Deformation Characteristics And Geochronology Of Migmatite In The North Dabie Unit Of The Dabie Orogenic Belt

The Dabie orogenic belt (DOB), located in east China, is a typical collision orogenic belt, formed in the process of collision between the South China Block and the North China Block. Large-scale post-orogenic magmatic activity and migmatization happened in the DOB, now migmatites mainly outcrop in the North Dabie unit. Previous dating works of migmatite showed migmatization occurred in the Early Cretaceous. However, some dating of migmatites didn’t get the Early Cretaceous age. Migmatite shows obvious deformation characteristics in the North Dabie unit, but whether the deformation and migmatization occured simultaneously still don’t have enough evidence. Obtainning deformation time of migmatites is very important for establishing the mechanism of crust thinning in post-orogenic stage.This work take detailed field observation for migmatites outcrop in the North Dabie unit divided into three regions according to differences of migmatite deformation:north of Tiantangzhai granites, south of Tiantangzhai granites and Yuexi region. Migmatites show banded distribution, strongly crumpled deformation and mainly banded with partitial crumpled deformation respectively in these three regions. Detailed observation under the microscope and Electron Backscattered Diffraction (EBSD) analysis show that although the migmatites display different deformation in outcrop scale, but deformation characterics are homologous in microscopic scale, which suggests all migmatites experienced the same deformation process. Except the strong crumpled deformation migmatites, all migmatites exhibit top-to-NW shear sense with NW-SE mineral stretching lineation. Microscopic observation show migmatites experienced two-stage deformation:higher temperature (> 600 ℃) deformation in the early stage and lower temperature (about 400 ℃) deformation in the late stage.Zircon LA-ICP MS U-Pb dating were carried out for five migmatites with different deformation strength. The width of rim of zircons are wider than the diameter of the laser spot from weak deformed migmatite D9 and D11 and get weighted average age of 131.3±1.8 Ma and 131.6±1.3 Ma respectively. The width of rim of zircons are lesser than the laser spot from crumpled deformed migmatite D21 and D49 and can’t get the Early Cretaceous age. A few of rim of zircons are wider than the laser spot from strong deformed migmatite D14 and get a weighted average age of 123.9±4.2 Ma. It is a conclusion can be drawn that strong deformation restrain the growth of the zircon and all migmatites experienced the same deformation event. Both the wide zircon rim from weak deformed migmatite and the narrow rim from strong deformed migmatite formed in the same event, so the zircon rim of weak deformed migmatite recorded the time of migmatite deformation. This work shows that the migmatite deformation occurred at about 131 Ma in the North Dabie unit.