橄榄石显微构造和中国东部上地幔流动应力

MICROSTRUCTURES OF OLIVINE AND FLOW STRESSES IN THE UPPER MANTLE BENEATH EASTERN CHINA

笔者对中国东部11个玄武岩产地的幔源包体(尖晶石或石榴石二辉橄榄岩)显微构造研究结果表明,橄榄石位错构造类型与目前世界上其它地方报道成果和高温高压岩石变形实验资料是相似的。橄榄石亚颗粒(100)、(001)分布广泛,形状规则,它是中国上地幔中典型稳态显微构造。大部分样品中,亚颗粒大小显示对数正态分布,计算说明中国东部上地幔高温蠕变流动应力为10～50MPa,局部地区伴随有较高差异应力,120～170MPa。低温高应变率{110}滑移条带在成因上与深源断层有关,但主要与幔源包体萃取过程有关。

We have examined the microstructure of spinel and garnet peridotite xeno-liths from 11 basalt localities in eastern China. The dislocation substructures of olivine in all of the specimens are similar to those reported previously from other areas of the world and are strongly reminiscent of the substructures produced in laboratory deformation of polycrystals at high temperature and pressure. Extensive measurements of (100) and (001) subgrain dimensions demonstrate that in almost all cases, the frequency of subgrain spacing follows a lognormal distribution with ( geometric ) mean spacings of 36-60μm. In one locality (Mingxi), two xenoliths show significantly narrower spacings of 15-20μm (indicating a much higher stress ) , while other from the same locality have the normal ( wider ) spacing and one specimen has a mixed distribution, indicating that the strain produced at higher stress was not sufficient to re-equilibrate the microstructure. The general correspondence between these results and those of previous studies conducted elsewhere in the world suggests that the general level of lithospheric stress of 10-50MPa has been essentially constant since the middle Tertiary or that the dynamic process of magma generation and/or extraction always leads to similar microstruc-tures.