OLIVINE MICROSTRUCTURES FROM MANTLE-DERIVED XENOLITH IN PINGNAN COUNTY, GUANGXI

doi:10.3321/j.issn:1000-2383.2001.01.002

Abstract：The equilibrium temperature of the mantle-derived spinellherzolite xenolith in the basalt in Pingnan County, Guangxi lies between 930 and 980℃, and its corresponding equilibrium depth between 59 and 74km. The kink-band sliding system of the olivine in the mantle-derived xenolith is often （010）[100] , but sometimes （001）[100]. However, the kink-band sliding system of orthopyroxene is （100）[001]. All these sliding systems occurred both at the high temperature and at the low strain rate, indicating that the upper mantle of the region experienced the plastic deformation both at the high temperature and at the low strain rate. Varieties of dislocations are present in the olivine, such as free dislocation, dislocation wall, dislocation bow, tangle, and (110) sliding system, indicating the plastic deformation features of the upper mantle. The size of the dislocation is used to estimate the following features of the upper mantle: the differential flow stress 24.5-42.1MPa，the flow rate 2.93×10-17-8.36×10-16s-1，the effective viscosity η 1.72×1023-2.80×1024Pa.s. All these upper-mantle features are in agreement with those in the Cenozoic eastern China, showing that they both occurred in the same tensile environment.