The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt(East-Central China)

摘要

The Xiaotian–Mozitan Shear Zone(XMSZ)is the boundary of the Dabie High-grade Metamorphic Complex(DHMC)and the North Huaiyang Tectonic Belt.It was deformed in ductile conditions with a top-to-NW/WNW movement.Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation,respectively,which indicates a retrograde process of evolution.A few groups of zircon U-Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone.Zircons from the felsic ultramylonites are all magmatic,producing a weighted mean 206Pb/238U age of 754±8.1 Ma,which indicates the time of magmatic activities caused by rifting in the Neoproterozoic.Zircons from the granitic veins,cutting into the mylonites,are also of magmatic origin,producing a weighted mean 206 Pb/238 U age of 130±2.5 Ma,which represents the time of regional magmatic activity in the Cretaceous.Zircons from the mylonitic gneisses are of anatectic-metamorphic origins and are characterized by a core-mantle interior texture,which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206 Pb/238 U age of 762±18 Ma,similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206Pb/238U age of 143±1.8 Ma,indicating the anatectic metamorphism in the Dabie Orogenic Belt(DOB).Based on integrated analysis of the structure,thermal conditions of ductile deformation and the contact relations of the dated rocks,the activation time of the Xiaotian–Mozitan Shear Zone is constrained between~143 Ma and 130 Ma,during which the DOB was undergoing a transition in tectonic regime from compression to extension.Therefore,the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process.This research also inclines us to the interpretation of it as an extensional detachment,with regard to the tectonic properties of the shear zone.It may also be part of a continental scale extension in the background of the North China Block’s cratonic destruction,dominated by the subduction and roll-back of the Paleo-Pacific plate,but more detailed work is needed in order to unravel its complicated development.