Characterization and origin of a compositionally zoned aluminous A-type granite from South India

The Pan-African Ambalavayal granite intrudes the high-grade metamorphic ter rain of northern Kerala, South India and is spatially associated with the M oyar and Calicut lineaments. The pluton was aligned nearly parallel to the northeast-southwest and east-west faults in the basement, consistent with m agma ascent along pre-existing deep-crustal lineaments in an extensional te ctonic regime. The pluton is characterized by the presence of iron-rich hyd rous mafic minerals, primary magnetite, fO(2) above the Ni-NiO buffer and h igh initial emplacement temperatures near 1000 degrees C. Modal and textura l analyses reveal two probable compositional zones within the pluton: outer and inner. Major element variations support this zoning and point to a per alkaline to metaluminous outer zone and a metaluminous to slightly peralumi nous inner zone. Both zones exhibit major and trace element characteristics of the A-type granites with the outer zone belonging to the A, subtype and the inner zone to the A, subtype of Eby. The trace element trends observed from outer zone to the inner zone suggests that crystal fractionation may have been the dominant process in the generation of high levels of the inco mpatible elements in the case of inner zone samples. The high initial Sr-87 /Sr-86 ratio (0.7135) and high Y/Nb ratios (Y/Nb > 1.2) are in the range ex pected for rocks derived from crustal protoliths. A petrogenetic model invo lving partial melting of a charnockitic, mafic to intermediate lower crust followed by limited fractional crystallization of the magma in a high-level magma chamber is proposed. The enrichment of HFSE and REE (except Eu) in t he inner zone is considered the ultimate product of crystal-melt and volati le activity during the final stage of crystallization in a highly silicic ( SiO2 > 74 %) magma chamber.