Evolution of the Paleo-Asian Ocean in south-eastern Inner Mongolia, China: Evidence from zircon U-Pb ages, Hf isotopes data and specific deformation of the Linxi and Sunjiafen formations

The crystallization age of the Sunjiafen Formation volcanic rocks is 253.9 +/- 1.1 Ma. Geochemical data show that these volcanic rocks belong to the shoshonitic and metaluminous series and have high Sr and low Y contents (461-582 and 5.60-6.74 ppm, respectively), MgO = 1.32-2.65 wt%, and Na2O/K2O = 0.81-1.40. In MgO-SiO2, Cr-SiO2, Sr/Y-Y and (La/Yb)(N)-Yb-N diagrams, data for the samples plot in the adakite and "adakite related to thickened lower crust" fields. As such, we suggest that the Sunjiafen Formation C-type adakitic andesites formed in a compressional tectonic setting. The youngest age (youngest peak age) of two sandstone samples from the Linxi Formation are 251 Ma (257 Ma) and 237 Ma (254 Ma). Detrital zircons with ages of >1500 Ma have e(Hf)(t) values of -5.87 to +6.31. A comparison of the age spectra and e(Hf)(t) values of the detrital zircons in the Linxi Formation with those of igneous zircons from the Central Asian Orogenic Belt and North China Craton indicates the formation was derived from both the belt and the craton. We identified "Jura-type Folds" that formed in peripheral foreland basin in the Linxi Formation, and along with plant fossil fragments (Paracalamites frigidus). These and previously published results indicate the Paleo-Asian Ocean closed along the Xar Moron-Changchun Suture zone during the Late Permian. The study area was in a continuous compressional setting during the Late Permian and formed a peripheral foreland basin that was related to continent-continent collision.