Evolution and time of formation of the Hohhot metamorphic core complex, North China: new structural and geochronologic evidence

摘要

The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructurcs of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous ~(40)Ar/~(39)Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directcd, early tangential, high-temperature ductile shearing probably was initiated during ca. 148-140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major cxtensional ductile shearing along the detachment zones took place during ca. 140-132 Ma. Using these new U-Pb ages, combined with previously published ~(40)Ar/~(39) cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).