西乡群三郎铺组和大石沟组火山岩U-Pb定年及岩石成因研究  

Geochemistry and petrogenesis of volcanic rocks from Sanlangpu Formation and Dashigou Formation

作　　者：徐学义[1] 陈隽璐[1] 李向民[1] 马中平[1] 王洪亮[1] 夏林圻[1] 夏祖春[1] 李平[1,2] 

机构地区：[1]中国地质调查局西安地质调查中心,西安710054 [2]长安大学,西安710069

出　　处：《岩石学报》2010年第2期617-632,共16页Acta Petrologica Sinica

基　　金：国土资源大调查项目(1212010611804;1212010610319);科技部科技支撑课题(2006BAB01A11)联合资助

摘　　要：三郎铺组不整合上覆于孙家河组之上,与大石沟组呈连续过渡关系,两者均为一套火山沉积岩系。三郞铺组火山岩主要为玄武岩和流纹岩,大石沟组火山岩包括玄武岩、安山岩、流纹岩。大石沟组流纹岩LA-ICPMS锆石U-Pb定年揭示流纹岩形成时代为803.0±5.3Ma,属新元古代岩浆作用产物。元素地球化学和三郎铺组特有的陆相火山岩结构表明火山岩形成大陆板内伸展环境,属亚碱性火山岩系。三郎铺组玄武岩和流纹岩组成双峰式火山岩套,微量元素比值对模拟结果表明玄武岩浆来源于岩石圈地幔中原始石榴石二辉橄榄岩的部分熔融,部分熔融程度约为7%;三郎铺组流纹岩浆源岩可能为白勉峡组下部的玄武质岩石,岩浆起源于斜长石稳定的下地壳源区或部分熔融形成的原生岩浆在上升过程中,经历了较为显著的斜长石结晶分异作用过程。大石沟组玄武岩、安山岩和流纹岩具有相同的微量元素比值、ε(t)值和t_(DM),为同一母岩浆分离结晶的产物。玄武岩浆起源于原始石榴石二辉橄榄岩约10%的部分熔融,玄武岩浆分离结晶后残余岩浆比例约70%时形成安山岩浆,残余岩浆比例约20%～30%时产生流纹岩浆;分离结晶的主要矿物包括斜长石和铁钛氧化物。三郎铺组和大石沟组均是新元古代晚期大陆裂谷作用的岩浆响应。We present a detailed geochemistry study,new age dating for Sanlangpu Formation and Dashigou Formation volcanic rocks from Xixiang Group,which provides significant insights into the petrogenesis of the volcanic rocks.Sanlangpu Formation volcanic rocks,consisting of basalt and rhyolite which can be considered as bimodal magma suite,lie uncomfortably with Sunjiahe Formation. Dashigou Formation volcanic rocks are composed of basalt,andesite and rhyolite,lie up comfortably with Sanlangpu Formation.A combined field,geochronological,and geochemical study of the volcanic rocks is reported here.This study provides the first reliable LA-ICPMS zircon U-Pb dating for rhyolite sample collected from Gangchang Village in Nanzhen County gave a weighted mean ^(206)Pb/ ^(238)U age of 803.0±5.3Ma,which documented report of Neoproterozoic magmatism in Xixiang area.All the volcanic rocks belong to subalkali magma and erupted within plate.Trace element ratio partial melting composition model suggest basalts from Sanlangpu Formation are derived from primary garnet Iherzolite with a partial melting degree of 7%.According to the similarity of the trace element ratios and patterns,we also suggest an impossible source of Baimianxia Formation basalts for rhyolites,and the rhyolite magma might undergone strong crystallization of plagioclase during its evolution.On the other hand,the basalt,andesite,and rhyolite in Danshigou Formation have the same trace element ratios,ε_(Nd)(t) and t_(DM),suggesting a same parent magma Basalts in Danshigou Formation are possible derived from primary garnet lherzolite with the partial melting degree of 10%,and andesite derived when the residual magma approach to 70%during crystallization,and rhyolites occurred when the residual magma approach from 20%to 30%. The basaltic parent magma underwent strong crystallization of plagioclase and Fe-Ti oxides.It is inferred that Sanlangpu Formation and Dashigou Formation volcanic rocks also be attributed to the products of Neoproterozoic continental break-up in northe

关 键 词：岩石成因 LA-ICPMS锆石U-PB定年 岩石地球化学 三郎铺组 大石沟组 西乡群 

分 类 号：P597.3[天文地球—地球化学]