Magmatism and tectonic evolution at an active continental margin:A case study of the Dunhuang Block
Continental margin arcs are the products of magmatic activity associated with subduction at convergent plate boundaries,which generally record many essential geological processes,i.e.,arc magmatism,orogenic process,and the formation and evolution of continental crust.As the southernmost tectonic unit in the middle part of the Central Asian Orogenic Belt(CAOB),the Dunhuang Block has traditionally been defined as a microcontinent/terrane with the Early Precambrian metamorphic crystalline basement.It subsequently underwent multiple phases of tectonothermal events related to the CAOB and was reactivated in the Paleozoic,resulting in a series of extensive arc magmatism and metamorphism events.However,the mechanism of Paleozoic arc magmatism and the tectonic evolution history of the block are not systematically studied.Based on the most recent research progress of the Dunhuang Block in the past decade,we comprehensively complied the type of rock association,chronology framework,and geochemical composition of Paleozoic igneous rocks,as well as the characteristics of contemporaneous metamorphism and sedimentary tectonthermal events,culminating in following findings.(1)The Paleozoic magmatic processes in Dunhuang Block show multi-stages intrusion characteristic,i.e.,episodic patterns.The Paleozoic magmatism gradually migrated from the northeast to the south and then back to the middle Dunhuang area,which can be approximately divided into five episodes:～510Ma,440～410Ma,390～360Ma,～330Ma and 280～245Ma.Whereas the metamorphism events are mainly concentrated at 450～360Ma.(2)The types of Paleozoic igneous rocks mainly include calc-alkaline 1-type granites,adakitic rocks,a small amount of S-type and high-potassium granitoids.The compositions of Paleozoic granitoids have gradually transformed from the low-potassium tholeiite in the Cambrian to the high-potassium and rich-silicon in the Permian.(3)Isotopic geochemical characteristics indicate that the development of subduction-related arc magmatism during the Cambrian-Devonian in the central and north part of the Dunhuang area significantly contributed to juvenile crust growth,accompanied by ancient crustal reworking events.In contrast,magmatism events in the southern part during the Devonian-Carboniferous period were primarily dominated by remelting of ancient crustal material.(4)Based on evidence from adakitic rocks,the Dunhuang Block experienced two significant crustal thickening events in the whole Paleozoic.The early stage may be caused by the collision between the Dunhuang and Shibanshan terrane/arc in the southern Beishan and mantle-derived magma underplating.While the later stage may have been related to the slab rollback or retreat.The thickness of the crust for two thickening events reached～55km.(5)The Dunhuang Block is an ancient microcontinent with the Precambrian crystalline basement in the southern CAOB,which was strongly remodified and destroyed by the orogenic events related to the subduction southward of the Paleo-Asian Ocean.As an active continental margin arc in the southern Paleo-Asian Ocean,the Dunhuang Block was modified and reactivated by the geological processes such as subduction-related magmatism,partial melting,crustal thickening and regional metamorphism,and that generated widely continental arc magmatism characterized by shows episode features.In conclusion,these insights provide new constraint for exploring the origin of the microcontinents within the CAOB and the tectonic evolution of orogen.
Continental margin arcEpisodic magmatismReactivationCrustal thickeningDunhuang BlockCentral Asian Orogenic Belt
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