COMPOSITION OF HYDROUS FLUIDS RELEASED BY DEHYDRATION OF OCEANIC ISLAND BASALTS DURING SUBDUCTION: CONSTRAINTS FROM THE ECLOGITE-FACIES HIGH-PRESSURE VEINS IN THE WESTERN TIANSHAN, NW CHINA

Abstract

Dehydration of subducted ocean crust plays a significant role in the generation of island arc volcanics. Hydrous eclogite-facies veins cutting through the host blueschists/eclogites in the western Tianshan high-pressure-low-temperature metamorphic belt documented a dehydration process within a Paleozoic subduction zone. The veins have a typical high-pressure mineral assemblage: garnet (>8%) + omphacite (15%) + quartz (> 55%) + glaucophane (5%) + barroisite (10%) ± zoisite ± calcite. Petrography and mineralogical chemistry of the high-pressure veins and their host rocks indicate that the vein-forming fluids were released at the blueschist to eclogite transitional prograde conditions during subduction. The major element analyses of the veins demonstrate that the composition of solutes in such fluids is dominated by SiO2. The analyses on trace and rare-earth element (REE) indicate the host rocks have OIB-affinity, suggesting that oceanic island basalts (OIB) have been subducted and then involved dehydration. Both the veins and host rocks show similar trace element distribution patterns and enrichments of Li, Be, Cs, Rb, Ba, Pb and La as compared with normal mid-ocean-ridge basalt (N-MORB). Model-simulation suggests the enrichment of fluid-mobile elements (Li, Be, Pb) and large-ion lithophile elements (LILE; Cs, Rb, Ba) and the depletion of high-field-strength elements (HFSE; Nb) and REE (Sm, Nd) with respect to N-MORB in veins, fluids in equilibrium with veins and modeled original fluids from which the veins were precipitated. This study demonstrates that Li, Be, LILE, La and Pb-enriched and HFSE and heavy rare-earth elements (HREE) -depleted fluids were released by dehydration of OIB during subduction of the South Tianshan Ocean in Paleozoic time.