Tectonic and crustal processes drive multi-million year arc magma evolution leading up to porphyry copper deposit formation in central Chile

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Simon J E Large, Chetan L Nathwani, Jamie J Wilkinson, Thomas R Knott, Simon R Tapster, Yannick Buret
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Abstract

Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu-(Mo-Au) deposits. Reading the magmatic record of individual arc segments, and constraining the rates of magmatic changes, are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf-isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher melt water content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world’s largest known Cu resource at Rio Blanco-Los Bronces.
构造和地壳过程推动了智利中部斑岩铜矿床形成前数百万年的弧岩浆演化过程
俯冲带岩浆活动是火山活动、现代大陆地壳的生成以及具有重要经济价值的斑岩型铜(钼-金)矿床形成的主要控制因素。解读单个弧段的岩浆记录,并制约岩浆变化的速率,对于充分理解和量化弧长期间俯冲环境中岩浆演化的驱动过程至关重要。这项研究的重点是智利中部的旧金山岩浆岩和里奥布兰科-洛斯布朗塞斑岩矿床群,它们提供了约 13.5 Myr 的弧演化火成岩记录。我们利用全岩地球化学、锆石地质年代学和Hf-同位素地球化学来追踪该弧段在地壳增厚和斑岩铜矿床形成过程中地壳岩浆系统的变化。通过将分析数据集与蒙特卡洛分型结晶和同化分型结晶模型相结合,我们对岩浆演化过程中地壳的重要参与模型进行了测试。随着时间的推移,岩浆中的Dy/Yb、La/Yb、V/Sc和Sr/Y持续系统地增加,表明主要分馏组合从斜长石为主过渡到闪石为主,这反映了更深的结晶和/或更高的熔融水含量。εHf和Th/La的相应减少以及Ba/Th的相应增加最能解释从低Ba/Th(Chilenia)到高Ba/Th(Cuyania)基底地层岩性的逐渐深入同化。我们的研究强调,在地壳增厚的驱动下,岩浆含水量不断增加,地壳岩浆分化和同化的位置不断加深,与此同时,构造辐合加剧,无地震的胡安-费尔南德斯海脊侵入,这些因素可以解释导致 Rio Blanco-Los Bronces 斑岩铜矿床形成的多年岩浆演化的所有调查方面。我们的研究结果证实了含水岩浆高压分异对安第斯型斑岩矿床形成的重要性。一旦有利于斑岩型铜矿化的岩浆在下地壳产生,多次有效的岩浆迁移到上地壳,在大约3.5百万年的时间里,为几个离散的浅层岩浆-热液系统提供了能量,从而在Rio Blanco-Los Bronces形成了世界上已知的最大铜资源。
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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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