Southeastern continuation of the Bangong-Nujiang suture zone: Constraints from Middle Jurassic–Early Cretaceous sedimentary rocks in the western Baoshan block, SW China

Abstract

The Bangong-Nujiang Ocean (BNO) separated the Lhasa and Qiangtang blocks in Tibet during the Mesozoic. Whether it separated the Tengchong and Baoshan blocks and how it evolved remain debated. The sedimentary rocks from the Middle Jurassic Mengjia and Liuwan formations, Late Jurassic Longhai Formation, and Early Cretaceous Longkan Formation of the western Baoshan block serve to investigate these issues. Geochemical signatures indicate that their sediments were deposited on a passive continental margin, with a collisional provenance identified in the Late Jurassic–Early Cretaceous formations. All the formations show similar detrital zircon U–Pb age groups with the exceptions of the Longhai Formation having minor 190–150 Ma zircons and the Longkan Formation containing abundant 190–150 and 140–120 Ma zircons. Provenance studies indicate that the sediments of the Middle Jurassic formations were sourced from the southern Qiangtang, Baoshan and Simao blocks and the Changning–Menglian suture zone. In contrast, the sediments of the Late Jurassic–Early Cretaceous formations were derived mainly from the Tengchong and Baoshan blocks, with an increasing contribution of the Tengchong-derived materials from Late Jurassic to Early Cretaceous. We infer that the absent, minor and significant contributions from Tengchong-derived sources in the provenance of the Baoshan strata were associated with the BNO that separated the Tengchong and Baoshan blocks during the Middle Jurassic, the initiation of the Tengchong-Baoshan collision in the Late Jurassic, and that the Tengchong and Baoshan blocks had been welded together in the Early Cretaceous, respectively. Therefore, the widely spreaded and subduction-related Early Cretaceous magmatic rocks in the Tengchong block were produced by the eastward subduction of the Neo-Tethyan Ocean.