西秦岭伯阳石英正长斑岩的地球化学、年代学及其地质意义  

Geochemistry and chronology of quartz orthophyre in Boyang of the Western Qinling Orogenic Belt,and their geological significance

作　　者：丁守卓 罗金海[1,2] 程佳孝[1,2] 韩奎 王师迪[1,2] 尤佳[1,2] 

机构地区：[1]大陆动力学国家重点实验室(西北大学) [2]西北大学地质学系,陕西西安710069 [3]中国石油长庆油田苏里格南作业分公司,陕西西安710018 [4]陕西省地质调查中心,陕西西安710016

出　　处：《地学前缘》2015年第4期247-254,共8页Earth Science Frontiers

基　　金：西北大学大陆动力学国家重点实验室科学技术部专项基金项目

摘　　要：西秦岭天水市伯阳乡北部出露的流纹岩（小河子火山岩）及其南侧出露的次火山岩石英正长斑岩在现有的文献中均认为是同源岩浆活动的产物,但是并未对次火山岩进行过地球化学和同位素年代学研究。本文对次火山岩的研究表明,该石英正长斑岩具有高SiO2（71.34%-72.32%,平均为71.67%）、高（Na2O＋K2O）（9.13%-9.68%,平均为9.44%）、低TiO2（0.19%-0.21%）、MgO和铝弱过饱和（A/CNK=1.04-1.12）的特征,属于弱过铝质的碱性亚碱性花岗岩系列岩石,具有Ⅰ型花岗岩的特征。该石英正长斑岩轻稀土强烈富集,（La/Yb）N值=27.5-31.5（平均29.5）,Eu明显负异常（δEu=0.11-0.14,平均0.12）;大离子亲石元素（LILE）K、Rb、U等相对富集,高场强元素（HFS）Nb、P、HREE等则相对亏损,显示壳源特征。对石英正长斑岩进行的锆石U-Pb LA-ICP-MS同位素年代学测定获得了206Pb/238 U加权平均年龄（209.3±5.8）Ma（MSWD=16）,说明石英正长斑岩形成于晚三叠世,而不是前人认为的古近纪。石英正长斑岩具有与小河子火山岩中流纹岩类似的地球化学特征,二者的形成时代基本一致,是晚三叠世期间在碰撞后伸展背景下西秦岭酸性火山作用的产物。The rhyolite （Xiaohezi volcanic rocks） in the north section of Boyang, Tianshui City of west Qinling Mountain and the subvolcanic rock-quartz-orthophyre of its south side were considered the results of comagmatic activities. But the geochemistry and isotopic geochronology of the subvolcanic rock have never been studied up to now. Our study of it shows that the quartz-orthophyre contains high level of SiO2 （71.34%-72. 32%, with an average of 71.67）, high level of Na2O＋K2O （9.13%- 9.68%, with an average of 9.44%）, and low level of TiO2 （0. 19%- 0. 21%）, and MgO and aluminium are weakly oversaturated, which belong to weakly pel＇aluminous peralkaline-subalkaline granites, with the characteristics of I-type granite. The quartz orthophyre shows strong enrichment in LREE, （La/Yb）N = 27. 5 - 31.5 （with an average of 29. 5） and has obviously negative Eu anomaly （SEu=0. 11 -0. 14, with an average of 0. 12）; LILE like K, Rb, U are relatively enriched. HFS such as Nb, P, HREE are relatively depleted, indicating continental crust origin. U-Pb LA- ICP-MS dating of zircon yielded a weighted ^206 pb/^238U age of 209.3± 5.8 Ma （MSWD= 16）, suggesting that the quartz orthophyre was formed in the Late Triassic, not in the Paleogene as regarded before. The quartzorthophyre has the similar geochemical characteristics as the rhyolite in Xiaohezi volcanic rocks; both were basically formed at the same time and resulted from the Late Triassic acid volcanism under the tectonic setting of post-collisional extension of the western Qinling orogenic belt.

关 键 词：西秦岭 小河子火山岩 石英正长斑岩 流纹岩 古近纪 晚三叠世 碰撞后伸展作用 

分 类 号：P588.12[天文地球—岩石学] P597[天文地球—地质学]