Geochronology, petrogenesis and tectonic implication of Late Paleozoic volcanic rocks from the Dashizhai Formation in Inner Mongolia, NE China

In this paper we present geochemical, zircon U-Pb and Hf isotopic data on the late Paleozoic volcanic rocks of the Dashizhai Formation, which are exposed along the northwestern margin of the Songnen terrane in eastern Inner Mongolia. Our aim is to constrain the petrogenesis and tectonic setting of the volcanic rocks and to unravel the late Paleozoic tectonic evolution of the northwestern part of the Songnen terrane, along the eastern segment of the Central Asian Orogenic Belt. Lithologically, the Dashizhai Formation is composed mainly of rhyolitic tuff, rhyolite, dacite, andesite, basaltic andesite and basalt, with minor basaltic trachyandesite. The zircons separated from these rocks are euhedral-subhedral, have high Th/U ratios (0.2-1.6), and display broad oscillatory growth zoning, indicating a magmatic origin. The results of zircon U-Pb dating indicate the volcanic rocks formed during the early Permian (295-283Ma). Geochemically, these volcanic rocks belong to the mid-K to high-K calc-alkaline series and are characterized by an enrichment in large ion lithophile elements (LILEs) and a depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti), similar to igneous rocks that form in active continental margin settings. Most magmatic zircons of the rhyolites show positive epsilon(Hf)(t) values (+3.65 to +13.0) and two-stage model ages (T-DM2) of 1396-551Ma. These geochemical characteristics indicate that the acidic volcanic rocks of the Dashizhai Formation were most likely derived from the partial melting of dominantly juvenile crustal components with a possible addition of old materials. In contrast, the basic to intermediate volcanic rocks were derived from the partial melting of a depleted lithospheric mantle that had been metasomatized by fluids derived from a subducted slab. These data, together with regional geological investigations, suggest that the generation of the early Permian volcanic rocks of the Dashizhai Formation was related to the southward subduction of the Paleo-Asian oceanic plate beneath the Songnen terrane. This also implies that the terminal collision between the Songnen and Xing'an terranes did not occur before the early Permian. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.