秦岭—祁连结合部陇山构造带地质演化探讨——早古生代晚期花岗岩和中基性包裹体地球化学和独居石、锆石U-Pb年龄证据  

A discussion on geological evolution of the Longshan tectonic zone in the joint part of Qinling—Qilian Orogenic Belts——Evidences of geochemical and U-Pb ages of monazite and zircon from the late Early Paleozoic granite and intermediate—basic inclusion

作　　者：畅莹磊 罗金海[1] 陈卓[1] CHANG Yinglei;LUO Jinhai;CHEN Zhuo(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an,710069)

机构地区：[1]大陆动力学国家重点实验室,西北大学地质学系,西安710069

出　　处：《地质论评》2023年第3期830-854,共25页Geological Review

基　　金：国家自然科学基金资助项目(编号:42072231)的成果。

摘　　要：笔者等对秦岭—祁连结合部陇山构造带内部侵入于陇山岩群中的片麻状二长花岗岩和其中的黑云斜长片麻岩包裹体进行了地球化学和独居石、锆石U-Pb年代学研究。片麻状二长花岗岩具有I型花岗岩的地球化学特征,对其进行的LA-ICP-MS独居石U-Pb测年获得年龄433.5±1.8 Ma,锆石U-Pb测年获得了3个主峰年龄(439.5 Ma、634.5 Ma和2265 Ma)和5个次级峰值年龄(568.7 Ma、1875 Ma、2112 Ma、2604 Ma和2738 Ma)。黑云斜长片麻岩的地球化学特征反映其原岩是经历过显著的结晶分异作用的中基性岩,在其中分选出了两类锆石:①环带清晰且经受了热液蚀变的岩浆锆石,对其U-Pb测年获得了一条很好的不一致线,不一致线的上、下交点年龄分别是2194±42 Ma和431.2±58.1 Ma;②CL图像整体很暗的变质锆石或热液蚀变锆石,在这类锆石中获得了峰值年龄513.1 Ma。结合区域地质资料,笔者等把片麻状二长花岗岩的形成时间限定为431.2~439.5 Ma,把黑云斜长片麻岩原岩的形成时间限定为2194±42 Ma。片麻状二长花岗岩与文献揭示的围岩(斜长角闪片岩)的形成时间基本一致,暗示431.2~439.5 Ma期间陇山地区大量发育中基性和酸性岩浆活动,岩浆作用具有显著的地区差异特点。北祁连洋盆和北秦岭西段洋盆向北的俯冲活动在513.1 Ma时可能影响到了本文研究区。2738 Ma、2604 Ma、2112 Ma~2265 Ma和1875 Ma等峰值年龄分别应该是华北克拉通在新太古代晚期的陆壳生长事件和古元古代早期克拉通裂解事件在研究区的反映。634.5 Ma期间研究区可能处于一种未知的特别的构造环境。总体来看,陇山构造带在前寒武纪时作为华北地块的一部分而演化,显生宙以来其构造演化过程受到秦岭造山带和北祁连造山带的显著影响。The paper studies monazite and zircon U-Pb geochronology,chemistry of the gneissic monzogranite and biotite plagiogneiss inclusions,which intrude into the Longshan Group in the joint site of the Qinling—Qilian orogenic belts.The gneissic monzogranite are characterized by the geochemical feature of I-type granite,and the age of 433.5±1.8 Ma is given by means of LA-ICP-MS monazite U-Pb dating,and the three peak ages(439.5 Ma、634.5 Ma and 2265 Ma)and five sub-peak ages(568.7 Ma,1875 Ma,2112 Ma,2604 Ma and 2738 Ma)are given by means of LA-ICP-MS zircon U-Pb dating.The geochemical features of the biotite plagiogneiss inclusions suggested that its protolith should be the meso—basic magmatite suffered from remarkably crystallization differentiation,and two types of zircons are selected:①Magmatic zircon with clear oscillatory zoning and suffered from hydrothermal alteration,and LA-ICP-MS zircon U-Pb dating obtains a nice line of inconsistency with the upper intercept age(2194±42 Ma)and the lower intercept age(431.2±58.1 Ma),②Metamorphic zircon and(or)hydrothermal alteration zircon without oscillatory zoning and gloomy CL image,and LA-ICP-MS zircon U-Pb dating obtains peak age of 513.1 Ma.Combined with regional geological data,the formation age of the gneissic monzogranite is defined as 439.5~431.2 Ma,and the formation age of the biotite plagiogneiss inclusions is defined as 2194±42 Ma.The formation age of the gneissic monzogranite is basically coincide with that of the surrounding rock(plagioclase amphibole schist)revealed in the literature,and it is suggested that intermediate—basic and acid magmatic activities broadly developed during 439.5~431.2 Ma in the Longshan area,and the significant regional difference of change of magmatism exists at that time.The northward subduction of the northern Qilian Ocean and the western segment of the northern Qinling Ocean had affected the studied area in 513.1 Ma.The peak ages of 2738 Ma,2604 Ma,2265~2112 Ma and 1875 Ma should correspond the continental crustal growth

关 键 词：秦岭—祁连结合部 二长花岗岩 中基性岩 独居石U-Pb测年 锆石U-PB测年 

分 类 号：P534.4[天文地球—古生物学与地层学] P588.12[天文地球—地质学] P597.3