The initial time of the Fen–Wei graben system: Constraints from geochronology of the Qifeng granite porphyry dikes in the Zhongtiaoshan Mountains

Abstract

The Fen–Wei graben system is an essential Cenozoic extensional fault–depression belt in the central part of the North China craton. Previous research achievements mainly focused on its formation process and dynamic mechanism, but the initial time has been unresolved due to the lack of quantitative dating objects. This paper first reports the granite porphyry dikes in the andesite of the Majiahe formation in the Xionger group of the Zhongtiaoshan orogenic belt adjacent to the Fen–Wei graben system. The dating results of the granite porphyry dikes indicate that the age sequence concentrates mainly in two stages, 1769.8 ± 8.7 Ma and 69.14 ± 0.85 Ma. The former is consistent with the primary age of the volcanic rocks of the Xionger group, representing the characteristics of inherited zircons, and the latter represents the formation age of the granite porphyry dikes. The Ga/Al value ratios of granite porphyry dikes of the Qifeng samples are all greater than 2.6, characterized by enriched silicon, alkali, potassium, depleted calcium, and high magnesium. The rare earth elements are enriched in light and depleted in heavy rare earth elements. The Qifeng granite porphyrite dikes are I-type granites, indicating that the tectonic setting is extensional. The research provides new evidence for the initial time of the southern part of Fenwei graben in the Cenozoic.

Keywords

• zircon u–pb geochronology
• geochemistry
• granite porphyry dikes
• zhongtiaoshan mountains
• fen–wei graben system