Sedimentary provenance and age of the Langshan Group in the northeastern Alxa Block: implications for Neoproterozoic tectonic evolution

Abstract

The origin and the Precambrian tectonic evolution of the Alxa Block have not been well studied, largely due to widespread desert coverage and a lack of high-resolution geochronological data. This paper reports zircon U–Pb ages and Hf isotopic data for the Langshan Group in the northeastern Alxa Block. Detrital zircons from the Langshan Group mainly provide U–Pb ages between ca. 2000 Ma and 1100 Ma, suggesting that their provenance was dominated by late Palaeoproterozoic to Mesoproterozoic rocks, which yield two main peak ages at 1592 Ma and 1745 Ma. These zircons possess eHf (t) values between − 15 and + 8, implying the derivation of their precursor magmas from old crust and juvenile materials. Zircon U–Pb dating yields ages of 898 ± 4 Ma and 808 ± 4 Ma for the granite cobbles in coarse conglomerate in the Fourth Formation and the diorite intruding into the Second Formation of the Langshan Group, which constrains the depositional age of the main part of the Langshan Group to between 898 and 804 Ma. Our data show that the Langshan Group was probably deposited in an intra-continental rift basin and originated from recycled orogens. The geochronology and palaeocurrent data indicate that the North China Craton (NCC) could not have provided source materials for the Langshan Group. Instead, the Langshan Group shares a similar origin and detrital zircon age spectrum with the Beishan Complex in the South Beishan Collage. Combining these findings with published data, we propose that the Alxa Block was not a part of the NCC during the Neoproterozoic but a Precambrian micro-continent block. The Alxa Block recorded footprints that are related to the late amalgamation of the Rodinia supercontinent and the initial breakup. We suggest that the Alxa Block laid between Australia and Laurentia in the Rodinia supercontinent during the Neoproterozoic.