北秦岭五垛山花岗岩锆石U-Pb年代学和地球化学特征及成因

Zircon U-Pb Geochronology, Geochemistry and Petrogenesis of Granites from the Wuduoshan Pluton, the North Qinling Terrane

五垛山岩体是位于北秦岭地体东部的大型复式岩体,花岗岩类型复杂,并呈多期次侵位特征。对岩体中黑云母花岗岩和二长花岗岩的代表性样品进行锆石U-Pb定年,显示其形成年龄为441~431Ma,部分样品保留古元古代至新元古代的继承锆石。五垛山岩体花岗岩具有高硅、低镁、富碱、弱过铝至强过铝特征。微量元素蛛网图显示富集Rb、Ba、K和Pb,亏损Nb、Ta、P、Ti。稀土元素的球粒陨石分配模式显示轻、重稀土分馏明显,δEu为0.36~1.45。五垛山岩体花岗岩的87Sr/86Sr(i)为0.70304~0.71290,εNd(t)值为-4.6^-1.9,两阶段Nd模式年龄(TDM2)为1.34~1.58 Ga。稀土元素判别图显示这些花岗岩主要是由部分熔融作用形成,同位素特征指示源区为北秦岭变沉积岩与幔源物质的混合,其中一部分花岗岩形成于下地壳的深度,另一部分花岗岩形成于中上地壳,而源区的幔源物质可能为早期侵位于地壳中的中基性岩体。结合北秦岭早古生代地壳演化背景,认为五垛山岩体花岗岩反映了早志留世北秦岭地体在后碰撞环境下加厚地壳的垮塌过程。

The Wuduoshan pluton,located in the eastern segment of the North Qinling terrane,is a large-scale composite granitic intrusion formed via multi-stage emplacement.Zircon U-Pb dating yields crystallization ages of 441-430 Ma for representative samples of biotite granite and monzogranite,and inherited zircons have been identified in some samples with ages ranged from Proterozoic to Neoproterozoic.Granites from the Wuduoshan pluton exhibit high silica,low magnesium,alkaline-rich,and weakly to strongly peraluminous signatures.On the primitive mantle-normalized trace element spidergrams,they show enrichment of Rb,Ba,K and Pb and depletion of Nb,Ta,P and Ti.Chondrite-normalized REE distribution patterns exhibit distinct fractionation between LREE and HREE,and withδEu values of 0.36-1.45.87Sr/86Sr(i)andεNd(t)values for granites from the Wuduoshan pluton are 0.70304-0.71290 and-4.6--1.9 respectively,with two-stage Nd model ages of 1.34-1.58 Ga.These granites were likely formed via a partial melting process by using REE discrimination diagram.Isotopic compositions indicate that they were originated from a source characterized by mixing of metasedimentary rocks from the North Qinling terrane and mantle-derived materials.Among them,one part of granites were formed in lower crust,and another part were generated in middle-upper crust.The mantle-derived materials in source region may be intermediate-basic plutons that intruded into the crust during an early stage. Combined with crustal evolution of the North Qinlingterrane during Early Paleozoic, we suggest that the granites from the Wuduoshan pluton likely reflect the collapse process of thethickened North Qinling crust under a post-collisional setting during Early Silurian.