Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Abstract

In the Ravipadu gabbro pluton (RGP), Prakasam province, Andhra Pradesh, phlogopites occur as coronal rims along the interfaces between plagioclase-olivine, pIagioclase-ilmenite and rarely plagioclase-clinopyroxene, and as independent grains grown at the expense of intercumulus pyroxenes.

The changes observed in the chemical composition of phlogopites are in the contents of MgO, FeO¹ and TiO₂ SiO₂ and Al₂O₃ do not show much variation and K₂O is almost constant, Na₂O is low and CaO is absent. Calculated end members are rich in phlogopite, Ti-phlogopite and eastonite components.

The phlogopites faithfully record the progressive iron enrichment trend of the original phases and consequently that of tholeiitic parental magma. It is established that in a single phlogopite the Mg number, and Mg, Fe, and Ti contents are controlled by its habit, whereas in a series of rocks the phlogopite composition is steered by the composition of host rock. Concentrations of A1 and K in the Ravipadu phlogopites are independent of chemistry of minerals that are consumed and also evolutionary history of the parental magma but are a signature of "chemical collaboration" of interacting fluid with original mineralogy. Textural features and geochemical traits of the phlogopites suggest probable attainment of perfect equilibrium between the fluid and primary phases during the formation of phlogopites in the RGP.

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