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Showing papers on "Granulite published in 1994"


Journal ArticleDOI
TL;DR: In this article, a series of experiments, between 100 and 2000 MPa, on the fluid-absent melting of a quartz-rich aluminous metagreywacke composed of 32 wt% plagioclase (Pl) (An22), 25 wt % biotite (Bt) (XMg45), and 41 wt percent quartz (Qtz), were carried out using a powder of minerals (≤5μm) and a glass of the same composition.
Abstract: Island arcs, active and passive margins are the best tectonic settings to generate fertile reservoirs likely to be involved in subsequent granitoid genesis. In such environments, greywackes are abundant crustal rock types and thus are good candidates to generate large quantities of granitoid magmas. We performed a series of experiments, between 100 and 2000 MPa, on the fluid-absent melting of a quartz-rich aluminous metagreywacke composed of 32 wt% plagioclase (Pl) (An22), 25 wt% biotite (Bt) (X Mg45), and 41 wt% quartz (Qtz). Eighty experiments, averaging 13 days each, were carried out using a powder of minerals (≤5μm) and a glass of the same composition. The multivariant field of the complex reaction Bt+Pl+Qtz⇔Grt/Crd/Spl+ Opx+Kfs+melt limited by the Opx-in and Bt-out curves, is located between 810–860°C at 100 MPa, 800–850°C at 200 MPa, 810–860°C at 300 MPa, 820–880°C at 500 MPa, 860–930°C at 800 MPa, 890–990°C at 1000 MPa, and at a temperature lower than 1000°C at 1500 and 1700 MPa. The melting of biotite+plagioclase+ quartz produced melt+orthopyroxene (Opx) +cordierite (Crd) or spinel (Spl) at 100, 200 and 300 MPa, and melt+orthopyroxene+garnet (Grt) from 500 to 1700 MPa (+Qtz, Pl, FeTi Oxide at all pressures). K-feldspar (Kfs) was found as a product of the reaction in some cases and we observed that the residual plagioclase was always strongly enriched in orthoclase component. The P-T surface corresponding to the multivariant field of this reaction is about 50 to 100°C wide. At temperatures below the appearance of orthopyroxene, biotite is progressively replaced by garnet with increasing P. At 850°C, we observed that (1) the modal proportion of garnet increases markedly with P; (2) the grossular content of the garnet increases regularly from about 4 mol% at 500 MPa to 15 mol% at 2000 MPa. These changes can be ascribed to the reaction Bt+Pl+Qtz ⇔ Grt+Kfs+melt with biotite +plagioclase+quartz on the low-P side of the reaction. As a result, at 200 MPa, we observed the progressive disappearance of biotite without production of orthopyroxene. These experiments emphasize the importance of this reaction for the understanding of partial melting processes and evolution of the lower continental crust. Ca-poor Al-metagreywackes represent fertile rocks at commonly attainable temperatures (i.e. 800–900°C), below 700 MPa. There, 30 to 60 vol.% of melt can be produced. Above this pressure, temperatures above 900°C are required, making the production of granitoid magmas more difficult. Thin layers of gneisses composed of rothopyroxene, garnet, plagioclase, and quartz (±biotite), interbedded within sillimanite-bearing paragneisses, are quite common in granulite terrains. They may result from partial melting of metagreywackes and correspond to recrystallized mixtures of crystals (+trapped melt) left behind after removal of a major proportion of melt. Available experimental constraints indicate that extensive melting of pelites takes place at a significantly lower temperature (850°C±20) than in Al-metagreywackes (950°C±30), at 1000 MPa. The common observation that biotite is no longer stable in aluminous paragneisses while it still coexists commonly with orthopyroxene, garnet, plagioclase and quartz, provides rather tight temperature constraints for granulitic metamorphism.

502 citations


Journal ArticleDOI
TL;DR: In this paper, the first detailed study of oxygen isotope ratios in zircon has been carried out and the results show that high-magnetism zircons are up to 2 ǫ lower in δ18O than low magnetism zirons from the same rock, consistent with the hypothesis that they represent nearly pristine samples of the parent magma.

331 citations


Journal ArticleDOI
01 Jul 1994-Science
TL;DR: Pseudotachylytes in the Bergen arcs of western Norway contain microlites including omphacite, garnet, plagioclase, and quartz that may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process.
Abstract: Pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust. Pseudotachylytes in the Bergen arcs of western Norway contain microlites including omphacite, garnet, plagioclase, and quartz. This eclogite facies assemblage is stable at temperatures of about 800°C and pressures of 18 to 19 kilobars, corresponding to depths of 60 kilometers or more. The pseudotachylytes are exposed in Grenvillian granulites that locally underwent fluid-induced eclogitization and corresponding volume reduction of approximately 10 percent during the Caledonian continental collision. The pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process.

255 citations


Journal ArticleDOI
TL;DR: In this paper, a U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma.
Abstract: Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite...

239 citations


Journal ArticleDOI
TL;DR: In the early sixties it was noted that the “Highland Series” rocks had been subjected to granulite facies metamorphism and the ‘Vijayan Series’ rocks to amphibolite facia metamorphisms as discussed by the authors, the former sediments having been laid down on a Vijayan floor.

216 citations


Journal ArticleDOI
TL;DR: The precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt and in Southern India as the Achankovil shear zones as discussed by the authors.
Abstract: The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six north-south trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100–150 km wide dextral shear zone of probable Pan-African age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic -ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.

210 citations



Journal ArticleDOI
TL;DR: In this paper, Baur et al. used upper-intercept ages of zircons from metapelites in the Highland Complex to establish a new geochronological framework, which is consistent with previously published work on SmNd model ages.

167 citations


Journal ArticleDOI
TL;DR: In this paper, compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec.
Abstract: Compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the Minto terrane of northern Quebec. The compositional maps provide clear evidence of late exchange of Fe/(Fe + Mg) after Ca in garnet and Al in orthopyroxene had been quenched-in. The extent of late Fe-Mg exchange was controlled by neighbouring minerals, with negligible Fe-Mg gradients against plagioclase and quartz, and substantial gradients against exchangeable Fe-Mg minerals. Cores of grains in contact with exchangeable Fe-Mg neighbours are progressively more reset in Fe/(Fe + Mg) as grain size decreases, whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in Fe/(Fe + Mg) than cores of the largest grains. Gradients of Ca in garnet and of Al in orthopyroxene in grains of uniform Fe/(Fe + Mg) preserve a high-temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular Fe-Mg diffusion in garnet and orthopyroxene was rapid enough to homogenize Fe/(Fe + Mg). The transition from efficient intergranular exchange at relatively high temperatures to local Fe-Mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock, possibly an internally generated dehydration melt phase. Implications for geothermometry of granulites include the following (numerical values are particular to this study): (1) core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in Fe/(Fe + Mg) relative to the most refractory compositions by an amount equivalent to 120d C; (2) Fe-Mg exchange thermometry using even the most refractory Fe/(Fe + Mg) compositions may not record peak granulite conditions, possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock; and (3) temperature-sensitive net transfer equilibria involving Al solubility in orthopyroxene yield temperatures up to 150d C higher than maximum Fe-Mg exchange temperatures, even in grains with flat Fe/(Fe - Mg) compositional profiles, making them a better means of estimating peak granulite temperatures than Fe-Mg exchange thermometry.

158 citations


Journal ArticleDOI
TL;DR: In this article, U-Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U-pb systems, high-pressure assemblages, local melting, and rapid cooling.
Abstract: While the position of the Grenville Front is placed at a structural feature associated with the onset of high-grade rocks when approached from the north, the time of first metamorphism in these rocks is, in general, pre-Grenvillian and highly variable. At Killarney, amphibolite to granulite grade rocks were metamorphosed, lineated, and foliated to their present “front-parallel” configuration mainly by 1453±6 Ma. North of North Bay, amphibolite grade migmatites have been subjected to a pre-2650 Ma metamorphism. Granulite grade gneisses south of the Grenville Front near Val d’Or record a 2640 Ma metamorphic event. South of Chibougamau, mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 Ma. In central Labrador a 1650 Ma foliated granite is present south of the Grenville Front, and at least some of the banding is this old. At the Atlantic coast, granulite facies metamorphism in front-parallel gneisses predates a 1790–1662 Ma deformation. In contrast, each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies NW directed thrusting or crustal loading. Near Killarney on Georgian Bay, titanite records an episode of 977±2 Ma partial resetting, the degree of which increases with distance into the Grenville Province. Here Grenvillian deformation, where present, is manifest as feldspar-porphyroclastic fabric with strong SE plunging lineations. Near North Bay, Archean pegmatites in the Grenville Front zone were similarly deformed at 988±10 Ma, and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987±2 Ma. A 991±4 Ma cooling age for titanite in this melt is analytically identical to the zircon growth age, hence rapid cooling occurred. South of Chibougamau, new zircons grew in high-pressure coronite in response to tectonic loading at 995±6 Ma, and in Central Labrador, melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995±2 Ma. Titanite in the granitic host rock has an analytically identical age of 992±4 Ma, again indicating rapid cooling. This second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651±5 Ma to 988±5 Ma two-age mixing line. Grenville resetting at about 1037±19 Ma in the Grenville Front zone on the Labrador coast is indicated, but major thrusting is pre-1662 Ma. U-Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U-Pb systems, high-pressure assemblages, local melting, and rapid cooling. Crustal loading and shearing, with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling, are required.

133 citations


Journal ArticleDOI
TL;DR: U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events as discussed by the authors.
Abstract: U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of ≥2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at ≥1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.

Journal ArticleDOI
TL;DR: Amphibole mixing properties and standard state properties for the end members tremolite, tschermakite, pargasite, ferrotremolite and ferrotchermakites have been refined through analysis of relevant phase equilibrium experiments, coexisting cummingtonite-hornblende pairs, and natural mineral parageneses.
Abstract: Amphibole mixing properties and standard state properties for the end members tremolite, tschermakite, pargasite, ferrotremolite, ferrotschermakite, and ferropargasite have been refined through analysis of relevant phase equilibrium experiments, coexisting cummingtonite–hornblende pairs, and natural mineral parageneses. When merged with the mineral properties given by R.G. Berman, these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades. The method was applied to texturally equilibrated garnet–pyroxene–hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet–biotite gneisses from the Kapuskasing structural zone. Thermometer resetting is apparent only in garnet–biotite gneisses, whereas touching garnet–pyroxene pairs in mafic gneiss show little evidence of retrograde Fe–Mg exchange. Computed metamorphic temperatures (garnet–clinopyrox...

Journal ArticleDOI
TL;DR: The amphibolite-facies gneisses consist of quartz, biotite, plagioclase, garnet, K-feldspar, sillimanite and were identified as metapelites, metagreywackes and quarzites with minor carbonates as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. The results of this study can be explained by melting of granulite-facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event.
Abstract: The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite-facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment-tapping-fractional-crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba-rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona-Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona-Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

Journal ArticleDOI
TL;DR: In this paper, the authors show that the exhumed deep crustal rocks in the Western Gneiss Region (WGR) of Norway experienced Caledonian high-pressure metamorphism during the Silurian, Scandian continental collision between Baltica and Laurentia.

Journal ArticleDOI
TL;DR: Two new U-Pb concordia zircon ages are reported for granulite-facies rocks from the Mozambique Orogenic Belt in eastern Tanzania as discussed by the authors.
Abstract: Two new U-Pb concordia zircon ages are reported for granulite-facies rocks from the Mozambique Orogenic Belt in eastern Tanzania. These rocks are thought to have been generated during collision-related crustal thickening process and exhumed by deep erosion. A two-pyroxene granulite sample from the Pare Mountains has yielded a concordia upper intercept age of 645 ± 10 Ma, whereas a granulite-facies meta-anorthosite sample from the Uluguru Mountains has given an upper intercept age of 695 ± 4 Ma. Evidence for the involvement of older crustal material (pre-Late Proterozoic) was not found. These zircon data indicate crystallization during the peak of granulite-facies metamorphism during the Pan-African orogeny ( c . 950 to c . 550 Ma) consistent with a subduction-collisional environment.

Journal ArticleDOI
TL;DR: In this article, strongly foliated eclogite-facies rocks in 30-150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite facies gabbroanorthosite terrain in the Bergen Arcs of Norway.

Journal ArticleDOI
TL;DR: In this article, a region across the Eastern Ghats Mobile Belt (EGMB) was mapped by using Landsat Thematic Mapper data to identify a number of shear zones, lineaments, and structural domes and basins.
Abstract: Regional mapping of a section across the Eastern Ghats Mobile Belt (EGMB) north of the Godavari graben in Eastern Peninsular India by using Landsat Thematic Mapper data enables recognition of a number of shear zones, lineaments, and structural domes and basins. A conspicuous megashear occurs at the western boundary of the granulite facies rocks of the EGMB adjacent to the Archean granite-greenstone craton. The confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature. The strike extensions of this shear belt extend through to the Elchuru alkaline complex, Prakasam District, Andhra Pradesh, and the syenite plutons of Koraput district, Orissa. The contrasting lithologies, metamorphism and structural history on either side of the shear zone suggests that it might be a Precambrian suture zone. The mesoscopic structural features in the EGMB include prominent foliation with moderate to steep dips, folds, faults/shears, S-C fabrics, pinch and swell structures and other linear fabric elements. These observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence. The major rock units in this part of EGMB comprise garnetiferous sillimanite gneisses, quartzites and calc-granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks. The contact of the two rock units is generally sheared and often migmatised. The structural fabric suggests two major tectonic events: an essentially horizontal tectonic regime resulting in thrust systems and associated structures, subsequently followed by strike-slip tectonics characterized by high shear strains. Features such as westward-verging thrusts, large-scale recumbent folds, major shear zones, structural domes and basins, indications of tectonic crustal shortening, extensive calc-alkali magmatism and widespread migmatization in the region are attributed to collisional processes during Proterozoic times. The spatial disposition of the EGMB and its linkage with the distribution of similar rock units during the late Precambrian time in a global tectonic scenario are discussed.

Journal ArticleDOI
TL;DR: In this paper, a retrograde P-T trajectory of the granulite complex at Anakapalle, which was metamorphosed at ~1000 Ma, was deduced.
Abstract: The granulite complex at Anakapalle, which was metamorphosed at ~1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite, mafic granulites, calc-silicate rock, spinel granulites, and two types of sapphirine granulites-one quartz-bearing and migmatitic and the other devoid of quartz and massive. Reaction textures in conjunction with mineral-chemical data suggest several continuous and discontinuous equilibria in these rocks. In orthopyroxene granulites, dehydration-melting of biotite in the presence of quartz occurred according to the reaction biotite+quartz= garnet (Py 37 )+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py 37 )+quartz = orthopyroxene + plagioclase. Subsequently, coronal garnet (Py 30 ) and quartz were produced by the same reaction but proceeding in the opposite direction. In spinel granulites, garnet (Py 42 ) and sillimanite were produced by the breakdown of spinel in the presence of quartz. In the two types of sapphirine granulites, garnet with variable pyrope content broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets from quartz-free sapphirine granulites compared with the quartz-bearing one (53 mol % pyrope). The calculated positions of the mineral reactions and diserete P-T points obtained by thermobarometry define a retrograde P-T trajectory during which a steep decompression of ~1.5 kbar from P-T max of 8 kbar and 900 °C was followed by near-isobaric cooling of ~300 °C. During this decompression, garnet with variable pyrope contents in different rocks broke down on intersection with various divariant equilibria. Near-isobaric cooling resulted in the formation of coronal garnet around second-generation orthopyroxene and plagioclase replacing earlier porphyroblastic garnet in orthopyroxene granulites. It has been argued that the deduced P-T trajectory originated in an extensional regime involving either a crust of near-normal thickness of a slightly overthickened crust owing to magmatic underaccretion.

Journal ArticleDOI
TL;DR: In this article, a multigrain U-Pb systematics of zircon and single-grain evaporation data of zircons in combination with Nd model ages are presented for pyroxene-free, felsic, garnetiferous granulites and orthopyroxene bearing granulite of felsitic, mafic and intermediate composition from the Blansky Les mountains granulita complex, S Bohemia.
Abstract: Conventional multigrain U-Pb systematics of zircon and single-grain evaporation data of zircons in combination with Nd model ages are presented for pyroxene-free, felsic, garnetiferous granulites and orthopyroxene-bearing granulites of felsic, mafic and intermediate composition from the Blansky Les mountains granulite complex, S Bohemia. Zircons from mafic and intermediate granulites yield Concordia intercept ages between 346 ± 5 and 351 ± 6 Ma as well as a single-grain evaporation age of 346 ± 12 Ma. These ages define the time of Variscan high-grade metamorphism in S Bohemia and are supported by an internal Sm-Nd garnet-zircon-plagioclase-whole rock isochron age of 343 ± 21 Ma. Euhedral zircons from felsic granulites of probable granitic derivation yield U-Pb ages of 366 ± 5 Ma and 373 ± 12 Ma interpreted as intrusion ages of the granitic precursor. An inherited Proterozoic crustal component is documented by poorly defined Concordia upper intercept ages around 1700 Ma and Nd-model ages between 1.3 and 2.1 Ga.

Journal ArticleDOI
TL;DR: In this article, the authors used a regression line in a SmNd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initial ϵNd-value of +2.3.

Journal ArticleDOI
TL;DR: There is widespread evidence for greenschist facies metamorphism in different rock types within all the Witwatersrand gold mines The silicate mineral assemblages suggest near-uniform peak metamorphic conditions equivalent to the chlorite zone.

Journal ArticleDOI
TL;DR: Petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined Highland Complex in Sri Lanka as discussed by the authors, which includes the occurrences of relict kyanite±quartz, staurolite, corundum±kyanite, hercynite+karnet, and sapphirine+kynite±spinel within garnet in the sillimanite-quartz-rich rocks.

Journal ArticleDOI
TL;DR: The authors showed that residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspathic mineralogy of the source.
Abstract: Melting experiments (P = 6.9 kbar, T = 850-950 deg C, NNO is less than fO2 is less than HM) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville basement of eastern North America. A graphite-bearing granulite gneiss did not melt. Water (H2O(+) = 0.60 to 2.0 wt %) is bound in low-grade, retrograde metamorphic minerals and is consumed during the earliest stages of melting. Most melts are water-undersaturated. Melt compositions range from metaluminous, silicic granodiorite (diorite starting composition) to peraluminous or weakly metaluminous granites (all others). In general, liquids become more feldspathic, less silicic, and less peraluminous and are enriched in FeO, MgO, and TiO2 with increasing temperature. Residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of the partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K-feldspar, a common restite phase in the charnockite and granulite (but not the diorite) should control the behavior of Ba and, possibly, Eu in these systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apatite, a common restite phase, is enriched in rare earth elements (REE), especially middle REE. Retention of apatite in the restite will result in steep, light REE-enriched patterns for melts derived from the diorite and charnockites.

Journal ArticleDOI
TL;DR: In this article, the authors define the P-T regimes of UHP metamorphism and describe mineralogical, petrological and tectonic characteristics for a few representative UHP terranes.
Abstract: Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).

Journal ArticleDOI
TL;DR: In the Pan-African granulite-facies Highland Complex of Sri Lanka, the metamorphic grade increases from the central northwestern part towards boththe east and the southeast as discussed by the authors.

Journal ArticleDOI
TL;DR: In this paper, Sarkar et al. showed that separating the structures of different generations is necessary for a proper understanding of the field features taken to indicate arrested formation of charnockites in South India and elsewhere.

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TL;DR: In this article, the authors obtained the maximum P -T estimates for about 115 locations from core compositions of garnet, orthopyroxene, and plagioclase in the Highland Complex (HC) of Sri Lanka.

Journal ArticleDOI
TL;DR: In this paper, it was shown that low-angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the Archaean-Proterozoic boundary.

Journal ArticleDOI
TL;DR: In the Palghat Gap region, the authors of as discussed by the authors reported a P-T of 9-10 kbar and 800-900d C using both conventional thermobarometric methods and the TWEEQU thermobarometry program, which is among the highest yet reported for South Indian and Sri Lankan granulites.
Abstract: The Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende-biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc-silicate. The P-T estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9-10 kbar and 800-900d C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. These P-T estimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates c. 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrograde P-T path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet-pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80d C Ma-1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.