Constraints for post-orogenic extension of the northern margin of the Qaidam Basin from the Late Silurian–Late Devonian igneous rocks in the Gahai–Nanshan area

Abstract

Objective The northern margin tectonic zone of the Qaidam Basin underwent a transition from oceanic subduction to continental subduction during the Early Paleozoic, resulting in the formation of the ultrahigh-pressure metamorphic zone known as the northern Qaidam tectonic zone (NQTZ). There has been a longstanding debate regarding the collapse time of the NQTZ. The Maoniushan Formation has long been regarded as a sign of the end of orogeny; however, recent studies show that the Maoniushan Formation has spanned a long time, and it is controversial when the orogenic belt began to collapse. This study aims to determine the age and genetic background of the Maoniushan Formation and to understand the tectonic transition process of the northern Qinghai–Tibet Plateau from the Proto-Tethys to the Paleo-Tethys. Methods Deep-earth samples, including volcanic and intrusive rocks, offer valuable insights into the activity of the crust during this period. In order to investigate the volcaniclastic rocks and granites in the Gahai–Nanshan area, located in the eastern section of the NQTZ, zircon LA–ICP–MS U–Pb chronology and Lu–Hf isotope methods were employed to explore the formation age of igneous rocks and the characteristics of source rocks. Results The zircon U–Pb chronology reveals that the volcaniclastic rocks of the Maoniushan Formation originated approximately 423 Ma. Furthermore, the intrusive granite was formed at 370 Ma, indicating that the volcanic eruption occurred during the Late Silurian, while the subsequent intrusion and crystallization of the granites occurred during the Late Devonian. The zircon Lu–Hf isotope data reveals that the εHf(t) values of the Late Silurian tuffs are concentrated within the range of –11.5 to –8.3, and the corresponding two-stage Hf model ages are primarily between 1945–2133 Ma. These results indicate that the volcanic rocks predominantly originated from partially melting ancient crustal materials. In contrast, the εHf(t) values of the Late Devonian intrusive granites exhibit a distribution within the range of 3.9–9.1, accompanied by two-stage Hf model ages primarily falling within the 792–1118 Ma range. The results suggest that the granites mainly resulted from partially melting Meso-Neoproterozoic crustal materials. Based on a comprehensive analysis of regional geological and petrological data, it is postulated that the Late Silurian–Early Devonian witnessed pronounced orogenesis resulting from continental deep subduction. This event led to substantial crustal thickening in the NQTZ, where the thickened crustal basement of the Oulongbruk experienced partial melting, ultimately giving rise to the volcanic rocks observed during this period. During the Late Devonian, delamination of the thickened crust facilitated the upwelling of the asthenosphere mantle, triggering regional crustal extension. The interaction between mantle material and crust results in the formation of granitic–volcanic rocks. Conclusion The Maoniushan Formation in the region encapsulates a significant period, making it unsuitable to represent the end of orogeny. Late Devonian igneous rocks indicate that the NQTZ entered a period of substantial crustal extension during this time. [ Significance ] The late Devonian igneous rocks of Maoniushan Formation regionally mark the end of orogeny and the beginning of the Paleo-Tethys tectonic domain.

Keywords

• tibetan plateau
• late paleozoic
• northern margin of the qaidam basin
• igneous rocks
• tectonic setting