The Zhaheba ophiolite complex in Eastern Junggar (NW China): Long lived supra-subduction zone ocean crust formation and its implications for the tectonic evolution in southern Altaids

Abstract

The composite Zhaheba ophiolite complex, exposed in Eastern Junggar in the Southern Altaids, records an unusually long record of oceanic crust and magmatic arc evolution. The Zhaheba ophiolite complex consists of ultramafic rocks, gabbro, diorite, basalt and chert intruded by diabase dikes and diorite porphyry. These rocks are overlain by a several-km-thick section of tuffaceous rocks, volcaniclastic sedimentary rocks, and intermediate volcanic rocks. The igneous rocks of the ophiolite complex show negative Nb and Ta anomalies and LREE enrichment relative to HREE, suggesting the influence of fluids derived from a subducting oceanic slab. The LA-ICPMS U–Pb age of zircons from gabbro is 495.1±3.5Ma. Zircon ages from diorite and basalt are 458.3±7.2Ma and 446.6±6.0Ma, respectively. The basalt is locally overlain by bedded chert. Diabase dikes and diorite porphyry yield the U–Pb ages of 421.5±4.1Ma and 423.7±6.5Ma, respectively. The age of stratigraphically lower part of the overlying volcanic–volcaniclastic section is constrained to be about 410Ma, the maximum depositional age of the tuffaceous sandstone from U–Pb detrital zircon ages. Late rhyolite at the top of the stratigraphic section yielded a U–Pb zircon age of 280.3±3.7Ma. The age and stratigraphic relationships for the Zhaheba ophiolite complex and related rocks suggest that the period of ~70Ma of initial supra-subduction magmatism was followed by construction of a mature island arc that spanned an additional 140Ma. Many other ophiolites in the southern Altaids appear to record similar relationships, and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra-subduction zone. The occurrence of the Zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic Tethyan SSZ ophiolites but similar to some ophiolites in North America. Although the Zhaheba ophiolite belt is flanked by the Dulate arc in the north and Yemaquan arc in the south, it cannot stand a suture between two arcs. It is suggested that Devonian–Carboniferous Dulate arc was built on the late Cambrian–middle Ordovician Zhaheba supra-subduction oceanic crust. The late Carboniferous rocks and early Permian rocks in Dulate arc are interpreted to form in the extensional process within Zhaheba–Dulate arc composite system.