东准噶尔北缘乌伦古地区晚二叠世A型花岗质岩墙成因及构造背景

Petrogenesis and tectonic setting of the Late Permian A-type granitic dyke swarm in Ulungur,East Junggar

中亚造山带西南缘的东准噶尔地区出露大量晚古生代花岗岩,是揭示该造山带地壳演化的良好对象。在东准噶尔北缘乌伦古西北的阿克吉拉识别出一套走向与区域构造线总体垂直的晚古生代花岗质岩墙,其时代和成因研究对深入理解本区的构造演化具有重要意义。SHRIMP锆石U-Pb测年结果显示其侵位于晚二叠世(266±2 Ma)。岩石地球化学组成上,它们具有高硅(SiO2=75.66%~76.69%)、富碱(Na2O+K2O=8.67%~9.16%)、低钙(CaO=0.06%~0.14%)和镁(MgO=0.04%~0.06%)的特征;具有明显的负Eu异常(δEu=0.16~0.18)相对亏损Ba、Sr、P、Ti等,明显富集Nb、Zr、Th、Ta、Hf等高场强元素具有较大的Ga/Al值(>2.6),指示该花岗岩属弱过铝质A1型花岗岩。岩石具有正的εNd(t)值(+4.7)和年轻的模式年龄(655 Ma),暗示源区物质主要为年轻幔源物质。综合分析,推测该岩墙形成于后碰撞环境,由地幔上涌引起新生下地壳部分熔融而成。

A large number of Late Paleozoic granites occur in the East Junggar on the southwest margin of the Central Asian Orogenic belt,providing a good chance to reveal the crustal evolution of the Central Asian Orogenic Belt.In this study,a series of Late Paleozoic granitic dykes striking vertically to the suture zone were recognized in Akejila,northwest of Ulungur of East Junggar.Their timing of emplacement and petrogenesis are critical to understand the tectono-crustal evolution of the study area.SHRIMP zircon U-Pb dating yielded a Late Permian age(266±2 Ma).Geochemical analyses indicate that they are characterized by relatively high concentration of silica(SiO2=75.66%~76.69%) and alkali(Na2 O+K2 O=8.67%~9.16%),but low calcium(CaO=0.06%~0.14%) and magnesia(MgO=0.04%~0.06%) content.The obvious negative Eu anomaly(8 Eu=0.16~0.18),relative depletion of Ba,Sr,P and Ti,and significant enrichment of Nb,Zr,Th,Ta and Hf,as well as high Ga/Al value(>2.6) characterize the weakly peraluminous A1 type granites.The positive εNd(t) value(+4.7) and a young model age(655 Ma) suggest that these dykes are mainly derived from the juvenile mantle.Therefore,the comprehensive analysis indicates that the dykes were formed by the partial melting of the juvenile lower crust caused by the upwelling of the mantle in a post-collision setting.