Geochemistry of some unusual early Archaean sediments from Dharwar Craton, India

Metamorphosed Archaean sediments comprise ∼ 35% of the 3.5 Ga old Holenarasipur schist belt (HNSB) and 90% of the 3.2 Ga old Javanahalli schist belt (JSB) of the Dharwar Craton. The high-alumina sediments of the HNSB are variable in their major, LIL and ferromagnesian trace elements. The REE patterns are variable, with both positive and negative Eu anomalies. Cr and Ni are more abundant than in most of the detrital sediments, at corresponding silica contents. Al and Cr show positive correlation. The Th/U, La/Th, Ba/La and Zr/La ratios and abundances show a large-scale scatter. The paragneisses of JSB also have a variable major element composition, but uniform REE patterns and abundances, with strong negative Eu anomalies similar to some Archaean tonalitic gneisses and Phanerozoic shales and quartzites. The para-amphibolites, stratigraphically above these gneisses, have depleted flat-REE patterns and positive Eu anomalies. The HNSB sediments are probably derived from a source consisting of mafic, ultramafic, anorthositic and acid volcanic rocks. In contrast, the source area for JSB sediments had considerable acid plutonic rocks. This indicates that the processes of sial formation started soon after the HNSB group had been deposited and continued through the development of the younger greenstone belts. Rapid nucleation and development of continental crust occurred during the interval 3.5 to 3.2 Ga.