ALBERT

Description: For this study Nd, Sr and Pb isotope compositions were analyzed for ortho- and paragneisses from the Eastern Ghats Belt of India in order to determine its crust formation and crustal evolution. This belt represents a Proterozoic orogen that extends along the east coast of Peninsular India and forms part of the mobile belts in East Gondwana and Rodinia. The Eastern Ghats Belt was affected by Mesoproterozoic granulite facies metamorphism in the western segment (Western Charnockite Zone) and a Grenvillian regional-scale high-grade event in the central and eastern segments (Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone) as well as a local Pan-African overprint. The results of the isotope studies are used for the large-scale reconstruction of the indo-antarctic part of the Rodinia supercontinent. Based on Nd model ages and Pb isotope ratios from leached feldspars four crustal domains can be distinguished in the Eastern Ghats Belt. These domains can in part be correlated with the lithological division of the belt: (1) The Western Charnockite Zone south of the Godavari Graben is characterized by Nd model ages between 2.3 and 2.5 Ga for orthogneisses and 2.6 and 2.8 Ga for metasediments (Domain 1). The Pb isotopes are primitive indicating reworking of dominantly Archean and mixing with minor Proterozoic material; (2) North of Godavari Graben Nd model ages for orthogneisses are significantly higher with values ranging from 3.2 to 3.9 Ga. The Pb isotopes are strongly retarded; (3) The north-eastern parts of the Charnockite Migmatite Zone and Western Khondalite Zone form a distinct and almost homogeneous crustal domain (Domain 3) with Nd model ages between 1.8 and 2.2 Ga; (4) Between the isotopically homogeneous terranes stretches a broad transition zone (Domain 2) enclosing parts of the Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone. The Nd model ages for metasediments (2.1–2.5 Ga) are younger than paragneiss ages of the adjoining Western Charnockite Zone. The Nd model ages for orthogneisses (1.8–3.2 Ga) display a large spread, which is consistent with the Pb isotope signatures that indicate mixing of Archean with Proterozoic material. The border zone between Domain 3 and the Archean Eastern Indian Craton forms a second transition zone (Domain 4) characterized by metasediments with Nd model ages between 2.2 and 2.8 Ga and orthogneisses with model ages around 3.2 Ga. Reworking of Archean crustal material is most intense along the border zones of the belt and ''juvenile' material is more dominant away from the orogenic front. This scenario is indicative of an active continental margin setting for the two Proterozoic episodes of orogenesis in the Eastern Ghats Belt. A correlation of Domain 3 with the Rayner Complex and the Prydz Bay region, Antarctica, for the early crustal evolution is supported by the similarity of the isotope signatures. The Napier Complex is very different to the Eastern Ghats Belt and an early joint evolution of these terranes is ruled out on the basis of the Pb–Pb and Sm–Nd systematics.

Description: High-MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh-temperature (UHT) metamorphism and near-isothermal decompression, the rocks were intruded in a lit-par-lit manner by felsic melts (charnockite), which caused local-scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions. The sequence of multiphase reaction textures in the high-MgAl xenoliths reflects the changes of physico-chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, 〉 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)1 + garnet (grt)1 + sapphirine (spr)1 + spinel (spl)1 + rutile (rt). A subsequent phase of near-isothermal decompression in the order of 1-2 kbar (stage 2) resulted in extensive replacement of grt1 and opx1 megacrysts by lamellar (opx2 + spr2) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg3 + opx2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx2 + spr2) symplectites by biotite and sillimanite and/or plg3, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain-size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4-7 kbar) as indicated by local growth of late garnet (grt5) at the expense of (opx + spr + plg) domains at static conditions. Recently published isotope data suggest that the multistage evolution of the high-MgAl granulites at Anakapalle followed a discontinuous P-T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep-crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid-crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high-T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.