青藏高原东部贡觉盆地新生代火山岩特征及其构造意义

Characteristics of Cenozoic Volcanic Rocks in Gongjue Basin,Eastern Tibet,and Their Tectonic Implications

藏东贡觉盆地古近纪火山岩形成于印度-欧亚板块碰撞造山后的伸展环境中,火山活动受水平剪切-走滑断裂带控制。火山岩以高Si O2、Al2O3、K2O、Na2O和低MgO、Ti O2为特征,属高钾钙碱性中酸性火山岩。火山岩中富集大离子亲石元素(LILE)、轻稀土元素(LREE),亏损高场强元素(HFSE)和一部分相容元素(Co、Ni、V、Sc);无Eu异常但有弱的Ce负异常。火山岩源区可能是经历了壳源混合作用的富集交代地幔。富集组分来自壳源,它们沿古俯冲带以再循环方式进入地幔楔,并与地幔发生交代作用。

Cenozoic volcanic rocks in Gongjue basin of eastern Tibet were formed in a post-collisional extensional tectonic setting, and volcanic activities were controlled by a strike-slip shear zone. Chemical composition of volcanic rocks indicates that they belong to the high-K calc-alkaline magmatic system, with volcanic rocks possessing relatively high SiO-2, Al-2O-3, K-2O and Na-2O and comparatively low MgO and TiO-2 components. Petrological and chemical studies also indicate that volcanic rocks are intermediate to acidic rocks. This suite of rocks are enriched in LILE (Rb,Ba,K,Sr,Th) and LREE, and must have experienced marked depletion of HFSE and a group of compatible elements (Cr,Ni,V,Sc), similar to characteristics of post-collisional arc-type rocks. There exist weak Ce anomalies but no Eu anomalies in these rocks. The sources of the volcanic rocks might be the enriched metasomtic mantle which experienced multiple mixture of components mainly derived from previously subducted continental/oceanic crust.