LA-ICP-MS zircon U-Pb ages of Xijianquanzi alkali-rich potassium-high granites in Beishan,Gansu Province,and their tectonic significance
ZHANG Wen,WU Tai-ran,HE Yuan-kai,FENG Ji-cheng and ZHENG Rong-guo(Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education; School of Earth and Space Sciences,Peking University,Beijing 100871,China)
Beishan area is located in the west of Inner Mongolia and Gansu Province,which is the conjunction zone of Tarim plate,Sino-Korea plate and Kazakhstan plate and plays an important role in the study of the Central Asia Orogenic Belt (CAOB).The relationship of the three plates is so complicated that there exists much controversy concerning Paleozoic tectonic evolution in this area.Granitoids,especially Late Paleozoic granitoids,are widely distributed in Beishan area,which are of great importance in the study of Paleozoic magmatic evolution.Through analyzing the spatial and temporal distribution of the Late Paleozoic granitoids,we can find the relationships between these granitoids and explore the crust-mantle interaction in Beishan area during Late Paleozoic,which is important for determining the geotectonic evolution of this area.In the southern belt of Beishan,the most important tectonic event in Permian is the formation of Yin'aoxia rift zone,which makes the study of Late Paleozoic evolution more complex.The results of studying Xijianquanzi granite lying in the southern belt of Beishan in such aspects as major elements,trace elements,REE and LA-ICP-MS zircon U-Pb geochronology have provided new evidence for regional tectonics.The LA-ICP-MS zircon U-Pb age of Xijianquanzi granite is 266.1±2.2 Ma,and the Hf modal ages are 746～871 Ma.εHf(t) values are +1.3～+4.7 with an average of +2.7,which implies the mixture of crustal and mantle-derived magmas.This feature coincides with that of other areas in the Central Asia Orogenic Belt (CAOB).Therefore,the granites in the belt may have similar magma sources.The granitic body mainly consists of monzonitic granites which belong to alkaline series with high content of SiO2 (74.33%～76.67%),Na2O+K2O (9.26%～9.57%,NK/A=0.9～1.08) and K2O (Na2O/K2O1).In addition,chondrite-normalized REE patterns and primitive mantle normalized geochemical patterns of all samples are approximately parallel to each other,which shows that they evolved from the same magma source.Chondrite-normalized REE patterns of the intrusive body are of "V" types and show enrichment of light rare earth elements (LREE) with low ratio of (La/Yb)N (3.18～5.29) and negative Eu anomalies (δEu =0.10～0.16).Except for Eu,the abundance of rare earth elements are demonstrably over 10 times that of chondrite.Depletion of Nb,Ba,Sr,Ti,P and enrichment of Rb,Th,K can be seen in the spidergram.Based on regional geological and geochemical characteristics,the authors have reached the conclusion that the Xijianquanzi intrusive body is an alkali-rich potassium-high granite which is a mixed product of crustal and mantle-derived magmas and was formed at the stage of rifting in the post-collisional extensional period,affected greatly by Yin'aoxia rift zone.This implies that the area entered into the stage of rifting in Middle Permian.
【Fund】： 中国地调局IGMA5000资助项目;; 国家自然科学基金资助项目(40672146)
【CateGory Index】： P588.121
【CateGory Index】： P588.121