Geochemistry and Origin of Megacrystic Granitoid Rocks from Eastern Ghats Granulite Belt

The Eastern Ghats Granulite Belt (EGGB) forms part of a continuous Precambrian metamorphic terrain in Gondwana. It is characterised by widespread development of an Archaean khondalite suite of metasedimentary rocks, Archaean to Late-Proterozoic charnockites and Late Proterozoic anorthositic, granitic and syenitic emplacements. A 1900 Ma megacrystic granitoid suite, containing varying proportions of charnockites and granites, forms an important and widely distributed litho-unit in the central khondalite and eastern migmatite zones of the EGGB. It contains metasedimentary enclaves, megacrystic K-feldspar, quartz, plagioclase ovoids, biotite, garnet (porphyroblasts and coronas), apatite, zircon, ilmenite, magnetite, etc. Hypersthene is present in the charnockite phase. Monazite is present in some garnet-free granites. It is characterised by low Na2O/K2O ratios, high alumina saturation index, CaO, MgO, and فREE, negative correlation of TiO2, Al2O3, Fe2O3t, MgO, MnO, CaO, P2O5, Ba, Sr, Zr and V with SiO2, positive correlation of K2O, REE, Th and Rb with SiO2, fractionated LREE, relatively flat HREE and negative Eu anomalies. ta suggest S-type nature of the suite. Fractionation of the granitic magma and local variations in pH2O and fCO2 caused the formation of megacrystic charnockites. Formation of the corona garnet is related to the reworking of the suite during late Proterozoic (ca. 1250 Ma) isothermal decompression associated with channelised CO2-rich fluid flux along narrow shear zones.