Influence of stretching and density contrasts on the chemical evolution of continental magmas: an example from the Ivrea-Verbano Zone
Influence of stretching and density contrasts on the chemical evolution of continental magmas: an example from the Ivrea-Verbano Zone
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拉伸和密度对比对大陆岩浆化学演化的影响：以伊夫雷亚-韦尔巴诺带为例
DOI：
10.1007/s004100050153
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发表时间：
1996
影响因子：
3.5
通讯作者：
J. Budahn
中科院分区：
文献类型：
--
作者：
S. Sinigoi;J. E. Quick;A. Mayer;J. Budahn
意大利西阿尔卑斯山南部伊夫雷亚-韦尔巴诺带是一个巨大的镁铁质杂岩，它侵入高级变质岩，而高级变质岩是在下地壳中形成的。通过地质填图和岩浆体化学变化研究了底侵地壳的演化。与火成岩基性岩石夹层的地壳岩石（隔膜）的碎片集中在复杂（副片麻岩带）深处的狭窄区域，并显示出部分熔融程度较高的证据。整个序列的火成岩的稀土元素模式和Sr同位素组成的变化是一致的，越来越多的地壳污染接近隔膜。因此，含副片麻岩带被认为是一个“同化区”的代表，在那里的地幔和地壳衍生的岩浆之间的原位相互作用，导致混合熔体的生产。浮力导致混合熔体向上迁移，这些熔体与最后的隔膜结合在一起，并储存在较高的水平，为上镁铁质杂岩提供了补给。同化区的同生岩浆拉伸促进了熔体的混合和均匀化。花岗岩类的化学成分变化表明，深隔比浅隔更亏损。这表明，第一次合并的隔膜密度比后来的，所需的高密度的第一次注入的镁铁质岩浆。据推测，镁铁质熔体和地壳岩石之间的密度差异起着至关重要的作用，大陆岩浆的混染过程。在厚的下板壳中，可能需要从下地壳中提取早期长英质/混合熔体，以增加下地壳的密度，并允许后期镁铁质岩浆穿透更高的地壳水平。
Abstract The southern Ivrea-Verbano Zone of the Italian Western Alps contains a huge mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust. Slivers of crustal rocks (septa) interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex (Paragneiss-bearing Belt) and show evidence of advanced degrees of partial melting. Variations of rare-earth-element patterns and Sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa. Therefore, the Paragneiss-bearing Belt is considered representative of an “assimilation region” where in-situ interaction between mantle- and crust-derived magmas resulted in production of hybrid melts. Buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels, feeding the Upper Mafic Complex. Synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts. Chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones. This suggests that the first incorporated septa were denser than the later ones, as required by the high density of the first-injected mafic magmas. It is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas. In thick under plated crust, the extraction of early felsic/hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels.