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

摘要： 根据地层与同位素年代确定华北第四纪火山的时代为更新世(Q_p),距今0.4Ma为火山活动高潮期。山西大同火山群东部火山为拉斑玄武岩,西部为碱性玄武岩;山东火山岩石碱性最强,为橄榄玄武岩,Sr同位素⁸⁷Sr-⁸⁶Sr为0.70347～0.70461,推测岩浆为地幔部分熔融的原始岩浆。对玄武岩中幔源包体的矿物进行了大量的电子探针分析,计算出其平衡温度为880～1100℃,平衡压力为0.8～2.0GPa。由此推导的上地幔地温线低于由第三纪玄武岩中包体推导的地温线。上地幔的差异应力为13～31MPa,并随温度的升高而降低;应变速率为7.78×10^-20～1.21×10^-14s^-1,并随温度的升高而增大。研究结果表明,第四纪时期仍然存在上地幔底辟上涌,但其规模和活动强度均变弱。

关键词: 第四纪火山作用, 上地幔流变学, 华北