Mineral chemistry and genesis of phlogopites from the gabbros of Ravipadu,Prakasam Province, Andhra Pradesh, India

In the Ravipadu gabbro pluton (RGP), Prakasam province, Andhra Pradesh, phl ogopites occur as coronal rims along the interfaces between plagioclase-oli vine, plagioclase-ilmenite and rarely plagioclase-clinopyroxene, and as ind ependent 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 a nd K2O is almost constant, Na2O is low and CaO is absent. Calculated end me mbers 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 T i contents are controlled by its habit, whereas in a series of rocks the ph logopite composition is steered by the composition of host rock. Concentrat ions of Al and K in the Ravipadu phlogopites are independent of chemistry o f 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 p hlogopites suggest probable attainment of perfect equilibrium between the f luid and primary phases during the formation of phlogopites in the RGP.