Ultrapotassic rocks along Late Ductile Shear Zones from the Eastern Ghats Belt, India

Abstract

Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, Khyphen; feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midhyphen; crust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K₂O/Na₂O ratio of ∼12 (molar) and K₂O/Al₂O₃ ratio of ∼1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity. However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to Khyphen; feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt posthyphen; dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Panhyphen; African age. The extensive Khyphen; feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.