Evolution of the early Paleozoic Hongguleleng–Balkybey Ocean: Evidence from the Hebukesaier ophiolitic mélange in the northern West Junggar, NW China

Abstract

The Hebukesaier ophiolitic mélange is located in West Junggar, northwest China. Based on detailed field mapping, we identified three rock assemblages in the mélange: ophiolite, seamount, and rhyolite porphyry. The ophiolite comprises mainly metaperidotites, pyroxenites, gabbros, dolerites, diabase dike, plagiogranites, basalts and cherts. Two gabbro and one plagiogranite samples have ages of 512 ± 9 Ma, 505 ± 5 Ma and 502 ± 5 Ma, respectively, which constrain the formation time of the oceanic crust. The geochemical characteristics of the basaltic rocks (gabbros and basalts) suggest that the ophiolite formed by intrusion of mid-ocean ridge melts. The ocean island basalts occur as tectonic blocks within the mélanges that coexist closely with marble, siliceous mudstone and chert, with formation characteristics of a seamount. The Early Silurian rhyolite porphyry samples (435 ± 2 Ma) are characterized by high Al2O3 (14.30–15.31 wt%), Sr/Y (22.26–78.42) and (La/Yb)N (20.27–24.24), similar to those of Late Silurian–Early Devonian adakitic rocks (420–411 Ma) from the Xiemisitai Mountains. Furthermore, the rhyolite porphyry samples display high positive εHf(t) values and low Mg values as well as low concentrations of compatible elements (Cr, Co and Ni), suggesting that the rhyolite porphyry originated from partial melting of lower juvenile crust. The rhyolite porphyry displays spatio-temporal characteristics similar to those of the post-collision plutons (434–405 Ma) of the Xiemisitai Mountains. The rhyolite porphyry intruded the ophiolite in the mélange, implying that the Hongguleleng–Balkybey Ocean closed prior to the Early Silurian. Our results, combined with other geological evidence, indicate that the northern West Junggar underwent ocean basin spreading, intra-oceanic subduction, arc–arc collision, the closure of the ocean and post-collisional extension during the Middle Cambrian–Early Devonian.