Origin and tectonic significance of the Hoboksar ophiolitic mélange in northern West Junggar (NW China)

Abstract

Although the Early Paleozoic tectonic framework of northern West Junggar is essential for understanding accretionary processes in the southwestern Central Asian Orogenic Belt, this setting is still not well understood. In order to address this issue, we employed detailed geochronological, geochemical and Sr-Nd-Hf-Os isotope analyses on the Hoboksar ophiolitic mélange in northern West Junggar. The Hoboksar ophiolitic mélange mainly consists of serpentinized peridotites, pyroxenites, gabbros, diabase, cherts, marbles, volcanic and pyroclastic rocks. It is characterized by a “block-in-matrix” structure with serpentinized peridotites as the matrix which is crosscut by later diorite porphyry and rhyolitic porphyry. Olivines and spinels in these serpentinized peridotites show high Fo# and Cr#, respectively. The clinopyroxenes in the pyroxenites exhibit high Mg#, low Al2O3 and Cr2O3. The serpentinized peridotites have extremely low rare-earth element (REE) abundances, U-shaped to flat REE patterns and positive Eu anomalies. Their particularly low Re/Os, initial 187Os/188Os and γOs(t) imply that they could represent residual phases after partial melting of upper mantle. The gabbros and diabases display slight fractionation between light REE and heavy REE, negligible Eu anomalies, depletion in Rb, Nb and Ta, enrichment in Th and U, low initial 87Sr/86Sr and positive εNd(t). The mineral and whole-rock geochemical and isotopic compositions of these ultramafic-mafic rocks demonstrate that the Hoboksar ophiolitic mélange originated from a depleted mantle in a fore-arc supra-subduction zone (SSZ) setting.Zircon UPb dating on diorite porphyry and rhyolitic porphyry yields weighted mean 206Pb/238U ages of 435 ± 4 Ma and 441 ± 2 Ma, respectively, postdating the closure of an ancient ocean represented by the Hoboksar ophiolite. These later intrusions resemble adakitic rocks with high Sr/Y. They exhibit mild depletion in Nb and Ti, low initial 87Sr/86Sr, positive εNd(t) and zircon εHf(t). Therefore the diorite porphyry and rhyolitic porphyry likely derived from juvenile lower crust. Based on our new data and other geologic evidence, we propose that the ancient Hoboksar Ocean resided between two Paleozoic arcs in northern West Junggar during the Early Ordovician, and this ocean was closed before 440 Ma. The emplacement of adakitic stitching intrusions implies a back-arc extensional setting that may be due to (present-day) northward subduction of the Junggar-Balkhash oceanic slab during the Early Silurian.