Geological Significance Of The Longshoushan Group In The Alxa Block : Constraint From Detrital Zircon Geochronology

The Alxa Block is located to the south margin of the Central Asian Orogenic Belt,which extends from the Beishan orogen and Tarim Craton in the west to the North China Craton in the east.Most scholars consider it as westward extension of the North China Craton.Recently,scholars have carried out U-Pb dating and Lu-Hf isotopic studies for detrital zircons from Ordovician and Devonian strata in the Alxa Block.The results suggest that the Alxa Block exhibited some tectonic-thermal events which were absent in the North China Craton.Thus,there is a controversy about the tectonic affinity of the Alxa Block.In this contribution,we have carried out studies on the Longshoushan Group of the western Alxa Block.Through our field observation,some gneiss samples have been collected for further geochemical analyzing.A total of 215 analyses of 200 grains were obtained for this sample,which produced most concordant ages dominated by Paleoproterozoic to Archean ages.Two age populations can be identified: 21~19 and 25~23Ga Ga.Archean to Paleo-proterozoic strata exposed in eastern Alax Block has been dated by zircon U-Pb dating methods.The results suggest the formation age of the Neoarchean Diebusige group at ca.2.7 Ga,Paleoproterozoic Bayanwulashan group at ca.2.5~2.26 Ga,Alax group at ca.1.37 Ga and 1.62 Ga.Our new data and previous studies implied that Archean to Paleoproterozoic detrital zircons are widely distributed in the eastern Alax block.In this paper,detrital zircons age distribution of Precambrian strata from other adjacent blocks are also exhibited for comparison,including Changchengxi strata in the North China Craton,Helanshan sandstone and Zhuozishan group in the Erdos Block,Qilian,Yangtze and Tarim Blocks.The age distributions are consistent with those of the Erdos Block of the North China Craton.Thus,we consider that the Alxa Block was connected with the North China Craton in the Paleoproterozoic.