高喜马拉雅晚新生代富铁火山岩的发现及其构造意义  

Discovery of Cenozoic magnetite volcanic rock in High Himalaya and its tectonic significance

作　　者：刘小汉[1] 仝来喜 史仁灯[1] 李广伟[3] 陈生生[1] 周学君[1] 刘兆[4] LIU XiaoHan;TONG LaiXi;SHI RenDeng;LI GuangWei;CHEN ShengSheng;ZHOU XueJun;LIU Zhao(Key Laboratory of Continental Collision and Tibetan Uplift,lnstitate of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi＇ an 710069,China;School of Earth Sciences,University of Melbourne,Melbourne 3010,Australia;State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China)

机构地区：[1]中国科学院青藏高原研究所大陆碰撞与高原隆升重点实验室,北京100101 [2]西北大学地质学系大陆动力学国家重点实验室,西安710069 [3]墨尔本大学地球科学学院 [4]中国科学院广州地球化学研究所同位素地球化学国家重点实验室,广州510640

出　　处：《岩石学报》2018年第7期1876-1884,共9页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41272216)资助

摘　　要：本文首次报道了在藏南高喜马拉雅变质带新发现的一种特殊的晚新生代超基性富铁火山岩,主要由铁橄榄石、氧化铁、及富钾玻璃基质组成,铁橄榄石中含有少量自形铁尖晶石,玻璃基质中含有少量铁石榴石雏晶。岩石具玻基斑状结构,气孔构造发育,具有典型鬣刺构造。全岩成分具有强烈硅不饱和(全岩SiO_2含量为18.8%~29.7%)和极端富铁(全岩Fe2OT3含量为56.2%~74.2%)的特征。地球化学分析表明其大离子亲石元素Th和U等强烈富集,高场强元素Nb和Ta及Ti元素相对亏损,Sr元素具有明显的负异常,显示与板块消减俯冲有关的地球化学特征。该火山岩切割区域片麻理,说明其形成于碰撞后的陆内伸展环境,而其富铁和硅极不饱和的特征表示其有可能属于不混溶作用的产物,也可能属于后期热液蚀变作用,以及由富铁原岩在深部发生熔融并在伸展环境沿裂隙喷发而形成。该火山岩的K/Ar年龄介于4.76~7.25Ma,被熔浆包裹的围岩的磷灰石裂变径迹(AFT)分析结果是2.04±0.21Ma,指示该火山岩可能是上新世-更新世喷发的(2～4Ma)。这些超基性富铁火山岩首次提供了高喜马拉雅构造带在新生代经历碰撞后发生伸展作用的火山岩证据,为认识青藏高原南部大地构造格局及其形成演化过程提供了新的依据。Late Cenozoic ultra-mafic magnetite-rich volcanic rocks in the High Himalaya metamorphic belt,southern Tibet,are firstly reported in this study. The rocks are mainly composed of Fe-olivine,Fe oxides,Fe-garnet and potassic-rich glasses. A few euhedral Fe-spinel inclusions are discovered in the Fe-olivine grains. Vesicular structure,vitro basic porphyritic texture and typical spinifex texture are normally observed in the rocks. The bulk rock composition of the rock has a feature of strong Si-unsaturation（ 18. 8% - 29. 7% SiO2） and extreme Fe-richness（ 56. 2% - 74. 2% Fe2 O3 T）. Geochemical analyses show that it is strong rich in LILE of Th and U,relative depletion in high field strong elements such as Nb,Ta and Ti,and obvious negative anomalies for Sr element,indicating a geochemical process related to subduction. The volcanic lava cuts across the regional gneissic foliation,implying that it was probably formed in a post-collisional intracontinental extensional environment. Further,the Fe-rich and Si-unsaturated feature also indicates that it might be formed as a result of eruption along extentional fault and melting of Fe-rich protolith from a deep source. The volcano probably erupted during Pliocene-Pleistocene（ 2 - 4 Ma）,based on its K/Ar ages between 4. 76 - 7. 25 Ma,and surrounding rocks wrapped by lava with an apatite fission track（ AFT） age of 2. 04 ± 0. 21 Ma. Generally,the volcanic rock provides first volcanic evidence for post-collisional extension in the High Himalaya tectonic belt during Late Cenozoic,which is critical to further understand the tectonic framework of the southern Tibet Plateau and its evolutionary processes.

关 键 词：高喜马拉雅 晚新生代 超基性富铁火山岩 大地构造意义 

分 类 号：P588.146[天文地球—岩石学] P597.3[天文地球—地质学]