Abstract

Plagiogranites occur on the eastern margin of the southern part of South Andaman in the outer sedimentary arc of the Andaman-Nicobar islands, a part of the Sunda-Burmese double-chain arc system, in the Bay of Bengal. They intrude gabbros that are associated with pillow basalt, East Coast volcanic rocks (basalt, basaltic andesite to acid differentiates), radiolarian cherts, conglomerate and grit.

The plagiogranite, characterized by the occurrence of interstitial vermicular and micrographic intergrowths of quartz and plagioclase, shows some geochemical similarities with ophiolitic plagiogranites, but Rb, Yb, Nb, Ta and Y abundances in the Andaman plagiogranite are characteristic of volcanic-arc granites rather than the ocean-ridge granites. It is suggested that the Andaman plagiogranite has been generated by low-pressure crystal fractionation of a low-K tholeiitic magma derived from a mantle source in a supra-subduction zone tectonic setting. Further, it is suggested that this plagiogranite intruded the gabbro of the Andaman ophiolite probably during Mid-Eocene times, as the Andaman ophiolitic rocks (Late Cretaceous to Early Eocene) are believed to have been obducted onto the leading edge of the Eurasian continent during the Mid-Eocene to Late Oligocene subduction event, prior to the currently active Andaman-Java subduction system, that was initiated during the Late Miocene.

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