Petrography, geochemistry, and U-Pb detrital zircon dating of early Permian sedimentary rocks from the North Flank of the North China Craton: Implications for the late Palaeozoic tectonic evolution of the eastern Central Asian Orogenic Belt

摘要

Sedimentary rocks from the Hugete and Sanmianjing Formations are exposed on the North Flank of the North China Craton (NCC), southeast of the Central Asian Orogenic Belt (CAOB), and contain key evidence for understanding the late Palaeozoic tectonic setting of the northern margin of the NCC. The Hugete Formation is conformably overlain by the Sanmianjing Formation, and the detrital modes of sandstones from these two formations indicate that they were mainly derived from a continental block source. The geochemical characteristics of sandstones and pelites from the Hugete Formation suggest deposition in a shallow marine environment and derivation from a source composed of predominantly felsic magmatic rocks. Detrital zircon ages from the sandstones of the Hugete Formation include age modes at approximately 2500 and 1800Ma, with smaller modes at 380-404 and 288.2 +/- 8.2Ma. Combined with previously reported age-diagnostic fossils, the youngest age of 288.2 +/- 8.2Ma constrains the depositional age of the Hugete and Sanmianjing Formations to the early Permian. Based on regional geology, these age data reveal that the following: (1) the sedimentary rocks of the Hugete and Sanmianjing Formations were sourced mainly from the NCC, located to the south; (2) the early Palaeozoic accretionary arc complex at the northern margin of the NCC experienced intense erosion before the early Permian and thus it did not appear as a concurrent hinterland uplift during the deposition of the Hugete and Sanmianjing Formations; and (3) no significant topography was generated between the study area and the NCC by the early Permian island arc and orogen. Age distribution patterns, as well as geochemical and petrographic data, indicate that the Hugete and Sanmianjing Formations were deposited in a newly opened intra-continental ocean basin formed in an extensional tectonic environment.