PETROLOGY OF POST - ARCHEAN MAGMATIC ROCKS IN THE EASTERN INDIAN CRATON

Minor rhyolite intrusives spatially associated with granophyric granit es occur along the eastern and southwestern fringe of the Archaean Sin ghbhum Granite batholith in the eastern Indian Craton. Some minor trac hyte dykes occur within the batholith and both are of post-Archaean ag e. On the basis of major and trace element studies it has been inferre d that a process more complex than simple fractional crystallisation h as controlled the observed chemical variation in both the rocks. Howev er, the two are genetically unrelated and have been generated from dif ferent sources. All the rhyolites, which are scattered as small bodies over an extensive area, are produced from similar source compositions . The chemical variation among the rhyolites is the result of a differ ence in degree of melting and the samples enriched in LREE and other i ncompatible elements represent a lower degree of partial melt. Composi tion of the rhyolites suggests that neither muscovite nor aluminosilic ates were present in the source. The likely source is therefore the tr ondhjemites and mafic-ultramafic rocks constituting the Archaean crust of the region. Similar source for rhyolites distributed over a large area indicates a homogeneous composition for the Archaean crust.