Cambrian-Early Ordovician intrusive bodies in Dunhuang Block: Records of Paleo-Asian Ocean subduction and implications for regional Early Paleozoic tectonic evolution

The lack of 1.6 similar to 0.46Ga geological records in the Dunhuang Block badly restricted the comprehensive understanding of its Early Paleozoic tectonic evolution. In 1/50, 000 regional geological survey, three Cambrian-Early Ordovician intermediate-acid intrusive bodies, namely the Xiaowanshan, the Shuishanzi and the Xiaowannan plutons, were discovered in the northeastern margin of the Dunhuang Block. The LA-ICP-MS zircon U-Pb ages of these plutons are 517 +/- 3Ma, 480 +/- 3Ma and 473 +/- 3Ma, respectively, and thus determined an earliest Early Paleozoic magmatism and Early Ordovician magmatism in the Dunhuang Block. Based on the analysis of petrography, petrogeochemical characteristics, petrogenesis and tectonic setting, these intrusions are sodium-rich I-type granites formed in the active continental margin environment related to oceanic crust subduction. Among them, the Xiaowanshan Pluton was formed by partial melting of a basaltic mantle wedge under low pressure and low temperature with a strong metasomatization by subduction fluids; while the Jieshanzi and Xiaowannan plutons were formed by partial melting of a new mafic oceanic crust under a condition of high pressure and low temperature together with rich water, and subsequently strongly contaminated by a mantle wedge. Although both of the later two plutons belong to a type of classic O-type adakite, their formation pressure and residual minerals during the partial melting were different. Combined with the regional geological information, the Early Paleozoic ocean-continent transition on the northern margin of Dunhuang Block can be divided into three stages: (1) the Late Sinian-Early Cambrian (574 similar to 518Ma): the passive continental margin stage of the Dunhuang Block; (2) the Cambrian II-Early Ordovician (517 similar to 471Ma): the active continental margin satge of the northern margin of the Dunhuang Block, when the ancient Asian Ocean subducted to the Dunhuang Block and the Shibanshan/Mazongshan blocks in two directions; (3) the Middle Ordovician-Early Devonian (464 similar to 412Ma): a satge of collisional orogen between the Dunhuang Block and the Shibanshan/Mazongshan blocks, when the southern branch of the Paleo-Asian Ocean closed.