Petrology of Post -Archaean Magmatic Rocks in the Eastern Indian Craton

Minor rhyolite intrusives spatially associated with granophyric granites occur along the eastern and southwestern fringe of the Archaean Singhbhum Granite batholith in the eastern Indian Craton. Some minor trachyte dykes occur within the batholith and both are of post-Archaean age. On the basis of major and trace element studies it has been inferred that a process more complex than simple fractional crystallisation has controlled the observed chemical variation in both the rocks. However, the two are genetically unrelated and have been generated from different 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 difference in degree of melting and the samples enriched in LREE and other incompatible elements represent a lower degree of partial melt. Composition of the rhyolites suggests that neither muscovite nor aluminosilicates were present in the source. The likely source is therefore the trondhjemites 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.