| The study of rheological behavior of continental lithosphere has become a frontier research topic in international geodynamic research,and the heterogeneity of rheological behavior is the main factor that causes the research difficulty.The deep crust of the orogenic belt provides a comprehensive record of continental rheology,rheological processes and tectonic dynamic,which is the core content of the rheological behavior study of continental lithosphere.The Paleoproterozoic orogenic belt in the northern part of the North China,known as the Khondalitic Belt,is characterized by extensive exposure of high grade metamorphic-anatectic rocks and the preservation of various rheological structural patterns derived from the deeper layers of the continental crust.Hence,it is a natural laboratory for us to explore the rheological characteristics of the continental lower crust and further explain the processes of crustal deformation and crust-mantle material exchange.This paper plans to carry out detail macro-and microstructural deformation characteristics,whole-rock geochemistry,zircon U-Pb chronology and EBSD fabric analysis of the widely developed migmatites in the Zhuozi-Liangcheng area,and combing the previous research data,we foucs on rocks types,partial melting and rheological features.On this basis,a rheological framework of the Paleoproterozoic orogenic belt of North China Craton was preliminarily established.The results show that the migmatites in Liangcheng-Zhuozi area exhibit different rock assemblages and macro-microstructural characteristics.In the Liangcheng area,the exposed rocks mainly consist of porphyritic charnockite,sillimanite garnet-gneiss and leucogranites,which display accumulation and shear structures,respectively.The rocks in the Dayushu area primarily comprise gneiss granitic granite and a small amount of leptytite,exhibiting a " schollen "-schlieren-homogeneous structure.Additionally,the exposed rocks in Zhuozi area are mainly leptytite,sillimanite garnet-gneiss,accomplied by a few garnet granites,with stromatic structures on the macro level.These characteristics indicate a transition from the accumulation series to diatexite series and then to the metatexite series,reflecting the deep to shallow crustal level of the orogenic belt.In the accumulated series,the macro-microstructure and deformation characteristics of porphyritic charnockite indicate a complex series of processes involving crystal accumulation and melt extrusion discharge before solidification.In local melt-rich zones,the minerals develop excellent directional mobility and act as strain concentrations to record strong deformation during the final melt migration.In this complex process,the minerals undergo a multi-stage transition in deformation mechanisms,including granular flow,grain boundary sliding,and dislocation creep.In the syn-melt shear zone(high strain zone),the sillimanite garnet-gneiss and leucogranites show significant shear deformation feature.The presence of numerous melt-pseudocone structures and the absence of mylonite microstructure(dynamic recrystallization)suggest that the high strain zone formed during the synchronous-anatexis period.The fine-grained felsic minerals in the leucogranites represent the micro-network channels formed by late melt pouring along the previously accumulated mineral crevices under the influence of synchrony-anatexis shear,representing the products of crystallization of different batch magma.Zircon U-Pb chronology results indicate that the collision of the Paleoproterozoic orogenic belt in the North China Craton likely occurred earlier than 1.95 Ga.Moreover,～1.95 Ga marks the onset of large-scale partial melting during the post-orogenic extension collapse.The orogenic crust formed a bottom-up layered structure through melt/solid segregation,magma mobilitiy and solid settling.The wide age span recorded in the migmatite was due to continuous heating of the lower crust of the orogenic belt and continuous or intermittent growth of zircons in a slow cooling environment from high to ultra-high temperatures.As a result of the asthenosphere(mantle source)material cushion(upsurge),this further heated the ’dry’ lower crust and led to ultra-high temperature metamorphism.During subsequent uplift process,a strong shear strain zone formed at the contact point between the accumulated series to diatexite series.This facilitated the drainage of melt and formation of the solid framework of the rock,resulting in ductile shear deformation.In the Paleoproterozoic orogenic belt,the deformation mechanism of rocks undergoes changes due to variations in melt content under conditions of anatexis within a tectonic regime characterized by horizontal crustal laminar flow.As the crustal level deepens,the degree of amatexis increases,resulting in a gradual increase in melt content.Consequently,the deformation mechanism shifts from dislocation creep to grain boundary sliding and eventually to granular flow.During the subsequent cooling and crystallization process,the melt content transitions from a state of magma,where crystals are suspended in the melt,to a crystal skeleton that provides support,and ultimately to a state of melt disconnection.Simultaneously,the deformation mechanism transforms from granular flow to grain boundary sliding,and eventually to dislocation creep.These changes in deformation mechanism play a crucial role in controlling macro-structural styles and the evolution processes of orogenic belts. |
