Early Permian Peperites in the Beishan Region, NW China: Implications for Regional Tectonic and Palaeoenvironmental Setting

English Abstract:

Peperites have been identified at the Lower Permian Upper Zhesi group in Beishan, China, which is significant for the reconstruction of Late Paleozoic evolution in the southern part of the Central Asian Orogenic Belt. Five types of peperites are distinguished in the Hongliuhe and Liuyuan profiles and interpreted as the products from basaltic lava bulldozing into wet, unconsolidated sediments at their basal contacts. Laser ⁴⁰Ar/³⁹Ar dating of the basalt yielding an age of 274±11Ma, as well as the occurrence of Early Permian brachiopod fossils in the carbonate sediments interbedded with the basalt confirm that the peperites formed in Early Permian. Identification of the peperites reveals that the basaltic lava flows were derived from autochthonous basaltic magmatism and formed as part of the Lower Permian succession. The peperites also distinguish these subaqueous basaltic lava flows are not dismembered ophiolitic components, formed as a product of oceanization in Early Permian. The clastic rocks in the Lower Zhesi group underlying the basaltic lava and peperites in the Hongliuhe and Liuyuan areas show a general fining-upwards sequence, indicating that they were deposited in a progressively deepening basin overlying the Devonian Hongliuhe suture zone. Therefore, the peperites were probably originated in a progressively subsiding-basin environment as a result of advanced rifting during Early-Middle Permian in the Beishan region. Subaqueous volcanism in rift basins, accompanied by coeval deposition of carbonate sediments and mudstones, built up the peperite-bearing volcanogenic-sedimentary successions. A detailed investigation of Permian peperites in the Beishan region provides a typical application in determining palaeoenvironment and thus constraining regional tectonic setting.