冈底斯带西段那木如岩体始新世岩浆作用及构造意义

冈底斯带西段狮泉河南部那木如岩体岩性变化较大,其中产出大量基性岩透镜体及暗色微细粒包体,空间上与花岗岩类呈渐变过渡接触关系。本文在详细野外调研的基础上,对狮泉河-札达一带那木如花岗岩及其中基性岩石进行了系统的岩石学、地球化学和同位素年代学研究。结果表明,那木如岩体主体岩性为黑云母花岗岩,其SiO₂为65%~76%,全碱含量较高,花岗岩中K₂O+Na₂O=5.50%~8.71%,基性岩石中则4.42%~6.7%。花岗岩类稀土元素最高含量为284.8×10^-6,最低只有105.4×10^-6;而基性岩类最高为120.4×10^-6,最低72.48×10^-6。两者稀土元素分配曲线均呈右倾平缓样式,花岗岩具有不明显Eu负异常,微量元素显示出花岗岩类和基性岩类具有相似的蛛网分布样式。两者均明显富K而亏损Nb、Ti等不活泼的HFS元素,显示出明显的岩浆混合作用趋势。4件花岗岩和基性岩样品所显示的LA-ICP-MS法锆石U-Pb年龄分别为46.11±0.78Ma、45.47±0.4Ma、46.7±2.9Ma和45.4±1.4Ma,变化在45.4~46.7Ma范围内,表明始新世早期(~46Ma)区域发生了岩浆混合作用。这一时限与冈底斯中、东部岩浆作用时代(40~52Ma)非常一致,表明始新世早期整个冈底斯发生了规模巨大的岩浆事件,暗示着印度-欧亚大陆碰撞作用在东西方向上所表现出的同时性。

The Neocene magmatism from Namuru intrusion in western Gangdese, Tibet and its tectonic significance

There are lots of granitic intrusions in the western Gangdese, Tibet. Namuru granite is one of the typical intrusions with various gabbro inclusions and mafic micro-granular enclaves (shortly MMEs). Field investigation has found the gradually transitional relationship between the gabbro inclusions and granite with abundant MMEs. It is lithologically biotite granite for Namuru intrusion. The chemical analyses show that the SiO₂ varies from 65%~76%, average 73% for the granite and 48.5%~55.6%, average 51%. The total alkali contents are high in both the granite(K₂O+Na₂O=5.50%~8.71%) and mafic rocks (4.42%~6.7%). The REE pattern is flat and slightly declining with no clearly Eu anomaly with the total content from up to 284.8×10^-6 and lowest of 105.4×10^-6 in the granite and up to 120.4×10^-6, and lowest of 72.48×10^-6 in the gabbro rocks. The normalized trace element spider is quite similar in the both with K element enriched and Nb, Ti depleted. Zircon LA-ICP-MS U-Pb dating for 4 samples both granite and gabbro inclusions gave the age of 46.11±0.78Ma, 45.47±0.4Ma, 46.7±2.9Ma and 45.4±1.4Ma respectively, falling into a range of 45.4~46.7Ma of crystalling age. All the characters indicated that magma mixing had happened between granite and mafic magma during the Neocene (45.4~46.7Ma), forming the vast granitic and gabbro rocks as an important magmatic event in western Gangdese. It happens to be consistent with the duration (40.0~52.5Ma) for the known magma mixing and underplating in eastern to middle Gangdese, such as Quxu and Xigarze. It probably represents the giant magma event with magma mixing and underplating in Gangdese during Early Neocene. The paper therefore inferred, on the basis of magmatic rocks, that the collision between India-Eurasian continents are acting simultaneously in both eastern and western Gangdese in Eocene, resulting in basaltic magma underplating below and then magma mixing along whole Gangdese belt and formation of the Gangdese giant magmatic belt.