Article Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling CHIARADIA, Massimo, et al. Abstract In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geo- chemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geo- chemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), andback-arc (Sumaco) chains. All rocks display typical sub- duction-related [...] Reference CHIARADIA, Massimo, et al. Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling. Contributions to Mineralogy and Petrology, 2009, vol. 158, no. 5, p. 563-588 DOI : 10.1007/s00410-009-0397-2 Available at: http://archive-ouverte.unige.ch/unige:18901 Disclaimer: layout of this document may differ from the published version. 1 / 1 Contrib Mineral Petrol (2009) 158:563–588 DOI 10.1007/s00410-009-0397-2 ORIGINAL PAPER Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling Massimo Chiaradia Æ Othmar Mu¨ntener Æ Bernardo Beate Æ Denis Fontignie Received: 23 September 2008 / Accepted: 25 February 2009 / Published online: 14 March 2009 Ó Springer-Verlag 2009 Abstract In the Northern Andes of Ecuador, a broad back-arc (Sumaco) chains. All rocks display typical sub- Quaternary volcanic arc with significant across-arc geo- duction-related geochemical signatures, such as Nb and Ta chemical changes sits upon continental crust consisting of negative anomalies and LILE enrichment. They show a accreted oceanic and continental terranes. Quaternary relative depletion of fluid-mobile elements and a general volcanic centers occur, from west to east, along the increase in incompatible elements from the front to the Western Cordillera (frontal arc), in the Inter-Andean back-arc suggesting derivation from progressively lower Depression and along the Eastern Cordillera (main arc), degrees of partial melting of the mantle wedge induced by and in the Sub-Andean Zone (back-arc). The adakite-like decreasing amounts of fluids released from the slab. We signatures of the frontal and main arc volcanoes have been observe widespread petrographic evidence of interaction of interpreted either as the result of slab melting plus primary melts with mafic xenoliths as well as with clino- subsequent slab melt–mantle interactions or of lower pyroxene- and/or amphibole-bearing cumulates and of crustal melting, fractional crystallization, and assimilation magma mixing at all frontal and main arc volcanic centers. processes. In this paper, we present petrographic, geo- Within each volcanic center, rocks display correlations chemical, and isotopic (Sr, Nd, Pb) data on dominantly between evolution indices and radiogenic isotopes, andesitic to dacitic volcanic rocks as well as crustal although absolute variations of radiogenic isotopes are xenolith and cumulate samples from five volcanic centers small and their values are overall rather primitive (e.g., 87 86 (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a eNd =?1.5 to ?6, Sr/ Sr = 0.7040–0.70435). Rare NW–SE transect at about 0° latitude and encompassing the earth element patterns are characterized by variably frac- frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and tionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We Communicated by T. L. Grove. interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers Electronic supplementary material The online version of this characterized by fractional crystallization and magma article (doi:10.1007/s00410-009-0397-2) contains supplementary mixing processes at different lower- to mid-crustal levels material, which is available to authorized users. as well as by assimilation of mafic lower crust and/or its M. Chiaradia (&) Á D. Fontignie partial melts. Thus, we propose that the adakite-like sig- Department of Mineralogy, University of Geneva, Rue des natures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb Maraıˆchers 13, 1205 Geneva, Switzerland values) are primarily the result of lower- to mid-crustal e-mail: [email protected] processing of mantle-derived melts, rather than of slab O. Mu¨ntener melts and slab melt–mantle interactions. The isotopic sig- Institut de Mine´ralogie et Ge´ochimie, Universite´ de Lausanne, natures of the least evolved adakite-like rocks of the active Anthropole, 1015 Lausanne, Switzerland and recent volcanoes are the same as those of Tertiary B. Beate ’’normal’’ calc-alkaline magmatic rocks of Ecuador sug- Escuela Politecnica Nacional, Quito, Ecuador gesting that the source of the magma did not change 123 564 Contrib Mineral Petrol (2009) 158:563–588 through time. What changed was the depth of magmatic Martin 1999; Smithies 2000; Kamber et al. 2002; Condie evolution, probably as a consequence of increased com- 2005; Martin et al. 2005). pression induced by the stronger coupling between the In a series of recent papers (e.g., Bourdon et al. 2002, subducting and overriding plates associated with subduc- 2003; Samaniego et al. 2002; Hidalgo et al. 2007; Hoffer tion of the aseismic Carnegie Ridge. et al. 2008), the adakite-like rocks of Ecuador have been considered the result of slab melting, but other works on Keywords Ecuador Á Quaternary Á Volcanism Á the same rocks suggest the possible role played by mag- Igneous fractionation Á Adakite Á Radiogenic isotopes matic processes at lower crustal levels (Kilian et al. 1995; Arculus et al. 1999; Monzier et al. 1999; Garrison and Davidson 2003; Chiaradia et al. 2004a; Bryant et al. 2006; Introduction Garrison et al. 2006), a petrogenetic process invoked also to explain the geochemical signatures of southern Colom- The Andean Cordillera is one of the best examples of bian volcanoes (e.g., Droux and Delaloye 1996; Calvache active continental arc magmatism on Earth. Along-arc and Williams 1997). variations in crustal thickness and composition as well as in In this study we report new petrographic, geochemical subduction styles produce magmatic rocks that differ in and isotopic (Pb, Sr, Nd) data of volcanic rocks from a geochemical and isotopic characteristics and indicate that transect across the Ecuadorian arc between latitudes various sources have contributed to their origin (e.g., James 0°300N and 0°300S including Quaternary volcanic centers 1982; Thorpe et al. 1983; Harmon et al. 1984). of the frontal arc (Pululagua, Pichincha), main arc (Cha- The Northern Andes of Ecuador are particularly inter- cana and Ilalo), and back-arc (Sumaco) (Fig. 1). Our new esting because a broad active volcanic arc has developed data show that, although slab melt metasomatism of the upon a crust consisting of both oceanic and continental mantle wedge cannot be ruled out, the observed adakite- accreted terranes (e.g., Feininger 1987; Litherland et al. like signatures of the volcanic rocks of Ecuador are best 1994), which allows evaluation of magma interaction with explained as the result of high-pressure fractionation of different crust types, and because of the occurrence of mantle-derived melts in the stability field of amphi- Quaternary magmatic rocks with adakite-like features (e.g., bole ± garnet, accompanied by assimilation of mafic lower Bourdon et al. 2003). Adakites were initially defined by crust and/or mixing with its partial melting products. Defant and Drummond (1990), following the study of Kay (1978), as magmatic rocks with distinctive geochemical signatures (e.g., Sr [ 400 ppm, Y B 18 ppm, Yb B Geological and geotectonic setting 1.8 ppm, La/YbN [ 10) derived from melting of subducted oceanic crust and many studies have applied this petro- Ecuador consists of six main physiographic domains (from genetic model to arc rocks with adakite-like signatures west to east: Coastal Plain, Western Cordillera, Inter- (e.g., Kay et al. 1993; Yogodzinski et al. 1995; Stern and Andean Depression, Eastern Cordillera, Sub-Andean Zone, Kilian 1996). However, it has also been proposed that the Amazon Basin), the four westernmost of which roughly same geochemical signatures of adakites can be formed coincide with proposed terranes of both continental and through other processes, such as partial melting of a mafic oceanic affinity (Fig. 1a, b). These terranes were accreted lower crust (e.g., Kay et al. 1987; Hildreth and Moorbath to the continental margin (Sub-Andean Zone and Amazon 1988; Atherton and Petford 1993) or high-pressure crys- Basin, Fig. 1b) between the Late Jurassic and the Late tallization of hydrous magmas (Mu¨ntener et al. 2001; Cretaceous (Feininger 1987; Megard 1989; Jaillard et al. Mu¨ntener and Ulmer 2006; Macpherson et al. 2006; Rod- 1990, 1997; Aspden and Litherland 1992; Litherland et al. rı´guez et al. 2007; Alonso-Perez et al. 2009). As pointed 1994; Kerr et al. 1997; Reynaud et al. 1999; Hughes and out by Garrison and Davidson (2003), among others, Pilatasig 2002; Mamberti et al. 2003; Kerr and Tarney magmatic rocks with adakite-like signatures can be any 2005; Vallejo et al. 2006). Geophysical (Feininger and rock that has formed from a basaltic protolith under pH2O– Seguin 1983) and geochemical (e.g., Mamberti et al. 2003) T conditions in which plagioclase is not stable and garnet data indicate the existence of an oceanic plateau basement (and/or amphibole) are stable.