INTEGRATED ANALYSES CONSTRAINING THE DEPOSITIONAL AGE AND PROVENANCE OF THE MAONIUSHAN FORMATION: IMPLICATIONS FOR THE COLLAPSE OF OROGENS IN THE NORTH QAIDAM, NW CHINA

The deposition age of the Maoniushan Formation has been controversial since it was founded and it records a significant history of the conversion of the tectonic regimes in the North Qaidam. Systematical outcrop study showed that the Maoniushan Formation consists of alluvial fan facies. Vertically, the formation shows a normal grading as a whole. Paleocurrent directions in Olongbuluke and Wanggaxiu are from northwest to southeast and from southeast to northwest, suggesting two ancient slopes in the two directions. Gravel counting indicates that in Olongbuluke area, the gravels consists of the supracrustal rock, Mainly the sandstone and carbonate which are indicative of shallow erosion. In Wanggaxiu area, the composition of gravels is complex, providing evidence for greater erosion. LA-ICP-MS U-Pb isotopic dating was used for detrital zircons from sandy fillings in gravels of the Maoniushan Formation, the results reveals that the the youngest detrital zircon U-Pb age is 365 ± 3 Ma. Combined with the paleontological evidence, the deposition age of the Maoniushan Formation was in the late Devonian. In Chengqianggou, Olongbuluke area, results yield three U-Pb age populations: late Neoproterozoic-late Devonian(365-627 Ma) with peak age at 381.5 Ma, late Mesoproterozoic-early Neoproterozoic(892-1035 Ma) with peak age at 925 Ma and 995 Ma and Paleoproterozoic(2353-2513 Ma) with peak age at 2479 Ma. In Wanggaxiu, the age of detrital zircons Mainly distributed between 1811~2531 Ma with Many peaks, a dominant peak at 2349 Ma. Combined with the paleocurrent directions and the composition of gravels, we suggest that in Chengqianggou profile, the Paleozoic source is the combination of the third and fourth phase granite intrusion in the North Qaidam. Also, substance in Shaliuhe Member in Neoproterozoic, the Delingha complex and the Dakendaban Member in Paleoproterozoic are also the source of the Maoniushan Formation. in Wanggaxiu profile, the main source derived from the mixture of Delingha complex and the Dakendaban member in Paleoproterozoic. Thus, the source and basement rocks must have experienced rapid uplift and exhumation during the late Devonian. The North Qaidam orogens had collapsed during the deposition of the Maoniushan Formation.