The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low fi...The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility(AMS) and the crystallographic preferred orientation(CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.3×10^(-4) to 3.314×10^(-2) SI, with the Jelínek's anisotropy values(Pj) ranges from 1.094 to 1.487. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction(EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the [001] axes along the lineation and the [100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.展开更多
The Wahongshan fault zone in Qinghai province is one of the most important faults in western China. In this paper, deformation and X-ray petrofabrics have been studied in the middle segment of the fault. The results s...The Wahongshan fault zone in Qinghai province is one of the most important faults in western China. In this paper, deformation and X-ray petrofabrics have been studied in the middle segment of the fault. The results show that the formation of the fault zones can be divided into two major stages: ductile shear deformation stage and brittle deformation stage. The early stage ductile shearing leads to the formation of the NW-NNW trending mylonite zones along the fault, which is intensely cut by the late-formed brittle faults. X-ray petrofabrics of rocks near the faults indicate that the minerals in the tectonites show a great degree of orientation in the alignment. The quartz, which is a very important mineral in the tectonites, is deformed by basal face gliding or near basal face gliding, and sometimes by prismatic face sliding, which indicates that the rocks are deformed in epithermal to mesothermal or mesothermal environment, and the dynamic recrystallization also plays an important role in the formation of the quartz alignment. The results also demonstrate that plutons formed in the Hercynian and Indosinian stages show no great ductile deformation as can be seen from the X-ray petrofabrics, so it is concluded that these rocks are formed after the formation of the ductile shear zones. Results of Structural deformation analysis and isotope geochronologic analysis of syntectonic muscovite indicate that Wahongshan ductile shear zones are formed in the late Silurian Period during the late Caledonian stage.展开更多
基金supported by the National Natural Science Foundation of China(Nos.41425012,41872230,41772222)the National Key Basic Research Program of China(No.2015CB856101)the MOST Special Fund from the State Key Laboratory of GPMR
文摘The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility(AMS) and the crystallographic preferred orientation(CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.3×10^(-4) to 3.314×10^(-2) SI, with the Jelínek's anisotropy values(Pj) ranges from 1.094 to 1.487. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction(EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the [001] axes along the lineation and the [100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.
文摘The Wahongshan fault zone in Qinghai province is one of the most important faults in western China. In this paper, deformation and X-ray petrofabrics have been studied in the middle segment of the fault. The results show that the formation of the fault zones can be divided into two major stages: ductile shear deformation stage and brittle deformation stage. The early stage ductile shearing leads to the formation of the NW-NNW trending mylonite zones along the fault, which is intensely cut by the late-formed brittle faults. X-ray petrofabrics of rocks near the faults indicate that the minerals in the tectonites show a great degree of orientation in the alignment. The quartz, which is a very important mineral in the tectonites, is deformed by basal face gliding or near basal face gliding, and sometimes by prismatic face sliding, which indicates that the rocks are deformed in epithermal to mesothermal or mesothermal environment, and the dynamic recrystallization also plays an important role in the formation of the quartz alignment. The results also demonstrate that plutons formed in the Hercynian and Indosinian stages show no great ductile deformation as can be seen from the X-ray petrofabrics, so it is concluded that these rocks are formed after the formation of the ductile shear zones. Results of Structural deformation analysis and isotope geochronologic analysis of syntectonic muscovite indicate that Wahongshan ductile shear zones are formed in the late Silurian Period during the late Caledonian stage.
