关键词: 幔源包体, 上地幔, 热结构, 流变学, 甘肃宕昌, 中国西部

Abstract: Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth surface may provide direct information about the upper mantle beneath the volcanic region. In eastern China there are more than one hundred reported localities of mantle xenoliths, on which a lot of studies have been carried out. In western China, however, only a few mantle xenolith localities have been reported, where less work has been done. This paper presents the results of study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the composition, thermal structure and rheology of the upper mantle beneath the region, and then to compare the obtained results with those obtained in North China. The rock types of mantle xenoliths in this region have confirmed that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52～75km), and garnet lherzolite at greater depth (greater than 75km), rather than of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of the North China, and is somewhat closer to the Oceanic geotherm. The crust mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel lherzolite. If taken 1 280℃ as the lowest temperature of asthenosphere then the lithosphere asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometer, while the strain rate and equivalent viscosity are determined by using high temperature flow law of peridotite. The constructed differential stress, strain rate and viscosity profiles indicate that asthenospheric diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, their scale and intensity are less than those occurred in North China. Moreover, numerous small scale shear zones with localized deformation may occur in the lithospheric mantle, as evidenced by the extensive occurrence of xenoliths with tabular granuloblastic texture. All these results may indicate that the region is located at the transition between the East China continental rift zone and Central China cratonic block group of Deng jinfu et al.(1996).

Key words: mantle xenoliths, upper mantle, thermal structure, rheology, Gansu Province, western China