Geochemical record of subduction initiation in the sub-arc mantle: Insights from the Loma Caribe peridotite (Dominican Republic)

Abstract

The Loma Caribe peridotite body is mainly composed of serpentinized spinel harzburgites and lherzolites and minor orthopyroxene-bearing dunites and dunites. Modal proportions, mineral and whole-rock major and trace element compositions generally coincide with those of abyssal mantle rocks from mid-ocean ridges for the lherzolites, and refractory supra-subduction peridotites for the harzburgites and dunites. The clinopyroxene-bearing harzburgites have intermediate compositions that overlap with those of residual mantle from both these settings. Major elements in the peridotites were mostly undisturbed by serpentinization and/or seafloor weathering whereas light rare earth elements (LREE) and large ion lithophile elements (LILE) were enriched by syn- and/or post-melting interaction with fluids/melts. Major element variations indicate that protoliths of the Loma Caribe peridotites mostly melted at 1–2GPa and 1250°C–1500°C, as normal mid-ocean ridge and supra-subduction zone mantle. The MREE/HREE fractionations in both whole rocks and clinopyroxene can be explained by initial low (5%–6%) fractional melting of a garnet lherzolite source followed by variable (5%–20%) melting in the spinel stability field. The lherzolites and clinopyroxene-bearing harzburgites are residues of increasing melting triggered by increasing addition of slab fluids to a spinel peridotite source, while melting of the harzburgite protoliths was likely the result of focused flux of slab hydrous melts. The dunites and orthopyroxene-bearing dunites are products of pyroxene dissolution in residual peridotites caused by reaction with two different subduction-related melts, probably the parental magmas of Early Cretaceous low-Ti island arc tholeiites (IAT) and boninites from Central Hispaniola, respectively. We conclude that the geochemical heterogeneity of the Loma Caribe peridotites records shifting conditions of melting during the development of subduction beneath the incipient Greater Antilles paleo-island arc in the Early Cretaceous.