福建明溪上地幔热结构及流变学特征

Thermal Structure and Rheology of the Upper Mantle Beneath Mingxi, Fujian Province

通过对采自福建明溪的幔源包体样品的详细研究,建立了该区上地幔的地温线,探讨其流变学特征。所获地温线高于大洋地温线,但稍低于中国东部和澳大利亚东南部地温线。由该地温线推导的壳幔边界为38 km左右,但尖晶石二辉橄榄岩在32 km左右即已开始出现,表明存在上地幔物质的底侵作用。同样,尖晶石二辉橄榄岩和石榴子石二辉橄榄岩包体平衡温度有所重叠,表明两者不是截然分开,其间存在有5～10 km的过渡带。包体的变形特征表明,该区上地幔曾发生过较强烈的变形作用,可能与软流圈的底辟上涌有关。它导致在60 km深度附近形成以高差异应力、高应变速率和低粘滞度为特征的剪切带。

The upper mantle geotherm and rheology for Mingxi, Fujian Province, has been constructed through a detailed study of mantle xenoliths collected from this area. The constructed geotherm is higher than the oceanic geotherm and slightly lower than those for eastern China and southeastern Australia. The crust-mantle boundary as derived from this geotherm is located at a depth of about 38 km, but spinel lherzolite was found at a depth of 32 km, indicating the under-plating of upper mantle materials. Moreover, the equilibrium temperatures of some spinel lherzo-lites are within the same range as those of some garnet lherzolites. It seems to indicate that there is no distinct boundary between spinel Iherzolites and garnet Iherzolites, and that there should be a transition zone 5~10 km thick between the two, instead. The deformation features of the xenoliths have revealed that a strong deformation event occurred within the upper mantle of this area. It might be related to upper mantle diapirism. As a result, some zones characterized by a high strain rate and a low viscosity occurred at a depth of about 65 km, which resemble the upper mantle shear zones proposed by Downes.