161 / 2015-08-28 22:37:47

OIB-type basats within West Junggar ophiolitic mélanges

ophiolite,seamount,accretion,Junggar, Central Asian Orogenic Belt

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 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 seamounts fragments in West Junggar accretionary complex, and may represent mantle transition zone plume-related magmatism within the Paleo-Asian Ocean.
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 seamounts accretion for the CAOB, which can better explain the tectonic evolution of the CAOB.