Lithos 218–219 (2015) 37–53

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Lithos

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The Yanaurcu volcano (Western Cordillera, Ecuador): A field, petrographic, geochemical, isotopic and geochronological study

Paul Béguelin a,⁎, Massimo Chiaradia a, Bernardo Beate b,RichardSpikingsa a Section of Earth and Environmental Sciences, University of Geneva, rue des Maraîchers 13, 1205 Geneva, Switzerland b Department of Geology, Escuela Politecnica Naciónal, Quito, Ecuador article info abstract

Article history: The Yanaurcu volcanic center in the Ecuadorian frontal arc is characterized by several epochs of activity from the Received 23 October 2014 Early Pliocene to approx. 61 ka, with important changes in geochemistry and isotope ratio values throughout its Accepted 22 January 2015 history. Most of its units have high Sr/Y and La/Yb signatures. We present a comprehensive study of this volcano Available online 31 January 2015 involving morphological and stratigraphical observations, sampling, petrography, whole rock and in situ geo- chemistry, whole rock radiogenic isotope analysis and 40Ar/39Ar geochronology. We identify three magmatic Keywords: fl Adakite series: (1) Early Pliocene andesitic and dacitic volcanics, (2) Pliocene andesitic ows (~3.6 Ma), and (3) two Andesite Pleistocene andesitic domes (~172 ka and ~61 ka). Radiogenic isotope data suggest an increasing amount of Crustal processes basement assimilation through the evolution of Yanaurcu, as well as lower rates of ascent and increasing re- Ecuador charge at upper crustal levels, with the Pleistocene domes representing a thermally more mature state of the Garnet fractionation crust (deep and mid-crust becoming warmer with time when fluxed by a continuous magmatic supply). Recharge We also investigate the deep and mid-crustal magma evolution of Yanaurcu by modeling REE + Sr + Y with the Monte Carlo approach in order to get constraints on the fractionating assemblage. We highlight the importance of fractionating garnet in the development of high Sr/Y and La/Yb signatures, and propose a changing regional stress to modify the depth of magma evolution, and therefore the stability of garnet in the fractionating assemblage. Switches from transtensional to compressional regimes in Ecuador have been highlighted by various studies based on tectonic and thermochronological evidences. We conclude that (1) Yanaurcu high Sr/Y and La/Yb ratios can be fully explained by crustal processes and do not need an enriched mantle source nor slab melting, and (2) local thermal maturation of the crust is responsible for an enrichment of incompatible elements through time via recharge processes, whereas heavy rare earth element (HREE) depletion is controlled by regional tectonics. © 2015 Elsevier B.V. All rights reserved.

1. Introduction necessary, arc-scale geochemical comparisons alone leading to ambigu- ous conclusions (Garrison and Davidson, 2003). A careful assessment of The study of andesite petrogenesis at convergent margins is a key each volcanic center based on field, petrographic, geochemical, isotopic matter in petrology, because it concerns, among others, the origin of and geochronological observations is of great importance to arc-scale continental crust (Grove and Kinzler, 1986; Kelemen et al., 2003; studies, as small scale processes in space and time can explain major Reubi and Blundy, 2009; Rudnick, 1995) and the formation of large por- geochemical anomalies, such as large ion lithophile element (LILE) phyry copper deposits (PCDs, Chiaradia et al., 2012; Richards, 2003, enrichment (Andronico et al., 2005; Chiaradia et al., 2011, 2014; 2009; Sillitoe, 1972). The understanding of continental arc magmatism Davidson et al., 2005). is a crucial problem regarding to the above, and the impossibility for di- The Ecuadorian Andes are part of the Northern Andes and are char- rect access to deep processes in subduction zones as well as the struc- acterized by the accretion of one or several basaltic terranes (oceanic tural, geochemical and thermal heterogeneity of the overlying crust plateau, Vallejo et al., 2009; Fig. 1) to the continental margin. This mar- make major questions such as melt production and differentiation in gin has a long-lived history of transient stress regimes, switching from arcs difficult to answer. A multidisciplinary and multiscale approach is transtensional to compressional on various timescales (Pedoja et al., 2006; Spikings et al., 2001; Tibaldi and Ferrari, 1992). The Ecuadorian Quaternary arc coincides with the southern part of the Northern Andean ⁎ Corresponding author at: 80 chemin de Saule, 1233 Bernex, Switzerland. Tel.: +41 77 Volcanic Zone. This arc segment is particularly broad between ~1°N and 4239022. ~1°S, possibly related to the subduction under Ecuador of the aseismic E-mail addresses: [email protected] (P. Béguelin), [email protected] (M. Chiaradia), [email protected] (B. Beate), Carnegie Ridge (originated from the Galápagos hot spot: Gutscher [email protected] (R. Spikings). et al., 1999). Quaternary Ecuadorian lavas, but also rocks from the

http://dx.doi.org/10.1016/j.lithos.2015.01.014 0024-4937/© 2015 Elsevier B.V. All rights reserved. 38 P. Béguelin et al. / Lithos 218–219 (2015) 37–53

COLOMBIA Pilavo Yanaurcu

Pululahua Cayambe 0°

Pichincha Reventador Basement Chacana Pallatanga Terrane Inter-Andean Depression Sumaco Eastern Cordillera Cotopaxi Illinizas Sub-Andean nappes and Amazon Basin 1°S Quaternary volcanoes Extinct volcano ECUADOR Potentially active volcano Tungurahua Active volcano Currently erupting volcano 2°S Sangay 0 60 km N

Fig. 1. Tectonic map of Ecuador. The main basement units described in the text are shown (modified after Litherland et al., 1994) as well as all the Ecuadorian Quaternary volcanic centers (data from Instituto Geofísico EPN Quito, 2010–2013).

Tertiary Macuchi arc display generally high Sr/Y and La/Yb signatures Geochemical heterogeneities between adjacent volcanoes have (Quaternary arc: see publications below, Macuchi arc: Chiaradia, been described by Chiaradia et al. (2011, 2014).Theyoverprint 2009). This latter feature has been attributed to adakitic slab melts by across-arc trends highlighted by Barragan et al. (1998), Bourdon et al. various researchers invoking a possible low-angle subduction under (2003), Bryant et al. (2006), Chiaradia et al. (2009), Chiaradia et al. Ecuador (Bourdon et al., 2002a,b; Hidalgo et al., 2007, 2012; Robin (2014) and Le Voyer et al. (2008), implying that various processes con- et al., 2009; Samaniego et al., 2002, 2010), but this point of view was trol arc magmatism in Ecuador on different spatial scales. Chiaradia et al. challenged by Garrison and Davidson (2003) (see also reply from (2011, 2014) propose crustal processes as the likely cause of local het- Bourdon et al., 2004), arguing that geophysical evidences for such an erogeneities emphasizing that magma geochemistry is controlled by angle are scarce, that correlations between geochemistry and lateral both mantle source and crustal evolution (fractionation, assimilation changes in slab geometry are weak to non-existent, and that geochem- and recharge). ical resemblance between sampled lavas and pristine slab melts is not In this study, we present a complete field, petrographic, geochemi- convincing. Bryant et al. (2006) and Chiaradia et al. (2004, 2009, 2011, cal, isotopic and geochronological study of the Quaternary Yanaurcu 2014) propose other potential processes to explain this signature, that volcanic edifice, part of the Ecuadorian Western Cordillera. Our aim is is lower crustal recycling or high-pressure fractional crystallization of to investigate the petrogenesis of Yanaurcu over the last five millions basaltic mantle melts inside the stability field of amphibole and garnet years, and to understand geochemical variations through Yanaurcu's and outside the stability field of plagioclase. Indeed, crustal thickness history. Our study also provides new data on the Ecuadorian continental under the Ecuadorian arc is proposed to range from 50 to 70 km arc, since Yanaurcu had not been studied before our investigation. This (Guillier et al., 2001), and progressive depletion in middle rare earth el- is relevant for the understanding of the evolution of the Ecuadorian ement (MREE) and heavy rare earth element (HREE) with enrichment arc at the regional scale. in Sr and light rare earth element (LREE) is frequently observed in Yanaurcu is characterized by a large variability in terms of petrogra- Ecuadorian Quaternary magmatic series (Fig. 2). Switches between nor- phy, geochemistry and isotope composition among its temporally- mal and high Sr/Y and La/Yb magmatism in Ecuador have been de- different units. These tend not to form trends in bivariate plots, each scribed to occur during the Late Eocene–Oligocene (Chiaradia, 2009) stage being characterized by discrete clusters of values of element in the Macuchi arc, in the Late Miocene to Pliocene in southern concentration, element ratios and isotope ratios. Yanaurcu rocks show Ecuador (Quimsacocha volcanic center: Beate et al., 2001)andat petrographic evidences for recharge processes in the crust, and whole b1 Ma in the Quaternary arc. It has been proposed that they are related rock radiogenic isotope data show a radiogenic shift through Yanaurcu's to changes in the regional state of stress (Chiaradia et al., 2004, 2009; history. Whole rock trace element geochemistry is characterized by Schütte et al., 2010) or to the onset of Carnegie ridge subduction high Sr/Y and La/Yb ratios, driving some of Yanaurcu units into the geo- (Bourdon et al., 2003). chemical field of “adakites”, at least in a Sr/Y versus Y plot. We propose Download English Version: https://daneshyari.com/en/article/4715749

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