OIB-type rocks within West Junggar ophiolitic mélanges: Evidence for the accretion of seamounts

The Central Asian Orogenic Belt (CAOB, also known as the Altaid Tectonic Collage), one of the largest and long-lived accretionary orogens in the world, developed from ca. 600Ma to 250Ma by multiple accretions of terranes of different origin. Despite extensive research aimed at understanding tectonic evolution of the CAOB and the evolution of the Paleo-Asian Ocean, the subduction–accretion mechanism of the CAOB remains controversial. The West Junggar, northwestern China, is located in the southwest of the CAOB and consists of Paleozoic ophiolitic mélanges, arcs and accretionary complexes.The ophiolitic mélanges in West Junggar are generally complicated and usually consist of metamorphic peridotite (serpentinite), cumulates, gabbros, basaltic lavas (pillows), and abyssal radiolarian cherts. However, sheeted dike complexes are extremely rare and, where present, usually form small, isolated bodies. The formation age of ophiolitic mélanges in West Junggar has been determined generally as early Paleozoic except for the Darbut and Karamay ophiolitic mélanges with the age ranging from Early–Middle Devonian to Late Carboniferous. The Darbut and Karamay ophiolitic mélanges have distinct unconformable or fault contacts with Lower Devonian to Lower Carboniferous volcano-sedimentary rocks consisting mainly of sandstone, basalt, tuff and volcanic breccia. The strata deposited on the Tangbale and Mayile ophiolitic mélanges are early Paleozoic and dominated by Ordovician–Silurian volcano-sedimentary rocks.The alkaline basalts in the Tangbale, Mayile, Darbut and Karamay ophiolitic mélanges occur as tectonic blocks within the mélanges which are composed of pelagic limestones, siliceous mudstone and radiolarian chert, possessing formation characteristics of seamounts (oceanic islands/plateaus). The alkaline basalts are characterized by LREE enrichment and HREE depletion, very weak or no Eu anomalies, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The OIB-type rocks in ophiolitic mélanges are considered as accreted seamount fragments in West Junggar accretionary complex, and may represent mantle transition zone plume-related magmatism within the Paleo-Asian Ocean. Therefore, we propose the following tectonic evolutionary history of the West Junggar.The seamounts formed in West Junggar as well as the Paleo-Asian Ocean at Neoproterozoic. With progressive evolution of the Paleo-Asian Ocean, subduction of the oceanic lithosphere commenced during late Cambrian to early Ordovician, resulted in the seamounts eventually accreted in the fore-arc with oceanic fragments forming the Tangbale and Mayile ophiolitic mélanges. As retreat of the subducting slab, the subduction–accretion processes may have continued in the late Paleozoic, given rise to form the Darbut and karamay ophiolitic mélange with the OIB-type rocks from seamounts. Based on our observations, and in combination with previous work, we suggest that the intra-plate magmatism in the CAOB might generally continuous during the development of the Paleo-Asian Ocean from the Late Neoproterozoic to the Mesozoic. Therefore, we present a new model that is multiple intra-oceanic subduction with seamount accretion for the CAOB, which can better explain the tectonic evolution of the CAOB.