Abstract: A reliable geotherm has been constructed for the lower crust and upper mantle beneath Hanuoba, Hebei Province, China through the detailed study of xenoliths from basalts. The construted geotherm is higher than the oceanic geotherm and that for old cratonic areas. It is comparable to that for the eastern margin of Australian craton, and seems to indicate the tectonic regime of the studied area. The petrologic section derived from this geotherm indicates that the lower crust of this area consists mainly of granulite facies rocks of mafic compositions. They span from about 27km to 42km depth. The upper mantle consists of lherzolite, and it seems that there is a transition zone between spinel and garnet lherzolites. The crust mantle boundary defined by this geotherm is at 42km depth and coincides well with the seismic Moho. However, the overlapping of the equilibrium temperatures of some granulite and spinel lherzolite xenoliths indicates that granulite, spinel lherzolites and garnet pyroxynite may be interlayered in a zone above the crust mantle boundary. This may explain the layered reflectors observed at the base of lower crust. The lithosphere-asthenosphere boundary can be derived to be at about 95km depth, and the underlying asthenosphere consists of garnet lherzolite incorporated by upper mantle fluids. The obtained results may provide basic constraints for the lithosphere mapping and the construction of seismogenic model of the area.

Key words: Lower crust, Upper mantle, Xenoliths, Geotherm, Hanuoba area, Hebei Province

摘要： 通过对采自河北汉诺坝玄武岩中的下地壳和上地幔包体的详细研究,建立了本区下地壳—上地幔地温线。该地温线高于大洋地温线和古老地盾地温线,接近克拉通边缘的地温线,符合该区的大地构造环境。由该地温线建立的下地壳—上地幔地质结构剖面表明,该区下地壳主要由不同类型的麻粒岩相岩石组成,其化学成分以镁铁质为主,深度范围为25～42km。上地幔由超镁铁质的二辉橄榄岩组成,在尖晶石二辉橄榄岩和石榴石二辉橄榄岩之间有一过渡层。由地温线确定的壳幔边界位于42km附近,与地震资料确定的莫霍面一致,但在壳幔边界之上的下地壳底部有下地壳麻粒岩和超镁铁质岩的互层。这一现象可以解释在下地壳底部常见的层状反射层。该区岩石圈底界大约在95km,其下的软流层仍由石榴石二辉橄榄岩组成。

关键词: 下地壳, 上地幔, 幔源包体, 麻粒岩包体, 地温线, 河北汉诺坝