Keywords: 
• Alxa /
• Siberian Plate /
• Yuanbaoshan Formation /
• sedimentary sequence /
• geochemistry /
• tectonic

Abstract: Through the peak age of detrital zircons(420±3 Ma and 419.6±3.6 Ma) and the fossils of graptolites and cephalopods in the Yuanbaoshan Formation in Hangwula area, the north of Alxa, the conclusion is that the age of Yuanbaoshan Formation is Devonian, not Early Silurian. The information of lithology, sediment structures of Yuanbaoshan Formation shows that the Yuanbaoshan Formation is bathyal- pelagic deposit, but the Bouma sequence is not complete, including the ae, abe, ade, abde and other combinations, and the Formation is divided into three sedimentary subfacies which are inner fan, middle fan and outer fan. The study area, located in the north of Alxa region which belongs to the southern margin of the Siberian plate, is the key part of the study to the closing time of Paleo-Asian Ocean. The characteristics of Early Paleozoic cherts indicate that the tectonic setting of the study area is a stable passive continental margin in the early Paleozoic. Since Late Paleozoic, the tectonic activity in the study area began to increase, and gradually the tectonic setting transformed from a passive continental margin to an active continental margin, to form a thick turbidite. Source area is the Siberian plate in the north of the study area, according to the flute cast and gloove cast in the turbidite. And according to the study of lithology, sedimentary characteristics and geochemical characteristics of the turbidite, the tubidite in the study area formed in the continental margin in the southern margin of the Siberian plate, and also is affected by the island arc, resulting in the turbidite with island arc attributes. It should be noted that the peak age of detrital zircons of the turbidite is in accord with the age of island arc diorite in the Sunite Left Banner, therefore, the subduction from Tarim plate(or the oceanic crust of Paleo-Asian Ocean) to the Siberia plate began in 420 Ma.