Petrology of ultramafic and mafic rocks from the South Andaman Ophiolite, Bay of Bengal: Evidence for an arc-related high-pressure origin

Minor ultramafic (dunite) and mafic (gabbroic) rock occurrences are exposed in South Andaman Island, Bay of Bengal. Dunite is in contact with serpentinite, while gabbroic rocks are in contact with the pyroxenite. Petrographic analysis using a petrographic microscope, major and trace element [including rare earth elements (REE)] analysis using an X-ray Fluorescence (XRF) spectrometer and the High Resolution Inductively Coupled Plasma Mass Spectrometer (HR-ICPMS), and mineral chemistry using an Electron Probe Micro-Analyzer (EPMA) were performed on selected ultramafic and mafic rocks. Petrographically, dunite is composed of olivine, clinopyroxene, and orthopyroxene, while olivine, clinopyroxene, orthopyroxene, and calcic plagioclase are present in olivine–gabbronorite. The bulk rock elemental relationship (Zr versus P₂O₅ and TiO₂ versus Zr/P₂O₅) indicate that the dunite and olivine–gabbronorite are tholeiitic in composition. The clinopyroxene with high Mg# [Mg²⁺/(Mg²⁺ + Fe²⁺)] and lower TiO₂ content is present in dunite, whereas the clinopyroxene with high Mg# and high TiO₂ content exists in olivine–gabbronorite. Cr₂O₃ versus Mg# in the clinopyroxene relationship and negative Nb, Ta, and Ti anomalies in these rocks imply high pressure arc related peridotite mantle source. Our results suggest that the dunite and gabbroic rocks were also intruded in the Andaman Ophiolitic suite of rocks during earlier subduction setting in Late Cretaceous time. Further, it is suggested that these ophiolites have been obducted on to the leading edge of the Eurasian continent during the Mid–Eocene to Late Oligocene event, prior to the current tectonically active Andaman–Java subduction, which was initiated in the Late–Miocene.