Geochemical cycling during subduction initiation: Evidence from serpentinized mantle wedge peridotite in the south Andaman ophiolite suite

Abstract

The ophiolite suite from south Andaman islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here we present new major, trace, rare earth (REE) and platinum group (PGE) element data for serpentinized and metasomatized peridotites (dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al2O3/TiO2 greater than 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with (La/Sm)N greater than 1 and (Gd/Yb)N greater than 1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of (i) hydration, metasomatism and serpentinization of depleted,MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and (ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signature collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd (4.4-12.2 ppb) with high sum of PPGE/sum of IPGE (2-3) and Pd/Ir (2-5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic melts into the depleted and residual mantle wedge peridotite during fore-arc extension at the initial stage of intraoceanic subduction. The PGE patterns with high Os+Ir (2-8.6 ppb) and Ru (2.8-8.4 ppb) also suggest rejuvenated by infiltration of melts derived by high degree of mantle melting. The trace, REE and PGE data presented in our study collectively reflect heterogeneous mantle compositions and provide insights into ocean-crust-mantle interaction and associated geochemical cycling within a suprasubduction zone regime.