中拉萨地块南缘孔隆晚白垩世火山岩成因及对地壳演化的约束  

Crustal Evolution Constraints from the Petrogenesis of Late Cretaceous Konglong Volcanics on Southern Margin of Central Lhasa Subterrane

作　　者：陈澍民 缪宇 廖驾 贺前平 成明 张珍力 吴绍安 章志明 CHEN Shumin;MIAO Yu;LIAO Jia;HE Qianping;CHENG Ming;ZHANG Zhenli;WU Shaoan;ZHANG Zhiming(Changsha Natural Resources Comprehensive Survey Centre,China Geological Survey,Changsha,Hunan 410600,China;Kunming Natural Resources Comprehensive Survey Centre,China Geological Survey,Kunming,Yunnan 650000,China)

机构地区：[1]中国地质调查局长沙自然资源综合调查中心,湖南长沙410600 [2]中国地质调查局昆明自然资源综合调查中心,云南昆明650000

出　　处：《现代地质》2021年第6期1713-1726,共14页Geoscience

基　　金：中国地质调查局地质调查项目(DD2016008003,DD2016008004)。

摘　　要：拉萨地块晚白垩世岩浆岩的研究集中于南北两缘,中部的岩浆-构造演化对班公湖—怒江洋或新特提斯洋演化过程的响应及地幔贡献度仍然缺乏精确约束。本次研究报道了中拉萨地块南缘孔隆地区发现的一套流纹质火山岩。锆石U-Pb年代学、全岩地球化学及Hf同位素分析结果显示,孔隆火山岩LA-ICP-MS锆石U-Pb年龄为(88.0±1.7)Ma,形成于晚白垩世早期。主量元素SiO_(2)及Al_(2)O_(3)含量较高,Mg^(#)较低;轻、重稀土分馏明显,表现为轻稀土相对富集,重稀土相对亏损;相对富集Rb、Th、K,相对亏损Ba、Eu、Sr、P、Ti;与周缘埃达克岩区别明显。锆石ε_(Hf)(t)在-10.2~-5.3之间,T_(DM2)模式年龄为1492~1804 Ma,结合Nb/La平均值(0.21)及Nb/Ta平均值(10.1)分析,认为孔隆火山岩可能直接源于古老的下地壳深熔,未经幔源物质混染而形成,其主要驱动力为新特提斯洋壳北向俯冲。本项研究为印亚大陆碰撞前拉萨地块演化提供了新的约束。Research of the Late Cretaceous magmatic⁃tectonic evolution is a focus on the northern and southern margins of the Lhasa terrane.The response to the evolution of the Bangong⁃Nujiang ocean or Neo⁃Tethys ocean,and the mantle contribution still lacks precise constraints on the central Lhasa terrane.This study reports the rhyolitic volcanic rock outcrops at Konglong on the southern margin of the central Lhasa subterrane.Results of LA⁃ICP⁃MS zircon U⁃Pb geochronology,whole⁃rock geochemistry and zircon Hf isotope analyses show that the Konglong rhyolite was formed in the early Late Cretaceous(88.0±1.7 Ma).The rocks have high contents of SiO_(2)and Al_(2)O_(3),and low Mg^(#),enriched in LREEs and relatively depleted in HREEs.The rocks are distinctly enriched in Rb,Th,and K,but relatively depleted in Ba,Eu,Sr,P,and Ti,which are clearly distinguished from the surrounding adakites.The zirconε_(Hf)(t)values range from-10.2 to-5.3,withT_(DM2)model age of 1492-1804 Ma.This,combining with the average Nb/La(0.21)and Nb/Ta(10.1)values,imply an ancient lower crustal source.The results indicate that the Konglong volcanic rocks were mostly derived directly from ancient lower crustal anatexis without mantle⁃sourced input or assimilation.The petrogenetic geodynamics may have been the north⁃dipping Neotethys subduction and the heating from lower crustal delamination in the north.This study provides new constraints on the evolution of the Lhasa terrane prior to the India⁃Asia continent⁃continent collision.

关 键 词：流纹质火山岩 地壳深熔 晚白垩世 拉萨地块 西藏 

分 类 号：P588.1[天文地球—岩石学]