Formation And Evolution Of The Dunhuang Block

The discovery and research of the Precambrian rocks is the vital important for understanding the formation and evolution of the early continental crust.The Dunhuang Block is located between the Tarim Craton(TC)and the North China Craton(NCC),and its surface is overwhelmingly covered by Quatenary sediments.The limited exposed rocks in the Dunhuang Block are mainly composed of the Precambrian metamorphic complex rocks and the Paleozoic rocks that related with the Central Asian Orogenic Belt(CAOB).Because it is situated at the junction area of North China and Tarim craton,thus its formation and evolution are hot issues in the field of geosciences.In this contribution,we carried out an intergrated study of zircon U-Pb dating,Lu-Hf isotope in zircons as well as whole rock major and trace elements on the Dunhuang complex in Dongbatushan area to discuss the formation and evolution of the Dunhuang Block.The main points of the results are summaried as fellows:(1)According to the geological survey,petrography and geochronology of the Dunhuang Complex,the protolith age of the Archean TTG gneiss in Duongbatushan area emplaced at 2612-2546Ma.More importantly,they recorded both early Paleoproterozoic(2461-2455Ma)and Paleozoic(?350Ma)metamorphic age.The protolith of Proterozoic granite gneiss emplaced at 2003Ma,metabasic rock was formed at 1991Ma.The protolith of Paleozoic amphibolites and granitic rocks were formed at the period of 413-362Ma.(2)The characteristics of geochemical studies indicate that the Archean granitic gneisses in Dongbatushan are brlong to typical Archean TTG rocks;they share similiar characteristics to Phanerozoic granite formed in island arc setting.The Paleoproterozoic granites present nature of Phanerozoic adakitic rock,which are likely formed in a convergence setting.The Paleozoic rocks have similar geochemical characteristics with those in island arcs setting,suggesting that they formed in the subduction-collision environment.(3)The Hf isotopic compositons in zircon suggest that the-2.7-2.5Ga tectonothernal events in the Dunhuang Block were manily involved in the reworking of the Paleoarchean rocks and growth of the new crust.Magmatic events of the?2.0-1.8Ga is mainly partial metlting of Archean rocks;while the 440-362 Ma tectonothernal events are likely related to the COAB,these rocks were produced by variable mixing of a juvenile materials with older crust.(4)Combined with available data,it is considered that the Dunhuang Block has formed to be a unified crystalline basement during the early Precambrian(prior to 1.78Ga).Due to it is located in margin area of a certain craton;alternatively,it is an independent small block,the block was strongly influenced and modified by the subduction and collision of the Paleozoic CAOB,and became part of it.But the Dunhuang Block is should't considered as an independent orogenic belt.