北秦岭西段岩湾加里东期碰撞型侵入体形成时代及地球化学特征

Dating and Geochemical Characteristics of the Yanwan Paleozoic Collisional Intrusion in the West Segment of Northern Qinling Mts

北秦岭造山带西段商丹断裂带北侧岩湾一带分布的含石榴石二长花岗岩以含富铝的石榴石、白云母等矿物为特征,属含白云母过铝质花岗岩类(MPG);SiO2含量为73.39%～74.4%,Na2O/K2O小于1,A/CNK(分子比)为1.07～1.18,里特曼指数(δ)为2.02～2.15,CIPW标准矿物中出现刚玉(C),具强过铝质钙碱性岩石系列的地球化学特征,为典型的"S"型花岗岩;稀土总量为121×10-6～151×10-6,轻重稀土比值为14.91～21,δEu为0.55～0.74,为δEu负异常明显的右倾型稀土分配模式;大离子亲石元素洋脊花岗岩标准化形式及各类构造环境判别图解均显示其形成于同碰撞环境,为大陆碰撞的地球动力学背景。岩体的LA-ICP-MS锆石U-Pb年龄为414.3±1.9Ma,属末志留世晚期,该侵入体应是早古生代晚期加里东运动碰撞造山过程的产物,其形成时代(414.3±1.9Ma)结合同一地区红花铺俯冲型侵入体出现相近的变质锆石的年龄(413.5±1.3Ma),较精确地约束了北秦岭西段加里东运动碰撞造山的时限。

The garnet-bearing monzonitic granite intrusions are distributed in the Yanwan area along the northern side of the Shangxian-Danfeng faults in the western segment of Northern Qinling Mts.They consist of Al-rich garnet and muscovite,and belong to muscovite-bearing per-aluminous granitic series with SiO2=73.39%～74.40%,Na2O/K2O〈1,A/CNK=1.07～1.18,and Rittman values（δ） ranging from 2.02 to 2.15.Moreover,corundum occurring in CIPW calculation geochemically indicates the Honghuapu intrusion is attributed to S-type granite.REE and trace elements study of the intrusion show the chondrite-normalized REE patterns are right-inclined with ∑REE=121×10-6～151×10-6,LREE/HREE=14.91～21,and negative δEu values（δEu=0.55～0.74）.The ORG-normalized large ion lithophile element patterns and the trace elements tectonic identification diagrams suggest that the intrusion formed at a collisional setting representing a continental collisional geodynamic framework.LA-ICPMS zircon U-Pb dating of the intrusion is 414.3±1.9Ma,indicating the Yanwan intrusions formed at the end period of Late-Silurian and are the products of Caledonian orogeny in the late Paleozoic.On the other hand,the age of 414.4±1.9Ma,together with the same metamorphic ages（413.5±1.3Ma） in the Honghuapu subduction intrusion using LA-ICPMS zircon U-Pb dating method,can accurately restrict the timing of Caledonian collisional orogeny in the western segment of Northern Qinling Mts..