• S K Sen

      Articles written in Journal of Earth System Science

    • The Indian granulites

      S K Sen

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    • Dehydration melting of micas in the Chilka Lake Khondalites: The link between the metapelites and granitoids

      S K Sen S Bhattacharya

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      Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks.

      A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks.

      The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na2O-K2O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different 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 leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.

    • Diverse signatures of deformation, pressure-temperature and anatexis in the Rayagada sector of the Eastern Ghats granulite terrane

      S K Sen S Bhattacharya

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      The granulites and granitoids around Rayagada in the north central part of the Eastern Ghats belt display structural and petrological differences when compared to similar rocks from Chilka and Jenapore in the northern Eastern Ghats. The impress of F1 deformation is almost erased while that ofF3 is muted. The metapelites have a restricted chemical range and are non-migmatitic. There are two varieties of leptynitic granitoids, one of which is interlayered with yet another S-type granite containing cordierite. The maximum recorded temperature from geothermometers is 780‡C, but the magnitude of pressure is comparatively low, the highest value being 6.3 kbar. Another distinctive feature of the pressuretemperature record is the absence of evidence of decompression in the lower realms of pressure and temperature. Metamorphic reactions that could be identified indicate cooling, a noteworthy reaction being the sillimanite to andalusite transformation. Integration of data from pressure-temperature sensors suggest cooling at two pressures, 6 and 5 kbar. The generation of two types of granitoids from metapelites is interpreted to be due to intersection with solidus curves for pelitic and graywacke-like compositions, constrained by recent experiments, at 6 and 5 kbar. The first melting occurred on a prograde path while the second one was due to increase in temperature during exhumation at tectonic rates.

      Thus inspite of a broad similarity in the geodynamic scenario across the northern part of the Eastern Ghat belt, differences in exhumation rates and in style of melting were responsible for producing different signatures in the Rayagada granulite terrane.

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