关键词: 第四纪火山作用, 上地幔流变学, 华北

Abstract: The Quaternary volcanoes in North China are distributed mainly in Shanxi and Shandong Provinces. In Shanxi Province, the famous Datong volcanic cluster consists of ten odd well preserved volcanic craters, volcanic cones, and volcanic necks. In Shandong Province, isolated volcanoes are distributed in Penglai and Wudi areas. Stratigraphic correlation has revealed that the volcanic products are middle to late Pleistocene in ages, while dating data have indicated that the ages of these volcanic products are in the range of 0.8～0.1Ma, and that the climax of eruption occurred in middle Pleistocene. The Quaternary volcanics in North China can be assigned to alkaline and tholeiitic basalts. The chemical compositions of these basalts indicate that they are the products of partial melting to different extents in mantle source area, having the basic features of primitive mantle-derived magma that has not been affected by crystallization differentiation. The basalts from three localities all contain mantle xenoliths comprising mainly spinel lherzolites of proto-granular and porphyroclastic textures. The rheological parameters of the upper mantle beneath these three regions have been inferred from the detailed study of these mantle xenoliths. The differential stresses are determined to be 21～31MPa, 23～26MPa and 13～24MPa for Datong, Wudi and Penglai, respectively, while the strain rates are 10^-17～10^-15s^-1, 10^-18～10^-17s^-1 and 10^-20～10^-14s^-1, respectively. The equilibrium temperatures are 900～1 050℃ for Datong xenoliths, 840～900℃ for Wudi xenoliths and 800～1 000℃ for Penglai xenoliths. The constructed differential stress and strain rate profiles of the upper mantle beneath these three areas coincide well with those of the upper mantle in continental extension zone or rift proposed by Ave Lallemant et al (1980). Volcanism is the product of the combination action of deep-seated tectonic and shallow tectonic processes. The Datong region is a part of Shanxi depression belt, Wudi volcano lies within the North China depression, and Penglai volcano is located on the outer margin of the Bohai depression. Tectonically, volcanism bears a close relation with fault depression and down-warping tectonics. Seismic sounding data and the study of mantle xenoliths have confirmed the existence of upper mantle uplift or diapir beneath these depressions. In consideration of the scale, range and distribution of Quaternary volcanism, as well as the results of the study of mantle xenoliths, it is suggested that the features of the upper mantle of North China in Quaternary, inherit the properties of the upper mantle in Tertiary time. The volcanism in Quaternary time, however, was much weaker than that in Tertiary time.

Key words: Quaternary volcanism, Upper mantle rheology, North China