ABSTRACT

The NE-SW, NW-SE and E-W trending Proterozoic Newer Dolerite dykes around Chaibasa, Jharkhand, India are found to intrude Singhbhum Granites of late Archean age. Clouded plagioclase, bastitised orthopyroxene and uralitized clinopyroxene, magnetite-ilmenite, quartz-feldspar granophyric intergrowth are the characteristic minerals present in the dolerite dykes which assign a low grade metamorphosed character to the dolerite. Both olivine normative and quartz normative varieties are observed in this suite of dykes. These dolerites have been classified into two groups on the basis of REE distribution patterns: one with LREE (Light Rare Earth Element) enriched pattern (Group I) and the other with flat REE pattern (Group II). A significant positive correlation among trace elements Nb, Zr, La, Ce for Group I dykes suggest variable degree of partial melting of the mantle source. In tectonic discrimination diagrams made with immobile incompatible trace elements Nb, Zr, Ti and Y, Group I dykes show arc-like geochemical signature while Group II dykes are similar to MORBs. Enrichment in some of the LILE and prominent depletion in Nb, P and Ti in Group I dykes suggest that the original melt was derived from a metasomatised mantle source. Group II dykes might derived from a MORB-type mantle source. Melting was triggered by volatiles in case of dykes of Group I while for the Group II dykes, melting was produced by thinning of the lithosphere and subsequent decompression.