由甘肃宕昌好梯幔源包体推导的上地幔物质组成、热结构和流变学特征

COMPOSITION,THERMAL STRUCTURES AND RHEOLOGY OF THE UPPER MANTLE INFERRED FROM MANTLE XENOLITHS FROM HAOTI,DANGCHANG,GANSU PROVINCE, WESTERN CHINA

新生代火山岩中的幔源包体可以提供有关上地幔的直接信息。通过对采自甘肃宕昌县好梯乡的幔源橄榄岩包体的详细研究 ,探讨该地区上地幔的热结构和流变学特征 ,并与华北地区进行对比。研究结果表明 ,该区自壳幔边界以下至 75km深处的上地幔主要由尖晶石二辉橄榄岩组成 ,75km以下则主要由石榴石二辉橄榄岩组成 ,方辉橄榄岩只占少数 ;与华北地区没有明显的差异。由包体平衡温度、压力建立的上地幔地温线低于华北地区地温线 ;确定的壳幔边界在 5 2km左右 ,莫霍界面附近为下地壳物质与尖晶石二辉橄榄岩的过渡带 ;岩石圈底界深度约为 12 0km左右。根据包体的研究结果建立了研究区上地幔的差异应力、应变速率和等效黏滞度剖面。结果表明 ,该区新生代时期存在软流层的底辟 ,以及相应的岩石圈的减薄 ,但其规模明显小于华北地区。这些结果表明该区处在中国东部大陆裂谷带和中国中部克拉通块体群的过渡带。

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).