Geochronology and petrogenesis of hornblende-bearing basalts (Rhön, Germany) - remnants of volcanic rocks from the lithospheric upper mantle)

拟议的项目涉及碱性橄榄玄武岩的地质年代学、地球化学和同位素组成。它们代表了Rhön地区（德国）火山岩的重要组成部分。与欧洲中部的其他火山相比，这些碱性火山岩大多含有毫米至厘米大小的角闪石斑晶，以及斜辉石（有些具有绿色岩心）和橄榄石。根据这些不寻常的矿物组成和地球化学特征，如TiO2含量高，可以认为角闪玄武岩的来源与其他中欧碱性岩石不同，即可能代表地壳下岩石圈地幔的熔体。研究了火山岩的Nd、Sr、Pb、Hf、Os和O全岩同位素，并用电子探针和离子探针测定角闪石斑晶的主微量元素组成。角闪石斑晶的Ar-Ar年龄测定将补充同位素数据。此外，还计划对角闪石斑晶进行Nd、Sr、Pb和O同位素分析。在这项研究中获得的结果将对裂谷相关地区的上地幔可能的非均质性进行限制，也可以用来区分二级过程（有或没有分离结晶、岩浆混合的地壳污染）和一级过程（有或没有含oh矿物的尖晶石或石榴石橄榄岩的部分熔融等）。因此，该项目非常适合在与裂谷有关的大陆环境中增加岩石圈动力学方面的知识。

The proposed project deals with the geochronology, geochemistry and isotope composition of alkali olivine basalts s. /. that represent a significant part of the volcanic rocks in the Rhön area (Germany). In contrast to other volcanic occurrences in central Europe, most of these alkaline volcanic rocks contain mm- to cm-sized hornblende phenocrysts beside clinopyroxene (some with green cores) and olivine. Based on these unusual mineralogical composition and some geochemical features, i.e. high TiO2 contents > 3.0 wt%, it can be suggested that the hornblendebearing basalts originate from sources distinct to those envisaged for other alkaline rocks from central Europe, i.e. probably representing melts of the subcrustal lithospheric mantle. The volcanic rocks will be investigated for Nd, Sr, Pb, Hf, Os and O whole rock isotopes and the major and trace element composition of the hornblende phenocrysts will be determined by using an electron microprobe and an ionprobe. Ar-Ar age determinations on hornblende phenocrysts will complement the isotope data. In addition, it is planned to analyse the hornblende phenocrysts for Nd, Sr, Pb and O isotopes. Results obtained during this study will place constraints on possible heterogeneities of the upper mantle in rift-related areas and can also be used to distinguish second order processes (crustal contamination with or without fractional crystallization, magma mixing) from first order processes (partial melting of spinel or garnet peridotite with or without OH-bearing minerals etc.). The project is therefore ideally suited to add knowledge on lithosphere dynamics in rift-related continental setting.