ALBERT

Description: The Andaman ophiolite occurs as thrust slices in the outer arc of the Andaman–Java subduction zone. This ophiolite preserves the mantle sequence, layered ultramafic–mafic rocks, intrusive and extrusive rocks. The mantle sequence is represented by serpentinized lherzolite and harzburgite, hosting dunite and chromitite pods. The low Cr-number (0.2–0.4), Cr-number–TiO2 relation of the chromites, oxygen fugacity (fO2) values (?log fO2 (FMQ) = -1.90) and trace elements of mantle peridotites indicate a mid-ocean ridge basalt–suprasubduction-zone (MORB–SSZ) setting. The MORB mantle underwent a low degree of melting (c. 10–15%) and interacted with the subduction-zone melts. Melt–rock interaction of the peridotites in a suprasubduction zone is demonstrated by the replacement of pyroxene grains by olivine grains (Fo90), composition of chromites and oxygen fugacity (?log fO2 (FMQ) = –1.90 to +2.16, where FMQ is the fayalite–magnetite–quartz buffer). The chromite composition of chromitite pods (Cr-number 0.72–0.75), fO2 levels and trace elements for layered peridotites, and occurrence of the extrusive rocks as low-Ca boninite and island arc tholeiitic (IAT) basalt indicate interplay of both boninite and IAT melts for the Andaman ophiolite. The MORB mantle of the subducting Indian plate accreted into the mantle wedge and then melting of the accreted mantle produced boninite melt at the first stage and tholeiitic melts at the second stage.Supplementary material: Sample co-ordinates from the Andaman ophiolite, mineral analysis data for all the samples, modal compositions of the units of the Andaman ophiolite and details of the laboratory analyses including details of the standard are available at http://www.geolsoc.org.uk/SUP18459.