Zircon Geochronology And Hf Isotopic Compositions For The Haisen Chulu,Aergelintai Granite In Alxa,Inner Mongolia And Their Geological Significances

Alxa is an important area for studying the tectonic evolution of the Central Asian orogenic belt and its adjacent areas.The age and composition of Precambrian rocks in Alxa area are significant in disscussing the boundaries of the Alxa block.In this paper,new constraints have been provided on disscussing the north boundary of the Alxa block by studies on Haisen Chulu gneiss,Ulanbulage gneiss and Aergelintaishan granite.Minor Precambrian rocks exposed in the northern Beidashan area of Alxa,which were traditionally considered as a part of the Longshoushan Group.The age and isotopic characteristics of these Precambrian rocks are important for the understanding of the local basement characteristics and the tectonic architecture of north Alxa area.A suite of orthogneiss ourcropped in the Haisen Chulu in the north Beidashan area.LA-ICPMS zircon U-Pb dating performed on the inner part of the zircons obtained an upper intercept age of 1408±29 Ma?MSWD=13?on the concordia diagram,which indicates that the protolith for the gneiss intruded at¹.4Ga.The zircons from Haisen Chulu gneiss show positive?_Hf?t?values between+0.13and+6.68 and the two-stage Hf model ages are in the range of 2.07 to 1.62 Ga,indicating that the protoliths were mainly derived from the juvenile crust.This study,in combination with previous works,indicate that the Precambrian rocks outcropped in northern Longshoushan-Beidashan area was formed in Mesoproterozoic or later,but not as Archean or Paleoproterozoic as previously proposed.A suite of monzonitic granites which were named Alglintaishan granite,intruded into the Ulanbulage gneiss.LA-ICPMS U-Pb dating show that the age of Ulanbulage gneiss is 1.9 Ga,The Aergelintaishani Mountains granite intrusive contact occurs in Ulanbulage gneiss with an age of 0.9 Ga.These two magmatic events developed extensively in the south of Tepai-Chagan Chulu ophiolite belt.The Mesoproterozoic gneiss and Neoproterozoic granite which in the shouth of Tepai-Chagan Chulu ophiolite belt,indicates that they are developed on the same block.Combined with previous studies,the two sets of rock bodies should be the result of magmatic events in Alxa block.The Heisen Chulu gneiss,which located to the north of the Tepai-Chagan Qulu ophiolite,shows the properties similar to the Precambrian rocks from the micro continental in the Central Asian orogenic belt,while the Ulan bulag gneiss and the Alglintai granite,which located in the south of the Tepai-Chagan Qulu ophiolite,have obvious internal characteristics of the Alxa block.Therefore,the north boundary of the Alxa Block may be Tepai-Chagan Qulu ophiolite.