Does the Liaoji granite represent basement in the Jiao–Liao–Ji Belt?

Abstract

Lack of knowledge of the basement structure of the Paleoproterozoic Jiao–Liao–Ji Belt (JLJB) of the North China Craton has hindered understanding of the tectonic evolution of the belt and, to an extent, the entire craton. Previous studies of the JLJB were based mostly on petrological, structural, and geochemical analyses, resulting in a lack of understanding of the deep geological structure of the belt. In this study, we used a combination of geophysical and geological methods to interpret the deep geological structure of the JLJB on the basis of high-precision non-seismic geophysical data. We collected high-precision gravity, magnetic, and magnetotelluric sounding data. We also compiled data on rock physical parameters for important geological bodies, and obtained high-precision Bouguer gravity anomaly data from the China Geological Survey. We interpreted these data to visualize the subsurface distributions of geological bodies (including sedimentary and metamorphic units and intrusive rocks) in the Kuandian–Huairen area of the JLJB. We found that the Liaoji granites are developed on a large scale in the deep part of the JLJB, showing a weak gravity anomaly (−25 to −6 mGal) and high resistivity (103.5 to 106 Ω·m). These granites may have intruded along a series of normal faults at the time of the initial formation of the belt, and expanded over time. Subsequently, the low-resistivity Liaohe Group (10 to 102.5 Ω·m) was deposited on the Liaoji granite across a sedimentary contact. Therefore, the Liaoji granite forms the basement of the JLJB. The presence of extensional faults in the deep part of the JLJB suggests that the belt was also subjected to tectonic extension. The Yuejiadian–Xiaohuanggou fault to the east of Kuandian may be the boundary between the South Liaohe Group and the North Liaohe Group. The South Liaohe Group is relatively thin and tightly folded, whereas the North Liaohe Group is thicker and characterized by relatively open folds.