Archean-Proterozoic Evolution of Indian Charnockites: Isotopic and Geochemical Evidence from Granulites of the Eastern Ghats Belt
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Abstract:
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.Keywords:
Charnockite
Pyroxene
Migmatite
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In southern Bahia, there are outcrops of migmatites and granulites in the Jequitinhonha Complex, which is part of the northern portion of the Araçuaí Orogen. Migmatites (garnet-cordierite diatexite) dominate the metamorphic rocks and host lenses and layers of felsic garnet granulite. The conditions of temperature and pressure of metamorphism were calculated using conventional thermobarometry and the software THERMOCALC. Values around 850 °C and 7 kbar were obtained with THERMOCALC. The calculations for the garnet-cordierite diatexite were made considering aH2O equal to 1, but the best results of calculations for the granulites are obtained with aH2O values of 0.3. Pressure values obtained with GAPES resulted in consistent values with THERMOCALC, but the pair garnet-orthopyroxene always produces low values for temperature and high ones for pressure. The results are consistent with the presence of the pair garnet and cordierite in diatexite and orthopyroxene in felsic granulite. From the tectonic point of view, the setting in which metamorphism of these rocks occurred requires high heat flow with a thermal anomaly in mid continental crust, as indicated by values of 7 kbar. Recent studies have favored the closure of a back-arc basin for this tectonic setting, but it does not solve the problem that the time span between metamorphic peak and the end of granite intrusions, involving large bodies of charnockite, is more than 80 million years. The model of tectonic switching is suggested here as it can explain the maintenance of high temperatures for a more extended interval of time.
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Charnockite
Cordierite
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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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Migmatite
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Charnockite
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Lile
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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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Summary This paper presents wet chemical analyses of forty-one pyroxenes (thirty-nine of them new) from pyroxene granulites and a few neighbouring rocks from Swat Kohistan and the adjoining Indus Valley. The granulites, considered to be derived from plutonic norites of an island arc tholeiitic nature, constitute one of the most extensive belts of its kind in the world. The pyroxenes are not unusual in any respect but they bear a closer resemblance to metamorphic than to igneous pyroxenes crystallized from deep-seated tholeiitic magmas. The distribution of Mg, Fe, and Mn (average K D Mg−Fe =0.57) and the tie-line intersections on the Wo-En join (generally from Wo 80.5 to Wo 76.5 ) between the coexisting eighteen pyroxene pairs from the granulites are discussed. Based on eight different methods of geothermometry and other considerations, it is concluded that the pyroxene granulites were metamorphosed at around 800 ° C and 7 to 8 kbar.
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Charnockite
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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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