西藏然巴地区十字石‒蓝晶石云母片岩变质作用研究:对始新世淡色花岗岩成因的启示  

Metamorphic Evolution of the Kyanite-staurolite Micaschist from the Ramba Dome,Tibet:Insights into the Petrogenesis of Himalayan Eocene Leucogranite

作　　者：李志泰 徐扛 王连峰 王强[1] 蒋映德[1] LI Zhitai;XU Kang;WANG Lianfeng;WANG Qiang;JIANG Yingde(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]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,广东广州510640 [2]中国科学院大学,北京100049

出　　处：《大地构造与成矿学》2020年第6期1190-1207,共18页Geotectonica et Metallogenia

基　　金：中科院前沿科学重点项目:青藏高原中北部新生代陆内岩石圈演化(QYZDJ-SSW-DQC026);第二次青藏高原科学考察研究计划(STEP)(2019QZKK0702)联合资助。

摘　　要：然巴穹隆及其周源发育不同期次淡色花岗岩与不同程度变质作用,两者之间存在何种成因关联尚不清楚。为揭示这一内在联系,本文对然巴穹隆中变形的淡色花岗岩与围岩中十字石‒蓝晶石片岩开展了系统的岩相学观察、主微量元素分析、变质相平衡模拟和锆石、独居石U-Pb年代学研究。研究表明然巴穹隆中变形花岗岩以脉状产出,为似斑状二云母花岗岩,其形成年龄约为45 Ma。这些花岗岩脉具有高CaO与Sr含量,相对较高的Sr/Y值、较低的Rb/Sr值,在Sr/Y-Y图解中落于埃达克岩范围内,暗示其成因与地壳加厚有关。穹隆围岩中十字石‒蓝晶石云母片岩具有峰期共生矿物组合:蓝晶石+十字石+石榴石+黑云母+白云母+斜长石+石英+金红石+钛铁矿。变质相平衡模拟结果显示其具有增温增压的进变质作用过程,且变质峰期温压条件为600~650℃/8×10^8~9×1^08 Pa。对该云母片岩开展独居石U-Pb测年,获得变质峰期年龄约为53 Ma。研究表明然巴地区在始新世存在着地壳加厚过程,造成加厚地壳的部分熔融从而形成区域围岩变形二云母花岗岩。本次研究为喜马拉雅造山带始新世淡色花岗岩与印度‒亚洲板块碰撞导致地壳加厚的成因关联提供了研究实例。The Ramba dome in the Tethyan Himalaya is characterized by polyphase intrusions of leucogranite and affected by variable metamorphism.The effects of regional metamorphism on the generation of the leucogranites have not yet been well documented.To address this issue,systematic petro-structural observation,geochemical analysis,metamorphic phase equilibrium modelling and U-Pb dating of zircon and monazite were carried out on a staurolite-kyanite micaschist and the associated deformed leucogranite dykes in the metamorphic envelope of the dome.The deformed leucogranite dykes are porphyritic two-mica granite with a U-Pb zircon crystallization age of ca.45 Ma.These dykes have high CaO and Sr contents,relatively high Sr/Y and low Rb/Sr ratios,compositionally similar to that of typical adakitic granite.These features imply that the leucogranite dykes most likely formed in a thickened crustal environment.The kyanite-staurolite micaschist has a peak mineral assemblage of kyanite+staurolite+garnet+biotite+muscovite+plagioclase+quartz+rutile+ilmenite.Metamorphic phase equilibrium modelling reveals a prograde P-T path associated with increases of temperature and pressure,reaching a peak stage of 600−650℃/8×10^8^−9×10^8 Pa.These findings suggest a burial evolution,probably reflecting progressive crust thickening.U-Pb dating of monazite from the micaschist yielded an age of ca.53 Ma,interpreted as the timing of the metamorphism.Taken together,the results from this study suggest a significant crustal thickening at ca.53 Ma in the Ramba area,and this process might account for the formation of the high Sr/Y leucogranite from the deep crust.This study provides a good example for generation of bulk Himalayan Eocene leucogranite in response to the India-Asia collision.

关 键 词：喜马拉雅造山带 然巴穹隆 淡色花岗岩 变质变形 

分 类 号：P511.5[天文地球—地质学] P581