Petrogenesis of the Early Paleozoic Dioritic–Granitic Magmatism in the Eastern North Qilian Orogen, NW China: Implications for Tethyan Tectonic Evolution

Abstract

Abstract The North Qilian Orogen witnessed the opening, subduction, and closure of the Proto-Tethys Qilian Ocean and the post-subduction of multiple exhumation events from Late Neoproterozoic to Early Paleozoic. The Early Paleozoic dioritic–granitic magmatic suites, prominently exposed in the eastern North Qilian Orogen, offer valuable insights into the evolution of the Proto-Tethys Ocean. However, their petrogenesis, magma source, and tectonic evolution remain controversial. Here, we investigate the Leigongshan, Zhigou, and Dalongcun intrusions and present geochronological, geochemical, and isotopic data, aiming to refine the comprehension of their timing and petrogenesis, which will contribute to understanding the tectonic evolution of the Proto-Tethys Ocean. Zircon U-Pb dating reveals mean ages of 471–427 Ma for these intrusions, consistent with compiled formation ages of dioritic–granitic intrusions in the eastern North Qilian Orogen, indicating close temporal links with the tectonic evolution of the Proto-Tethys Ocean during the Early Paleozoic. The studied magmatic rocks could be categorized into two major types: granitoids and diorites. The granitoids are majorly I-type granitoids that are generated through partial melting of the mafic lower crust and fractional crystallization at the middle-upper crust, with the involvement of mantle-derived materials. The diorites underwent limited crustal contamination and fractionation of hornblende, plagioclase, and some accessory minerals. They were derived mainly from the mixture of fertile mantle and reworked crustal components, with minor contributions from subduction-related slab fluids and sediment melts. In addition, all the studied Early Paleozoic dioritic–granitic intrusions (ca. 471–427 Ma) formed within subduction-related arc settings. Combined with the tectonic evolution of the Early Paleozoic Qilian orogenic system, we interpret these Cambrian to Silurian dioritic–granitic intrusions as tectonic responses to the subduction (ca. 520–460 Ma) and closure (~440 Ma) of the Proto-Tethys Ocean, whereas the Devonian Huangyanghe intrusion witnessed the final stage of extensional collapse of the Qilian orogenic system at ca. 400–360 Ma.