Petrogenesis of the Ravipadu Gabbro Pluton, Prakasam Province, Andhra Pradesh

The Ravipadu gabbro pluton (RGP) is one of the basic intrusives occurring within the deep Precambrian crust composed of amphibolites and granulites in the Prakasam igneous province juxtaposed against the Eastern Ghats mobile belt and Cuddapah basin. The pluton is a post-kinematic lopolith emplaced either permissively into the preheated country rocks as crystal mushes or emplaced by convective stirring. The intrusion has induced very high temperature thermal metamorphism of the pelites occurring on the fringes of the pluton. Dolerite and basaltic dykes traverse the RGP.

The RGP is composed chiefly of olivine gabbro, olivine norite, troctolite and anorthosite in the decreasing order of areal extent. Sporadically developed layering is recorded in olivine norites, which may represent quiescent zones of magma chamber. The rocks exhibit cumulate, poikilitic and sub-ophitic textures. Olivine, plagioclase, orthopyroxene and clinopyroxene are the essential minerals crystallised in that order, and their variable proportions and combinations have produced the observed lithology. Sub-solidus diffusion along olivine-plagioclase and Fe-Ti oxide-plagioclase interfaces has produced spectacular coronas.

The composition of olivine ranges from Fo_56.70 to Fo_67.09 and lower Fo content is attributed to derivation of parental liquids from a more Fe-rich source than the normal mantle. The composition of plagioclase varies from An_55.31 to An_75.48 Orthopyroxene is bronzite showing restricted variation from En_68.82 Fs_29.37 to En_71.66 Fs_27.19. Wollastonite component is low and erratic in orthopyroxenes. Clinopyroxene is Al-rich titanaugite and its end-member composition varies from En_41.95 Wo_44.45 Fs_15.23 to En_47.05 Wo_38.91 Fs_11.03. llmenite composition ranges from Il_87.83 to II_89.09 with very little haematite component. The ilmenites represent re-equilibrated compositions.

The Ravipadu gabbroic rocks are primarily hypersthene and olivine normative; they do not contain quartz or nepheline in the norm. Textures, mineral assemblages, and mineral- and whole-rock chemistry indicate that the Ravipadu gabbros could be formed by crystal-liquid fractionation of subalkaline tholeiitic magma under reducing (anhydrous) conditions. The tholeiitic nature is also re-affirmed by Fe-enrichment as the parental magma evolved. The chondrite normalized REE patterns show enriched LREE, convex downward MREE and almost flat HREE trends with positive Eu anomalies for all the rocks.

Petrographic features and geochemical traits accord cumulate status to the rocks and they do not represent melt compositions. Parental melt REE concentrations are calculated by utilising exploratory geochemical inversion techniques. The calculated parental liquids display enriched LREE and flat HREE trends. The variation in the concentrations of REE calculated from different cumulate rocks represents progressive evolution of a parental magma by fractional crystallization. Petrogenetic modelling suggests that the parental liquids are derived by 5 to 10% melting of an LREE enriched komatiite/tholeiite source in the deep crust. This source could be an underplated Late Archaean crust and this deep crustal melting might have been induced by the heat released from a mantle plume.