【作者】 曾罡；

【导师】 徐夕生； 陈立辉；

【作者基本信息】 南京大学 ， 矿物学、岩石学、矿床学， 2011， 博士

【摘要】 钠质碱性玄武岩是中国东部新生代火山作用的主要岩石类型,分布十分广泛,为探讨大陆板内碱性玄武岩的成因、地幔源区的性质以及引发玄武质岩浆活动的深部动力学机制提供了理想的研究对象。本文选择华北克拉通东南缘山东、苏皖两区域的新生代碱性玄武岩作为我们的研究对象,开展了系统的岩石地球化学研究(主量元素、微量元素和Sr-Nd-Hf同位素),并结合玄武岩中橄榄岩捕掳体的微区原位矿物化学(电子探针和LA-ICPMS)分析,揭示了钠质碱性玄武岩成因上的多样性,建立了地幔化学不均一性与源区岩性之间的联系,为探讨玄武质岩浆形成的深部动力学机制提供了新的线索。山东新生代碱性玄武岩可分为晚期的强碱性玄武岩与早期的链状弱碱性玄武岩两类。稀土倒转模式的计算结果显示,强碱性玄武岩的部分熔融程度(1-3%)要明显低于链状弱碱性玄武岩(3-10%)。强碱性玄武岩具有比链状弱碱性玄武岩偏低的SiO2(39.2-45.1wt.%)、Al2O3(10.3-13.8wt.%)含量,偏高的全碱(Na2O+K2O=4.3-8.6wt.%)、CaO(8.0-12.6wt.%)含量,偏高的Ca/Al(0.7-1.3)比值,以及更富集的绝大多数不相容元素含量(Zr、Hf、Ti和K除外),且在微量元素蛛网图上呈现出强烈的K、Zr、Hf和Ti的负异常(Hf/Hf*=0.59-0.77,Ti/Ti*=0.46-0.71)。这些特征与碳酸岩相似,暗示强碱性玄武岩的源区曾经遭受过碳酸盐质流体或熔体的交代,强碱性玄武岩是碳酸盐化的地幔低程度熔融的产物。而链状弱碱性玄武岩具有比强碱性玄武岩相对富集的Sr-Nd-Hf同位素组成,且同位素比值之间存在相关性,表明其源区存在三个不同的地幔端元。亏损端元的地球化学特征与大山玄武岩相似,暗示其也来源于碳酸盐化的软流圈地幔。其它两个富集端元均具有高的SiO2含量、低的CaO含量、低的Ce/Pb、Th/La和Zr/Hf比值、以及解耦的Nd-Hf同位素组成,与榴辉岩或石榴子石辉石岩组分熔融的特征相类似,暗示链状弱碱性玄武岩的地幔源区还存在重循环的古老下地壳。此外,链状弱碱性玄武岩的双链之间还存在系统的、小尺度的地球化学差异,暗示它们源区的榴辉岩组分并不均一,而榴辉岩组分之间细微的差别则是由于重循环的大陆地壳在早期经历过不同程度的部分熔融引起的。苏皖地区的盘石山和塔山玄武岩含大量地幔捕掳体,且具有区别苏皖地区其它玄武岩独特的地球化学特征,包括偏低的sc含量(10.3-17.8ppm)和Ca/Al比值(0.4-0.6),偏高的Na/Ti(?)匕值(2.8-5.0)等,暗示它们在形成过程中明显受单斜辉石的影响。这些玄武岩中含大量橄榄岩捕掳体,它们由岩性不同的核部与边部组成,其边部的辉石含量要明显低于核部,且在显微镜下和电子探针背散射图像中均见单斜辉石呈明显的筛状边结构,暗示这些橄榄岩捕掳体在随寄主岩浆的上升过程中发生了部分熔融。而玄武岩独特的单斜辉石效应特征,则是继承自橄榄岩捕掳体部分熔融所释放的熔体。此外,橄榄岩在显微镜下还可见橄榄石包裹斜方辉石现象,暗示橄榄岩捕掳体与寄主岩浆之间发生了反应,消耗斜方辉石生成橄榄石,并引起寄主玄武岩的Si02含量增加。因此,盘石山和塔山玄武岩独特的地球化学特征是上升过程中玄武岩与橄榄岩捕掳体相互作用的产物。更多还原

【Abstract】 Cenozoic volcanism in eastern China is wide-spread, and the most important rock type in this area is the sodic alkaline basalt. It provides an ideal setting in which to study the genesis of intra-continental alkaline basalts, the characteristics of the mantle source, and the potential geodynamic mechanism inducing continental volcanism. Here we present major, trace-element, and Sr-Nd-Hf isotopes data for the Cenozoic alkaline basalts from Shandong and Jiangsu-Anhui area in the southeastern margin of North China Craton, as well as the major, trace-element data for minernals in peridotite xenoliths hosted in Tashan basalts (one of the volcanoes in Jiangsu-Anhui area), to argue the multiple genesis of the sodic alkaline basalt. We also suggest that the geochemical heterogeneity of the mantle is induced by different lithologies in the source, and it provides a new clue to discuss the potential geodynamic mechanism for the Cenozoic continental volcanism in eastern China.The Cenozoic alkaline basalts in Shandong can be divided into two types:the strongly alkaline basalts and the weakly alkaline basalts. Inverse rare earth element (REE) modeling suggests that the strongly and the weakly alkaline basalts represent melting amounts of1.5-3%and3-10%, respectively. In comparison with the weakly alkaline basalts, these strongly alkaline basalts have relatively lower contents of SiO2(39.2-45.1wt.%) and Al2O3(10.3-13.8wt.%), higher contents of alkalis (Na2O+K2O=4.3-8.6wt.%) and CaO (8.0-12.6wt.%), higher Ca/Al ratios (0.7-1.3), and higher concentrations of most incompatible elements. On the whole, primitive-mantle normalized spidergrams reveal that the strongly alkaline basalts have stronger negative K, Zr, Hf, and Ti anomalies (Hf/Hf*=0.59-0.77, Ti/Ti*=0.46-0.71) than do the weakly alkaline basalts. These main characteristics of the strongly alkaline basalts resemble those of carbonatites. Therefore, we suggest that the carbonatitic "fingerprints" of these rocks are inherited from an asthenospheric source that had undergone enrichment with carbonatitic liquids, and our observations indicate that the source rocks of the strongly alkaline basalts in Shandong were mainly carbonated peridotite. Sr-Nd-Hf isotopic compositions for the weakly alkaline basalts form separate binary mixing arrays which converge on the compositions of Dashan, an isolated, nephelinitic volcano with the most depleted isotopic signature. The two enriched endmembers have higher SiO2contents, lower CaO contents, lower Ce/Pb, Th/La, Zr/Hf ratios, and significantly diverging isotopic enrichment trends from this common endmember. Both trends deviate from the normal Nd-Hf mantle array toward higher εHf values. All these features point to a recycled eclogitic source for the weakly alkaline basalts in Shandong. In addition, the isotopic compositions of basalts from the two volcanic chains show two slightly different mixing trends, which indicates that the enriched endmembers of the two chains are different. Such differences reflect an additional, small-scale chemical and isotopic difference between the recycled crustal components in the respective mantle sources. These are explained by different proportions of melts extracted from eclogite during the first (Mesozoic) stage of evolution.Panshishan and Tanshan are two Cenozoic volcanoes in the Jiangsu-Anhui area, which are characterized with abundant mantle xenoliths. Basalts from these two volcanoes have lower Sc contents (10.3-17.8ppm), lower Ca/Al ratios (0.4-0.6) and higher Na/Ti ratios (2.8-5.0) than those for other alkaline basalts in Jiangsu-Anhui area, indicating the influence of clinopyroxene in the genesis of these basalts. Peridotite xenoliths from these two volcanoes are usually composed of core and rim with different lithologies. The core of peridotite xenoliths generally contains more amounts of pyroxene than the rim. The spongy textures are observed in clinopyroxene, which indicates that it has been melted during the ascending process, and the "clinopyroxene fingerprints" are inherited from these melts during melting. In addition, we observed that the orthopyroxenes are included in the olivine, suggesting the reaction between peridotite and basaltic melts. This reaction can consume the orthopyroxenes, produce the olivines, and raise the SiO2contents of basalts. Therefore, the Panshishan and Tashan basalts are produced by the interactions between basaltic magma and peridotite xenoliths.更多还原