Cumulate gabbros in the South Andaman Island ophiolite suite (India): their bearing on the tectonic setting

Abstract

The Andaman ophiolite of southeastern India is located on the outer arc of the Andaman–Java subduction zone. It is represented by thrust slices formed in the Mesozoic Neo-Tethys Ocean. Lithologically, it consists of dismembered mafic and ultramafic rocks and associated oceanic pelagic sediments. The present study focuses on the mafic cumulate rocks of the Andaman ophiolite preserved in the Kodiaghat and Munda Pahar area of South Andaman Island. The mafic cumulates are represented by olivine-bearing and olivine-free gabbros. The sequence of crystallization in the gabbros is olivines (Fo∼80) ± chromian spinels (Cr# = 57–59), plagioclases (An61–95), clinopyroxenes (Mg# = 82–89), and amphiboles (Mg–hornblende, edenite, and pargasite). Major oxide and trace element whole-rock geochemistry and mineral compositions are consistent with a hybrid signature of island arc tholeiite (IAT) – mid-oceanic ridge basalt (MORB). Geochemical modelling shows that trapped melt fractions of 0%–20% can produce the observed trace element signatures of these gabbros. Our findings suggest that the gabbroic cumulates of the Andaman ophiolite were formed in an oceanic back-arc and oceanic arc setting developed in the Neo-Tethys oceanic domain between the Indian and Burmese plates during the Late Cretaceous.