Neoarchean tectonics: Insight from the Baijiafen ductile shear zone, eastern Anshan, Liaoning Province, NE China

Abstract

The North China Craton is one of the major Archean to Paleoproterozoic cratons in the world and oldest craton in China, which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics. The controversy over the Archean tectonic regimes has lasted several decades centering around horizontal and vertical tectonics, the two classical tectonic models for Archean times. Thus, more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. This study provides an example for revealing of Archean tectonics. The NWN-trending, ENE-dipping Baijiafen ductile shear zone is located in the eastern Anshan of the northeastern North China Craton and mainly comprises two types of gneisses, including the Chentaigou porphyritic granitic gneiss and the Baijiafen trondhjemitic gneiss. In this study, we have carried out a detailed study on the macrostructure, microstructure and fabric characteristics of the two main types of deformed gneisses within the shear zone. The ribbon structures formed by intensely elongated quartz grains are widespread in these gneisses. Well-developed mineral stretching lineations and asymmetric fabrics indicate an ESE-directed downward shearing. The quartz c-axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non-coaxial deformation with active rhomb 〈a〉 slip and basal 〈a〉 slip. Deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400–500°C under greenschist facies metamorphic conditions. Dislocation creep is the main rock deformation mechanism within the shear zone. Finite strain measurement results suggest that the strain types of the shear zone are generally related to elongate-plane deformation, and the tectonites change from L-S- to LS-type across the shear zone. The strain intensity increases obviously towards east, and the Baijiafen gneiss unit is located within the high-strain zone. Kinematic vorticity values of gneisses indicate that the deformed rocks were produced by steady-state simple-shear dominated general shear. Based on abundant structural evidence and previous studies, we infer that the deformation of the Baijiafen ductile shear zone may have resulted from the downward adjoining rock flow during the sagduction of the banded iron formations and synchronous Archean granite dome emplacement, supporting a vertical tectonic regime in Archean times.