The Protolith Deposition Ages, Source Provenance And Tectonic Significance Of Metasedimentary Rocks From The Dunhuangblock

The Dunhuang Block is located between the Tarim and the North China cratons,which is linked to sourthernmost part of Central Asian Orogenic Belt to the north and Qilian Orogenic belt to the sourth.Therefore,the Dunhuang Block is a key area to reveal and understand the formation and evolution of the Central Asian Ogenic Belt and related Precambrian blocks.The detritus from sedimentary rocks are originated from different source provenance,which can be used to reveal tectonic-thermal events of the source provenance as well as to contrain the protolith deposition ages of meta-sedimentary rocks.In this contribution,we will focus on meta-sedimentary rocks exposed in Sanweishan,Hanxia and Hongliuxia area in Dunhuang Block.Based on a synthesis of the field investigation,lithological,detrital zircon chronology and the whole rock geochemical componsitions of the meta-sedimentary rocks from Dunhuang Block,we obtain some conclusions as follows:1)The meta-sedimentary rocks of the Dunhuang Group in the Dunhuang Block are mainly composed of terrigenous clastic and carbonate rocks,their protoliths have undergone moderate weathering.2)We obtained the youngest magmatic zircon age from three mica-quartz schist in Sanweishan area where the Dunhuang Group is firstly named is 481-386Ma;meanwhile the youngest magmatic zircon age of three mica-quartz schist in Hanxia is 394-386Ma;and one magmatic zircon with age of 386 Ma is also recognized from two-mica-quartz schist of Hanxia area.Combined with the intrusive ages of granitoid rocks,we infer that the deposition ages of meta-sedimentary rocks from the Dunhuang Group mainly have been deposited in the period of Early Ordovician to Late Devonian.3)According to the lithologies and the characteristics of whole-rock geochemical compostions in the Dunhuang Group,the tectonic setting of the protolith of meta-sedimentary rocks is interpreted as depositting in a basin of active continental margin,which is closely related to the southward subduction of the Paleozoic Hongliuhe-Niujuanzi-Xichangjing ocean intervene the Dunhuang and the Beishan blocks.4)Detrital zircon U-Pb Age-bin histograms and age-distribution curves of mtea-sedimentary rocks of the Dunhuang Group from the Dunhuang Block present a few unique peaks of Mesoproterozoic-Neoproterozoic,which obviously distinguish from thoseof Tarim,Yangtze and North China cratons,it indicate that the Dunhuang Block is not obvious affinity with those cratons at least prior to the Precambrian.