Study On The Rheological Behaviors Of Anatectic Rocks From The Deep Crustal Level In The Daqingshan Area,Inner Mongolia

The Daqingshan-Wulashan areas in Inner Mongolia is a typical area of early Paleoproterozoic orogenic belt in the North China Craton,in where a large number of high grade metamorphic-anatectic rocks are widely exposed,and it preserved abundant rheological structural patterns derived from the deeper level of continental crust.It therefore becomes a natural laboratory for the study of rheological behavior and mechanism of partially molten rock.Previous studies suggest the rock structural patterns from the area are very complicate,and the metamorphic and plutonic units interspersed with each other in space.The rheological structures are mostly distributed in a planar manner on the macroscopic level,and the boundary is indistinct.This study plans to carry out detail macroscopic-microscopic deformation characteristics,EBSD fabric analysis and zircon U-Pb chronology of high-grade metamorphic-anatectic rocks from the Hujigou-Damio-Xuehaigou cross-section in the Daqingshan area,and combining the previous research data,we focus on the coupling relationships between the rock types,partial melting and rheological features,as well as the process of partial melting and later its structural response.The results presented here suggest that the deformation pattern is characterized by the deeper crustal level in the Xuehaigou area from the Daqingshan Paleoproterozoic orogenic belt,whereas,the relative shallower crustal deformation pattern is recorded from the Daomiao-Hujiigou area.The macroscopic deformation and microfabric characteristics indicate that rocks at different tectonic levels in the orogenic belt exhibit different structural rheological behaviors.Under advanced metamorphic-anatectic conditions at the deep crustal level(e.g.Xuehaigou area),macroscopically,various forms of light-colored veins are involved in rock rheology or close-range rheological migration,usually showing a network of intermittent discontinuities formed by melt flow entraining mineral particle flow strip structure,with L-structure rocks and A-type sheath folds developed.Microscopically,single minerals are affected by diffusion creep,grain boundary slip and particle flow(superplastic flow),instead of obvious deformation fabric.Therefore,there is no or very weak CPO(Crystallographic Preferred Orientation)development for the grains.In contrast,under relatively weak metamorphic-anatectic conditions in the middle-deep crustal level(e.g.Daomiao-Hujiigou area),the melt content is relatively lower due to the weak anatexis,and some stripe structures are formed macroscopically.A small amount of melt is connected and spread along the direction of the flow surface,which constitutes the melting lineation.Microscopically,distinct mineral(e.g.plagioclase,amphibole and pyroxene)usually suffers dislocation creep deformation and develops strong CPO.Significantly,the amphibole shows obvious dislocation creep characteristics at different structural levels,and the CPO is not closely affected by the anatexis.Comparatively speaking,in the same tectonic environment,it is easier for orthopyroxene to form obvious CPO than clinopyroxene.The above deformation patterns and mechanisms reflect the characteristics of horizontal layered viscoplastic flow deformation of high grade metamorphic-anatectic rock in the solid-liquid two-phase medium at the root of the orogenic belt.Subsequently,the plastic flow deformation of the anatectic rocks gradually hardens following the exhumation process during the extension and thinning of the thickened orogenic crust.