Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: Paleogeodynamic implications

The mantle section of the Andaman Ophiolite between Rutland Island in the south and north Andaman in the north has been studied. The restitic peridotite of Rutland Island is comprised of depleted harzburgite to clinopyroxene-bearing harzburgite whereas that in middle and north Andaman is mostly less-depleted, lherzolite-dominated mantle. Chromian spinels from the mantle section of the Andaman Ophiolite belong to four major groups: Group-1 — spinels from Rutland chromitite pods, characterized by high Cr# [= Cr/(Cr + A1) atomic ratio] (~ 0.71); Group-2 — spinels from north Andaman chromitite pods with two subgroups, 2A — characterized by high Cr# (0.70–0.79) and 2B — characterized by medium Cr# (0.48–0.51) and high Mg# [= Mg/(Mg + Fe2+) atomic ratio] (0.74–0.77); Group-3 — residual accessory grains from Rutland peridotite, characterized by medium Cr# (0.37–0.55) and low Mg# (0.41–0.54); and Group-4 — residual accessory grains from middle and north Andaman peridotites, characterized by low Cr# (0.09–0.23) and high Mg# (0.71–0.81).Group-1 chromian spinels of Rutland Island and Group-2A of north Andaman are likely to have crystallized from boninitic melts, formed by relatively high degrees of partial melting, whereas Group-2B varieties from north Andaman are from tholeiitic melts, formed by lower degrees of melting. The chemistry of the residual accessory chromian spinels (Groups 3 and 4) suggests that the mantle peridotites of Rutland Island towards the south are similar to fore-arc peridotites of suprasubduction zone environments whereas those of north Andaman are less depleted. This spatial and/or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time. The coexistence of both chromitite types, with high- and low-Cr spinels, in the same area from north Andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages. It is inferred that a similar geodynamic setting to the present-day also existed during the Cretaceous period with a fore-arc at Rutland Island and a back-arc towards the north at north Andaman.

► Chemistry of accessory Cr-spinels of mantle restites displays directional variation
► Mantle restites towards south are increasingly akin to more depleted peridotites
► The same sliver of oceanic mantle underwent different styles of melting
► A fore-arc–back-arc setting during Cretaceous is inferred