西藏班公湖地区晚白垩世构造体制由挤压向伸展的转换:来自岩浆岩的证据  

Conversion of tectonic regimes from compression to extension in the Bangong Lake area,Xizang during the Late Cretaceous: Evidence from magmatic rocks

作　　者：秦雅东[1] 李德威[2] 林仕良[1] 江丹 刘德民[1] 张硕 

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610081 [2]中国地质大学(武汉),湖北武汉430074 [3]武汉市测绘研究院,湖北武汉430022 [4]江苏地质调查院,江苏南京210018

出　　处：《沉积与特提斯地质》2015年第4期92-105,共14页Sedimentary Geology and Tethyan Geology

基　　金：中国地质调查局西藏1:5万聂拉木等4幅区域地质调查(No.1212011121242);中国地质调查局云南1:5万马树等4幅区域地质矿产调查(No.1212011220401)

摘　　要：班公湖蛇绿混杂岩带内分布着一系列小型斜长花岗斑岩和花岗闪长岩岩体,锆石U-Pb年龄分别为97.4±1.1Ma和91.94±0.78Ma,具埃达克质岩特征,高Si O2、Al2O3和Sr,低Y和Yb,Sr/Y〉35,轻重稀土分异明显,亏损Nb、Ta和Ti,Cr和Ni含量很低,推测为玄武质岩浆底侵加厚下地壳部分熔融形成。辉石闪长岩脉分布于南侧日土花岗岩内,推测年龄为80-76Ma,岩石地球化学显示亏损Zr、Hf、Ti、Y等高场强元素,富集大离子亲石元素,且具较高的Sc、Y、Cr、Co、HREE和Mg#值（〉40）,源区为经过熔体交代的上地幔。结合前人资料,本文认为班公湖地区在97-92Ma仍处于持续碰撞造山、地壳加厚过程中;92Ma之后,构造体制从碰撞期的挤压转变为碰撞后的板内伸展;80-76Ma,板内的伸展进一步加剧。The Late Cretaceous intermediate to acidic rocks in the ophiolite melanges from the Bangong Lake area in Xizang are dominantly made up of plagiogranite porphyry and granodiorite, which give the zircon U-Pb ages of 97.4 ± 1.1 Ma and 91.94 ±0.78 Ma, respectively. These plagiogranite porphyry and granodiorite have the features of adakitic rocks, and are characterized by high SiO2, Al2O3 and Sr, low Y and Yb, Sr/Y 〉 35, highly differentiation in LREE and HREE, depletion of Nb, Ta and Ti, and very low contents of Cr and Ni, implying that they were formed by the partial melting of thickened lower crust caused by the underplating of the basaltic magmas. The pyroxene diorite dated at the ages of 80 to 76 Ma in the Rutog granites in the southern part of the Bangong Lake area displays the characteristics of the depletion of the high field strength （HFSE） such as Zr, Hf, Ti and Y, enrichment of the large-lon lithophile elements （LILE）, and higher contents of Sc, Y, Cr, Co and HREE, and higher Mg# values （ 〉 40 ）, indicating a upper mantle origin of pyroxene diorite. The sustained collision and mountain building still took place in the Bangong Lake area during the periods of 97 to 92 Ma. After that, there were conversions of tectonic regimes from the compression during the collision stages to the post-collision intraplate extension. Finally the further extension persisted from 80 Ma to 76 Ma.

关 键 词：班公湖 埃达克岩 辉石闪长岩 地壳伸展 

分 类 号：P542[天文地球—构造地质学]