西藏阿索地区目思旦角凝灰熔岩形成时代、岩浆成因及构造背景  

Geochronology,magmatic genesis and tectonic setting of the Musidanjiao tufflava in the Asa area,Tibet.

作　　者：李航 王明[1,2] 曾孝文 曾先进[1,2] 申迪 LI Hang;WANG Ming;ZENG Xiaowen;ZENG Xianjin;SHEN Di(College of Earth Science,Jilin University,Changchun 130061,Jilin,China;Key Laboratory of Mineral Resources Evaluation in Northeast Asia,Ministry of Natural Resources,Jilin University,Changchun 130061,Jilin,China)

机构地区：[1]吉林大学地球科学学院,吉林长春130061 [2]自然资源部东北亚矿产资源评价重点实验室,吉林长春130061

出　　处：《地质通报》2021年第8期1302-1313,共12页Geological Bulletin of China

基　　金：中国博士后科学基金第九批特别资助项目《藏北羌塘南部晚古生代基性岩墙群的成因及构造意义研究》(编号:2016T90248);青年科学基金项目《青藏高原羌塘南部埃迪卡拉纪地层研究》(批准号:41602230);中国地质调查局项目《西藏尼玛县尼则等三幅区域地质调查》(班公湖-怒江成矿带铜多金属矿产资源基地调查)(编号:DD20160026);自然资源部东北亚矿产资源评价重点实验室自主基金(编号:DBY-ZZ-18-11)。

摘　　要：为了进一步探讨西藏中部阿索地区的白垩纪岩浆活动演化历史,于阿索南部的目思旦角附近采集了一期早白垩世流纹质岩屑晶屑凝灰熔岩样品,对其进行了年代学及全岩地球化学测试。测得目思旦角凝灰熔岩中锆石206 Pb/238 U年龄加权平均值为129.3±1.9 Ma(n=17,MSWD=0.039)。地球化学测试结果表明,凝灰熔岩具有极高的SiO 2(76.27%~78.70%)含量,中等的Al 2 O 3(11.23%~12.21%)及全碱(5.60%~6.30%)含量,很低的全铁(1.16~1.50%)、CaO(0.62%~1.40%)、MgO(0.19%~0.24%)、TiO 2(0.09%~0.15%)和P 2 O 5(0.02%~0.02%)含量;且它们表现出富集U、Th、Pb等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,并具有富集的轻稀土元素,平缓的重稀土元素,以及明显的负Eu异常。这些特征与陆缘弧地区发育的典型I型花岗质岩石的地球化学属性相似,指示它们的源区可能为部分熔融的中地壳,并在后期经历了显著的结晶分异作用。结合阿索地区已报道的120 Ma左右的陆缘岛弧岩浆活动,以及105 Ma左右的伸展型岩浆活动,认为阿索地区130 Ma左右的早白垩世流纹质岩屑晶屑凝灰熔岩可能形成于班公湖-怒江洋南向俯冲的构造背景。In order to further explore the evolution history of Cretaceous magmatism in the Asa area,central Tibet,samples were collected from Early Cretaceous rhyolitic tufflava near Musidanjiao,south of the Asa area,to carry out the geochronological and geochemical studies.The mean zircon 206 Pb/238 U age of Musidanjiao tufflava yields 129.3±1.9 Ma(n=17,mswd=0.039).The geochemical test results show that the tufflava has extremely high SiO 2 content(76.27%~78.70%),moderate Al 2 O 3(11.23~12.21%),total alkali content of 5.60%~6.30%,very low total iron(1.16%~1.50%),CaO(0.62%~1.40%),MgO(0.19%~0.24%),TiO 2(0.09%~0.15%),and P 2 O 5(0.02%~0.02%)contents.Moreover,they show enrichment of large ion lithophile elements such as U,Th and Pb,and depletion of high field strength elements such as Nb,Ta and Ti,as well as enrichment of LREE,flat HREE and obvious negative Eu anomaly.These characteristics indicate that they might be originated from the partial melting of middle crust and experienced significant crystallization differentiation in the later period,which is geochemically similar to the typical I-type granitoids developed in the continental margin arc region.In combination with the reported continental margin island arc magmatism of~120 Ma and the extensional magmatism of~105 Ma in the Asa area,it is suggested that the Early Cretaceous rhyolitic taffflava magmatism of~130 Ma in the Asa area might be formed in the background of the southward subduction of Bangong-Nujiang Ocean.

关 键 词：西藏阿索 凝灰熔岩 班公湖-怒江洋 LA-ICP-MS锆石U-PB定年 

分 类 号：P534.53[天文地球—古生物学与地层学] P597[天文地球—地质学]