Abstract: This paper deals with ultramafic xenoliths from 6 occurrences, found in Neogene and Quaternary alkaline basalts rather than in Paleogene subalkaline basalts. The xenoliths are mainly distributed along the Tancheng-Lujiang fault zone and its both sides in eastern North China and in submeridional gravity gradient belt of western North China, that is, in both eastern and western border belts belonging to a regional inhomogeneous uplift of upper mantle in North China. These ultramafic xenoliths were squeezed up and carried out by alkaline magma along the deep-seated fault from the asthenosphere to the surface. The xenoliths with coarsegrained texture constitute relatively large portion among the studied ultramafic xenoliths from western part of the North China fault block. The value of stress estimated from the size of neoblasts is in a ranges from 250 to 300 bars. And the xenoliths from eastern part of the North China fault block are mainly of porphyroclastic texture. The value of stress from these xenoliths ranges in 300-600 bars. The equilibrating P-T conditions of ultramafic xenoliths obtained from the mineral assemlage of the xenoliths appear in 950-1160℃ and 15-25kb (50-80km), Both the interface M₃ estimated from the converted seismic wave sounding and the highly conducting layer in the upper mantle obtained from magnetotelluric sounding are at a depth of 60-80km which may represent the top of asthenosphere in this region. It follows that the ultramafic xenoliths might be the rocks occurring at the top of mantle's asthenosphere. The equilibrating P-T conditions of the xenoliths from this region show itself as a oceanic type of paleogeotherm, and trend to the geotherm of continental rift. Combining the tectontic development and magmatism coming into being since Cenozoic time in the North China region on the large scale inhomogeneous uplift in the upper mantle and the relatively high geothermal gradient, we conclude that this region is characterized by the block-faulting since Cenozoic time which is similar to the continental rifting in nature.

摘要： (二)由新变晶大小估计地幔应力近些年来,据天然橄榄石的变形结构、显微亚构造及实验研究的结果,提出了多种估计上地幔应力的方法。Nicolas(1978)^[11]曾对此进行过综合评述。归纳起来有两类方法,一类是建立在位错显微构造基础上的方法,如根据橄榄石的位错密度和位错曲率与应力之间的经验关系或亚颗粒大小与应力之间的关系来估算应力值;另一类方法是由动力重结晶的橄榄石晶粒(新变晶)的大小来估计上地幔应力的方法^[11-13]。