Re–Os isotopic and platinum group elemental constraints on the genesis of the Xiadong ophiolitic complex, Eastern Xinjiang, NW China

Abstract

The Xiadong ultramafic complex, Eastern Xinjiang, NW China, was tectonically emplaced in Proterozoic rocks of the Middle Tianshan Terrane in the Central Asian Orogenic Belt. It consists of harzburgite, dunite and websterite and is associated with amphibolite. The amphibolite is interpreted as metamorphosed oceanic crust, and contains igneous zircon grains with an age of 368±3Ma.All of the ultramafic rocks have highly variable PGE contents. Both the harzburgite and dunite have high PGEs but low Cu and Ni, whereas the dunite and websterite show significant depletion of Ir and Pt. The dunite and harzburgite generally have low 187Re/188Os ratios (0.074–0.81) with relatively uniform 187Os/188Os ratios from 0.1320 to 0.1375, slightly higher than the primitive upper mantle (PUM) value (0.1296), except for one harzburgite sample with much higher 187Re/188Os (24.6) and 187Os/188Os ratios (0.4581). The websterite has significantly high 187Re/188Os (202–1661) and radiogenic 187Os/188Os (1.166–19.90) ratios relative to the dunite and harzburgite.PGE patterns and Re–Os isotopic compositions clearly indicate that the Xiadong complex is a part of an ophiolitic mantle sequence, and the PGE geochemistry suggests a relatively reduced mantle source. Rocks from the complex may have undergone two stages of formation; early partial melting at a mid-ocean ridge generated a depleted mantle sequence with low Re/Os ratios. The harzburgite represents the residuum after partial melting of the lherzolite, whereas the dunite was formed by later melt/rock interaction. The websterites are cumulates derived from SSZ magmas above an intra-oceanic subduction zone during the second stage of formation. This second stage of magmatism resulted in metasomatism and enrichment of Re/Os ratios and 187Os/188Os isotope ratios of the mantle rocks. Our new data suggest that the Xiadong complex records the former presence of a major ocean basin between the Tarim Block and the Middle Tianshan Terrane at ~368Ma. The ophiolite was emplaced shortly after the second stage of magmatism.