Geochemistry and U-Pb detrital zircon ages of late Permian to Early Triassic metamorphic rocks from northern Liaoning, North China: Evidence for the timing of final closure of the Paleo-Asian Ocean

This paper reports LA-ICP-MS U-Pb zircon ages and geochemical data from the Zhaobeishan Formation on the northern margin of the North China Craton (NCC). These data are used to better constrain the depositional age, provenance characteristics and tectonic evolution regarding particularly the final closure of the Paleo-Asian Ocean in the eastern segment of the Central Asian Orogenic Belt (CAOB). Major, trace, and rare earth element (REE) characteristics show that the Zhaobeishan Formation formed in an active continental margin environment. Clastic particles in the formation display varied provenance sources from basic to intermediate acid arc igneous rocks. Twenty-two zircon rim analyses from a diopside granulite yield a weighted mean Pb-206/ U-238 age of 245 +/- 2 Ma (MSWD = 3.2), which represents the timing of metamorphism, whilst 45 concordant zircon core analyses yield apparent ages of 257-2592 Ma that fall into eight age populations. The depositional age of the Zhaobeishan Formation is constrained to be between 257 and 245 Ma, based on the youngest concordant Pb-206/U-238 age of the zircon cores and the metamorphic age of zircon rims. The provenance of the formation is dominated by Paleozoic magmatic arcs of the northern margin of the NCC, with material also derived from the interior of the NCC and the Songliao Block (SLB). Mixed detrital zircons from the NCC and SLB, which were separated by the Paleo-Asian Ocean, indicate a short distance between the NCC and SLB, and a narrow ocean at the time of sedimentation. The Paleo-Asian Ocean still existed in the eastern segment of the CAOB during the late Permian to Early Triassic, and the metamorphic age (ca. 245 Ma) of zircon rims in the diopside granulite sample may represent the timing of final closure of the Paleo-Asian Ocean.