Petrogenesis of Middle Silurian Shandan diorites in North Qilian Orogenic Belt, NW China: Insights into postcollisional slab breakoff

Gang Chen ¹ Dechao Li ^1,2* XIJUN LIU ^1,2,3* Pengde Liu ³ Zhihan Bai ¹ Xiao Liu ^1,2 Rongguo Hu ^1,2 Hao Tian ¹ Yande Liu ¹ Wenmin Huang ¹ Yao Xiao ¹

• ¹ Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, College of Earth Sciences, Guilin University of Technology, Guilin, China
• ² Guilin University Technology, Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources in Guangxi, Guilin, China
• ³ National Key Laboratory of Arid Area Ecological Security and Sustainable Development, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences (CAS), Ürümqi, Xinjiang Uyghur Region, China

Paleozoic intrusive rocks are exposed in the Longshoushan area in NW China, in the Northern Qilian Block and on the southern edge of the Alxa Block. Understanding the petrogenesis and tectonic setting of these intrusive rocks is crucial for reconstructing the tectonic evolution and tectonomagmatic processes that occurred along the North Qilian Orogenic Belt between the Alxa and Central Qilian blocks. This study presents an integrated analysis of petrology, zircon U-Pb geochronology, whole-rock geochemistry, along with Sr-Nd-Pb isotopic data and zircon Hf-O isotopic analyses for these intrusive rocks. The Shandan intrusive rocks consist primarily of calc-alkaline quartz diorite (~430 Ma) and diorite (~403 Ma). These diorites are enriched in largeion lithophile elements (e.g., Ba and U) and depleted in high field strength elements (e.g., Nb, Ta, and Ti), similar to subduction-related magmas. The Shandan diorites have enriched Sr and Nd isotopic compositions, with high initial 87 Sr/ 86 Sr ratios (0.705247-0.70618), variable εNd(t) values (-1.58 to -3.53), positive zircon εHf(t) values (+0.08 to +3.55) and low zircon δ 18 O values (5.75‰-6.38‰). The older zircon grains (430 Ma) yield εHf(t) values of +0.14 to +6.58 and the younger grains (403 Ma) yield negative εHf(t) values (+2.24 to -11.0). The geochemical and isotopic data suggest that the diorites were derived through low-degree partial melting of enriched subcontinental lithospheric mantle with the addition of crustal material and subduction-related sediment-derived melts. We suggest that the formation of the Shandan diorites was dominated by slab breakoff at ~430 Ma, which created a window that enabled the upwelling of asthenospheric material and induced partial melting of the subcontinental lithospheric mantle and crust. At ~403 Ma, slab breakoff was nearing end, leading to weaker asthenospheric upwelling.