Deformation characteristics of the high-grade metamorphic and anatectic rocks in the Daqingshan Paleoproterozoic orogenic belt, Inner Mongolia: A case study from the Shijiaqu-Xuehaigou area

Abstract

The high-grade granulite-phase metamorphic and anatectic rocks are widespread in the Daqingshan area, which is an archetypical of the Early Paleoproterozoic Orogenic Belt (POB) in the North China Carton (NCC), providing an ideal object to delineate the rheology of partially molten lower crust. The multiple approaches of SEM-EBSD fabric analysis, electron probe microanalyses (EPMA) and LA-ICP-MS zircon U-Pb geochronology have been carried out on the metamorphic and anatectic rocks from the Shijiaqu-Xuehaigou area in the Daqingshan. The results of zircon U-Pb dating show that the anatexis process in the POB took place more or less prior to ca. 1920 Ma, subsequently, the main plastic deformation event occurred at ca. 1915–1870 Ma. The macro- and microscopic structures suggest that the Xuehaigou and Shijiaqu areas show the characteristics of the diatexite and metatexite zones, respectively. In the diatexite zone, appearance of abundant melts tends to form schollen and schlieren structures in the high-grade metamorphic and anatectic rocks. The deformation mechanism of plagioclase is mainly represented by diffusion creep and particle flow. In contrast, in the metatexite zone, the deformation pattern is represented by stromatic structure with light-colored bands paralleling to the main gneissosity, and the deformation mechanism of plagioclase is characterized by dislocation creep. Besides, the deformation mechanism of amphibole in the lower crustal rocks from both zones is mostly dominated by dislocation creep. Significantly, the presence of melts in the high-grade metamorphic and anatectic rocks probably weaken their strength, and may have aided to form the sub-horizontal channel, contributing significantly to the subsequent exhumation of the lower crustal rocks in the orogenic belt.