青藏高原拉萨地块新生代超钾质岩与南北向地堑成因关系  

The relationship between the NS-trending grabens and the ultrapotassic volcanic rocks in Lhasa block,Qinghai-Tibetan Plateau

作　　者：陈建林[1] 许继峰[1] 王保弟[1,2] 康志强[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：《岩石矿物学杂志》2010年第4期341-354,共14页Acta Petrologica et Mineralogica

基　　金：973资助项目(2009CB421004,2002CB412602);国家自然科学基金资助项目(40872055,40930316);中科院知识创新工程重要方向性资助项目(KZCX2-YW-Q04-02)

摘　　要：青藏高原拉萨地块广泛分布有新生代超钾质岩，岩石地球化学和Sr-Nd—Pb同位素特征表明这些超钾质岩来源于与古俯冲环境有着密切联系的含金云母的富集地幔源区，它们主要喷发于25～10Ma。同时在拉萨地块分布有多条南北向地堑（裂谷），且它们的切割深度可能到达下地壳的深部甚至岩石圈地幔，它们主要形成于23～8Ma。拉萨地块大多数超钾质岩沿着新生代的南北向地堑（裂谷）分布，并且它们在形成时代和空间分布上存在着明显的耦合性，结合沿着印度-雅鲁藏布江缝合带分布的中新世埃达克质岩，笔者认为这些超钾质岩很可能与中新世早期北向俯冲的印度岩石圈沿着印度-雅鲁藏布江缝合带附近发生断离，以及由此而引起拉萨地块东西向伸展构造活动产生的南北向地堑（裂谷）系统有关。Cenozoic ultrapotassic volcanic rocks which are widely distributed in Lhasa block of southern Qinghai- Tibetan Plateau were mainly erupted between 25 Ma and 10 Ma. These ultrapotassic volcanic rocks have typical lamproite characteristics such as high SiO2, MgO, K2O and TiO2, low Al2O3 and Na2O, and high K2O/Na2O ratios. Their high abundances of incompatible trace elements like large ion lithophile elements （LILE） and light rare earth elements （LREE） also reach the extreme levels typical of lamproites. The lamproites show high radiogenic ^87Sr/^86Sr and low unradiogenic ^143Nd/^144Nd, low ^206pb/^204pb, and elevated radiogenic ^207pb/^204pb and ^208pb/^204Pb ratios. Geochemical and Sr-Nd-Pb isotopic characteristics of these ultrapotassic volcanic rocks in Lhasa block suggest that they were probably derived from a phlogopite-bcaring enriched mantle source related closely to metasomatism during earlier subduction events. The ultrapotassic rocks in southern Tibet may imply the uplift and extension of Tibetan plateau after the collision of Indian plate and Eurasian plate. Meanwhile, there are a number of N-S grabens （rifts） mostly formed in 23-8 Ma in Lhasa block, which might have cut the lower crust or lithosphere into the depth, suggesting the extension of southern Tibet in Miocene. Furthermore, these ultrapotassic volcanic rocks are coincident with N-S grabens （rifts） in space and time. In addition, the tectonic activities in western Lhasa block （such as Karakorum fault） in 17 Ma and in eastern Lhasa block （such as J iali fault and Red River-Ailao Mountain fault zone） in 23- 17 Ma imply that at about 20 ± 3 Ma the tectonic stress field in southern and southern-eastern parts of Qinghai-Tibetan Plateau underwent a considerable change from the NS-trending compression to the EW-trending extension. Combined with contemporary adakitic rocks along the Yarlung Zangbo suture, the authors hold that the northward subducted Indian lithospheric mantle might have experienced break-off beneath the Lhasa b

关 键 词：新生代 超钾质岩 南北向地堑(裂谷) 拉萨地块 青藏高原 

分 类 号：P588.14[天文地球—岩石学] P542[天文地球—地质学]