秦岭商丹带晚中生代同构造花岗岩脉的年代学及其构造意义

Geochronology of Late Mesozoic syn-tectonic granite veins in the Shangdan tectonic belt of the Qinling Orogen and its tectonic implications

秦岭造山带于晚中生代发生了强烈的陆内造山作用,出露大量晚中生代花岗岩。关于这些花岗岩形成的构造环境及动力学背景还存在争议,限制了我们对秦岭陆内造山作用以及陆内构造变形时间的进一步认识。商丹构造带中同构造花岗岩脉的年龄可有效限定陆内构造变形时间。针对该花岗岩脉我们开展了锆石U-Pb年代学以及Hf同位素研究。镜下观察显示花岗岩脉具有明显的糜棱岩化现象,矿物定向明显且具有中-高温固态变形特征,如石英颗粒的颗粒边界迁移或亚颗粒旋转重结晶以及长石边部出现的蠕英结构。CL图像揭示锆石阴极发光强度较弱,内部为海绵状结构。同时,锆石Th/U比值低,集中于0.03~0.07之间,重稀土元素富集。结合(Sm/La)N-La和Ce/Ce^(*)-(Sm/La)_(N)锆石类型判别图解,我们可判断锆石形成于岩浆与热液的过渡阶段,为流体交代改造型锆石。锆石U-Pb定年结果显示花岗岩脉热液锆石形成于130~124 Ma,代表一期重要的构造热事件。花岗岩脉ε_(Hf)(t)值变化范围为-9.47~-0.99,对应的二阶段模式年龄为1752~1218 Ma,可能为秦岭群和宽坪群岩石部分熔融产物。结合前人已有成果,我们认为秦岭造山带燕山期陆内构造变形至少存在两幕,而沙沟街韧性剪切带内同构造花岗岩脉年龄对应了第二幕(150~120 Ma)陆内构造变形。

Intense intracontinental orogeny occurred in the Qinling Orogen during Late Mesozoic,and the ages of syn-tectonic granite veins founded in the Shangdan tectonic belt can effectively define the time of intracontinental tectonic deformation.In this paper,zircon U-Pb dating and Hf isotope analysis were conducted on the granite veins.The microstructural observations suggest that the granite veins are characterized by mylonitization,preferred orientation of minerals and medium to high-temperature solid-state deformations,such as the dynamic recrystallization of quartz by grain boundary migration and subgrain rotation,and myrmekite growth textures at the edge of feldspar.CL images reveal that the zircons are marked by spongeous texture with weak cathodoluminescence.Meanwhile,the zircons have lower Th/U ratios in the range of 0.03 to 0.07 and are enriched in heavy rare-elements.According to the discriminant diagrams of(Sm/La)N-La and Ce/Ce^(*)-(Sm/La)_(N),it can be concluded that the zircons of syn-tectonic granite veins were formed in the transition stage from magmatic to hydrothermal process,belonging to the hydrothermal fluid metasomatic alteration type zircon.Zircon U-Pb dating yields an age of 130~124 Ma for the hydrothermal zircons,representing an important tectonic thermal event.Furthermore,the zircons of granite veins have concentrated negativeε_(Hf)(t)values of-9.47~-0.99 and Hf two-stage model ages of 1752~1218 Ma,probably resulting from the partial melting of the Qinling Group and the Kuanping Group.Combined with previous works,this study reveals that the Yanshanian intracontinental orogeny in Qinling Orogen is characterized by two episodic tectonic deformations,and the age of the syn-tectonic granite veins in Shagoujie ductile shear zone corresponds to the second episode of the intracontinental tectonic deformation.