西藏纳木错早白垩世流纹岩锆石U-Pb年龄、地球化学及其构造意义  

Zircon U-Pb Age,Geochemical and Tectonic Significance of Early Cretaceous Namtso Rhyolite in Tibet,China

作　　者：魏永峰[1,2] 肖倩茹 李有波[1,2] 罗巍 冉杰[1] 林美英 WEI Yongfeng;XIAO Qianru;LI Youbo;LUO Wei;RAN Jie;LIN Meiying(Regional Geological Survey Party,Sichuan Bureau of Geology and Mineral Resources Development,Shuangliu,Sichuan 610213,China;College of Earth Sciences,Chengdu University of Technology,Chengdu,Sichuan 610059,China)

机构地区：[1]四川省地质矿产勘查开发局区域地质调查队,四川双流610213 [2]成都理工大学地球科学学院,四川成都610059

出　　处：《现代地质》2019年第3期487-500,共14页Geoscience

基　　金：中国地质调查局项目“西藏班公湖—怒江成矿带地质矿产调查”(1212011221083)

摘　　要：西藏纳木错流纹岩构造上处于北冈底斯南缘,地层角度不整合于中—晚侏罗世拉贡塘组之上,归属于卧荣沟组。其中流纹岩的LA-ICP-MS锆石U-Pb年龄为120~112 Ma,时代为早白垩世中期。岩石具高Si O2、富Al2O3和K2O,贫Ti O2、MgO、Ca O特征,A/CNK=0.94~1.91(平均1.16),属于高钾钙碱性-钾玄岩系列过铝质岩石。(La/Yb)N为1.49~11.08,δEu=0.12~0.55,球粒陨石标准化稀土元素分布模式显示为略右倾V字形。大离子亲石元素及高场强元素Rb、K、Th、Ta、Ce、Zr、Hf相对富集,Nb相对亏损,Ba、Sr、P、Ti强烈亏损,成岩岩浆源自地壳,可以与多尼组酸性火山岩对比。流纹岩具同碰撞型火山特征,不具传统岛弧环境特征,研究认为在早白垩世中期(120 Ma左右),北冈底斯发生了一次重大的构造事件,即班公湖—怒江新特提斯洋俯冲已经结束,冈底斯带与羌塘地块陆-陆碰撞开始,碰撞过程中下地壳增厚并部分发生熔融,纳木错流纹岩是碰撞后部分熔融的产物。The Namtso rhyolite is located in the southern margin of the North Gangdese Belt.It belongs to the Woronggou Formation and overlies the Middle-Late Jurassic Lagongtang Formation along an angular unconformity.The rhyolite is LA-ICP-MS zircon U-Pb dated to be Early Cretaceous(120-112 Ma).The rocks are characterized by high SiO2,Al2O3and K2O,and low Ti O2,MgO and CaO,A/CNK=0.94-1.91(average 1.16).The rhyolite is high-K calc-alkaline and peraluminous.The chondrite-normalized patterns show a gently right-inclining((La/Yb)N=1.49 to 11.08)"V"shape and weakly negative Eu anomalies(δEu=0.12 to 0.55).The elements of Rb,K,Th,Ta,Ce,Zr and Hf are enriched,while Nb and Ba,Sr,P and Ti are strongly depleted,suggesting a crustal inheritance.The geochemistry can be compared with the Duoni Group felsic volcanic rocks.The Namtso rhyolite does not have the characteristics of typical island arc rhyolites,but mimics those of collisional felsic volcanic rocks.We considered that during the middle Early Cretaceous(ca.120 Ma),a major tectonic event may have occurred in the North Gangdese Belt,i.e.,subduction of Neo-Tethys Ocean in the Bangong Lake and Nujiang River region had ended,and the continent-continent collision in the Gangdese and Qiangtang region had begun.During the collision,the lower crust may have thickened and partially melted,forming the Namtso rhyolite.

关 键 词：锆石U-PB年龄 早白垩世 流纹岩 卧荣沟组 纳木错 北冈底斯带 中国西藏 

分 类 号：P588.1[天文地球—岩石学]