accurate release paths for the rock.

  6. Particle velocity experiments in anorthosite and gabbro

    NASA Astrophysics Data System (ADS)

    Boslough, M. B.; Ahrens, T. J.

    Shock wave experiments were conducted in San Gabriel anorthosite and San Marcos gabbra 10 11 GPa using a 40 mm-borne propellant gun. Particle velocities were measured directly at several points in each target by means of electromagnetic gauges. Hugoniot states were calculated by determining shock-transit time from the gauge records. Sound speeds indicate a loss of shear strength upon sock compression for both rocks, with the strength loss persisting upon release to zero stress om the anorthosite. Stress-density release paths in the anorthosite indicate possible transformation of albite to jadeite + (quartz or coesite), with the amount of material transformed increasing as a function of shock stress. Electrical interferene effects in the gabbro precluded the determination of accurate release paths for the rock.

  7. Impact cratering and spall failure of gabbro

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.; Boslough, M. B.

    1984-01-01

    Both hypervelocity impact and dynamic spall experiments were carried out on a series of well-indurated samples of gabbro. The impact experiments carried out with 0.04 to 0.2 g, 5-6 km/sec projectiles produced deci-centimeter-sized craters and demonstrated crater efficiencies of 6/10 to the -9 g/erg, and order of magnitude greater than in metal and some two to three times that of previous experiments on less strong igneous rocks. Most of the crater volume (some 60 to 80 percent) is due to spall failure. Distribution of cumulative fragment number, as a function of mass of fragments with masses greater than 0.1 gram yield values of b = d(log10N sub f)dlog10(m) of -0.5 to -0.6, where N sub f is the cumulate number of fragments and m is the mass of fragments. These values are in agreement or slightly higher than those obtained for less strong rocks and indicate that a large fraction of the ejecta resides in a few large fragments.

  8. Impact cratering and spall failure at gabbro

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.; Boslough, M. B.

    1983-01-01

    Both hypervelocity impact and dynamic spall experiments were carried out on a series of well-indurated samples of gabbro. The impact experiments carried out with 0.04 to 0.2g, 5-6 km/sec projectiles produced deci-centimeter-sized craters and demonstrated crater efficiencies of 6/10 to the - 9 g/erg, and order of magnitude greater than in metal and some two to three times that of previous experiments on less strong igneous rocks. Most of the crater volume (some 60 to 80%) is due to spall failure. Distribution of cumulative fragment number, as a function of mass of fragments with masses greater than 0.1 gram yield values of b = d(log10N sub f)dlog10(m)of -0.5 to -0.6, where N sub f is the cumulate number of fragments and m is the mass of fragments. These values are in agreement or slightly higher than those obtained for less strong rocks and indicate that a large fraction of the ejectra resides in a few large fragments.

  9. The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix V.; Sablukov, Sergei M.; Sablukova, Ludmila I.; Zakharchenko, Olga D.

    2009-10-01

    In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area - Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos - this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the 'Transbrasiliano Lineament'. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic.

  10. Explosively Erupted Gabbro From Kilauea Volcano's Summit Magmatic System

    NASA Astrophysics Data System (ADS)

    Rose, T. R.; Fiske, R. S.; Swanson, D. A.

    2001-12-01

    Gabbro clasts up to 13cm in diameter were blasted to the surface of Kilauea volcano by a powerful pyroclastic eruption 1500-1000 years b.p. They occur in the Kulanaokuaiki 3 tephra and as sparsely distributed lag lithics over 70 square km of the volcano's south flank where the tephra has been eroded away. These holocrystalline and glass-bearing gabbros make up about 12 per cent of the 1100 lithic clasts studied. Some are enclosed in cored bombs, but most occur as monolithologic clasts. Lag-lithic isopleths from an array of 110 south-flank sites suggest that this eruption emanated from the volcano's summit. The gabbros are remarkably fresh and consist chiefly of plagioclase + clinopyroxene + opaque oxides with varying percentages of olivine and rare orthopyroxene. Plagioclase and clinopyroxene crystals range up to 5 mm in length in the coarsest examples. Interstitial glass comprises as much as 25 modal per cent of the rock. In some samples this glass is moderately vesicular, reflecting an abrupt pressure drop accompanying eruption. The interstital glass contains 2.0 to 0.5 wt. per cent MgO and some has locally abundant apatite needles. Major element compositional trends of interstitial glass and glass melt inclusions in olivine suggest that the gabbros formed by crystallization along multiple liquid lines of descent. Whole-rock XRF compositions of 11 samples suggest that the gabbros are of two distinct types. Nine have 11 to 15 wt. per cent MgO and are samples of magma bodies that were crystallizing or had crystallized at depth. Two gabbros contain 6 wt. per cent MgO; these probably originated as segregation veins in as yet unknown parent bodies. All of the gabbros texturally resemble segregation veins from Kilauea Iki and other Kilauean lava lake deposits, but only the two low-MgO samples likely had such an origin. Most of the gabbros are fragments torn from partially to entirely crystallized bodies adjacent to the Kulanaokuaiki 3 parent magma body. We infer that

  11. On the origin of rhythmic layering in layered gabbros

    NASA Astrophysics Data System (ADS)

    Wang, Z.; Zhang, J.

    2015-12-01

    Rhythmic layering of silicates (plagioclase, pyroxene and olivine), ilmenite and magnitite is a common feature in mafic-ultramafic intrusions. The origin of rhythmic layering has been hotly debated in the literatures. Proposed mechanisms include gravity differentiation, double-diffusive convection, oscillatory crystallization of magma, repeated injection and supplement of magma, etc. Here we provide detailed FTIR and EBSD studies on the water content and deformation microstructure of gabbros from the Panzhihua intrusion and experimentally deformed synthetic gabrros and magnetite aggregates with a volume ratio of 6:4. The FTIR analyses revealed a significant amount of hydroxyls in both clinopyroxene (411-775 ppm) and plagioclase (328-716 ppm), suggesting a high water content mantle plume source. The EBSD analyses show similar fabrics in constitutent minerals of natural and experimental specimens: a weak clinopyroxene fabric of (100) parallel to foliation and [001] parallel to lineation; a strong plagioclase fabric of (010) parallel to foliation and [100] parallel to lineation, a weak ilmenite fabric of (001) parallel to foliation and [hk0] parallel to lieantion; and a near random magnitite fabric. There is an obvious rhythmic layering in sheared gabrros and magnetite aggregates similar to natural observations. Our results revealed strong layer-parallel shearing deformation during the formation of the Panxi layered intructions. There is a significant strength contrast between gabbro and Fe-Ti oxides. We propose that the formation of the rhythmic layering in mafic-ultramafic intrusions is caused mainly by rheological stratification of Fe-Ti oxides and gabbros.

  12. Fluid infiltration of the Tudor Gabbro during regional metamorphism

    SciTech Connect

    Dunn, S.R.; Valley, J.W.

    1985-01-01

    The Tudor Gabbro (TG), an ovate body (4 x 9 km) 40 km SE of Bancroft. Ontario, was metamorphosed to upper greenschist facies along with surrounding sediments and volcanics. Allen (1976) delineated concentric isograds around the gabbro, including +sphene, +tremolite (to 1.5 km), +tremolite + clinozoisite, +diopside (approx. 120 m), and +garnet (approx. 80 m). Metamorphic conditions are inferred to be 490+/-50/sup 0/C, 5 kb with no thermal gradient. Allen suggested that H/sub 2/O infiltration of the marble and calc-schist accounts for the isograds. The gabbro mineralogy of titanaugite, andesine to labradorite, and minor hornblende is extensively recrystallized to albite and/or oligoclase + actinolite + epidote + ilmenite + calcite (up to 4 wt%) +/- biotite +/- chlorite +/- sphene +/- scapolite. Isotopic analyses of calcite from 39 TG samples show delta/sup 18/O = 9.4 to 16.6 and delta/sup 13/C = -1.9 to 3.4. Bulk silicate delta/sup 18/O of TG range from 7.1 to 10.2. Calcites in metasediment have delta/sup 18/O = 18.1 to 25.3 and delta/sup 13/C = 1.3 to 5.6. Two whole rock silicate analyses of a skarn developed locally at the contact show intermediate delta/sup 18/O of 16.2 and 17.3. The stability of Czo component in epidote requires H/sub 2/O-rich fluids. The delta/sup 13/C of TG calcites average +0.7 nearly identical to the average of 178 carbonates from Grenville marbles (+1.0), showing that metasediment-derived CO/sub 2/ pervasively infiltrated the TG. The infiltration of H/sub 2/O into both the TG and the metasediment suggests that H/sub 2/O-rich fluids migrated upward along the contact.

  13. The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke

    NASA Astrophysics Data System (ADS)

    Shaw, Cliff S. J.

    1997-07-01

    The Coldwell alkaline complex is a large (> 350 km 2) gabbro and syenite intrusion on the north shore of Lake Superior. It was emplaced at 1108 Ma during early magmatic activity associated with the formation of the Mid-Continent Rift of North America. The eastern gabbro forms a partial ring dyke on the outer margin of the complex and consists of at least three discrete intrusions. The largest of these is the layered gabbro that comprises a 300 m thick fine- to medium-grained basal unit overlain by up to 1100 m of variably massive to layered gabbroic cumulates which vary from olivine gabbro to anorthosite. Several xenoliths of Archaean metamorphic rocks that range in size from 10's to 100's of meters are present in the central part of the intrusion. Within discrete horizons in the layered gabbro are many centimeter- to meter-scale, gabbroic xenoliths. The main cumulus minerals, in order of crystallization, are plagioclase, olivine and clinopyroxene ± Fe-Ti oxides. Biotite and Fe-Ti-oxide are the dominant intercumulus phases. Orthopyroxene occurs not as a cumulus phase but as peritectic overgrowths on cumulus olivine. A detailed petrographic and mineral chemical study of samples from two stratigraphically controlled traverses through the layered gabbro indicates that the stratigraphy cannot be correlated along the 33 km strike of the ring dyke. Mineral compositions show both normal and reversed fractionation trends. These patterns are interpreted to record at least three separate intrusions of magma into restricted dilatant zones within the ring dyke possibly associated with ongoing caldera collapse. Calculations of parental melt composition using mineral — melt equilibria show that even the most primitive gabbros crystallized from an evolved magma with mg# of 0.42-0.49. The presence of orthopyroxene overgrowths on cumulus olivine suggests rising silica activity in the melt during crystallization and implies a subalkaline parentage for the layered gabbro.

  14. Microstructures and deformation mechanisms of experimentally deformed gabbro

    NASA Astrophysics Data System (ADS)

    Zhou, Yongsheng; He, Changrong

    2015-04-01

    The natural gabbro samples were deformed at temperature ranging from 700 to 1150 °C with strain rate steps of 1 × 10-4, 2.5 × 10-5, 6.3 × 10-6 s-1. The mechanical data show that sample experiences gradual transition from semi-brittle flow to plastic flow, corresponding to a systematically decreasing stress exponent n with the increasing temperature ranging from 16.5 to 4.1 (He et al. Sci China (D) 46(7):730-742, 2003). We investigate microstructures and deformation mechanisms of experimentally deformed gabbro under transmission electron microscope in this study. For low temperature of 700 °C to 950 °C, the deformation is mainly accommodated with dislocation glide and mechanical twinning, corresponding to stress exponent lager than 5, which means semi-brittle deformation. Whereas with higher temperature up to 1000 °C-1150 °C, the deformation is accommodated mainly with dislocation glide and climb corresponding to stress exponent of 4.1, which means plastic deformation. Evidence of dislocation climb has been found as dislocation walls in plagioclase. The observed slip system in plagioclase is (001)1/2[110] and that in clinopyroxene are (100)[001] and (010)[001]. The (010)[001] slip system in clinopyroxene is newly found in this work. Melt was found at temperature of 950 °C-1050 °C. The melt glass distributed both in melt thin film between two grain boundaries and melt tubules of triangular along three grain boundaries at temperature of 950 °C-1000 °C. The melt triangular interconnected to the melt film at temperature of 1050 °C-1150 °C, where the melt chemical composition differentiated into iron-rich dark dots and silicate-rich matrix.

  15. Contribution of oceanic gabbros to sea-floor spreading magnetic anomalies.

    PubMed

    Kikawa, E; Ozawa, K

    1992-10-30

    The contribution of oceanic gabbros, representative rocks for layer 3 of the oceanic crust, to sea-floor spreading magnetic anomalies has been controversial because of the large variation in magnetic properties. Ocean Drilling Program (ODP) Leg 118 contains a continuous 500.7-meter section of oceanic gabbro that allows the relations between magnetization and petrologic characteristics, such as the degree of metamorphism and the magmatic evolution, to be clarified. The data suggest that oceanic gabbros, together with the effects of metamorphism and of magmatic evolution, account for a significant part of the marine magnetic anomalies. PMID:17777035

  16. Petrology and mineralogy of dredged gabbro from Gettysburg Bank, eastern Atlantic

    NASA Astrophysics Data System (ADS)

    Prichard, H. M.; Cann, J. R.

    1982-04-01

    Gabbros have been dredged from Gettysburg Bank, 110 km west of Portugal on the Azores/Gibraltar fracture zone. Primary minerals in olivine, pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals. Igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation. Amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions. Of the amphiboles analysed, chlorine was present in the green amphiboles but below 0.05% in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green.

  17. Physical, chemical, and thermal interactions in the Pleasant Bay Layered Gabbro-Diorite Intrusion, Maine

    NASA Astrophysics Data System (ADS)

    Patwardhan, K.; Algeo, J.

    2012-12-01

    The approximately 3 km thick Pleasant Bay Layered Intrusion (PBLI) is interpreted to have formed (420 Ma) by repeated intrusions of gabbroic magma into a partly solidified dioritic magma chamber (Wiebe, 1993; Waight et. al., 2001) during the earliest stage of the Acadian orogeny (Tucker et. al., 2001). Typical field relationships in the PBLI include gradational gabbro-diorite layers of variable thicknesses with pipes of silicic composition intruding along the chilled lower contacts of gabbroic layers, chilled gabbroic blobs encased within diorite, and composite dikes consisting of intermingled gabbro blobs of rounded and/or angular geometries within a dioritic or granitic matrix. Detailed studies of similar relationships in the nearby Isle au Haut Igneous Complex (IHIC) have indicated that where diorite underlies gabbro, residual silicic melt was extracted from the underlying partly solidified diorite by compaction and migrated upwards to form a thin layer of buoyant melt that underwent a Rayleigh-Taylor (R-T) type instability producing the silicic pipes which were subsequently arrested by solidification of the overlying gabbro (Chapman & Rhodes, 1992; Patwardhan & Marsh, 2011). Whereas in the IHIC pipes typically are 11-12 cm in diameter and spaced ~30 cm apart, instability features in the PBLI occur at varying scales with pipe diameters commonly ranging from 4 to 100 cm and pipe spacing varying between 20 to 150 cm. Strong compositional differences between the gabbro (~48 wt.% SiO2), diorite (~57 wt.% SiO2), and pipes (~66 wt.% SiO2) correspond to significant differences in estimated density, liquid viscosity, and temperature (near liquidus gabbro: 2.68 g/cm3, 102.3 poise, 1210 oC, partly crystallized diorite: 2.61 g/cm3, 990 oC, and silicic melt derived from partly crystallized diorite: 2.36 g/cm3, 105 poise). The thickness of the buoyant silicic layer and the ascent rate of the pipes emanating from this layer are calculated using estimated viscosity ratios and

  18. Basalts and gabbros from Mare Crisium - Evidence for extreme fractional crystallization

    NASA Technical Reports Server (NTRS)

    Lu, F.; Taylor, L. A.; Jin, Y.

    1989-01-01

    Petrographic and electron microprobe techniques were used to examine igneous fragments in Luna 24 samples 24088 and 24105. It is the complex chemistry of the pyroxenes that distinguishes the different rock types. Basaltic pyroxenes exhibit an Fe-enrichment trend; the evolutionary trends are more complex in the gabbros, with enrichments in both Fe and Ti and a depletion in Cr. These chemical evolutionary trends are displayed by a progressive variation in rock types from Mg-rich olivine-gabbro to olivine-gabbro, and to ferrogabbro and ferrotroctolite. The low TiO2 content of the primary melt, possibly represented by the least-evolved Mg-rich olivine-gabbro, retarded the formation of early ilmenite and spinel, such that 'Fenner Trend' Fe enrichment occurred. The ferrotroctolite is probably the end product of chemical evolution by extreme fractional crystallization, controlled primarily by olivine and pyroxene crystallization.

  19. The gabbro (shoshonitic)-monzonite-granodiorite association of Khankandi pluton, Alborz Mountains, NW Iran

    NASA Astrophysics Data System (ADS)

    Aghazadeh, Mehraj; Castro, Antonio; Omran, Nematallah Rashidnejad; Emami, Mohamad Hashem; Moinvaziri, Hossien; Badrzadeh, Zahra

    2010-05-01

    The Khankandi pluton forms part of a group of gabbro-granodiorite intrusions in the Alborz Mountains of NW Iran. A petrographical and geochemical study of this plutonic association reveals the existence of several magmatic cycles with different origins and slight differences in age. The oldest cycle (C1) is represented by granodiorites. A second cycle (C2) is formed by a gabbro-monzonite association, with a clear shoshonitic affinity, that dominates most of the intrusive volume. Gabbros and monzonites form a co-magmatic association. Zircons from the monzonites were analyzed by LA-ICP-MS for U-Pb dating. An average age of 28.9 Ma, ranging from 23.7 to 33.6 Ma was obtained. Gabbros, monzonites and granodiorites share a nearly common isotopic ratio for Sr and Nd. Both initial Sr and Nd ratios are clustered within a narrow range from 0.7045 to 0.7047 for the 87Sr/ 86Sr ratio and ɛNd from 1.46 to 1.89. Comparison with experimental studies, together with mantle-like isotopic ratios and comparisons of REE patterns, points to an origin by variable melting rates from a common metasomatised mantle source for gabbros and monzonites. Melting of a subducted mélange is suggested for the granodiorite magmas predating the gabbro-monzonite intrusion. The two sources, a metasomatised mantle and ascending silicic plumes, are direct consequence of subduction.

  20. The permeability of gabbro in oceanic core complexes

    NASA Astrophysics Data System (ADS)

    Titarenko, S.; McCaig, A. M.

    2013-12-01

    ridges in the Pacific, and volcanic plateaux in the Atlantic, suggest that the topmost basalts are extremely permeable even in crust up to 60 Ma, with estimates ranging from 10-13 to 10-9 m2. This has profound effects on the thermal structure of the crust, particularly where bare seamounts allow access of seawater to this shallow aquifer. In the Atlantic, up to 50% of the cuts has formed in the detachment mode of seafloor spreading, and lacks a continuous basaltic layer. The most prominent bare seamounts are often oceanic core complexes exposing gabbro and serpentinite. It follows that the hydrological and thermal regime in the Atlantic is likely to be inhomogeneous and unpredictable. Additionally, our data show that even in a tectonically active massif <1.2 M.y. in age, the large scale permeability of gabbro at low temperature is much less than that normally used in black smoker modelling (10-14 to 10-12 m2). Sheeted sill models of crustal construction at fast spreading ridges require removal of heat by deep circulation of seawater close to the ridge crest. Any permeability allowing such deep circulation is likely to be extremely transient in nature.

  1. Cohesive Zone Length of Gabbro at Supershear Rupture Velocity

    NASA Astrophysics Data System (ADS)

    Fukuyama, E.; Xu, S.; Mizoguchi, K.; Yamashita, F.

    2014-12-01

    We investigated the shear strain field ahead of a supershear rupture. The strain data was obtained during large-scale biaxial friction experiments conducted at NIED in March 2013. We conducted friction experiments using a pair of meter-scale gabbro rock specimens whose simulated fault area was 1.5m x 0.1m. We applied 2.6MPa normal stress and loading velocity of 0.1mm/s. At the long side of the fault edge, which is parallel to the slip direction, 32 2-component semi-conductor strain gauges were installed at an interval of 50mm and 10mm off the fault. The data are conditioned by high frequency strain amplifiers (<0.5MHz) and continuously recorded at an interval of 1MHz with 16-bit resolution. Many stick slip events were observed and a unilateral rupture event was chosen in this analysis that propagated with supershear rupture velocity. One of the reasons for this selection was that the strain field ahead of the supershear rupture was not contaminated by elastic waves. Focusing on the rupture front, stress concentration was observed and sharp stress drop occurred immediately inside the rupture. We found that the stress concentration becomes mild as the rupture propagates and length of the stress concentration area becomes longer. This observation is quite interesting because in this experiment the rupture propagated at a constant speed close to root two times the shear wave velocity and thus a longer stress concentration region suggests more energy dissipation. We could speculate that such longer stress concentration area suggests longer plastic region ahead of the rupture (or longer cohesive distance). I.e. the cohesive zone length becomes longer as the rupture propagates to maintain constant rupture velocity propagation. We empirically obtained the relation Lc = 1.8x10^-5 L for 0.1

  2. A pristine eucrite-like gabbro from Descartes and its exotic kindred

    NASA Technical Reports Server (NTRS)

    Marvin, U. B.; Warren, P. H.

    1980-01-01

    A coarse-grained plagioclase-pyroxene gabbro (61224,6) with a cumulate texture suggestive of a slowly cooled plutonic rock was recovered from the 4-10 mm fraction of an Apollo 16 soil. The rock is uncommonly poor in feldspar and rich in Na for a lunar highlands lithology. Trace element analyses show extremely low siderophile element concentrations which confirm the pristine character indicated by the texture. The composition of 61224,6 is compared with those of 3 other pristine, exceptionally mafic, nonmare gabbros and of certain eucrites. 61224,6 and the three other gabbros have notable chemical differences but share relatively high ratios of Ti/Sm and Sc/Sm which suggest a possible genetic relationship. We conclude that 61224,6 represents a Na-rich cumulate from a layered intrusion within the highlands crust.

  3. Structural-temporal approach for dynamic strength characterization of gabbro-diabase

    NASA Astrophysics Data System (ADS)

    Petrov, Y.; Bragov, A.; Evstifeev, A.; Cadoni, E.

    2015-09-01

    This paper presents some of the results of dynamic compression, splitting and direct-tensile tests of gabbro-diabase using two split Hopkinson bar apparatus. The results of laboratory experiments on specimens without and with notch for a wide range of loading parameters are given. Strength and deformation characteristics of gabbro-diabase were determined experimentally at high strain rates up to 103 s-1. Mechanical characteristics were found to be strongly depended on the strain rate. A uniform interpretation of the rate effects of fracture of the tested gabbro-diabase is given on the basis of structural-temporal approach based on a set of fixed material constants. It is shown that temporal dependences of both the compressive and tensile strengths of the tested material can be effectively calculated using the incubation time fracture criterion.

  4. Stream water chemistry in a gabbro/granite watershed, Quabbin reservation, central Massachusetts

    SciTech Connect

    Reid, J.B. Jr.; Gallant, J.; Christensen, C.; Mengason, M. . School of Natural Science)

    1993-03-01

    While monitoring pH-alkalinity relationships in tributaries of the Quabbin Reservoir, the authors have discovered an anomalous brook whose waters become progressively more acidic downstream. The watershed's bedrock is roughly half Prescott hornblende gabbro and half Cooleyville granitic gneiss with the contact crossing the watershed diagonally; gabbroic bedrock dominates the stream's upper reaches. Outcrop density and topography suggest relatively thin till cover (< [approximately]2m). All parts of the stream get some contribution from both bedrock types, through gabbro contribution diminishes smoothly downstream. Springs in gabbro (pH [approximately]7, alk 20--30mg/1, cond [approximately]50[mu]mho) and in granite (pH [approximately]5, alk 2--6 mg/1, cond [approximately]15[mu]mho) retain these characteristics through dry and wet seasons; the stream's response to high rain events is more complex and can be used to estimate where surface water and groundwater each make their greatest contributions. Each point along the brook can be assigned a value of %gabbro characterizing the bedrock proportions in the watershed upstream of it; plots of major cations, alkalinity and conductivity vs. %gabbro show strong positive correlation. Two-week leaching experiments (initial pH = 4) with A, B and C soil horizons from both sides of the contact show greatest rises (to pH = 5.5) in gabbro soils distant from the contact, and progressively smaller increases crossing the contact to granite soils (pH rises to 4.5). The data suggest that bedrock and soil chemistry are primarily responsible for stream chemistry; topography and residence time here play secondary roles.

  5. Shock wave properties of anorthosite and gabbro. [to model hypervelocity impact cratering on planetary surfaces

    NASA Technical Reports Server (NTRS)

    Boslough, M. B.; Ahrens, T. J.

    1985-01-01

    Huyoniot data on San Gabriel anorthosite and San Marcos gabbro to 11 GPA are presented. Release paths in the stress-density plane and sound velocities are reported as determined from partial velocity data. Electrical interference effects precluded the determination of accurate release paths for the gabbro. Because of the loss of shear strength in the shocked state, the plastic behavior exhibited by anorthosite indicates that calculations of energy partitioning due to impact onto planetary surfaces based on elastic-plastic models may underestimate the amount of internal energy deposited in the impacted surface material.

  6. Characteristic features of ore gabbro formation in the third layer of the oceanic crust

    NASA Astrophysics Data System (ADS)

    Eskin, A. E.

    2009-07-01

    Peculiarities of ore gabbro formation in slow-spreading mid-ocean ridges exemplified by the non-transform Sierra Leone fault and Marathon fault areas are considered. The formation of ore gabbros is most often connected with the rheologically weakened oceanic lithosphere mainly with active fault zones, in which basic magmatic melts intruded. Those faults provide ways for migration of differentiated melts at different depth levels. When intruding and moving along fault zones, melts interacting with the mantle and crust host rocks are often already hydrated. Such interaction occurs largely in conditions of subsolidus deformations resulting in enrichment of melts with volatile components.

  7. Comparison of Lunar Basalts and Gabbros with those of the Terrestrial Ocean Crust

    NASA Astrophysics Data System (ADS)

    Natland, J. H.

    2012-12-01

    Initial studies of lunar samples returned from the Apollo and Luna missions took place before rocks of the Earth's lower ocean crust, chiefly varieties of gabbro cumulates, were widely known or understood. Continuing exploration of the ocean crust invites some new comparisons. When volcanic rocks and glass from Apollo 11 and 17 were discovered to have very high TiO2 contents (8-14%), nothing comparable was known from Earth. The high-TiO2 lunar samples were soon described as primary melts derived from considerable depths in the lunar mantle. Other lunar samples have only very low TiO2 contents (~0.2%) and very low concentrations of highly incompatible elements such as Zr and Sr. Today, dredging and drilling results indicate that oxide gabbros rich in magmatic oxides and sulfides and with up to 12% TiO2 comprise a significant percentage of the gabbroic portion of the ocean crust especially at slowly spreading ridges. These are very late stage differentiates, and are commonly juxtaposed by high-temperature deformation processes with more primitive olivine gabbros and troctolites having only ~0.2% TiO2 and low concentrations of Zr and other incompatible elements. The rocks are mainly adcumulates, with very low concentrations of incompatible elements set by proportions of cumulus minerals, and with little contribution from the liquids that produced them. In addition, some lunar gabbros with highly calcic plagioclase (~An93-98) are similar to gabbros and troctolites found in island arcs. All of these similarities suggest that very few lunar basaltic rocks are pristine; instead they all could be nearly complete shock fusion products produced by meteorite impact into a diverse assemblage of lunar gabbros that included both low- and high-TiO2 gabbroic facies. On this hypothesis, no lunar basalt is a primary melt derived from the Moon's mantle. Although magmatic environments on the ancient Moon and in the modern ocean crust were different in important ways, the general

  8. Paleoproterozoic gabbro-diorite-granite magmatism of the Batomga Rise (NE Aldan Shield): Sm-Nd isotope geochemical evidence

    NASA Astrophysics Data System (ADS)

    Kuzmin, V. K.; Bogomolov, E. S.; Glebovitskii, V. A.

    2016-02-01

    The geochemical similarity and almost simultaneous (2055-2060 Ma) formation of Utakachan gabbro-amphibolite, Jagdakin granodiorite-diorite, Khoyunda granitoid, and Tygymyt leucogranite complexes, which inruded metamorphic formations of the Batomga Group are evidence of their formaton from unified magmatic source. All this makes it possibble to combine aforementioned complexes into the unified Early Proterozoic diferentiated gabbro-diorite-granite complex.

  9. Lherzolite, anorthosite, gabbro, and basalt dredged from the mid-Indian ocean ridge.

    PubMed

    Engel, C G; Fisher, R L

    1969-11-28

    The Central Indian Ridge is mantled with flows of low-potassium basalt of uniform composition. Gabbro, anorthosite, and garnet-bearing lherzolite are exposed in cross fractures, and lherzolite is the bedrock at the center of the ridge. The Iherzolites are upper-mantle rock exposed by faulting. PMID:17775570

  10. Lherzolite, anorthosite, gabbro, and basalt dredged from the Mid-Indian Ocean Ridge

    USGS Publications Warehouse

    Engel, C.G.; Fisher, R.L.

    1969-01-01

    The Central Indian Ridge is mantled with flows of low-potassium basalt of uniform composition. Gabbro, anorthosite, and garnet-bearing lherzolite are exposed in cross fractures, and lherzolite is the bedrock at the center of the ridge. The lherzolites are upper-mantle rock exposed by faulting.

  11. Iodp Exp 345: Orthopyroxene Omnipresent in Layered Gabbros from the Hess Deep, EPR

    NASA Astrophysics Data System (ADS)

    Koepke, J.; Abe, N.; Akizawa, N.; Falloon, T.; Hoshide, T.; Jean, M. M.; Maeda, J.; Snow, J. E.

    2013-12-01

    Drilling studies of the lower oceanic crust in the framework of the IODP (Integrated Ocean Drilling Program) have the objective of understanding the construction processes of the ocean crust. Currently, much attention is focused on the relatively inaccessible fast spreading lower ocean crust (IODP Hole 1256D, ODP Hole 894G, this study). At the Hess Deep Rift, propagation of the Cocos Nazca Ridge into young, fast-spreading East Pacific Rise crust generates extensive exposures of the plutonic lower crust. IODP Exp. 345 drilled a dismembered section of lower crust at ~4850 m water depth under challenging borehole conditions. We drilled three main holes between 35-110 mbsf with ~30% recovery, returning cores relevant to the igneous evolution of the lower crust. The drilled cores show spectacular modal and/or grain size layering present in >50% of the recovered core. Typical rocks are primitive (Mg# 75-89) olivine gabbro, troctolitic gabbro and olivine gabbronorite. A very significant first-order observation from this expedition is that orthopyroxene was found as an abundant phase in many of the layered primitive gabbroic rocks. This was unexpected, since experiments on the liquid line of descent of MORB show that orthopyroxene always crystallizes late, at low melt fractions, and commonly interstitially. Recovered rocks at Site U1415, however, show that orthopyroxene crystallization is common: both as an interstitial phase in primitive troctolite (crystallization even before clinopyroxene), as a cumulus phase coexisting with clinopyroxene in primitive olivine gabbros and as monomineralic bands in primitive gabbros of the Multitextured Layered Gabbro series in Hole U1415P. None of these features are commonly observed in ocean floor gabbros. The discovery of orthopyroxene in primitive lithologies in high quantities in the lower crustal section at Hess Deep is a novel finding, although the presence of orthopyroxene in one primitive gabbro sample from Hess Deep was reported

  12. Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

    USGS Publications Warehouse

    Harlan, S.S.; Geissman, J.W.

    1998-01-01

    The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1??, I = -41.9?? (k = 94, ??95 = 3.3??, N = 21 sites) and a paleomagnetic pole at 21.1?? S, 221.1 ??E, (K= 89, A95 = 3.4??). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47-1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5-1.4 Ga poles of the North American apparent polar wander path.

  13. Shape-preferred orientation (SPO) of oceanic gabbros at IODP Hole 1256D: Implications for magmatic processes

    NASA Astrophysics Data System (ADS)

    Trela, J.; Ferré, E. C.; Deans, J.; Anma, R.; Morris, A.; Expedition 335 Scientific Party

    2012-04-01

    Magmatic fabrics in oceanic gabbros close to the sheeted dike-gabbro transition at superfast spreading centers (>200 mm/y) remain poorly investigated. At ~1407 mbsf at IODP Hole 1256D, Expedition 312 recovered non-azimuthally oriented core samples from two gabbro bodies of undetermined shapes: Gabbro 1; 52 m-thick and Gabbro 2; 24 m-thick. Oceanic gabbros in the upper part of the plutonic complex are expected to be macroscopically isotropic, therefore investigating the existence and strength of a fabric requires a highly sensitive method. The Intercept method (Launeau and Robin, 1996), based on quantification of shape-preferred orientation (SPO) in 3-D, has a remarkably high sensitivity for shape anisotropy (0.3%). This method has been used on 33 oceanic gabbro samples from Hole 1256D as a function of depth for each mineral phase. Image analysis using the Intercept method on gabbroic fabrics provides new constraints on crustal accretion mechanisms, the timing of deformation relative to crystallization, i.e., pre-full crystallization fabric vs crystal plastic strain fabric. Observed fabrics provide constrains to test the two-end member ocean accretion models: (A) the "Gabbro Glacier" model; (B) the "Sheeted Sills" model or a hybrid model between (A) and (B). The following lines of evidence are distinct and allow testing of the two models respectively: Model A- The "Gabbro Glacier" model - Microstructures including fractured grains and mechanical twins in plagioclase grains should indicate increasing amounts of strain. SPO is expected to increase as a function of depth. A transitional zone from steeply foliated cumulate gabbros to horizontally layered gabbros is anticipated to occur below the sheeted dike complex. Model B- The "Sheeted Sills" model - No specific correlation between SPO and depth is expected unless geochemical investigations indicate Gabbros 1 and 2 were convecting. Fine-grained margins are expected to occur along the tops and bottoms of the sills

  14. Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite

    USGS Publications Warehouse

    Komor, S.C.; Elthon, D.

    1990-01-01

    Rhythmically layered anorthosite and gabbro are exposed in a 4-10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20-30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300-301 gabbros, it is estimated to have been ~40??C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. -from Authors

  15. Effect of Pressure and Stress on Water Transport in Intact and Fractured Gabbro and Granite

    SciTech Connect

    Trimmer, D.; Bonner, B.; Heard, H.C.; Duba, A.

    1980-12-10

    New laboratory data are reported on the effect of confining pressure (to 60 MPa), pore-water pressure (to 30 MPa), and stress difference (to 0.88 of the fracture stress) on permeability of intact and fractured White Lake gneissic granite. Westerly granite, and Creighton gabbro. Permeabilities as low as 10/sup -24/ m/sup 2/ (10/sup -2/ darcy) have been measured using a transient technique. Fracture displacement, electrical conductance, compressional velocity, and pulse amplitude are determined simultaneously. The loads applied to the 0.15-m-diameter by 0.28-m-length test sample are controlled automatically, and most data are taken by microprocessor. Tests on the intact gneissic granite indicated permeabilities of 10/sup -22/ to 10/sup -24/ m/sup 2/ that appeared to be unaffected either by effective pressure or by stress. The granite yielded permeabilities of 4 +- 10/sup -20/ m/sup 2/ that decreased by a factor of 2 as effective pressure increased to 25 MPa and varied by a factor of 2 as stress was increased to 0.5 of the fracture stress. Permeability of the gabbro linearly decreased from 2 x 10/sup -22/ to 8 x 10/sup -24/ m/sup 2/ with effective pressures to 25 MPa. Loading of the gabbro up to 0.88 of the fracture stress increased permeability by a factor of 7. The introduction of a throughgoing fracture increased the apparent permeability by 10/sup 6/ to 10/sup 9/ over the intact values in both granite and gabbro. When compared to the initial value, compressional velocities increased by 5% with pressure to 30 MPa in the gneissic granite. For granite, pressurization from 2 to 25 MPa increased the velocity and pulse amplitude by 5 and 30%, and decreased the conductance by 50%. Velocity, amplitude, and conductance were weakly dependent on pressure in gabbro. The addition of stress decreased velocity and amplitude while increasing conductance markedly on both granite and gabbro.

  16. Percussive tool use by Taï Western chimpanzees and Fazenda Boa Vista bearded capuchin monkeys: a comparison.

    PubMed

    Visalberghi, Elisabetta; Sirianni, Giulia; Fragaszy, Dorothy; Boesch, Christophe

    2015-11-19

    Percussive tool use holds special interest for scientists concerned with human origins. We summarize the findings from two field sites, Taï and Fazenda Boa Vista, where percussive tool use by chimpanzees and bearded capuchins, respectively, has been extensively investigated. We describe the ecological settings in which nut-cracking occurs and focus on four aspects of nut-cracking that have important cognitive implications, namely selection of tools, tool transport, tool modification and modulation of actions to reach the goal of cracking the nut. We comment on similarities and differences in behaviour and consider whether the observed differences reflect ecological, morphological, social and/or cognitive factors. Both species are sensitive to physical properties of tools, adjust their selection of hammers conditionally to the resistance of the nuts and to transport distance, and modulate the energy of their strikes under some conditions. However, chimpanzees transport hammers more frequently and for longer distances, take into account a higher number of combinations of variables and occasionally intentionally modify tools. A parsimonious interpretation of our findings is that morphological, ecological and social factors account for the observed differences. Confirmation of plausible cognitive differences in nut-cracking requires data not yet available. PMID:26483529

  17. Characteristics and petrogenesis of Alaskan-type ultramafic-gabbro intrusions, southeastern Alaska

    SciTech Connect

    Loney, R.A. ); Himmelberg, G.R. Univ. of Missouri, Columbia, MO )

    1993-04-01

    Alaskan-type ultramafic-gabbro intrusions occur along a belt that extends from Duke Island to Klukwan in southeastern Alaska and fall into two age groups, 400 to 440 Ma and 100 to 110 Ma. Most of the smaller bodies are magnetite-bearing hornblende clinopyroxenite; the larger ones consist of dunite, wehrlite, olivine clinopyroxenite, with some gabbro, in addition to hornblende clinopyroxenite and hornblendite. Textural, mineralogical, and chemical characteristics of the Alaskan-type ultramafic bodies indicate that they originated by fractional crystallization of a basaltic magma and accumulation in a crustal magma chamber. The Al[sub 2]O[sub 3] content of clinopyroxene shows a marked enrichment with differentiation, suggesting crystallization from progressively more hydrous melts like those characteristics of arc magmas. REE abundance levels and patterns are markedly similar for given rock units in all the bodies studied suggesting that all the bodies were derived by differentiation of closely similar parent magmas under near identical conditions. The exact composition of the primary melt is uncertain but the authors' preferred interpretation is that the parental magma of most Alaskan-type bodies was a subalkaline hydrous basalt. The striking similarity between the REE abundance levels and patterns of the Alaskan-type clinopyroxenites and gabbros, and the clinopyroxenite xenoliths and plutonic gabbros associated with Aleutian Island Arc volcanism, further suggests that the primary magma was probably a hydrous olivine basalt similar to the primary magma proposed for the Aleutian arc lavas. The mineral chemistry and phase equilibria of the ultramafic bodies suggest that they crystallized in magma chambers at depths greater than about 9 km. Except for the Duke Island body, which has sedimentary structures and shows evidence of ubiquitous current activity, most of the other bodies appear to have accumulated under static conditions.

  18. Re-Os geochemistry and geochronology of the Ransko gabbro-peridotite massif, Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Pašava, Jan; Erban, Vojtěch

    2013-10-01

    The Ransko gabbro-peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex, which hosts low-grade Ni-Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites, and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase-rich peridotites. Gabbro, troctolite, peridotite and Ni-Cu ores from the Jezírka Ni-Cu (PGE) deposit, considered to be a typical example of the liquid segregation style of mineralization, were analyzed for Re-Os concentrations and isotopic ratios. Seven barren and mineralized samples from the Jezírka deposit yielded a Re-Os regression of 341.5 ± 7.9 Ma (MSWD = 69). Strongly mineralized peridotite with mantle-like initial 187Os/188Os ratio of 0.125 suggests that Os as well as other PGE present in the Ni-Cu mineralization are predominantly of mantle origin. On the other hand, barren and low-mineralized samples have radiogenic initial 187Os/188Os ratios of 0.14-0.16 suggesting some import of Re and/or radiogenic 187Os most likely through contamination by continental crust during magma emplacement. The Re-Os age of the Ransko Massif is significantly younger than the previously suggested Lower Cambrian age, but it is similar to and/or younger than the age of metamorphism of the adjacent Kutná Hora crystalline complex and the Moldanubian unit. Therefore, it is likely that the emplacement of the Ransko massif and its Ni-Cu mineralization was closely connected with the late-stage evolution of the Kutná Hora crystalline complex.

  19. The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

    NASA Technical Reports Server (NTRS)

    Namiki, Noriyuki; Solomon, Sean C.

    1992-01-01

    Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

  20. The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

    NASA Astrophysics Data System (ADS)

    Namiki, Noriyuki; Solomon, Sean C.

    1992-12-01

    Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

  1. The Santiago Peak Volcanic rocks of the Peninsular Ranges batholith, Southern California: volcanic rocks associated with coeval gabbros

    NASA Astrophysics Data System (ADS)

    Tanaka, H.; Smith, T. E.; Huang, C. H.

    1984-03-01

    Analyses of the latest Jurassic Santiago Peak volcanic rocks from the western zone of the Peninsular Ranges batholith reveal the existence of two independent groups; one comprising basalts and andesites of the island arc tholeiitic series, and the second made up of the dacites and rhyolites of the subalkaline (calc-alkaline?) series or silicic series. The basalts and andesites have V, Co and Ni contents similar to those estimated for the residual melts in equilibrium with the Peninsular Ranges gabbros. This fact together with the tholeiitic nature of the gabbros and the intimate spatial and temporal relationship between the SPV and the gabbros suggests that the basalts and andesites had a common origin with the gabbros. The mafic volcanism and plutonism seems to have occurred in a youthful island arc and the silicic volcanism in a mature island arc or a continental margin.

  2. The origin of layered gabbros from the mid lower ocean crust, Hess Deep, East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Cheadle, M. J.; Brown, T. C.; Ceuleneer, G.; Meyer, R.

    2014-12-01

    IODP Exp. 345 Holes U1415 I & J cored a ~30m thick unit of conspicuously layered gabbroic rocks from the lower plutonic crust at Hess Deep. These rocks likely come from >1500m below the dike gabbro transition and thus provide an unique opportunity to study the origin of layering and the formation of relatively deep, fast spread plutonic crust formed at the EPR. Here we report the initial results of a comprehensive high-resolution petrologic, geochemical and petrographic study of this unit, which focuses on a fairly continuous 1.5m long section recovered at Hole I. The rocks consist of opx-bearing olivine gabbro, olivine gabbro and gabbro and exhibit 1-10cm scale modal layering. Some layers host spectacular 2-3 cm diameter cpx oikocrysts encapsulating partially resorbed plagioclase laths. Downhole variations in mineral chemistry are complicated. Olivine, cpx and opx Mg#'s partly reflect equilibration and show a subtle metre-scale variation (1-2 Mg#), whereas, for example, plagioclase anorthite, and cpx TiO2 contents reveal a more complicated 10-20 cm-scale variation (2-4 An, and 0.2 TiO2). Mineral zonation, for all but Mg# in equilibrated olivine, is of higher magnitude than downhole variations in average mineral compositions. Trace element geochemistry reveals rather homogeneous plagioclase and opx compositions; however cpx exhibits variation at the mineral scale. Cpx shows an increased range of, and highest REE concentrations, in the more olivine rich, near cotectic, composition gabbros, whereas the more plagioclase rich, cumulates show no variation of, and low REE, concentrations.Plagioclase fabrics are moderate to weak and partially modally controlled, but the strength of the plagioclase crystallographic preferred orientation (CPO) varies dramatically, within the 1.5m core showing a significant part of the variation recorded by Oman ophiolite plutonic crust. Plagioclase shape preferred orientation and CPO match well suggesting that diffusion enabled compaction

  3. Oxygen-isotope exchange and mineral alteration in gabbros of the Lower Layered Series, Kap Edvard Holm Complex, East Greenland

    SciTech Connect

    Fehlhaber, K.; Bird, D.K. )

    1991-08-01

    Multiple intrusions of gabbros, mafic dikes, and syenites in the Kap Edvard Holm Complex gave rise to prolonged circulation of meteoric hydrothermal solutions and extreme isotope exchange and mineral alteration in the 3,600-m-thick Lower Layered Series gabbros. In the Lower Layered Series, {delta}{sup 18}O of plagioclase varies from +0.3{per thousand} to {minus}5.8{per thousand}, and it decreases with an increase in the volume of secondary talc, chlorite, and actinolite. In the same gabbros, pyroxenes have a more restricted range in {delta}{sup 18}O, from 5.0{per thousand} to 3.8{per thousand}, and values of {delta}{sup 18}O{sub pyroxene} are independent of the abundance of secondary minerals, which ranges from 14% to 30%. These relations indicate that large amounts of water continued to flow through the rocks at temperatures of < 500-600C, altering the gabbros to assemblages of talc + chlorite + actinolite {plus minus}epidote {plus minus}albite and causing significant oxygen-isotope exchange in plagioclase, but not in pyroxene. The extensive low-temperature secondary mineralization and {sup 18}O depletion of plagioclase in the Lower Layered Series are associated with the later emplacement of dikes and gabbros and syenites, which created new fracture systems and provided heat sources for hydrothermal fluid circulation. This produced subsolidus mineral alteration and isotope exchange in the Lower Layered Series that are distinct from those in the Skaergaard and Cuillin gabbros of the North Atlantic Tertiary province, but are similar to those observed in some oceanic gabbros.

  4. Structure and petrology of the La Perouse gabbro intrusion, Fairweather Range, southeastern Alaska.

    USGS Publications Warehouse

    Loney, R.A.; Himmelberg, G.R.

    1983-01-01

    The gabbro was intruded during the Middle Tertiary into a Mesozoic granulite-facies metamorphic environment dominated by strike-slip fault movement, compression and possible minor subduction. The asymmetric funnel form of the intrusion is due to subsidence from magmatic loading at high T, coupled with control from pre-existing structures, and not from tectonic compression. The intrusion is 12 X 27 km and has exposed cumulate layering of approx 6000 m. Probe analyses of olivines (24), Ca-poor pyroxenes (28), augites (22) and plagioclases (35) are tabulated. Cumulus mineral compositions in the basal cumulates are: olivine Fo86-71, plagioclase An81-63, bronzite Ca3Mg82Fe15 - Ca4Mg75Fe21, augite Ca45Mg47Fe8 - Ca42Mg48Fe10. The layered gabbro above the basal cumulates consists dominantly of lenticularly interlayered plagioclase-augite-orthopyroxene-olivine, plagioclase-augite- olivine and plagioclase-orthopyroxene-augite cumulates, the composition ranges being olivine Fo75-50, plagioclase An78-42, orthopyroxene and inverted pigeonite Ca2.8Mg76.4Fe20.8 - Ca1.4Mg31.0Fe67.6, augite Ca43.1Mg46.9Fe10.0 - Ca40.5Mg27.1Fe32.4. The most iron-rich pyroxene and albite-rich plagioclase occur in a zone near the margin of the intrusion and are probably related to exchange reactions with the country rock. It is considered that the gabbro did not accumulate by simple fractional crystallization of a single or even several large batches of magma, but by numerous influxes of previously fractionated magma from a deeper reservoir. Conditions of crystallization are interpreted as approx 1055oC, 5.4 kbar and fO2 near the wustite-magnetite buffer.-R.A.H.

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

    USGS Publications Warehouse

    Gilotti, J.A.; Elvevold, S.

    1998-01-01

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

  6. The Atlantis Bank gabbro-suite was not a "normal" magma-chamber that produced basalts

    NASA Astrophysics Data System (ADS)

    Kvassnes, A. J.; Dick, H. J. B.; Grove, T. L.

    2003-04-01

    The differentiation of the basalts sampled at Atlantis II Fracture Zone, South-West Indian Ridge, is not the result of simple fractionation of gabbroic mineral-assemblages like those recovered from the adjacent Atlantis Bank and ODP Hole 735B. Large mineral data sets for the gabbros (Dick, et al 2002) are now available for analysis and comparison to spatially associated basalts. We have used Melts and pMelts (Ghiorso and Sack, 1995) to estimate the fractional crystallization trend gabbros from a primitive mantle melt or of the AII F.Z. MORB. Thermodynamic models (Grove et al (1992), Putirka (1999)) were also used to model the glasses hypothetical mafic and felsic mineral equilibrium-compositions. Our results show that while the basalts suggest 30-50% crystallization, the gabbros indicate 35-90% crystallization of a primary melt. It is therefore unlikely that the gabbros sampled from Atlantis Bank are the fossil magma-chambers that expelled melts that formed the spatially associated basalts. The models also show that the most primitive gabbros have elevated clinopyroxene Mg#s (Mg/(Mg+Fe)) relative to the coexisting plagioclase An%. This was unexpected, as the clinopyroxene frequently occurs as oikocrysts surrounding the plagioclase and encloses rounded olivine chadacrysts, indicating that the clinopyroxene precipitated late. Elthon (1992) noted the same problem for Cayman Trough gabbros; suggesting that this was the result of intermediate pressure fractionation. In our models, pressure does have some effect up to 5kbar, but is not enough to explain the discrepancy. We propose a model where melts are modified in a porous network or mush. Plagioclase-olivine networks form by accumulation of buoyant glomerocrysts and then work as filters as new melts pass through. Dissolution of the minerals would make the new melt appear to be more primitive with regards to increased Mg#s, as the dissolution happens fast without complete internal re-equilibration with the gabbro

  7. Magmatic accretion and thermal convection at the sheeted dike complex-gabbro boundary in superfast spreading crust, ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Trela, Jarek; Ferré, Eric C.; Launeau, Patrick; Bartz, Daniel M.; Morris, Antony

    2015-10-01

    The magmatic processes responsible for accretion of new oceanic plutonic crust at fast-spreading centers remain unclear. Expedition 312, at Ocean Drilling Program Hole 1256D, on the eastern flank of the East Pacific Rise, barely drilled through the uppermost gabbros but offers unparalleled opportunities to constrain deep magmatic processes and the mode of construction of the gabbroic layer. Gabbro 1 and gabbro 2, respectively 52 and 24 m-thick, were intruded in an altered and baked sheeted dike complex. Both gabbros lack a macroscopic fabric and were until now considered isotropic. Here, we use digital 3-D fabric analysis of plagioclase phenocrysts in 33 non-azimuthally oriented samples. In contrast with previous observations, the two gabbros display a distinct and consistent shape-preferred orientation. Both gabbros exhibit magmatic, equilibrated microstructures, with minor submagmatic microstructures. The calculated Rayleigh numbers range between 1015 and 1018, proving that the gabbroic magma was initially undergoing turbulent magmatic flow. Magmatic foliations dip moderately, with a mean of 48° in gabbro 1 and 61° in gabbro 2. Magmatic lineations plunge in gabbro 1 between 1 and 44° (mean 28°), while in Gabbro 2, plunges range from 6 to 69° (mean 41°). These inclined magmatic structures, combined with the inclined magmatic contacts between the gabbros and their host-rock, show that these two gabbro bodies, instead of being horizontal sills, more likely are inclined intrusions that cut obliquely through the sheeted dike complex. Seismic reflection data around Hole 1256D shows inclined reflectors similar to those produced by gabbroic saucer-shaped intrusions in passive margins. On the basis of the inclined fabrics, inclined contacts and the regional seismic data, we propose that the uppermost gabbroic intrusions at fast-spreading centers form with a saucer-shape. This new shape of intrusion, never reported before in the oceanic environment, may constitute a

  8. sup 40 Ar- sup 39 Ar dating of the Beja gabbro: Timing of the accretion of southern Portugal

    SciTech Connect

    Ruffet, G. )

    1990-11-01

    The {sup 40}Ar-{sup 39}Ar dating of the amphibole from the Beja gabbro (Southern Portugal) yields a plateau age at 336.4 {plus minus} 0.8 Ma (2{sigma} level). The corresponding calculated isotopic closure temperature is around 800C. The comparison of this temperature with the magnetic blocking temperature ({approximately}570C) allows an estimation of a probable thermoremanent acquisition age for the characteristic magnetization component of the Beja gabbro between 335Ma and 315Ma, assuming cooling rates between 10C/Ma and 100C/Ma. These results, combined wtih paleomagnetic results from the Beja gabbro and Late Paleozoic rocks from Southern Portugal (Perroud et al., 1985), suggest that the southermost part of Spain and Portugal was separated from Northern Iberia in Early Carboniferous times and was accreted to Europe during the Late Carboniferous.

  9. Zoned Cr, Fe-spinel from the La Perouse layered gabbro, Fairweather Range, Alaska

    USGS Publications Warehouse

    Czamanske, G.K.; Himmelberg, G.R.; Goff, F.E.

    1976-01-01

    Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr2O3 to rims with less than 11 wt.% Cr2O3. Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al2O3 and TiO2. Also notable are the concentrations of MnO and V2O3, which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900??C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma. ?? 1976.

  10. Origin and age of the Eisenkappel gabbro to granite suite (Carinthia, SE Austrian Alps)

    PubMed Central

    Miller, C.; Thöni, M.; Goessler, W.; Tessadri, R.

    2011-01-01

    The northern part of the Karawanken plutonic belt is a gabbro–granite complex located just north of the Periadriatic lineament near the Slovenian–Austrian border. Petrographic and geochemical studies of the Eisenkappel intrusive complex indicate that this multiphase plutonic suite developed by a combination of crystal accumulation, fractional crystallization and assimilation processes, magma mixing and mingling. The mafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from an enriched mantle source. The mafic melts triggered partial melting of the crust and the formation of granite. The granitic rocks are alkalic, metaluminous and have the high Fe/Fe + Mg characteristics of within-plate plutons. Temperature and pressure conditions, derived from amphibole-plagioclase and different amphibole thermobarometers, suggest that the analysed Eisenkappel gabbros crystallized at around 1000 ± 20 °C and 380–470 MPa, whereas the granitic rock crystallized at T ≤ 800 ± 20 °C and ≤ 350 MPa. Mineral-whole rock Sm–Nd analyses of two cumulate gabbros yielded 249 ± 8.4 Ma and 250 ± 26 Ma (εNd: + 3.6), garnet-whole rock Sm–Nd analyses of two silicic samples yielded well-constrained ages of 238.4 ± 1.9 Ma and 242.1 ± 2.1 Ma (εNd: − 2.6). PMID:26525511

  11. VERY High Temperature Hydrothermal Record in Plagioclase of BLACK Gabbros in Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.; Mainprice, D.; Nicolas, A. A.

    2014-12-01

    The lower crustal section in Oman ophiolite includes 'black gabbros' that have escaped the common medium-low temperature hydrous alteration. Their plagioclases are totally fresh, but contain in their mass, nebulous inclusions most times below the resolution of optical microscope, or expressed as solid silicate phases clinopyroxene and pargasitic amphibole, up to 10 µm sized, having T equilibrium above 900°C with their host plagioclase. These gabbros have a well-expressed magmatic foliation, relayed by plastic strain marked by stretched olivine crystals, and pinching twins in plagioclase. In addition to major elements analyses, the crystallographic relationships of these Mg silicate inclusions to their host plagioclase are explored by Electron Back Scattering Diffraction (EBSD) processing. - Diopsidic clinopyroxene inclusions are dominant over pargasitic amphibole that tend to locate close to the margins of host plagioclase (Fig 1). Some inclusions are mixed clinopyroxene-amphibole, separated by a non-indexed phase that could represent a pyribole-type structure, suggesting transformation from clinopyroxene to amphibole during cooling. High chlorine content in the amphibole sign the seawater contamination at least during the development of this phase. - Preliminary statistical pole figures (Fig. 2) in the six joined plagioclase grains studied, show that both plagioclase and diopside inclusions have a strong crystal preferred orientation (CPO) connected such that the strong [010]pl maximum coincide with the strong [100]di. In addition, a coincidence appears between three sub-maxima of [100]pl and [001]di. These interesting relationships are refined. It is inferred that clinopyroxene developed through corrosion of the plagioclase by a Mg-bearing hydrous fluid, penetrating possibly via twin interface and diffusing at T~1100°C, upper limit of clinopyroxene stability in hydrous conditions. Development of pargasite implies increasing hydration during cooling.

  12. Frictional properties of gabbro at conditions corresponding to slow slip events in subduction zones

    NASA Astrophysics Data System (ADS)

    Mitchell, E. K.; Fialko, Y.; Brown, K. M.

    2015-11-01

    We conducted a series of experiments to explore the rate and state frictional properties of gabbro at conditions thought to be representative of slow slip events (SSEs) in subduction zones. The experiments were conducted using a heated direct shear apparatus. We tested both solid and simulated gouge samples at low effective normal stress (5-30 MPa) over a broad range of temperatures (20-600°C) under dry and hydrated conditions. In tests performed on dry solid samples, we observed stable sliding at low temperatures (20-150°C), stick slip at high temperatures (350-600°C), and a transitional "episodic slow slip" behavior at intermediate temperatures (200-300°C). In tests performed on dry gouge samples, we observed stable sliding at all temperatures. Under hydrated conditions, the gouge samples exhibited episodic slow slip and stick-slip behavior at temperatures between 300 and 500°C. Our results show a decrease in the rate parameter (a - b) with temperature for both solid and gouge samples; friction transitions from velocity strengthening to velocity weakening at temperature of about 150°C for both solid and gouge samples. We do not observe transition to velocity-strengthening friction at the high end of the tested temperature range. Our results suggest that the occurrence of slow slip events and the downdip limit of the seismogenic zone on subduction megathrusts cannot be solely explained by the temperature dependence of frictional properties of gabbro. Further experimental studies are needed to evaluate the effects of water fugacity and compositional heterogeneity (e.g., the presence of phyllosilicates) on frictional stability of subduction megathrusts.

  13. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge. PMID:11343114

  14. Mineral chemistry as a tool for understanding the petrogenesis of Cryogenian (arc-related)-Ediacaran (post-collisional) gabbros in the western Arabian Shield of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Surour, Adel A.; Ahmed, Ahmed H.; Harbi, Hesham M.

    2016-07-01

    Metagabbros and gabbros in the Ablah-Shuwas belt (western Saudi Arabia) represent part of significant mafic magmatism in the Neoproterozoic Arabian Shield. The metagabbros are Cryogenian, occasionally stratified and bear calcic amphiboles (hornblende, magnesio-hornblende and actinolite) typical of calc-alkaline complexes. These amphiboles suggest low pressure ( 1-3 kbar), high f_{O2 } and crystallization temperature up to 727 °C, whereas it is 247-275 °C in the case of retrograde chlorite. Rutile and titanite in metagabbros are Fe-rich and replace Mn-bearing ilmenite precursors at high f_{O2} . On the other hand, younger gabbros are fresh, layered and comprised of olivine gabbro and olivine-hornblende gabbro with an uppermost layer of anorthositic gabbro. The fresh gabbros are biotite-bearing. They are characterized by secondary magnetite-orthopyroxene symplectitic intergrowth at the outer peripheries of olivine. The symplectite forms by deuteric alteration from residual pore fluids moving along olivine grain boundaries in the sub-solidus state. In fresh gabbros, ortho- and clinopyroxenes indicate crystallization at 1300-900 and 800-600 °C, respectively. Geochemically, the Cryogenian metagabbros ( 850-780 Ma) are tholeiitic to calc-alkaline in composition and interpreted as arc-related. Younger, fresh gabbros are calc-alkaline and post-collisional ( 620-590 Ma, i.e., Ediacaran), forming during the late stages of arc amalgamation in the southern Arabian Shield. The calc-alkaline metagabbros are related to a lithospheric mantle source previously modified by subduction. Younger, fresh gabbros were probably produced by partial melting of an enriched mantle source (e.g., garnet lherzolite).

  15. Comparison of the electrical response of dry and hydrosaturated gabbro as a function of uniaxial stress

    NASA Astrophysics Data System (ADS)

    Dahlgren, R.; Johnston, M. J.; Freund, F. T.; Nakaba, R. N.; Vanderbilt, V. C.

    2012-12-01

    Suggestions that substantial charge generation occurs in the Earth's crust as a result of tectonic loading and variations in this loading prior to earthquakes have been controversial because the presence of fluids in crustal materials is thought to preclude the storing of charge. A series of laboratory experiments was set up to investigate the charge response during repetitive step loading of a suite of dry and saturated rocks. The dry rocks include those with possible semiconductor properties (granite, sandstone, gabbro) and those without (marble, limestone). Saturated samples (gabbro) were prepared by firstly vacuum removing all fluids and gas from open pores, microcracks and fractures and then saturating them for several months with fluids similar to those observed in and around fault zones. Brine with an electrical conductivity matched to that observed in and near fault zones (2 S/m) was obtained by preparing a 0.56 molar NaCl solution. Electrical response from dry rocks during repeated loading cycles from a pedestal stress of 5.6 MPa to 22.7 MPa and back to 5.6 MPa indicates 1) a short-term transient current response of a few tens of pA during loading and unloading with decay time constants of seconds consistent with generation and decay of electrical charge from various alternative physical processes, 2) a net negative current with increasing load in most cases of a few pA or less, and 3) a similar response for rocks with and without semiconductor properties. For the fluid-saturated sample tested, resistance was much lower as expected, and the resistance was increased such that it was on par with the dry sample with a 100 Mohm padding resistor. The baseline was slowly drifting, and continuous currents were observed from self-potential (SP) and electrokinetic effects, but step-loading and unloading produced no observable changes in current generation. This is consistent with self discharge expected in the electrically conductive crust near active faults during

  16. 'Melt welt' mechanism of extreme weakening of gabbro at seismic slip rates.

    PubMed

    Brown, Kevin M; Fialko, Yuri

    2012-08-30

    Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (∼0.1-1 m s(-1)), and at moderate normal stresses (1-10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting. The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood. The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt ('melt welts'), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1-0.4 MW m(-2), while the average temperature of the shear zone is well below the solidus (250-450 °C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics. PMID:22932388

  17. Elastic wave velocities in anorthosite and anorthositic gabbros from Apollo 15 and 16 landing sites

    NASA Technical Reports Server (NTRS)

    Chung, D. H.

    1973-01-01

    Laboratory measurements of ultrasonic velocities in lunar samples 15065, 15555, 15415, 60015, and 61016 as well as in synthetic materials corresponding to compositions of anorthositic gabbros are presented as a function of hydrostatic pressure to about 7 kb. The author examined the seismic velocity distributions in the moon with reference to the variations to be expected in a homogeneous medium. The lunar mantle begins about 60 km, and the velocity of P waves in this area is about 7.7 km/sec. Variation of the seismic parameter with depth in the upper crust (about 20 km thick) is much too rapid to be explained by compression of a uniform material and the departure from expectation is so great that no reasonable adjustment of the material parameters can bring agreement; therefore, this author concludes that this result in this region of the moon is not due to self-compression but to textural gradients. In the lower crust (about 40 km thick), the region is shown to be relatively homogeneous, consisting probably of anorthositic rocks.

  18. The Permian Dongfanghong island-arc gabbro of the Wandashan Orogen, NE China: Implications for Paleo-Pacific subduction

    NASA Astrophysics Data System (ADS)

    Sun, Ming-Dao; Xu, Yi-Gang; Wilde, Simon A.; Chen, Han-Lin; Yang, Shu-Feng

    2015-09-01

    The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~ 275 Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na2O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures (87Sr/86Sri = ~ 0.7065, εNd(t) = ~- 0.5, 208Pb/204Pbi = ~ 38.05, 207Pb/204Pbi = ~ 15.56, 206Pb/204Pbi = ~ 18.20 and zircon εHf(t) = ~+ 5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~ 290 Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~ 275 Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous-Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia.

  19. Distribution of Sc, Ta, Hf, Zr, Co, and Fe in the crust of weathering of metalliferous gabbro-norites in volodarsk-volyn rock body

    SciTech Connect

    Borisenko, L.F.; Chudinov, V.I.

    1986-09-01

    Nuclear physics methods are used to determine the Sc, Ta, Hf, Zr, Co, and Fe contents in gabbro-norites and the component minerals of these rocks, as well as in the various zones of the crusts of weathering developed on gabbro-norites. It has been established that Sc, Ta, Hf, and Zr accumulate in the kaolinite zone, but Co is partly washed out of it.

  20. Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

    USGS Publications Warehouse

    Bacon, Charles R.; Vazquez, Jorge A.; Wooden, Joseph L.

    2012-01-01

    Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular

  1. IODP Expedition 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Ildefonse, Benoit; Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam

    2014-05-01

    Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor. However, owing to the nearly continuous overlying extrusive upper crust, sampling in situ the lower crust is challenging. Hence, models for understanding the formation of the lower crust are based essentially on geophysical studies and ophiolites. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant sections of primitive, modally layered gabbroic rocks from the lowermost plutonic crust formed at a fast-spreading ridge, and exposed at the Hess Deep Rift (Gillis et al., Nature, 2014, doi:10.1038/nature12778). Drilling Site U1415 is located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic rocks, and gabbronorite. All rock types are primitive to moderately evolved, with Mg# 89-76, and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolites. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics

  2. IODP Exp 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Gillis, K. M.; Snow, J. E.; Klaus, A.

    2013-12-01

    Plutonic rocks from the lower ocean crust formed at fast-spreading ridges provide a record of the history of melt transport and crystallization between the mantle and the seafloor. Despite the significance of these rocks, sampling them in situ has proven extremely challenging. This means our models for understanding the formation of the lower crust are based largely on geophysical studies and ophiolites that did not form at typical mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant recovery of primitive modally layered gabbroic rocks from the lowermost plutonic crust from a fast-spreading ridge exposed at the Hess Deep Rift. Drilling was focused at Site U1415, located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. All rock types are primitive (Mg# 76-89) and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolite complexes. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the first robust estimate of the bulk composition of crust formed at a

  3. Geochemistry of gabbros and basaltic dykes from the upper/lower oceanic crust boundary: New data from IODP Expedition 335 (ODP Site 1256, Cocos Plate)

    NASA Astrophysics Data System (ADS)

    Godard, M.; Adachi, Y.; Miyashita, S.; Kurz, M. D.; Roy, P.

    2012-12-01

    ODP Hole 1256D (Cocos plate), a borehole in a 15 m.y old oceanic crust formed at a superfast spreading ridge, was deepened down to 1521.6 meters below seafloor (mbsf) during IODP Expedition 335. The lower part of the borehole (>1340 mbsf) crosscuts a series of variably altered lithologies interpreted as marking the transition from upper to lower crust (top to bottom): basaltic sheeted then granoblastic dykes, a ~50 m gabbro screen (Gabbro 1), a ~20 m interval of granoblastic dykes (Dyke Screen 1), a ~20 m gabbro screen (Gabbro 2) then a second interval of granoblastic dykes. We present the results of an XRF and ICPMS study performed on 3 cored granoblastic dikes from this last interval, and on a basalt, 5 granoblastic dikes and two gabbroic rocks retrieved during junk basket runs during Expedition 335. The basalt and granoblastic dikes have MORB type compositions similar to that of the variably altered basalt samples and granoblastic dikes cored during the previous expeditions at Hole 1256D. Expedition 335 granoblastic dikes are characterized by their depletion in lithophile trace elements (Yb~2-3 ppm) similar to those of the granoblastic dikes cored below Gabbro 2, which represent the most depleted end-member of the basaltic dikes previously found at Site 1256 (Yb~2.5-6 ppm). We interpret this signature as evidence that the Expedition 335 granoblastic dikes come from the bottom of Hole 1256D (at Gabbro 2 lower interface and below) rather than from shallower levels. The two gabbros (olivine gabbronorite and olivine gabbro) have high LOI indicating that they were more affected by low temperature hydration processes than the neighboring granoblastic dikes. Their composition is similar to that of the less evolved end-members of the gabbroic rock suite previously sampled at Hole 1256D, which were found in the Gabbro 1 interval. They have relatively high Mg# (70-72) and Ni (200-280 ppm), reflecting their modal olivine content. Although they are slightly depleted

  4. An ultramafic primary magma for a low Si, high Ti-Fe gabbro in the Panxi region of the Emeishan large igneous province, SW China

    NASA Astrophysics Data System (ADS)

    Gregory Shellnutt, J.; Wang, Kuo-Lung

    2014-01-01

    The Kelang gabbro (256 ± 3 Ma) is in contact with the peralkaline syenite of the Late Permian (˜260 Ma) Fe-Ti oxide-bearing Baima igneous complex and contains high contents of TiO2 (˜4.0 wt.%) and Fe2O3t (˜20 wt.%) and low SiO2 (˜42 wt.%) and Al2O3 (˜11 wt.%) content. The gabbros are primarily composed of plagioclase, clinopyroxene, olivine (Mg# ˜50), ilmenite, magnetite, orthopyroxene (Mg# ˜60), biotite and apatite. The Sr-Nd whole rock isotope data (ISr = 0.7046, ɛNd(T) = +2.5) and positive primitive mantle normalized anomalies of Ba, Sr and Ti and high Eu/Eu* (˜1.1) of the Kelang gabbro suggest a possible connection to the Baima cumulate gabbro (ISr = 0.7047-0.7052, ɛNd(T) = +1.6 to +4.2, Eu/Eu* = 1.5-3.6) but the bulk Cr content (˜4 ppm) and mineralogy are very different. Mass-balance, least-squares modeling suggests that the low Cr content of the Kelang gabbro is likely a result of fractionation of Cr-spinel from a picritic primary magma. After early fractionation of Mg-rich mafic silicates and Cr-spinel, the residual liquid will be enriched in Ti and Fe, depleted in Cr, have low Si and Al and resemble the bulk composition of the Kelang gabbro. Consequently, the Kelang gabbro may be the uppermost portion of a large mafic-ultramafic intrusion unrelated to the Baima igneous complex.

  5. Strength and stability of frictional sliding of gabbro gouge at elevated temperatures

    NASA Astrophysics Data System (ADS)

    He, Changrong; Yao, Wenming; Wang, Zeli; Zhou, Yongsheng

    2006-12-01

    To investigate the strength of frictional sliding and stability of mafic lower crust, we conducted experiments on oven-dried gabbro gouge of 1 mm thick sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) at slip rates of 1.22 × 10 - 3 mm/s and 1.22 × 10 - 4 mm/s and elevated temperatures up to 615 °C. Special attention has been paid to whether transition from velocity weakening to velocity strengthening occurs due to the elevation of temperature. Two series of experiments were conducted with normal stresses of 200 MPa and 300 MPa, respectively. For both normal stresses, the friction strengths are comparable at least up to 510 °C, with no significant weakening effect of increasing temperature. Comparison of our results with Byerlee's rule on a strike slip fault with a specific temperature profile in the Zhangbei region of North China shows that the strength given by experiments are around that given by Byerlee's rule and a little greater in the high temperature range. At 200 MPa normal stress, the steady-state rate dependence a - b shows only positive values, probably still in the "run-in" process where velocity strengthening is a common feature. With a normal stress of 300 MPa, the values of steady-state rate dependence decreases systematically with increasing temperature, and stick-slip occurred at 615 °C. Considering the limited displacement, limited normal stress applied and the effect of normal stress for the temperatures above 420 °C, it is inferred here that velocity weakening may be the typical behaviour at higher normal stress for temperature above 420 °C and at least up to 615 °C, which covers most of the temperature range in the lower crust of geologically stable continental interior. For a dry mafic lower crust in cool continental interiors where frictional sliding prevails over plastic flow, unstable slip nucleation may occur to generate earthquakes.

  6. Low-sulfide PGE ore in the Volchetundra gabbro-anorthosite pluton, Kola Peninsula, Russia

    NASA Astrophysics Data System (ADS)

    Chashchin, V. V.; Petrov, S. V.

    2013-09-01

    The internal structure of the Volchetundra gabbro-anorthosite massif is considered, including localization of low-sulfide PGE mineralization and its mineralogy. The Volchetundra massif 24 km long and 0.5-4.0 km wide occupies the middle part of the Main Range complex, which extends for 75 km in the nearly meridional direction. The main and marginal zones are distinguished in the massif. The marginal zone 20-400 m wide extends along the entire eastern contact of the massif and is primarily composed of mediumgrained meso- and leucocratic norite, gabbronorite, plagioclasite, and less fequent orthopyroxenite. The main zone consists of coarse-grained leucogabbro and gabbronorite with an anorthosite zone in the axial part of the massif. The PGE mineralization of the Volchetundra massif is distinctly subdivided into two types substantially differing in localization, mineralogy, geochemistry, and economic importance. Mineralization of the first type is localized in the marginal zone and characterized by the highest resource potential. Mineralization hosted in the main zone belongs to the second type. The PGE ore of marginal zone is spatially and genetically related to the pyrite-pentlandite-chalcopyrite-pyrrhotite sulfide mineralization (1-5%) in the form of fine inequigranular interstitial disseminations, and less frequent larger grains and pockets localized within two ore zones each up to 2 km in extent. The thickness of separate mineralized layers varies from 0.5 to 3.0 m and up to 45 m in bulges. The average Pt + Pd grade is 1.37 gpt at Pd/Pt = 3.1. The mineralization of the second type has been penetrated by boreholes. Separate intersections do not correlate with one another and are limited in extent both along the strike and down the dip. The PGE mineralization is related to finely dispersed pentlandite-pyrite-pyrrhotite-chalcopyrite sulfides, sulfide emulsions, and less abundant stringer-disseminated sulfide ore. The orebodies vary from 2 to 7 m in thickness. The

  7. Recycled gabbro signature in Upper Cretaceous Magma within Strandja Massif: NW Turkey

    NASA Astrophysics Data System (ADS)

    Ulusoy, Ezgi; Kagan Kadioglu, Yusuf

    2016-04-01

    Basic magma intrusions within plate interiors upwelling mantle plumes have chemical signatures that are distinct from mid-ocean ridge magmas. When a basic magma interact with continental crust or with the felsic magma, the compositions of both magma changes, but there is no consensus as to how this interaction occurs. Here we analyse the mineral behavior and trace element signature of gabbroic rocks of the samples collected from the Strandja Massif. Srednogorie magmatic arc is a part of Apuseni- Banat-Timok-Srednogorie magmatic belt and formed by subduction and closure of the Tethys Ocean during Upper Cretaceous times. Upper Cretaceous magmatic rocks cutting Strandja Massif in NW Turkey belong to eastern edge of Srednogorie Magmatic arc. Upper Cretacous magmatic rocks divided into four subgroup in Turkey part of Strandja massif: (I) granitic rocks, (II) monzonitic rock, (III) syenitic rocks and (IV) gabbroic rocks. Gabbroic rocks outcropped around study area in phaneritic - equigranular texture. According to mineralogic - petrographic studies gabbros have mainly holocrystalline texture and ophitic to subophitic texture composed of plagioclase, amphibole, pyroxene, and rarely olivine and opaque minerals. Also because of special conditions there have been pegmatitic texture on mafic minerals with euhedral form up to 3 cm in size and orbicular texture which reach 15cm in size and rounded - elliptical form. Confocal Raman Spectroscopy studies reveals that plagioclase are ranging in composition from labradorite to bytownite, the pyroxene are ranging in composition from diopside to augite acting with uralitization processes and the olivine are generally in the composition of forsterite. Petrographic and mineralogical determination reveals some metasomatic magmatic epidote presence. Confocal Raman Spectroscopy studies on anhydrous minerals within gabbroic rocks shows affect of hydrous process because of magma mixing. The gabbroic rocks have tholeiitic and changed towards

  8. Sedimentation in Magma Chambers: Evidence From the Geochemistry, Microstructure and Crystallography of Troctolite and Gabbro Cumulates, Rum Layered Intrusion, Scotland.

    NASA Astrophysics Data System (ADS)

    Lo Ré, F. C.; Cheadle, M. J.; Swapp, S. M.; Coogan, L. A.

    2003-12-01

    The formation of igneous cumulates remains poorly understood. In particular, petrologists disagree about the relative importance of crystal accumulation by sedimentation or in-situ growth, partly because post-cumulus processes often overprint evidence for the primary mechanism. We address this problem with a detailed geochemical, microstructural and crystallographic study of gabbros and troctolites from the Eastern Layered Series of the Rum Layered Intrusion, NW Scotland. We collected samples, approximately every 50cm, through the complete 11m sequence of foliated gabbros and troctolites from Unit 9. The samples were quantitatively analyzed for crystal shape and size, shape preferred orientation (SPO), crystallographic preferred orientation (CPO), modal mineralogy, and whole rock and mineral chemistry. We present the following results: Within cumulate layers just cm's apart, olivine crystal shape can vary from sub-equant to tabular, and crystal diameter can vary from 1-8mm. This suggests at least one olivine population was derived elsewhere in the magma chamber. Complex anorthite zoning is also consistent with an extended history of plagioclase crystal transport. A one-crystal thick olivine layer between feldspathic layers of differing grain size is interpreted to be a lag deposit. Two possible examples of cross bedding also exist. Taken together, these observations suggest sedimentation was the primary method of crystal accumulation. The data also provide constraints on post-cumulus processes. Complex plagioclase zoning suggests that processes of viscous compaction and/or recrystallization were not extensive. The preservation of magmatic and deformation twins and absence of a polygonal fabric support this conclusion. In addition, the CPO and SPO are the same. Lacking evidence for compaction and recrystallization lead us to believe similar CPO and SPO are the preserved artifact of a sedimentary foliation. Therefore, we conclude the gabbros and troctolites of Unit 9

  9. Petrogenesis of the Zhangmatun gabbro in the Ji'nan complex, North China Craton: Implications for skarn-type iron mineralization

    NASA Astrophysics Data System (ADS)

    Xie, Qiuhong; Zhang, Zhaochong; Hou, Tong; Santosh, M.; Jin, Ziliang; Han, Liu; Cheng, Zhiguo

    2015-12-01

    The Zhangmatun gabbro is part of the Ji'nan complex in the North China Craton. It is spatially and temporally associated with the Zhangmatun skarn-type high-grade iron deposit. U-Pb dating of zircons from the Zhangmatun gabbro yielded an age of 130.2 ± 1.8 Ma, indicating the emplacement time as Early Cretaceous. The Zhangmatun gabbro displays porphyritic texture with plagioclase and clinopyroxene as the major phenocryst phases, together with minor biotite, hornblende, and magnetite. The rocks show a restricted range of SiO2 (50.1-52.7 wt.%) and Al2O3 (14.7-16.0 wt.%), low TiO2 (0.69-0.86 wt.%), relatively variable FeOT (2.87-8.62 wt.%), high MgO (6.50-8.01 wt.%) and Na2O (3.36-5.13 wt.%). The gabbro is also characterized by marked enrichment of LREEs and other LILEs (e.g., Rb, Sr and Ba), and strong depletion in HFSEs (e.g., Nb, Ta, Ti, Zr and Hf), similar to arc-related magma. The plagioclase and clinopyroxene from the Ji'nan gabbro, Zhangmatun gabbro, as well as Yaoshan and Huashan gabbroic diorite exhibit higher oxygen isotopes than those of the primitive mantle, indicating involvement of crustal materials. The similarity in geology, geochronology and petrology of these rocks suggests that they are co-magmatic. The parent magma of the Zhangmatun gabbro was produced by partial melting of amphibole-bearing spinel lherzolite in the upper mantle which has been modified by the lower crustal materials beneath the North China Craton. In combination with the Sr-Nd isotopic compositions of the Zhangmatun gabbro, we recognize EMI-type mantle source beneath the Ji'nan area. Moreover, the relatively high values as well as the relatively large variation of εHf(t) (-9.1 to +1.8) imply that asthenospheric material was also involved in the mantle-derived melt. The formation of iron concentration in the hydrothermal fluids is attributed to the exsolution of Fe-rich magmatic fluids and leaching from the solidified Zhangmatun gabbro at the retrograde alteration stage.

  10. The Fazenda Gavião granodiorite and associated potassic plutons as evidence for Palaeoproterozoic arc-continent collision in the Rio Itapicuru greenstone belt, Brazil

    NASA Astrophysics Data System (ADS)

    Costa, Felipe G.; Oliveira, Elson P.; McNaughton, Neal J.

    2011-08-01

    Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, São Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gavião granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with Archaean migmatite-gneiss basement. The pluton is mostly composed of hornblende granodiorite, occasionally crosscut by syn-plutonic mafic dykes. The FGG is metaluminous, medium- to high-K calc-alkaline with relatively constant silica abundances (SiO2 ˜ 63-66 wt%), high Sr (900-800 ppm) and high Ba (1000-1500 ppm). The associated mafic dykes are ultrapotassic, with high abundances of Ba, Sr, MgO, Ni, Cr, and light rare earth elements, suggesting derivation from partial melts of an enriched mantle source. The FGG originated probably by fractional crystallization from a primitive K-rich mafic magma that interacted with crustal melts. Its zircon U-Pb SHRIMP age of 2106 ± 6 Ma indicates that the FGG is younger than the early (2163-2127 Ma) tonalite-trondhjemite-granodiorite (TTG) and calc-alkaline arc plutons of the greenstone belt, and is closely related in time and space with potassic to ultrapotassic plutons (ca. 2110-2105 Ma). The negative ɛNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive ɛNd(t) of the older arc plutons, indicating a major change of isotope signatures in granites of the Rio Itapicuru greenstone belt with time. This isotope shift may be related to magma contamination with older continental material and/or derivation of the parental potassic magma from enriched lithospheric mantle sources. We suggest that the K-rich plutons were emplaced during or shortly after Palaeoproterozoic arc-continent collision.

  11. Comagmatic contact relationships between the Rock Creek Gabbro and Round Valley Peak granodiorite, central Sierra Nevada, CA

    SciTech Connect

    Christensen, C.C.; Bown, C.J. . School of Natural Science)

    1993-03-01

    The Rock Creek Gabbro (RCG) in Little Lakes Valley, near Tom's Place, CA abuts three granodiorites with distinctive contact characteristics. Against within a cm in most places. The contact with Round Valley Peak (RVP) on the north, however, is a zone at least 3 km wide and records a mode of mafic magmatic enclave formation. A northward traverse of the zone begins 300--400 m within the RCG with progressively lighter, though still uniform rock. Next is a 100--200m wide jumble of sharp-edged angular 10--30m gabbroic xenoliths, variable in grainsize and plastic deformation and interspersed with stretched partially disaggregated enclaves in normal RVP granodiorite. Xenoliths are essentially absent from the RVP from here north; stretched enclaves with very consistent strikes paralleling (within 20[degree]) the mapped RCG-RVP contact and high angle dips (70--90[degree]), occur singly and in dense swarms and fall from 4% to 0.5% of outcrop area in the remaining traverse. Rock Creek gabbros including xenoliths at the contact cluster chemically with RVP enclaves on all major and trace element plots, suggesting a common parentage; some of each group show evidence of plagioclase flotation. Trace element data (esp. Zr/Nb) suggests that fractional crystallization dominates mixing in the evolution of the gabbroic/enclave magma.

  12. 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.

  13. A mineralogical investigation of the Late Permian Doba gabbro, southern Chad: Constraints on the parental magma conditions and composition

    NASA Astrophysics Data System (ADS)

    Shellnutt, J. Gregory; Lee, Tung-Yi; Yang, Chih-Cheng; Hu, Shin-Tai; Wu, Jong-Chang; Iizuka, Yoshiyuki

    2016-02-01

    The late Permian Doba (257 ± 1 Ma) gabbro was discovered at the base of an exploration well through the Cretaceous Doba Basin of Southern Chad. The gabbro is at least 250 m thick, has cumulus mineral textures and consists of plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxide, apatite and quartz. The composition of the silicate minerals (i.e. plagioclase, pyroxenes) from the lowest part of the intrusion tend to be more primitive (i.e. An59, Wo40-44En33-47Fs12-25) than the upper part (i.e. An41, Wo39-43En30-44Fs14-29) suggesting the magmatic system differentiated internally by crystal fractionation and crystal redistribution. Based on the chemistry of the pyroxenes, the parental magma was compositionally similar to a Ti-rich, within-plate continental tholeiite that had a bulk Mg# of ˜44. The relative oxidation state of the magma was initially moderately reducing (i.e. ΔFMQ-0.3) and then became more reducing after a period of fractionation (i.e. ΔFMQ-1.2). The estimated initial magma temperature was at least 1100 °C. The within-plate composition suggests there was rifting-related magmatism near the boundary of the Saharan Metacraton that could be related to edge-driven mantle convection.

  14. Magmatism and metamorphism at the sheeted dyke-gabbro transition zone: new insight from beerbachite from ODP/IODP Hole 1256D and Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Python, Marie; Abily, Bénédicte; France, Lydéric

    2014-05-01

    During IODP Expedition 335, two-pyroxenes bearing granulites (beerbachites) were extensively recovered as drilling cuttings at the gabbro-sheeted dyke transition zone of ODP Hole 1256D (East Pacific Rise, 6°44.163'N, 91°56.061'W). This lithology results from high-temperature metamorphism of previously hydrothermally altered diabases, basalts and/or gabbros; the heat source likely stems from the melt lens located at the top of the magmatic chambers imaged along present-day fast-spreading ridges. This lithology, associated with gabbroic bodies, characterises the transition zone between the sheeted dyke complex and the uppermost gabbroic section and represents the interface between magmatic and hydrothermal convecting systems in an oceanic crust formed at fast-spreading ridges. Samples acquired during IODP Exp. 335 show a particularly high degree of recrystallisation and are characterised by the absence of hydrous phases like amphibole, suggesting very high-T metamorphism. The Beerbachites mineral chemical characteristics are rather homogeneous compared to gabbros or dolerite from the sheeted dyke but pyroxenes Mg#, Ti, Al and Cr contents as well as the anorthite content of plagioclase are closer to gabbro than dolerite. This similarity may be explained by two hypothesis: either beerbachites in Hole 1256D are metamorphosed gabbros, or they underwent a melt-rock reaction process with the gabbros parental magma and were re-equilibrated at high temperature until their mineral composition become similar to that of gabbros. The gabbro-sheeted dyke transition zone in the Oman ophiolite is also outlined by the presence of high grade metamorphic rocks. Fine grained granulites and amphibolites that may be derived from the transformation of altered sheeted dyke diabases are in direct contact with fresh gabbroic and troctolitic bodies which are themselves cross-cut by dolerite dykes. The observation of textures show that high-T recrystallisation occurred in the fine grained

  15. The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement

    NASA Astrophysics Data System (ADS)

    Petri, B.; Mohn, G.; Štípská, P.; Schulmann, K.; Manatschal, G.

    2016-05-01

    Contact aureoles rimming plutonic rocks are the locus of metamorphism and deformations witnessing magma emplacement mechanisms in the crust. In this study, structural and petrological observations are combined to mineral equilibria modelling to unravel the polyphase tectono-metamorphic history of the Permian Sondalo gabbro and its host rock, the Campo unit (Eastern Alps). The Campo unit consists of Grt-St-Ms-Bt-Pl-Qtz ± Sil ± And ± Crd mica schists attesting of a Carboniferous prograde P- T path, reaching 6 kbar/600 °C and subsequently 5.6 kbar/650 °C. This metamorphism is coeval with the formation of a sub-vertical NE-SW trending foliation (S1) and its overprint by a sub-vertical NW-SE trending foliation (S2). The heat brought by the Permian intrusives subsequently caused heating of the Campo unit at around 3-4 kbar/540 °C reflected by regional static crystallization of cordierite and andalusite porphyroblasts. During the intrusion of the Sondalo gabbro, thermal peak conditions are recorded by Grt-Sil-Spl-Crd-Ilm granulitic xenoliths at ~5.5 kbar/930 °C, subsequently exhumed at ~4 kbar during the development of a new foliation (S3). This foliation is localized around the pluton and moderately dips away from the centre of the pluton. In the migmatitic contact aureole, Grt-Sil-Bt-Pl-Qtz-Ilm and Grt-Sil-Crd-Spl-Bt-Kfs-Ilm residual rocks bear the new foliation (S3) and document a decompression from 6 kbar/750 °C to 5 kbar/725 °C and from 5.2 kbar/800 °C to reach 4.8 kbar/770 °C, respectively. The P- T- d paths recorded by the host rock and the xenoliths point to a two-step magma emplacement. First the Sondalo gabbro intruded the Campo unit causing heating of the host rock without deformation at 3-4 kbar. Second, the ductile flow along the pluton margins developed a new foliation (S3) during exhumation of the pluton and its immediate contact aureole from 6 to 4 kbar. Altogether, it indicates a progressive increase in mechanical coupling between the pluton

  16. Fe-Ti-oxide textures and microstructures in shear zones from oceanic gabbros at Atlantis Bank, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Till, Jessica; Morales, Luiz F. G.; Rybacki, Erik

    2016-04-01

    Ocean drilling expeditions at several oceanic core complexes formed at slow- and ultra-slow-spreading ridges have recovered cores containing numerous zones of oxide-rich gabbros containing ilmenite and magnetite. In these cores, high modal concentrations of Fe-Ti-oxides are systematically associated with high-temperature plastic deformation features in silicates. We present observations of Fe-Ti-oxide mineral structures and textural characteristics from a series of oxide-rich shear zones from Atlantis Bank (ODP Site 735B) on the Southwest Indian Ridge aimed at determining how oxide mineral abundances relate to strain localization. Fe-Ti-oxide minerals in undeformed oxide gabbros and in highly deformed samples from natural shear zones generally have morphologies characteristic of crystallized melt, including highly cuspate grains and low dihedral angles. Anisotropy of magnetic susceptibility in oxide-rich shear zones is very strong, with fabrics mainly characterized by strong magnetic foliations parallel to the macroscopic foliation. Crystallographic preferred orientations (CPO) in magnetite are generally weak, with occasionally well-defined textures. Ilmenite typically displays well-developed CPOs, however, the melt-like ilmenite grain shapes indicate that at least part of the crystallographic texture results from oriented ilmenite growth during post-deformation crystallization. The oxides are hypothesized to have initially been present as isolated pockets of trapped melt (intercumulus liquid) in a load-bearing silicate framework. Progressive plastic deformation of silicate phases at high-temperature mainly produced two features: (i) elongated melt pockets, which crystallized to form strings of opaque minerals and (ii), interconnected networks of melt regions. The latter lead to intense strain localization of the rock, which appears as oxide-rich mylonites in the samples. In some samples, abundant low-angle grain boundaries in both magnetite and ilmenite suggest

  17. Sr-Isotope Composition of Feldspar: Implication for age and Evolution of Gabbros from Uralian-Alaskan Type Complexes in the Ural Mountains, Russia

    NASA Astrophysics Data System (ADS)

    Bruegmann, G. E.; Krause, J.; Pushkarev, E.

    2006-12-01

    Compared to whole-rock or bulk mineral analyses single mineral composition could reveal disequilibrium features between crystals or within individual crystals which provide important information on magma chamber processes. We applied a LA-ICPMS technique to determine Sr isotopes in plagioclase and K-feldspar of gabbroic rocks from the Nizhnii Tagil and Kytlym complexes in the Ural Mountains in Russia. These complexes are Uralian-Alaskan-type zoned mafic-ultramafic complexes, consisting of dunite, clinopyroxenite and gabbro bodies. The rock association is considered to represent a cumulate sequence of a single parental melt feeding a magma chamber system. The instruments used were a NU Plasma MC-ICPMS and a solid-state 193 and 213 nm Nd:YAG laser ablation system from New Wave. The data of the present study includes up to five spot analyses (100-120 μm spot size) of each mineral in a thin section. Baselines for analyses were measured on-peak for 20 s while flushing the sample chamber with He. About 300 to 500 isotope ratios were measured during the ablation time of 80-100 s. Raw data were exported to an external data reduction spreadsheet and corrected for interferences (Kr, Rb) and mass bias using 87Sr/^{88}Sr=0.1194. During the time of analyses we measured the Sr isotope composition of an in-house marine carbonate reference material. Its 87Sr/86Sr of 0.70923±6 (2SD) agrees within error limits with our TIMS measurements. The gabbros have porphyric textures with clinopyroxene phenocrysts in a matrix of olivine, clinopyroxene and spinel. Two gabbro types can be distinguished based on additional matrix minerals. One type of gabbro is silica saturated, contains plagioclase (An57-88) and in places orthopyroxene as matrix phases. The second gabbro type is silica undersaturated and contains in the matrix plagioclase (An29-56) and pseudoleucite, a fine grained intergrowth of nepheline and K-feldspar (Or30-81). In this gabbro plagioclase has generally higher Sr

  18. Granite, gabbro and mafic microgranular enclaves in the Gejiu area, Yunnan Province, China: a case of two-stage mixing of crust- and mantle-derived magmas

    NASA Astrophysics Data System (ADS)

    Cheng, Yanbo; Spandler, Carl; Mao, Jingwen; Rusk, Brian G.

    2012-10-01

    Geochronological, geochemical, whole-rock Sr-Nd, and zircon Hf isotopic analyses were carried out on the Jiasha Gabbro, mafic microgranular enclaves (MME) and host Longchahe Granite samples from the Gejiu area in the southeast Yunnan province, SW China, with the aim of characterizing their petrogenesis. Compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin. The Longchahe Granite largely comprises metaluminous quartz monzonite with distinctive K-feldspar megacrysts, but also contains a minor component of peraluminous leucogranite. The MME have spheroidal to elongated/lenticular shapes with sharp, crenulated and occasionally diffuse contacts with the host granite, which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma. Field observations, geochronology, geochemistry, Sr-Nd and zircon Hf isotopic compositions point to a complex petrogenesis for this granite-MME-gabbro association. Zircon 206Pb/238U ages determined by LA-ICP-MS for a mafic enclave, its host granite and the gabbro body are 83.1 ± 0.9 Ma, 83.1 ± 0.4 Ma and 83.2 ± 0.4 Ma, respectively, indicating coeval crystallization of these igneous rock units. Crystal fractionation processes can explain much of the compositional diversity of the Jiasha Gabbro. The geochemical features of the gabbro, such as high Mg# (up to 70) and Cr (up to 327 ppm), enrichment in LILEs (e.g., Rb, Ba, K2O) and LREEs, and depletion in HFSE (e.g., Nb, Ta, Ti), together with initial 87Sr/86Sr ratios of 0.708-0.709 and negative ɛNd(t) values (-5.23 to -6.45), indicate they were derived from a mantle source that had undergone previous enrichment, possibly by subduction components. The Longchahe Granite has a large range of SiO2 (59.87-74.94 wt%), is distinctly alkaline in composition, and has Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i > 0.712, ɛNd(t) = -6.93 to -7

  19. Origin of the 'Gabbro' Signature in Ocean Island Basalts: Constraints from Osmium Isotopic Ratios of Galapagos Basalts

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; Dale, C. W.; Geist, D.; Harpp, K. S.

    2014-12-01

    The Re-Os isotope system has become increasingly used as a tracer of lithological heterogeneity in the convecting mantle, with radiogenic 187Os/188Os in high-Os oceanic basalts and picrites widely interpreted as evidence of a melt contribution from ancient recycled oceanic crust. When combined with 206Pb/204Pb and O isotopes, 187Os/188Os ratios have been used to identify distinct lithological units (i.e. sediments, gabbros and basalts). We report new 187Os/188Os for basalts with high Os (>40 ppt) and MgO from Galápagos, which range from near primitive mantle values (0.130) to highly radiogenic (0.155). While co-variations in 187Os/188Os and 206Pb/204Pb for some Galápagos basalts (Floreana-type) are HIMU like, and consistent with melting of ancient recycled oceanic crust, others have variable 187Os/188Os ratios and primitive to depleted mantle like 206Pb/204Pb. Similar variations in Os and Pb isotopic space have been interpreted in other OIB suites as melts from recycled ancient oceanic gabbros, entrained by upwelling mantle plumes. Nevertheless, a marked east-west spatial variation in 187Os/188Os of Galápagos basalts does not correlate with postulated lithological variations in the Galápagos plume (Vidito et al., 2013). We show that basalts in eastern Galápagos with elevated 187Os/188Os and positive Sr anomalies occur in the vicinity of over-thickened 10 Ma gabbroic crust, that formed when the Galápagos plume was on-axis. We propose the elevated 187Os/188Os of Galápagos basalts are due to in-situ assimilation of young gabbroic lower crust, with high Re/Os, rather than melting of ancient recycled material in the Galápagos plume. In western Galápagos recent plume accreted crust is thick but more mafic, the melt flux higher and assimilation more sporadic. The contamination thresholds of Os and MgO in Galápagos basalts occur at higher contents than for many global OIBs (Azores, Iceland, Hawaii) and may reflect both a relatively low melt flux into the crust

  20. Sm-Nd and Rb-Sr Ages for Northwest Africa 2977, A Young Lunar Gabbro from the PKT

    NASA Technical Reports Server (NTRS)

    Nyquist, L. E.; Shih, C.-Y.; Reese, Y. D.; Irving, A. J.

    2009-01-01

    Northwest Africa (NWA) 2977 is an olivine gabbro cumulate equivalent to one of the lithologies in lunar mare breccia NWA 773 [1,2,3]. The Ar-39-Ar-40 age is 2.77+/-0.04 Ga based on the last approx.57% of the gas release [4], similar to results for NWA 773 [5]. A Sm-Nd age (T) of 2.865+/-0.031 Ga and Epsilon(sub Nd) = -7.84+/-0.22 for the NWA 773 gabbro reported by [6] has been revised to T = 2.993+/-=0.032 Ga, Epsilon(sub Nd) -4.5+/-0.3 [7]. Sm-147-Nd-143 isochron for NWA 2977: Whole rock, pyroxene, olivine, plagioclase, whole rock leachate (approx.phosphate) and the combined leachates from the mineral separates yield a well defined Sm-Nd isochron for an age T = 3.10+/-0.05 Ga and Epsilon(sub Nd-CHUR) = -3.74+/-0.26 [8], or Epsilon(sub Nd-HEDR) = -4.61+/-0.26 [9]. Rb-87-Sr-87 isochron: NWA 2977 contains only a modest amount of Rb and/or Sr contamination. The Sr-isotopic composition of the contaminant closely resembles that of seawater. The whole rock residue after leaching combined with leach residues for plagioclase and pyroxene define an isochron age of 3.29+/-0.11 Ga for initial Sr-87/Sr-86 = 0.70287+/-18. The olivine residue, with lower Sr abundance of approx 1.5 ppm, is only slightly displaced from the isochron. The relatively small uncertainties of the Rb-Sr isochron parameters and near-concordancy with the Sm-Nd age indicate that both the Rb-Sr and the Sm-Nd ages are reliable.

  1. Magnetic and microscopic features of silicate-hosted Fe-oxide inclusions in an oceanic gabbro section

    NASA Astrophysics Data System (ADS)

    Till, Jessica

    2015-04-01

    The magnetic mineralogy of oceanic gabbros is typically dominated by magnetite, which occurs in several forms: as a cumulus or intercumulus phase, as a secondary phase formed through alteration, or as exsolved inclusions in plagioclase and pyroxene. This study characterizes the contribution of magnetic inclusions in plagioclase and pyroxene to the bulk rock remanence and examines changes in the distribution of remanence carriers with crustal depth. Selected samples were taken throughout a 1500-m-long section of drilled oceanic gabbro cores collected from the Oceanic Drilling Program Site 735B at Atlantis Bank on the Southwest Indian Ridge. Hysteresis parameters and curves of isothermal remanence acquisition were measured for plagioclase and clinopyroxene mineral separates and compared with whole rock measurements for samples from various depths to determine the relative contributions of each to the bulk sample remanence properties. In whole-rock samples, bulk saturation magnetization decreases and coercivity distributions become dominated by harder magnetic components with increasing depth. The changes in rock magnetic properties with depth are interpreted to result from variations in composition as well as cooling rates. Coercivity distributions in both plagioclase and pyroxene systematically shift to higher coercivities with increasing depth in the section, although the change is more pronounced in plagioclase, indicating that the size distributions of magnetic inclusions in plagioclase become progressively finer. First-order reversal curves for plagioclase separates provide a striking example of non-interacting single-domain particles. Variations in exsolution textures and compositions of the inclusions were also investigated by microanalysis and electron microscopy. Microscopic examination revealed unexpected complexity in the structure of exsolution features, with several oxide phases commonly present as inclusions in plagioclase and multiple generations of

  2. Petrography and Geochemistry (Trace, Ree and Pge) of Pedda Cherlo Palle Gabbro-Diorite Pluton, Prakasam Igneous Province, Andhra Pradesh, India

    NASA Astrophysics Data System (ADS)

    Subramanyam, K. S. V.; Reddy, U. V. B.; Balaram, V.; Roy, Parijat

    2015-09-01

    Prakasam Igneous Province (PIP) is an important geological domain in the Eastern Dharwar Craton (EDC), found in the junction zone between the EDC and Eastern Ghat Mobile Belt (EGMB). The Pedda Cherlo Palle (PCP) gabbros are massive, leucocratic-mesocractic, and show cumulus textures with minerals plagioclase, cpx, and amphiboles. Compositionally, plagioclase is a labradorite-bytownite, cpx is diopside to augite, olivines are hyalosiderites and amphiboles are magnesiohornblendes. PCP gabbros have normal SiO2, high Al2O3, moderate to high TiO2, Na2O and medium Fe2O3, so, classified as subalkaline tholeiitic gabbros. Fractionated rare earth element (REE) patterns, high abundance of large ion lithofile elements (LILE) and transitional metals coupled with light REE (LREE) relative enrichment over heavy REE (HREE) and Nb are characteristics of partial melting of depleted mantle and melts that have undergone fractional crystalisation. These partial melts are enriched in LREE and LILE, due to the addition of slab derived sediment and fluids. PCP gabbros contain low abundance (5.1 to 24.6 ng/g) of platinum group elements (PGE), and show an increase in the order Ir>Os>Pt>Ru»Pd>Rh. We propose that the subduction related intraoceanic island arc might have accreted to the southeastern margin of India to the east of Cuddapah basin in a collisional regime that took place during Ur to Rodinia amalgamations.

  3. Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain, Medicine Lake volcano, California: Implications for fractionation mechanisms and the formation of composition gaps

    USGS Publications Warehouse

    Brophy, J.G.; Dorais, M.J.; Donnelly-Nolan, J.; Singer, B.S.

    1997-01-01

    The rhyolite of Little Glass Mountain (73-74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54-61% SiO2) and partially crystalline cumulate hornblende gabbro (53-55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54-61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide??olivine, +/-orthopyroxene, +/-hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53-55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70-74% SiO2). The gabbros record a two-stage crystallization history of plagioclase + olivine + augite (Stage I) followed by plagioclase+orthopyroxene + hornblende + Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the

  4. Magmatic ^18O in Zircons From Gabbros and Serpentinized Peridotite at the Mid-Atlantic Ridge (ODP Leg 153)

    NASA Astrophysics Data System (ADS)

    Cavosie, A. J.; Kita, N. T.; Valley, J. W.

    2005-12-01

    Zircons from gabbros and serpentinized ultramafic rocks from the Mid-Atlantic Ridge near the Kane Transform (MARK area) drilled during Leg 153 of the Ocean Drilling Program were analyzed for δ18O in situ in rock chips by ion microprobe. The gabbros contain clinopyroxene, plagioclase, apatite, Fe-oxides, with zircon occurring primarily along grain boundaries and as inclusions in other minerals (e.g. plagioclase). The mineralogy of the serpentinites is more complex, as they are comprised predominantly of a serpentine matrix that is cross-cut by multiple generations of intrusions, including zircon-bearing magmas emplaced as cm- to mm-scale gabbroic dikes and also lower temperature hydrothermal veins. The gabbroic dikes were pervasively altered at greenschist facies conditions, leaving zircon ± apatite as the only preserved magmatic phases. In some serpentinites it is difficult to distinguish altered magmatic veins from lower temperature hydrothermal veins. Zircons in serpentinite and gabbroic samples yield average δ18O values of 4.94±0.80‰ VSMOW (2 SD, N=33 analyses on 12 grains). This value would be in high temperature, magmatic equilibrium with MORB if δ18O (WR) ~ 5.3‰, or mantle peridotite if δ18O (Ol) = ~4.8‰. Equilibrium fractionation factors for δ18O between zircon-water (Zrc-H2O) were calculated by combining fractionation factors for Zrc-quartz (Valley et al. 2003) and quartz-H2O (Clayton et al., 1972; Matsuhisa et al., 1979). Over the temperature range of the calibrated fractionation factors (i.e. 500-800°C), calculated δ18O (zircon) values would be < 1 ‰ for zircon in equilibrium with previously measured MARK hydrothermal fluids (e.g. δ18O = 2.3‰). Zircon in equilibrium with seawater with δ18O = 0.0‰ would have 2.3‰ lower values. We note that extrapolation of the Zrc-H2O fractionation factors to temperatures below the calibrations of Zrc-Qtz and Qtz-H2O (e.g. <500°C) does not yield a typical mineral-water `crossover' at lower

  5. Stick-slip behavior of Indian gabbro as studied using a NIED large-scale biaxial friction apparatus

    NASA Astrophysics Data System (ADS)

    Togo, Tetsuhiro; Shimamoto, Toshihiko; Yamashita, Futoshi; Fukuyama, Eiichi; Mizoguchi, Kazuo; Urata, Yumi

    2015-04-01

    This paper reports stick-slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention (NIED), Tsukuba, Japan. The apparatus consists of the existing shaking table as the shear-loading device up to 3,600 kN, the main frame for holding two large rectangular prismatic specimens with a sliding area of 0.75 m2 and for applying normal stresses σ n up to 1.33 MPa, and a reaction force unit holding the stationary specimen to the ground. The shaking table can produce loading rates v up to 1.0 m/s, accelerations up to 9.4 m/s2, and displacements d up to 0.44 m, using four servocontrolled actuators. We report results from eight preliminary experiments conducted with room humidity on the same gabbro specimens at v = 0.1-100 mm/s and σ n = 0.66-1.33 MPa, and with d of about 0.39 m. The peak and steady-state friction coefficients were about 0.8 and 0.6, respectively, consistent with the Byerlee friction. The axial force drop or shear stress drop during an abrupt slip is linearly proportional to the amount of displacement, and the slope of this relationship determines the stiffness of the apparatus as 1.15 × 108 N/m or 153 MPa/m for the specimens we used. This low stiffness makes fault motion very unstable and the overshooting of shear stress to a negative value was recognized in some violent stick-slip events. An abrupt slip occurred in a constant rise time of 16-18 ms despite wide variation of the stress drop, and an average velocity during an abrupt slip is linearly proportional to the stress drop. The use of a large-scale shaking table has a great potential in increasing the slip rate and total displacement in biaxial friction experiments with large specimens.

  6. Carbon Species in Serpentinites and Gabbros Underlying the Lost City Hydrothermal Field, Southern Atlantis Massif (30°N, MAR)

    NASA Astrophysics Data System (ADS)

    Delacour, A.; Schaeffer, P.; Bernasconi, S. M.; Frueh-Green, G. L.

    2006-12-01

    Serpentinization of oceanic peridotites results in the production of volatile-rich (methane and hydrogen) fluids and other light hydrocarbons, and is characteristic of the low-temperature (<90°C) fluids actively venting at the Lost City Hydrothermal Field (LCHF; 30°N near the MAR). Carbon contents and carbon isotope compositions have been measured from serpentinized peridotites and gabbros in the basement of the LCHF with the goal to better understand carbon sources and carbon cycling during serpentinization and hydrothermal venting. The serpentinites have total carbon contents (TC) of 60 to 820 ppm, and up to 1.7 wt% in samples containing carbonate veins. C-isotope compositions of the TC range from -24.9 to +2.3‰, whereby positive δ13C values are correlated with serpentinites with carbonate veins and indicate a marine carbon input. The non-carbonate carbon content (TOC: total organic carbon and graphite residual after HCl dissolution) of the serpentinites is from 55 to 280 ppm, with δ13CTOC ranging from -29.5 to -21.5‰. The gabbros show a wider range of TC and δ13CTC, but have δ13CTOC in the same range as the serpentinites. The TOC isotopic compositions may reflect hydrocarbon production during serpentinization or the presence of organic compounds within the samples. To constrain the origin further, organic compounds were extracted from selected serpentinites. The saturated hydrocarbon fractions show a predominance of n-alkanes and an unresolved complex mixture. Long chain n- alkanes show no odd over even carbon number predominance, indicating no contamination by higher plants waxes from recent material. Although n-alkanes can be biogenic or be produced abiotically by Fischer Tropsch-type reactions, the occurrence of the isoprenoids norpristane, pristane, phytane and squalane clearly indicate a biogenic origin, possibly incorporated into the serpentinites during fluid-rock interaction. Compound- specific C-isotope analyses show relatively constant

  7. Slow Slip Predictions Based on Gabbro Dehydration and Friction Data Compared to GPS Measurements in Northern Cascadia

    NASA Astrophysics Data System (ADS)

    Rice, J. R.; Liu, Y.

    2008-12-01

    For episodic slow slip transients in subduction zones, a large uncertainty in comparing surface deformations predicted by rate and state friction modeling [Liu and Rice, JGR, 2007] to GPS measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along the fault. In this study, we apply petrological data [Peacock et al., USGS, 2002; Hacker et al., JGR 2003; Wada et al., JGR, 2008] and recently reported friction data [He et al., Tectonophys, 2006, 2007] for gabbro, as a reasonable representation of the seafloor, to a Cascadia-like 2D model in order to produce simulations which show spontaneous aseismic transients. We compare the resulting inter-transient and transient surface deformations to GPS observations along the northern Cascadia margin. An inferred region along dip of elevated fluid pressure is constrained by seismological observations where available, and by thermal and petrological models for the Cascadia and SW Japan subduction zones. For the assumed a and a-b profiles, we search the model parameter space, by varying the level of effective normal stress σ, characteristic slip distance L in the source areas of transients, and the fault width under that low σ, to identify simulation cases which produce transient aseismic slip and recurrence interval similar to the observed 20-30 mm and 14 months, respectively, in northern Cascadia. Using a simple planar fault geometry and extrapolating the 2D fault slip to a 3D distribution, we find that the gabbro gouge friction data allows a much better fit to GPS observations than is possible with the granite data [Blanpied et al., JGR, 1995, 1998] which, for lack of a suitable alternative, has been used as the basis for most previous subduction earthquake modeling, including ours. Nevertheless, the values of L required to reasonably fit the geodetic data during a transient event are somewhat larger than 100 microns, rather than in the range of 10 to a few 10s of microns as might be

  8. Gravity and magnetic expression of the San Leandro gabbro with implications for the geometry and evolution of the Hayward Fault zone, northern California

    USGS Publications Warehouse

    Ponce, D.A.; Hildenbrand, T.G.; Jachens, R.C.

    2003-01-01

    The Hayward Fault, one of the most hazardous faults in northern California, trends north-northwest and extends for about 90 km along the eastern San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km-long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75??-80?? northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented at one time, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the roof thrust of this tectonic wedge was rotated to near vertical, and in places, the strike-slip Hayward Fault probably reactivated or preferentially followed this pre-existing feature. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, we qualitatively explore mechanical models to explain how this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the

  9. Gravity and Magnetic Expression of the San Leandro Gabbro with Implications for the Geometry and Evolution of the Hayward Fault Zone, Northern California

    NASA Astrophysics Data System (ADS)

    Ponce, D. A.; Hildenbrand, T. G.; Jachens, R. C.

    2002-12-01

    The Hayward Fault, one of the most hazardous faults in northern California, trends NNW and extends for about 90 km along the eastern margin of the San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75-80o northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity, that extends to a depth of about 12 km, is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented by Late Cretaceous to early Tertiary, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the strike-slip Hayward Fault probably reactivated or preferentially followed this pre-existing feature in the late Tertiary. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the Hayward Fault will provide additional constraints on seismic

  10. Mineralogy and petrology of lunar meteorite Northwest Africa 2977 consisting of olivine cumulate gabbro including inverted pigeonite

    NASA Astrophysics Data System (ADS)

    Nagaoka, Hiroshi; Karouji, Yuzuru; Takeda, Hiroshi; Fagan, Timothy J.; Ebihara, Mitsuru; Hasebe, Nobuyuki

    2015-12-01

    Lunar meteorite Northwest Africa (NWA) 2977 is identified as an olivine cumulate gabbro (OC), consisting of coarse cumulate olivine crystals up to 1 mm with low-Ca and high-Ca pyroxenes, plagioclase, and interstitial incompatible element-rich pockets of K-feldspar, Ca-phosphates, ilmenite, and troilite. These minerals and textures are similar to those of the OC clasts of the NWA 773 clan of meteorites. NWA 2977 contains a variety of pyroxene textures and compositions including augite, pigeonite, and rare orthopyroxene, all having exsolution lamellae. Some of the orthopyroxene has abundant augite lamellae with compositions indicating formation by inversion of pigeonite. This pigeonite was inverted at 1140 °C according to the pigeonite eutectoid reaction (PER) temperatures. Inverted pigeonite has not been found previously in the NWA 773 clan of meteorites. The presence of inverted pigeonite indicates that NWA 2977 cooled more slowly than most other OC clasts of the NWA 773 clan. The relatively slow cooling of NWA 2977 can be explained by formation in a deeper level of the original igneous body of the NWA 773 clan OC lithology.

  11. Panafrican basement and Mesozoic gabbro in the Zagros orogenic belt in the Dorud-Azna region (NW Iran): Laser-ablation ICP-MS zircon ages and geochemistry

    NASA Astrophysics Data System (ADS)

    Shakerardakani, Farzaneh; Neubauer, Franz; Masoudi, Fariborz; Mehrabi, Behzad; Liu, Xiaoming; Dong, Yunpeng; Mohajjel, Mohammad; Monfaredi, Behzad; Friedl, Gertrude

    2015-04-01

    The Dorud-Azna region of the Sanandaj-Sirjan metamorphic zone (SSMZ) in NW Iran is part of the Zagros orogenic belt and exposes an amphibolite-grade metamorphic succession of mixed continental (e.g., the granitic Galeh-Doz orthogneiss) and oceanic or rift (e.g., various amphibolites) origin, which is intruded by the Darijune gabbro. Laser-ablation ICP-MS U-Pb zircon ages of 608 ± 18 Ma and 588 ± 41 Ma of the granitic Galeh-Doz orthogneiss indicate a Panafrican basement similar to that of the Yazd block of Central Iran. We therefore interpret this part of the SSMZ to represent a previously subducted part of the Panafrican basement of the Iranian microcontinent. Based on geochemistry, amphibolites and metagabbros from the Amphibolite unit representially potentially the cover on the Panafrican Galeh-Doz orthogneiss are interpreted to have their origin in a likely Upper Paleozoic to Mesozoic rift or E-MORB ophiolite succession. One of the metagabbros, here termed Dare-Hedavand metagabbro yield a 206Pb/238U mean age of 314.6 ± 3.7 Ma. Talc-bearing greenschists to epidote amphibolites of the Triassic June complex formed in a similar, but younger environment. Furthermore, the metamorphic complex is intruded by the Darijune gabbro, which yields a mean ICP-MS U-Pb zircon age of 170.2 ± 3.1 Ma. The final stage of likely Jurassic greenschist facies-grade metamorphism postdates the gabbro intrusion. We interpret the Darijune gabbro to have resulted from the initial subduction of Neotethyan oceanic lithosphere in a continental arc setting. The new data combined with data from literature indicate that the SSMZ represents a tectonic element of a mixed continental and oceanic origin, likely accreted to Central Iran during a Jurassic tectonic process. The new data also constrains thick-skinned nappe stacking within the Sanandaj-Sirjan zone.

  12. Subduction of Indian continent beneath southern Tibet in the latest Eocene (˜ 35 Ma): insights from the Quguosha gabbros in southern Lhasa block

    NASA Astrophysics Data System (ADS)

    Ma, Lin; Wang, Qiang; Li, Zheng-Xiang; Wyman, Derek; Jiang, Zi-Qi

    2016-04-01

    Geophysical data illustrate that the Indian continental lithosphere has northward subducted beneath the Tibet Plateau. However, when the Indian continental lithosphere started to subduct remain unclear. Here we report new results from the Quguosha gabbros of southern Lhasa block, southern Tibet. LA-ICP-MS zircon U-Pb dating of two samples gives a ca. 35 Ma formation age (i.e., the latest Eocene) for the Quguosha gabbros, which are within a magmatism gap of the Lhasa block. The Quguosha gabbros samples exhibit Sr-Nd isotopic compositions ([87Sr/86Sr]initial = 0.7056-0.7058 and ɛNd(t) = -2.2 - -3.6) different from those of the Jurassic-Eocene magmatic rocks with depleted Sr-Nd isotopic characteristics, but somewhat similar to those of Oligocene-Miocene K-rich magmatic rocks with enriched Sr-Nd isotopic characteristics. We interpret the Quguosha mafic magmas to have been generated by partial melting of lithospheric mantle metasomatized by subducted continental sediments, which entered continental subduction channel(s) and then probably accreted or underplated into the overlying mantle during the northward subduction of the Indian continent. Our data also suggest that the Indian continental crust has started to be underthrusted beneath the mantle lithosphere of the southern Lhasa sub-block by at least ca. 35 Ma.

  13. Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

    NASA Astrophysics Data System (ADS)

    Hidas, Károly; Varas-Reus, Maria Isabel; Garrido, Carlos J.; Marchesi, Claudio; Acosta-Vigil, Antonio; Padrón-Navarta, José Alberto; Targuisti, Kamal; Konc, Zoltán

    2015-05-01

    We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1-3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2-0.5 GPa) in a 7-16 km thick

  14. Occurrences of Orthopyroxene in the "Multi-textured" Layered Gabbros from the Hess Deep Rift, East Pacific Rise (the Site U1415P, IODP Expedition 345)

    NASA Astrophysics Data System (ADS)

    Hoshide, T.; Machi, S.; Maeda, J. I.

    2015-12-01

    IODP Exp.345 drilled three main holes (Holes U1415 I, J & P) from the lowermost plutonic crust exposed at the Hess Deep Rift, East Pacific Rise and primitive layered gabbroic rocks were newly discovered from these holes (Gillis et al., 2014). One of the mysteries about the layered gabbros is the fact that Opx, which is considered to appear in the late stage of crystallization on the basis of crystallization experiments of MORB, occurs as a dominant phase in many of the layered gabbros. In this presentation, we report the occurrence of Opx from the Hole U1415P and consider the significance of Opx in the origin of the layered gabbros. Hole U1415P (about 100m in thickness) is divided into two units, the upper Multi-textured Layered Gabbro Series (MLGS) and lower Troctolite Series (TS). Gabbroic rocks from the MLGS contain Opx (< 4 vol%) and are macroscopically classified into Opx-bearing olivine gabbro. However, these rocks are mesoscopically (on cm scale) inhomogeneous and have a great variation of mode, grain size and texture. On the other hand, TS consists of homogeneous troctolites and Opx rarely occurs in the series. The occurrences of Opx from the MLGS are as follows: (i) coarse-grained Opx+Cpx+Pl vein parallel to the layered structure of the surrounding troctolite (ii) undeformed Opx+Pl veinlets in kinked Ol (iii) Opx in the concave of anhedral Ol (iv) Opx rimming Cr-spl crystals in contact with Ol. The occurrence of Opx like (ii) and (iii) resembles the texture which is considered to be formed by the reaction between mantle peridotite and a SiO2-saturated melt (e.g, Piccardo et al., 2007). The facts that Opx is often found in association with Cr-spl and Cr-spl lamellae occur in pyroxenes of the Opx+Cpx+Pl vein suggest that the SiO2-saturated melt which reacted with Ol was rich in chromium. In addition, Cr-spl crystals rimmed by Opx contain multiphase-solid inclusions. The inclusions should be key in understanding the chemical composition of the reacted melt.

  15. Timing and nature of the Xinlin-Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing'an Range, eastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Feng, Zhiqiang; Liu, Yongjiang; Liu, Binqiang; Wen, Quanbo; Li, Weimin; Liu, Qing

    2016-03-01

    Jifeng ophiolitic mélange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing'an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin-Xiguitu Ocean associated with the collision between the Erguna block and Xing'an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic mélange, the gabbros from newly discovered the Jifeng ophiolitic mélange are studied with zircon U-Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U-Pb dating from the ophiolitic gabbros yields U-Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic mélange. The gabbros display low SiO2, TiO2, K2O contents, high Na2O, LREE contents and indistinctive REE fractionation [(La/Yb)N = 1.97-2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the ɛ Hf ( t) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages ( T DM1) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic mélange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin-Xiguitu Ocean) existed between the Erguna block and Xing'an block. The Ocean's formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing'an block.

  16. Petrographical and geochemical characteristics of the sheeted dyke-gabbro transition zone in ODP/IODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Python, M.; France, L.; Abily, B.; Abe, N.; Alt, J. C.; Godard, M. M.; Ildefonse, B.; Koepke, J. H.; Kurtz, M. D.; Oizumi, R.; Payot, B. D.

    2012-04-01

    During IODP Expedition 335, high grade granoblastic hornfels were extensively recovered as drilling cuttings at the gabbro-sheeted dyke transition zone of ODP Hole 1256D (East Pacific Rise, 6°44.163'N, 91°56.061'W). This lithology probably results from high-temperature metamorphism of previously hydrothermally altered diabases and/or basalts; the heat source likely stems from the melt lens located at the top of the magmatic chambers imaged along present-day fast-spreading ridges. This lithology, associated with gabbroic bodies, characterises the transition zone between the sheeted dyke complex and the uppermost gabbroic section, and represents the interface between magmatic and hydrothermal convecting systems in an oceanic crust formed at fast-spreading ridges. In this study, 14 samples from the junk basket (cuttings) and 2 samples from cores obtained during Expedition 335 were observed and analysed. The petrological and chemical characteristics of 5 granoblastic samples collected during Expedition 312 at the root of the sheeted dyke complex and between two gabbroic horizons were also acquired for comparison. Samples collected during IODP Expedition 335 are mainly fine grained oxide grabbronorites composed of two pyroxenes, plagioclases and oxides (ilmenite, magnetite) with more or less amphiboles, sulphides, quartz and accessory minerals. Orthopyroxene Shape (roundish or anhedral), the amount of oxide inclusions in clinopyroxene and plagioclase morphology (laths or triple-junction mosaic) indicate various recrystallisation degrees. Plagioclases show a strong zoning in the less recrystallised samples, which tend to disappear with increasing recrystallisation degree. Samples show usually low alteration (less than 10%) with moderate transformation of pyroxenes into talc or actinolite. Samples from Expedition 312 show finer grains, higher degree of alteration (up to 30%), and weaker recrystallisation. They are mainly composed of plagioclases, amphibole and oxides

  17. 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

    , the random CPO of Pl suggests dominant diffusion creep deformation. The CPO of Amph suggests that Amph re-orients by rigid body rotation during deformation forming aligned (100) planes with the shear plane. Mineral chemistry and EBSD analysis shows that shear zone formation in the gabbro is associated with initial stage of cracking and grain size reduction under water deficient conditions. The cracking of Opx at lower crustal conditions (700 OC, 1 GPa) requires high differential stresses which indicate a higher strength of leuco-gabbro at the onset of subduction. Absence of newly formed Opx in the Omph corona suggests that cracking and recrystallization of Opx begins before peak PT-conditions. During exhumation, Omph is replaced by fine grained Cpx+Pl. The drastic change in grain size due to breakdown of Omph and replacement of Cpx by Amph facilitates diffusion creep deformation that may accommodate large strains during deformation. This study demonstrates the change in deformation mechanism from initial cracking of orthopyroxene to grain size sensitive diffusion creep due to high-pressure metamorphism and fluid infiltration.

  18. From Gabbro to Granulite to Kyanite- and bimineralic Eclogite: A petrological, geochemical and mass balance approach to mantle eclogites

    NASA Astrophysics Data System (ADS)

    Sommer, H.; Jacob, D.

    2013-12-01

    In this study, we present the phase transition from gabbro into granulite and finally into kyanite- bearing and bimineralic eclogite. The investigated rock sample is a heterogeneous kyanite- bearing and bimineralic eclogite from the earth's mantle collected at the Roberts Victor Diamond mine in South Africa. Plagioclase of the former granulite reacted completely out under low H2O activity (fH2O) to form this kyanite- bearing and bimineralic eclogite. To quantify the phase transitions of the original gabbroic precursor, which was first metamorphosed under H-T granulite facies conditions followed by metamorphism under Earth's mantle conditions into both types of eclogite, a petrological, geochemical and a mass balance approach has been made. i) The results from our petrological approach show that Ca-rich garnet, which is coexistent with Ca-rich omphacite are the metastable phases from the original granulite in the kyanite-bearing relict while Mg-rich garnet, coexistent with Na-rich omphacite are the stable phases in the bimineralic eclogite part which shows equilibration conditions of ~5.5 Gpa and ~1200°C. ii) Our geochemical results show a positive Eu anomaly in garnet from the kyanite-bearing part, which indicates that the igneous precursor of the granulite was a gabbro, probably oceanic crust. Most of the rare earth elements show an excellent correlation with the major elements of the rock forming minerals during the plagioclase-out reaction of the former granulite. The LREE in garnet are removed during the formation of the bimineralic eclogite due to loss of the anorthite component in plagioclase of the former granulite. Contrary, the HREE are enriched in garnets in the bimineralic part of the eclogite compared to those in the kyanite zone, and correlate with the Mg-Ca exchange between both garnet generations. iii) The results from our mass balance approach indicate that garnet in bimineralic eclogite was formed by 0.925 mole of garnet and 0.075 mole of

  19. Magnesium isotopic compositions of altered oceanic basalts and gabbros from IODP site 1256 at the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Huang, Jian; Ke, Shan; Gao, Yongjun; Xiao, Yilin; Li, Shuguang

    2015-08-01

    To investigate the behaviour of Mg isotopes during alteration of oceanic crust and constrain the Mg isotopic compositions of the altered oceanic crust (AOC), high-precision Mg isotope analyses have been conducted on forty-four altered basalts and gabbros recovered from IODP site 1256, which represent the carbonate-barren AOC formed at the East Pacific Rise (EPR). These samples were altered by interaction of seawater-derived fluids with oceanic crust at different temperatures and water/rock ratios. With the exception of one sample that has a slightly heavier Mg isotopic composition (δ26Mg = 0.01 ± 0.08‰), all the other samples have relatively homogenous and mantle-like Mg isotopic compositions, with δ26Mg ranging from - 0.36 to - 0.14‰ (an average value of - 0.25 ± 0.11‰, 2SD, n = 43). This suggests that limited Mg isotope fractionation occurred during alteration of oceanic crust at the EPR at bulk rock scale, irrespective of highly variable alteration temperatures and variable water/rock ratios. Thus, our study suggests that the offset of δ26Mg values between seawater and global runoff dominantly results from the formation of marine dolomite as a sink for Mg. The mantle-like Mg isotopic composition further indicates that recycling of carbonate-barren AOC would not result in Mg isotope heterogeneity of the mantle at global scale. Consequently, the light Mg isotopic compositions of the mantle at local scale must result from incorporation of recycled Mg isotopically light carbonates.

  20. Hornblende gabbro sill complex at Onion Valley, California, and a mixing origin for the Sierra Nevada batholith

    NASA Astrophysics Data System (ADS)

    Sisson, T. W.; Grove, T. L.; Coleman, D. S.

    1996-12-01

    The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite, individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad, lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling, show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ˜0.7065, ɛNd ˜-4

  1. The Easternmost Southwest Indian Ridge: A Laboratory to Study MORB and Oceanic Gabbro Petrogenesis in a Very Low Melt Supply Context

    NASA Astrophysics Data System (ADS)

    Paquet, M.; Cannat, M.; Hamelin, C.; Brunelli, D.

    2014-12-01

    Our study area is located at the ultra-slow Southwest Indian Ridge, east of the Melville Fracture Zone, between 61 and 67°E. The melt distribution in this area is very heterogeneous, with corridors of ultramafic seafloor where plate separation is accommodated by large offset normal faults [Sauter, Cannat et al., 2013]. These ultramafic corridors also expose rare gabbros and basalts. We use the major and trace elements composition of these magmatic rocks to document the petrogenesis of MORB in this exceptionnally low melt supply portion of the MOR system. Basalts from the easternmost SWIR represent a global MORB end-member for major element compositions [Meyzen et al., 2003], with higher Na2O and Al2O3 wt%, and lower CaO and FeO wt% at a given MgO. Within this group, basalts from the ultramafic corridors have particularly high Na2O, low CaO and FeO wt%. Best fitting calculated liquid lines of descent are obtained for crystallization pressures of ~8 kbar. Gabbroic rocks recovered in the ultramafic corridors include gabbros, oxide-gabbros and variably impregnated peridotites. This presentation focuses on these impregnated samples, where cpx have high Mg#, yet are in equilibrium with the nearby basalts in terms of their trace element compositions. Plagioclase An contents vary over a broad range, and there is evidence for opx resorption. These characteristics result from melt-mantle interactions in the axial lithosphere, which may explain several peculiar major element characteristics of the basalts. Similar interactions probably occur beneath ridges at intermediate to slow and ultraslow spreading rates. We propose that they are particularly significant in our study area due to its exceptionnally low integrated melt-rock ratio.

  2. Elastic moduli, thermal expansion, and inferred permeability of Climax quartz monzonite and Sudbury gabbro to 500/sup 0/C and 55 MPa

    SciTech Connect

    Page, L.; Heard, H.C.

    1981-03-17

    Young's modulus (E), bulk modulus (K), and the coefficient of thermal linear expansion (..cap alpha..) have been determined for Climax quartz monzonite to 500/sup 0/C and pressures (P) to 55 MPa and for Sudbury gabbro to 300/sup 0/C and 55 MPa. For each rock, both E and K decreased with T and increased with P in a nonlinear manner. In the monzonite, E and K decreased by up to 60% as P decreased from 55.2 to 6.9 MPa isothermally, while the gabbro indicated a decrease up to 70% over the same pressure range. As T increased isobarically, E and K for the monzonite decreased by up to a factor of approx. 80% from 19 to 500/sup 0/C. The moduli of the gabbro decreased by as much as 70% from 19 to 300/sup 0/C. ..cap alpha.. for the monzonite increased with T and decreased with P in a nonmonotonic fashion, with most measured values for ..cap alpha.. greater than values calculated for the crack-free aggregate. Depending on P, ..cap alpha.. in the monzonite increased from 8 to 11.10/sup -6/ /sup 0/C/sup -1/ at 40/sup 0/C to 22 to 25.10/sup -6/C/sup -1/ at 475/sup 0/C. For the gabbro, ..cap alpha.. also generally decreased with increasing P. Values ranged from 6 to 11.10/sup -6/ /sup 0/C/sup -1/, showing a nonlinear trend and very little net increas over the T range from 19 to 300/sup 0/C. Calculated permeability of these rocks based on the ..cap alpha.. determinations indicated that permeabilities may increase by up to a factor of 3 over the temperature interval 19 to 300/sup 0/C, and the permeability of the monzonite is inferred to increase by up to a factor of 8 by 500/sup 0/C. In both rocks, most measurements are consistent with microcracks controlling the thermoelastic response by opening with T and closing with sigma and P.

  3. Study on the Hymenoptera parasitoid associated with Lepidoptera larvae in reforestation and agrosilvopastoral systems at Fazenda Canchim (Embrapa Pecuária Sudeste) São Carlos, SP, Brazil.

    PubMed

    Pereira, A G; Silva, R B; Dias, M M; Penteado-Dias, A M

    2015-11-01

    The aim of this study was to characterize the local fauna of Hymenoptera parasitoids associated with Lepidoptera larvae in areas of reforestation and agrosilvopastoral systems at Fazenda Canchim (Embrapa Pecuária Sudeste, São Carlos, SP, Brazil). Lepidoptera larvae collected with entomological umbrella were kept in the laboratory until emergence of adults or their parasitoids. From those collected in the agrosilvopastoral system, emerged 267 specimens of hymenopteran parasitoids belonging to 16 genera: Braconidae, Agathidinae (Alabagrus), Braconinae (Bracon), Microgastrinae (Cotesia, Diolcogaster, Glyptapanteles, Pholetesor and Protapanteles), Orgilinae (Orgilus); Ichneumonidae, Campopleginae (Casinaria, Charops and Microcharops); Chalcididae, Chalcidinae (Brachymeria and Conura); Eulophidae, Entedoninae (Horismenus), Eulophinae (Elachertus and Euplectrus). From the Lepidoptera larvae collected in the reforestation, emerged 68 specimens of hymenopteran parasitoids, belonging to 8 genera: Chalcididae, Chalcidinae (Conura); Ichneumonidae, Pimplinae (Neotheronia), Campopleginae (Charops and Microcharops) and Braconidae, Microgastrinae (Apanteles, Diolcogaster, Distatrix, Glyptapanteles and Protapanteles). The results of this study suggest the occurrence of a wide variety of Hymenoptera parasitoids in the studied environments. PMID:26602340

  4. A Neoproterozoic age for the chromitite and gabbro of the Tapo ultramafic Massif, Eastern Cordillera, Central Peru and its tectonic implications

    NASA Astrophysics Data System (ADS)

    Tassinari, Colombo C. G.; Castroviejo, Ricardo; Rodrigues, Jose F.; Acosta, Jorge; Pereira, Eurico

    2011-12-01

    The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites show a tholeiitic affiliation and a flat REE spider diagram, with a slight LREE depletion and a positive Eu anomaly suggesting magmatic accumulation of plagioclase, in an ocean ridge or ocean island environment. Sm-Nd isotopic analyses were performed on chromite as well as on whole rock from the gabbro. All samples yielded an Sm-Nd isochrone age of 718 ± 47 Ma with an initial 143Nd/ 144Nd of 0.51213 ± 0.00005. The ɛNd (718 Ma) values calculated for both chromite and gabbro are in close agreement, around 8.0, implying that they were formed at the same time from the same mantelic magma source. Furthermore a K-Ar age on amphibole of 448 ± 26 Ma was obtained, interpreted as the cooling age of a younger orogenic event. These rocks represent slices of oceanic crust (from a dismembered ophiolitic complex), metamorphosed and later overthrust on upper Palaeozoic continental formations.

  5. Olivine-gabbros and olivine-rich troctolites genesis through melt-rock reactions in oceanic spreading lithosphere: an experimental study up to 0.7 GPa

    NASA Astrophysics Data System (ADS)

    Francomme, Justine E.; Fumagalli, Patrizia; Borghini, Giulio

    2016-04-01

    Extensive melt-rock reaction and melt impregnation significantly affect not only the physical and chemical properties at mantle-crust transition, but also control the evolution of migrating melts. We performed reactive dissolution and crystallization experiments at pressure ≤ 0.7 GPa in a piston-cylinder apparatus to provide experimental constraints on genesis of olivine-rich troctolites and olivine-gabbros at mantle-crust transition in oceanic spreading lithosphere by melt-rock reaction. Our experiments are carried out by using Salt-Pyrex-Graphite-Magnesium assemblies and graphite-lined platinum capsules. Experimental charges are prepared with three layers: (1) basalt powder, (2) fine powder (1-10μm) of San Carlos olivine (Fo90.1), and (3) carbon spheres used as a melt trap. Three synthetic MORB-type melts have been used, two tholeiitic basalts (Mg#: 0.62, SiO2: 47.70 wt%, Na2O: 2.28 wt% and Mg#: 0.58, SiO2: 49.25 wt%, Na2O: 2.49 wt%) and a primitive one (Mg#: 0.74, SiO2: 48.25 wt%, Na2O: 1.80 wt%), in order to investigate the effect of melt composition. A rock/melt ratio of 0.7 has been kept fixed. Experiments have been conducted at temperatures from 1200 to 1300°C, at both step cooling and isothermal conditions for different run durations (from 12 to 72 hrs). They resulted in layered samples in which all the initial San Carlos olivine powder, analog of a dunitic pluton infiltrated by basaltic melt, is replaced by different lithologies from olivine-rich troctolite to olivine gabbro. In isothermal experiments, reacted melts have been successfully trapped in the carbon spheres allowing their chemical analysis; as expected the reacted melt has a higher Mg# than the initial one (e.g. from Mg#=0.62 to 0.73). Across the different lithologies Mg# of olivine is decreasing from the olivine-rich troctolite to the gabbro. Replacive olivine-rich troctolite has a poikilitic texture with rounded euhedral olivine and interstitial poikilitic plagioclase and clinopyroxene

  6. U-Pb dating and composition of inclusions in zircon from ophiolitic gabbro of the Klyuchevsk massif (Middle Urals): Results and geological interpretation

    NASA Astrophysics Data System (ADS)

    Smirnov, V. N.; Ivanov, K. S.; Koroteev, V. A.; Erokhin, Yu. V.; Khiller, V. V.

    2016-06-01

    The U-Pb (SHRIMP) dating of zircon from the layered complex of ophiolitic gabbro in the Klyuchevsk massif yielded an age of 456 ± 6 Ma corresponding within the limits of error to zircon dates obtained for other petrographic varieties from this massif. The investigation of the composition of silicate inclusions in dated zircon grains revealed that they are represented by typical metamorphic minerals: albite, zoisite, and secondary amphiboles. The data indicate that zircon was crystallized during metamorphic transformations of gabbroids and its U-Pb age (Late Ordovician-Silurian) is characteristic of all rocks in the ophiolite association of the Klyuchevsk massif indicating the age of metamorphism, not their formation time.

  7. Petrology and tectonic significance of gabbros, tonalites, shoshonites, and anorthosites in a late Paleozoic arc-root complex in the Wrangellia Terrane, southern Alaska

    SciTech Connect

    Beard, J.S. ); Barker, F. )

    1989-11-01

    Plutonic rocks intrusive into the late Paleozoic Tetelna Formation of southern Alaska are the underpinnings of the late Paleozoic Skolai arc of the Wrangellia Terrane. There are four groups of intrusive rocks within the Skolai arc: (1) Gabbro-diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition; (2) Silicic intrusions including tonalite, granodiorite, and granite; (3) Monzonitic to syenitic plutonic rocks of the Ahtell complex and related dikes and sills; (4) Fault-bounded bytownite anorthosite of uncertain age and association. These anorthosites may be related to post-Skolai, Nikolai Greenstone magmatism. The silicic rocks yield discordant U-Pb zircon ages of 290-320 Ma (early to late Pennsylvanian). The monzonitic rocks of the Ahtell complex have shoshonitic chemistry. Similar shoshonitic rocks are widespread in both the Wrangellia terrane and the neighboring Alexander terrane and intrude the contact between the two. In modern oceanic arcs, shoshonitic rocks are typically associated with tectonic instability occurring during the initial stages of subduction or just prior to or during termination or flip of an established subduction zone. The nature of any tectonic instability which may have led to the cessation of subduction in the Skolai arc is unclear. Possibilities include collision of the arc with a ridge, an oceanic plateau, another arc, or a continental fragment. One possibility is that the shoshonitic magmatism marks the late Paleozoic amalgamation of Wrangellia and the Alexander terrane. The scarcity of arc rocks predating the shoshonites in the Alexander terrane supports this possibility, but structural corroboration is lacking.

  8. Formation of plagioclase-bearing peridotite and a peridotite-wehrlite-gabbro suite through melt-rock reaction: An experimental study

    NASA Astrophysics Data System (ADS)

    Saper, L.; Liang, Y.

    2012-12-01

    Plagioclase-bearing peridotites are observed among abyssal peridotites, massif peridotites, and mantle sections of ophiolites of lherzolite subtype. Formation of plagioclase-bearing peridotites is often attributed to basalt impregnation into host harzburgite or lherzolite in a thermal boundary layer. During transport through asthenospheric mantle, melt generated in the deep mantle will inevitably interact with the overlying mantle column through reactive dissolution and may leave geochemical imprints on plagioclase-bearing peridotites. To assess the role of melt-rock reaction on the formation of plagioclase-bearing peridotites and its implications for lithosphere composition, we conducted dissolution experiments in which a 88% spinel lherzolite + 12% basalt starting mixture was juxtaposed against a primitive MORB in a graphite-lined molybdenum capsule. The reaction couples were run at 1300°C and 1 GPa for 1 or 24 hrs, and then stepped cooled to 1050°C and 0.7 GPa over the next several days. Cooling promotes in situ crystallization of interstitial melts, allowing us to better characterize the mineral compositional trends produced and observed by melt-rock reaction and crystallization. A gabbro and a plagioclase-bearing peridotite were observed in the two halves of the reaction couple after the experiments were completed. The peridotite from the 24 hr reaction experiment is mostly composed of subhedral to euhedral olivines (10-50 μm in size, Mg# 75-83), poikilitic clinopyroxene (~100 μm in size, Mg# 73-83) with olivine and spinel chadocrysts, and interstitial plagioclase (An# 68-78) and melt. In a control experiment quenched after a 24 hour reaction at 1300°C the basalt completely dissolved the pyroxenes and spinels leaving a residue of rounded olivine grains (10-100 μm in size) surrounded by a relatively large melt fraction. Textural results from the step-cooling experiments suggest the following crystallization sequence from the olivine+melt mush: olivine

  9. Hyperextension of continental lithospheric mantle to oceanic-like lithosphere: the record of late gabbros in the Ronda subcontinental lithospheric mantle section (Betic Cordillera, S-Spain)

    NASA Astrophysics Data System (ADS)

    Hidas, Karoly; Garrido, Carlos; Targuisti, Kamal; Padron-Navarta, Jose Alberto; Tommasi, Andrea; Marchesi, Claudio; Konc, Zoltan; Varas-Reus, Maria Isabel; Acosta Vigil, Antonio

    2014-05-01

    Rupturing continents is a primary player in plate tectonic cycle thus longevity, stability, evolution and breakup of subcontinental lithosphere belongs for a long time to a class of basic geological problems among processes that shape the view of our Earth. An emerging body of evidences - based on mainly geophysical and structural studies - demonstrates that the western Mediterranean and its back-arc basins, such as the Alborán Domain, are hyperextended to an oceanic-like lithosphere. Formation of gabbroic melts in the late ductile history of the Ronda Peridotite (S-Spain) - the largest (ca. 300 km2) outcrop of subcontinental lithospheric mantle massifs on Earth - also attests for the extreme thinning of the continental lithosphere that started in early Miocene times. In the Ronda Peridotite, discordant gabbroic veins and their host plagioclase lherzolite, as well as gabbroic patches in dunite were collected in the youngest plagioclase tectonite domains of the Ojén and Ronda massifs, respectively. In Ojén, gabbro occurs as 1-3 centimeter wide discordant veins and dikes that crosscut the plagioclase tectonite foliation at high angle (60°). Within the veins cm-scale igneous plagioclase and clinopyroxene grains show a shape preferred orientation and grow oriented, subparallel to the trace of high temperature host peridotite foliation and oblique to the trend of the vein. In contrast to Ojén, mafic melts in the Ronda massif crystallized along subcentimeter wide anastomozing veins and they often form segregated interstitial melt accumulations in the host dunite composed of plagioclase, clinopyroxene and amphibole. Despite the differences in petrography and major element composition, the identical shape of calculated REE patterns of liquid in equilibrium with clinopyroxenes indicates that the percolating melt in Ronda and Ojén shares a common source. However, unlike gabbros from the oceanic lithosphere that shows clinopyroxene in equilibrium with LREE-depleted MORB

  10. Insights into Oceanic Crust Accretion from a Comparison of Rock Magnetic and Silicate Fabrics from Lower Crustal Gabbros from Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    Horst, A. J.; Morris, A.; Friedman, S. A.; Cheadle, M. J.

    2014-12-01

    The mechanisms of lower crustal accretion remain a long-standing question for those who study fast-spreading mid-ocean ridges. One of the goals of Integrated Ocean Drilling Program (IODP) Expedition 345 is to test accretionary models by investigating the structure of the lower oceanic crust exposed within the Hess Deep Rift. Located near the tip of the westward-propagating Cocos-Nazca spreading center, Hess Deep Rift exposes crust formed at the East Pacific Rise. During IODP Expedition 345, primitive gabbroic rocks were recovered from a dismembered lower crustal section at ~4850 meters below sealevel. Constraints on physical processes during magmatic accretion are provided by the relative orientation and strength of rock fabrics. We present anisotropy of magnetic susceptibility (AMS) fabric data from gabbros recovered from the two deepest holes (U1415J and U1415P). AMS measurements provide petrofabric data that may be used to constrain magma emplacement and subsequent magmatic flow. Bulk susceptibility ranges from 1.15 x 10-4 to 5.73 x 10-2 SI, with a majority of the samples having susceptibility greater than 10-3 SI, suggesting magnetite is the dominant contributor to the AMS signal. Low-temperature demagnetization data show Verwey transitions near 125K indicating the presence of nearly stoichiometric magnetite in most samples. AMS reveals dominantly oblate fabrics with a moderate degree of anisotropy (P') ranging from 1.01 to 1.38 (average P' = 1.13). Fabric strength varies within each of the petrologically-defined units recovered from different crustal blocks. Additional remanence anisotropy fabric analyses of a few specimens reveal nearly identical directions of principal axes compared to AMS, but with larger degrees of anisotropy. Electron backscatter diffraction (EBSD) data from one sample shows a moderate plagioclase crystallographic preferred orientation best defined by a b-axis maxima that is coincident with the AMS minimum principal axis. This comparison

  11. Natural History of a Complex Hybrid Picrite-Ferrobasalt with Gabbro Clots at IODP Site U1349, Shatsky Rise, Western Pacific

    NASA Astrophysics Data System (ADS)

    Natland, J. H.; Hellebrand, E.

    2011-12-01

    Strongly altered basalt from IODP Site U1349, Shatsky Rise, shows remarkable lithologic and mineralogic evidence for mixing between hot and primitive spinel-bearing magma on the one hand, and much cooler, strongly differentiated basalt on the other. The spinel survived almost unscathed but host olivine was nearly completely obliterated by the alteration, and is now replaced by secondary minerals. Most spinel has low CrNo (~20) but variable MgNo (70-40), which suggests an initial stage of mixing, before encountering the differentiated basalt. The differentiated basalt, which in places on the cored surface forms swirls that can be distinguished from the capturing picrite, has abundant microphenocrysts of plagioclase and clinopyroxene, many of which are intergrown and which thus crystallized along a low-pressure cotectic at a much lower temperature than the olivine and spinel. Also present are coarse-grained plagioclase-clinopyroxene clots in ophitic intergrowths, some of which enclose irregularly shaped ilmenite-magnetite plates, now modified by oxyexsolution to other oxide minerals. These clots give the rocks an "ophimottled" texture in thin section. Recalculation of clinopyroxene compositions using program QUILF (1) indicates that the clots re-equilibrated to subsolidus temperatures from 100-300 degrees lower than phenocrysts, so that they now resemble pyroxenes in abyssal gabbros from both slow- and fast-spreading ridges. The pyroxene is strongly zoned, however, with Cr-rich inner zones and Ti-rich outer zones revealing the effects of former percolating and differentiating intergranular melts. Picritic magma thus scavenged bits of a frozen but still hot magma body containing oxide gabbro before reaching a shallow reservoir or rift system laden with basaltic magma of intermediate composition. The rocks contain streaks and swirls of picrite entangled with the more differentiated lava, and erupted as pillow lava at or near sea level, with the differentiated lava

  12. Unraveling protolith ages of meta-gabbros from Samos and the Attic-Cycladic Crystalline Belt, Greece: Results of a U-Pb zircon and Sr-Nd whole rock study

    NASA Astrophysics Data System (ADS)

    Bröcker, Michael; Löwen, Kersten; Rodionov, Nikolay

    2014-06-01

    The focus of this study is on meta-ophiolitic rocks from Samos and the Attic-Cycladic Crystalline Belt, Greece. SHRIMP U-Pb zircon geochronology, Sr-Nd isotope and bulk-rock geochemistry have been applied to meta-gabbros that occur as blocks and lenses in blueschist-facies mélanges on Samos and Evia, and in the greenschist-facies Upper Unit on Tinos. The geodynamic significance of these meta-ophiolite fragments within the overall pattern of the Eastern Mediterranean region is unclear. Regional correlations within the Cyclades archipelago and with the Jurassic meta-ophiolites of the Balkan region or the Cretaceous occurrences in Turkey are uncertain. Although field, petrological and geochemical similarities among some mélange occurrences suggest a common genetic relationship, such interpretations remain speculative if not supported by robust geochronological data. SHRIMP U-Pb zircon dating of three meta-gabbro blocks from Samos yielded Cretaceous ages with weighted mean 206Pb/238U ages of 78.3 ± 1.3 Ma, 76.8 ± 1.4 Ma and 77.8 ± 1.4 Ma and almost identical intercept ages, interpreted to indicate the time of magmatic crystallization. These results further substantiate models suggesting a correlative relationship with mélanges on the islands of Syros and Tinos, central Aegean Sea, where similar rocks yielded almost identical U-Pb zircon ages. Published and new Sr-Nd isotope data of meta-gabbros from Andros, Samos, Tinos (Lower Unit) and from mainland Greece (Pindos, Othris) reveal distinctive differences among ion probe-dated samples with Jurassic and Cretaceous protolith ages. Three groups can clearly be distinguished in a 87Sr/86Sr vs. 143Nd/144Nd diagram. However, these geochemical parameters do not allow assigning tentative age estimates for yet undated meta-gabbros from southern Evia and the Upper Unit of Tinos. The situation is further complicated by the observation that the Jurassic and Cretaceous meta-gabbros do not show other discriminating geochemical

  13. The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite

    USGS Publications Warehouse

    Barker, F.; Wones, D.R.; Sharp, W.N.; Desborough, G.A.

    1975-01-01

    , Mass., and syenite of Kungnat, Greenland, among others - allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K2O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na2O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid - mostly olivine, clinopyroxene and plagioclase. ?? 1975.

  14. Origin of Silurian gabbros and I-type granites in central Fujian, SE China: Implications for the evolution of the early Paleozoic orogen of South China

    NASA Astrophysics Data System (ADS)

    Zhang, Qiao; Jiang, Yao-Hui; Wang, Guo-Chang; Liu, Zheng; Ni, Chun-Yu; Qing, Long

    2015-02-01

    The early Paleozoic orogen of South China is possibly one of the few examples of intraplate orogeny in the world. It is characterized by an angular unconformity between post-Silurian cover and pre-Devonian strata and by the intensive and extensive early Paleozoic granitic plutonism. However, synchronous mafic-ultramafic rocks have not been well-studied, but they are crucial for understanding the nature and evolution of the orogen. In this paper, we present the first detailed LA-ICP-MS zircon U-Pb dating, major and trace element geochemical and Sr-Nd-Hf isotopic data for a Silurian gabbroic pluton (Dakang) and a coeval granitic pluton (Guiyang), that have recently been identified in central Fujian, in the southeastern part of the orogen. We assess the origin of these mafic and granitic rocks and their relationship to the evolution of the early Paleozoic orogen in South China. LA-ICP-MS zircon U-Pb dating shows that the Dakang and Guiyang plutons were emplaced at 441-438 Ma and 431 Ma, respectively. The Dakang pluton consists of gabbros and minor intermediate rocks (diorite, monzodiorite and monzonite). These rocks are all metaluminous and potassic, and are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE). They have initial 87Sr/86Sr of 0.7066-0.7098, εNd (T) of - 7.0 to - 3.3 and εHf (T) (in-situ zircon) of - 5.2 to - 4.4. Geochemical data suggest that the Dakang gabbroic magmas were derived by partial melting of previously subduction-modified lithospheric mantle in the spinel-garnet transition zone at a high temperature (~ 1300 °C). These primary magmas underwent fractionation crystallization of clinopyroxene and amphibole, forming the evolved gabbros and intermediate rocks. The Guiyang pluton consists of weakly peraluminous granites, which show low Ga/Al ratios (10,000*Ga/Al < 2.6) and so can be classed as I-type granites. They have initial 87Sr/86Sr of 0.7095-0.7113, εNd (T) of - 6.7 to - 5.9 and εHf (T

  15. Dual origin of Fe-Ti-P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion

    NASA Astrophysics Data System (ADS)

    Namur, Olivier; Charlier, Bernard; Holness, Marian B.

    2012-12-01

    We present a detailed study of two ca. 200 m-thick apatite-bearing ferrogabbro horizons of the Sept Iles layered intrusion (Canada). These rocks are the most evolved cumulates of the megacyclic units (MCU) I and II, and mark the transition between basaltic and silicic magmatism. They are made up of plagioclase (An55-34), olivine (Fo66-21), clinopyroxene (Mg#75-55), ilmenite, magnetite, apatite, ± pigeonite and are a significant source of Fe-Ti-P ore. Ferrogabbros have relatively uniform bulk-rock compositions in MCU I but are bimodal in MCU II. The liquid lines of descent for major elements in equilibrium with cumulates of MCU I and II have been calculated using a forward model formalism. Both trends evolve towards SiO2-enrichment and FeOt-depletion after saturation in Fe-Ti oxides. However, because of magma mixing in MCU II, they do not follow the same path. Evolved liquids from MCU II are shown to enter the experimentally-determined two liquid stability field, while MCU I liquids do not. Immiscibility in MCU II and its absence in MCU I are supported by the presence of contrasted reactive symplectites in cumulate rocks. Apatite-bearing ferrogabbros in MCU II have crystallized from distinct immiscible Fe-rich and Si-rich silicate melts which have physically segregated in the slow-cooling magma chamber. Two different types of cumulate rocks are thus produced: leucocratic and melanocratic gabbros. This is consistent with the presence of Si-rich and Fe-rich melt inclusions in apatite. In contrast, homogeneous ferrogabbros from MCU I were produced by simple fractional crystallization of a homogeneous liquid. Our data suggest that immiscibility could also explain the large geochemical variability of ferrogabbros in the Upper Zone of the Bushveld Complex (South Africa).

  16. Geological structure and ore mineralization of the South Sopchinsky and Gabbro-10 massifs and the Moroshkovoe Lake target, Monchegorsk area, Kola Peninsula, Russia

    NASA Astrophysics Data System (ADS)

    Pripachkin, Pavel V.; Rundkvist, Tatyana V.; Miroshnikova, Yana A.; Chernyavsky, Alexey V.; Borisenko, Elena S.

    2015-08-01

    The South Sopchinsky massif (SSM), Gabbro-10 (G-10) massif, and Moroshkovoe Lake (ML) target Monchegorsk area, Kola Peninsula, are located at the junction of the Monchepluton and Monchetundra layered intrusions. The intrusions were studied in detail as they are targets for platinum-group element (PGE) mineralization. The rocks in these targets comprise medium- to coarse-grained mesocratic to leucocratic gabbronorites, medium-grained mesocratic to melanocratic norites and pyroxenites, and various veins mainly comprising norite, plagioclase-amphibole-magnetite rocks, and quartz-magnetite rocks. The veins contain Ni-Cu-PGE mineralization associated with magnetite and chromite. In all targets, the contacts between gabbronorite and norite-pyroxenite are undulating, and the presence of magmatic (intrusive) breccias suggests that these rocks formed through mingling of two distinct magmatic pulses. In places, the gabbronorites clearly crosscut the modal layering of the norites and pyroxenites. Trace element data indicate that the gabbronorites have similar compositions to rocks of the upper part of the Monchetundra intrusion, whereas the norites and pyroxenites resemble rocks from the lower to intermediate stratigraphic levels of the Monchepluton, such as in the Nude-Poaz and Sopcha massifs. Sulfide mineralization in the studied targets principally consists of secondary bornite, millerite, and chalcopyrite. In contrast, the primary sulfide assemblage within the layered sequence of the adjacent Monchepluton is characterized by pentlandite, chalcopyrite, and pyrrhotite. Therefore, the mineralization in the studied targets is interpreted to be of a contact style. We argue that the studied area represents the contact zone between gabbronorites of the Monchetundra intrusion and norites and pyroxenites of the Monchepluton. In addition, the rocks were overprinted by postmagmatic veining and remobilization of contact style sulfide and PGE mineralization.

  17. Hydrothermal alteration products of gabbros help accommodate exhumation-related deformation in mantle-derived ultramafics exposed at the Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Picazo, S.; Cannat, M.; Escartin, J.; Gibert, B.; Delacour, A.; Silantyev, S.

    2011-12-01

    Outcrops of deeply-derived ultramafic rocks and gabbros are widespread along slow spreading ridges but the rheology and dynamics of the exhumation faults and of their uplifted footwalls are still poorly known. Previous studies of samples collected within meters of exposed exhumation fault surfaces in the Atlantic have shown that a gabbroic component was added to the primarily ultramafic material in the fault zone, allowing for the growth of abundant amphibole, chlorite and talc. The nature of this component (altered magmatic intrusions or metasomatic hydrothermal fluids) could not, however, be ascertained in the pervasively sheared fault material. In this abstract we report on a set of 474 samples collected at the Mid-Atlantic Ridge (MAR) during the Serpentine cruise (2007; RV Pourquoi Pas? PI Y. Fouquet) next to the ultramafic-hosted Ashadze (13°N) and Logatchev (14°45'N) vent fields. Most of these 474 samples are weakly to moderately deformed and are interpreted as representing the upper few hundred meters below their respective exhumation fault zone, rather than the fault zone itself. The large number of samples, and their overall moderate degree of deformation gives us a chance to propose a semi-statistical study of plastic, brittle-plastic and brittle deformation in ultramafic rocks next to a MAR exhumation system, in relation with the magmatic and hydrothermal history. Our primary finding is that significant brittle-plastic deformation systematically involves amphibole±chlorite±talc-bearing ultramafic lithologies. Serpentine is commonly present in these deformed assemblages, but we did not find serpentine-only shear zones. Amphibole (in successive generations ranging from hornblende to tremolite) and chlorite occur in veins, many of which also contain zircon and some relict plagioclase, indicating a magmatic origin. Relicts of primary peridotite minerals in the most amphibole-rich samples indicate that magmatic injection followed on an episode of

  18. Halogens and noble gases in Mathematician Ridge meta-gabbros, NE Pacific: implications for oceanic hydrothermal root zones and global volatile cycles

    NASA Astrophysics Data System (ADS)

    Kendrick, Mark A.; Honda, Masahiko; Vanko, David A.

    2015-12-01

    Six variably amphibolitised meta-gabbros cut by quartz-epidote veins containing high-salinity brine, and vapour fluid inclusions were investigated for halogen (Cl, Br, I) and noble gas (He, Ne, Ar, Kr, Xe) concentrations. The primary aims were to investigate fluid sources and interactions in hydrothermal root zones and determine the concentrations and behaviours of these elements in altered oceanic crust, which is poorly known, but has important implications for global volatile (re)cycling. Amphiboles in each sample have average concentrations of 0.1-0.5 wt% Cl, 0.5-3 ppm Br and 5-68 ppb I. Amphibole has Br/Cl of ~0.0004 that is about ten times lower than coexisting fluid inclusions and seawater, and I/Cl of 2-44 × 10-6 that is 3-5 times lower than coexisting fluid inclusions but higher than seawater. The amphibole and fluid compositions are attributed to mixing halogens introduced by seawater with a large halogen component remobilised from mafic lithologies in the crust and fractionation of halogens between fluids and metamorphic amphibole formed at low water-rock ratios. The metamorphic amphibole and hydrothermal quartz are dominated by seawater-derived atmospheric Ne, Ar, Kr and Xe and mantle-derived He, with 3He/4He of ~9 R/Ra (Ra = atmospheric ratio). The amphibole and quartz preserve high 4He concentrations that are similar to MORB glasses and have noble gas abundance ratios with high 4He/36Ar and 22Ne/36Ar that are greater than seawater and air. These characteristics result from the high solubility of light noble gases in amphibole and suggest that all the noble gases can behave similarly to `excess 40Ar' in metamorphic hydrothermal root zones. All noble gases are therefore trapped in hydrous minerals to some extent and can be inefficiently lost during metamorphism implying that even the lightest noble gases (He and Ne) can potentially be subducted into the Earth's mantle.

  19. Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

    NASA Technical Reports Server (NTRS)

    Takeda, Hiroshi; Bogard, Donald D.; Mittlefehldt, David W.; Garrison, Daniel H.

    2000-01-01

    We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures

  20. 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

  1. Zircon U-Pb age and Sr-Nd-Hf isotope geochemistry of Permian granodiorite and associated gabbro in the Songliao Block, NE China and implications for growth of juvenile crust

    NASA Astrophysics Data System (ADS)

    Liu, Shen; Hu, Ruizhong; Gao, Shan; Feng, Caixia; Feng, Guangying; Coulson, Ian M.; Li, Cai; Wang, Tao; Qi, Youqiang

    2010-02-01

    Post-orogenic granitic and associated mafic rocks from northeastern (NE) China consist of granodiorite and gabbro intrusions. We report here upon new U-Pb zircon ages, geochemical data and Sr-Nd-Hf isotopic data for these rocks. LA-ICP-MS U-Pb zircon analysis yields an age of 262.8 ± 1.0 Ma for the granitic rocks, and a uniform age of 262.1 ± 0.7 Ma for the gabbro. Most of the granitic and mafic rocks are characterised by low K 2O + Na 2O, and pertain to the subalkaline series in the total alkali-silica diagram. The granodiorite samples show low ( 87Sr/ 86Sr) i ranging from 0.700 to 0.705, positive ɛNd( t) values from +0.3 to +0.8, and large variation in ɛHf( t) values of between - 4.0 and +2.5, indicating that both newly underplated basalt (70-80%) and ancient lower crustal sources (20-30%) contributed to their origin. Furthermore, positive ɛHf( t) values with two-stage model ages ( TDM2) of 1123-1260 Ma, together with Nd model ages (960-1000 Ma), suggest an important episode of crustal growth during the Meso-Neoproterozoic beneath the Songliao Block. In contrast, the investigated gabbro is characterised by relatively high ( 87Sr/ 86Sr) i ratios (0.707-0.708), negative ɛNd( t) (- 5.9 to - 5.3) and ɛHf( t) values (- 5.0 to - 2.3), implying that this was derived from an enriched mantle source. The geochemical data indicate that the granitic magmas underwent separation of clinopyroxene, hornblende, K-feldspar, plagioclase, Ti-bearing phases (e.g., rutile, ilmenite, titanite), apatite and zircon during their evolution. Whereas the gabbro is characterised by low MgO (2.92-3.92 wt.%), Mg# (35-41) and compatible elements content, such as Cr (10-68 ppm), Co (16-31 ppm) and Ni (5.7-33 ppm), features of a more evolved mafic magma. There is no evidence that the granitic and mafic rocks were affected by crustal contamination during emplacement. Our interpretation is that the two coeval intrusive suites were both formed in a post-orogenic extensional setting, related to

  2. Crystallization ages of the A-type magmatism of the Graciosa Province (Southern Brazil): Constraints from zircon U-Pb (ID-TIMS) dating of coeval K-rich gabbro-dioritic rocks

    NASA Astrophysics Data System (ADS)

    Vlach, Silvio R. F.; Siga, Oswaldo; Harara, Ossama M. M.; Gualda, Guilherme A. R.; Basei, Miguel A. S.; Vilalva, Frederico C. J.

    2011-12-01

    Zircon U-Pb (ID-TIMS) ages for gabbro-dioritic rocks and for a monzogranite from the Graciosa Province of A-type granites and syenites, southern Brazil, are presented. Two gabbro-dioritic samples gave concordant ages (580 ± 2, 583 ± 3 Ma), while less precise upper intercept ages were obtained for another one (584 ± 8 Ma) and the monzogranite (585 ± 12 Ma). The best results indicate crystallization ages of the basic-intermediate magmas around 580-583 Ma. Micro-structural evidences and structural relationships between the mafic-intermediate rocks and the much more abundant granites and syenites demonstrate that magmas mingled and partially mixed with one another during emplacement and crystallization. Therefore, the results are representative of the overall magmatism. The extensional A-type magmatism occurred ca. 10-30 Ma after (1) emplacement and crystallization of the high-K calc-alkaline syn- to late-collisional batholiths, (2) peak regional metamorphism, and (3) final amalgamation of the Luis Alves, Curitiba, and Paranaguá terrains. It is suggested that the whole magmatism occurred in a short time interval and there is no evidence of spatial or temporal migration of the magmatic foci. ID-TIMS zircon dating of coeval basic-intermediate rocks may yield better emplacement and crystallization ages than direct dating of granitic and syenitic rocks, which are more susceptible to inheritance and/or late- to post-magmatic disturbances.

  3. Protótipo do primeiro interferômetro brasileiro - BDA

    NASA Astrophysics Data System (ADS)

    Cecatto, J. R.; Fernandes, F. C. R.; Neri, J. A. C. F.; Bethi, N.; Felipini, N. S.; Madsen, F. R. H.; Andrade, M. C.; Soares, A. C.; Alonso, E. M. B., Sawant, H. S.

    2004-04-01

    A interferometria é uma poderosa ferramenta usada para investigar estruturas espaciais de fontes astrofísicas fornecendo uma riqueza de detalhes inatingível pelas técnicas convencionais de imageamento. Em particular, a interferometria com ondas de rádio abre o horizonte de conhecimento do Universo nesta ampla banda do espectro eletromagnético, que vai de cerca de 20 kHz até centenas de GHz já próximo ao infravermelho, e que está acessível a partir de instrumentos instalados em solo. Neste trabalho, apresentamos o interferômetro designado por Arranjo Decimétrico Brasileiro (BDA). Trata-se do primeiro interferômetro a ser desenvolvido no Brasil e América Latina que já está em operação na fase de protótipo. Apresentamos o desenvolvimento realizado até o momento, o sítio de instalação do instrumento, o protótipo e os principais resultados dos testes de sua operação, as perspectivas futuras e a ciência a ser desenvolvida com o instrumento nas fases II e III. Neste trabalho é dada ênfase ao desenvolvimento, testes de operação e principais resultados do protótipo. É discutida brevemente a ciência que pode ser feita com o instrumento. Tanto os detalhes técnicos quanto os principais parâmetros estimados para o instrumento nas próximas fases de desenvolvimento e o desempenho do protótipo serão publicados em breve.

  4. Timing, petrogenesis and tectonic setting of the Late Paleozoic gabbro-granodiorite-granite intrusions in the Shalazhashan of northern Alxa: Constraints on the southernmost boundary of the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Shi, Xingjun; Wang, Tao; Zhang, Lei; Castro, Antonio; Xiao, XuChang; Tong, Ying; Zhang, Jianjun; Guo, Lei; Yang, Qidi

    2014-11-01

    The Late Paleozoic tectonic setting and location of the southernmost boundary of the Central Asian Orogenic Belt (CAOB) with respect to the Alxa Block or Alxa-North China Craton (ANCC) are debated. This paper presents new geochronological, petrological, geochemical and zircon Hf isotopic data of the Late Paleozoic intrusions from the Shalazhashan in northern Alxa and discusses the tectonic setting and boundary between the CAOB and ANCC. Using zircon U-Pb dating, intrusions can be broadly grouped as Late Carboniferous granodiorites (~ 301 Ma), Middle Permian gabbros (~ 264 Ma) and granites (~ 266 Ma) and Late Permian granodiorites, monzogranites and quartz monzodiorites (254-250 Ma). The Late Carboniferous granodiorites are slightly peraluminous and calcic. The remarkably high zircon Hf isotopes (εHf(t) = + 6-+ 10) and characteristics of high silica adakites suggest that these granodiorites were mainly derived from "hot" basaltic slab-melts of the subducted oceanic crust. The Middle Permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle. The Middle Permian granites are slightly peraluminous with high-K calc-alkaline and low εHf(t) values from - 0.9 to + 2.9. These granites were most likely derived from juvenile materials mixed with old crustal materials. The Late Permian granodiorites, monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous, with variable Peacock alkali-lime index values from calc-alkalic to alkali-calcic. These rocks were mainly derived from juvenile crustal materials, as evidenced by their high εHf(t) values (+ 3.3 to + 8.9). The juvenile sources of the above intrusions in the Shalazhashan are similar to those of the granitoids from the CAOB but distinct from the granitoids within the Alxa Block. These findings suggest that the Shalazhashan Zone belongs to the CAOB rather than the Alxa Block and that its boundary with the Alxa block can be

  5. Reaction of seawater with fresh mid-ocean ridge gabbro creates ';atypical' REE pattern and high REE fluid fluxes: Experiments at 425 and 475 °C, 400 and 1000 bar

    NASA Astrophysics Data System (ADS)

    Beermann, O.; Garbe-Schönberg, D.; Holzheid, A. D.

    2013-12-01

    High-temperature MOR hydrothermalism significantly affects ocean chemistry. The Sisters Peak (SP) hydrothermal field at 5°S on the slow-spreading Mid-Atlantic Ridge (MAR) emanates fluids >400°C [1] that have high concentrations of H2, transition metals, and rare earth elements (REE) exhibiting ';atypical' REE pattern characterized by depletions of LREE and HREE relative to MREE and no Eu anomaly [2]. This is in contrast to the ';typical' LREE enrichment and strong positive Eu anomaly known from many MOR vent fluids observed world-wide [e.g., 3]. Besides temperature, the seawater-to-rock ratio (w/r ratio) has significant control on the fluid chemistry [e.g., 4, 5]. To understand how vent fluid REE-signatures are generated during water-rock interaction processes we reacted unaltered gabbro with natural bottom seawater at 425 °C and 400 bar and at 425 and 475 °C at 1000 bar at variable w/r (mass) ratios ranging from 0.5-10 by using cold seal pressure vessels (CSPV). The run durations varied from 3-72 h. Reacted fluids were analysed for major and trace elements by ICP-OES and ICP-MS. In our experiments, ';atypical' REE fluid pattern similar to those of SP fluids were obtained at high w/r ratio (5 and 10) that might be characteristic for focused fluid-flow along e.g., detachment faults at slow-spreading MOR [6]. In contrast, more ';typical'-like REE pattern with elevated LREE and slightly positive Eu anomalies have been reproduced at low w/r ratio (0.5-1). Results of numerical simulations imply that strong positive Eu anomalies of fluids and altered gabbro from high temperature MOR hydrothermal systems can be created by intense rock leaching processes at high w/r ratio (5-10). This suggests that hydrothermal circulation through the ocean crust creates ';typical' REE fluid pattern with strong positive Eu anomalies if seawater reacts with gabbroic host rock that has been already leached in REE at high fluid fluxes. Simulations of the temporal chemical evolution of

  6. Magmatic relationships between depleted mantle harzburgites, boninitic cumulate gabbros and subduction-related tholeiitic basalts in the Puerto Plata ophiolitic complex, Dominican Republic: Implications for the birth of the Caribbean island-arc

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Castillo-Carrión, Mercedes; Pérez-Estaún, Andrés

    2014-05-01

    The Lower Cretaceous Puerto Plata ophiolitic complex (PPC) occurs west of the main collisional suture between the Caribbean and North American plates in the northern Dominican Republic, and imposes important constraints on the geochemical and tectonic processes associated with the birth of the Caribbean island-arc. The PPC exposes a tectonically dismembered 3.0-km-thick section of upper mantle harzburgites, lower crustal cumulate gabbroic rocks and upper crustal basaltic pillow lavas, volcanic breccias and pelagic sediments. The harzburgites exhibit a highly depleted signature in terms of their modal compositions, mineral chemistry and whole rock major and trace element contents, suggesting that they are residues after high-degrees of partial melting. Melt modeling suggests that they were similar in trace element characteristics to a boninite. In the crustal sequence, three magmatic episodes have been recognized based on field, mineral and geochemical data. The first phase is composed of the lower layered gabbronorites, which are variably deformed and recrystallized at high-temperature conditions. Trace element modeling suggests that the gabbronorites crystallized from LREE-depleted island-arc tholeiitic (IAT) melts. The second phase is composed of the intermediate layered troctolites (126 Ma), which are undeformed and preserve igneous cumulate textures. Modeling indicates that the troctolites crystallized from boninitic melts. The gabronorite-troctolite substrate was intruded by a third, supra-subduction zone tholeiitic magmatic phase at < 126 Ma, which formed the upper olivine gabbros and gabbronorites. These gabbroic rocks formed from melts similar in composition to the IAT basalts and basaltic andesites of the overlying Los Caños Fm. Contemporaneous Aptian to lower Albian mafic volcanic rocks of the Los Ranchos Fm and Cacheal complex have comparable IAT geochemical and isotopic signatures, suggesting that all of them may have erupted over a single piece of the

  7. Alteration and fluid flow recorded in a 1400 m section of oceanic gabbros beneath a detachment fault at 30 oN mid-Atlantic Ridge, (IODP Site U1309)

    NASA Astrophysics Data System (ADS)

    McCaig, A.; Frost, B. R.; Beard, J. S.; Abratis, M.; Andreani, M.; Delacour, A.; Drouin, M.; Fryer, P.; Ozaka, T.; Ohara, Y.

    2005-12-01

    IODP Hole U1309D was drilled north of the summit of the Atlantis inside corner high at 30 oN during Expedition 304/305. The hole is located on a corrugated surface assumed to be a low angle detachment fault. Small samples of talc-tremolite-chlorite schist recovered at the top of the hole are believed to represent fault rock associated with slip on the detachment fault system. 1400m of gabbro with subordinate diabase, troctolite and ultramafic cumulates were penetrated with over 75% recovery. A superb alteration profile is preserved in the core, with pervasive alteration above 380mbsf overprinted by localised metasomatic alteration around veins and breccia zones. Below 380mbsf, most alteration is localised around veins, faults, igneous contacts and late leucocratic dikelets, and may involve magmatic fluids in some cases. A down-temperature sequence of alteration is seen comprising: 1) Narrow granulite-facies shear zones containing Ti-amphiboles. 2) Widespread but minor amphibolite facies hornblende growth. 3) Low amphibolite to upper greenschist facies coronitic reactions between olivine and plagioclase, 4) Coupled serpentinization of olivine and replacement of plagioclase by prehnite and hydrogarnet, 5) Fracture controlled zeolite and clays. The coronitic reaction went to completion in olivine-bearing rocks above 330 mbsf, and the serpentine-prehnite-hydrogarnet assemblage is only common below this depth, indicating continued downward penetration of water during cooling. Zeolite-facies metamorphism is occurring today at the base of the hole, where temperatures exceed 120 oC. Permeability in the core was generated both by thermal and tectonic cracking, and by metamorphic reactions. In particular, volume increase in the coronitic reaction is manifested by radial chlorite-filled cracks in plagioclase surrounding pseudomorphs after olivine. In diabase, complex zoning in amphiboles after pyroxene suggests dissolution and precipitation with generation of porosity. In a

  8. Primitive layered gabbros from fast-spreading lower oceanic crust.

    PubMed

    Gillis, Kathryn M; Snow, Jonathan E; Klaus, Adam; Abe, Natsue; Adrião, Alden B; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J; Faak, Kathrin; Falloon, Trevor J; Friedman, Sarah A; Godard, Marguerite; Guerin, Gilles; Harigane, Yumiko; Horst, Andrew J; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M; John, Barbara E; Koepke, Juergen; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E; McCaig, Andrew M; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P

    2014-01-01

    Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt. PMID:24291793

  9. Aquisição fonológica do português brasileiro por crianças ouvintes bilíngues bimodais e surdas usuárias de implante coclear

    PubMed Central

    Cruz, Carina Rebello; Finger, Ingrid

    2014-01-01

    Resumo O presente estudo investiga a aquisição fonológica do Português Brasileiro (PB) por 24 crianças ouvintes bilíngues bimodais, com acesso irrestrito à Língua Brasileira de Sinais (Libras), e por 6 crianças surdas que utilizam implante coclear (IC), com acesso restrito ou irrestrito à Libras. Para a avaliação do sistema fonológico das crianças em PB, foi utilizada a Parte A, Prova de Nomeação, do ABFW – Teste de Linguagem Infantil (ANDRADE et al. 2004). Os resultados revelaram que as crianças ouvintes bilíngues bimodais e a criança surda usuária de IC com acesso irrestrito à Libras apresentaram processo de aquisição fonológica esperada (normal) para a sua faixa etária. Considera-se que a aquisição precoce e o acesso irrestrito à Libras podem ter sido determinantes para o desempenho dessas crianças no teste oral utilizado. PMID:25506105

  10. The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

    NASA Technical Reports Server (NTRS)

    Namiki, Noriyuki; Solomon, Sean C.

    1993-01-01

    The hypothesis is explored that the crust-mantle boundary of Venus is not in phase equilibrium but rather is rate-limited by the temperature-dependent volume diffusion of the slowest ionic species. The 1D thermal evolution problem is solved assuming that the mountains formed by uniform horizontal shortening of the crust and the lithospheric mantle at a constant rate. The time-dependent density structure and surface elevation are calculated by assuming a temperature-dependent reaction rate and local Airy isostatic compensation. For a horizontal strain rate of 10 exp -15/s or greater, the temperature increase at the base of the crust during mountain formation is modest to negligible, the deepening lower crust is metastable, and the surface elevation increases as the crust thickens. For strain rates less than 10 exp -16/s, crustal temperature increases with time because of internal heat production and the lower crust is more readily transformed to the dense eclogite assemblage. For such models, a maximum elevation is reached during crustal shortening.

  11. Petrology and crystal chemistry of poikilitic anorthositic gabbro 77017. [lunar rocks

    NASA Technical Reports Server (NTRS)

    Mccallum, I. S.; Mathez, E. A.; Okamura, F. P.; Ghose, S.

    1974-01-01

    Aspects of mineralogy are considered, taking into account the occurrence and the characteristics of plagioclase, pyroxene, and olivine. Attention is also given to oxides, opaque minerals, and glass components. Questions regarding the temperature of formation and the origin of the considered lunar poikilitic rocks are discussed. It is pointed out that the presented hypothesis may not be applicable to other poikilitic lunar rocks.

  12. Chronology and isotopic geochemistry of apollo 14 basalts and Skaergard Gabbro, Eastern Greenland

    NASA Technical Reports Server (NTRS)

    Dasch, E. J.

    1986-01-01

    Work completed on Apollo 14 basalts has been published. The two dates obtained from these rocks comprised the oldest and two of the three oldest ages (4.1 and 4.3 billion years) known for lunar maria basalts; thus their ages are important in understanding the moon's earliest history. Owing to the antiquity of these rocks, two more fragments have been dated as part of a second ASEE/NASA SFF program. The new ages are 3.95 and 4.12 billion years, thus further establishing and amplifying the earlier results. This work, although perhaps more interesting for its chronologic information, was begun as a test of chemical and petrographic models. Fragments of Apollo basalt were placed into five categories, based on petrologic and chemical, especially rare-earth element, composition. Isotopic studies were begun in an attempt to determine if the five groups of basalts were related by age or initial isotopic composition (isotopic composition of lava at time of extrusion). Although a few of the representatives of the five groups have the same age and/or initial strontium-isotopic composition, within the analtytical uncertainties, most apparently are unrelated. Petrologic implications of these data will be published in an appropriate journal.

  13. A view into the Cretaceous geomagnetic field from analysis of gabbros and submarine glasses

    NASA Astrophysics Data System (ADS)

    Granot, Roi; Tauxe, Lisa; Gee, Jeffrey S.; Ron, Hagai

    2007-04-01

    The nature of the geomagnetic field during the Cretaceous normal polarity superchron (CNS) has been a matter of debate for several decades. Numerical geodynamo simulations predict higher intensities, but comparable variability, during times of few reversals than times with frequent reversals. Published geomagnetic paleointensity data from the CNS are highly scattered suggesting that additional studies are required. Here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the Troodos ophiolite, Cyprus (92.1 Ma old). Together with recently published data from the Troodos upper crust we obtain three independent paleointensity time-series. These sequences reveal quasi-cyclic variations of intensities about a mean value of 54 ± 20 Z Am 2, providing insight into the fluctuating nature of the Cretaceous magnetic field. Our data suggest the CNS field was both weaker and more variable than predicted by geodynamo simulations. The large amplitudes of these variations may explain the wide range of dipole moments previously determined from the CNS.

  14. Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

    NASA Astrophysics Data System (ADS)

    Shu, Qiao; Brey, Gerhard P.; Hoefer, Heidi E.; Zhao, Zhidan; Pearson, D. Graham

    2016-02-01

    An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and δ 18O values lower than the Earth's mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4-6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.

  15. Rare earth abundances and Rb-Sr systematics of basalts, gabbro, anorthosite and minor granitic rocks from the Indian Ocean Ridge System, Western Indian Ocean

    USGS Publications Warehouse

    Hedge, C.E.; Futa, K.; Engel, C.G.; Fisher, R.L.

    1979-01-01

    Basalts dredged from the Mid-Indian Ocean Ridge System have rare earth, Rb, and Sr concentrations like those from other mid-ocean ridges, but have slightly higher Sr87/Sr86 ratios. Underlying gabbroic complexes are similar to the basalts in Sr87/Sr86, but are poorer K, Rb, and in rare earths. The chemical and isotopic data, as well as the geologic relations suggest a cumulate origin for the bulk of the gabbroic complexes. ?? 1979 Springer-Verlag.

  16. O Romantismo Brasileiro: Leitura e Transformacao Social (Brazilian Romanticism: Reading and Social Transformation).

    ERIC Educational Resources Information Center

    Augusti, Valeria

    1997-01-01

    Considers the set of representations that configure the images of the literature of Brazilian Romanticism. Notes the capacity of this literature to transform both values and social patterns of behavior. (PA)

  17. Age constraints on felsic intrusions, metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil

    USGS Publications Warehouse

    Mello, E.F.; Xavier, R.P.; McNaughton, N.J.; Hagemann, S.G.; Fletcher, I.; Snee, L.

    2006-01-01

    U-Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152??6 Ma in monazite crystals and 2,155??9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130??7 Ma and 2,128??8 Ma in zircon crystals derived from the Teofila??ndia tonalite. The emplacement age of the syntectonic Ambro??sio dome as indicated by a 2,080??2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar-Ar plateau age of 2,080??5 Ma obtained in hornblendes from an amphibolite and with a U-Pb SHRIMP age of 2,076??10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambro??sio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937??16, 3,111??13 and 3,162??13 Ma. Ar-Ar plateau ages of 2,050??4 Ma and 2,054??2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar-Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen. ?? Springer-Verlag 2006.

  18. Um satélite brasileiro para observação do diâmetro solar

    NASA Astrophysics Data System (ADS)

    Emilio, M.; Leister, N. V.; Benevides Soares, P.; Teixeira, R.; Kuhn, J.

    2003-08-01

    Propomos uma missão espacial para medir a forma e o diâmetro solar com o objetivo de ajudar a determinar o potencial gravitacional do Sol e a sua rotação com precisão, testar modelos teóricos de variação de energia e pela primeira vez medir os modos g de oscilação. As observações serão obtidas através do instrumento denominado APT (Astrometric and Photometric Telescope) descrito por Kuhn(1983). A sensibilidade do instrumento é de 0,2 mas em 27 dias para as observações do diâmetro solar feitas a cada minuto. Esta é uma missão de três anos de duração e pode complementar as medidas que serão feitas pelo satélite PICARD (a ser lançado em 2007). Outros parâmetros físicos podem ser obtidos com as mesmas imagens o que certamente interessará à comunidade de física solar. Um primeiro contato foi realizado com a agência espacial brasileira que pretende lançar um satélite científico a cada dois anos.

  19. Maskelynite: Formation by explosive shock

    USGS Publications Warehouse

    Milton, D.J.; De Carli, P. S.

    1963-01-01

    When high pressure (250 to 300 kilobars) was applied suddenly (shock-loading) to gabbro, the plagioclase was transformed to a noncrystalline phase (maskelynite) by a solid-state reaction at a low temperature, while the proxene remained crystalline. The shock-loaded gabbro resembles meteorites of the shergottite class; this suggests that the latter formed as a result of shock. The shock-loading of gabbro at 600 to 800 kilobars raised the temperature above the melting range of the plagioclase.

  20. Graphitization of Organic Material in a Progressively Metamorphosed Precambrian Iron Formation.

    PubMed

    French, B M

    1964-11-13

    Organic matter in the sedimentary Biwabik iron formation in northern Minnesota shows a progressive increase in crystallinity where the formation is metamorphosed by the intrusive Duluth gabbro complex. X-ray diffraction of acid-insoluble residues shows that there is a complete range in crystallinity, from amorphous material in the unmetamorphosed sediments to completely crystalline graphite adjacent to the gabbro. PMID:17777057

  1. Platinum potential of mafic-ultramafic massifs in the western part of the Dambuka ore district (Upper Amur Region, Russia)

    NASA Astrophysics Data System (ADS)

    Melnikov, A. V.; Stepanov, V. A.; Moiseenko, V. G.

    2016-02-01

    New data on the Pt potential of mafic-ultramafic massifs of the Khani-Maya, Uldegit, and Dzhalta complexes in the western part of the Dambuka ore district are discussed. The Khani-Maya Complex is represented by metamorphosed gabbro, gabbronorites, gabbro anorthosites, subordinate pyroxenites, hornblendites, and peridotites. The Uldegit Complex is composed of pyroxenites, hornblendites, gabbro, gabbronorites, norites, troctolites, peridotites, dunites, actinolite-tremolites, serpentinites, anthophyllites, and tremolite-plagioclase rocks. The Dzhalta Complex is formed of peridotites, gabbro, eclogitized gabbro, hornblendites, cortlandites, and pyroxenites. All these complexes differ from each other by the concentrations of Ni, Cu, Co, Au, and platinoids depending on the composition of the constituting rocks and the presence of sulfide minerals.

  2. Sr isotopic tracer study of the Samail ophiolite, Oman

    SciTech Connect

    Lanphere, M.A.; Coleman, R.G.; Hopson, C.A.

    1981-04-10

    We have measured Rb and Sr concentrations and Sr isotopic compositions in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite diabase dikes, and gabbro and websterite dikes within the metamorphic peridotite. Ten samples of cummulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean /sup 87/Sr//sup 86/Sr ratios and standard deviations of 0.70314 +- 0.00030 and 0.70306 +- 0.00034, respectively. The dispersion in Sr isotopic composition may reflect real heterogeneities in the magma source region. The average Sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt. The /sup 87/Sr//sup 86/Sr ratios of noncumulate gabbro, plagiogranite, and diabase dikes range from 0.7034 to 0.7047, 0.7038 to 0.7046, and 0.7037 to 0.7061, respectively. These higher /sup 87/Sr//sup 86/Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater. Mineral separates from dikes that cut harzburgite tectonite have Sr isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.

  3. Mechanical Properties of Shock-Damaged Rocks

    NASA Technical Reports Server (NTRS)

    He, Hongliang; Ahrens, T. J.

    1994-01-01

    Stress-strain tests were performed both on shock-damaged gabbro and limestone. The effective Young's modulus decreases with increasing initial damage parameter value, and an apparent work-softening process occurs prior to failure. To further characterize shock-induced microcracks, the longitudinal elastic wave velocity behavior of shock-damaged gabbro in the direction of compression up to failure was measured using an acoustic transmission technique under uniaxial loading. A dramatic increase in velocity was observed for the static compressive stress range of 0-50 MPa. Above that stress range, the velocity behavior of lightly damaged (D(sub 0) less than 0.1) gabbro is almost equal to unshocked gabbro. The failure strength of heavily-damaged (D(sub 0) greater than 0.1) gabbro is approx. 100-150 MPa, much lower than that of lightly damaged and unshocked gabbros (approx. 230-260 MPa). Following Nur's theory, the crack shape distribution was analyzed. The shock-induced cracks in gabbro appear to be largely thin penny-shaped cracks with c/a values below 5 x 10(exp -4). Moreover, the applicability of Ashby and Sammis's theory relating failure strength and damage parameter of shock-damaged rocks was examined and was found to yield a good estimate of the relation of shock-induced deficit in elastic modulus with the deficit in compressive strength.

  4. Sr isotopic tracer study of the Samail ophiolite, Oman.

    USGS Publications Warehouse

    Lanphere, M.A.; Coleman, R.G.; Hopson, C.A.

    1981-01-01

    Rb and Sr concentrations and Sr-isotopic compositions were measured in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dykes, and gabbro and websterite dykes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have 87Sr/86Sr ratios of 0.70314 + or - 0.00030 and 0.70306 + or - 0.00034, respectively. The dispersion in Sr- isotopic composition may reflect real heterogeneities in the magma source region. The average Sr-isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern MORB. The 87Sr/86Sr ratios of noncumulate gabbro, plagiogranite, and diabase dykes range 0.7034-0.7047, 0.7038-0.7046 and 0.7037- 0.7061, respectively. These higher 87Sr/86Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with sea-water. Mineral separates from dykes that cut harzburgite tectonite have Sr-isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dykes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.-T.R.

  5. Dating the growth of oceanic crust at a slow-spreading ridge.

    PubMed

    Schwartz, Joshua J; John, Barbara E; Cheadle, Michael J; Miranda, Elena A; Grimes, Craig B; Wooden, Joseph L; Dick, Henry J B

    2005-10-28

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately 25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately 5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank. PMID:16254183

  6. Maskelynite: Formation by Explosive Shock.

    PubMed

    Milton, D J; de Carli, P S

    1963-05-10

    When high pressure (250 to 300 kilobars) was applied suddenly (shock-loading) to gabbro, the plagioclase was transformed to a noncrystalline phase (maskelynite) by a solid-state reaction at a low temperature, while the proxene remained crystalline. The shock-loaded gabbro resembles meteorites of the shergottite class; this suggests that the latter formed as a result of shock. The shock-loading of gabbro at 600 to 800 kilobars raised the temperature above the melting range of the plagioclase. PMID:17737107

  7. Dating the growth of oceanic crust at a slow-spreading ridge

    USGS Publications Warehouse

    Schwartz, J.J.; John, Barbara E.; Cheadle, Michael J.; Miranda, E.A.; Grimes, Craig B.; Wooden, J.L.; Dick, H.J.B.

    2005-01-01

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ???25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ???5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

  8. The Archean Dongwanzi ophiolite complex, North China craton: 2.505-billion-year-old oceanic crust and mantle.

    PubMed

    Kusky, T M; Li, J H; Tucker, R D

    2001-05-11

    We report a thick, laterally extensive 2505 +/- 2.2-million-year-old (uranium-lead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramafic rocks, a mafic-ultramafic transition zone of interlayered gabbro and ultramafic cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dike-pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting. PMID:11349144

  9. Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)

    NASA Astrophysics Data System (ADS)

    Leuthold, J.; Blundy, J. D.; Holness, M. B.; Sides, R.

    2014-07-01

    We present a detailed microstructural and geochemical study of reactive liquid flow in Unit 9 of the Rum Eastern Layered Intrusion, Scotland. Unit 9 comprises an underlying lens-like body of peridotite overlain by a sequence of troctolite and gabbro (termed allivalite), with some local and minor anorthosite. The troctolite is separated from the overlying gabbro by a distinct, sub-horizontal, undulose horizon (the `major wavy horizon'). Higher in the stratigraphy is another, similar, horizon (the `minor wavy horizon') that separates relatively clinopyroxene-poor gabbro from an overlying gabbro. To the north of the peridotite lens, both troctolite and gabbro grade into poikilitic gabbro. Clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro. The poikilitic gabbros contain multiple generations of clinopyroxene, with Cr-rich (~1.1 wt% Cr2O3) anhedral cores with moderate REE concentrations ( core1) overgrown by an anhedral REE-depleted second generation with moderate Cr (~0.7 wt% Cr2O3) ( core2). These composite cores are rimmed by Cr-poor (~0.2 wt% Cr2O3) and REE-poor to -moderate clinopyroxene. We interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine-phyric picrite to form the discontinuous lenses that comprise the Unit 9 peridotite. Loss of clinopyroxene-saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene-poor gabbro. The spatial extent of clinopyroxene loss is marked by the minor wavy horizon. A second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite, with the major wavy horizon marking the extent of melting during this episode. The poikilitic gabbro formed from clinopyroxene-saturated melt moving upwards and

  10. Petrology of gabbroic xenoliths in 1960 Kilauea basalt: crystalline remnants of prior (1955) magmatism

    USGS Publications Warehouse

    Fodor, R.V.; Moore, R.B.

    1994-01-01

    The 1960 Kapoho lavas of Kilauea's east rift zone contain 1-10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl??ol??opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo82.5, cpx Mg# 86.5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall 'open' textures to suggest that they represent 'mush' and 'crust' of a magma crystallization environment. Their phase compositions are more evolved (Fo80-70, cpx Mg# 82-75, and An73-63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7-5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal-liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ???8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (???5 wt% MgO) through ???28-34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (???6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. ?? 1994 Springer-Verlag.

  11. Luna 24 - Opaque mineral chemistry of gabbroic and basaltic fragments from Mare Crisium

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.

    1977-01-01

    Spinels and ilmenites are relatively sparse in the Luna 24 gabbro and basalts. Spinel compositions show some affinities to those of spinels in Apollo 12, Apollo 14 and Luna 16 basalts; a characteristic feature is high Al2O3, reaching a maximum of 19.8 wt%. A comparison of spinels in the Luna 24 gabbro with those in other deep-seated lunar intrusive rocks shows a characteristic trend for Fe/Mg. This trend is systematic from gabbro to anorthosite to troctolite and is interpreted to be P-T dependent. Compositions of spinels in the gabbro fall within the Cr/Al trend defined by the spinels of the basalts, but form a Fe/Mg trend parallel to that of the basalts; this relationship suggests that both the gabbro and the basalts are derived from a closely similar source region, with the basalts originating at a slightly greater depth than the gabbro. The spinels in both rock types are considered to have formed at high crustal levels, at low pressures. The Luna 24 data suggest that the compositional discontinuities which exist between chromian spinels and titanian spinels in a large proportion of mare basalts are the result of nucleation of chromian spinels at high crustal levels prior to eruption, and of titanian spinels during melt crystallization at the lunar surface.

  12. Metamorphism in oceanic layer 3, Gorringe Bank, eastern Atlantic

    NASA Astrophysics Data System (ADS)

    Mevel, Catherine

    1988-12-01

    Gorringe Bank is an anomalously high structure of the eastern part of the north Atlantic, which was known to be composed of mantle-derived peridotites (layer 4) and gabbros (layer 3). During the submersible cruise CYAGOR II in 1981, the contact between layer 4 and layer 3 was observed on Mount Gettysburg and interpreted as tectonic. The overlying series of gabbro was extensively sampled on both mounts composing the bank, Gettysburg and Ormonde. Coarse-grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks (ferrogabbros and diorites). Cumulate gabbros are missing. The gabbroic section sampled is therefore interpreted as the upper part of the plutonic section. Most samples were strongly recrystallized during two distinct events. Metamorphism occurred close to the ridge axis, from interaction of a seawater-derived fluid with still hot gabbros. High temperature shear zones favoured fluid circulation, but the water/rock ratio — estimated from the sodium input — was very small in undeformed rocks (<1). The low W/R ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals. Low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom.

  13. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-12-01

    The debate over the processes of igneous accretion of gabbroic lower crust at submarine spreading centers is centered on two end-member hypotheses: Gabbro Glaciers and Sheeted Sills. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present newly published data (VanTongeren et al., 2015 EPSL v. 427, p. 249-261) on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite, Oman. Based on our results we provide five critical observations that any model for the accretion of the lower oceanic crust must satisfy: (1) There is a distinctive change in the orientation of the outcrop-scale layering from near-vertical to sub-horizontal that is also reflected in the plagioclase fabrics in the uppermost ~1000-1500 m of the gabbroic crust; (2) The distinction between the upper gabbros and lower gabbros is not a geochemical boundary. Rather, the change in outcrop-scale orientation from near-vertical to sub-horizontal occurs stratigraphically lower in the crust than a change in whole-rock geochemistry; (3) There is no systematic difference in plagioclase fabric strength in any crystallographic axis between the upper gabbros and the lower gabbros; (4) Beneath the abrupt transition from sub-vertical to sub-horizontal fabric, there is no systematic change in the geographic orientation of the plagioclase fabric, or in the development of a dominant lineation direction within the upper gabbros or the lower gabbros; (5) In the lower gabbros, the obliquity between the (010) and the modal layering remains approximately constant and indicates a consistent top to the right sense of shear throughout the stratigraphy. Our observations are most consistent with the Sheeted Sills hypothesis, in which the majority of lower crustal gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with

  14. The origin of rhythmic layering in the Cape Neddick Plutonic Complex, Maine

    SciTech Connect

    Shipley, J. . Dept. of Geology)

    1993-03-01

    The Cape Neddick Plutonic Complex located along the southwestern coast of Maine is a small layered gabbroic body. It contains four concentric gabbros of differing composition, From the center to the exterior these are Cortlandtitic, Anorthositic, Normal, and Pegmatitic gabbros. They vary slightly in the proportion of essential minerals; plagioclase, clinopyroxene, hornblende, and biotite [+-] opaques and olivine. Rhythmic layering is seen in all four gabbros. This layering is also concentric around the center of the complex. The best exposure of the layering is along the coast where non-graded and graded rhythmic layering is seen extensively in the Normal gabbro. Only non-graded layering is seen in the Anorthositic gabbro. Non-graded layers were sampled at two localities. One locality is on Cape Nubble Island in the Normal gabbro. The second locality is in the Anorthositic gabbro along the northern coast. The layers are roughly 5--8 cm in width and are continuous around the complex. They are identified on weathered surfaces as alternating bands of felsic and mafic minerals. Layers are not obvious in hand sample. Approximately 15--20 layers were sampled perpendicular to layering. Petrographic and geochemical studies will help constrain the origin of rhythmic layering in the Cape Neddick Complex. Crystal settling or structural processes seem highly unlikely due to the small distance between the layers and the fact that they are not graded. Possible models include multiple pulses of magma within the same magma chamber, density currents, or in situ fractionation by a nucleation-diffusion process. Modal and chemical analysis of mineral phases within individual layers will allow comparison of the bulk composition of each layer. Analysis of coexisting pyroxene and plagioclase can be used to estimate compositional variations in the parent liquid.

  15. Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks

    NASA Astrophysics Data System (ADS)

    Ji, Shaocheng; Shao, Tongbin; Salisbury, Matthew H.; Sun, Shengsi; Michibayashi, Katsuyoshi; Zhao, Weihua; Long, Changxing; Liang, Fenghua; Satsukawa, Takako

    2014-11-01

    Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing >80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: Vp(Z)/Vp(Y) ≥ 1 and Vp(Z)/Vp(X) ≥ 1 for anorthosites while 0.8 < Vp(Z)/Vp(Y) ≤ 1 and 0.8 < Vp(Z)/Vp(X) ≤ 1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of Vp(X) > Vp(Y) than the gabbros. Laminated anorthosites with Vp(X) ≈ Vp(Y) ≪ Vp(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by Vp(X) ≈ Vp(Y) > Vp(Z) and Vp(X) > Vp(Y) > Vp(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.

  16. Chemistry and petrology of Luna 24 lithic fragments and less than 250-micron soils - Constraints on the origin of VLT mare basalts

    NASA Technical Reports Server (NTRS)

    Ma, M.-S.; Schmitt, R. A.; Taylor, G. J.; Warner, R. D.; Lange, D. E.; Keil, K.

    1978-01-01

    Results are reported on a combined INAA-petrologic study of 17 small (0.2-1.5 mg) Luna 24 lithic and mineral fragments and INAA study of 5 bulk soils and mineral separates from gabbro 24170. Lithic and mineral fragments are classified into VLT mare basalts (ferrobasalt and metabasalts), low-Ti, variolitic mare basalt, gabbros, melt rock and soil breccia. Data indicate 5 possible magma types, represented by: (1) VLT ferrobasalt and gabbro fragments, with low-TiO2 (about 1%), slightly bow-shaped REE pattern, and low REE concentrations (5-10X chondritic); (2) a ferrobasalt (Laul et al., 1978) and metabasalt fragments with major and trace element contents similar to (1), but positive Eu anomalies; (3) one gabbro fragment with distinctive pyroxene compositional trend (increasing Ti with nearly constant Fe/Fe + Mg) and highest REE contents of any Luna 24 mare basaltic sample, (4) a gabbro fragment with considerably less V and Cr2O3 than ferrobasalt and metabasalt fragments; and (5) variolitic basalt fragment with higher Ti2(2.3%) than other Luna 24 basalts and pyroxene that has increasing then decreasing Ti with increasing Fe/Fe + Mg. Trace element data place constraints on the nature of the source region and possible parent magmas for the Luna 24 VLT ferrobasalt.

  17. Breccias 73215 and 73255 - Siderophile and volatile trace elements

    NASA Technical Reports Server (NTRS)

    Morgan, J. W.; Petrie, R. K.

    1979-01-01

    Fifteen siderophile and volatile trace elements (Os, Re, Ir, Pd, Ni, Au, Sb, Ge, Se, Ag, In, Zn, Cd, Bi, Tl) and U were determined by radiochemical neutron activation analysis in a spheroidal aphanitic clast and a clast of coarse-grained anorthositic gabbro from breccia 73215 and in three types of aphanite and two clasts of fine-grained anorthositic gabbro from breccia 73255. In common with most Apollo 17 fragment-laden melt rocks, the aphanites from 73215 and 73255 predominantly contain a Group 2 meteoritic component, which is apparently derived from the Serenitatis impact. All aphanitic lithologies contain the same meteoritic component, and are probably cogenetic. The clasts of fine-grained anorthositic gabbro contain substantial amounts (2% to 6% Cl equivalent based on Au) of a pre-Serenitatis Group 3 component. The clast of coarse-grained anorthositic gabbro is low in siderophile elements (0.4% Cl equivalent), and the meteoritic component (Group 5) is not well-defined. A strong correlation exists between Ir and Au in both the aphanites and the anorthositic gabbro clasts, which argues against the breccias 73215 and 73255 being open systems for Au

  18. Formation of composite dykes by contact remelting and magma mingling: The Shaluta pluton, Transbaikalia (Russia)

    NASA Astrophysics Data System (ADS)

    Litvinovsky, B. A.; Zanvilevich, A. N.; Katzir, Y.

    2012-10-01

    A unique opportunity to study the source areas, from which composite dykes were injected, occurs in the Shaluta pluton, Transbaikalia, Russia. The major quartz syenite pluton was intruded by several synplutonic gabbro bodies of various sizes. Investigations of the contact zones between gabbro and host syenite showed that liquid basalt magma intruded the incompletely crystallized coarse-grained quartz syenite with T = 700-720 °C and caused contact remelting of the silicic rock at about 900-950 °C. Mechanical interaction between newly formed silicic melt and partially crystallized mafic magma resulted in extensive magma mingling. Chemical interaction was exhibited by migration of MgO, CaO, FeO∗, Sr, H2O and Cl from the basalt magma, whereas silica, alkalis, Rb and Ba migrated from the silicic refusion zone into the crystallized gabbro. Presence of melt inclusions with homogenization temperature ranging from 640 to 790 °C in quartz and attaining 850-900 °C in late clinopyroxene indicates that at least part of newly formed minerals crystallized from the hybrid melt. Mingled magmatic material was squeezed out inwards, into the host solid quartz syenite pluton and formed dyke-like apophyses that can be traced for a distance of 60-70 m from the contact zone. Apophyses have the same dimensions, structure and composition as typical composite dykes that are common in the roof pendant over the gabbro bodies and nearby the gabbro exposures.

  19. Evolution of rodingites along stratigraphic depth in the Iti and Kallidromon ophiolites (Central Greece)

    NASA Astrophysics Data System (ADS)

    Tsikouras, Basilios; Karipi, Sofia; Hatzipanagiotou, Konstantin

    2013-08-01

    Rodingitised rocks were collected from the neighbouring Iti and Kallidromon ophiolites. They comprise metasomatic assemblages after serpentinised lherzolite and harzburgite, gabbro and dolerite dykes. The main mineral phases in the metasomatised mantle and gabbroic rocks include grossularitic garnet, chlorite and diopside whereas epidote group minerals were mainly developed at the expense of the dolerite dykes. The fluid phase involved in the metasomatic reactions was highly alkaline in the metasomatising peridotites and less alkaline in the altered gabbro and dolerite. Under such conditions, Ti is assumed to have remained immobile in the whole process while Zr remained constant in the stratigraphically upper rodingitised gabbro and dolerite. Transportation of rare earth elements, Zr, Cr and Ni from the altered mantle segment to the upper gabbro level was assisted by the presence of carbonate and hydroxyl ligands. After consequent breakdown of the carbonate complexes, these elements were deposited in the rodingitised gabbro and dolerite. Rare earths were mainly integrated in the neoblastic diopside whereas Cr and Ni likely formed insoluble hydroxides in that less reducing environment. The metasomatic event happened in the mantle wedge close to the subduction of the Pelagonian carbonates that strongly enriched the fluid phase in CO2. The entrance of an externally derived hydrothermal fluid was responsible for the reduction of pH at the higher levels and the calculated influx of Si in the system.

  20. Spatiotemporal Distribution and Population Structure of Monokalliapseudes schubarti (Tanaidacea: Kalliapseudidae) in an Estuary in Southern Brazil

    PubMed Central

    Freitas-Júnior, Felipe; Christoffersen, Martin Lindsey; de Araújo, Joafrâncio Pereira; Branco, Joaquim Olinto

    2013-01-01

    Monokalliapseudes schubarti is an endemic tanaidacean microcrustacean from southeastern Brazil to Uruguay inhabiting low energy estuaries. Saco da Fazenda is located in the estuary of the Itajaí-Açú River, state of Santa Catarina, Brazil. It is exposed to strong anthropic impact and receives intensive flows of domestic wastewater, solid residues, and drainage activities. Specimens of M. schubarti were collected monthly, in the intertidal and subtidal regions of Saco da Fazenda, in four stations defined as a function of the physiography of the environment during the period of July 2003 to June 2004. Fecundity values were high, with continuous reproductive activity during the whole period of study. The greatest population densities were observed in the intertidal region, where they are nevertheless intensely consumed by birds, swimming crabs, and fish. This species represents a fundamental link in the food chain of Saco da Fazenda, transferring energy from the detritus level to higher trophic levels. Habitat disturbance and high organic matter may represent factors controlling the distribution of populations of M. schubarti. For this reason, the species may be used to monitor anthropic effects in estuarine areas. PMID:24298211

  1. Basaltic volcanism on the eucrite parent body - Petrology and chemistry of the polymict eucrite ALHA80102

    NASA Technical Reports Server (NTRS)

    Treiman, A. H.; Drake, M. J.

    1985-01-01

    The polymict eucrite meteorite ALHA80102 is an unequilibrated breccia of basaltic and gabbroic clasts in a fragmental matrix. Clasts include basalts of many textural types, cumulate gabbro, black 'glass', and ferroan troctolite (plagioclase, silica, Fe-rich olivine, ilmenite, mesostasis). Ferroan troctolite has not been previously reported from eucrites or howardites; it is interpreted as the end-product of fractional crystallization of eucritic magmas. Bulk and trace element compositions (by electron microprobe and INAA) of clasts and matrix from ALHA80102 are similar to those of other eucrites; the meteorite contains clasts similar to Juvinas and to Stannern. A clast of cumulate eucrite gabbro is enriched in the light rare earths (La/Lu = 2XCI). This clast is interpreted as an unrepresentative sample of metamorphically equilibrated gabbro; LREE-enriched magmas need not be invoked. ALHA80102 is similar to other polymict eucrites from the Allan Hills and may be paired with ALHA76005, ALHA77302, and ALHA78040.

  2. Formation and emplacement ages of the Masirah ophiolite, Sultanate of Oman

    SciTech Connect

    Smewing, J.D. ); Abbotts, I.L. ); Dunne, L.A. ); Rex, D.C. )

    1991-05-01

    Hornblende separates from gabbros and biotite separates from crosscutting potassic granites of the Masirah ophiolite have been analyzed by conventional K-Ar techniques. The gabbros have Late Jurassic-Early Cretaceous ages (126-158 Ma), and the granites have broadly similar ages (124-146 Ma). Because the gabbros are likely to give ophiolite formation ages and the granites ophiolite emplacement ages, it can be concluded that the Masirah ophiolite was emplaced very soon after formation. The Late Jurassic-Early Cretaceous ages contrast with the middle Cretaceous ages (90-95 Ma) of the Semail ophiolite, confirming earlier suggestions that the Masirah ophiolite cannot simply be a right-laterally displaced fragment of the Semail ophiolite, but is instead an uplifted block of Indian Ocean crust. The formation and emplacement of the Masirah ophiolite are discussed in the context of the late Mesozoic evolution of the western Indian Ocean.

  3. Magma Mixing in Uralian-Alaskan Type Complexes in the Ural Mountains, Russia: Implications from the Trace Element Variation in Clinopyroxene

    NASA Astrophysics Data System (ADS)

    Krause, J.; Bruegmann, G. E.; Pushkarev, E.

    2006-12-01

    The distinctive geologic and petrographic feature of classical Uralian-Alaskan-type zoned mafic-ultramafic complexes is a zonal distribution of mafic and ultramafic rocks, with a central dunite body that grades outward into clinopyroxenite and gabbroic lithologies. This rock association is considered to represent cumulates of a single parental melt feeding a magma chamber system. In this study we discuss in-situ trace element variations (LA-ICPMS analyses) in clinopyroxene from gabbros of the Nizhnii Tagil and Kytlym complexes in the Ural Mountains in Russia. The gabbros have porphyric textures with clinopyroxene phenocrysts in a matrix of olivine, clinopyroxene and spinel. Two gabbro types can be distinguished based on additional matrix minerals. One type of gabbro is silica saturated, contains plagioclase (An57-88) and in places orthopyroxene as matrix phases. This type occurs in all complexes. The second gabbro type is silica undersaturated and contains in the matrix plagioclase (An29-56) and pseudoleucite, a fine grained intergrowth of nepheline and K- feldspar (Or30-81). It is only observed in Nizhnii Tagil and the western part of the Kytlym Complex, however, it is interbedded on a scale of decimeter to tens of meters with the silica-saturated gabbro. Clinopyroxene of gabbros from Nizhnii Tagil and the western part of the Kytlym Complex is enriched in LREEN (4.4-33.6; normalized to the primitive mantle values of Hofmann (1988)) relative to HREEN (1.4-8.4) and have high Sr concentrations (130-470 ppm). Phenocryst cores from pseudoleucite-bearing gabbros tend to have higher La/Lu (30-35) than those of silicate-saturated gabbros (La/Lu: 17-30). Towards the phenocryst rims the La/Lu increases up to 45 in the pseudolucite bearing gabbros monitoring the trend of fractional crystallization. However, the continuous increase of La/Lu from silica-saturated to silica-undersaturated gabbro traces the mixing of two different parental magmas. Clinopyroxene from gabbros of

  4. Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

    NASA Astrophysics Data System (ADS)

    Godard, M.; Abratis, M.; Awaji, S.; Brunelli, D.; Christie, D.; Hansen, H.; Hellebrand, E.; Johnson, K.; Maeda, J.; Yamasaki, T.; Kato, Y.

    2007-12-01

    IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge (MAR) 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex. The main hole, Hole U1309D, is the second deepest hole drilled into an intrusive slow-spread oceanic lithosphere: it penetrated 1415.5 mbsf (75% recovery). We present here the results of a bulk rock geochemical study (major and trace elements - ICPMS -) carried out on 234 samples representative of the different lithologies sampled at Site U1309. Over 96% of Hole U1309D is made up of gabbroic rocks, cross-cut by late diabases and basaltic dykes in the upper part of the section. Diabases and basalts have depleted MORB compositions (La/Yb ~0.8 and Yb ~ 3.5 ppm) similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic evidence of melt impregnation. They have relatively fertile compositions, similar to MARK peridotites, with Mg# (100xMg/(Mg+Fe)) of 89-90, Ni>2400ppm and Yb 0.03-0.11 ppm. Gabbroic rocks span a wide range of lithologies and geochemical compositions. They comprise olivine-rich troctolites (>70% modal olivine), troctolites, olivine and troctolitic gabbros (5 to 50% modal olivine), gabbros (including gabbronorites) and oxide gabbros (>2% modal Fe-Ti oxides), which represent respectively 5.4 %, 2.7%, 25.5%, 55.7% and 7% of the core recovered at Hole U1309D. Minor felsic ("leucocratic") dikes cross- cutting gabbros were also sampled. Troctolites and olivine-rich troctolites have high Mg# (82-89), high Ni (up to 2300 ppm) and low trace element contents (Yb 0.06-0.8 ppm). They overlap in composition with peridotites sampled at Atlantis Massif and with impregnated peridotites drilled along the MAR (e.g., ODP Site 1271 (MAR 15°20'N)). Gabbros and olivine gabbros have high Mg# (60-86) and low trace element contents (Yb 0.125-2.5 ppm - (La/Yb)CN ~ 0.4-0.7); these gabbros are among the most primitive and depleted yet sampled along

  5. The Origin of Chemical Heterogeneity in the Mantle Source of Kilauea Lavas

    NASA Astrophysics Data System (ADS)

    Burns, D. H.; Pietruszka, A. J.; Marske, J. P.; Norman, M. D.; Garcia, M. O.

    2014-12-01

    Differences in lava composition between Hawaiian volcanoes likely result from the melting of a heterogeneous mantle source containing variable amounts and/or types of ancient recycled oceanic crust (sediment, basalt, and/or gabbro). Our previous model calculations based on average incompatible trace element abundances suggest that the mantle sources of Hawaiian volcanoes contain ~8-21% recycled oceanic crust (ROC) [1]. The ROC within the Hawaiian mantle plume (basalt and gabbro, with no marine sediment) was altered by interaction with seawater or hydrothermal fluids and then variably dehydrated in an ancient subduction zone. The uppermost portion of the residual slab (gabbro-free and strongly dehydrated) is best expressed in the source of Loihi lavas, whereas the lowermost portion of the residual slab (gabbro-rich and weakly dehydrated) is best expressed in the source of Mauna Loa and Koolau lavas. Here we investigate the origin of chemical heterogeneity within the mantle source of a single Hawaiian volcano based on new modeling of trace element abundances in 80 Kilauea lavas and tephra erupted over the last millenium. The results using our simplest model [1] suggest that the amount of recycled basalt (dehydrated from ~2-9%) and gabbro within the mantle source tapped by Kilauea has ranged from ~9-14% and ~0-3%, respectively. The total amount of ROC correlates with the inferred melt fraction (~3-9%), which indicates that the fertility of the mantle source is the primary control on the degree of partial melting at Hawaiian shield volcanoes. These model results for Kilauea bridge those for the neighboring active volcanoes (Mauna Loa and Loihi), confirming that the Hawaiian plume is heterogeneous on a small scale with a NW-SE oriented spatial gradient in the amount, type (i.e., basalt vs. gabbro) and extent of dehydration of the ancient ROC [1]. [1] Pietruszka et al. (2013) EPSL 361, 298-309.

  6. Luminescence of apollo 11 lunar samples.

    PubMed

    Greenman, N N; Gross, H G

    1970-01-30

    Luminescence measurements were made of four lunar rocks, two terrestrial rocks (granite and gabbro), and one terrestrial mineral (willemite) by comparing the spectral curves with the curve of a barium sulfate standard. Efficiencies with 3000 angstrom excitation were < 6 x 10(-5) for the lunar samples, < 8 x 10(-5) for gabbro of very similar composition to the lunar samples, approximately 10(-4) for granite, and approximately 2 X 10(-2) for willemite. If these are typical values for other ultraviolet excitation wavelengths, the Apollo 11 site appears to contribute little to the observed lunar luminescence. PMID:17781561

  7. Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in california.

    PubMed

    Coleman, D S; Glazner, A F; Frost, T P

    1992-12-18

    Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism. PMID:17836186

  8. Gabbroic and Peridotitic Enclaves from the 2008 Kasatochi Eruption, Aleutian Islands, Alaska

    NASA Astrophysics Data System (ADS)

    Kentner, A.; Nadin, E. S.; Izbekov, P. E.; Nye, C. J.; Neill, O. K.

    2012-12-01

    Kasatochi volcano of the Andreanof Islands in the western Aleutian Arc violently erupted over a two day period from August 7-8, 2008. The eruption involved multiple explosive events generating pyroclastic flows, which included abundant mafic and ultramafic enclaves that have since weathered out and accumulated in talus along the coast. These and other mafic enclaves sampled by modern island arc lavas provide insight into subduction magmatism because they emerge from a section of the subduction system that is less likely than shallower zones to be modified by magmatic processes such as mixing, assimilation, or fractionation. We present new whole rock, clinopyroxene, amphibole, plagioclase, and melt compositions from Kasatochi enclaves of the 2008 eruption. The highly crystalline (~40 vol. % phenocryst content), medium-K basaltic andesite host rock contains ~52-55 wt. % SiO2 and 0.6-0.9 wt. % K2O, and is composed of plagioclase, ortho- and clinopyroxene, amphibole, and Ti-magnetite in a microlite-rich groundmass. Upon eruption, this magma sampled two distinct enclave populations: gabbro and peridotite. The gabbro has abundant amphibole (mostly magnesio-hastingsite) and plagioclase with minor clinopyroxene, olivine, and magnetite, while the peridotite is composed of olivine with minor amounts of clinopyroxene and orthopyroxene. There is little textural variation amongst the peridotitic samples collected, but the gabbroic samples vary from layered to massive and cover a range in grain size from fine-grained to pegmatitic. The layered gabbros display centimeter-scale bands of alternating plagioclase- and amphibole-rich layers, with a strong preferential alignment of the amphibole grains. The coarser-grained samples are very friable, with ~10% pore space; disaggregation of these upon host-magma ascent likely formed the amphibole and plagioclase xenocrysts in the andesitic host. Based on the textural and compositional differences, we divide the enclaves into four groups

  9. Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in California

    USGS Publications Warehouse

    Coleman, D.S.; Frost, T.P.; Glazner, A.F.

    1992-01-01

    Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

  10. Geochemistry of Fast-Spreading Lower Oceanic Crust: Results from Drilling at the Hess Deep Rift (ODP Leg 147 and IODP Expedition 345; East Pacific Rise)

    NASA Astrophysics Data System (ADS)

    Godard, M.; Falloon, T.; Gillis, K. M.; Akizawa, N.; de Brito Adriao, A.; Koepke, J.; Marks, N.; Meyer, R.; Saha, A.; Garbe-Schoenberg, C. D.

    2014-12-01

    The Hess Deep Rift, where the Cocos Nazca Ridge propagates into the young, fast-spread East Pacific Rise crust, exposes a dismembered, but nearly complete, lower crustal section. The extensive exposures of the plutonic crust were drilled at 3 sites during ODP Leg 147 (Nov. 1992-Jan. 1993) and IODP Expedition 345 (Dec. 2012-Feb. 2013). We report preliminary results of a bulk rock geochemical study (major and trace elements) carried out on 109 samples representative of the different drilled lithologies. The shallowest gabbroic rocks were sampled at ODP Site 894. They comprise gabbronorite, gabbro, olivine gabbro and gabbronorite. They have evolved compositions with Mg# 39-55, Yb 4-8 x chondrite and Eu/Eu* 1-1.6. Olivine gabbro and troctolite were dominant at IODP Site U1415, with minor gabbro, gabbronorite and clinopyroxene oikocryst-bearing troctolite and gabbro. All U1415 gabbroic rocks have primitive compositions except for one gabbronorite rubble that is similar in composition to the shallow gabbros. Olivine gabbro, gabbro and gabbronorite overlap in composition: they have high Mg# (79-87) and Ni (130-570 ppm), low TiO2 (0.1-0.3 wt.%) and Yb (1.3-2.3 x chondrite) and positive Eu anomaly (Eu/Eu*=1.9-2.7). Troctolite has high Mg# (81-89), Ni (260-1500 ppm) and low TiO2 (<0.1 wt.%) and Yb (~0.5xchondrite) and large Eu/Eu* (>4). ODP Site 895 recovered sequences of highly depleted harzburgite, dunite and troctolite (Yb down to <0.1xchondrite) that are interpreted as a mantle-crust transition zone. Basalts were recovered at Sites 894 and U1415: they have low Yb (0.5-0.9xN6MORB) and are depleted in the most incompatible elements (Ce/Yb=0.6-0.9xN-MORB). The main geochemical characteristics of Site U1415 and 894 gabbroic rocks are consistent with formation as a cumulate sequence from a common parental MORB melt; troctolites are the most primitive end-member of this sequence. They overlap in composition with the most primitive of slow and fast spread crust gabbroic rocks.

  11. 138-121 Ma asthenospheric magmatism prior to continental break-up in the North Atlantic and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Schärer, Urs; Girardeau, Jacques; Cornen, Guy; Boillot, Gilbert

    2000-09-01

    Along the Galicia and Gorringe banks and in the Iberia Abyssal Plain of the North Atlantic, unroofed sub-continental mantle fills the gap between 'true' oceanic crust and the continental crust margin. These lithospheric peridotites are intruded by gabbros and dolerites, and locally covered by basalts. Primary magmatic zircons extracted from gabbros and meta-gabbros of the two banks were dated by the U-Pb chronometer, and initial hafnium isotope signatures ( ɛHf i) were determined on the same grains. For Mt. Gettysburg at Gorringe, gabbro emplacement ages of 137.5±0.5 (2σ) Ma and 135.7±0.8 Ma are obtained, and corresponding ɛHf i lie at +20.5±0.3 (2σ) and +19.5±0.4, substantiating magma formation from severely LILE-depleted mantle domains. Gabbro zircons from Mt. Ormonde at Gorringe yield a much younger age of 77.1±0.4 Ma and the Hf isotopes document an intermediately LILE-depleted mantle source having a ɛHf i of +7.6±0.4. Given its age and Hf signature, emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the Iberian Continent in Upper Cretaceous time. Concerning the Galicia section, zircons from a meta-gabbro yield an emplacement age of 121.7±0.4 Ma and a ɛHf i of +14.0±0.2, and a ɛHf i of +14.6±0.2 is obtained for zircons from a previously dated meta-gabbro of identical age. These results indicate magma extraction from mantle reservoirs that are slightly less LILE-depleted than those sampled by the about 20 Myr older Gorringe gabbros. The data demonstrate that magmatism occurring prior to complete separation of Europe from America was essentially of asthenospheric origin. Both the 138-135 Ma ages for the Gorringe gabbros and 122 Ma ages for the Galicia gabbros are at least 5 Myr older than the oldest sediments on Gorringe, and the break-up unconformity at the Galicia Bank, respectively. Magma source signatures of the syn-rift gabbros are in agreement with values expected for differently depleted Cretaceous

  12. Detachment Fault Initiation and Control by Partially Molten Zones in the Lower Ocean Crust

    NASA Astrophysics Data System (ADS)

    Dick, H. J.; Natland, J. H.; MacLeod, C. J.; Robinson, P. T.

    2012-12-01

    The close association of oxide gabbro and deformation in interleaved ferrogabbro and olivine gabbro at Atlantis Bank on the SW Indian Ridge explains the formation of this enormous single-domed gabbroic oceanic core complex. ODP Holes 735B and 1105A show that the stratigraphy is defined by 100's of zones of intense deformation and strain localization in the upper 500-m where various melts percolated including late-stage iron-titanium rich melts. The latter created highly deformed oxide-rich gabbro zones at scales from millimeters to over 100 meters. Mapping by ROV, over-the-side rock drilling, dredging, and submersible shows that this stratigraphy exists uniformly over the bank. Deep drilling and sampling up the headwalls of major landslips cutting into the core complex show that the fault zone was imbricate, likely reflecting relocation of the active slip plane due to cyclic intrusion in the lower crust. The detachment originated as a high-angle fault on the rift valley wall that propagated into a zone of partially molten gabbro beneath the sheeted dikes. This zone then pinned the footwall block, creating a plutonic growth fault along which gabbro intruded beneath the ridge axis was continuously uplifted and exposed on the Antarctic plate for ~3.9 myr. The overlying basaltic carapace spread more slowly to the north on the African Plate. Textural evidence, particularly that provided by iron-titanium oxides, shows that melts migrated along complex shear zones in which several creep mechanisms operated, ranging from crystal plastic dislocation creep, diffusion creep, grain boundary sliding, and brittle deformation. More than one of these mechanisms may have occurred concurrently. Subsequently, these zones localized later solid-state creep, often producing texturally complex rocks where separation of the timing and duration of different creep mechanisms is difficult to unravel. As uplift of the plutonic section progressed, the footwall passed through the zone of diking

  13. California earthquakes: why only shallow focus?

    PubMed

    Brace, W F; Byerlee, J D

    1970-06-26

    Frictional sliding on sawcuts and faults in laboratory samples of granite and gabbro is markedly temperature-dependent. At pressures from 1 to 5 kilobars, stick-slip gave way to stable sliding as temperature was increased from 200 to 500 degrees Celsius. Increased temperature with depth could thus cause the abrupt disappearance of earthquakes noted at shallow depths in California. PMID:17759338

  14. Low-Velocity Zone of the Earth's Mantle: Incipient Melting Caused by Water.

    PubMed

    Lambert, I B; Wyllie, P J

    1970-08-21

    Experimental phase diagrams for the systems gabbro-water and peridotite-water indicate that, if there is any water in the upper mantle, then traces of hydrous interstitial silicate magma will be produced at depths corresponding to the beginning of the low-velocity zone. This explanation for the zone is more satisfactory than others proposed. PMID:17820306

  15. The crustal section of the Siniktanneyak Mountain ophiolite, Brooks Range, Alaska

    SciTech Connect

    Bickerstaff, D.; Harris, R.A.; Miller, M.A. . Dept. of Geology and Geography)

    1993-04-01

    Fragments of the upper crustal section of the Brooks Range Ophiolite on the west flank of Siniktanneyak Mountain expose important contact relations and paleohorizontal indicators. The nearly complete crustal sequence faces northwest. Based on field observations, the crustal units encountered at Siniktanneyak Mountain from bottom to top are: (1) layered gabbro, (2) isotropic gabbro, (3) high level and late-stage intrusions of diorite and diabase, (4) rare sheeted dikes, (5) basalt, and (6) a bedded volcanic tuff. Potassium feldspar-bearing pegmatites are also found. Of particular interest is the orientation of the layered gabbro, sheeted dikes, and the bedded volcanic tuff. The steeply dipping gabbro layers strike N-S, the adjacent vertical sheeted dikes strike NE-SW. Bedded volcanic tuff and lavas are flat lying. Contacts within the upper crust units are often covered by talus. Contacts between various plutonic rocks are both sharp and gradational, suggesting syn- and post-cooling intrusions. Contacts between plutonic rock and higher volcanic rock appear to be fault contacts.

  16. Shallow drilling investigation of contact relationships in the Wichita Mountains igneous province

    SciTech Connect

    Gilbert, M.C.; Hogan, J.P. . School of Geology and Geophysics); Luza, K. )

    1993-02-01

    Within the Wichita Mountains Igneous Province, a variety of mineralogically, texturally and compositionally diverse hybrid rock types (i.e. gabbro-diorites, monzonites and granodiorites) crop out at gabbro-grants contacts. Possible coeval sedimentary rocks associated with crustal rifting are restricted to a few scattered, isolated exposures of a mineralogically variable group of meta-quartzites (Meers Quartzite). Typically these outcrops of meta-quartzite are of limited areal extent and are surrounded by either gabbro, granite, rhyolite or a combination of these rock types. However, the origin of both the hybrid rock types and the Meers Quartzite remains enigmatic because outcrops containing complete and clear contact relationships are extremely rare. At present, direct testing of models is difficult as complete exposure of contacts between these units is extremely rare due to deposition of younger sedimentary units and severe degradation by weathering. Poor condition of existing samples has hampered geochemical and other petrologic methods in evaluating models. Four potential drilling sites have been selected where critical contacts between major geologic units are interpreted to be present in the shallow subsurface (<300 ft.). Objectives of drilling are (1) direct observation of contacts between rock units by retrieval of a complete core sample from the drill hole, (2) retrieval of freshest possible rock material for petrographic and geochemical analysis and (3) retrieval of a complete transect beginning in Mount Scott Granite or Meers Quartzite across the hybrid rock zone and into the substrate gabbro to document variations associated with the transition.

  17. Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas

    PubMed

    Sobolev; Hofmann; Nikogosian

    2000-04-27

    The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace-element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust. PMID:10801125

  18. Magma mixing and mingling on Deer, Niblack, and Etolin Islands, southeastern Alaska

    SciTech Connect

    Lindline, J.; Crawford, W.A.; Crawford, M.L. . Geology Dept.)

    1993-03-01

    Intimately associated 20 m.a. hornblende-biotite granites and olivine gabbro norites occur on Etolin, Niblack and Deer Islands, southwest of Wrangell, Alaska. The field relationships suggest multiple injections of mafic and felsic phases within this igneous complex. Ellipsoidal to angular mafic magmatic enclaves occur in the granite, ranging in number from sparse to tightly packed swarms. Slightly curved decimeter sized rafts of fine grained mafic enclaves comprise a frozen fountain of mafic magma in the felsic host. Course-grained felsic dikes containing gabbroic zenoliths and ubiquitous fine-grained mafic pillows exhibiting sharp and sutured chilled borders intrude the layered gabbro. Synplutonic northeast trending fine-grained mafic and fine-grained felsic dikes mutually cross-cut the felsic pillow-bearing dikes. The granite consists of green hornblende, dark brown biotite, plagioclase and quartz. The mafic mineral assemblage changes from olivine, orthopyroxene, clinopyroxene, and plagioclase in the gabbro through intermediate-grained phases containing altered clinopyroxene, brown hornblende, red-brown biotite, plagioclase and quartz. The increase in proportion of hydrous mafic minerals from the gabbro to the fine-grained mafic enclaves and changes in pleochroic colors of biotite and hornblende from the intermediate-grained phases to the fine-grained mafic enclaves suggest chemical interaction between the mafic enclaves and their felsic host.

  19. Macquarie island and the cause of oceanic linear magnetic anomalies.

    PubMed

    Varne, R; Gee, R D; Quilty, P G

    1969-10-10

    Macquarie Islands is formed of probably Pliocene oceanic crust. Intruded into pillow lavas is a belt of harzburgite and layered gabbro mnasses cut by dike swarms. Similar belt-like structures may cause the linear magnetic anomalies of the ocean. PMID:17731490

  20. Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

    USGS Publications Warehouse

    Whitney, D.L.; Tepper, J.H.; Hirschmann, M.M.; Hurlow, H.A.

    2008-01-01

    The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88-80) with relatively sodic plagioclase (An75-60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc. ?? 2008 Geological Society of America.

  1. Influence of water on rheology and strain localization in the lower continental crust

    NASA Astrophysics Data System (ADS)

    Getsinger, A. J.; Hirth, G.; Stünitz, H.; Goergen, E. T.

    2013-07-01

    We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnøya, Norway. Over a distance of ˜1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (˜1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

  2. Feldspar basalts in lunar soil and the nature of the lunar continents

    NASA Technical Reports Server (NTRS)

    Reid, A. M.; Ridley, W. I.; Harmon, R. S.; Warner, J.; Brett, R.; Jakes, P.; Brown, R. W.

    1974-01-01

    It is found that 25% on the Apollo-14 glasses have the same composition as the glasses in two samples taken from the Luna-16 column. The compositions are equivalent to feldspar basalt and anorthosite gabbro, and are similar to the feldspar basalts identified from Surveyor-7 analysis for lunar continents.

  3. The abundances of components of the lunar soils by a least-squares mixing model and the formation age of KREEP.

    NASA Technical Reports Server (NTRS)

    Schonfeld, E.; Meyer, C., Jr.

    1972-01-01

    A least-square mixing model incorporating mare basalts, KREEP basalts, anorthosites, anorthositic gabbros, ultramafics, granites, and meteorites was used to estimate the abundances of rock components in lunar soil from the Apollo 11, 12, 15, Luna 16, and Surveyor 5 and 6 landing sites. The predominance of iron-rich mare basalt at the sites is indicated.

  4. Petrology of gabbroic rocks from the Mid-Cayman rise spreading center

    NASA Technical Reports Server (NTRS)

    Elthon, Don

    1987-01-01

    Mineral analyses of oxide and silicate phases from a suite (collected with the DSRV Alvin in January 1976 and July 1977) of 48 gabbroic rocks collected from the vicinity of the Mid-Cayman Rise spreading center are reported. Mineral compositions of these anorthosites, leuco-troctolite, leuco-olivine gabbros, olivine gabbros, leuco-gabbros, and gabbros indicate the cumulate rocks have been produced by the crystal fractionation of basaltic liquids. Certain features of these rocks are inconsistent with the occurrence of this fractionation at low pressures (1 atm to 2 kbar). Although the experimental data are not available to conclusively demonstrate that the effects seen were produced at moderate pressures (5-10 kbar), the effects are similar to those predicted thermodynamically and to those observed in limited experimental studies within this pressure range. It is therefore suggested that the most likely scenario for the production of these gabbroic rocks is the moderate-pressure crystallization of basaltic magmas within deep-seated magma chambers underneath this slow-spreading center.

  5. Geochemical and zircon U-Pb dating analysis of metamagmatic rocks from the Yuli belt in Taiwan

    NASA Astrophysics Data System (ADS)

    Hung, Chia-Chia

    2016-04-01

    The Tananao Schist Complex of the oldest rocks in Taiwan is exposed at the eastern limb of Backbone Range. Based on the lithologic and metamorphic characteristics, the complex can be divided into the Tailuko and Yuli belts. The Tailuko belt consists of marble, gneiss, and subordinate scattered metabasite; the Yuli belt is composed of greenschist, serpentinite, meta-tuff, meta-gabbro, metabasite, and glaucophane-schist blocks which enclosed by spotted schist of the host rocks. The metamorphic belts were inferred as a Mesozoic mélange. It's still controversial due to the difficulty of analyzing metamorphic rocks. In this study, we focus on the zircon U-Pb dating, geochemistry, and petrographic analysis of spotted schist, metabasite, meta-gabbro, and meta-tuff in order to constrain the formation and crystallization ages and interpret its tectonic setting. Based on zircon U-Pb dating, the host rocks of spotted schist and the exotic blocks of meta-tuff, meta-gabbro (the peak age of 14.4, 15.8, and 16.7 Ma), and metabasite occurred at Miocene. Geochemical characteristics for metabasite and meta-gabbro blocks show Ta-Nd-Ti depletion and LREE depletion in spidergram occurring volcanic arc and N-MORB type affinities, respectively. Results as above mentioned, we suggest that the metamagmatic rocks in the Yuli belt occur within a mélange during the Eurasia continental margin subduction at the Middle-Late Miocene.

  6. Geochemical characteristics of hydrous basaltic magmas due to assimilation and fractional crystallization: the Ikoma gabbroic complex, southwest Japan

    NASA Astrophysics Data System (ADS)

    Koizumi, N.; Okudaira, T.; Ogawa, D.; Yamashita, K.; Suda, Y.

    2016-02-01

    To clarify the processes that occur in hydrous basaltic magma chambers, we have undertaken detailed petrological and geochemical analyses of mafic and intermediate rocks from the Ikoma gabbroic complex, southwest Japan. The complex consists mainly of hornblende gabbros, hornblende gabbronorites, and hornblende leucogabbros. The hornblende leucogabbros are characterized by low TiO2 and high CaO contents, whereas the hornblende gabbronorites have high TiO2 and low CaO contents. The initial 87Sr/86Sr ratios (SrI) of the hornblende gabbronorites and hornblende gabbros are higher than those of the hornblende leucogabbros and plagioclase, and they may have resulted from a higher degree of assimilation of metasediments. The geochemical features of the hornblende leucogabbros and hornblende gabbronorites can be explained by accumulation of plagioclase and ilmenite, respectively, in a hybrid magma that formed by chemical interaction between mafic magma and metasediment, whereas the hornblende gabbros were produced by a high degree of crustal assimilation and fractional crystallization of this hybrid magma. As a result of the density differences between crystals and melt, the Ikoma gabbroic rocks formed by the accumulation of plagioclase in the middle of the magma chamber and by the accumulation of ilmenite in the bottom of the chamber. Taking into account the subsequent assimilation and fractional crystallization, our observations suggest an enriched mantle (SrI = ~0.7071) as the source material for the Ikoma gabbros.

  7. Triassic mafic and intermediate magmatism associated with continental collision between the North and South China Cratons in the Korean Peninsula

    NASA Astrophysics Data System (ADS)

    Yi, Sang-Bong; Oh, Chang Whan; Lee, Seung-Yeol; Choi, Seon-Gyu; Kim, Taesung; Yi, Keewook

    2016-03-01

    Triassic coeval mafic and intermediate magmatism occurred in the area suggested to be the southern margin of the North China Craton (NCC) in the Gyeonggi Massif (GM) of the Korean Peninsula. This study investigates aspects of the mafic and intermediate magmatism using SHRIMP zircon ages and whole-rock chemical and isotopic Sr-Nd data. The mafic and intermediate rocks intruded into a basement paragneiss in three areas (Yangpyeong, Odesan and Yangyang) within the GM at ca. 230 Ma. The paragneiss was metamorphosed in both the Paleoproterozoic and Triassic. Gabbros (hornblende gabbro and pyroxene-mica gabbro) from the study areas exhibit strong light REE (LREE) enrichment relative to chondrite (LaN/YbN = 11.1-30.6) and a high LILE/HFSE pattern, Ta-Nb-P-Ti troughs and positive Ba-K-Pb-Sr spikes on the N-MORB-normalized multi-element variation diagram. These features are typical characteristics of arc-related gabbros. The gabbros also show strongly enriched initial isotopic compositions (87Sr/86Sr(i) = 0.7100-0.7137; εNd(t) = - 13.1 to - 19.7). The coeval intermediate intrusive rocks also exhibit whole-rock chemical and isotopic features (87Sr/86Sr(i) = 0.7099-0.7143; εNd(t) = - 10.8 to - 18.6) similar to those of the gabbros. The mafic and intermediate intrusive rocks plot in the within-plate and/or post-collisional fields on tectonic discrimination diagrams. These data indicate that the mafic and intermediate magmatism in the study areas occurred during the Triassic post-collisional relaxation period via partial melting of sub-continental lithospheric mantle (SCLM) that was enriched in a subduction environment prior to (or during) the Permo-Triassic continental collision between the NCC and the South China Craton (SCC). The highly enriched mantle signatures revealed by the gabbros from the study areas are matched to the enriched features identified in Cretaceous mafic igneous rocks (ca. 130 Ma) on the southern margin of the NCC. Thus, this study suggests that the

  8. Lithological and structural investigations of the Finero back thrust

    NASA Astrophysics Data System (ADS)

    Palzer, M.; Österle, J.; Klötzli, U.

    2012-04-01

    The Ivrea-Verbano-Zone (IVZ, Southern Alps, NW Italy) constitutes a renowned cross-section through the continental crust. It is one of the few places in the world where a complete crustal transect from the palaeo-surface to granulite facies lower crustal conditions and accompanying mantle melt intrusions can be studied directly. It has thus gained an enormous amount of interest and generated a wealth of literature. But the litho-tectonic evolution of the IVZ is still only partly understood and numerous problems remain unsolved. The IVZ is tectonically confined by the Insubric Line to the north and west and by the Cossato-Mergozzo-Brissago Line (CMB) as well as by the Pogallo Line to the east. The outcropping rocks are interpreted as a part of the Adriatic continental crust emplaced during the Alpine orogeny. Lithologies comprise a stratigraphically upper amphibolite facies "kinzigite series" and a lower granulite facies "stronalite series" (both with metapelites, basites, calcsilicate rocks), numerous ultrabasic mantle tectonites and a widespread suite of Permian mantle melt intrusions, the so-called "mafic complex". The largest mantle tectonite of the IVZ is the peridotite body of Finero. This comprises three main lithologies: phlogopite peridotite, "internal gabbro", hornblende peridotite (and minor amounts of pyroxenites, gabbroic dikes, ect.). In spite of many studies, the answers to numerous questions concerning the structure and history of the ultrabasic and basic rocks are still unsatisfying and need to be questioned. Peridotites, gabbros, stronalites and kinzigites can be found from north to south, interpreted as an extensionally thinned intersection through the lower crust. Problems arise from the difficult distinction between the "internal gabbro" within the peridotite body and the "external gabbro", as part of the "mafic complex", the unsatisfying structural interpretations concerning the peridotite body and the relationship between peridotite and "mafic

  9. Decoupling of Serpentinization and Prehnitization in Lower East Pacific Rise Crust at Hess Dee

    NASA Astrophysics Data System (ADS)

    Deasy, R. T.; Wintsch, R. P.; Meyer, R.; Bish, D. L.; Gasaway, C.; Heimdal, T.

    2014-12-01

    Our down-hole mineralogical and geochemical analyses from the East Pacific Rise fast-spreading lower oceanic crust indicate that alteration of olivine to serpentine and of plagioclase to prehnite were independent, and neither alone monitors the total "alteration." The results are based on representative channel sub-samples recovered from every Hole J core during IODP Expedition 345 to the Hess Deep tectonic window. Samples have been analyzed for trace element, Sr isotopic, and quantitative mineralogical compositions (the latter by Rietveld refinement using X-ray diffraction data). Hole J is the most representative rock succession drilled at the Hess Deep as it penetrated the two principle plutonic lithologies: an upper gabbro and a lower troctolite. Units are significantly distinguished by XRD modal mineralogy and trace element abundances. The more heterogeneous gabbro contains 23-32 wt% clinopyroxene (cpx), 34-54 wt% plagioclase (plag), and <4 wt% olivine (ol). The troctolite contains 3-11% cpx, 14-36% plag, and ≤6% ol. Alteration minerals comprise together 18-31% in the gabbro versus 55-80% of the troctolite. The most abundant alteration products are prehnite and chlorite. Gabbro samples with lowest abundances of alteration minerals (18-20 wt%) preserve 87Sr/86Sr ratios (0.70275-0.7028) consistent with unaltered mantle. The abundance of plag in the gabbro, the major host for Sr, suggests retention of mantle Sr isotopic compositions there is due to the large reservoir of magmatic Sr. 87Sr/86Sr ratios of 0.70300-0.70342 in the troctolite samples indicate seawater interaction, even where olivine is most abundant, and serpentine is at or below the ~1% detection limit by XRD. Significant alteration of the deep crust by seawater thus predates the first appearance of serpentine. These data suggest that the timing and operation of prehnite- and serpentine-producing alteration reactions are independent.

  10. Timescale of emplacement of the Panzhihua gabbroic layered intrusion recorded in giant plagioclase at Sichuan Province, SW China

    NASA Astrophysics Data System (ADS)

    Cheng, Lilu; Zeng, Ling; Ren, Zhongyuan; Wang, Yu; Luo, Zhaohua

    2014-09-01

    Giant plagioclase crystals carried into the Panzhihua gabbroic layered intrusion from a deeper magma chamber can help constrain the timescales of emplacement of the Panzhihua intrusion in the Emeishan large igneous province (LIP). In this study, we present the petrographic textures and chemical compositions of giant plagioclase and fine-grained gabbro samples along a ~ 50 m horizontal outcrop of the low zone of the Panzhihua deposit. The giant plagioclase gabbro (GPG) dykes mostly intrude into the fine-grained gabbros without significant contact. Both types of gabbros have the similar bulk-rock major and trace element compositions. However, the mineral composition shows that most of the plagioclase megacrysts contain less An than the plagioclase in fine-grained gabbro samples. In situ analyses of Sr isotopes from core-to-rim transects of plagioclase megacrysts are constant, indicating that there are no recycled crystals. The textural characteristics of samples combined with petrological modeling using MELTS suggest that the plagioclase megacrysts should crystallize and grow in a deeper magma chamber. Textual studies of the GPG indicate that these plagioclase megacrysts mostly plot as straight lines on a classic crystal size distribution (CSD) diagram. For a plagioclase growth range of 10- 11-10- 10 mm/s, the plagioclase should have a growth time of 530-8118 years. In a 100 km3 magma chamber releasing thermal energy at a rate of 1000 MW, the Panzhihua intrusion should reach 50% crystallization after ~ 2400 years. The growth time recorded by the megacrysts in the GPG and numerical modeling may indicate that the emplacement and crystallization of the Panzhihua intrusion may have taken place in thousands of years. In the Emeishan LIP, therefore the combination of large volumes of Fe-rich magma flux and efficient metal precipitation led to the formation of a giant Fe-Ti-V oxide deposit in a short period.

  11. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-10-01

    Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

  12. Chemical heterogeneity in the Hawaiian mantle plume from the alteration and dehydration of recycled oceanic crust

    NASA Astrophysics Data System (ADS)

    Pietruszka, Aaron J.; Norman, Marc D.; Garcia, Michael O.; Marske, Jared P.; Burns, Dale H.

    2013-01-01

    Inter-shield differences in the composition of lavas from Hawaiian volcanoes are generally thought to result from the melting of a heterogeneous mantle source containing variable amounts or types of oceanic crust (sediment, basalt, and/or gabbro) that was recycled into the mantle at an ancient subduction zone. Here we investigate the origin of chemical heterogeneity in the Hawaiian mantle plume by comparing the incompatible trace element abundances of tholeiitic basalts from (1) the three active Hawaiian volcanoes (Kilauea, Mauna Loa, and Loihi) and (2) the extinct Koolau shield (a compositional end member for Hawaiian volcanoes). New model calculations suggest that the mantle sources of Hawaiian volcanoes contain a significant amount of recycled oceanic crust with a factor of ˜2 increase from ˜8-16% at Loihi and Kilauea to ˜15-21% at Mauna Loa and Koolau. We propose that the Hawaiian plume contains a package of recycled oceanic crust (basalt and gabbro, with little or no marine sediment) that was altered by interaction with seawater or hydrothermal fluids prior to being variably dehydrated during subduction. The recycled oceanic crust in the mantle source of Loihi and Kilauea lavas is dominated by the uppermost portion of the residual slab (gabbro-free and strongly dehydrated), whereas the recycled oceanic crust in the mantle source of Mauna Loa and Koolau lavas is dominated by the lowermost portion of the residual slab (gabbro-rich and weakly dehydrated). The present-day distribution of compositional heterogeneities in the Hawaiian plume cannot be described by either a large-scale bilateral asymmetry or radial zonation. Instead, the mantle source of the active Hawaiian volcanoes is probably heterogeneous on a small scale with a NW-SE oriented spatial gradient in the amount, type (i.e., basalt vs. gabbro), and extent of dehydration of the ancient recycled oceanic crust.

  13. Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Sinigoi, Silvano; Quick, James E.; Demarchi, Gabriella; Klötzli, Urs S.

    2016-05-01

    The Permian Sesia Magmatic System of the southwestern Alps displays the plumbing system beneath a Permian caldera, including a deep crustal gabbroic complex, upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera. Isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts, whereas granites define a distinct, more radiogenic cluster (Sri ≈ 0.708 and 0.710, respectively). AFC computations starting from the best mafic candidate for a starting melt show that Nd and Sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of ≈ 30% of partially depleted crust and ≈ 15%-30% gabbro fractionation. Trace elements of the deep crustal gabbro cumulates require a further ≈ 60% fractionation of the andesitic basalt and loss of ≈ 40% of silica-rich residual melt. The composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt. Chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the Mafic Complex. During the growth of the Mafic Complex, progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation. Anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion. Upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping. At each cycle of mafic intrusion and incorporation, residual and anatectic melts are produced in roughly constant proportions, because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt. Such a process can explain the

  14. Pyasino-Vologochan intrusion: Geological structure and platinum-copper-nickel ores (Norilsk Region)

    NASA Astrophysics Data System (ADS)

    Sluzhenikin, S. F.; Krivolutskaya, N. A.

    2015-09-01

    Rocks enriched in olivine, such as olivine and picritic gabbro-dolerite and troctolite are abundant in the Pyasino-Vologochan intrusion. In contrast to Norilsk type ore-bearing massifs, ultramafic rocks of the intrusion are dominated by troctolite. Picritic gabbro-dolerite and troctolite are located between olivine and olivine-bearing gabbro-dolerite. These rocks are related by gradual transitions. The rocks are enriched in olivine, but depleted in magnesium relative to the Norilsk type ore-bearing massifs. The intrusion is also distinguished by low chromium content in the rocks. Most disseminated Pt-Cu-Ni ores are confined to troctolite and picritic gabbro-dolerite, and to a lesser extent, to contact and olivine gabbro-dolerite. The ore is mainly observed as low-sulfur assemblages of sulfides with troilite, Fe-rich hexagonal pyrrhotite, Fe-rich chalcopyrite, talnakhite, Fe-rich pentlandite, and cubanite. The total content of platinum group elements (PGE) in the ore (ppm) is as follows: from 0.45-3.47 to 7.8; gold, from 0.05-0.24 to 0.49; and silver, from 0.53-4.50 to 8.30. Disseminated ores of the Pyasino-Vologochan intrusion contain the following Pt and Pd minerals: sobolevskite, Te-sobolevskite, Te-insizwaite, maslovite, paolovite, zvyagintsevite, atokite, niggliite, mertieite II, guanglinite, and Pd2(As,Sb), Pd2(Sn,As), Pd3Sb, (Pd,Ni)2As minerals. The similarities and differences between the Pyasino-Vologochan intrusion and Norilsk type ore-bearing massifs are discussed in this paper.

  15. ESR dating of teeth from Brazilian megafauna

    NASA Astrophysics Data System (ADS)

    Oliveira, L. C.; Kinoshita, A.; Barreto, A. M. F.; Figueiredo, A. M.; Silva, J. L. L.; Baffa, O.

    2010-11-01

    The study of radiation defects created in biomaterials, such as bone and teeth, can be used in dating with importance to palaeontology and archaeology. Two Stegomastodon teeth (AL1 and AL2) from north-eastern Brazilian megafauna were studied by electron spin resonance (ESR) spectroscopy. The samples were collected in Fazenda Ovo da Ema, (913349 / 3714965) UTM, Alagoas state, Brazil. The dating of these samples can contribute to the better knowledge of megafauna presence in this region as well as to the events associated to the extinction of these species.

  16. Monitoring of reforested areas using LANDSAT data. [Ribas do Rio Pardo, Mato Grosso do Sul, Brazil

    NASA Technical Reports Server (NTRS)

    Dejesusparada, N. (Principal Investigator); Filho, P. H.; Shimabukuro, Y. E.

    1981-01-01

    Imagery obtained with channels 5 and 7 was visually interpreted in an effort to determine the spatial, spectral, and temporal characteristics of a 105,000 hectare area of Fazenda Mutum which was reforested with various species of pine and eucalyptus. It was possible to map a reforested area as small as 6 hectare in its initial implantation using contrast with the surrounding targets. Five classes were mapped: nondeforested areas, partially deforested areas, deforested areas, partially reforested areas, and fully reforested areas. In 1979, 12,000 hectare were deforested, 4,330.83 hectare were partially reforested, and 42,744.71 hectare were reforested.

  17. A comparative petrological study of the Siberian and Ethiopian Large Igneous Provinces (LIPs) and a case study on Triassic mafic rocks in Chukotka, NE Russia

    NASA Astrophysics Data System (ADS)

    Desta, M. T.; Ishiwatari, A.; Machi, S.; Ledneva, G. V.; Sokolov, S. D.; Artem, M. V.; Bazylev, B. A.

    2012-12-01

    This study is mainly targeted to find the possible eastern marginal extension of Siberian LIP and to compare them with the central Siberian LIP and is also aimed to compare and contrast the geochemical and petrological characteristics of Siberian LIP (~ 250 Ma) with the Ethiopian LIP (~ 30 Ma) to consider the mantle and crustal processes in view of magmatic diversity among those LIPs. A review of previous geochemical data from the Siberian and Ethiopian LIP confirms notable differences in their major and trace element compositions. Siberian LIP comprises a variety of rocks (such as basalts, basaltic andesites, picrites and meimechites) with a wide range of SiO2 (40-62 wt. %). In contrast, Ethiopian LIP is characterized by bimodal volcanism with the absence of intermediate rock. The Ethiopian high-Ti basalts and picrites have higher TiO2 (3-6 wt. %), lower CaO/Al2O3 (0.5-1.5) and MgO (5-26 wt. %) than the Siberian high-Ti picrites and meimechites (2-4, 1.8-2.3 and 13-36 wt. % respectively). Siberian LIP shows more significant depletion in HFSE (mainly Nb) and higher La/Sm ratios than Ethiopian LIP. This may suggest contamination of Siberian LIP magma by crustal component. Triassic volcanic and intrusive rock samples are collected in 2009 from the Chukotka province (Northeast Russia), where geographically far to the east from the central Siberian flood basalt province. The petrography of the studied samples includes basaltic rocks (i.e. hornblende basalt, lamprophyre, pyroxene basalt, and ankaramite) and gabbroic rocks (i.e. amphibole-bearing type-I and amphibole free type-II). Clinopyroxenes (diopside to augite) both from basalts and gabbros show only low-Ti (<1 wt. %) characteristics. Amphiboles are pargasite in lamprophyres and type-I gabbros, tschermakite in hornblende-basalt, edenite in ankaramite and edenite-hornblende in type-II gabbros. Plagioclase ranges from An100 to An45.6 in type-I gabbros, An54.5 to An0.9 in type-II gabbros and An73.4 to An1.3 in

  18. Contrasted Origins For Gabbroic Cumulates Along The Mid-Atlantic Ridge.

    NASA Astrophysics Data System (ADS)

    Nonnotte, P.; Benoit, M.; Ceuleneer, G.

    2005-12-01

    Gabbroic cumulates sampled along mid-ocean ridges are classically viewed as fractional crystallization products of primitive mid-ocean-ridge basalts (MORB) in shallow axial magma chambers. Recent observations of deep horizons of the ocean crust have shown that, in slow spreading environments, crustal building proceeds by repeated injections of modest amounts of variously evolved melt batches. Such a discontinuous character of magma emplacement favors the development of complex petrogenetic evolutions triggered by the migration of various types of melts and fluids into formerly crystallized - or still crystallizing - gabbroic cumulates and into residual peridotite. In some occurrences, however, gabbros are clearly not issued from fractional crystallization of MORB-like melts. In order to better constrain the origin of various kinds of gabbronorites emplaced along the Mid-Atlantic Ridge (MAR), we performed a geochemical study (trace elements and Sr-Nd isotopes) of clinopyroxene (cpx) separates leached with a severe analytical protocol designed to remove the geochemical effects of post-crystallization water rock interaction. We focus on two sites: MAR off the FAMOUS area (DSDP Site 334) and MAR on both sides of the 15°20 N fracture zone (MODE98-Leg1 cruise). Cumulates sampled at DSDP Site 334 are orthopyroxene-rich gabbronorites which call for parent melts richer in SiO2, more "andesitic", than MORB at a given MgO content. They also show an extreme depletion in most incompatible major, minor and trace elements compared to MORB. We measured isotopic signatures to trace the source of the DSDP Site 334 cumulates. We find that Site 334 cpx depart from the global mantle correlation: normal MORB values for the 143Nd/144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr/86Sr (0.7034-0.7067) ratios. These signatures show that the parent melts of Site 334 cumulates are issued from an ultra-depleted MORB source and that contamination with seawater occurred at

  19. Bi-cycles petrographic association in middle part of East Pana PGE layers deposit

    NASA Astrophysics Data System (ADS)

    Asavin, Alex; Veksler, Ilya; Gorbunov, Artem

    2016-04-01

    The PGE mineralization in the East Pana layered gabbroic intrusion forms three discrete layers at different stratigraphic levels, which are traditionally labeled as zones A, B and C. In order to investigate possible relationships of mineralization with magmatic layering we sampled a 120 m long drill core section across zone B in the middle part of the intrusion and carried out detailed petrographic, mineralogical and geochemical studies of the samples. The ore zone is located in medial part of the of East's Pana deposite. The samples represent mainly from a layered sequence of gabbro and gabbro-norite. This zone is composed of interlayers of gabbroic sequences and gabbro-norite of various color, with different structures and different relationship of rock-forming minerals of Ol-Opx-Cpx-Pl. We studied one of key's drill-hole section of ore zone, in which is located two ore horizons. Fundamental feature layered intrusions are presence in cross-section cycles includes of stable petrographic association. In section of ore zone it is possible to select two most contrast petrographic types. Whole-rock analyses and petrographic observations reveal two units of modal layering comprising, from bottom to top, melanocratic gabbro grading upwards into mesocratic gabbro and gabbro-norite overlain by pegmatoidal, gabbroic rock with has sharp footwall and hanging wall contacts.There is also an olivine-bearing gabbro at the bottom of the lower unit. The ore horizons are located in same gabbro-norite type rock. The ore horizons are located in same gabbro-norite type part. The second upper ore zone located in more differential species types. There is the common trend of system evolution of well distinguished on triangle of Ol-Pl-Di, Ol-Pl-Q and other. However composition of the rocks in the two parts of our section show us similar, but independent trends. For example on diagram differentiation of rocks composition, with normative content of anorthite on the X axis, trends of

  20. The Igneous Architecture of IODP Hole U1309D: Constructing Oceanic Crust from Multiple Sills

    NASA Astrophysics Data System (ADS)

    Christofferson, C. A.; John, B. E.; Cheadle, M. J.; Swapp, S. M.; Grimes, C. B.

    2010-12-01

    Gabbroic rocks comprise a significant portion of the footwall of many oceanic core complexes. The decreasing age of these gabbros from the breakaway to the termination suggests that they are continuously accreted as the bounding detachment fault slips. But, questions remain; how are these large (>1 km diameter) gabbroic bodies constructed and at what scale and frequency is melt added to the system? Here we report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif core complex (30° N, MAR). We present new, high resolution (1m scale) downhole lithological diagrams, compiled from observations of the archive core, refined using other available data including magnetic susceptibility. These data show that the thickness of the individual magmatic units is on the order of 1 to a few 10’s of m. Contacts between many of these units are sharp; many of the units are interpreted to result from small injections of melt as opposed to in-situ fractionation. However, these units are often disaggregated by later intrusive bodies and hence, it is likely that individual gabbro units were initially somewhat thicker. Downhole plots of magmatic fabric dip from both shipboard data and new electron backscatter diffraction (EBSD) data are consistent with the units having initially been intruded as sills, and subsequently rotated by ~40-50°, as constrained by paleomagnetic data (Morris et al., 2009). Using all of the available data, we propose the IODP Hole 1309D gabbro section is a composite body that grew episodically by relatively small (10’s m) repeated sill-like injections of melt. We conclude that the melt lens that formed these gabbros was relatively small at any one time. The EBSD data also provide textural constraints on the model for crustal accretion. Gabbroic samples analyzed so far show a moderate to weak plagioclase and clinopyroxene foliation, similar to those in continental mafic intrusions and are thus interpreted to be magmatic fabrics

  1. Paleomagnetic evidence bearing on Tertiary tectonics of the Tihamat Asir coastal plain, southwestern Saudi Arabia

    USGS Publications Warehouse

    Kellogg, K.S.; Blank, H.R.

    1982-01-01

    Paleomagnetic directions determined for an upper Oligocene to lower Miocene dike swarm and from two lower Miocene layered gabbros in the Tihamat Asir coastal plain of southwestern Saudi Arabia are used to test several hypotheses concerning the tectonics of rifting along the eastern margin of the Red Sea. The dikes and gabbros were emplaced during the initial phases of Red Sea rifting and may mark the transition between continental and oceanic crust. Although these rocks have been hydrothermally altered to varying degrees, reliable remanent directions after alternating-field demagnetization were obtained for 23 dikes and for gabbros at Jabal at Tirf and Wadi Liyyah. Twelve of the dikes are reversely magnetized. After the directions of the reversely magnetized dikes are inverted 180?, the mean direction calculated for the normal dikes is approximately 24? more downward than that calculated for the reversed dikes. This result is similar to that found for the As Sarat volcanic field, 100 km to the north, and may be due to a displaced dipole source for the field. The unrotated mean remanent direction for the dikes (inverting reversed dike directions 180?) is D (declination) = 353.2? and I (inclination) = 6.8? with a95 (radius of the cone of 95 percent confidence) = 8.9? whereas directions from the Jabal at Tirf and Wadi Liyyah gabbros lie at D = 176.2?, I = -1.6? (a95 = 7 1 ?) and D = 17.1?, I = 16.3? (a 95 = 8.7?), respectively. Comparing these results with the results from the As Sarat volcanic field, all the paleomagnetic evidence supports a model for approximately 20 ? of westward tilting of the Wadi Damad and Wadi Jizan areas after the emplacement of the Jabal at Tiff gabbro. The Wadi Liyyah area may have been tilted even more toward the Red Sea. The paleomagnetic directions from three widely separated localities in the Jabal at Tirf gabbro are not significantly different, a fact which indicates that the body cooled in approximately its present bowl shape. Evidence

  2. Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system

    NASA Astrophysics Data System (ADS)

    Gao, Ruohan; Lassiter, John C.; Barnes, Jaime D.; Clague, David A.; Bohrson, Wendy A.

    2016-05-01

    The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., "gabbro glacier" vs. "multiple sills"). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the "multiple sills" model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC. Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5-5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt-crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially

  3. An evolved axial melt lens in the Northern Ibra Valley, Southern Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Loocke, M. P.; Lissenberg, C. J.; MacLeod, C. J.

    2014-12-01

    The axial melt lens (AML) is a common feature lying at the base of the upper crust at fast-spreading mid-ocean ridges. It is thought to play a major role in the evolution of MORB and, potentially, accretion of the plutonic lower crust. In order to better understand the petrological processes that operate in AMLs we have examined the nature and variability of the horizon equivalent to the AML preserved in the Oman ophiolite. We present the results of a detailed investigation of a section east of Fahrah in the Ibra Valley. Here, a suite of 'varitextured' gabbros separates the sheeted dykes above from foliated gabbros below. It comprises 3 distinct units: an ophitic gabbro with pegmatitic patches (patchy gabbro; 70 m thick), overlain by a spotty gabbro (50 m), capped by a quartz-diorite (120 m). The sheeted dykes are observed to root in the quartz-diorite. Contacts between the plutonic units are gradational and subhorizontal. All of the units are isotropic. A total of 110 samples were collected for detailed petrographic and chemical analysis. With the exception of a small number of the diorites, all of the samples have a 'cumulate' component. Primary igneous amphibole is ubiquitous, present even as a minor phase in the foliated gabbros beneath, and indicating extensive differentiation and/or the presence of water in the primary liquid. France et al. (2014, Lithos) report patches of granoblastic material from this horizon in the Fahrah area, and suggest they represent the restites of partially melted pieces of the sheeted dykes. We did not, however, find any such granoblastic material, nor can the quartz-diorites represent partial melt; instead, preliminary geochemical modeling suggests that all of the units can be related by simple progressive fractional crystallization of an Oman axial ('V1' or 'Geotimes') melt. Along with the field relationships, as well as the basaltic andesite to dacite composition of the overlying sheeted dykes, this suggests that the AML was the

  4. Geophysical Investigations Along the Hayward Fault, Northern California, and Their Implications on Earthquake Hazards

    NASA Astrophysics Data System (ADS)

    Ponce, D. A.; Graymer, R. W.; Hildenbrand, T. G.; Jachens, R. C.; Simpson, R. W.

    2007-12-01

    Geophysical studies indicate that the Hayward Fault follows a pre-existing basement structure and that local geologic features play an important role in earthquake seismicity. The recent creeping trace of the Hayward Fault extends for about 90 km from San Pablo Bay in the northwest to Fremont in the southeast, and together with its northern extension, the Rodgers Creek Fault, is regarded as one of the most hazardous faults in northern California. The Hayward Fault is predominantly a right-lateral strike-slip fault that forms the western boundary of the East Bay Hills and separates Franciscan Complex rocks on the southwest from Coast Range Ophiolite and Great Valley Sequence basement rocks on the northeast. The Hayward Fault is characterized by distinct linear gravity and magnetic anomalies that correlate with changes in geology, structural trends, creep rates, and clusters of seismicity. These correlations indicate the existence of fault-zone discontinuities that probably reflect changes in mechanical properties. These fault-zone discontinuities may play a role in defining fault segments--locations where recurring seismic ruptures may tend to nucleate or terminate. Along the central part of the Hayward Fault, a prominent gravity and magnetic anomaly correlates with an exposed gabbro body, the San Leandro gabbro. Modeling of these anomalies reveals that the San Leandro gabbro is much more extensive in the subsurface than the outcrop pattern suggests, extending to a depth of about 6-8 km. The inferred extent of the San Leandro gabbro, it's geologic setting, and associated seismicity suggest that the Hayward Fault evolved from a pre-existing basement feature, similar to the ancestral Coast Range Fault. Combined modeling and relocated double-difference seismicity data indicate that the dip of the fault surface varies from near vertical in the north to about 75 degrees in the central part to about 50 degrees in the south near Fremont and ultimately connects with the

  5. Magmatic origin of alkaline meta-igneous rocks from Chamberlindalen, SW Svalbard

    NASA Astrophysics Data System (ADS)

    Goluchowska, Karolina; Barker, Abigail; Manecki, Maciej; Czerny, Jerzy; Majka, Jaroslaw

    2014-05-01

    This study focuses on the late Neoproterozoic meta-igneous rocks of SW Svalbard to determine their magmatic evolution, conditions of magma storage and origin. The samples from the Chamberlindalen area form an alkaline igneous suite, from which thin dikes and intrusive bodies have been collected. The rocks in question intrude Late Neoproterozoic metasediments and are surrounded by occurrences of Neoproterozoic metabasalts in contrast to highly alkaline the Chamberlindalen intrusions. The rocks from Chamberlindalen are divided into two groups based on their geochemistry, mineralogy and field relationships. The dikes, classify as minettes, belonging to the lamprophyre group and contain mainly euhedral, elongated phlogopite and additionally clinopyroxene and feldspar. The rest of the samples are highly magnesian and are classified as alkali gabbro. The alkali gabbros contain primary magmatic minerals such as clinopyroxene, calcic amphibole and mica in different proportions. The alkali gabbros are enriched in LREE and HFSE and depleted in P, K and HREE. The minette dikes are always more enriched in HFSE and REE in comparison to the alkali gabbros. The mineral chemistry of the alkali gabbros reveals that pyroxenes are represented by diopside with Wo46-51 En35-46 Fs6-14, and calcic amphibole by kaersutite. The Mg# number for diopside is from 72 - 88, whereas for kaersutite Mg# number is 51 - 74. Thermobarometry calculations for diopside and kaersutite have been performed. In the alkali gabbros from Chamberlindalen, diopside crystallized between 0.7 - 8 kbar and 1152 - 1233°C. Results for kaersutite reveal that they crystallized between 5 - 17 kbar and 1043 - 1215°C. For diopside the main crystallization was between 10 and 38 km, whereas for kaersutite, the main crystallization was between 30 and 50 km. Clinopyroxene and minor kaersutite also show a zone of crystallization at 2 to 10 km. This reflects a main crystallization zone at 10 - 50 km throughout the continental

  6. Magnetism of the oceanic crust: Evidence from ophiolite complexes

    SciTech Connect

    Banerjee, S.K.

    1980-07-10

    The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomanies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.

  7. AFBC bed material performance with low-rank coals

    SciTech Connect

    Goblirsch, G.M.; Benson, S.A.; Karner, F.R.; Rindt, D.K.; Hajicek, D.R.

    1983-01-01

    The purpose of this paper is to describe the reasons for carefully screening any candidate bed material for use in low-rank coal atmospheric fluidized-bed combustion, before the final selection is made. The sections of this paper describe: (1) the experimental equipment used to obtain the data, as well as the experimental and analytical procedures used in evaluation; (2) the results of tests utilizing various bed materials with particular emphasis on the problem of bed material agglomeration; and (3) the conclusions and recommendations for bed material selection and control for use with low-rank coal. Bed materials of aluminum oxide, quartz, limestone, dolomite, granite, gabbro, and mixtures of some of these materials have been used in the testing. Of these materials, gabbro appears most suitable for use with high available sodium lignites. 17 figures, 8 tables. (DMC)

  8. Luna 24 - Mineral chemistry of 90-150 micron clasts

    NASA Technical Reports Server (NTRS)

    Meyer, H. O. A.; Hwang, J.-Y.; Mccallister, R. H.

    1978-01-01

    The mineralogy, composition and source relations of monomineralic clasts in the size range 0.09 to 0.15 mm have been studied for seven grain mounts from the Luna 24 core, obtained in Mare Crisium. One of the core horizons, which showed the greatest number of mafic mineral clasts, apparently represents a less reworked level or one which received a greater average influx of gabbroic and/or basaltic ejecta. Most of the mafic minerals in the core were probably derived from the comminution of clasts of very low titanium basalts and/or gabbros. A small number of mineral clasts may have originated from a Mg-rich gabbro. A mixture of local mare basalts and ejecta from Fahrenheit crater probably makes up most of the regolith at the Luna 24 site.

  9. Origin and modal petrography of Luna 24 soils

    NASA Technical Reports Server (NTRS)

    Basu, A.; Mckay, D. S.; Fruland, R. M.

    1978-01-01

    Petrographic modal analyses of polished grain mounts of fractions in the 20 to 250 micron size range from Luna 24 soil samples are presented and used to infer the nature and relative contributions of source rocks. It is found that more than 90% of the identifiable rock fragments are mare basalts, with about 11% of the soil consisting of the crystalline form. Soil breccias, which make up nearly 10% of the soil, are found to be immature. Electron probe analysis of glass particles reveals principle clusters conforming to anorthosite, anorthositic gabbro and mare basalts. More than half of the soil is composed of monomineralic particles, with pyroxene as the most abundant mineral. It is concluded that 85% of the regolith is derived from local mare basalts and gabbros and about 10% is derived from early cumulates of local mare basalt magma. Highland sources are considered to contribute not more than 3% of the regolith.

  10. Laser /39/Ar-/40/Ar dating of two clasts from consortium breccia 73215

    NASA Technical Reports Server (NTRS)

    Eichhorn, G.; Schaeffer, O. A.; James, O. B.; Mueller, H. W.

    1978-01-01

    A laser Ar-39-Ar-40 study of the components of an ANT-suite anorthositic gabbro and a black aphanite from a consortium breccia is reported. A wide range of K-Ar ages is found for the plagioclase in the anorthositic gabbro; at the centers of the largest grains is material showing the greatest age (older than 4.11 billion years) while the youngest material (3.81-3.88 billion years) is found near the grain margins. Partial outgassing of the clasts upon incorporation into the breccia could account for the age patterns. The black aphanite clast appears to be cogenetic with the aphanite that forms the breccia matrix. The time of crystallization of a lunar granite has also been measured by the laser technique.

  11. Petrology of lunar rocks from the Sea of Fertility

    NASA Technical Reports Server (NTRS)

    Pavlenko, A. S.; Tarasov, L. S.; Shevaleyevskiy, I. D.; Ivanov, A. V.

    1974-01-01

    Based on a comparative analysis of the petrochemistry of an integrated sample of gabbro basalts and a fine fraction from regolith returned by Luna 16 automatic station from the Sea of Fertility, with the mean compositions of various types of mare basalts, anorthosites, and regolith from the Sea of Tranquillity and the Ocean of Storms, with reference to several data on rare elements, the nature of the fine fractions is discussed. It is shown that the integrated sample of gabbro basalt from the coarse fraction in the lower part of the core can be represented as a mixture of mare basalts of the Sea of Tranquillity and nonmare basalts of the krip type in the ratio of about 3 to 2. It is confirmed that the compositions of the Apollo 11 and Apollo 12 regolith are complementary with the compositions of basalts and anorthosites of the Sea of Tranquillity and the Ocean of Storms.

  12. Results from the Apollo passive seismic experiment

    NASA Technical Reports Server (NTRS)

    Latham, G.; Nakamura, Y.; Dorman, J.; Duennebier, F.; Ewing, M.; Lammlein, D.

    1977-01-01

    Recent results from the Apollo Seismic Network suggest that primitive differentiation occurred in the outer shell of the moon to a depth of approximately 300 km and the central region of the moon is presently molten to a radius of between 200 and 300 km. If early melting to a depth of 300 to 400 km was a consequence of accretional energy, very short accretion times are required. It was shown that the best model for the zone of original differentiation is a crust 40 to 80 km thick, ranging in composition from anorthositic gabbro to gabbro, and overlying an ultramafic cumulate about 250 km thick. The best candidate for the molten core appears to be iron or iron sulphide. A new class of seismic signals recently were identified that may correspond to shallow moonquakes. These are rare, but much more energetic than the more numerous, deep moonquakes.

  13. Possible types of rotations and translations in the Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Abrahamsen, Niels

    1985-06-01

    Folding, faulting and large scale translation nappe movements are well-known classical collision zone features, which are also found in the Scandinavian Caledonides where lateral movements of up to a few hundred kilometres have been suggested. In contrast to this, azimuthal rotations around vertical axes are rarely or never observed geologically. In a few cases, palaeomagnetic data, however, show abnormal directions of the remanent magnetic components, which may indicate vertical axes of rotations. Different types and scales of palaeomagnetic interpretations are classified and discussed in relation to 5 examples from the Scandinavian Caledonides (the Särv nappe, the Lönset dome, the Fongen gabbro, the Sulitjelma gabbro, and the Trollfjord-Komagelv fault), and various models to explain segmental rotations of the crust are suggested.

  14. Results from the Apollo passive seismic experiment

    NASA Technical Reports Server (NTRS)

    Lathum, G.; Nakamura, Y.; Dorman, J.; Duennebier, F.; Ewing, M.; Lammlein, D.

    1974-01-01

    Recent results from the Apollo seismic network suggest that primitive differentiation occurred in the outer shell of the moon to a depth of approximately 300 km; and the central region of the moon is presently molten to a radius of between 200 and 300 km. If early melting to a depth of 300 to 400 km was a consequence of accretional energy, very short accretion times are required. The best model for the zone of original differentiation appears to be a crust 40 to 80 km thick, ranging in composition from anorthositic gabbro to gabbro; overlying an ultramafic cumulate (olivine-pyroxene) about 250 km thick. The best candidate for the molten core appears to be iron or iron sulphide. A new class of seismic signals has recently been identified that may correspond to shallow moonquakes. These are rare, but much more energetic than the more numerous, deep moonquakes.

  15. The relationships between geology and soil chemistry at the Apollo 17 landing site

    NASA Technical Reports Server (NTRS)

    Rhodes, J. M.; Rodgers, K. V.; Bansal, B. M.; Wiesmann, H.; Shih, C.; Nyquist, L. E.; Hubbard, N. J.

    1974-01-01

    Within the wide compositional range of the Apollo 17 soils, three distinct chemical groups have been recognized, each one corresponding broadly with a major geological and physiographic unit. These groups are: (1) Valley Floor type soils, (2) South Massif type soils, and (3) North Massif type soils. The observed chemical variations within and between these three groups is interpreted by means of mixing models in terms of lateral transport and mixing of prevailing local rock types, such as high-titanium basalts, KREEP-like noritic breccias, anorthositic gabbro breccias and orange glass. According to these models, North Nassif types evolved on the lower slopes of the North Massif and Sculptured Hills where anorthositic gabbro predominates over noritic breccia and where lateral mixing with basalt is effective, whereas the South Massif type soils originally developed on the upper slopes of the South Massif, where anorthositic breccia and noritic breccias are equally abundant, and where lateral mixing with basalt was minimal.

  16. Alpine-type sensu strictu(ophiolitic) peridotites: Refractory residues from partial melting or igneous sediments? A contribution to the discussion of the paper: "The origin of ultramafic and ultrabasic rocks" by P.J. Wyllie

    USGS Publications Warehouse

    Thayer, T.P.

    1969-01-01

    Although Alpine peridotites and basaltic lavas are widely associated in eugeosynclines and oceanic areas, their genetic ties are obscure. Three major characteristics of olivine-rich Alpine peridotite and dunite-relict cumulus textures, aggregated masses of chromitite, and intimate association with magnesium-rich gabbro - cannot be explained by partial melting of garnet peridotite to form tholeiite. Association of magnesium-rich gabbro with the chromite-bearing and so-called high-temperature Alpine peridotites is believed to present problems that have not been considered by advocates of the partial-melting hypothesis. The chromite-bearing Alpine peridotites and related feldspathic rocks are believed to have formed near the top of the mantle by gravitational differentiation processes which are largely independent of the melting processes that produce basaltic magma at depths of 50 km or more. ?? 1969.

  17. Mesosiderite clasts with the most extreme positive europium anomalies among solar system rocks

    NASA Technical Reports Server (NTRS)

    Mittlefehldt, David W.; Rubin, Alan E.; Davis, Andrew M.

    1992-01-01

    Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.

  18. Compositions of Normal and Anomalous Eucrite-Type Mafic Achondrites

    NASA Technical Reports Server (NTRS)

    Mittlefehldt, D. W.; Peng, Z. X.; Mertzman, S. A.

    2016-01-01

    The most common asteroidal igneous meteorites are eucrite-type mafic achondrites - basalts and gabbros composed of ferroan pigeonite, ferroan augite, calcic plagioclase, silica, ilmenite, troilite, Ca-phosphate, chromite and Fe-metal. These rocks are thought to have formed on a single asteroid along with howardites and diogenites. However, high precision O-isotopic analyses have shown that some mafic achondrites have small, well-resolved, non-mass-dependent differences that have been interpreted as indicating derivation from different asteroids. Some of these O-anomalous mafic achondrites also have anomalous petrologic characteristics, strengthening the case that they hail from distinct parent asteroids. We present the results of bulk compositional studies of a suite of normal and anomalous eucrite-type basalts and cumulate gabbros.

  19. Mantle flow patterns and magma chambers at ocean ridges: Evidence from the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolas, A.; Boudier, F.; Ceuleneer, G.

    1988-12-01

    As a result of an extensive program of structural mapping in the ultramafic section of the Oman ophiolite, maps of mantle flow below the spreading center of origin have been drawn. They reveal a mantle diapiric system in which the uppermost mantle flow diverges from diapirs 10 15 km across, which could have been spaced by an average distance of 50 km. Some diapirs could have been located off-axis. The rotation of flow lines in the diapirs occurs within the few hundred meters of the transition zone separating the mantle and crustal formations. The importance of this zone is stressed. The structure of the layered gabbros of the crustal unit in most places reflects a large magmatic flow induced by the solid state flow in the underlying peridotites. The magmatic foliation of the gabbros steepens upsection and becomes parallel to the sheeted dike attitude. A new model of a tent-shaped magma chamber is derived from these structural data.

  20. Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr-Nd-Hf isotopic characterization

    NASA Astrophysics Data System (ADS)

    Liu, Tong; Zhai, Qing-guo; Wang, Jun; Bao, Pei-sheng; Qiangba, Zhaxi; Tang, Suo-han; Tang, Yue

    2016-02-01

    The Dongqiao ophiolite occurs in the central segment of the Bangong-Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30 years' studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U-Pb dating yielded ages of 188 ± 1 Ma for the layered gabbro and 181 ± 1 Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong-Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong-Nujiang suture zone. If so, the Tethys Ocean of the Bangong-Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

  1. The evolution of the Bangong-Nujiang Neo-Tethys ocean: Evidence from zircon U-Pb and Lu-Hf isotopic analyses of Early Cretaceous oceanic islands and ophiolites

    NASA Astrophysics Data System (ADS)

    Fan, Jian-Jun; Li, Cai; Xie, Chao-Ming; Wang, Ming; Chen, Jing-Wen

    2015-08-01

    We conducted in situ U-Pb analyses of zircons from three basalts and one gabbro from the Zhonggang oceanic island, one basalt from the Zhaga oceanic island, and one gabbro from the Kangqiong ophiolite (all located in the middle segments of the Bangong-Nujiang suture zone of Tibetan Plateau), as well as in situ Hf isotope analyses of zircons from one gabbro from the Zhonggang oceanic island to constrain the tectonic evolution of the Bangong-Nujiang Neo-Tethys ocean. All samples contain numerous inherited zircons, and all the zircons contain magmatic oscillatory zoning and have Th/U ratios exceeding 0.4. Moreover, the average ΣREE content of these zircons is less than 2000 ppm, and they display clear negative Eu and variable positive Ce anomalies, indicating a magmatic origin. LA-ICP-MS U-Pb dating of the zircons revealed three clear peaks in the age distribution, at 248-255, 162-168, and 117-120 Ma; Lu-Hf isotopic analyses of zircons from the gabbro of the Zhonggang oceanic island yielded a 269 Ma crust-mantle separation age. Taking into account the regional geology, previous data, and our new analyses, we infer that the middle and western segments of the Bangong-Nujiang Neo-Tethys ocean had initially opened in the late Permian (254-269 Ma) and that the ocean opened substantially between the late Permian and the Early Triassic (248-255 Ma). In addition, we infer that the initiation of subduction of the Bangong-Nujiang Neo-Tethys ocean took place at ~ 162-168 Ma, which is Middle Jurassic. The 117-120 Ma age is the time when the oceanic islands and ophiolites were formed, indicating that the Bangong-Nujiang Neo-Tethys ocean was, to some extent, still open at that time.

  2. Mechanical and transport properties of rocks at high temperatures and pressures. Task I, the physical nature of fracturing at depth. Technical progress report No. 1, 1 March 1980-30 November 1980

    SciTech Connect

    Carter, N.L.

    1980-12-15

    Research progress is reported in the following areas: (1) the delineation of the boundary separating elastic-brittle and transient-1 semibrittle behavior of granite and of its volcanic and metamorphic equivalents, rhyolite and granite gneiss; (2) the variation of fracture permeability in Sioux Quartzite, Westerly Granite and a fine-1 grained gabbro as a function of effective pressure and hydrothermal alterations; and (3) determine the mechanical properties of selected rocks at high temperatures and pressures. (ACR)

  3. The petrographic criteria of selection of geological environments for building high-level waste (HLW) repository

    SciTech Connect

    Omelyanenko, B.I.; Petrov, V.A.; Yudintsev, S.V.; Zaraisky, G.P.; Starostin, V.I.

    1993-12-31

    Igneous rocks of basic composition (basalts, diabases, gabbro-dolerites, dunites, etc.) are an appropriate geological environment for high-level waste disposal. During interaction with hot ground waters their isolation ability will increase due to the decrease of hydraulic permeability and increase of their sorption ability. According to petrophysical characteristics, such rocks are viscous-rigid media with the highest mechanical stability and do not undergo any changes in properties over the whole temperature range, which is possible in a HLW repository.

  4. Solonker ophiolite in Inner Mongolia, China: A late Permian continental margin-type ophiolite

    NASA Astrophysics Data System (ADS)

    Luo, Zhi-wen; Xu, Bei; Shi, Guan-zhong; Zhao, Pan; Faure, M.; Chen, Yan

    2016-09-01

    The Solonker ophiolite is exposed along the border between Mongolia and China within the Solonker zone, the southeastern Central Asian Orogenic Belt (CAOB), and it is composed dominantly of serpentinized peridotite with subordinate gabbro, basaltic lava, radiolarian-bearing siliceous rocks, and minor plagiogranite. Meanwhile, layered mafic-ultramafic cumulates are not ubiquitous. In this study, zircon grains from two gabbros and a plagiogranite yield 206Pb/238U ages of 259 ± 6 Ma, 257 ± 3 Ma and 263 ± 1 Ma. These data were interpreted to represent the formation age of the Solonker ophiolite. The studied gabbros and basalts have a tholeiitic composition, showing a MORB affinity. They are also characterized by enrichment of Pb and depletion of Nb relative to La and Th. Furthermore, the studied gabbros contain inherited zircon grains and display a large range of zircon Hf isotopes (εHf(t) = - 5.27 to + 10.19). These features imply that crustal contamination played an important role in the generation of these mafic rocks. Major elements derived from the radiolarian-bearing siliceous rocks suggest a continental margin setting. This is confirmed by rock association. Terrigenous rocks (sandstones and siltstones) interstratified with siliceous rocks. U-Pb dating of detrital zircon grains in sandstones from both the northern and southern sides of the Solonker ophiolite belt, along with published data, reveals that the Late Carboniferous-Early Permian strata in fault contact with the Solonker ophiolite was deposited above Early Paleozoic orogens. The lines of petrological, geochemical, geochronological, and isotopic evidence led us to propose that the Solonker ophiolite is a Late Permian continental margin-type body formed during the early stages of opening of an ocean basin, following rifting and break-up of the Early Paleozoic orogens. Accordingly, the Permian Solonker zone is characterized by an intra-continental extensional setting.

  5. Mineral resources of the Prospect Mountain Wilderness Study Area, Carbon County, Wyoming

    SciTech Connect

    du Bray, E.A.; Bankey, V.; Hill, R.H.; Ryan, G.S.

    1989-01-01

    The Prospect Mountain Wilderness Study Area is about 20 mi east-southeast of Encampment in Carbon County, Wyoming. This study area is underlain by middle Proterozoic gabbro, granite, and hornblende gneiss, which is locally cut by pegmatite dikes. There are no identified resources and no potential for undiscovered energy resources in this study area. Resource potential for all undiscovered metallic commodities and for industrial mineral is low.

  6. The Mars Hill Terrane: An enigmatic southern Appalachian terrane

    SciTech Connect

    Raymond, L.A.; Johnson, P.A. . Dept. of Geology)

    1994-03-01

    The Mars Hill Terrane (MHT) in the Appalachian Blue Ride Belt is bordered by complex, locally reactivated thrust and strike-slip faults. On the east, the MHT is bounded by the allochthonous, ensimatic Toe Terrane (TT) across the diachronous, ductile Holland Mountain-Soque River Fault System. The MHT is separated on the northwest from ensialic Laurentian basement (LB), by the Fries-Hayesville Fault System. On the south, the MHT is truncated by the Shope Fork Fault. The MHT is characterized by migmatitic biotite-pyroxene-hornblende gneiss, but contains 1--1.8 b.y. old quartz-feldspar gneisses, plus ultramafic rocks, calc-silicate rocks, mica schists and gneisses, and Neoproterozoic Bakersville gabbros. This rock assemblage contrasts with that of the adjoining terranes. The only correlative units between the MHT and adjoining terranes are Neoproterozoic gabbro, Ordovician-Devonian granitoid plutons, and ultramafic rocks. Gabbro links the MHT with LB rocks. Apparently similar calc-silicate rocks differ petrographically among terranes. During Taconic or Acadian events, both the TT and MHT reached amphibolite to granulite metamorphic grade, but the LB did not exceed greenschist grade. The data conflict. The O-D plutons, ultramafic rocks, and metamorphic histories suggest that the TT had docked with the MHT by Ordovician time. The premetamorphic character of the Holland Mtn.-Soque River Fault System supports that chronology. Neoproterozoic gabbros suggest a MHT-LB link by Cambrian time, but the LB experienced neither O-D plutonism nor Paleozoic amphibolite-granulite facies metamorphism.

  7. Isotopic evidence of source variations in commingled magma systems: Colorado River extensional corridor, Arizona and Nevada

    SciTech Connect

    Metcalf, R.V.; Smith, E.I.; Martin, M.W. . Dept. of Geoscience); Gonzales, D.A.; Walker, J.D. . Isotope Geochronology Lab.)

    1993-04-01

    Mixing of mantle derived mafic and crustal derived felsic magmas is a major Province-wide process forming Tertiary intermediate magmas within the Basin and Range. Major variations in magma sources, however, may exist in temporally and spatially related systems. Such variations are exemplified by two closely spaced plutons within the northern Colorado River extensional corridor. The 15.96 Ma Mt. Perkins pluton (MPP) was emplaced in three major phases: phase 1 (oldest) gabbro; phase 2 quartz diorite to hornblende granodiorite; and phase 3 biotite granodiorite ([+-]hbld). Phases 2 and 3 contain mafic microgranitoid enclaves (MME) that exhibit evidence of magma mingling. Combined data from phase 2 and 3 rocks, including MMW, shows positive [sup 87]Sr/[sup 86]Sr and negative [var epsilon]Nd correlations vs. SiO[sub 2] (50--72 wt %). Phase 2 rocks, which plot between phase 2 MME and MME-free phase 3 granodiorite, represent hybrid magmas formed by mixing of mantle and crustal derived magmas. Phase 1 gabbro falls off isotope-SiO[sub 2] trends and represents a separate mantle derived magma. The 13.2 Ma Wilson Ridge pluton (WRP), <20 km north of MPP, is cogenetic with the river Mountains volcano (RMV). In WRP an early diorite was intruded by a suite of monzodiorite to quartz monzonite. The monzodiorite portion contains MME and mafic schlieren representing mingled and mixed mafic magmas. The WRP and MPP represent two closely spaced isotopically distinct and separate magma systems. There are five magma sources. The two felsic mixing end members represent two different crustal magma sources. Two mantle sources are presented by MPP phase 1 gabbro and phase 2 MME, reflecting lithospheric and asthenospheric components, respectively. The latter represents the oldest reported Tertiary asthenospheric component within the region. A single lithospheric mantle source, different from the MPP gabbro, is indicated for the mafic mixing end member in the WRP-RMV suite.

  8. Apollo 17, Station 6 boulder sample 76255 - Absolute petrology of breccia matrix and igneous clasts

    NASA Technical Reports Server (NTRS)

    Warner, J. L.; Phinney, W. C.; Simonds, C. H.

    1976-01-01

    The matrix of 76255 is the finest-grained, most clast-laden, impact-melt polymict breccia sampled from the Station 6 boulder. The paper speculates on how the matrix of 76255 fits into and enhances existing thermal models of breccia lithification. Emphasis is on the detailed petrology of five lithic clasts, two of which display mineralogical and textural affinities to mare basalts, while three, a gabbro, a norite, and a troctolite are considered primitive plutonic rocks.

  9. Breccias from the lunar highlands - Preliminary petrographic report on Apollo 16 samples 60017 and 63335.

    NASA Technical Reports Server (NTRS)

    Kridelbaugh, S. J.; Mckay, G. A.; Weill, D. F.

    1973-01-01

    Lunar samples 60017,4 and 63335,14 are composed of microbreccias and devitrified glass. These components are predominantly anorthositic, with the exception of a cryptocrystalline clast found in the microbreccia portion of 63335,14 which contains 2.7% potassium oxide and 66.7% silicon dioxide. The samples have been subjected to extreme shock and thermal metamorphism. The parent materials of the microbreccias include both a coarse-grained anorthosite and a fine-grained subophitic anorthositic gabbro.

  10. Apollo 17 petrology and experimental determination of differentiation sequences in model moon compositions

    NASA Technical Reports Server (NTRS)

    Hodges, F. N.; Kushiro, I.

    1974-01-01

    Experimental studies of model moon compositions are discussed, taking into account questions related to the differentiation of the outer layer of the moon. Phase relations for a series of proposed lunar compositions have been determined and a petrographic and electron microprobe study was conducted on four Apollo 17 samples. Two of the samples consist of high-titanium mare basalts, one includes crushed anorthosite and gabbro, and another contains blue-gray breccia.

  11. Magnetic anomalies in Bahia Esperanza: A window of magmatic arc intrusions and glacier erosion over the northeastern Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Galindo-Zaldívar, Jesús; Ruiz-Constán, Ana; Pedrera, Antonio; Ghidella, Marta; Montes, Manuel; Nozal, Francisco; Rodríguez-Fernandez, Luis Roberto

    2013-02-01

    Bahia Esperanza, constituting the NE tip of the Antarctic Peninsula, is made up of Paleozoic clastic sedimentary rocks overlain by a Jurassic volcano-sedimentary series and intruded by Cretaceous gabbros and diorites. The area is located along the southern part of the Pacific Margin magnetic anomaly belt. Field magnetic researches during February 2010 contribute to determining the deep geometry of the intermediate and basic intrusive rocks. Moreover, the new field data help constrain the regional Pacific Margin Anomaly, characterized up to now only by aeromagnetic and marine data. Field magnetic susceptibility measurements of intrusive intermediate and basic rocks, responsible for magnetic anomalies, ranges from 0.5 × 10- 3 SI in diorites to values between 0.75 × 10- 3 SI and 1.3 × 10- 3 SI in gabbros. In addition, a significant remanent magnetism should also have contributed to the anomalies. The regional magnetic anomaly is characterized by a westward increase from 100 nT up to 750 nT, associated with large intrusive diorite bodies. They probably underlie most of the western slopes of Mount Flora. Gabbros in the Nobby Nunatak determine local residual rough anomalies that extend northwards and westwards, pointing to the irregular geometry of the top of the basic rocks bodies below the Pirámide Peak Glacier. However, the southern and eastern boundaries with the Buenos Aires Glacier are sharp related to deep glacier incision. As a result of the glacier dynamics, magnetic anomalies are also detected north of the Nobby Nunatak due to the extension of the anomalous body and the presence of gabbro blocks in the moraines. The Bahia Esperanza region is a key area where onshore field geological and magnetic research allows us to constrain the shape of the crustal igneous intrusions and the basement glacier geometry, providing accurate data that complete regional aeromagnetic research.

  12. Tsumoite (BiTe) and associated Ni-PGE mineralization from Gondpipri mafic-ultramafic complex, Bastar Craton, Central India: mineralogy and genetic significance

    NASA Astrophysics Data System (ADS)

    Dora, M.; Singh, H.; Kundu, A.; Shareef, M.; Randive, K.; Joshi, S.

    2014-12-01

    This paper reports the occurrence of Tsumoite (a bismuth telluride) in the Heti Cu-Ni-PGM prospect, Gondpipri mafic-ultramafic complex, Central India. The Gondpipri complex consists of several tectonically dismembered gabbronorite-gabbro-anorthositic gabbro — olivine gabbro -websterite disposed in ˜10 km long tonalite-trondhjemitegranodiorite (TTG) and charnockite-enderbite suite of rocks. The mineralization occurs in the sulphide zone hosted by gabbro variants. The host rocks have been deformed and metamorphosed to granulite grade and subjected to various degrees of hydrothermal alteration. The mineralization comprises chalcopyrite, pentlandite, pyrrhotite, cubanite, millerite, and pyrite. In addition to these, occur (1) tsumoite (2) PGM in the form of moncheite, merenskyite, Pd-mellonite, and Pt-Pd-Te-Bi-Fe-S alloy. The present study indicates that the mineralization occurs in two stages related to: (i) magmatic and (ii) hydrothermal remobilization and transport of Cu-rich sulphides, tsumoite and PGM, and their re-deposition in hydrosilicate alteration zones. It is possible that the mineralization at Heti formed at different stages of bismuth activity under variable fS2, T, and fTe2 conditions due to change in total concentration of Te and S and /or cooling. Since the role of S is limited, Te and cooling are important factor influencing mineralogy and composition of tsumoite and associated mineralization. Mineralization occurs in two different modes of occurrences. The early mineralisation occur as blebs, specks and dissemination of sulphides, viz. pyrrhotite, chalcopyrite, pentlandite and minor pyrite ± PGM, whereas later mineralisation occur as stringers, minor veins of sulphides viz. pyrite, millerite, cubanite, sijenite, tsumoite and ± PGM. Mineral assemblages and textural relationships at Heti has indicated precipitation of tsumoite and associated PGM along fractures and secondary silicates, which confirms their hydrothermal origin.

  13. The petrogenesis of late Neoproterozoic mafic dyke-like intrusion in south Sinai, Egypt

    NASA Astrophysics Data System (ADS)

    Azer, M. K.; Abu El-Ela, F. F.; Ren, M.

    2012-08-01

    New field, petrographical and geochemical studies are presented here for the late Neoproterozoic Rimm intrusion (˜15 km long) exposed in the southern Sinai Peninsula, Egypt in the northernmost Arabian-Nubian Shield (ANS). Field relations indicate that the Rimm intrusion is younger than the surrounding metamorphic rocks and calc-alkaline syn-tectonic granodiorite and it was not affected by regional metamorphism. The anorogenic peralkaline granite of Gebel Serbal crosscuts the Rimm intrusion. The Rimm intrusion is made up of several consanguineous rock types with gradational contacts. It is composed chiefly of pyroxene-hornblende gabbro, hornblende gabbro and minor quartz diorite. The chemical composition of the mafic minerals indicated that the studied rocks derived from calc-alkaline mafic magma. Geochemically, the studied rocks are characterized by enrichment in LILE relative to HFSE and LREE relative to HREE [(Ce/Yb)N = 4.50-6.36]. Quartz diorite display slightly concave HREE pattern and slightly negative Eu-anomaly [(Eu/Eu*)n = 0.91] which may be the result of fractionation of amphibole and plagioclase from the source melt, respectively. The Rimm intrusion evolved from mafic mantle magma into different type rocks by fractional crystallization with minor crustal contamination. The initial magma corresponds to pyroxene-hornblende gabbro and the crystallization of hornblende was caused by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene and Ca-rich plagioclase. Amphiboles geobarometer indicate that the gabbroic rocks of the Rimm intrusion crystallized at pressures between 4.8 and 6.4 Kb, while quartz diorite crystallized at 1.3-2.1 Kb. Crystallization temperatures range between 800 and 926 °C for the gabbros and between 667 and 784 °C for the quartz diorite. The Rimm intrusion represents a post-orogenic phase formed during the crustal thinning and extension of the Arabian-Nubian Shield.

  14. New Sm Nd ages for the Ivrea Verbano Zone, Sesia and Sessera valleys (Northern-Italy)

    NASA Astrophysics Data System (ADS)

    Mayer, Adriano; Mezger, Klaus; Sinigoi, Silvano

    2000-02-01

    The Ivrea-Verbano Zone (IVZ) is a slice of South-Alpine lower crust intruded by mantle magma. The main episode of magmatic underplating occurred during the Permian time and was related to continental rifting. We have carried out mineral Sm-Nd isochron dating of samples from the IVZ, including a gabbro dyke on which apparent Oligocene zircon ages have been reported, and a metapelite. The gabbro dyke sampled yielded three isochrons (cpx, opx, plag, amph and whole rocks) with the same initial 143Nd / 144Nd and an age of 274 ± 11 Ma (95% c.l., MSWD = 1.31) for the last major thermal event. Since this dyke is an embayment of the surrounding layered gabbro in the Balmuccia Peridotite, this age also represents the minimum age for the cooling of mafic-ultramafic sequence and for the emplacement of the Balmuccia peridotite in the IVZ. These conclusions are in contrast with those of Gebauer et al. (1992), who reported Oligocene U-Pb ages for magmatic zircons extracted from the same rocks. In order to evaluate the effect of Alpine intrusions on the IVZ, a sample of amphibole gabbro from the upper Sessera Valley was collected near Oligocene magmatic rocks. An internal Sm-Nd isochron (amph + cpx + plag + wr) from this sample yielded an age of 267 ± 21 Ma (95% c.l., MSWD = 0.716). Thus, despite the vicinity of the Oligocene magmatic activity, no thermal overprint (at T>650°C) occurred in this part of the IVZ more recently than 267 ± 21 Ma. Furthermore, a paragneiss septum within a shallower part of the intrusion in the lower Sessera Valley yielded a middle Triassic Sm-Nd garnet, biotite, plagioclase and whole rock age of 244 ± 4 Ma (95% c.l).

  15. Consortium wins major Brazilian gas contract

    SciTech Connect

    O`Driscoll

    1994-08-16

    An international consortium of BHP of Australia, Tenneco Gas of the U.S. and British Gas was selected Monday by Petroleo Braileiro SA (Petrobras) to Monday by Petroleo Brasileiro SA (Petrobras) to develop a $2 billion natural gas pipeline linking reserves in Bolivia with markets in southern and southeastern Brazil.

  16. MOBRAL--Seminario Interamericano de Educacion de Adultos (MOBRAL--Interamerican Seminar on Adult Education). Final Report.

    ERIC Educational Resources Information Center

    Ministerio da Educacao e Cultura, Rio de Janeiro (Brazil). Movimento Brasileiro de Alfabetizacao.

    The report contains the substance of the MOBRAL (Movimento Brasileiro de Alfabetizacao)--Interamerican Seminar on Adult Education held in Rio de Janeiro from April 9th to 18th, 1973, for invited representatives from 21 Latin America and Caribbean countries. The object was to make a contribution to the collective task of identifying, defining, and…

  17. Practical control strategy eliminates FCCU compressor surge problems

    SciTech Connect

    Campos, M.C.M.M.; Rodriques, P.S.B. )

    1993-01-11

    This paper reports that the control system originally designed for the fluid catalytic cracking unit (FCCU) compressor at Petroleo Brasileiro SA's (Petrobras) Presidente Bernardes refinery, in Sao Paulo, Brazil, was inadequate. The system required almost permanent flow recirculation to prevent surge. An improved antisurge control strategy was implemented in mid-1990. Since then, the unit has operated without the former surge problems.

  18. Photoproduction of charm particles at Fermilab

    SciTech Connect

    Cumalat, John P.

    1997-03-15

    A brief description of the Fermilab Photoproduction Experiment E831 or FOCUS is presented. The experiment concentrates on the reconstruction of charm particles. The FOCUS collaboration has participants from several Central American and Latin American institutions; CINVESTAV and Universidad Autonoma de Puebla from Mexico, University of Puerto Rico from the United States, and Centro Brasileiro de Pesquisas Fisicas in Rio de Janeiro from Brasil.

  19. Feruvite from the Sullivan Pb-Zn-Ag deposit, British Columbia

    USGS Publications Warehouse

    Jiang, S.-Y.; Palmer, M.R.; McDonald, A.M.; Slack, J.F.; Leitch, C.H.B.

    1996-01-01

    Feruvite, an uncommon Ca- and Fe2+-rich tourmaline species, has been discovered in the footwall of the Sullivan Pb-Zn-Ag deposit (British Columbia) near gabbro sills and dikes. Its chemical composition varies according to occurrence: feruvite from the shallow footwall has lower Ca, higher Al, and higher X-site vacancies than that from the deep footwall. The major chemical substitution involved in the feruvite is the exchange vector CaMgO???-1Al-1(OH)-1. The most important factor controlling feruvite formation at Sullivan is likely the reaction of Fe-rich hydrothermal fluids with Ca-rich minerals in gabbro and host rocks. This reaction led to the breakdown of Ca-rich minerals (plagioclase and hornblende), with release of Ca to solution and its incorporation into feruvite. This process probably postdated the main stages of formation of fine-grained, intermediate schorl-dravite in the tourmalinite pipe in the footwall, and is attributed to postore intrusion of gabbro and associated albite-chlorite-pyrite alteration.

  20. Thermal diffusivity of igneous rocks at elevated pressure and temperature

    SciTech Connect

    Durham, W.B.; Mirkovich, V.V.; Heard, H.C.

    1987-10-10

    Thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 /sup 0/C and pressures of 200 MPa. The measuring method was based on the concept of cylindrical symmetry and periodic heat pulses. The seven rocks measured were Westerly (Rhode Island) granite, Climax Stock (Nevada) quartz monzonite, Pomona (Washington) basalt, Atikokan (Ontario, Canada) granite, Creighton (Ontario, Canada) gabbro, East Bull Lake (Ontario, Canada) gabbro, and Stripa (Sweden) granite. The diffusivity of all the rocks showed a positive linear dependence on inverse temperature and, excluding the East Bull Lake gabbro, showed a linear dependence on quartz content. (Quartz content varied from 0 to 31% by volume.) Diffusivity in all cases rose or remained steady with increasing confining pressure. The pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 MPa, depending on rock type. The pressure effect (measured as a percentage change in diffusivity) is stronger in the four rocks of granite composition than in the three of basaltic composition. Our results agree well with existing thermal diffusivity measurements at atmospheric pressure.

  1. The lunar highland melt-rock suite

    NASA Technical Reports Server (NTRS)

    Vaniman, D. T.; Papike, J. J.

    1978-01-01

    Size can be used as a criterion to select 18 large (larger than 1 cm) samples from among 148 melt-rock fragments of all sizes. This selection provides a suite of large samples which represent the important chemical variants among highland melt rocks; each large sample has enough material for a number of sample-destructive studies, as well as for future reference. Cluster analysis of the total data base of 148 highland melt rocks shows six distinct groups: anorthosite, gabbroic anorthosite, anorthositic gabbro ('highland basalt'), low K Fra Mauro, intermediate-K Fra Mauro, and high-K. Large samples are available for four of the melt-rock groups (gabbroic anorthosite, anorthositic gabbro, low-K Fra Mauro, and intermediate-K Fra Mauro). This sample selection reveals two subgroups of anorthositic gabbro (one anorthite-poor with negative Eu anomaly and one anorthite-rich without Eu anomaly). There is a sharp distinction between those Apollo 16 melt rocks and glasses which have both been classified as 'gabbroic anorthosite'.

  2. Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

    PubMed

    Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

    1999-09-01

    Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin. PMID:10504137

  3. New evidence for Early Paleozoic orogeny in the eastern Klamath terrane, northern California

    SciTech Connect

    Masson, P.H.

    1993-04-01

    The Grey Rocks outlier, in the east-central part of the Trinity mafic-ultramafic complex, has two localities in which metamorphosed and strongly deformed rocks lie unconformably between the Trinity complex and overlying, less deformed, volcanic and sedimentary sections. At one locality, a bouldery metaconglomerate and schistose sedimentary melange consist of detritus derived from Trinity serpentine, gabbros, and dikes. Foliation is vertical, striking discordantly beneath mafic submarine flows and a thick argillite section assigned questionably to the Mississippian Bragdon formation. At the second locality, interbedded argillite, thin discontinuous flows, and hyaloclastic lapilli tuffs are metamorphosed and isoclinally folded. Steep foliation is truncated beneath the gently dipping base of the Grey Rocks submarine volcanic pile. Undeformed dikes cut the folded sequence and enter the Grey Rocks volcanics, possibly as late feeders. The Grey Rocks lavas and volcanic breccias have been compared with the Early Devonian Copley greenstone of the Redding section. Timing of the orogenic event or events is poorly constrained. Accepting a questionable correlation between the submarine volcanics and the Copley greenstone, the orogenic event(s) would be Early Devonian or older. Clasts in the schists and conglomerates at the first locality are clearly derived from gabbros and dikes considered by some workers to be Silurian. The folded sequence at the second locality contains no Trinity complex detritus and rests on a strong, low-angle, schist and mylonite zone that truncates the Silurian( ) dike and gabbro complex.

  4. Age of the Mulcahy Lake intrusion, northwest Ontario, and implications for the evolution of greenstone-granite terrains

    NASA Technical Reports Server (NTRS)

    Morrison, D. A.; Bogard, D. D.; Phinney, W. C.; Davis, D. W.; Wooden, J. L.; Ashwal, L. D.; Maczuga, D. E.

    1985-01-01

    An investigation of zircon data from the Mulcahy Lake gabbro, a 63 sq km layered mafic intrusion in the Wabigoon subprovince of Ontario, which show that the gabbro crystallized at 2733.2 +1.0, -0.9 Ma, is considered. It is shown that the gabbro intrudes tholeiites of the Crow Lake-Savant Lake greenstone belt. Whole rock samples and mineral separates from the Mulcahy Lake intrusion are dated by Rb-Sr, Sm-Nd, and Ar-30-Ar-40 techniques. Disturbances in the system are revealed by the Rb-Sr data and an initial Sr ratio of 0.7007 for an age of 2733 Ma is indicated by samples with low Rb/Sr ratios. The age determined for the Sm-Nd data is 2744 + or 55 Ma with an epsilon Nd value of +2.6 + or - 1.2 which indicates a source region depleted in a light rare earth element. Primary hornblende is analyzed for Ar-40/Ar-39 and an age of 2703 + or - 20 is obtained. Some implications for the development of greenstone-granite belts are discussed.

  5. Middle Jurassic U-Pb crystallization age for Siniktanneyak Mountain ophiolite, Brooks Range, Alaska

    SciTech Connect

    Moore, T.E. ); Aleinikoff, J.N.; Walter, M. )

    1993-04-01

    The authors report here a U-Pb age for the Siniktanneyak Mountain Ophiolite klippe in the west-central Brooks Range, the first U-Pb ophiolite age in northern Alaska. Like klippen of mafic and ultramafic rocks in the Brooks Range, the Siniktanneyak Mountain klippe is composed of a lower allochthon of Devonian and younger( ) diabase and metabasalt with trace-element characteristics of seamount basalts and an upper allochthon of ophiolite. The ophiolite is partial, consisting of (1) abundant dunite and subordinate harzburgite and wehrlite; (2) cumulate clinopyroxene gabbro, and (3) minor noncumulate clinopyroxene gabbro and subordinate plagiogranite; no sheeted dikes or volcanic rocks are known in the ophilitic allochthon. The plagiogranite forms small dikes and stocks that intrude the noncumulate gabbro and consists of zoned Na-rich plagioclase + clinopyroxene with interstial quartz and biotite. Five fractions of subhedral, tan zircon from the plagiogranite yield slightly discordant U-Pb data with an upper intercept age of 170 [+-] 3 Ma. The U-Pb data indicate that the Siniktanneyak Mountain ophiolite crystallized in the Middle Jurassic and was emplaced by thrusting onto mafic accretionary prism rocks within about 10 m.y. of crystallization. The U-Pb data provide an upper limit to the age of initiation of the Brookian orogeny.

  6. Igneous intrusions in coal-bearing sequences

    SciTech Connect

    Gurevich, A.B.; Shishlov, S.B.

    1987-08-01

    Intrusions of various compositions, sizes, and shapes have been observed in 115 out of 620 coal basins or deposits on all the continents. They are mainly subvolcanic and hypabyssal, with depths of emplacement estimated as ranging from a few hundred meters to 6 km, but usually 3-4 km. Compositionally, 42% are basic, 31% intermediate, 23% acid, and 4% ultrabasic. Mafic (and related) rock types include dolerites, trachydolerites, gabbro-dolerites, gabbro-monzonites, monzonites, diabases, gabbrodiabases, and less often gabbros and basalts (subvolcanic bodies). These mafic intrusions occur in coal formations of various ages from Carboniferous through Neogene, but predominate in Paleozoic (47%) and Cenozoic beds (45%). They also occur in coal formations of all genetic types, apart from those on ancient stable platforms, where there are no signs of intrusive activity. The mafic intrusions are almost everywhere associated with comagmatic lavas and tuffs (mainly in the younger strata), and the coal beds themselves are to some extent enriched in pyroclastic material, particularly in the upper horizons. This paper gives a worldwide review of igneous intrusions in coal beds. 24 references.

  7. Petrology of four clasts from consortium breccia 73215

    NASA Technical Reports Server (NTRS)

    James, O. B.; Hammarstrom, J. G.

    1977-01-01

    One felsite ('granite') and three ANT-suite anorthositic gabbro clasts extracted from breccia 73215 are described. The felsite clast has two components - fragments of crystalline felsite and veins and patches of felsic glass. The crystalline felsite, which consists largely of a vermicular intergrowth of quartz and Ba-K-feldspar, crystallized from a highly differentiated melt between 3.90 and 4.05 b.y. The felsic glass component consists of crystallized brown and colorless glasses and uncrystallized colorless glass which are all K and Si rich. The relation of glass features to past heating and the breccia-forming event is considered. In the three anorthositic gabbros, which have similar mineralogies and gradational textures, plagioclase is dominant, and olivine and orthopyroxene are the major mafic minerals. The petrologic data suggest that the gabbros formed as heated, partly melted, and/or recrystallized polymict breccias. It is possible that the approximately 4.25 b.y. age obtained for the three rocks is the date of the melting/recrystallization event.

  8. 40Ar/ 39Ar ages for the alkaline volcanism and the basement of Gorringe Bank, North Atlantic ocean

    NASA Astrophysics Data System (ADS)

    Féraud, Gilbert; Gastaud, Janine; Auzende, Jean-Marie; Olivet, Jean-Louis; Cornen, Guy

    1982-01-01

    Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast. We applied the 40Ar/ 39Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma). Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events. The basement rocks of Gorringe Bank reveal distrubed 40Ar/ 39Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.

  9. Possible petrogenetic associations among igneous components in North Massif soils: Evidence in 2-4 mm soil particles from 76503

    NASA Technical Reports Server (NTRS)

    Jolliff, Bradley L.; Bishop, Kaylynn M.; Haskin, Larry A.

    1992-01-01

    Studies of Apollo 17 highland igneous rocks and clasts in breccias from the North and South Massifs have described magnesian troctolite, norite, anorthositic gabbro, dunite, spinel cataclasites, and granulitic lithologies that may have noritic anothosite or anorthositic norite/gabbro as igneous precursors, and have speculated on possible petrogenetic relationships among these rock types. Mineral compositions and relative proportions of plagioclase and plagioclase-olivine particles in samples 76503 indicate that the precursor lithology of those particles were troctolitic anorthosite, not troctolite. Mineral and chemical compositions of more pyroxene-rich, magnesian breccias and granulites in 76503 indicate that their precursor lithology was anorthositic norite/gabbro. The combination of mineral compositions and whole-rock trace-element compositional trends supports a genetic relationship among these two groups as would result from differentiation of a single pluton. Although highland igneous lithologies in Apollo 17 materials have been described previously, the proportions of different igneous lithologies present in the massifs, their frequency of association, and how they are related are not well known. We consider the proportions of, and associations among, the igneous lithologies found in a North Massif soil, which may represent those of the North Massif or a major part of it.

  10. Geology of the Zambales ophiolite, Luzon, Philippines

    USGS Publications Warehouse

    Rossman, D.L.; Castanada, G.C.; Bacuta, G.C.

    1989-01-01

    The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite