Closure time of the eastern Paleo-Asian Ocean: Constraint from Permian-Triassic strata and detrital zircon geochronology at southern Great Xing'an Mountains, Inner Mongolia (China)

Closure time of the eastern Paleo-Asian Ocean (PAO) has long been debated, since one important reason is the absence of coeval sedimentation study. Recently, we found well preserved Permian-Triassic (P-T) strata, the Upper Permian Linxi and Lower Triassic Laolongtou formations, at Ar Horchin Banner, Inner Mongolia (China), in southern Great Xing'an Range that can provide detailed record for this process. In this study, the sedimentation and detrital geochronology study of this newly discovered strata are carried out to determine the precise closure time of the eastern PAO. The Upper Permian Linxi Formation is a set of lacustrine sequence, whereas the Lower Triassic Laolongtou Formation a set of fluvial and lacustrine sequence, and the bottom of the later is a conglomerate layer of braided alluvial fan, marking the boundary of the two formations. The sedimentary environment near the P-T boundary were distinct, with warm and humid in ending Permian and hot and arid in earliest Triassic. The accumulation of Linxi Formation occurred at Late Permian Changhsingian Period, while the Laolongtou Formation at Early Triassic Olenekian Period, and they are unconformable contacted with a short sedimentation hiatus between them. The sandstone samples of the Linxi Formation content various kinds of heavy minerals that mainly come from felsic to mafic magmatic rocks, and minor from metamorphic and sedimentary rocks. The heavy minerals from the Laolongtou Formation sandstones are rare, derived from medium-acid magmatic and minor metamorphic rocks. The mean ages of youngest group of the two clastic samples from Linxi Formation are 255 +/- 2Ma and 255 +/- 1Ma, respectively, with epsilon(Hf) (t) values ranging from -22.84 to +13.17. While the two clastic rock samples of Laolongtou Formation have youngest mean ages of 248 +/- 1Ma and 249 +/- 1Ma, respectively, with epsilon(Hf) (t) values of 7.95 similar to 11.28. The heavy minerals, detrital zircon ages and Hf isotopes suggested that the sedimentations of the Linxi Formation have complex provenance that derived mainly from the Xing'an-Mongolia Orogenic Belt, and a small amount of North China Craton (NCC), with long-distance transport and recycling deposit. Whereas the provenance of the Laolongtou Formation is mainly from the Xing'an-Mongolia Orogenic Belt that is characterized by a near source sedimentation with short distance transportation. The comprehensive study of the regional detrital zircon geochronology information of the Linxi and Laolongtou formations indicate that the NCC and Central Asian Orogenic Belt (CAOB) converged and collided along the Xar Moron Suture Zone at ending Permian accompanied with the closure of eastern PAO, and this process lasted at least to Early Triassic. A huge orogenic event due to the collision and amalgamation of NCC and CAOB should have happened during the accumulation of the Laolongtou Formation, which provided right products for this formation.