New 40Ar-39Ar age constraints on the deformation along the Machaoying fault zone: Implications for Early Cambrian tectonism in the North China Craton

The Machaoying fault zone extends along the southern margin of the North China Craton (NCC) andcontrolled the regional structures and hydrothermal mineral systems in this area. The fault underwent atleast two major deformational phases, as revealed by macro- and micro-structural observations from a welldevelopedsegment of the fault in the Hongzhuang–Baitu area, located south of the Xiong'er Mountains. Earlyductile deformation is characterized by thrusting from north to south, which was subsequently overprintedby late brittle faulting. Syntectonic strain shadows of biotite are preserved around rotated porphyroclasts ofquartz amygdales in mylonite. The biotite yields a 40Ar–39Ar plateau age of 524.9±1.9 Ma, which isinterpreted as the time of regional thrusting along the Machaoying fault zone. The thrusting may betemporally correlated with an Early Cambrian discontinuity in sedimentation observed in the rockssequences of the NCC, suggesting a compressional regime in this area and a craton-wide tectonic event. Many540–500 Ma tectonic events have been previously identified in the Qinling–Qilian–Kunlun Orogenic Belt ofcentral China and in massifs in northeastern China, both of which surround the NCC, and some of these wereinterpreted to be associated with assembly of Gondwana. However, paleomagnetic data indicate that the NCCwas unlikely to have been connected with Gondwana in the Early Cambrian and thus our new biotite datecannot record deformation along the Gondwanan margin. Dating of K-feldspar from a quartz–K-feldspar veinformed along one of the brittle faults of the Machaoying fault zone yields a much younger 40Ar–39Ar plateauage of 119.5±0.7 Ma. This is a minimum age for the brittle deformation along the southern margin of theNCC, which also overlaps the age of widespread gold and molybdenum mineralization in the region.© 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.