Rheology, Fabric And Seismic Anisotropy Study Of Mantle Peridotite Xenoliths In Wangqing, Jilin Province

Spinel lherzolite is the major type of mantle peridotite xenoliths in Wangqing, Jilinprovince, and also a few harzbergite and dunite existed. The characteristics of the xenolithsindicate the equilibrium temperature is within a range of1009～1140℃, pressure is about0.8～2.0GPa and differential stress is about33.8～69.6MPa. Strain rate (1.5×10-13～5.9×10-15s-1) and viscosity (1.6×1020～1.9×1021Pa·s) are calculated from the hightemperature flow law for the upper mantle, showing a property similar to the asthenosphere.The oxidation decoration technique revealed the microstructure of the upper mantleperidotites and suggested a plastic deformation dominated by high temperature dislocationcreep. The LPOs of olivine, measured by the EBSD, generally show A-type fabricsindicating a low stress and anhydrous lithospheric mantle condition. There is also a B-typefabric found in one sample. However, four existing models (at high-stress in the presenceof water, at high-temperature in the presence of melt, during diffusion-creep in thepresence of orthopyroxene and at very high pressure under dry conditions) cannot explainthe B-type fabric. Therefore, further research is needed. Seismic anisotropy calculatedfrom the fabric data of the xenoliths suggests that the direction of the fast wave in this areais subparallel to the shear direction of the upper mantle. Together with the geophysical data,we calculate the thickness of the anisotropy layer and it is about100～135km. As noobvious correlations between rock structure, fabric strength, and differential stress, thelithospheric mantle may have undergone a complex and inhomogeneous deformation. Therheological properties, fabric characteristics and seismic anisotropy of the mantle peridotitexenoliths provide not only fundamental information and constrain for the further researchof the upper mantle in Wangqing area, but also rheological implication for the possibleasthenosphere upwelling and the bottom lithosphere dioptrically intruded the shallowupper mantle.