DEHYDRATION MELTING OF MICAS IN THE CHILKA LAKE KHONDALITES - THE LINK BETWEEN THE METAPELITES AND GRANITOIDS

Garnet-sillimanite gneisses, locally known as khondalites, occur abund antly in the Chilka Lake granulite terrane belonging to the Eastern Gh ats Proterozoic belt of India. Though their chemistry has been modifie d by partial melting, it is evident that the majority of these rocks a re metapelitic, with some tending to be metapsammitic. Five petrograph ically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios - the m ain chemical discriminant separating the five groups. The variations i n Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes a nd spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclas e and quartz, commonly referred to as leptynites in Indian granulite t erranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous cha racter of the leptynites and presence of sillimanite trails within gar nets in some of them suggest derivation of leptynites by partial melti ng of khondalites. Here we examine this connection in the light of res ults derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical com positions in a (MgO + FeO)-Na2O-K2O projection match the composition o f liquids derived by biotite and muscovite dehydration melting, when c orrected for co-products of melting reactions constrained by mass bala nce and modal considerations. The melt components of the leptynites de scribe four clusters in the M-N-K diagram. One of them matches melts p roduced dominantly by muscovite dehydration melting, while three clust ers correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different p aths that pelitic and psammitic protoliths are expected to generate du ring dehydration melting. Thus the leptynites evidently represent gran itoids which were produced by dehydration melting in metapelites of di fferent compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor . Disequilibrium in terms of major elements is illustrated by leucosom es within migmatites developed in a group of metapelites. But the disc rete leptynites that have been compared with experimental melts approa ch equilibrium melt compositions closely.