Ultrapotassic rocks along late ductile shear zones from the Eastern Ghats Belt, India

Ultrapotassic rock is reported for the first time from the polycyclic Eastern Chats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, K-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 mid-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 similar to 12 (molar) and K2O/A2O(3) ratio of similar to 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 K-feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt post-dales an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Chats Belt and is possibly of Pan-African age. The extensive K-feldspathisation in the Eastern Chats belt could also be linked with this ultrapotassic melt.