Zircon U-Pb Ages and Hf Isotope Characteristics of the Volcanic Rocks from Queershan Group in the Hazhudongshan Area of Beishan, Inner Mongolia and Their Geological Significance

The Beishan orogenic belt has always been one of study hotspots on the time of closure of the Paleoasian Ocean. Previous studies on the Hongshishan-Baiheshan-Pengboshan ophiolite mélange belt are more focused on a direct constraint on the time of the oceanic subduction, subduction polarity, and the origin and structural attributes of Carboniferous volcanic rocks, resulting in a lack of understanding of the tectonic setting and magmatic evolution of the Devonian volcanic rocks. In this paper, we present new detailed petrologic, geochemical, U-Pb ages and Hf isotope analyses on the Devonian volcanic rocks from the Hazhu formation of the Queershan Group in the Hazhudongshan area of Beishan. Combining with the previous works in this region, our study reveals that the volcanic rocks from the Hazhu Formation of the Queershan Group from Hazhudongshan formed in the Middle Devonian (386.9±1.7 Ma), which is characterized by enriched Na and low K of the low potassium tholeiite to calc-alkaline series. In the aspect of trace elements, these rocks are depleted in high field strength elements of Nb, Ta, P and Ti. These rocks were probably formed in the island arc environment, representing products of the southward subduction of the Paleoasian Ocean. The ε_Hf(t) values of the rhyolite sample are uniform, ranging from 4.4 to 15, with the two-stage Hf model ages between 424 Ma and 1 109 Ma. Thus, the rhyolite of Queershan Group should be derived from partial melting of Early Paleozoic and Meso-Neoproterozoic juvenile crust. However, the magma of andesite of Queershan Group is derived from depleted mantel wedge metasomatized by fluid from subducted plate. Meanwhile the product created by the rising magma has experienced a certain fractional crystallization and assimilation and contamination of upper crustal material. According to previous regional geological studies, the Hongshishan-Baiheshan-Pengboshan ocean should be formed in the extension setting of a Queershan back-arc basin after the Late Devonian.