晓天-磨子潭剪切带的早白垩世活动及其对北大别穹隆演化的制约

晓天-磨子潭剪切带是大别造山带内北大别杂岩带与北淮阳浅变质岩带的地表分界线,其构造演化特征可以为北大别穹窿的形成演化提供重要制约.本次由此韧性剪切带内4个未变形岩体的锆石样品进行LA-ICP MS U-Pb年龄测定得知,该韧性剪切带形成于早白垩世130～120 Ma期间,为后造山伸展断层.构造分析表明,这条剪切带形成于北大别穹窿深处,为一条平缓韧性拆离剪切带.其形成指示北大别单元在早白垩世初的造山带重力垮塌中发生了深部地壳物质的韧性侧向流动,而该剪切带则是地壳物质韧性侧向流动的上部运动边界.在随后的大规模岩浆上升与浮力增加中,该剪切带与北大别穹窿一起发生隆升而变陡,并随着不均匀隆升而使剪切带出现现今的几何学特征.晓天-磨子潭剪切带的演化显示北大别穹窿是由大别造山带晚造山伸展过程中形成的变质核杂岩组成的.

Motion of the Xiaotian-Mozitan shear zone in Early Cretaceous and its constraints for evolution of North Dabie dome

As the northern boundary of the North Dabie complex unit, the motion of the Xiaotian-Mozitan shear zone (XMSZ) in Early Cretaceous can be used to understand evolution of the North Dabie dome. For obtaining deformation time of the XMSZ, this paper used LA-ICP MS zircon U-Pb geochronology to study 4 undeformed intrusions from Xiaotian-Mozitan ductile shear zone based on detailed field survey. Detailed geochronological data show that these intrusions formed between 130-120 Ma. Therefore, a conclusion can be drawn that the ductile shear zone in XMSZ formed in the earliest Early Cretaceous associating with structural evidences. The XMSZ initiated as the same flat-lying ductile detachment shear zone at a deeper crustal level. Development of the flat-lying detachment shear zone was associated with lower crustal lateral flow under gravitational collapse in the earliest Early Cretaceous, and the detachment shear zone represents upper boundary of the lower crustal flow. The following large-scale magmatic intrusion and resultant buoyancy increase led to exhumation of the North Dabie dome and warping of the detachment shear zone. The present geometry of the detachment shear zones was caused by the inhomogeneous exhumation of the dome. The evolution of the XMSZ demonstrates that the North Dabie dome is a metamorphic core complex developed in the Dabie orogenic belt during post-orogenic extension.