Petrology and geochronology of a suite of pelitic granulites from parts of the Chotanagpur Granite Gneiss Complex , eastern India : Evidence for Stenian‐Tonian reworking of a late Paleoproterozoic crust
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The north‐eastern part of the Chotanagpur Granite Gneiss Complex (CGGC) in the East Indian shield contains enclaves of migmatitic pelitic granulites (PG) within felsic orthogneiss (FOG). Field observations, petrology and geochronology (LA–MC–ICP–MS U–Pb dating of zircon and EPMA Th–U–total‐Pb dating of monazite) of the PG suggest two distinct metamorphic events. The earliest event M1, which is characterized by high‐temperature (>850°C) granulite facies metamorphism, occurred in the timespan of ~1680–1580 Ma. Extensive dehydration melting of biotite + sillimanite + quartz‐rich protoliths led to stabilization of the restitic assemblage (garnet + alkali‐feldspar + quartz + sillimanite + ferrian‐ilmenite) together with large volumes of felsic melts (leucosomes). Collisional tectonics followed by delamination and asthenospheric upwelling could have triggered the M1 event. Subsequently, at ~1470–1400 Ma, the igneous protolith of the host FOG intruded and hydrated the PG. Thereafter, a second metamorphic event, M2, accompanied by compressional structures, affected both the rock types. A clockwise P–T path that culminated at ≥10 kbar ~760–850°C and is followed by a steeply decompressive retrograde path characterizes this event. The P–T path and the inferred geothermal gradient (<27°C/km) are compatible with a continent–continent collisional setting. Geochronological findings suggest a protracted orogeny for the M2 event with its major pulse during ~970–950 Ma. When combined with the published information, this study supports the view that a large (if not the entire) portion of the Indian shield and the granulite terranes of east Antarctica share similar tectonothermal events that led to the formation of two supercontinents, Columbia and Rodinia.Keywords:
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Pyroxene granulites and charnockites are associated with metasedimentary khondalite (garnet-sillimanite gneiss) and migmatites in the Eastern Ghats mobile belt. Various stages of in situ granulitization similar to those observed in southern India and Sri Lanka are present. Geochemically the granulite suite around Visakhapatnam is bimodal, with acid and mafic variants. There is an increase in the concentration and fractionation of the rare earth elements (REE) from pyroxene granulites to charnockites, and six Sm-Nd models ages () range from 2.86 to 2.35 Ga, with 2.1 Ga for a younger granite. The charnockites, a pyroxene granulite, and granite define a Pb/Pb isochron age of . U-Pb ages of zircons and monazites from a charnockite near Phulbani, Orissa, yield near concordant data with ages of and , respectively. The new isotopic data and the available ages from the northeastern part of the Eastern Ghats in Orissa suggest a major charnockite formation event between ca. 1170 and 950 Ma. This event is contemporaneous with that of Sri Lanka but much younger than the late Archean granulite event in southern India.
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Charnockite and cordierite-bearing gneisses occur in the Male-Mahadeshwara Hills (MM Hills) in Karnataka, southern India. Petrographic observations indicate a prograde Mg - cordierite partial breakdown reaction to give rise to garnet + sillimanite + quartz and a retrograde reaction to sillimanite + biotite + quartz. Geothermobarometric data document a peak metamorphic temperature of 850-860-degrees-C at pressures of 7-8.2 kb for the charnokites. The cordierite gneisses record retrograde conditions of 550-690-degrees-C and 6-6.3 kb. High density carbonic inclusions (0.900 - 1.120 g/cc) occur in these granulites. Textural studies combined with CO2 density data indicate isobaric cooling of the granulite complex suggesting a midcrustal metamorphism caused by magmatic heat advection.
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Summary The charnockite series consisting of pyroxene granulites and charnockites are not formed by magmatic differentiation, although both units had their antecedents in igneous material. The solid-state recrystallization of pyroxene granulite and the liquid-state crystallization of charnockite are inferred from the field criteria. The pyroxene granulites with the associated paragneisses are formed from the original sedimentary-volcanic sequence under granulite facies of metamorphism. The magmatic charnockites emplaced into these rocks resulting in the development of intermediate varieties by the partial assimilation and incorporation of the pyroxene granulites. The stability of the minerals is not affected by this migmatization, which is therefore thought to have taken place under granulite facies conditions. The evolutionary trend of the charnockite series is one of retrogression from the earlier granulite facies to the later almandine-amphibolite facies, caused by the subsequent granite invasion.
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Sapphirine granulite occurring as lenses in charnockite at Anantagiri, Eastern Ghat, India, displays an array of minerals which developed under different P-T-X conditions. Reaction textures in conjunction with mineral chemical data attest to several Fe-Mg continuous reactions, such as spinel+rutile+quartz+MgFe−1=sapphirine+ilmenite cordierite=sapphirine+quartz+MgFe−1 sapphirine+quartz=orthopyroxene+sillimanite+MgFe−1 orthopyroxene+sapphirine+quartz=garnet+MgFe−1 orthopyroxene+sillimanite=garnet+quartz+MgFe−1 orthopyroxene+sillimanite+quartz+MgFe−1=cordierite. Calculated positions of the reaction curves in P-T space, together with discrete P-T points obtained through geothermobarometry in sapphirine granulite and the closely associated charnockite and mafic granulite, define an anticlockwise P-T trajectory. This comprises a high-T/P prograde metamorphic path which culminated in a pressure regime of 8·3 kb above 950°C, a nearly isobaric cooling (IBC) path (from 950°C, 8·3 kb, to 675°C, 7·5kb) and a terminal decompressive path (from 7·5 to 4·5 kb). Spinel, quartz, high-Mg cordierite, and sapphirine were stabilized during the prograde high-T/P metamorphism, followed by the development of orthopyroxene, sillimanite, and garnet during the IBC. Retrograde low-Mg cordierite appeared as a consequence of decompression in the sapphirine granulite. Deformational structures, reported from the Eastern Ghat granulites, and the available geochronological data indicate that prograde metamorphism could have occurred at 30001±00 and 2500±100 Ma during a compressive orogeny that was associated with high heat influx through mafic magmatism. IBC ensued from Pmax and was thus a direct consequence of prograde metamorphism. However, in the absence of sufficient study on the spatial variation in P-T paths and the strain histories in relation to time, the linkage between IBC and isothermal decompression (ITD) has remained obscure. A prolonged IBC followed by ITD could be the consequence of one extensional mechanism which had an insufficient acceleration at the early stage, or ITD separately could be caused by an unrelated extensional tectonism. The complex cooled nearly isobarically from 2500 Ma. It suffered rapid decompression accompanied by anorthosite and alkaline magmatism at ∼1400–1000 Ma.
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The Bakhuis belt, one of the major granulite-facies domains in the Guiana Shield, consists of a core of banded rocks of the charnockite suite, metamorphosed and migmatized under granulite-facies conditions. Pelitic gneiss intercalations locally show sapphirine-quartz and orthopyroxene-sillimanite-quartz assemblages, with up to 10% Al2O3 in the orthopyroxene. These assemblages point to ultrahigh-temperature (UHT) metamorphism. P-T conditions are difficult to deduce, because the assemblages contain Fe3+ in sapphirine and sillimanite, and do not contain coeval garnet, but spinel, Feand Fe-Ti oxides instead. P-T conditions for the peak UHT metamorphism are estimated to have been 950 °C and 8.5-9 kb. An assemblage of impure corundum associated with quartz may also have formed during the UHT metamorphism. Unravelling the assemblages indicates a counterclockwise P-T path from an early cordierite-sillimanite assemblage via a subsequent sapphirine-quartz assemblage to the peak metamorphic assemblage orthopyroxene-sillimanitequartz. Retrogressive assemblages show an isobaric to near isobaric cooling path for the UHT occurrence. Single zircon Pb-evaporation and whole rock Sm-Nd dating were carried out on the Bakhuis granulites. Three zircons from a garnet-sillimanite-bearing gneiss yielded an age of 2072 ± 4 Ma, but a fourth grain provided increasing ages up to 2.10 Ga. An enderbitic granulite gave zircon ages in the range of 2.15-2.09 Ga. Zircons from a garnet-bearing gneiss defined an age of 2055 ± 3 Ma, while those from a garnet-bearing pegmatite layer gave ages ranging between 2085 Ma and 2058 Ma. A mylonitic orthopyroxene-bearing granite (a true charnockite) furnished a Pb-Pb zircon age of 2065 ± 2 Ma this is the first indication that the high-grade metamorphism was (at least locally) associated with melting and production of magmatic charnockite. Zircons from two samples of a discordant recrystallized basic dyke yielded ages of 2060 ± 4 Ma and 2056 ± 4 Ma, respectively. A discordant sheared pegmatite vein provided an age of 2059 ± 3 Ma. In conclusion, the 2072–2055 Ma ages are interpreted as the age of granulite metamorphism in the Bakhuis Mountains, and ages older than 2.07 Ga, and up to 2.15 Ga, are considered to reflect an inherited component from the early Transamazonian protoliths of the granulite. TDM model ages for the Bakhuis samples range from 2.40 to 2.19 Ga. These data, together with positive to slightly 175 GEOLOGIE DE LA FRANCE, N° 2-3-4, 2003 GEOLOGY OF FRANCE AND SURROUNDING AREAS, N° 2-3-4, 2003 The Bakhuis ultrahigh-temperature granulite belt (Suriname): I. petrological and geochronological evidence for a counterclockwise P-T path at 2.07-2.05 Ga La ceinture de granulites d’ultra-haute temperature des Monts Bakhuis (Suriname) : I. mise en evidence d’un chemin P-T anti-horaire a 2,07-2,05 Ga Geologie de la France, 2003, n° 2-3-4, 175-205, 12 fig., 6 tabl. Mots cles : Roche metamorphique, Facies granulite, Condition pression temperature, Datation, Pb-Pb, Sm-Nd, Zircon, Roche totale, Paleoproterozoique, Suriname, Bouclier guyanais.
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The Eastern Ghats Granulite Belt, India, with two major lithological associations: charnockites and meta sedimentary granulites, is characterized by polyphase deformation and complex, possibly multiple granulite events. Barring the cratonic margins in the north and west, two distinct crustal domains have been identified: the Eastern Ghats Province (EGP) and Ongole domain, separated by the Godavari graben. These domains also have distinct geochronological record of granulite event: in the EGP the first granulite event has been recorded as between 1.2 and 0.9 Ga; while in the Ongole domain the granulite event is recorded as 1.6 - 1.7 Ga. However, charnockite-massifs in both the domains, interpreted as product of deep crustal anatexis under granulite facies conditions, could provide a link in tectonic evolution of the EGB as a whole. LA-ICP-MS analysis of zircon spot ages of two charnockite massifs reveals vestiges of the1.6 Ga charnockite magmatism in the EGP as identical to that in the Ongole domain. Another charnockite massif in the EGP records concordant zircon spot age of 940 Ma, but single spot age of 990 Ma could indicate a prolonged UHT event. Thus magmatic charnockites of intracrustal melting origin could represent two granulite events, at ca. 1.6 and 1.0 Ga in the Eastern Ghats Belt. Also, accretionary orogenic processes of the Supercontinent Columbia might have encompassed the Eastern Ghats Belt with Australia, Antarctica and Laurentia.
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The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c . 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.
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