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Geochemical and Isotopic Variations in a Frontal Arc Volcanic Cluster (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Ecuador)

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Geochemical and Isotopic Variations in a Frontal Arc Volcanic Cluster (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Ecuador) Article Geochemical and isotopic variations in a frontal arc volcanic cluster (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Ecuador) CHIARADIA, Massimo, et al. Abstract Volcanic arc clusters are groups of adjacent volcanic edifices that allow the investigation of geochemical changes occurring through time within a limited area (< few hundreds of km2). As such they may increase our understanding of processes that lead to magma differentiation in arcs. Geochemical changes over time in volcanic clusters can be related to source or intracrustal processes. Here, we show that magmatic rocks of 9 edifices of the Chachimbiro-Pulumbura-Pilavo-Yanaurcu volcanic cluster, in the frontal arc of Ecuador, display temporal changes of major and trace elements as well as Pb isotopes during their ~13 Ma long life history (13 Ma to 6 ka). Additionally, geochemical compositions of magmatic rocks of these edifices also become more homogeneous through time. Fractionation, assimilation and recharge models suggest that the changes in geochemical composition and in the compositional spread of erupted materials of the cluster are controlled by an increased depth of magma evolution since ~300–400 ka ago. We propose two speculative scenarios to explain the deepening of magmatic evolutionary processes since [...] Reference CHIARADIA, Massimo, et al. Geochemical and isotopic variations in a frontal arc volcanic cluster (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Ecuador). Chemical Geology, 2021, vol. 574, no. 120240 DOI : 10.1016/j.chemgeo.2021.120240 Available at: http://archive-ouverte.unige.ch/unige:150996 Disclaimer: layout of this document may differ from the published version. 1 / 1 Chemical Geology 574 (2021) 120240 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Geochemical and isotopic variations in a frontal arc volcanic cluster (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Ecuador) Massimo Chiaradia *, Maria Teresa Bellver-Baca, Viviana Valverde, Richard Spikings Department of Earth Sciences, Rue des Maraîchers 13, 1205 Geneva, Switzerland ARTICLE INFO ABSTRACT Editor: Catherine Chauvel Volcanic arc clusters are groups of adjacent volcanic edificesthat allow the investigation of geochemical changes occurring through time within a limited area (< few hundreds of km2). As such they may increase our under­ Keywords: standing of processes that lead to magma differentiation in arcs. Geochemical changes over time in volcanic Volcanic cluster clusters can be related to source or intracrustal processes. Here, we show that magmatic rocks of 9 edificesof the Recharge Chachimbiro-Pulumbura-Pilavo-Yanaurcu volcanic cluster, in the frontal arc of Ecuador, display temporal Fractional crystallization changes of major and trace elements as well as Pb isotopes during their ~13 Ma long life history (13 Ma to 6 ka). Geochemistry Ecuador Additionally, geochemical compositions of magmatic rocks of these edifices also become more homogeneous Chachimbiro through time. Fractionation, assimilation and recharge models suggest that the changes in geochemical composition and in the compositional spread of erupted materials of the cluster are controlled by an increased depth of magma evolution since ~300–400 ka ago. We propose two speculative scenarios to explain the deep­ ening of magmatic evolutionary processes since ~300–400 ka in the studied cluster. Nonetheless, a higher geochronological and geochemical resolution is needed to determine the cause and exact timing of such a switch as well as its synchronicity or diachroneity with respect to the geochemical changes observed in other volcanic centers along and across the Ecuadorian arc. 1. Introduction et al., 2006; Klemetti and Grunder, 2008; Walker et al., 2010). In fact, volcanic clusters comprise several volcanic edifices occurring not only Arc magmas display first order major and trace element as well as within a relatively small geographic area (few hundreds of km2, e.g. isotopic changes across and along-arc, which are ascribed to changing ~700 km2 in the case of the Aucanquilcha cluster: Grunder et al., 2006) slab-mantle wedge interactions and/or to mantle and crust composi­ but also encompassing variably long lifetimes, which can be up to tional heterogeneities (Barragan et al., 1998; Patino et al., 2000; several millions of years (e.g., 11 Ma at the Aucanquilcha volcanic Chiaradia and Fontbot´e, 2002; Bryant et al., 2006; Mamani et al., 2010; cluster: Grunder et al., 2006; Klemetti and Grunder, 2008; Walker et al., Turner et al., 2016; Ancellin et al., 2017; Chiaradia et al., 2020). 2010). Therefore, volcanic clusters allow monitoring of geochemical However, geochemical changes may also occur within the same volcanic and isotopic changes through time in a spatially limited area. The edificeor at adjacent volcanoes and may derive from processes changing variability and typology of chemical signatures of magmas from vol­ both in space and time (Samaniego et al., 2002, 2010; Chiaradia et al., canoes in a cluster can be related to: (i) changes in the mantle source 2011; Kayzar et al., 2014; Weber et al., 2020). Understanding the through time: these may be difficult to evaluate if primitive magmatic meaning of such changes is important to fully characterize the genesis of rocks are lacking; (ii) magmatic processes occurring at deep crustal arc magmas and their relationships to Earth-scale significant processes levels (hot zones of Annen et al., 2006); (iii) magmatic processes like the formation of continental crust (e.g., Tang et al., 2019) and of occurring at shallow levels prior to eruption. economic porphyry-type deposits (e.g., Lee and Tang, 2020; Chiaradia In this work, we report and discuss data on a cluster in the frontal and Caricchi, 2017; Chiaradia, 2020). part of the Ecuadorian arc (Chachimbiro-Pulumbura-Pilavo-Yanaurcu, Under this point of view, volcanic clusters, i.e., occurrences of in­ CPPY, cluster) that comprises 9 volcanic edificeswith an overall lifetime dividual volcanic edificesspatially adjacent to each other, are useful to of ~13 Ma (Figs. 1 and 2). Our results show that volcanic edificesof the understand the meaning of temporal changes at a local scale (Grunder cluster display changes of major and trace elements as well as Pb * Corresponding author. E-mail address: [email protected] (M. Chiaradia). https://doi.org/10.1016/j.chemgeo.2021.120240 Received 19 October 2020; Received in revised form 15 March 2021; Accepted 9 April 2021 0009-2541/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M. Chiaradia et al. Chemical Geology 574 (2021) 120240 isotopes that are broadly correlated with time. They also indicate an overall reduction in the compositional spread of individual volcanic edifices (i.e., a tendency to more homogeneous compositions) through time. We use geochemical modelling and mass balance calculations carried out with a Monte Carlo approach to explain the magmatic pro­ cesses responsible for the above systematic changes in major and trace elements, radiogenic isotopes and compositional spread of magmatic rocks. We finallyspeculate on the mechanisms that could be responsible for such changes. 2. Geological setting The volcanic arc of Ecuador is associated with the subduction of the 12–20 Ma old Nazca plate and the overlying aseismic Carnegie Ridge formed by the Galapagos hotspot (Fig. 1). The arc in Ecuador is >100 km wide and consists of several sub-parallel chains of volcanoes in the Western Cordillera (frontal arc), in the Interandean Valley and Eastern Cordillera (Main arc) and in the Amazon basin (back-arc) (Fig. 1). The cluster here investigated (Fig. 2) includes the volcanic centers of Parulo (<20–40 ka: Chiaradia et al., 2011), Pilavo (<20–40 ka: Chiar­ adia et al., 2011), Chachimbiro (400–6 ka: Bellver-Baca et al., 2020), Yanaurcu (~5 Ma-61 ka; B´eguelin et al., 2015), and Pulumbura (~12–13 Ma; Valverde, 2018). The composite Yanaurcu edifice can be split into two older edifices (Dacitic Old Yanaurcu, DOY, ~5 Ma, Andesitic Old Yanaurcu: AOY, ~3.6 Ma) and 2 younger domes ~ (Nagnaro:~ 172 ka; Cerro Negro: 61 ka). The Chachimbiro edificeconsists of 4 different constructional phases (Bellver-Baca et al., 2020) that can be split temporally and compositionally into three main parts, an older andesitic one (Chachimbiro 1, CH1: 406–300 ka), an intermediate andesitic-dacitic one (Chachimbiro 2–3, CH2-CH3: 121–22 ka), and the Fig. 2. Simplified geological map of the Chachimbiro volcanic cluster (modi­ youngest rhyodacitic one (Chachimbiro 4, CH4: 5–6 ka). fied from BGS and CODIGEM, 1999). The volcanic cluster, which occupies a surface area of about 360 km2, – sits above the ~40 50 km thick crust of the Western Cordillera of 2.1. Pulumbura Ecuador (Guillier et al., 2001). The latter consists of a lower part made up by mafic rocks of the ca 20 km thick Pallatanga terrane, which was Pulumbura is an old, eroded dome complex covering an area of derived from the large Caribbean-Colombian Oceanic Plateau (CCOP) approximately 38 km2 (Fig. 2). According to the stratigraphy, the NW and was accreted to the continental margin of Ecuador during Late flankcorresponds to the oldest andesitic-dacitic lava flowsof the edifice Cretaceous times (Vallejo et al., 2006, 2009). The Pallatanga terrane is and an old andesitic dome (Old Pulumbura) (Valverde, 2018). The NE overlain by maficrocks of the Late Cretaceous Rio Cala island arc and by flank is composed of several andesitic-domes, and block-and-ash flows a thick detrital sequence derived from the erosion of the Jurassic to derived from their explosive activity (Sunirumi Group). The southern Proterozoic metamorphic and intrusive rocks of the Eastern Cordillera flanks are composed of phenocryst-rich, porphyritic dacitic domes and and Amazon craton (Silante Formation, Angamarca Group) (Vallejo block-and-ash deposits (Avisagala-Chaparumi Group) (Valverde, 2018). et al., 2009; Chiaradia et al., 2020). 40Ar/39Ar dating of non-magnetic groundmass fractions of 2 sam­ ples, one from the Old Pulumbura (VPM-01) and one (VPM-48a) from Fig.
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