西藏林周盆地设兴组玄武岩地球化学特征及意义  

The geochemical characteristics and significance of the basalt from Shexing Formation in Linzhou basin,southern Tibet

作　　者：李晓雄[1,2] 江万[3] 梁锦海 赵志丹[2] 刘栋[2] 莫宣学[2] 

机构地区：[1]中国建筑材料工业地质勘查中心陕西总队,西安710003 [2]地质过程与矿产资源国家重点实验室,中国地质大学地球科学与资源学院,北京100083 [3]中国地质科学院地质力学研究所,北京100081 [4]黑龙江三道湾子金矿有限公司,黑河164300

出　　处：《岩石学报》2015年第5期1285-1297,共13页Acta Petrologica Sinica

基　　金：国家973项目(2011CB403102;2015CB452604);国家危机矿山办公室项目(200809951);中国地质调查局项目(1212011220937;1212011121260;1212011121066);国家自然科学基金项目(41273044;41225006)联合资助

摘　　要：青藏高原南部拉萨地块分布的晚白垩世岩浆活动,对了解特提斯洋的构造演化及约束印度-亚洲大陆初始碰撞时限等具有重要的意义。本文在西藏林周盆地林子宗火山岩的下伏晚白垩世设兴组地层中首次发现了玄武岩夹层,并系统开展了玄武岩的Ar-Ar年代学、全岩主量和微量元素地球化学、以及Sr-Nd-Pb同位素地球化学研究。设兴组玄武岩呈夹层状产出于设兴组顶部的红色砂岩中,其斜长石Ar-Ar年龄为90.6±1.8Ma。玄武岩属于高钾钙碱性系列,富集轻稀土元素、强烈富集Ba、Th、U等大离子亲石元素(LILEs),亏损Nb、Ta、Ti等高场强元素(HFSEs),具有明显的大陆边缘弧的火山岩成分特征。玄武岩的Sr-Nd-Pb同位素成分比较均匀,(87Sr/86Sr)t为0.706568~0.706620,εNd(t)为+0.72^+4.75,(206Pb/204Pb)t比值为18.649~18.675,(207Pb/204Pb)t比值为15.640~15.666,(208Pb/204Pb)t比值为39.055~39.108,岩石总体表现出来自于亏损地幔的特征。综合岩石学和地球化学研究结果,设兴组玄武岩可能是晚白垩世(约90.6Ma)俯冲于拉萨地块之下的新特提斯洋壳在重力拖拉下发生板片回转,导致软流圈物质上升提供热量,从而诱发了俯冲流体交代的地幔楔的部分熔融形成的。本文结果进一步证明,之前普遍认为缺少晚白垩世火山岩的拉萨地块南缘,存在新特提斯洋的俯冲作用及其岩浆作用产物。The Late Cretaceous magmatism,which is distributed in the Lhasa terrane in southern Tibet,is of great significance to understand the tectonic evolution of the Neo-Tethys Ocean and to constrain the initial continental collision between India and Asia. This paper presents the first discovery,and systematical studies of Ar-Ar dating,whole-rock major and trace elements,and Sr-Nd-Pb isotope data of the basalt from Shexing Formation which is underlying uncomfortably in the bottom of Linzizong volcanic rocks in Linzhou basin,southern Tibet. The Shexing basalts are interbedded within the sandstone in the upper Late Cretaceous Shexing Formation. The Ar-Ar dating of plagioclase of the basalts yield the age of 90. 6 ± 1. 8Ma. The basalts are high-K calc-alkaline,with enriched light rare earth elements,large ion lithospheric elements（ LILEs,such as Ba,Th,U,etc.） and depleted high field stretch elements（ HFSEs,such as Nb,Ta,Ti,etc.）,which is similar to those of island arc volcanic rocks. The Sr-Nd-Pb isotopes of basalts display narrow range of variations,with（87Sr /86Sr）tranging from 0. 706568 to 0. 706620,εNd（ t） from ＋ 0. 72 to ＋ 4. 75,（206Pb /204Pb）tfrom18. 649 to 18. 675,（207Pb /204Pb）tfrom 15. 640 to 15. 666,and（208Pb /204Pb）tin the range of 39. 055 ～ 39. 108. These isotopic data show that the rocks are from a depleted upper mantle. These petrological,elemental and isotopic signatures indicate that the basalts are from continental marginal arc tectonic setting. The Shexing basalt may be caused by the partial melting of the fluids modified upper mantle wedge,during the subduction of Neo-Tethys oceanic lithosphere,and was induced by slab roll-back since ca. 90. 6Ma. This study further supports that the Late Cretaceous volcanic magmatism（ ～ 90Ma） was existing in southern Lhasa terrane,instead of rarely found in previous studies.

关 键 词：林周盆地 设兴组 玄武岩 年代学和地球化学 特提斯洋俯冲 拉萨地体 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学]