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甘肃北山西涧泉子富碱高钾花岗岩体的锆石LA-ICP-MS定年及其构造意义
LA-ICP-MS zircon U-Pb ages of Xijianquanzi alkali-rich potassium-high granites in Beishan, Gansu Province, and their tectonic significance
  
中文关键词:北山  富碱高钾花岗岩  裂谷作用  锆石U-Pb年龄  Hf同位素
英文关键词:Beishan  alkali-rich potassium-high granites  rifting  zircon U-Pb age  Hf isotope
基金项目:中国地调局IGMA5000资助项目;国家自然科学基金资助项目(40672146)
作者单位E-mail
张 文 造山带和地壳演化教育部重点实验室 北京大学 地球与空间科学学院 北京 100871 zhangwen08@pku.edu.cn 
吴泰然 造山带和地壳演化教育部重点实验室 北京大学 地球与空间科学学院 北京 100871 twu@pku.edu.cn 
贺元凯 造山带和地壳演化教育部重点实验室 北京大学 地球与空间科学学院 北京 100871  
冯继承 造山带和地壳演化教育部重点实验室 北京大学 地球与空间科学学院 北京 100871  
郑荣国 造山带和地壳演化教育部重点实验室 北京大学 地球与空间科学学院 北京 100871  
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中文摘要:
      位于甘肃北山南带的西涧泉子花岗岩体锆石的LA-ICP-MS年龄为266.1±2.2 Ma、其Hf同位素的模式年龄为746~871 Ma,εHf(t)=+1.3~+4.7,平均+2.7。西涧泉子花岗岩体以二长花岗岩为主,具有较高的SiO2含量(74.33%~76.67%)和Na2O+K2O含量(9.26%~9.57%),NK/A=0.9~1.08,富钾(Na2O/K2O<1),属碱性系列;球粒陨石标准化分配模式图整体呈V字型,具相对平缓的富集轻稀土元素稀土配分模式,重稀土元素分馏不明显且相对亏损,Eu具有明显的负异常,δEu =0.10~0.16。在微量元素原始地幔标准化蛛网图上,亏损Nb、Ba、Sr、Ti,富集Rb、Th、K。结合区域地质背景,认为西涧泉子富碱高钾花岗岩体受音凹峡二叠纪裂谷带的影响强烈,为壳幔混合成因,是后碰撞环境裂谷作用阶段的产物,进而确定该地区在中二叠世进入裂谷作用阶段。
英文摘要:
      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 averageof +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/K2O<1) . 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.
张 文,吴泰然,贺元凯,冯继承,郑荣国,2010,甘肃北山西涧泉子富碱高钾花岗岩体的锆石LA-ICP-MS定年及其构造意义[J].岩石矿物学杂志,29(6):719~731.
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