Deformation mechanisms of high–grade metamorphic rocks at middle–lower crustal levels: A case study of Jingjiagou Formation in Qingyuan Area, NE China

Abstract

The early continental lithosphere of the Earth has experienced complex tectonic-magmatic processes, leading to significant change of its thermal structure and rheological properties, which are highly heterogeneous. The Qingyuan area in northern Liaoning Province in NE China is a natural laboratory for studying the rheological properties and deformation mechanisms of high–grade metamorphic rocks in the middle and lower crust, characterized by widespread amphibolite–granulite facies metamorphic rocks and associated structural phenomena and varying rheology. This study presents detailed investigations of the middle–lower crustal rocks of Jingjiagou Formation in the Qingyuan area, including field and microstructures, EBSD (Electron Back Scatter Diffraction) analysis, zircon U–Pb dating and electron microprobe analyses. The peak P–T conditions are constrained at 1 ∼ 1.25 GPa/910 to 1050 °C for garnet pyroxenite, 0.8 ∼ 1.1 GPa/800 to 870 °C for pyrigarnite, and 0.85 ∼ 1.0 GPa/770 to 800 °C for amphibolite. Crystallographic preferred orientation statistics of pyroxene, amphibole and plagioclase indicate that deformation in the amphibolite–granulite facies rocks was dominated by dislocation creep of clinopyroxene and orthopyroxene during the peak metamorphic stage, with (110)[001]/(010)[001] and (100)[001] slip systems, respectively. During retrogression, amphibole deformation shifted to diffusion creep along (100)[001] slip system, with plagioclase also developing diffusion creep. At the Neoarchean/Paleoproterozoic boundary (2494 ± 12 Ma), these deformation mechanisms reflect the flow deformation characteristics of high-grade metamorphic rocks in the Jingjiagou Fm. under the influence of vertical tectonics. Supracrustal rocks subducted into TTG magmas and were transformed within amphibolite-granulite facies conditions, with deformation driven by dislocation creep. As TTG magma ascended and temperatures and pressures dropped, rock deformation transitioned to diffusion creep at various crustal levels. This shift in mineral deformation mechanisms influenced the macroscopic structural patterns and associated deformation processes in the Jingjiagou Fm. of the Qingyuan area, NE China.