Chemical evolution of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy
Chemical evolution of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy
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意大利北部伊夫雷亚-韦尔巴诺带下地壳大型镁铁质侵入体的化学演化
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发表时间：
1994
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通讯作者：
G. Rivalenti
中科院分区：
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作者：
S. Sinigoi;J. E. Quick;D. Clemens;A. Mayer;G. Demarchi;M. Mazzucchelli;L. Negrini;G. Rivalenti
Ivrea-Verbano和邻近的Strona-Ceneri带被统称为大陆地壳的一部分。当位于下地壳时，Ivrea-Verbano带的斜长角闪岩到麻粒岩相副片麻岩被大量的镁铁质到中间深成岩侵入，这些岩石被归类为镁铁质杂岩。基性岩杂岩的生长与拉张环境下的超固相线变形有关。靠近该结构轴的同位素和微量元素变化表明，结晶来自受到地壳物质广泛污染的地幔熔融物。前人的研究表明，该污染物具有87 Sr/86 Sr> 0.71，δ 18 O = 10-12.5‰，Eu正异常的特征。在本研究中，污染物也被证明已富集在Ba相对于Rb和K。镁铁质杂岩下部与副片麻岩隔有关的碳镁矾具有适当的化学性质，可作为污染物质的样品。这些化学特征可以解释为麻粒岩相副片麻岩的熔融，这是以前亏损的K和Rb的早期熔融事件。镁铁质杂岩核部Ba的富集可以通过一个小岩浆房内的富集-分采-结晶（RTF）过程来模拟，该岩浆房被地幔熔体反复补充，并被富Ba紫苏花岗岩污染。非常高的Ba/K在较低的部分复杂的初步归因于堆晶框架和渗透深熔熔体从下面的副片麻岩隔膜之间的化学交换。与镁铁质杂岩相反，相邻的Strona-Ceneri带中的同时代花岗岩的化学成分反映了来自地壳岩石的成分，这些岩石在以前的熔融事件中没有被显著耗尽。值得注意的是，不兼容的微量元素丰度的镁铁质杂岩和Strona-Ceneri花岗岩是类似的模型组成的下地壳和上地壳，分别。
The Ivrea-Verbano and adjacent Strona-Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite-facies paragneiss of the Ivrea-Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle-derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr > 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite-facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment-tapping-fractional-crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba-rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona-Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona-Ceneri granites are similar to model compositions for the lower and upper crust, respectively.