Evolution of the petrological and seismic Moho-implications for the continental crust-mantle boundary
Evolution of the petrological and seismic Moho-implications for the continental crust-mantle boundary
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DOI：
10.1111/j.1365-3121.1992.tb00455.x
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发表时间：
1992
期刊：
影响因子：
2.4
通讯作者：
K. Mengel;H. Kern
中科院分区：
文献类型：
--
作者：
K. Mengel;H. Kern
通过对比伊夫雷亚带和北黑森凹陷包体岩套中的镁铁质麻粒岩和变质辉长岩，讨论了下陆壳镁铁质岩石的化学和岩石学组成。这两个地区都含有不同类型的变质镁铁质岩性(I)以前的玄武岩，其微量元素模式从类MORB到与俯冲有关的或板内岩浆，以及(U)富Ca和Al的斜长岩，显示出由复杂的地壳深部岩浆作用产生的正Eu异常，如分馏、斜长石和辉石的堆积以及地壳污染。这些岩石中没有典型的石榴石-绿辉石共生晶体，表明在古生代造山过程中未达到榴辉岩稳定场。利用实验测定的镁铁质麻粒岩、辉长岩、榴辉岩和橄榄岩的纵波速度和密度的汇编，评价了大陆-大陆碰撞引起的晶体增厚过程中下地壳镁铁岩的关键物理性质。在一步一步的情景中，对于镁铁质岩石在造山增厚的地壳底部转化为埃尔多岩的情况，证明了地震莫霍面(定义为一级速度不连续)和岩石学莫霍面(定义为非橄榄岩地壳岩石和橄榄石为主岩石之间的边界)的位置不同。榴辉岩的P波速度与橄榄岩的P波速度基本重叠，尽管其密度明显高于普通上地幔岩石。因此，折射地震场研究可能不会检测到作为地壳岩石的埃尔多岩。这意味着折射地震场研究探测到的地震莫霍面出现在埃尔多斯岩和上覆地壳单元之间的上边界。由于埃尔多岩通常比橄榄岩具有更高的密度，它们可能被循环到更深的岩石圈，从而将多余的Eu转移到上地幔。这一过程可能有助于理解上陆壳的Eu负异常，这种负异常显然不能被普通镁铁质地壳岩石的丰度所定量地平衡。
The chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta-gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta-mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB-Iike to subduction-related or intra-plate tholeutes and (u) Ca-and Al-enriched, plagiodase-dominated gabbroic rocks showing positive Eu-anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet-omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes. A compilation of experimentally determined P-wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent-continent collision. In a step-by-step scenario it is demonstrated that the position of the seismic Moho (defined as a first-order velocity discontinuity) and the petrological Moho (defined as the boundary between non-peridotitic crustal rocks and olivine-dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P-wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks. As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks.