拉萨地块北部白垩纪多尼组火山岩的地球化学：形成的构造环境  

Geochemistry of Cretaceous Volcanic Rocks of Duoni Formation in Northern Lhasa Block:Discussion of Tectonic Setting

作　　者：康志强[1,2] 许继峰[1] 王保弟[1,2] 董彦辉[1,2] 王树庆[1,2] 陈建林[1,2] 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学和地球化学实验室,广东广州510640 [2]中国科学院研究生院,北京100039

出　　处：《地球科学（中国地质大学学报）》2009年第1期89-104,共16页Earth Science-Journal of China University of Geosciences

基　　金：国家重点基础研究973项目(Nos.40721063,40425003,9014001)

摘　　要：拉萨地块上广泛分布有中生代的岩浆活动,研究它们对于认识特提斯构造域的演化和理解整个青藏高原的形成过程有着重要的启示.对出露于拉萨地块北部的白垩纪多尼组火山岩进行了系统的年代学以及元素地球化学研究,达过和马跃乡地区的多尼组火山岩主体为酸性岩,包括少量的玄武岩和玄武安山岩(中基性岩).火山岩的地球化学组成表明多尼组中基性岩具有高的Cr(平均为184×10-6)和Ni(平均为43×10-6),富集强不相容性元素Rb、Th、U,而亏损Nb、Ta、Ti的特点,显示明显的弧火山岩成分特征,可能来源于具有较厚陆壳背境下的大陆边缘地幔楔的部分熔融;而酸性岩具有相对高的SiO2、K2O、K2O/Na2O以及低的Mg#(最大值为32.4),富集大离子亲石元素Rb、Ba以及Th、U、LREE等,具有很高的Rb/Sr(1.06～4.47,平均为2.73),并且与拉萨地块南部林子宗群酸性火山岩具有非常相似的蛛网图曲线,表明多尼组酸性岩与中基性岩可能来自不同的岩浆源区,前者很可能来源于上覆地壳的部分熔融.通过对两个酸性岩样品中锆石的U-PbLA-ICPMS定年,分别获得了116±1Ma和115±1Ma的年龄值,显示多尼组火山岩形成于早白垩世中期.该地区多尼组火山岩形成的构造环境仍然存在两种可能性,其一是新特提斯班公湖-怒江洋岩石圈向南俯冲导致了该火山岩的产生;其二可能是早白垩世该地段的构造伸展作用导致加厚地壳的广泛部分熔融形成酸性火山岩,并伴随陆下地幔的熔融事件产生的少量中基性火山岩.It can better understand the geological evolution of the Tethys and construction of the whole Tibet plateau through studying Mesozoic volcanic rocks widely exposed in Lhasa block. In present study, we report new major trace of elemental compositions and zircon U-Pb dating results of the Duoni Formation volcanic rocks in north Lhasa block. Duoni Formation volcanic rocks in Daguo and Mayuexiang areas mainly contain acid rocks and a few basalts, andesitic-basalts （intermediate-basic group）. The intermediate-basic rocks with high Cr（184×10^-6 in average） and Ni （43×10^-6 in average）, showing geochemical characteristics similar to the volcanic rocks that in arcs or active continental margins, such as LILE＇s enrichment and HFSE＇s depletion, which thought to be the melting products of continental marginal mantle wedge below a thickened crust due to southward subduction of the Bangongco-Nujiang ocean. The acid rocks have relatively high SiO2, K2O, K2O/Na2O and very low Mg#（the maxim is 32.4）, enrich in Rb, Ba, Th, U and LREE, high Rb/Sr （1.06±4.47, the average is 2.73）, and have spidergram patterns similar to the acid rocks of the Linzizong Group derived from a crust source. Thus, the acid rocks in Duoni Formation were came from a magmatic source different from those of the intermediate-basic rocks. Zircons from two acid rocks in Duoni Formation were dated by LA-ICPMS, obtaining age of 116±1Ma and 115±1Ma, respectively. We suggest that there are probably two possibilities for the formation tectonic setting of Duoni Formation.. the southward subduction of the Bangongco-Nujiang ocean, or the tectonic extension in Early-Cretaceous in the Lhasa terrane.

关 键 词：拉萨地块 多尼组 早白垩世 班公湖 怒江洋 地球化学 

分 类 号：P591[天文地球—地球化学]