西藏安多早侏罗世火山岩岩石成因及对班公湖-怒江特提斯洋俯冲消减的制约  

Petrogenesis of Early Jurassic volcanic rocks in the Amdo area,Tibet: Evidences for the subduction of the Bangong-Nujiang Tethys ocean

作　　者：李小波[1,2] 王保弟[1,3] 刘函[1,3] 王立全[1,3] 陈莉[1,3] 闫国川 周放[1,4] 

机构地区：[1]中国地质调查局成都地质调查中心,成都610081 [2]四川省地质矿产勘查开发局106地质队,成都611130 [3]中国地质调查局青藏高原地质研究中心,成都610081 [4]中国地质大学,北京100083

出　　处：《岩石学报》2017年第7期2073-2084,共12页Acta Petrologica Sinica

基　　金：国家重点研究发展计划项目(2015CB452601;2016YFC0600305);四川省基金项目(2014JQ0025);国家自然科学基金项目(41603038);中国地质调查局地质调查项目(DD20160016;1212011121262)联合资助

摘　　要：拉萨地块广泛分布着中生代的岩浆活动,研究它们对于认识特提斯洋的演化过程有着重要的启示。本文报道安多地区新识别出的早侏罗世岛弧型火山岩LA-ICP-MS锆石U-Pb年龄、锆石原位Hf同位素、全岩主微量元素分析和Sr-Nd同位素结果。英安岩206Pb/238U年龄为180.7±4.7Ma(MSWD=0.22,n=10),指示其形成时代为早侏罗世。这些火山岩的SiO_2含量为62.50%~76.15%,MgO含量为0.33%~3.05%,Fe_2O_3T含量为2.35%~6.69%;所有样品都显示轻稀土元素相对富集,具有铕的弱负异常,具有富集大离子亲石元素、亏损高场强元素的地球化学特征;同时这些火山岩锆石原位εHf(t)值均为正值(+15.1^+17.1);全岩Sr-Nd同位素εNd(t)值也均为正值(+7.1^+13.2),指示这些火山岩很可能是特提斯洋俯冲消减过程中新生下地壳部分熔融的产物。结合区域地质资料,认为安多火山岩不可能是龙木错-双湖古特提斯洋或雅鲁藏布江新特提斯洋俯冲消减的产物,而是班公湖-怒江特提斯洋俯冲消减的产物,区域上很可能与改则北、日土地区的侏罗纪侵入岩一起构成了班公湖-怒江特提斯洋北侧的岩浆弧带。由此本文认为安多早侏罗世岛弧型火山岩是班公湖-怒江特提斯洋北向俯冲消减的产物,从而为班公湖-怒江特提斯洋的俯冲消减过程提供了直接的火山岩证据。It can better understand the geological evolution of the Tethys through studying Mesozoic volcanic rocks widely exposed in the Lhasa Terrane. This paper report LA-ICP-MS zircon U-Pb geochronology and zircon in situ Lu-Hf isotope,whole-rock analysis of major and trace elements and Sr-Nd isotope for the volcanic rocks in the Amdo area. LA-ICP-MS zircon U-Pb analysis suggest that the volcanic rocks formed at 180. 7 ± 4. 7Ma,and reveal that they formed in the Early Jurassic. The SiO2,MgO and Fe2O3 Tof these volcanic rocks range from 62. 50% to 76. 15%,0. 33% to 3. 05%,2. 35% to 6. 69%,respectively. They are enriched LREE in the chondrite-normalized REE patterns with weak negative Eu anomalies,and are enriched in large ion lithophile elements（ LILEs） and depleted in high field strength elements（ HFSEs）. Zircon εHf（ t） and whole-rock εNd（ t） of these volcanic rocks have positive values and are from ＋ 15. 1 to ＋ 17. 1,from ＋ 7. 1 to ＋ 13. 2,respectively. These features suggest that Amdo volcanic rocks were probably produced by partial melting of new lower crust by the subduction of Tethys oceanic crust. Combining with regional geologic data,they were not originated from the subduction of Longmu Tso-Shuanghu Paleo-Tethys ocean or Yarlung Zangbo Neo-Tethys ocean,but Bangong-Nujiang Tethys ocean. Combining with the Jurassic magmatic arc in the Gerze and Rutog,they form the magmatic arc belt in the north side of Bangong-Nujiang Tethys ocean. Therefore,we suggest that the Early Jurassic island arc volcanic rocks in the Amdo area are the products of northward subduction of the Bangong-Nujiang Tethys ocean. These results provide direct evidence of the volcanic rocks for the subduction of the Bangong-Nujiang Tethys ocean.

关 键 词：火山岩 班公湖-怒江特提斯洋 锆石U-PB定年 西藏安多 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]