透辉石对长英质深成岩化学和机械分异的作用(意大利西阿达梅罗)

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Thomas Grocolas, Othmar Müntener
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The Western Adamello tonalite exhibits a coarse-grained, equigranular texture and is mainly composed of hornblende partially replaced by biotite, plagioclase, quartz and alkali feldspar. Within the tonalites, several types of schlieren textures, crystal accumulation zones and dikes are found, comprising: (i) hornblende-biotite-gabbros, spatially-related to (ii) plagioclase- and quartz-rich leucotonalites; and (iii) quartz-, albite- and alkali-feldspar-rich domains forming aplitic to pegmatitic dikes indicative of melt segregation and extraction. Hornblende, biotite and plagioclase phenocrysts have essentially the same compositional range in the tonalites, gabbros and leucotonalites. Together with field observations, this indicates that deformation-driven crystal–melt segregation controls the modal variation within the host tonalite. 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引用次数: 0

摘要

火山与板岩的联系对岩浆系统起着根本性的作用,它将结晶板岩、火山活动、挥发性溶解和矿床联系在一起。然而,我们对这些联系的性质的了解受到限制,因为很少有连续的露头显示出为火山岩提供养分的柱状根之间的明确关系。要更好地描述火山与深成岩之间的联系,一种方法是量化结晶的黑云母岩到花岗闪长岩深成岩中的熔体偏析量,并将其与近期的硅质喷发进行比较。在这里,我们研究了意大利钙碱性 Western Adamello 暴长岩中的间隙熔体偏析过程。Western Adamello辉长岩呈粗粒等粒状构造,主要由角闪石组成,部分被斜长石、斜长石、石英和碱性长石所取代。在碳酸盐岩中,发现了几种裂隙纹理、晶体堆积带和岩峰,包括:(i) 角闪石-生物辉石-辉长岩,在空间上与(ii) 富含斜长石和石英的白云石相关;(iii) 富含石英、白云石和碱性长石的区域,形成阳起石-伟晶岩岩峰,表明熔体分离和提取。在辉长岩、辉长岩和白云母中,角闪石、斜长石和斜长石表晶的成分范围基本相同。结合实地观测结果,这表明变形驱动的晶体-熔体偏析控制着主英安岩内部的模态变化。计算得出的与原始闪长岩处于平衡状态的熔体,其微量元素组成与寄主辉长岩相同,含量在5-10%以内,这表明辉长岩没有经历大量的熔体损失。定量模态成分和结晶分异计算表明,辉长岩的演化受控于斜长石和角闪石的结晶,然后是生物岩形成的围岩反应。这种围岩反应可以写成熔体1 +闪石 = 熔体2 + 斜长石 + 石英 + 斜长石,在一个较小的温度区间(约 50 °C)内将剩余的间隙熔体部分从 40% 降低到 15%,从而降低了大规模熔体偏析的温度窗口。在中低 K 值的钙碱性分异(如加利福尼亚州的 Western Adamello 和 Peninsular Ranges 岩床)中,生物玢岩形成反应在弱刚玉常态成分中开始,而在中高 K 值的挛辉石常态熔体(如加利福尼亚州的 Tuolumne 侵入岩套)中似乎不存在。这种差异可能受初始铝饱和度指数和母体熔体在中下地壳内的分化路径控制。纹理观察和质量平衡模型表明,75-88%的斜长石和石英以及30-70%的间隙熔体被机械地从Western Adamello辉长岩中移除,形成角闪石-生物辉长岩,而白云母则是由40-80%的斜长石和石英堆积而成。在300-400立方公里的西阿达梅洛碳酸盐岩中,只有约0.8-2.4立方公里的岩石类型与物理偏析过程有关,这表明熔体萃取有限。在全球范围内都可以观察到这种在辉长岩到花岗闪长岩柱岩中的晶体-熔体偏析过程,它有助于花岗岩液体的提取。在 Western Adamello tonalite 中观察到的这种机制可能有助于晶体贫乏的流纹岩的堆积和富含金属的盐水的偏析。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
The role of peritectic biotite for the chemical and mechanical differentiation of felsic plutonic rocks (Western Adamello, Italy)
The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of continuous outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount of melt segregation within crystallising tonalitic to granodioritic plutonic rocks, and to compare those with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello pluton (Italy). The Western Adamello tonalite exhibits a coarse-grained, equigranular texture and is mainly composed of hornblende partially replaced by biotite, plagioclase, quartz and alkali feldspar. Within the tonalites, several types of schlieren textures, crystal accumulation zones and dikes are found, comprising: (i) hornblende-biotite-gabbros, spatially-related to (ii) plagioclase- and quartz-rich leucotonalites; and (iii) quartz-, albite- and alkali-feldspar-rich domains forming aplitic to pegmatitic dikes indicative of melt segregation and extraction. Hornblende, biotite and plagioclase phenocrysts have essentially the same compositional range in the tonalites, gabbros and leucotonalites. Together with field observations, this indicates that deformation-driven crystal–melt segregation controls the modal variation within the host tonalite. The calculated melt in equilibrium with the primitive amphiboles has the same trace element composition as the host tonalite to within 5–10 %, indicating that the tonalite did not experience substantial melt loss. Quantitative modal compositions and crystallisation–differentiation calculations suggest that the evolution of the tonalite is controlled by plagioclase and hornblende crystallisation followed by a biotite-forming peritectic reaction. This peritectic reaction can be written as melt1 + amphibole = melt2 + biotite + quartz + plagioclase and decreases the remaining interstitial melt fraction from 40 to 15 % in a small temperature interval (~50 °C), therefore reducing the temperature window for large-scale melt segregation. The biotite-forming reaction initiates in weakly corundum-normative compositions in low to intermediate K calc-alkaline differentiation (e.g., Western Adamello and Peninsular Ranges batholith, California), whereas it seems absent in intermediate to high K, clinopyroxene-normative melts (e.g., Tuolumne intrusive suite, California). This difference is likely controlled by the initial aluminium saturation index and the differentiation path of the parental melt within the middle to lower crust. Textural observations and mass balance models indicate that 75–88 % plagioclase and quartz and 30–70 % interstitial melt was mechanically removed from the Western Adamello tonalite to form hornblende-biotite-gabbros, whereas the leucotonalites result from the accumulation of 40–80 % plagioclase and quartz. Of the emplaced 300–400 km3 of Western Adamello tonalite, only about 0.8–2.4 km3 represent rock types related to physical segregation processes, indicating limited melt extraction. Such crystal–melt segregation processes in tonalitic to granodioritic plutons are observed worldwide and facilitate the extraction of granitic liquids. This mechanism as observed in the Western Adamello tonalite potentially contributes to the accumulation of crystal-poor rhyolites and the segregation of metal-rich brines.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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