The Wulian metamorphic core complex: A newly discovered metamorphic core complex along the Sulu orogenic belt, eastern China

Abstract

Combined with field studies, microscopic observations, and EBSD fabric analysis, we defined a possible Early Cretaceous metamorphic core complex (MCC) in the Wulian (五莲) area along the Sulu (苏鲁) orogenic belt in eastern China. The MCC is of typical Cordilleran type with five elements: (1) a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rocks with (2) middle crust metamorphic rocks, (3) syn-kinematic plutons, (4) an upper plate of weakly deformed Proterozoic metamorphic rocks, and (5) Cretaceous volcanic-sedimentary rocks in the supradetachment basin. Some postkinematic incursions cut across the master detachment fault zone and two plates. In the upper plate, Zhucheng (诸城) Basin basement consists of the Proterozoic Fenzishan (粉子山) Group, Jinning period granite (762–834 Ma). The supradetachment basin above the Proterozoic rocks is filled with the Early Cretaceous Laiyang (莱阳) (∼135–125 Ma) and Qingshan (青山) groups (120–105 Ma), as well as the Late Cretaceous Wangshi (王氏) Group (85–65 Ma). The detachment fault zone is developed at the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss downward. Microstructure and quartz preferred orientation show that the mylonites formed at high greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian MCC are chiefly UHP (ultrahigh pressure) metamorphic rocks. Syntectonic rocks developed simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of North China Craton and adjacent area.