Structural characteristics and geochronology of migmatites in the North Dabie Complex unit: Timing of post-collisional deformation

Abstract

Extensive migmatization and large-scale post-collisional intrusions occurred in the Dabie orogen, east-central China, during the Early Cretaceous, characterized by distinct deformation preserved in migmatites in the North Dabie Complex unit. The North Dabie Complex unit can be subdivided into three areas based on detailed field observations: the north of the Tiantangzhai pluton, the Luotian area and the Yuexi area. Banded migmatites crop out in the north of the Tiantangzhai pluton while anisotropically deformed migmatites occur in the Luotian area, and both types coexist in the Yuexi area. Microscopy reveals similar micro-structures are in migmatites from the north of the Tiantangzhai pluton, the Yuexi area and border of the Luotian area, while static recrystallization appears in migmatites from the core of the Luotian area. The Lattice-Preferred Orientation of dynamically recrystallized quartz grains in the migmatites are measured using electron backscattered diffraction, revealing prism slip or slip in migmatite from the north of the Tiantangzhai pluton and the Yuexi area and in one sample from the Luotian area. A Type I crossed girdle is developed in another sample from the Luotian area, indicating top-to-the-SE shearing that developed under greenschist facies conditions. Zircon U-Pb dates from four migmatites reveal that mainly Early Cretaceous ages are from the north of the Tiantangzhai pluton, only four Early Cretaceous ages are from the Luotian area, and all zircons from the Yuexi area record Indosinian metamorphic ages. Although zircon U-Pb results show multiple migmatization events, all samples record a migmatization age of about 132 Ma, suggesting an orogen-scale event at that time. The subsolidus deformation in migmatites indicates that deformation were soon after migmatization. Overall, this study shows that deformation in migmatites of the North Dabie Complex unit occurred somewhat later than the migmatization (~132 Ma) at about 131 Ma. The most likely mechanism for thinning of the thickened crust in the Dabie orogen involved removal of the upper along the detachment fault and ductile adjustment of lower crust during development of the North Dabie metamorphic core complex.