对西秦岭疑似为燕山期花岗岩的舟曲峰迭和夏河桑日卡岩体进行岩相学研究和锆石U-Pb同位素地质年龄测定,获得锆石U-Pb年龄分别为201.3±0.9Ma和232.6±2.2Ma,表明2个岩体均属早中生代印支期造山作用岩浆活动的产物,澄清了有关地质图(1∶25万陇东幅地质图和1∶25万临夏市幅建造构造图)中2个岩体的时代归属.通过研究认为,西秦岭内部无论南带或北带基本不存在燕山期花岗岩,其花岗岩主体为出露于北带的印支期花岗岩体.因此,西秦岭可以与东秦岭的南秦岭构造单元对比,在构造带的划分上相当于南秦岭的西延.结合前人研究成果,从西秦岭与南秦岭花岗岩形成时代与同位素地球化学特征看,两者的岩浆源区相似并具有扬子地块基底属性.西秦岭缺少燕山期花岗岩的原因归咎于它的构造位置与东秦岭尤其是燕山期花岗岩极发育的小秦岭完全不同,后者燕山期岩浆作用得以盛行,可能与华北克拉通岩石圈破坏或与中生代中晚期华北地块向秦岭造山带的陆下俯冲有关.%In this paper,the authors petrographically studied both Fengdie and Sanrika granitic plutons in Zhouqu County and Xiahe County of West Qinling Orogen and determined their zircon U-P ages.These two plutons which used to be recognized as Yansha-nian (Middle-Late Mesozoic) intrusions yielded zircon U-Pb ages of 201.3±0.9Ma and 232.6±2.2Ma,respectively,suggesting that they should be products of the Indosinian (Early Mesozoic) magmatism and thus clarifying their timing in certain geological maps (1 ∶ 250000).Combining previous age data with the authors' study,the authors argue that there is basically no Yanshanian granite exposed in West Qinling Orogen and the majority of granites of West Qinling Mountain are Indosinian in age and spatially distributed in the northern part of West Qinling Orogen.Therefore,West Qinling can be tectonically compared with South Qinling tectonic unit of East Qinling,and the former is the western extension of the latter.It can be seen from both the age and the geochemical data that Indosinian granites of West Qinling and South Qinling were derived from the similar sources and possessed characteristics of the basement of the Yangtz block.The reason for lacking Yanshanian granite in West Qinling can be attributed to obvious difference of the tectonic location between West Qinling and East Qinling,especially the Xiaoqinling where Yanshanian granites were well developed.The formation of Yanshanina granites in the Xiaoqinling was probably related to the destruction of the North China cratonic lithosphere or to the intracontinental subduction of the North China block.
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