https://doi.org/10.1130/G46998.1 Manuscript received 6 February 2019 Revised manuscript received 11 November 2019 Manuscript accepted 5 December 2019 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 17 January 2020 Multi-stage arc magma evolution recorded by apatite in volcanic rocks Chetan L. Nathwani1,2, Matthew A. Loader2, Jamie J. Wilkinson2,1, Yannick Buret2, Robert H. Sievwright2 and Pete Hollings3 1 Department of Earth Science and Engineering, Imperial College London, Exhibition Road, South Kensington Campus, London SW7 2AZ, UK 2 London Centre for Ore Deposits and Exploration (LODE), Department of Earth Sciences, Natural History Museum, Cromwell Road, South Kensington, London SW7 5BD, UK 3 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada ABSTRACT However, accessory minerals such as apatite are Protracted magma storage in the deep crust is a key stage in the formation of evolved, capable of capturing discrete periods of melt hydrous arc magmas that can result in explosive volcanism and the formation of economi- evolution during differentiation. For example, cally valuable magmatic-hydrothermal ore deposits. High magmatic water content in the apatite has been shown to record the Sr con- deep crust results in extensive amphibole ± garnet fractionation and the suppression of pla- tent of the melt at the time of its crystallization, gioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the which has been used to reconstruct host-rock melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite compositions in provenance studies (Jennings trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These et al., 2011; Bruand et al., 2016). rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore In this study, we present a novel approach for deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete understanding arc magma evolution by combin- stages of arc magma evolution. We apply fractional crystallization modeling to show that ing the trace element compositions of apatites early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signa- from volcanic rocks with fractional crystallization ture that is predetermined by amphibole-dominated fractional crystallization in the lower modeling. We show that apatite records both deep crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage crustal fractionation and shallow crustal crystal- in the shallow crust causes competition for trace elements in the melt that leads to apatite lization processes, and tracks parameters valuable compositions diverging from bulk-magma chemistry. Understanding this decoupling behav- to understanding the generation of hydrous arc ior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for magmas and metallogenically fertile arcs. interpretation of provenance in detrital studies. STUDY AREA INTRODUCTION arc crust and is commonly associated with por- The Andes of central Chile are an ideal re- The chemical diversity observed in the rock phyry Cu ore deposits because impeded magma gion for a study of regional arc magma evo- record of volcanic arcs is determined by a multi- ascent from the lower crust facilitates volatile lution and metallogeny because volcanic rock tude of processes operating between the magma accumulation and enrichment in ore-forming sequences (Fig. 1) record an extended period source region and the surface. A fundamental components (Chiaradia, 2015; Chiaradia and of subduction-related magmatism that culmi- step in producing this variability is fractional Caricchi, 2017). The high Sr/Y values that result nated in high-Sr/Y magmatism (Sr/Y > 50) and crystallization, assimilation, and melting in can be used to provide insights into arc magma the genesis of three major porphyry Cu deposits the lower crust which drive magmas to more evolution (Macpherson et al., 2006; Rodriguez between 12.3 and 4.3 Ma (Perelló et al., 2012; evolved and hydrous compositions (Hildreth and et al., 2007), evaluate whether a magmatic sys- Toro et al., 2012; Spencer et al., 2015): Los Moorbath, 1988; Annen et al., 2006; Davidson tem has the potential to form a porphyry-related Pelambres, Rio Blanco–Los Bronces, and El et al., 2007). During extensive fractionation of ore deposit in exploration (i.e., is “metallogeni- Teniente (Kay and Mpodozis, 2001). This tran- hydrous magmas in the lower crust, amphi- cally fertile”; Richards, 2011; Loucks, 2014) and sition in magma chemistry has been attributed bole (±garnet) is stabilized in the fractionat- track crustal thickness (Chapman et al., 2015). to shallowing of the subducting slab, linked to ing assemblage and plagioclase is suppressed However, this deep fractionation history may be subduction of the Juan Fernández Ridge from (Müntener et al., 2001), resulting in melts with obscured due to differentiation and mixing upon ca. 26 Ma which led to substantial compression elevated Sr, an absence of strong negative Eu ascent to the shallow crust (Reubi and Blundy, and crustal thickening that peaked at ca. 5 Ma anomalies (both elements being compatible in 2009). Because arc rocks are a product of this (Stern and Skewes, 1995; Kay et al., 1999). plagioclase), and depleted Y (compatible in am- multi-stage, polybaric process, unravelling the The studied volcanic formations are all phibole and garnet). Such magma evolution is complete history of magmatic evolution using calc-alkaline, were deposited between 124 and promoted in strongly compressional, thickened bulk-rock chemistry alone can be challenging. 3.9 Ma (Fig. 1B; Hollings et al., 2005), and CITATION: Nathwani, C.L., et al., 2020, Multi-stage arc magma evolution recorded by apatite in volcanic rocks: Geology, v. 48, p. 323–327, https://doi.org/10.1130/G46998.1 Geological Society of America | GEOLOGY | Volume 48 | Number 4 | www.gsapubs.org 323 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/4/323/4972121/323.pdf by guest on 30 September 2021 71°W 70°W elements in apatite were determined by electron A B microprobe and trace elements by laser ablation SAL inductively coupled plasma mass spectrometry LP A P 32°S O T at the Natural History Museum, London (UK). C E Figure 1. (A) Simplified B A L Detailed rock descriptions, data tables, and ad- L - map of studied volcanic B R 10 units (FAR—Farellones ditional information on the analytical methods Formation; LA COPA—La are provided in the GSA Data Repository1. Copa Rhyolite; LC—Las P L Chilcas Formation; LP— Los Pelambres Formation; APATITE TRACE ELEMENT FAR SAL—Salamanca Forma- COMPOSITIONS tion) in central Chile, with Apatite Sr and Y show a broad negative cor- sample localities marked, 20 relation (Fig. 3) and exhibit both differences be- and locations of major ) tween samples (inter-sample) and within-sam- a porphyry Cu-Mo depos- LC LP R M A ple trends (intra-sample). Apatite Sr/Y (Sr/Y ) ( ap P its (LP—Los Pelambres; F L e RB-LB—Rio Blanco–Los varies between 0.1 and 11 (Fig. 4A), with low g A Bronces; ET—El Teni- 33°S Sr/Y ( 2) present in the most mafic ( 61% RB-LB LA ap < < ente). After Hollings et al. SiO ) and felsic (>70%) samples, and higher COPA (2005). Inset: Location 2 Sr/Yap (>2) exclusively found in the intermedi- L 30 of study area in South A S America. (B) Published ary whole-rock compositions (62%–66% SiO2). 60 age ranges for forma- The highest Sr/Yap cannot be explained by high tions studied (Rivano Sr alone, but requires reduced Y concentrations et al., 1993; Hollings et al., 2005; Piquer et al., (<200 ppm; Fig. 3), and thus any mechanism 2017) and major porphyry to explain the presence of high Sr/Yap must ac- Cu-Mo deposits (Perelló count for the simultaneous enrichment in Sr and et al., 2012; Toro et al., 120 depletion in Y. A correlation between Sr/Yap and FAR 2012; Spencer et al., 2015). Eu/Eu* is present that forms a well-defined C ap L logarithmic curve, in which samples from the Volcanic Porphyry 34°S Farellones and Los Pelambres Formations dis- ET formations deposits play higher Eu/Eu*ap (Eu/Eu*ap >0.5) and the La Copa, Salamanca, and Las Chilcas samples exhibit lower values (Fig. 4A). represent magmas that followed different evo- tures (Harrison and Watson, 1984) that typically Experimental work shows that Mg in apa- lution paths in the arc crust. The Las Chilcas, exceed 900°C, suggest that apatite is a relative- tite (Mgap) is proportional to the Mg content of Salamanca, Farellones, and Los Pelambres For- ly early-saturating phase in the samples. Major the melt (Prowatke and Klemme, 2006), which mations show predominantly low Sr/Y (<50; suggests that Mgap can be used as a tracer of Fig. 2) and pre-date the period of high-Sr/Y magmatic differentiation during apatite crystal- magmatism and mineralization (Fig. 1B). The 200 lization. Using this approach, our data reveal La Copa Los Pelambres Formation displays some higher 180 Farellones three systematic intra-sample trends in Sr/Yap Sr/Y values that have been attributed to com- Los Pelambres with magmatic differentiation (Fig. 4B). Within 160 Salamanca pression from localized shallowing of the sub- 140 Las Chilcas samples, decreasing Sr/Yap with differentiation ducting slab in this region (Yáñez et al., 2001). Miocene porphyries (decreasing Mg ) occurs where Sr/Y is initial- 120 Cenozoic volcanics ap ap The La Copa Rhyolite is found in an immedi- 100 ly elevated. Conversely, where Sr/Yap is initially ately post-ore diatreme (ca.