Geochemistry of mafic volcanics in the Bayingou ophiolitic mélange, Western Tianshan, NW China: Implications for magma genesis and tectonic setting

Abstract

This paper reports on the occurrence of mid-ocean-ridge basalts (MORB), boninites, and mafic volcanics with affinities to ocean island basalts (OIB) in the Bayingou Palaeozoic ophiolites, Western Tianshan, NW China, which is one of the best preserved ophiolitic sequences, and are the youngest ophiolites reported in the Western Tianshan during the Late Palaeozoic. The MORB samples have comparatively low TiO2 concentrations (0.84–1.20 wt%) and total rare earth element (REE) contents (25.30–55.97 ppm). The ISr values and εNd(t) values of MORBs range from 0.70480 to 0.70540 and + 3.58 to +6.90, respectively. The boninite sample has high SiO2 (53.79 wt%), very low TiO2 (0.22 wt%), high MgO (14.71 wt%), and a Mg# (100 × Mg2+/(Mg2++Fe2+)) value of 75. The boninite sample also has higher Cr (983 ppm) and Ni (230 ppm) contents and lower high field strength element content (e.g. Zr, Y, V, and heavy REEs) in comparison to the MORBs. The light REE (LREE) content of the boninite is slightly enriched, whereas most MORB samples display a depletion of LREEs ((La/Yb)N = 0.63–0.86), thus indicating that there are normal-type MORBs in this area. The OIB samples are characterized by a high concentration of TiO2 (1.89–3.15 wt%) and total REE content (100.68–151.07 ppm) with enriched LREEs ((La/Yb)N = 5.62–6.80). The ISr values and εNd(t) values of OIBs range from 0.70520 to 0.70541 and − 0.24 to +2.21, respectively. The whole rock Sr, Nd, and Pb isotopic compositions of OIB samples are plotted in the field between enriched mantle II (EMII) and PREMA. Such geochemical variation implies two mantle sources at the spreading zone: possibly, one source from the MORB and several sources from EMII-type mantle (mantle plum) in combination with characteristics of trace elements (Zr, Hf, Nb, Ta, Pb, and Ce).Petrogenetic modelling suggests that MORB may have derived from partial melting (ca. 10–20%) of a mantle peridotite that had previously experienced MORB extraction. For boninites, petrogenetic modelling suggests that these rocks are a product of about 20% partial melting of a source that previously experienced about 12% melt extraction from the source that formed the normal-type MORB. A hot plume may have been the thermal factor influencing the generation of the boninites. Volcanic rocks with OIB affinities in the Bayingou ophiolites are generated by 4.4–5.4% partial melting of the deeper mantle (EMII) and has a garnet lherzolite composition. The source of OIBs is affected by contamination of ancient oceanic lithosphere and terrigenous sediments, and is the same as other OIBs that occur in Palaeozoic ophiolites in Western Tianshan, indicating that these ophiolites may have formed in the same subduction event during the Palaeozoic. Our findings indicate that basaltic rocks in the Bayingou ophiolites formed in the supra-subduction zone (SSZ) environment in response to the subduction of Southern Tianshan Ocean during the late Early Carboniferous. The northward subduction of Southern Tianshan Ocean occurred until at least the late Early Carboniferous. In addition, the Northern Tianshan Ocean developed into a relatively mature ocean basin at the same time as the occurrence of MORB in the Bayingou ophiolites.