Structural Characteristics And Regional Tectonic Significance Of The Liaoji Granite

The Liaoji Paleoproterozoic Orogenic Belt is located in the East of the North China Craton,its main composition includes the Liaohe Group and the Liaoji Granite.The Liaohe Group includes the North Liaohe Group in the northern part and the South Liaohe Group in the southern part.Because there are still a lot of controversies on the genesis and tectonic environment of the Liaoji Granite and its relationship with the Liaohe Group,it is very important for understanding the geotectonic attribute and orogenic evolution to find out the structural characteristics of the Liaoji Granite.In this paper,we conducted comprehensive studies on the petrology,petrography,geochronology,tectonic deformation and metamorphism based on the Simenzi Liaoji Granite and its surrounding rocks in the west of the orogen.There is a typical dome structure in the Simenzi area,in which the core is a typical granite complex and the mantle and the shell is the South Liaohe Group.The striation structure of the Simenzi Liaoji Granite is subdivided into three types from its mineral components,magnetite-type,hornblende-type and biotite-type.From the center area to the margin of the Simenzi Liaoji Granite the magmatic fabric is gradually transformed into submagmatic fabric and plastic deformation fabric.The contact zone between the Simenzi Liaoji Granite and its surrounding rocks consists of a ductile shear zone with strong plastic deformation.The contact zone is composed of gneiss,migmatite and mylonite.The main deformation characteristics were the weak deformation,"overall ordered and locally disordered",flow folds and the secondary ductile shear zone in the South Liaohe Group.All the rocks from the core,mantle and shell zone of the Simenzi Granite Gneiss Dome have identical attitude and sense of shearing.Zircon U-Pb dating shows that the crystallization age of the Simenzi Liaoji Granite is2164～2132Ma,while a syntectonic dike from the coeval rocks is dated as 1910～1855Ma.Combining present and previous geochronological data studies we established chronological framework of tectonic evolution of Liaoji Paleoproterozoic Orogenic Belt,i.e.a pre-orogenic stage(2.25～2.10Ga),an magmatic interval stage(2.10～1.94Ga),a syn-orogenic stage(1.94～ 1.86 Ga)and a post-orogenic stage(< 1.86～ 1.84 Ga).It is considered that the Paleoproterozoic geological event sequence and the basic tectonic attribute of the event are resulted from the effect of deep crust-mantle coupling at different structure levels,the result from the Liaoji orogenic belt is consistent with those of a hot orogen.The formation of the Simenzi granite gneiss dome provides effective ways exploring the genesis of a hot orogen.We believe that the gneiss dome is a typical structural style of the Hot Orogen.The above studies provide important clues to reveal the regional tectonic environment and tectonic evolution for the Liaoji Paleoproterozoic Orogenic Belt,and to understand the deep crust flow.