Magma Mixing In The Genesis Of The Qingshanbao Granitoids In The Longshoushan Area: Implications For The Tectonic Evolution Of The North Qilian Orogenic Belt, Nw China

The geochemical differences of magmatic rocks can be influenced by melt source and magmatic processes, including partial melting, fractional crystallization and magma mixing. New whole-rock geochemical analyses, zircon U-Pb ages and Hf-isotope data on granitoids from the Qingshanbao complex in the Longshoushan belt highlight a significant Late Ordovician-Early Silurian magmatic episode during the collision between the Alxa and Qilian-Qaidam blocks, in which magma mixing was a fundamental process. The Qingshanbao complex comprises K-feldspar granite, granodiorite and diorite emplaced coevally from 442 to 433 Ma. They exhibit peraluminous to metaluminous I-type features, are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in high-field-strength elements (HFSEs). Chondrite-normalized REE patterns are fractionated (La-N/Yb-N = 10.2-34.9) with weakly negative Eu anomalies (Eu/Eu* = 0.66-0.94). The Lu-176/Hf-177 zircon two-stage model ages (T-DM2) and the epsilon(Hf)(t) on zircons extracted from the K-feldspar granite (1613-2,316 Ma, epsilon(Hf)(t) = -14.4 to -3.0) and granodiorite (1640-2015 Ma, epsilon(Hf)(t) = -9.6 to -3.6) suggest they were formed by the mixing of melts produced by the melting of Mesoproterozoic crustal materials, most likely meta-basic rocks from the Longshoushan Group, and to a lesser extent mantle materials. In contrast, zircons in the diorite yield T-DM2 ages ranging from 964 to 1767 Ma and epsilon(Hf)(t) values of -5.5 to 7.1, implying melting of depleted mantle rocks with subordinate participation of lower crust materials. We suggest that the Qingshanbao complex was emplaced when the North Qilian area was in a transitional stage from a compressional to an extensional environment as early as approximately 442 Ma.