南秦岭中段西乡群火山岩岩石成因  

Petrogenesis of Volcanic Rocks from Xixiang Group in Middle Part of South Qinling Mountains

作　　者：夏林圻[1] 夏祖春[1] 马中平[1] 徐学义[1] 李向民[1] 

机构地区：[1]西安地质矿产研究所,陕西西安710054

出　　处：《西北地质》2009年第2期1-37,共37页Northwestern Geology

基　　金：中国地质调查局综合研究项目(1212010611804)

摘　　要：南秦岭中段新元古代中期(730～845Ma)西乡群(自下而上包括孙家河组、大石沟组和白勉峡组)火山岩喷发于大陆板内裂谷环境。它们极有可能与导致Rodinia超大陆裂谷化裂解的地幔柱活动有关。根据岩石地球化学数据,南秦岭中段新元古代中期西乡群裂谷基性熔岩总体上属于低Ti/Y(LT,Ti/Y<500)岩浆类型。LT熔岩又可进一步划分为LT1和LT2等2个亚类。LT1熔岩以高Nb/La(0.87～0.98)、低ThN/NbN(≈1)、缺乏Nb-Ta和Ti的亏损、具有"大隆起"式微量元素原始地幔标准化分配型式、(87Sr/86Sr)(t)=0.703869、εNd(t)=4.83为特征,属于拉斑玄武质岩浆系列;LT2熔岩以低Nb/La(<0.75)、高ThN/NbN(>1.4)、Nb-Ta和Ti亏损明显和Sr-Nd同位素比值变化较大为特征。元素和同位素数据表明,西乡群裂谷火山岩的化学变化不是由一个共同的母岩浆结晶分异作用所产生。孙家河组、大石沟组和白勉峡组中TiO2含量大于1.09%的火山岩的母岩浆经受了辉长岩质结晶分离作用,而白勉峡组中TiO2含量小于0.69%的基性熔岩的化学演化则是受控于单斜辉石(cpx)±橄榄石(ol)结晶分离作用。西乡群火山岩系中,基性、中性和酸性熔岩间为分异结晶关系。南秦岭中段新元古代中期西乡群裂谷火山岩系极有可能是源于共同的地幔柱,该地幔柱组分的成分为:εNd(t)≈+5,87Sr/86Sr(t)≈0.704,La/Nb≈0.7。南秦岭中段新元古代中期西乡群裂谷基性熔岩存在空间上的地球化学变化:LT1熔岩的母岩浆,没有受到明显的大陆岩石圈混染,保存了鲜明的地幔柱信号;而大陆地壳或大陆岩石圈混染作用对于LT2熔岩的形成则有着重要贡献。研究揭示,南秦岭中段新元古代中期西乡群裂谷基性熔岩的母岩浆总体上产生于上涌地幔柱上部层位(地幔柱头)3GPa石榴子石稳定区(深度≈100km)。三湾岩组可能不应当归属于西乡群。三湾组火山岩的岩石地球化学特点与亏损型洋脊玄The volcanic rocks from Xixiang Group （including Sunjiahe Formation, Dashigou Formation, and Baimianxia Formation） situated in the middle part of South Qinling Mountains were formed during the mid-Neoproterozoic （730--827 Ma） and in an intracontinental rift setting. They are most likely related to the proposed mantle plume that led to the rifting-breakup of the Rodinia supercontinent. On the basis of petrogeochemical data, the rift-related basic lavas from the mid-Neoproterozoic Xixiang Group in middle part of South Qinling Mountains generally belong to a low-Ti/Y （LT, Ti/Y 〈 500） magma type, which can be classified into two subtypes： LT1 and LT2. The LT1 lavas are characterized by a high Nb/La （0.87 --0.98）, a low ThN/NbN （≈1）, without any depletion of Nb-Ta and Ti; in addition, the distribution of primitive mantle-normalized trace element shows an eminence pattern and is characterized by （^87Sr/^86Sr） （t） = 0. 703869 and εNd （t） = 4.83. So the LT1 belongs to the tholeiitic series. The LT2 lavas exhibit a low Nb/La （〈0. 75）, a high ThN/NbN （〉1.4）, an obvious depletion in Nb-Ta and Ti, and a variable Sr-Nd isotopic radio. Elemental and isotopic data suggest that the chemical variation of the rift-related volcanic rocks cannot be explained by crystallization from a common parental magma. The parental magmas of Sunjiahe Formation, Dashigou Formation volcanic rocks and Baimianxia Formation volcanic rocks with a TiO2 content greater than 1.09% were subjected to gabbroic fractionation; however, the chemical variation of the basic lavas from Baimianxia Formation with the TiO2 content smaller than0.69 is controlled by a clinopyroxene （cpx） ± olivine （ol） fractionation. In the Xixiang Group volcanic rock serioes, there is a differentiation crystallization relationship between basic, intermediate, and silicie lavas. Therefore, the rift-related volcanic rock series from Xixiang Group in middle part of South Qinling Mountains is most likely originated from a common

关 键 词：新元古代中期裂谷火山岩 西乡群 岩石地球化学 岩石成因 地幔柱 地壳混染 南秦岭 

分 类 号：P588.1[天文地球—岩石学]