75. Hydrothermal Activity in the NE Portion of the Sewell Rise, Central Andaman Trough: Evidences from Multibeam and Petrographic Studies

The Sewell consists of NE-SW to nearly E-W trending flat topped mounts with intervening valleys with a more steeper SE slope than the NW due to the presence of boundary faults as indicated by the profiles studies. The subvolcanic textured rock is rhyolitic in composition with additional mineral phases of calcite, calcite veins, Fe-oxide veins, albite (An0-10), apatite and titanite which are identified as part of hydrothermal veins. Furthermore, this subvolcanic texture indicate these hydrothermal veins were emplaced at subcrustal level for sometime followed by migration to the surface along the crustal scale faults associated with the Sewell Rise. Fe-Mnmicronodules, Fe (19.81 to 30.82 wt%)-Mn (17.72 to 29.93 wt%) encrustations with Fe-Mn coronas having more or less the same mineral chemistry and close proximity of the Andaman Back Arc Spreading Centre (ABSC) suggests a large scale hydrothermal activity and consequent deposition of initial phase of development of ABSC related sea floor massive sulphide deposits formed at ~4 m.a at different pulses for a prolonged period on the Sewell Rise.