【作者】 肖媛媛；

【导师】 高山；

【作者基本信息】 西北大学 ， 地球化学， 2009， 硕士

【摘要】 本文通过中国东部承德盆地大北沟组底部高Ti安山岩的研究,在汪方跃研究基础上,进行了更进一步的地球化学分析,包括对承德盆地野外个别样品的重新补采(08CD21,08CD22,08CD23、XYY(仅做辅助对比之用))、锆石U-Pb、Lu-Hf原位微区LA-MC-ICP MS联机分析(08CD21,08CD22,08CD23)、部分样品主量、微量(CD-13、DBG-30、DBG-31、DBG-32、08CD21、08CD22、08CD23、XYY)以及Sr-Nd-Pb同位素(CD-13、DBG-30、DBG-31、DBG-32)的重新测试或补测。根据锆石形态学和Th/U比研究,推断高Ti安山岩仅得到的几颗锆石年龄代表了上涌岩浆捕获围岩中的锆石年龄,最年轻一颗(08CD22:Sp3)年龄120±1Ma,限定了高Ti安山岩形成的上限时间。样品08CD21安山岩的加权平均年龄113.8±1.2Ma(n=26),与汪方跃所得到的顶部安山岩年龄(113.6±0.87Ma)相近,证明大北沟组这一系列岩石基本为同时期形成。根据各主量元素间的相关性、微量元素、稀土元素配分模式以及高场强元素含量,特别是Ti、P元素明显的含量变化,推断玄武岩与顶部安山岩为同一源区岩浆的演化系列,而高Ti-P安山岩与玄武岩系列则可能源于不同源区。高Ti安山岩可能为软流圈地幔在上涌过程中与地壳轻微反应后、岩浆再次快速上升的产物。玄武岩则反映下地壳发生了更进一步较强烈的局部熔融、并混染部分演化程度较高的软流圈地幔的过程。大北沟组火山岩的形成可能反映了120～113Ma软流圈地幔与下地壳间相互作用逐渐增强的过程。更多还原

【Abstract】 Based on Wang’s previous studies, this paper focuses on the studies of high Ti content andesites of Dabeigou Group from the Chengde Basin in North China which is first found. To acquire more geochemical data, we collected samples from the Chengde Basin again (08CD21, 08CD22, 08CD23, XYY), and analyzed age of zircons by LA-MC-ICP MS (08CD21, 08CD22, 08CD23), concentrations of major elements and trace elements (CD-13、DBG-30、DBG-31、DBG-32、08CD21、08CD22、08CD23、XYY (only for reference)), Sr-Nd-Pb isotope (CD-13、DBG-30、DBG-31、DBG-32) as well.Based on CL (cathodoluminescence) features and Th/U ratios, the very few zircon ages obtained from high Ti content andesites (08CD22: sp3) is suggested to inherited from wall rocks. The youngest age of 120±1Ma constrain the maximum age of high Ti andesites. The weighted mean age of another andesite sample (08CD21) is 113.8±1.2 Ma (n=26), which is similar to the age of 113.6±0.87Ma obtained by Wang. It indicates that rocks from Dabeigou Group are all formed during the same period.According to the correlations of different major element concentrations, trace element distribution models and High Field Strength Elements (HFSE), especially the distinctive Ti and P abundances, it is inferred that the basalts and the top andesites share a common source, but is different from the high Ti andesites. The latter one is evolved from the former one. The high Ti andesites may have originated from the upwelling asthenospheric mantle which reacted with lower crust, while the basalts may be caused by further melting of crust with some contamination from highly evolved asthenospheric mantle.Though the thinning mechanism of North China Craton is still controversial, the interaction between lower crust and asthenospheric mantle may have played an important role.更多还原