大别造山带内部变沉积岩锆石LA-ICP MS U-Pb定年及其大地构造意义

LA-ICP MS Zircon U-Pb Dating of Metasedimentary Rocks in Dabie Orogenic Belt and Its Tectonic Implications

大别造山带内部零星出露有浅变质岩,其与造山带其他单元内高级变质岩的共生状态可以为造山带的演化过程提供信息。北大别单元东北缘沈桥地区出露有浅变质岩。为了获取该浅变质岩的形成时间、物源信息及与北大别单元岩石之间的接触关系,在详细的野外观察基础上,对出露于沈桥地区的浅变质岩进行了详细的显微鉴定和碎屑锆石LA-ICP MS U-Pb定年。研究发现,沈桥地区浅变质岩具有与佛子岭群诸佛庵岩组一致的岩石学特征,碎屑锆石的最小年龄为431 Ma,年龄频谱特征与佛子岭群变沉积岩一致,表明沈桥地区浅变质岩为佛子岭群的一部分。显微鉴定表明,沈桥变沉积岩中石英动态重结晶型式均表现为完全的亚颗粒旋转(SGR)重结晶,黑云母具有明显的生长边,表明在其形成过程中曾经历了绿帘—角闪岩相变质—变形事件;变沉积岩在露头上显示的韵律层在沉积岩原岩形成时就已经存在,之后经过浅变质作用,岩石中的矿物相发生了改变:原岩中的泥质层变质为颜色较深的层状黑云母,砂质层变质为颜色较浅的动态重结晶石英。

Low-grade metamorphic rocks concentrated in the Beihuaiyang unit and were sporadically in other units of the Dabie orogenic belt,whose coexistence with other rocks in the orogenic belt can provide evidence for the evolution of the Dabie orogenic belt. A small number of metasedimentary rocks crop out in the Shenqiao area,northeast of the North Dabie unit. In order to obtain the forming age and provenance of the low-grade metamorphic rocks and their relationship with the rocks in the North Dabie unit,detailed field observations,systematic microscopic identification and detrital zircon LA-ICP MS U-Pb dating were presented for metasedimentary rocks in the Shenqiao area. The results show that metasedimentary rocks in the Shenqiao area have similar petrological characteristics as low-grade metamorphic rocks of the Zhufoan Formation in the Foziling Group. The minimum age of detrital zircon is 431 Ma,which is also similar as the Foziling Group. All these reveal that these low-grade metamorphic rocks in the Shenqiao area may be a part of the Foziling Group. Detailed observations on thin-sections show that quartz are characterized by Subgrain Rotation（ SGR） recrystallization and biotite has obvious growth rims,which indicates that the metasedimentary rocks experience an epidote-amphibolite facies metamorphic and deformation event. The rhythmic layers on the outcrop existed before experiencing low-grade metamorphism,during which the argillaceous layer in the protolith transferred to the darker layered biotite and the sandy layer to lighter and recrystallized quartz.