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

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1 / 1 Chemical Geology 574 (2021) 120240

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Chemical Geology

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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. 1. (a) Location of Ecuador in South America. Also shown are main topographic features of the PacificOcean: CR = Cocos Ridge; CGR = Carnegie Ridge; NR = Nazca Ridge; JFR = Juan Fernandez Ridge; Chile rise. (b) Geotectonic map of Ecuador showing the location of the Chachimbiro volcanic cluster investigated in this study.

2 M. Chiaradia et al. Chemical Geology 574 (2021) 120240 the youngest volcanic group (Avisagala-Chaparumi) of the edifice has amounts of amphibole, clinopyroxene and plagioclase phenocrysts. yielded ages of ~13 Ma and ~ 11.5 Ma, respectively (see below). It is Quartz xenocrysts surrounded by a clinopyroxene reaction rim are important to highlight that the SiO2 ranges of the three different units of frequently encountered. the Pulumbura volcanic edificeare comparable (SiO2 of Old Pulumbura Pilavo is older than ~6 ka (the age of the youngest tephra from the = 59.68–66.08 wt%; SiO2 of Sunirumi = 60.27–67.00 wt%; SiO2 of Chachimbiro complex covering Pilavo) and younger than 20–40 ka Avisagala-Chaparumi = 61.88–69.17 wt%), suggesting that throughout (Chiaradia et al., 2011), because it is not affected by the intense erosion its history this volcanic edificehas erupted products with a rather wide of the pleniglacial period (20–40 ka) that characterizes the adjacent SiO2 range. CNG of the Yanaurcu edifice(60.0 ± 20.0 ka old: see above) and stage 1 of Chachimbiro (CH1; 300–400 ka old). 2.2. Yanaurcu

The Yanaurcu volcanic complex (YVC) consists of various units 2.5. Parulo spanning a time interval of ~5 Ma (B´eguelin et al., 2015) (Fig. 2). The andesitic Tatacho–Corcomaco Group (TCG) and the Dacitic Old Parulo is a mono-compositional small volcano occupying a surface Yanaurcu (DOY), both arguably part of the Pugaran´ unit, a regional area of approximately 4 km2 (Fig. 2). The central part of the edificeis a group of volcanic products dated at 5.0 ± 2.9 Ma by BGS and CODIGEM dome, and around it there are approximately five lava-flows with a (1999), form the oldest basal units of the edifice. The Tata­ maximum run-out of 1 km (Valverde, 2018). Similar to Pilavo, no py­ cho–Corcomaco Group consists of porphyritic andesite while the Dacitic roclastic material was found associated with this volcano. The rocks Old Yanaurcu consists of dacite flows and pyroclastic deposits. Dacitic composing the flows are very homogeneous basaltic andesites with Old Yanaurcu lava flows have been sampled and investigated by abundant amphibole, and minor clinopyroxene and plagioclase pheno­ B´eguelin et al. (2015) and crop out at the northernmost and southern­ crysts very similar to Pilavo rocks. Because of similar morphological most margins of the Yanaurcu volcanic complex. observations to those of Pilavo applicable also to Parulo, the age of The Andesitic Old Yanaurcu represents the middle part of the Parulo should be > ~ 6 ka and < 20–40 ka. However, there is no Yanaurcu edifice and consists of a group of eroded Pliocene andesitic stratigraphic evidence for relative age relationships between the two flows dated by B´eguelin et al. (2015) at 3.58 ± 0.03 Ma by 40Ar/39Ar. edifices. ˜ This unit crops out at the feet of the Nagnaro˜ dome in the northern part of the Yanaurcu volcanic complex. 3. Sampling and analytical methods The northern part of the Yanaurcu volcanic complex is occupied by ˜ ˜ the ~1 km wide and 200 m high andesitic Nagnaro Dome (NND) dated Most of the rocks investigated in this study (127 out of a total of 209) ´ ± 40 39 by Beguelin et al. (2015) at 171.6 20.5 ka by Ar/ Ar (Fig. 2). are from previous works carried out within the research group of the The southern part of the Yanaurcu volcanic complex consists of a Universities of Geneva and Lausanne and the EPN of Quito (Yanaurcu; × significantly larger andesitic dome (~3 1.5 km for 500 m high), the Beguelin´ et al., 2015; Pilavo: Chiaradia et al., 2011; Chachimbiro: ´ ± Cerro Negro Group (CNG), dated by Beguelin et al. (2015) at 60.6 Bellver-Baca et al., 2020; Chiaradia et al., 2020). Additional data (N = 40 39 20.0 ka by Ar/ Ar (Fig. 2). The Cerro Negro Group has been strongly 35) were gathered from other available studies for Pilavo (Ancellin eroded by glacial activity. et al., 2017: N = 27) and Chachimbiro (Ancellin et al., 2017: N = 8). All ˜ ˜ Petrographically, both the Cerro Negro Group and Nagnaro Dome these data are reported in the Supplementary material 1. To these we are porphyritic andesites with plagioclase, amphibole, biotite and quartz add geochemical and radiogenic isotope data of 47 new rocks of the phenocrysts in a well crystallized groundmass of 25–400 μm elongated Pulumbura, Pilavo and Parulo centers (Supplementary material 1). Each plagioclase microliths. volcanic edifice is extensively represented (Pulumbura N = 24; Yanaurcu N = 35; Chachimbiro N = 86; Pilavo N = 54; Parulo N = 7) and 2.3. Chachimbiro investigated samples cover the bulk variability of stratigraphy and stages of development of the edifices.The low number of Parulo samples The Chachimbiro volcanic complex (Fig. 2) is a composite volcano is due to the very small volume of this volcanic edifice (Fig. 2) and its that in the last ~400 ka has blanketed an area of 246 km2 hosting a monotonous lithology. minimum amount of ~50 km3 of volcanic products divided into four main units (Bellver-Baca et al., 2020): the andesitic Chachimbiro 1 ± ± (405.7 20.0–298.6 32.9 ka), the andesitic to dacitic Chachimbiro 2 3.1. Rock sample preparation (121.75 ± 23.2–36.08 ± 2.8 ka) and Chachimbiro 3 (36.08 ± 2.8–22.73 ± ± 1.2 ka), and the rhyodacitic Chachimbiro 4 (5.76 0.03 ka). Cha­ Geochemical and radiogenic isotope analyses were carried out on chimbiro 1 consists of several andesitic flows accounting for about 32 ~2–5 kg bulk rock samples collected in the field. Before carrying out 3 km . Chachimbiro 2 and 3 consist of andesitic to dacitic domes and analyses, the rock samples were prepared in the following way. 3 associated explosive products accounting for a total of 17 km of erupted Weathered surfaces of rock samples were removed with a circular dia­ 3 material (Chachimbiro 3 is only 0.25 km ). Chachimbiro 4 is the last mond saw at the Department of Earth Sciences (University of Geneva, < 3 event and represents a lateral blast accounting for 0.002 km of Switzerland). The “cleaned” rocks were cut into approximately 5x5x10 erupted material. cm parallelepipeds, which were subsequently fragmented into a jaw crusher to fragments <1–2 cm in size. The jaw crusher was carefully 2.4. Pilavo cleaned between the crushing of each sample. The fragmented material of each sample was collected into a clean plastic bag and quarted to Pilavo volcano (Fig. 2) consists of about 30 lava flows up to 8 km obtain a representative aliquot of a few hundreds of grams of fragmented long, with a thickness of more than 40 m in many cases (Chiaradia et al., material out of the initial sample weighing several kg. A macroscopic 2011). Its morphology is closer to that of a shield than to that of a strato- rock witness was kept for each sample for additional work (e.g., petro­ volcano, with a total average basal diameter of about 8 km and a height graphic investigations). The representative aliquots of each sample were of ~650 m. There are no pyroclastic deposits associated with Pilavo. finely powdered using an agate mill for geochemical and isotopic ana­ Lava flowshave basaltic andesite to andesite composition with variable lyses detailed below.

3 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

3.2. XRF and LA-ICPMS geochemistry of whole rocks (major, minor, and Department of Earth Sciences (University of Geneva, Switzerland). Be­ trace elements) tween 100 and 150 mg of whole rock powder were dissolved during 7 days in screw cap-sealed Savillex® Teflon vials using 4 ml of concen­ ◦ ◦ Powdered rock samples were dried in an oven at 100 C during one trated HF and 1 ml of 14 M HNO3, at a temperature of 140 C and with night. After this, loss on ignition (LOI) was calculated from the mass the help of ultrasonication for 30 min twice a day. Subsequently, sam­ difference of a weighted aliquot (2–3 g) of this dried powder before and ples were dried and re-dissolved for 3 days (also with 30 min ultra­ ◦ after heating at 900 C. X-Ray-Fluorescence (XRF) measurements of sonication twice a day) in 3 ml of 14 M HNO3 and dried again. Sr, Nd, major and trace elements were carried out on fused glass beads and and Pb were then separated using cascade columns with Sr-Spec, TRU- pressed pellets respectively. Fused glass beads were prepared by mixing Spec and Ln-Spec resins according to a protocol modifiedfrom Pin et al. about 1.2 g of rock powder with 6 g of Li2B4O7 and melting this mixture (1994). Finally, the material was redissolved in 2% HNO3 solutions and ◦ at 1150 C in platinum crucibles. Pressed pellets were prepared by ratios were measured using a Thermo Neptune PLUS Multi-Collector mixing 12 g of rock powder with 3 g of wax and pressing the mixed ICP-MS in static mode on Faraday cups connected to 1011 Ohm re­ material inside a mold with a hydraulic press. sistors. In order to monitor internal fractionation, we used 88Sr/86Sr = XRF analyses were carried out at the Institute of Earth Sciences of the 8.375209 for the 87Sr/86Sr ratio, 146Nd/144Nd = 0.7219 for the University of Lausanne (Switzerland), using a Philips PW-2400 X-ray 143Nd/144Nd ratio and 203Tl/205Tl = 0.418922 for the three Pb ratios (a fluorescencespectrometer. Matrix effects were corrected using the alpha Tl standard was added to the sample solution). Long-term reproduc­ theoretical method (De Jongh, 1973) and data quality control was ibility of the measurements was controlled by repeated measurements (1 guaranteed by repeated measurements of the NIM-G, BHVO, QTW10.6, every 6 unknown samples) of the external standards SRM987 (87Sr/86Sr and QLO standards (Govindaraju, 1994). This technique allows precise = 0.710248, McArthur et al., 2001, long-term external reproducibility: 143 144 quantitative measurements of major and minor elements (SiO2, TiO2, 17 ppm, 1SD), JNdi-1 ( Nd/ Nd = 0.512115; Tanaka et al., 2000; Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5) with relative un­ long-term external reproducibility: 14 ppm, 1SD), and SRM981 (Baker certainties (2σ) of 0.5–1.1% and the measurement of 16 trace elements et al., 2004) for Pb (long-term 1SD external reproducibility of 83 ppm with relative uncertainties (2σ) of 2–20%. Because several of these trace for 206Pb/204Pb, 71 ppm for 207Pb/204Pb and 95 ppm for 208Pb/204Pb). elements (e.g., the few measured REE, Th, U, Pb) are measured with a low Because of a systematic difference between measured and accepted precision and accuracy and most of the REE as well as Ta, Nb, Th, U, Pb standard ratios (Supplementary material 1), 87Sr/86Sr, 143Nd/144Nd and cannot be measured because of their low concentrations, we obtained the Pb isotope ratios were further corrected for external fractionation by full spectrum of trace element analyses by LA-ICPMS. In order to do so, values of 0.021‰, +0.051‰ and + 0.36‰ amu respectively. In­ the glass beads used for XRF major and minor element analyses were terferences at strontium masses 84 (84Kr), 86 (86Kr) and 87 (87Rb) were broken after the XRF analyses and one of the central glass shards of each corrected by monitoring 83Kr and 85Rb, 144Sm interference on 144Nd was sample was measured at the Institute of Earth Sciences of the University of monitored on the mass 147Sm and corrected by using a 144Sm/147Sm Lausanne by LA-ICPMS (see Eggins, 2003). Analyses were performed with value of 0.206700 and 204Hg interference on 204Pb was corrected by an Agilent ICP mass spectrometer coupled to an UP-193FX 193 nm monitoring 202Hg. Total procedural blanks were < 500 pg for Pb and < Excimer ablation system. 33 trace elements as well as CaO, Al2O3, TiO2 100 pg for Sr and Nd which are insignificantcompared to the amounts of and MnO were measured. Operating conditions of the laser were set at 10 these elements purified from the whole rock samples investigated. Hz laser pulse repetition rate, 7.0 J/cm2 on-sample energy density, and a Corrections for time integrated decays of the parents of the radio­ spot diameter of 120 μm. Helium (1 l/min flow)was used as a carrier gas, genic isotopes measured (i.e., 238U for 206Pb, 235U for 207Pb, 232Th for transporting the ablated material into an Ar-plasma torch for ionisation. 208Pb, 87Rb for 87Sr, 147Sm for 143Nd) were carried out only for the ~13 The NIST SRM 612 glass standard was used as an external standard Ma old samples of Pulumbura, since for all other samples, due to their (concentration values from Pearce et al., 1997) to calibrate the spec­ young ages, they resulted to be insignificant. Results are presented in trometer drift and the instrument mass fractionation. Raw data were Supplementary material 1. reduced off-line using the LAMTRACE software (Jackson, 2008) and CaO concentrations, previously measured by XRF, were used as an internal 3.4. 40Ar/39Ar geochronology standard. For each sample 70–80s background acquisition was followed by 3 spot analyses at different places in the sample glass shard, each one The non-magnetic groundmasses of two samples of the Pulumbura lasting about 50–60s. Each measurement was separated by at least 15 s of edificewere dated by the 40Ar/39Ar method. One sample is from the Old washout. The finalconcentration value of each sample was calculated as Pulumbura group (VPM-01) and the other one (VPM-48a) is from the the average concentration value of the 3 measurements. The NIST SRM youngest volcanic group of the Pulumbura edifice (Avisagala- 612 standard was measured every 5 unknown samples and this mea­ Chaparumi). surement consisted of 70–80 s of background acquisition followed by 2 After separation with gravimetric and magnetic methods the non- spot measurements on the NIST SRM 612 glass lasting 50–60s each and magnetic groundmasses were hand-picked and rinsed in deionized separated by at least 15 s of washout. An average of the 2 measurements of water in an ultrasonic bath. The separates were subsequently packed in the standard was calculated each time and 1σ standard deviations on copper foil and irradiated in the CLICIT facility of the TRIGA reactor at these averages were all <3%. For the unknown samples, the 1σ un­ Oregon State University. J values were determined by measurement of certainties on the mean values of the 3 repeated measurements were < 5% simultaneously irradiated Fish Canyon Tuff sanidine, using an age of for all measured elements. Long-term measurements of rock powder 28.201 ± 0.092 Ma (Kuiper et al., 2008). Samples were degassed by standards AGV-2 and BCR-2 prepared as lithium tetraborate glasses, are CO2-IR laser step heating, and the extracted gas was passed through a ◦ listed in Supplementary material 1. The results show an accuracy better cold trap chilled to ~150 Kelvin and purified in a stainless steel than ±10% for most elements, and ± 15% for all elements except for Cr, extraction line fitted with AP10 and GP50 (ST101) getters. Argon iso­ which is usually overestimated; the reason is not well understood. topes were analysed using a GV Instruments Argus mass spectrometer Therefore, in our routine work, we use Cr values obtained by XRF. Results fittedwith four high-gain 1 × 1012 Ω Faraday detectors and one 1 × 1011 are presented in Supplementary material 1. Ω faraday detector (40Ar). Dates were calculated using the 40K decay constant of Steiger and Jager¨ (1977). Data reduction was performed 3.3. Radiogenic isotopes (Pb, Sr, Nd) using the ArArCALC 2.4 software (Koppers, 2002), plateau dates were determined using the criteria of Darlymple and Lanphere (1974), and Radiogenic isotope ratios of Sr (87Sr/86Sr), Nd (143Nd/144Nd), and Pb analytical data and calculated dates are presented in Supplementary (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) were measured at the material 2. Sample VPM01 yielded a weighted plateau age of 12.9 ±

4 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

0.12 Ma and sample VPM48a yielded a weighted plateau age of 11.55 ± Na2O contents increase concomitant with increasing SiO2 at each 0.22 Ma (Supplementary material 2). volcanic center with a slight decrease only in the SiO2-richest samples (Fig. 3b). Individual volcano edifices display also different degrees of 4. Results MgO and especially Ni depletion at similar SiO2 contents (Figs. 3c-d). In particular, older edifices (Pulumbura, Dacitic Old Yanaurcu, Andesitic The investigated volcanic edifices range in age from ~13 Ma to ~6 Old Yanaurcu) definetrends characterized (at similar SiO2 contents) by ka. The oldest edifice of the cluster is Pulumbura (~12.9–11.6 Ma: this the highest MgO and Ni contents, Chachimbiro 1–2-3 definetrends with study), followed by the Dacitic Old Yanaurcu (DOY, ~5 Ma: Beguelin´ intermediate depletions in these elements, and younger edifices et al., 2015; Boland et al., 2000), the Andesitic Old Yanaurcu (AOY, 3.58 (Nag˜ naro˜ Dome, Cerro Negro Group, Pilavo, Parulo) definetrends with ± 0.02 Ma: Beguelin´ et al., 2015), the Chachimbiro 1 stage of the Cha­ the strongest MgO and especially Ni depletions. Y contents decrease chimbiro Complex (~406–300 ka: Bellver-Baca et al., 2020), the systematically with SiO2 (Fig. 3e) although the oldest edifices (Pulum­ ˜ Nagnaro˜ Dome (NND, 171.6 ± 20.5 ka: B´eguelin et al., 2015), the bura, Dacitic Old Yanaurcu, Andesitic Old Yanaurcu) display a visibly Chachimbiro 2 stage of the Chachimbiro Complex (122–36 ka: Bellver- shallower slope with respect to the younger edifices. Also Sr/Y values ´ Baca et al., 2020), the Cerro Negro Group (CNG, 60.6 ± 20 ka: Beguelin display broad increases with SiO2 along slopes that are shallower for the et al., 2015), Pilavo and Parulo (<20–40 ka: Chiaradia et al., 2011), and oldest edifices compared to the younger ones (Figs. 3f). finallythe Chachimbiro 3 (36–23 ka) and Chachimbiro 4 (~6 ka) stages In plots of MgO versus various incompatible elements (e.g., Th, U, La, of the Chachimbiro Complex (Bellver-Baca et al., 2020). Sr) (Fig. 4a-d) the volcanic edificesare split into two contrasting trends: The rocks of the volcanic cluster range in composition from basaltic one (defined by Pulumbura, Dacitic Old Yanaurcu, Andesitic Old andesite to high-SiO2 dacite (54–70 wt% SiO2; Fig. 3a). Within this large Yanaurcu, Chachimbiro 1, and most of the Chachimbiro 2–3 samples) compositional spread, individual volcanic edificesdisplay variable SiO2 characterized by a very small increase of incompatible elements (from ranges, from <2 wt% up to ~10 wt% (Fig. 3a-f). ~1 to ~3 ppm) with decreasing MgO (sub-horizontal trend) and another

12 6 Trachyte ab 10 Trachy- Trachydacite 5 8 andesite Rhyolite

O (wt.%) Basaltic

2 trachy-

6 andesite O (wt.%) 4 2 Na O + K 4

2 Dacite Andesite Basaltic 3 Na 2 andesite

0 2 50 55 60 65 70 75 53 55 57 59 61 63 65 67 69 SiO2 (wt.%) SiO2 (wt.%) 6 140 c d 5 120 100 4 80 3 60 Ni (ppm) MgO (wt.%) 2 40

1 20

0 0 53 55 57 59 61 63 65 67 69 53 55 57 59 61 63 65 67 69 SiO2 (wt.%) SiO2 (wt.%) 25 ef120 20 100 15 80

10 Sr/Y

Y (ppm) Y 60

5 40

0 20 53 55 57 59 61 63 65 67 69 53 55 57 59 61 63 65 67 69 SiO2 (wt.%) SiO2 (wt.%)

Parulo Cerro Negro Group Chachimbiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Fig. 3. Geochemical plots of the rocks investigated in the present study: (a) TAS diagram for the classificationof the investigated rocks (Le Bas et al., 1986); (b-f) SiO2 versus Na2O, MgO, Ni, Y, Sr/Y.

5 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

16 7 ~25 ppm a b 14 6

12 5 10 4 8 3 U (ppm) Th (ppm) 6 2 4 2 1 0 0 0123456 0 1 2 3 4 5 6 MgO (wt.%) MgO (wt.%)

80 c d 1300

1100 60

900 40 Sr (ppm) 700 La (ppm) 20 500

300 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 MgO (wt.%) MgO (wt.%)

Parulo Cerro Negro Group Chachimbiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Fig. 4. Plots of incompatible elements versus MgO showing two distinct trends definedby samples of the older edifices(Pulumbura, Dacitic Old Yanaurcu, Andesitic Old Yanaurcu, stage 1 of Chachimbiro) plus stage 4 of Chachimbiro, and by samples of the younger edifices edifices (Nag˜ naro˜ Dome, Cerro Negro Group, Pilavo, Parulo). Stages 2–3 of Chachimbiro display mixed behaviour, with samples plotting on both trends. For discussion see text. one (some Chachimbiro 2–3 samples, Cerro Negro Group, Nag˜ naro˜ similar andesitic-dacitic composition. Stage Chachimbiro 4 (~6 ka) is Dome, Pilavo, Parulo) characterized by a vertical increase of incom­ much more felsic (rhyodacitic) than the previous stages Chachimbiro patible elements (up to extremely high values of ~25 ppm Th, 80 ppm 1–2-3 and is very homogeneous compositionally (Fig. 3). It shows a clear La, 7 ppm U, 1400 ppm Sr) at MgO contents between ~3 and ~ 4 wt% compositional and temporal gap with respect to previous rocks of Cha­ (Fig. 4). chimbiro (Fig. 3), which has been interpreted as the result of the erup­ Radiogenic isotopes (Fig. 5a-b) display relatively restricted ranges tion of a poorly crystalline rhyodacitic melt following thermal within each volcanic edifice, independent from the SiO2 ranges, result­ rejuvenation of a magma mush (Bellver-Baca et al., 2020). This stage of ing in clustered compositions of volcanic edificesin isotope ratio spaces. the Chacimbiro volcano has not been taken into account in the averages When considering the whole sample population, 206Pb/204Pb values of volcanic edifices to avoid a bias of this oversampled unit, which is 3 display broad correlations with major elements (Na2O, Al2O3, Fe2O3) volumetrically trivial (<0.002 km ) because derived from a single and Th/La (Figs. 5c-f). In contrast, Sr and Nd isotopes do not display any lateral blast eruption (see above and Bellver-Baca et al., 2020). significant correlation with major and trace elements (not shown). The plots of Figs. 6a-d show that average Pb isotope ratios of indi­ In order to compare the geochemical compositions of the different vidual edifices correlate with average major elements (Na2O, Fe2O3, volcanic edifices and evaluate their internal variability, we have Al2O3) as well as with average Th/La values. Additionally, the compo­ calculated average values and associated standard deviations (SD) for sitional variability (SD) of major elements (e.g., SiO2, Na2O) strongly elements and radiogenic isotopes of the sample populations of each correlates both with average compositions of major elements (Fig. 6e) volcanic center (Supplementary material 1; Fig. 6). Volcanoes have been and with average Pb isotopes (Fig. 6f): the larger is the major element ´ sampled thoroughly in all their units (Chiaradia et al., 2011; Beguelin spread of the rocks of a volcanic edifice(e.g., larger SD for Na2O: Fig. 6f) et al., 2015; Valverde, 2018; Bellver-Baca et al., 2020) and we estimate the less radiogenic are the average 206Pb/204Pb values of those rocks that the representativeness of the data is acceptable. (Fig. 6f). Pb isotopes display, within each volcano edifice,quite constant We have grouped together the three stages of the Pulumbura vol­ values, which result in low SD values (<0.05 for 206Pb/204Pb) that are canic edificebecause they display a similarly broad compositional range significantly smaller than inter-volcano differences (Figs. 5 and 6). despite their formation encompasses a temporal range of ~2 Ma (see The plots of Fig. 7 show that the average SiO2, Na2O, Ni values of the above and Supplementary material 1). We have split the Chachimbiro volcanic edifices broadly decrease through time (Figs. 7a-c), whereas 206 204 complex into two different stages (Chachimbiro 1 and Chachimbiro 2–3) average values of Pb/ Pb (Fig. 7d) and those of Fe2O3, Al2O3, CaO, reflecting the changing composition (from the andesitic Chachimbiro 1 Th, Y, HREE, Sr, Cs, and Th/La (not shown) increase through time. MgO, to the andesitic-dacitic Chachimbiro 2–3) through time of the different in contrast, remains roughly constant. Figs. 7e-f show that also the stages of this volcanic edifice (Bellver-Baca et al., 2020). Stages Cha­ variability (SD) of chemical elements, measured on the sample pop­ chimbiro 2 (~122–36 ka) and Chachimbiro 3 (~36–23 ka) have been ulations of each volcanic edifice, broadly decreases through time. In grouped together because their discrimination in the field is not contrast, the variability of Pb isotopes does not show any correlation straightforward, they are geochronologically continuous with the tem­ with time, and, as mentioned above, is small for all volcanic edifices(SD poral resolution currently available for this edifice, and they have very < 0.05 for 206Pb/206Pb).

6 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

19.2 19.2 a b

19.1 19.1 Pb Pb 204 204 19.0 19.0 Pb/ Pb/ 206 206 18.9 18.9

18.8 18.8 0.7038 0.7040 0.7042 34567 87Sr/86Sr eNd-CHUR 19.2 19.2 c d

19.1 19.1 Pb Pb 204 204 19.0 19.0 Pb/ Pb/ 206 206 18.9 18.9

18.8 18.8 2.5 3.0 3.5 4.0 4.5 5.0 14 15 16 17 18 19 Na2O (wt.%) Al2O3 (wt.%) 19.2 19.2 ef

19.1 19.1 Pb Pb 204 204 19.0 19.0 Pb/ Pb/ 206 206 18.9 18.9

18.8 18.8 345678910 0.1 0.2 0.3 0.4 0.5 Fe2O3 (wt.%) Th/La

Parulo Cerro Negro Group Chachibiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Fig. 5. Plots showing radiogenic Pb, Sr and Nd isotope compositions of the investigated rocks and correlations of 206Pb/204Pb with major and trace elements.

It is important to note that changes in the average geochemical with these volcanic edifices, through processes of fractional crystalli­ compositions and variability (SD) of the volcanic edifices appear to be zation, recharge, mixing and partial assimilation. Here, we discuss the roughly linear through time in Fig. 7 because the reported time scale is differences of geochemical compositions and their temporal evolution logarithmic. Looking in detail at the rate of such changes, it appears that within the cluster in the light of new geochronological and geochemical the three oldest volcanic edifices (Pulumbura, Dacitic Old Yanaurcu, data of Pulumbura, Pilavo and Parulo, and the integration of additional, Andesitic Old Yanaurcu), with ages from ~13 to ~3.58 Ma, have similar already published, geochemical data on volcanoes of the cluster (see average geochemical compositions and variability (SD values). In above and Supplementary material 1). ˜ contrast, the centers younger than ~172 ka (Nagnaro˜ Dome, Cerro The incompatible element (e.g., Th, U, Sr, La) versus MgO plots Negro Group, Pilavo, Parulo) are characterized by geochemical com­ (Fig. 4) highlight two strikingly different geochemical systematics. The positions significantly different and variability significantly lower than older edifices (Pulumbura, Dacitic Old Yanaurcu, Andesitic Old older volcanic edifices.The Chachimbiro 1 and Chachimbiro 2–3 stages Yanaurcu, Chachimbiro 1, all having ages between ~300 ka and ~ 13 of Chachimbiro, with ages slightly older to partially overlapping with Ma) display very shallow increases of Th, U, Sr, La with decreasing MgO. ˜ those of the younger edifices (400–22 ka), display intermediate com­ In contrast, the overall younger Nagnaro˜ Dome, Cerro Negro Group, positions (Figs. 4, 6, 7). Pilavo, Parulo (with ages <172 ka) plot along a nearly vertical trend of increasing Th, U, Sr, La within a narrow MgO interval between 3 and 4 5. Discussion wt% MgO (Fig. 4). The vertical trends in Fig. 4 are mostly controlled by samples of Pilavo, which show the broadest variability in terms of ˜ ˜ The volcanic edifices of the cluster have been investigated in detail incompatible elements, but also Parulo, Nagnaro Dome, and Cerro for their petrographic features, bulk rock and mineral compositions by Negro Group display higher incompatible element contents compared to previous studies (Chiaradia et al., 2011; B´eguelin et al., 2015; Valverde, older edifices in the 3–4 wt% MgO interval (Fig. 4). The oldest of the ˜ ˜ 2018; Bellver-Baca et al., 2020). All the above studies suggested a multi- young group of volcanic edifices (Nagnaro Dome, 172 ka) has stage, polybaric crustal evolution of the magmatic systems associated geochemical compositions that are close to the trends defined by older

7 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

a b 19.1 19.1 Pb Pb 204 204 19.0 19.0 Pb/ Pb/ 206 206

18.9 18.9

2.5 3.0 3.5 4.0 4.5 5.0 15 16 17 18 Na O (wt.%) 2 Al2O3 (wt.%) c d 19.1 19.1 Pb Pb 204

19.0 204 19.0 Pb/ Pb/ 206 206 18.9 18.9

456789 0.1 0.2 0.3 0.4 0.5 Fe2O3 (wt.%) Th/La 5.0 e f 19.1 4.5 Pb 4.0 204 19.0 O (wt.%) Pb/ 2 3.5 206 Na

3.0 18.9

2.5 01230.0 0.1 0.2 0.3 0.4 1SD SiO2 (wt.%) 1SD Na2O (wt.%)

Parulo Cerro Negro Group Chachibiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Fig. 6. (a-d) Correlations of average major element contents and Th/La with average Pb isotope compositions of the sample populations of the volcanic centers 206 204 investigated; (e-f) correlations of average values of Na2O and Pb/ Pb of the volcanic edificeswith major element spreads expressed as standard deviations (SD) of the populations. volcanic edifices (Fig. 4). Samples of Chachimbiro 2–3 stages of Cha­ Crystallization (REAFC) model of Lee et al. (2014) applied to a set of chimbiro (with unconstrained ages between 122 and 23 ka) display compatible and incompatible elements (Ni, Sc, Sr, Gd, Yb, Th) that are mixed behaviors, with some samples plotting along the trend of the differently incorporated into the main mineral phases crystallizing in young edifices,and most of the other samples plotting along the trend of calc-alkaline arc magmas (Fig. 8). The above elements allow us to the older edifices. The youngest Chachimbiro 4 stage of Chachimbiro evaluate the role of fractional crystallization of olivine±orthopyroxene (~6 ka) plots at the most fractionated end of the low incompatible (Ni), amphibole and clinopyroxene (Sc and Gd), plagioclase (Sr), garnet element trend defined by the older edifices. plus accessory phases like zircon and apatite (Gd, Yb) in the REAFC Lee et al. (2014) have shown that, in plots of incompatible versus model. Based on the examination of binary plots of major and trace el­ compatible elements, like those of Fig. 4 (e.g., Fig. 3 in Lee et al., 2014), ements (Figs. 3, 4, 8) we have subdivided the REAFC processes of the shallow trends correspond to a nearly pure fractional crystallization magmatic systems associated with the older edifices (Pulumbura, trend with limited recharge and crustal assimilation, and high evacua­ Dacitic Old Yanaurcu, Andesitic Old Yanaurcu, Chachimbiro 1) into tion rates. In contrast, near vertical incompatible element trends can three steps and those of the younger edifices (Nag˜ naro˜ Dome, Cerro only be reproduced by high recharge with little or no evacuation of the Negro Group, Pilavo, Parulo) into two steps (Fig. 8). As already dis­ magma chamber and some crustal assimilation. cussed above for the trends of Fig. 4, Chachimbiro 2–3 stages display mixed behaviors also in the plots of Fig. 8, with most samples plotting 5.1. Trace element geochemical modelling along the trends of the old edificesand some plotting along the trends of the young edifices. Because MgO cannot be used for reliable quantitative modelling, since no partition coefficients can be attributed to major elements, we 5.1.1. Step 1 have used the Recharge, Evacuation, Assimilation, and Fractional The plot of Fig. 8a displays strikingly different Ni depletions at equal

8 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

68 3.0 ab 66 2.5 64 2.0 (wt.%)

62 2 1.5 (wt.%)

2 60 1.0 58 SiO 1SD SiO 56 0.5

54 0.0 0.01 0.1 1 10 0.01 0.1 1 10

5.0 0.4 c d 4.5 0.3

4.0 O (wt.%) 2 0.2 O (wt.%)

2 3.5

Na 0.1

3.0 1SD Na

2.5 0.0 0.01 0.1 1 10 0.01 0.1 1 10

100 30 ef

80 20 60

40

Ni (ppm) 10 1SD Ni (ppm) 20

0 0 0.01 0.1 1 10 0.01 0.1 1 10

0.05 gh 19.1 0.04 Pb 204 Pb 0.03 204 19.0 Pb/

206 0.02 Pb/ 206

18.9 1SD 0.01

0.00 0.01 0.1 1 10 0.01 0.1 1 10 Age (Ma) Age (Ma)

Parulo Cerro Negro Group Chachibiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Fig. 7. Temporal changes of element and Pb isotope compositions of the investigated volcanic centers expressed as averages of the sample population of each center and of their standard deviations (SD).

9 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

200 Fig. 8. Plots of trace elements and trace element

ab1300 60 ratios with trends modelling Recharge, Evacua­ 160 tion, Assimilation, Fractional Crystallization 1100 80 (REAFC) processes as indicated in the legend. For 120 80 discussion see text. The percentages of fraction­ 60 900 58 90 ating minerals are from Tables S1-S2 in the Sup­ Ni (ppm) 80 Sr (ppm) 700 94 plementary material 3. Abbreviations for 38 97 60 48 70 80 minerals: ol = olivine; opx = orthopyroxene; cpx 40 17 500 80 = clinopyroxene; amph = amphibole; gar = 6 = = = 0 90 300 garnet; plag plagioclase; ap apatite; zirc 0 5 10 15 20 25 30 35 0 40 80 120 160 200 zircon; mt = magnetite. Numbers in italic indi­ Sc (ppm) Ni (ppm) cate the percentage of remaining melt. 30 6 c d 25 5 40 20 10 4 15 60 30 Gd/Yb Th (ppm) 3 40 70 10 80 70 78 2 20 5 90 97 99 4 94 0 1 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Ni (ppm) Ni (ppm)

6 e Fractionating assemblages of trends in old edifices (%) 5 60 Steps 1 70 ol 0.8, cpx 27.3, amph 10.5, plag 61.5 (based on 4 amph/gar = 1.6 Sc, Ni, Sr) 30 80 40 2 ol 1.4, cpx 24.6, amph 6.4, plag 61.1, ap 1.4, zirc Gd/Yb 3 60 0.1, mt 4.9 (based on Sc, Ni, Sr, Gd, Yb) 90 amph/gar = 5 3 cpx 7.9, amph 16.9, plag 68.7, ap 1.5, zirc 0.2, 2 80 mt 4.7 (based on Sc, Ni, Gd, Yb) 90 80 1 70 0 4 8 12 16 20 24 24 Fractionating assemblages of trends in young edifices (%) Th (ppm) 1 ol 62, opx 5, cpx 23.5, amph 2.4, gar 4.7, ap 2.2, zirc 0.08 (based on Sc, Ni, Sr, Gd, Yb, Th)

2 ol 2.7-2.8, cpx 62.6-65.3, amph 24-14.9, gar 4.8-9.4, mt 5.9-7.6 (based on Ni, Gd, Yb, Th, Sr)

Parulo Cerro Negro Group Chachibiro 4 Chachimbiro 1 Dacitic Old Yanaurcu Pilavo Ñagñaro Dome Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura

Sc contents for the old (Pulumbura, Dacitic Old Yanaurcu, Andesitic Old continental crust (~1.2 GPa: Müntener et al., 2001; see also Foden and Yanaurcu, most Chachimbiro 1 samples) and the young (Pilavo, Parulo, Green, 1992). Such pressures are consistent with the 40–50 km crustal ˜ Nagnaro˜ Dome, Cerro Negro Group) edifices. The Chachimbiro 2–3 thickness in the western Cordillera of Ecuador (e.g., Guillier et al., stages plot between these two end-member trends, although they are 2001). In terms of the other trace elements modelled (Sr, Gd, Yb, Th), closer to the Ni-depleted trend of the young edifices in the Sc-Ni space step 1 results in an increase of Sr and Th and in a slight decrease of Gd/ (Fig. 8a). Yb (Fig. 8b-e), which allows the quantificationof amphibole and garnet Making the plausible assumption that the parental magma of all fractionation in the model (see Supplementary material 3 and Tables S1- these edifices (Tables S1-S2) is an arc basalt with typical Ni (180 ppm) S2 therein). Modelling of step 1 for the young edificesalso requires high and Sc (32 ppm) contents (Kelemen et al., 2004) the different compo­ recharge (mass recharge/ mass fractionation = 0.8), low evacuation sitions of the two least evolved rocks from which originate the different (mass evacuation/mass crystallization = 0.002), and some crustal trends in the Sc-Ni space for the young and old edifices require clearly assimilation (mass assimilation/mass crystallization = 0.1: Table S2 in different REAFC processes (Fig. 8a). Model trends for the initial step 1 Supplementary material 3), which are all consistent with the deep reported in Fig. 8a (see Supplementary material 3 for details of the evolution of magmas associated with the young edifices in this step. modelling and how abundances of fractionating minerals were deter­ In contrast, the fractionating assemblage of step 1 for the old edifices mined) are indeed consistent with fractionation of dominant olivine (Fig. 8a- b) is characterized by dominant plagioclase, abundant clino­ (~62%) and clinopyroxene (~23%) plus subordinate amounts of pyroxene, subordinate amounts of amphibole and olivine, and no amphibole (~2%), garnet (~5%), and orthopyroxene (5%) for the trend garnet, and is, therefore, consistent with a shallow crustal evolution. of young edifices, whereas they require fractionation of dominant Mass balance parameters (Table S1 in Supplementary material 3) are plagioclase (~61%) and clinopyroxene (~25%) with subordinate also consistent with conditions typical of shallow level REAFC evolution amounts of amphibole (10%) and olivine (1%) for the old edifices (see Lee et al., 2014), with significantlylower recharge (mass recharge/ (Fig. 8a). The fractionation of olivine, clinopyroxene, orthopyroxene mass fractionation = 0.2), higher evacuation (mass evacuation/mass and minor amounts of garnet and amphibole of the young edifices is crystallizion = 0.02) and no assimilation, compared to step 1 of typical of basaltic melts evolving at high pressure at the base of the magmatic systems associated with the young edifices.

10 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

5.1.2. Step 2 proportion of the zircon fraction (a mineral that strongly fractionates The modelling of step 2 has been carried out using as parental HREE from MREE: Table S3 in Supplementary material 3), which still magmas the two end points of step 1 modelling (Fig. 8). Modelling has remains completely plausible in the model (~0.2% zircon) and is been carried out in order to fit the trends defined, on one hand by all consistent with the evolved composition of these rocks. ˜ rock samples of the young edifices( Nagnaro˜ Dome, Cerro Negro Group, Pilavo, Parulo), and, on the other hand, by rocks of the old edifices 5.2. Indications from average major element and Pb isotope compositions (Pulumbura, Dacitic Old Yanaurcu, Andesitic Old Yanaurcu, Chachim­ of the volcanic edifices biro 1) plus Chachimbiro 2–3, down to Ni values of 20 ppm. In fact, in the Ni interval 20–30 ppm some rocks of Chachimbiro 2–3 (and rocks of Our results show that individual volcanic edifices of the cluster the 6 ka old CH4 stage) show a strong increase in Gd/Yb values (Figs. 8d- display different average geochemical compositions of major elements e) which has been modelled in step 3 (see below). (e.g., Na2O, Al2O3, Fe2O3, SiO2) and Pb isotopes, and also different Modelling of step 2 in the Ni-Sc-Sr-Gd-Yb space for the old edifices geochemical variability (expressed as the standard deviations, SD) of (Fig. 8; Supplementary material 3) is consistent with an almost identical those parameters (Fig. 6). Additionally, they also show that the degree of main mineral assemblage as step 1, i.e., 61% plagioclase and 25% cli­ variability (SD) of major element compositions correlate with the nopyroxene, with the additional requirement of minor amounts of average major element and Pb isotope compositions of the edifices accessory mineral fractionation (~1.4% apatite and 0.1% zircon), which (Fig. 6). For instance, the volcanic edificesthat display a larger SiO2 and is plausible in these rocks, which are more evolved with respect to step Na2O variability (i.e., a broader compositional range) are characterized 206 204 1. In addition, step 2 is also characterized by low mass recharge/mass in average by higher Na2O and less radiogenic Pb/ Pb values fractionation values of 0.002, high mass evacuation/mass crystallization (Fig. 6). Because SiO2 and Na2O variability (SD) is arguably the result of values (0.2), and no assimilation (Table S1 in Supplementary material intracrustal magmatic differentiation, this points towards intracrustal 3). Therefore, step 2 geochemical and mass balance parameters are processes, rather than mantle source ones, as responsible for the major again consistent with an evolutionary process occurring at shallow element and Pb isotopic heterogeneities within the volcanic cluster, in crustal levels. agreement with the trace element models discussed above. In contrast, Figs. 8b-e show that modelling of step 2 for the young In order to test whether the REAFC models applied to trace elements edifices (Table S2 in Supplementary material 3) requires a completely can also explain average compositions and spreads (SD) of major ele­ different fractionating assemblage consisting of clinopyroxene ments of the volcanic edifices, we have used simple mass balance con­ (63–65%), amphibole (15–24%), garnet (5–9%) and magnetite (6–8%). straints to model a process of fractional crystallization followed by The range of variable values for the fractionating minerals explains the recharge and mixing of the recharge with the magma fractionated from two different trends of step 2 in the Th-Gd/Yb space for Pilavo rocks the previous step, repeating the process for 10 fractional crystallization/ (Fig. 8e), which require slightly different amphibole/garnet ratios in the recharge/mixing cycles (see Supplementary material 3 for details). In fractionating assemblage. The mineral assemblage modelled for step 2 of fact, the REAFC model used for trace elements cannot be applied to the young edifices is consistent with phenocrysts observed in Pilavo major elements for which partition coefficient values cannot be used. rocks (Chiaradia et al., 2011) and with deep fractionation (at least 0.8 The boundary conditions (parental magma, recharging magma, GPa) of hydrous andesitic melts (e.g., (Alonso-Perez et al., 2009). High recharge/fractionation ratios, compositions of fractionating minerals) of H2O pressure fractionation of Pilavo magmas is further supported by the such model are summarized in Tables S4-S6 of Supplementary material anorthite-rich (An80–92) cores of rare plagioclase phenocrysts (Chiaradia 3 and the model is described in detail in the Supplementary material 3. et al., 2011). Mass balance parameters of REAFC modelling (Table S2 in The proportions of fractionating minerals were derived from trace Supplementary material 3) needed to reproduce the step 2 trends of element modelling and varied systematically between a plagioclase-rich young rocks (mass recharge/mass fractionated = 0.44; mass evac­ assemblage for the high fractionation/recharge systems typical of the uation/mass crystallization =0.0022; mass assimilation/mass crystalli­ old edifices evolving at shallow levels, and an amphibole- and garnet- zation = 0.22) are also consistent with a deep crustal evolution of these bearing assemblage for the low fractionation/recharge systems typical magmas (Lee et al., 2014). of deep crustal evolution of the young systems (Table S6 in Supple­ The conclusions drawn from step 2 modelling of young edificesfully mentary material 3). For the sake of simplicity, no assimilation was agree with the model proposed by Chiaradia et al. (2011) for Pilavo. The considered for the modelling of major elements. interpretation of an intracrustal origin of the incompatible element We were able to reproduce the different average values of SiO2, enrichment in Pilavo lavas is supported by petrographic observations Na2O, Al2O3 and the different degrees of compositional spread (SD) of and mineral chemistry showing correlations of modal abundances of individual volcanic edifices by changing systematically the ratio be­ amphibole and thicknesses of amphibole reaction rims with Th and Sr tween the amount of recharge and the amount of fractional crystalli­ contents of bulk rocks (Chiaradia et al., 2011). These correlations clearly zation (R/FC; Fig. 9 and Table S7 in Supplementary material 3). As point out to an intracrustal origin of the incompatible element enrich­ expected, low R/FC values (<1) correspond to a larger spread in SiO2 ment in Pilavo lavas as quantified by the model discussed above. values with a typically high SD (Fig. 9). This reproduces well the chemical variability of the older magmatic edifices and is consistent 5.1.3. Step 3 with an evolution dominated by fractional crystallization of plagioclase The modelling of step 3 has been carried out to reproduce the high and clinopyroxene at shallow levels, where recharge is inefficient Gd/Yb values in some rocks of Chachimbiro 1 and Chachimbiro 2–3, and because magma cools rapidly due to the low enthalpy of the system. in all rocks of Chachimbiro 4 stages of Chachimbiro below 30 ppm Ni Under these conditions the magmatic system evolves through a thermal (Fig. 8d). Note that also rocks of the young edifices(in particular Pilavo) decline and with a variable enrichment in SiO2 for each new injected display variably high Gd/Yb values (Figs. 8d-e), but, as shown by Fig. 8e, batch of magma, resulting in a high SD value for the SiO2 content of the the processes leading to the high Gd/Yb values in these rocks (frac­ volcanic edifice.Gradually increasing R/FC (up to >20) corresponds to tionation of variable amounts of garnet and amphibole) are substantially an increasing efficiencyof recharge (because the recharged magma does different from those leading to high Gd/Yb values in rocks of the Cha­ not cool rapidly and is not evacuated), which is a process typically chimbiro 1–3 stages. In fact, modelling of trace element trends of step 3 associated with a deeper magma evolution (Lee et al., 2014; Weber can be accounted by fractionation of high proportions of plagioclase et al., 2020). This results in gradually decreasing ranges of SiO2 and (69%), significant amphibole (17%), minor amounts of clinopyroxene decreasing SD values of the SiO2 population (Fig. 9). (8%) and trace amounts of apatite and zircon. In particular, the strong We do not claim that the quantitative reproduction of the trace and increase of Gd/Yb values in these rocks is associated with an increased major element data by our models (Figs. 8-9) is the exact combination of

11 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

5.0 18 R/FC = 0.55-0.83 b 5.7-13.3 8.3-25 3.3-5 4.5

1.1-1.7 17 (wt.%)

4.0 3

O (wt.%) 2.2-3.3 O 2 2 3.5 2.2-3.3 5.7-13.3 3.3-5 16 1.1-1.7

3.0 8.3-25 mean Na R/FC = 0.55-0.83 2.5 15 5456 58 60 6264 66 68 5456 58 60 6264 66 68 mean SiO2 (wt.%) mean SiO2 (wt.%) 5.0 18 c R/FC = 0.55-0.83 d 5.7-13.3 3.3-5 4.5

(wt.%)17 8.3-25 Al mean 3 4.0 0.1 O

O (wt.%) 0.2 2.2-3.3 2 2 R/FC = 0.55-0.83 0.3 3.5 16 0.4

3.0 0.5 mean Al 1.1-1.7 mean Na

2.5 15 0123 0123 SD SiO2 (wt.%) SD SiO2 (wt.%) 4.0 0.5 e f R/FC = 0.55-0.83 R/FC = 0.55-0.83 0.4 3.0

0.3

(wt.%) 1.1-1.7 2 2.0 O (wt.%) 2 0.2 1.1-1.7 2.2-3.3 5.7-13.3 1.0 3.3-5 SD SiO 5.7-13.3 SD Na 0.1 8.3-25 8.3-25 3.3-5 2.2-3.3

0.0 0.0 5456 58 60 6264 66 68 2.5 3.5 4.5 mean SiO2 (wt.%) mean Na2O (wt.%)

Parulo Cerro Negro Group Chachimbiro 2-3 Andesitic Old Yanaurcu Pulumbura Pilavo Ñagñaro Dome Chachimbiro 1 Dacitic Old Yanaurcu Model

Fig. 9. Modelling of the average SiO2, Na2O and Al2O3 compositions of the investigated centers and of their standard deviations through a model of fractional crystallization, recharge and mixing in ten steps with different ratios of recharging to fractional crystallized magma (digits on the side of model symbols). See Supplementary material 3 and text for discussion. real parametric values since there are many variables that can act that is favoured by the increasing enthalpy of the deep magmatic sys­ together. However, we think that the occurrence of two broadly con­ tems (e.g., Annen et al., 2006). These deep systems become isotopically trasting processes (i.e., fractional crystallization and efficient recharge homogeneous (low SD of the 206Pb/204Pb values of the young volcanic with no evacuation at deep crustal levels versus dominant fractional edifices) through the repeated recharge, fractional crystallization and crystallization with evacuation at shallower levels) is supported by assimilation process, which also results in major element homogeneous successful independent modelling of both trace and major elements. compositions. The shift towards higher 206Pb/204Pb compositions of the Following the above modelling, we can also interpret the correla­ young edifices is not accompanied by a change in 207Pb/204Pb values, tions of average Pb isotope compositions with average major element which is consistent with assimilation of mafic rocks of the oceanic compositions (and Th/La) and major element spreads (SD) of the indi­ plateau Pallatanga terrane forming the lowermost part of the crust in the vidual volcanic edifices( Figs. 5 and 6). Figs. 5 and 6 show that average Western Cordillera of Ecuador (Chiaradia et al., 2020). Due to the broad 206Pb/204Pb values of the volcanic edificesbecome systematically more compositional fieldof these lower crustal rocks and the relatively small radiogenic with increasing Al2O3, Fe2O3, Th/La, decreasing Na2O and isotopic range displayed by the volcanic edifices in the cluster, calcu­ with decreasing compositional spread of major elements. In other words, lations of assimilation would be strongly biased by the largely uncon­ average 206Pb/204Pb compositions of volcanic edifices become more strained composition of the assimilant. radiogenic in edificesassociated with magmatic evolution at increasing Older edifices also display restricted Pb isotope changes, suggesting depths (corresponding for instance to increasing Al2O3 in Fig. 9). The that crustal assimilation was limited and that their isotopic composition systematic increase of 206Pb/204Pb values with depth of evolution can be broadly reflects that of the parental magma. The shallower level evo­ explained by an increasing assimilation of crustal rocks at deeper levels, lution of the older magmatic system (as discussed above) is consistent

12 M. Chiaradia et al. Chemical Geology 574 (2021) 120240 with a reduced assimilation due to the lower enthalpy of the system at petrogenetic processes of old and young edifices. Clearly a better shallow crustal levels. geochronological resolution coupled with geochemical data is needed It is significant that, among the radiogenic isotope systems, only Pb for the various stages of Chachimbiro to better understand how they fit displays correlations with element geochemistry. This is probably the in the overall temporal shift of geochemical signatures of the cluster. result of a higher Pb isotopic and concentration contrast between the There have been previous studies showing that a temporal transition crustal rocks and mantle-derived melts compared to Sr and Nd. in magma geochemical compositions, from normal arc to adakite-like signatures, is recurrent in several volcanic edifices of Ecuador (e.g., 5.3. Temporal changes Cayambe, Pichincha) during the last 1 Ma with a switch occurring at <~200 Ka (e.g., Samaniego et al., 2002, 2010), i.e. in the same temporal Although magmatism in the cluster is not continuous through time range as that occurring in the investigated cluster. This has been and has variably long-lasting lulls, the available data suggest that a attributed to the effects of the Carnegie ridge subduction, either because major change in geochemical features and petrogenetic processes of the younger magmas reflect the onset of slab melting (e.g., Bourdon et al., edificesoccurs around ~300–400 ka (Fig. 7). As discussed above, older 2003; Samaniego et al., 2002) or because the Carnegie ridge subduction volcanic centres (> ~ 300–400 ka: Chachimbiro 1; Dacitic Old has determined an increased compressional stress in the overriding plate Yanaurcu, Andesitic Old Yanaurcu, Pulumbura) display a variably large (Chiaradia et al., 2009, 2011, 2020). The onset of the Carnegie ridge compositional diversity, and are characterized by shallow level evolu­ subduction is debated, with proposed ages ranging from 1 to 15 Ma tion dominated by plagioclase-rich fractional crystallization with high (Lonsdale and Klitgord, 1978; Gutscher et al., 1999; Spikings et al., evacuation rates (Fig. 10). 2001). More recent studies suggest an old (5–6 Ma) onset of the ridge In contrast, geochemical compositions of young (<~172 ka) volca­ subduction (e.g., Collot et al., 2019), which would discount the nic edifices are consistent with evolution at lower to mid-crustal levels geophysical justificationto the hypothesis of slab melting as the cause of through olivine±orthopyroxene, clinopyroxene, amphibole and garnet the <1 Ma old geochemical shift of geochemical signatures in the fractionation, and low evacuation rates (Fig. 10). Samples of the Cha­ Ecuadorian arc volcanoes. chimbiro 2–3 stages (122–23 ka) present mixed geochemical features We propose two speculative explanations to the temporal shift of (Fig. 10) which could record the switching point between the geochemical and isotopic signatures in the volcanic edifices of the

Fig. 10. Cartoon showing the increase of depth of magma evolution in the Chachimbiro cluster through time as inferred from geochemical modelling of Figs. 8 and 9. For discussion see text. Abbreviations: amph = amphibole; cpx = clinopyroxene; gra = garnet; opx = orthopyroxene; plag = plagioclase

13 M. Chiaradia et al. Chemical Geology 574 (2021) 120240

Chachimbiro clusters that could also apply to the approximately coeval above two end-members. geochemical changes recorded by other volcanic edificesof Ecuador (see We speculate two possible scenarios to explain the swift deepening of above): magmatic evolution for the volcanic centers younger than ~300–400 ka: (1) an increased compression, possibly associated with subduction of (1) The deeper evolution of young magmatic systems (<~300–400 topographic anomalies of the Carnegie ridge, or (2) a self-constrained ka) could be related to increased crustal thickness (Chiaradia, change of the plumbing system due to prolonged and focused magma 2015; Farner and Lee, 2017) or increased compression (e.g., flow that has produced a strong thermal maturation of the crust and Cembrano and Lara, 2009). Crustal thickening is not consistent consequent deepening of the magma evolution. The timing of the with the swift change of geochemical signatures within the last 1 geochemical changes occurring in the Chachimbiro cluster broadly co­ Ma in the Chachimbiro cluster and at other Ecuadorian volcanic incides with that occurring at other volcanic edifices (e.g., Cayambe, centers. Therefore, increased compression is a more plausible Pichincha: see above), but more geochronological studies coupled with explanation, that would need further testing. Compression slows geochemical and structural investigations should be carried out to better down the ascent of magma from depth and increases the average resolve the timing(s) and understand the reason(s) of these changes depth of magma evolution within the crust compared to exten­ within the cluster and at the scale of the Ecuadorian arc. sion (e.g., Cembrano and Lara, 2009). Analogic modelling has So far, temporal changes in the geochemical composition of Ecua­ shown that subduction of aseismic ridges causes an increased dorian Quaternary volcanoes have been seen mostly at the level of compression in the overriding plate (Espurt et al., 2008; Martinod transition from “normal” to “adakite-like” signatures (high Sr/Y, La/Yb) et al., 2013). Although the Carnegie ridge subduction is older during the last 1 Ma. Our results suggest that changing intracrustal than 1 Ma (see above), the present-day topography of the Car­ magmatic processes during the last few hundreds of ka can also lead to negie ridge shows that the ridge is segmented with topographic significantdifferences in the major element composition of the erupted highs and lows. Such irregularities in the topography of the magmas with potential consequences on the eruption styles of Ecua­ Carnegie ridge could also exist in its subducted portion and might dorian volcanoes. have resulted in different degrees of compression in the past (e.g., ´ Hernandez et al., 2020). Declaration of Competing Interest (2) Another possible explanation for the deepening of the crustal level of magmatic evolution in the young volcanic edificescould The authors declare that they have no known competing financial be related to a self-constrained evolution of the plumbing system interests or personal relationships that could have appeared to influence due to long-lived magmatic activity in the cluster. Under this the work reported in this paper. point of view, coalescence of magma reservoirs due to structur­ ally focussed injection of melt into the same crust volume of the Acknowledgements Ecuadorian crust through a prolonged period may have led to a deepening of the level of magmatic evolution within the crust We thank two anonymous reviewers and Associate Editor Catherine (Biggs and Annen, 2019; Bellver-Baca et al., 2020). The obser­ Chauvel for their constructive comments that helped us to improve vation that the geochemical signatures typical of deep crustal significantly a previous version of our work. We are also grateful to evolution are mostly recorded by rocks of the three adjacent Alexey Ulianov for support with LA-ICPMS analyses, Mich`ele Senn for centers of Pilavo, Parulo and young Yanaurcu stages (Fig. 1) the chemical preparation of bulk rock samples for isotopic analyses, and could support such a hypothesis. Danijela Miletic for running 40Ar/39Ar analyses. This study was funded by the Swiss National Science Foundation (Project N. 200020_162415). More geochronological, geochemical and structural studies are needed to investigate in detail the timing and causes of such a change in Appendix A. Supplementary data the Chachimbiro cluster and in other Ecuadorian volcanic edifices. Supplementary data to this article can be found online at https://doi. 6. Conclusions org/10.1016/j.chemgeo.2021.120240.

We have presented and discussed geochemical and radiogenic References isotope data on 9 volcanic edificesfrom a long-lived (~13 Ma) volcanic cluster situated in the frontal arc of Ecuador. Trace element modelling Alonso-Perez, Rachel, Müntener, Othmar, Ulmer, Peter, 2009. Igneous garnet and shows that evolution of the magmatic systems feeding the volcanic ed­ amphibole fractionation in the roots of island arcs: experimental constraints on H2O undersaturated andesitic liquids. Contributions to Mineralogy and Petrology 157, ifices within the cluster is consistent with strikingly different processes 541–558. occurring within the crust. Edifices older than ~300–400 ka resulted Ancellin, M.-A., Samaniego, P., Vlastelic,´ I., Nauret, F., Gannoun, A., Hidalgo, S., 2017. from magmas that have evolved through plagioclase-dominated frac­ Across-arc versus along-arc Sr-Nd-Pb isotope variations in the Ecuadorian volcanic arc. Geochem. Geophys. Geosyst. 18 https://doi.org/10.1002/2016GC006679. tional crystallization at shallow levels. Under these conditions, such Annen, C., Blundy, J.D., Sparks, R.S.J., 2006. The genesis of intermediate and silicic magmas cooled rapidly and erupted a broad range of compositions at magmas in deep crustal hot zones. J. Pet. 47, 505–539. Baker, J., Peate, D., Waight, T., Meyzen, C., 2004. Pb isotopic analysis of standards and individual volcanic edifices. Edifices younger than ~172 ka evolved 207 204 ± samples using a Pb– Pb double spike and thallium to correct for mass bias with through olivine orthopyroxene, clinopyroxene, amphibole and garnet a double-focusing MC-ICPMS. Chem. Geol. 211, 275–303. fractionation at lower to mid-crustal levels. Under these conditions deep Barragan, R., Geist, D., Hall, M., Larsen, P., Kurz, M., 1998. Subduction controls on the magmatic systems were less prone to compositional differentiation compositions of lavas from the Ecuadorian Andes. Earth Planet. Sci. Lett. 154, – because cooling is much slower at deep crustal levels, especially as long 153 166. Beguelin,´ P., Chiaradia, M., Beate, B., Spikings, R.A., 2015. The Yanaurcu volcano as continuous recharges feed the system. This resulted in very homo­ (Western Cordillera, Ecuador): a field, petrographic, geochemical, isotopic and geneous compositions of the erupted materials from deep reservoirs. The geochronological study. Lithos 218-219, 37–53. ´ high enthalpy of the young magmatic systems favoured assimilation of Bellver-Baca, M.T., Chiaradia, M., Beate, B., Beguelin, P., Deriaz, B., Mendez- Chazarra, N., Villagomez, D., 2020. Geochemical evolution of the Quaternary the deep crust of the Western Cordillera of Ecuador, which consists of Chachimbiro Volcanic complex (Frontal Magmatic Arc of Ecuador). Lithos 356-357, mafic lithologies of the Pallatanga accreted oceanic plateau terrane, as 105237. https://doi.org/10.1016/j.lithos.2019.105237. indicated by Pb isotope systematics. The Chachimbiro 2–3 stages of the Biggs, J., Annen, C., 2019. The lateral growth and coalesence of magma systems. Phil. Trans. R. Soc. A 377, 20180005. https://doi.org/10.1098/rsta.2018.0005. Chachimbiro edifice, with ages between the older edifices and over­ Boland, M.P., Pilatasig, L.F., Ibandango, C.E., McCourt, W.J., Aspden, J.A., Hughes, R.A., lapping with the younger ones, display mixed behaviour between the Beate, B., 2000. Informe No. 10, Proyecto de Desarrollo Minero y Control Ambiental,

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