Metamorphism of cordierite gneisses from Eastern Ghat Granulite Terrain, Andhra Pradesh, South India

Cordierite-bearing metapelites of the Eastern Ghat granulite terrain occur in close association of Khondalites, quartzites, calc-silicate rocks and charnockites. Rocks occurring between Bobbili in the north and Guntur in the south of Andhra Pradesh are studied. The association of the mineral and textural relationships suggest the following metamorphic reactions: Garnet + sillimanite + quartz = cordierite, hypersthene + sillimanite + quartz = cordierite, sillimanite + spinel = cordierite + corundum, and biotite + quartz + sillimanite = cordierite + K=feldspar. Generally the minerals are not chemically zoned except garnet-biotite showing zoning when they come in close contact with one another. The potential thermometers are provided by the Fe-Mg distribution of coexisting biotite-garnet and cordierite-garnet. Conflicting interpretation of the P/T dependence of these reactions involving cordierite are due to H2O in the cordierite. The presence of alkali feldspar-quartz assemblage which is common in these gneisses will be constrained from melting only if H2O activity is less than 0.5. The piezometric array inferred is convex towards the temperature array, indicating a rapid and isothermal crustal uplift probably aided by thrust tectonics

A long-lived enriched mantle source for two Proterozoic carbonatite complexes from Tamil Nadu, southern India

We report new neodymium and strontium isotopic data for two Proterozoic carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 Ga (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neodymium isotopic compositions, together with oxygen isotope data, suggest the involvement of a single long-lived enriched mantle source in their origin. The isotopic evolution of this source indicates that it formed approximately contemporaneously with the accretion and metamorphism of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoic until at least 770 Ma ago. The older of the two carbonatites was intruded into young crust that was not older than about 150 Ma at the time of emplacement. The isotopic data contrast with those from carbonatites of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochemically unique lithospheric source for carbonatites

Zircon U-Pb age, Lu-Hf isotope, mineral chemistry and geochemistry of Sundamalai peralkaline pluton from the Salem Block, southern India: implications for Cryogenian adakite-like magmatism in an aborted-rift

Abstract not availableM.L. Renjith, M. Santosh, Tang Li, M. Satyanarayanan, M.M. Korakoppa, T. Tsunogae, D.V. Subba Rao, A. Kesav Krishna, S. Nirmal Chara