Metamorphism And Deformation History Of The Basement Rocks From The Northeastern Alxa Block And Their Tectonic Implications

The Bayanwulashan-Langshan fault zone is a major fault zone trending NE in the northeastern margin of the Alxa Block and extends about500km. Long-term studies suggest that the large fault zone experienced complicated evolution history and recorded geodynamic processes of the Alxa Block and the North China Craton(NCC). The Precambrian metamorphic basement is widely exposed in the Alxa Block, and the structures from late Paleozoic are conserved well. However, though lots of international geologists paid more attention to the strike-slip history and origin of the Bayanwulashan-Langshan area, especially the relation between it and the NCC, but their viewpoints about that were different. Some workers considered the fault zone as a eastern part of the Alxa arc tectonic belt and was formed by the compression from the Central Asian Orogenic Belt. Some workers suggest that the fault zone is the northeast boundary fault and originated as a result of the collision between Alxa Block and the NCC in Devonian.This paper show detailed studies on the physical components, structural deformation characteristics, geometry characteristics, kinematics characteristics and different isotope geochronology research of the Bayanwulashan-Langshan fault zone, the predominate boundary of diving Alxa Block and the NCC.This paper show metamorphism research on Bayanwulshan Metamorphic Complex. Multiple stage metamorphisms have been identified within the metamorphic basement rocks based on the mineral assemblage of plagioclase and amphibole, the chemical variations of amphiboles, and the replacement of amphibole by chlorite and epidote. We speculate that the metamorphic basement rocks underwent retrograde metamorphic episodes from amphibolites facies to greenschist facies and overprinted the preexisting petrogenesis information. Based on newly-formed mineral assembles and deformation behaviors of quartz in mylonites, these works show that deformation temperatures of Bayanwulashan-Langshan faulting ale separately500-600℃. Thermodynamic computations show that the metamorphic conditions of the amphibole plagioclase gneisses are719-755℃at0.26-0.48Gpa. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of the metamorphic basement rocks were at ca.2.30-2.24Ga and the two metamorphic events were at ca.1.95-1.91Ga and ca.l.88-1.85Ga respectively.According to lots of field works-two phases of strike-slip ductile shear belts were found in the Bayanwulashan-Langshan fault zone. Steep mylonite foliation in the sinistral shear belts strikes NE-NNE, its stretching lineation is gentle. Steep mylonite foliation in the dextral shear belts strikes NE or EW. Its stretching lineation is NE-SW or NEE-SWW. Such microstructures as S-C fabrics, rotated feldspar porphyroblasts, and mica fish and quartz c-axis fabrics all indicate two different shear sense. LPO of quartz exhibit (0001)<a>,{1101}<a> and{1010}<a> slip systems, indicating a NE-directed shear at middle-high temepreatures. Rock deformation mechanisms are mainly the creep of the dislocation glide system and fast recrystallization-recovery This work selected muscovite from mylonites in southwestern segment of Langshan fault zone, for40Ar/39Ar dating. In this study, muscovite40Ar/39Ar ages of356.7±2.5Ma and379±4.7Ma were obtained.The formation temperatus of all these mylonite are more than the closure temperature of muscovite, so these ages are cooling ages of the Langshan ductile shear zone,and the sinistral shear must have occurred after these time. U-Pb zircon dating results indicate that the magmatic age of the syn-tectonic granite is330.8±2.8Ma, and the age of late granite dikes are269.8±1.8Ma and280.5±1.5Ma, espectively.