Hohhot thermal upwelling-extensional structure: Recognition of "metamorphic core complexes" in North China

The North China Craton (NCC) underwent significant intracontinental tectonic shortening and crustal thickening in the Mid-Late Jurassic, followed by extensive magmatism, extensional dome generation, graben basins, and large-scale normal faulting in the Early Cretaceous. In the past 30 years, these extensional domes within the NCC have been interpreted as metamorphic core complexes (MCCs) in the North American Cordilleran. However, their structure, deformation mechanism and deep processes are evidently distinct from those in the Basin and Rang Province of North America. In this study, we investigated the Hohhot granite dome in the northern NCC and clarified the spatio-temporal relationship between the granitic core, ductile shear zone, and low-angle detachment system through geological mapping, structural analysis, and geochronology studies to unveil a thermal upwelling-extensional structure. The Hohhot granite dome is situated within the Daqingshan Jurassic orogen located in the eastern Yinshan tectonic belt. It is composed of a Late Jurassic (ca. 152 Ma) adakitic coarse-grained monzonitic granite (K1 eta gamma(3)(5)), an Early Cretaceous medium-grained monzonitic granite (K-1 eta gamma(3)(5)) (ca. 136 Ma), and an Early Cretaceous fine-grained granite (K-1 gamma(3)(5)) (ca. 127 Ma), distributed concentrically from outer to inner rings, respectively. The footwall granitic complex and metamorphic basement are separated from the hanging wall Early Cretaceous graben basin by an ENE-trending low-angle detachment faults on the southeastern foothill. Our structural mapping reveals a dome-shaped ductile shear zone (S-1) with variable foliations around the Early Cretaceous granite core and regionally constant NW-SE lineations, leading to transformed the Late Jurassic adakitic granite and metamorphic basement into mylonite. Kinematic indicators, including S-C fabrics and sigma-type feldspar porphyroclasts, suggest a top-to-the-SE shear. The results of kinematic vorticity indicate that the S-1 is characterized by simple-shear-dominated shear. In addition, zircon U-Pb ages of granitic mylonite and post-kinematic pegmatite dike constrain the deformation time to 152-141 Ma, corresponding to the time of regional Mid-Late Jurassic intracontinental orogeny in the NCC. Owing to the Early Cretaceous magma diapir, variable foliations and constant NW-SE lineations indicate that S-1 was transformed into a dome-type from initial possibly sub-horizontal type. Consequently, we separated the shear zone from the extensional system and proposed the Hohhot thermal upwelling-extensional structure. Our results support to divide the Hohhot extensional dome into three structural units: (1) A 136-127 Ma granite core: Composed of Early Cretaceous A-type granite; (2) a pure-shear-dominated shear zone (S-2), with high temperature (>500 degrees C) deformation mainly distributed in the granite core periphery and obvious foliation (no lineation), which may be caused by the volume expansion of the magma during emplacement; (3) a 134-130 Ma low-angle detachment system characterized by a low-angle detachment separating the footwall top-to-the-SE ductile structures and microbreccias zone from the hanging-wall Cretaceous graben basin. The quartz lattice preferred orientations (LPO) of extensional shear zone show Prism slip near the core granite and basal slip away from the core granite, indicating high temperature (>500 degrees C) deformation and low temperature (ca. 300 degrees C) deformation, respectively. We assume that the evolution and deformation behavior of the detachment system are controlled by the emplacement of the core granite. We highlight the role of magmatic diapir and thermal metamorphism in generating the Hohhot thermal upwelling-extensional structure, which sheds new insights into the crustal extension in the NCC.