Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

Abstract

Abstract. -Situated in the fore-arc of the Northern Volcanic Zone (NVZ) of the Andes in Ecuador, Pichincha volcano is an active edifice where have been erupted unusual magmas as adakites and high-Mg andesites. The particular geodynamic setting of the ecuadorian margin (i.e. the flat subduction of the Carnegie Ridge) suggests that thermo-barometric conditions for the partial melting of the oceanic crust are accomplished beneath this volcano. Pichincha adakites possess all the geochemical and isotopic characteristics of slab melts described in various other arc settings. High-Mg andesites with geochemical characteristics close to those of adakites present strong enrichments in MgO that suggest that, once they were produced by ca. 10 % partial melting of the downgoing subducted slab, some adakites en route to the surface strongly interacted with the peridotitic mantle wedge. Adakitic magmas could then represent, as in many other arcs where slab melting occurs, the principal metasomatic agent of the mantle in the NVZ in Ecuador. Résumé. -Situé dans la zone volcanique nord des Andes en Equateur, le volcan Pichincha est un édifice actif entré ré-cemment dans un nouveau cycle éruptif. La position exacte de la plaque plongeante sous l'Equateur est difficile à dé-terminer en raison de l'absence de sismicité intermédiaire dans la zone de subduction. Gutscher et al. Les adakites et les andésites riches en[1999] ont cependant récemment proposé qu'il puisse exister sous la marge une portion de plaque horizontale portée par la ride de Carnegie de faible densité qui est actuellement en subduction. D'autre part, Gutscher et al.[2000] ont observé à l'échelle du globe une relation étroite entre les portions de plaque subductée horizontale et la production de magmas adakitiques. Cette association suggère que l'horizontalisation de la plaque subductante permet une modification importante de la structure thermique du coin de manteau et le maintien de la lithosphère subductante dans les conditions de pression et de température compatibles avec la fusion de la croûte océanique. Un échantillonnage du volcan Pichincha a permis de mettre en évidence que les produits émis sont constitués d'andésites et de dacites moyennement potassiques possédant toutes les caractéristiques des adakites (c'est-à-dire des produits de fusion directe de la croûte océanique) telles que des enrichissements en Na 2 O, de fortes teneurs en Sr, mais de faibles concentrations en yttrium et en terres rares lourdes. Par ailleurs, on trouve parmi ces laves des andésites magnésiennes possédant à la fois les caractéristiques des adakites mais également des concentrations en magnésium éle-vées. Minéralogiquement, les laves du Pichincha contiennent essentiellement des cristaux de plagioclase, d'amphibole, d'orthopyroxène, de clinopyroxène et de magnétite. L'amphibole est généralement absente dans les andésites magné-siennes où le pyroxène devient le minéral dominant.Une modélisation géochimique montre qu'il est parfaitement possible d'expliquer la formation des adakites ...

Across‐arc versus along‐arc Sr‐Nd‐Pb isotope variations in the Ecuadorian volcanic arc

“…A large part of these data have been published in two regional‐oriented papers that discussed the trace element [ Schiano et al ., ] and Sr‐Nd‐O isotopic variations [ Hidalgo et al ., ] of the Ecuadorian volcanic arc. Additional data have been published by our group and other authors, in papers dealing with several large Ecuadorian volcanoes such as Sangay [ Monzier et al ., ], Antisana [ Bourdon et al ., b], Cayambe [ Samaniego et al ., ], Illinizas [ Hidalgo et al ., ]; Puyo and Pan de Azucar [ Hoffer et al ., ], Reventador [ Samaniego et al ., ], Mojanda‐Fuya Fuya [ Robin et al ., ], Pichincha [ Bourdon et al ., ; Robin et al ., ; Samaniego et al ., ], Cotopaxi [ Garrison et al ., ], Pilavo [ Chiaradia et al ., ], Chimborazo [ Samaniego et al ., ], and Tungurahua [ Samaniego et al ., ]. Some data were also published in regional studies such as those of Barragan et al .…”

“…To illustrate our approach, we selected one volcano from each alignment: Pichincha (Western Cordillera), Cotopaxi (at the boundary between the Inter‐Andean valley and the Eastern Cordillera), Tungurahua (Eastern Cordillera), and Sumaco (back arc). These volcanoes are well‐documented for their lead isotopic compositions: 20 samples for Pichincha [this paper; Chiaradia et al ., ; Bourdon et al ., ; Bryant et al ., ], 16 for Cotopaxi [ Garrison et al ., ; Bryant et al ., ], 64 for Tungurahua (this paper; and Nauret, personal communication) and 23 for Sumaco [this paper; unpublished IRD data; Chiaradia et al ., ; Bryant et al ., ; Bourdon et al ., ]. For each volcano, we show mean isotopic composition and individual data points in Figure , allowing a comparison of intravolcano and intervolcano isotopic variability.…”

“…Fluid-mobile elements (e.g., Ba, Pb, Li, Cs) are enriched with respect to fluid-immobile elements (e.g., Th, Nb, La) in the frontal part of the Ecuadorian arc, as pointed out above. It is noteworthy that fluid-mobile to [Barragan et al, 1998;Bourdon et al, 2002aBourdon et al, , 2002bBourdon et al, 2003;Samaniego et al, 2005;Bryant et al, 2006;Hidalgo et al, 2007;Robin et al, 2008;Hoffer et al, 2008;Chiaradia et al, 2009;Schiano et al, 2010;Samaniego et al, 2010;Samaniego et al, 2011;Chiaradia et al, 2011;Garrison et al, 2011;Hidalgo et al, 2012] and samples from this study are displayed (see supporting information).Volcano geological setting is color coded according to: blue 5 Western Cordillera (WC), Yellow 5 Inter-Andean Valley (IV), Green 5 Eastern Cordillera (EC), Orange 5 Back-Arc (BA). Arc zonation is visible as alkaline content at given SiO 2 increases away from the trench.…”

Evolution of the late Pleistocene Mojanda–Fuya Fuya volcanic complex (Ecuador), by progressive adakitic involvement in mantle magma sources

“…Apatite inclusions occur in amphibole, biotite, pyroxene and plagioclase. (Bourdon et al 2002b). Delineated fields are lavas from the Andean CVZ (same references as Fig.…”

“…In fact, 87 Sr/ 86 Sr (0.70407-0.70423) and 143 Nd/ 144 Nd (0.51285-0.51291) isotopic ratios are close to sea water altered MORB values (Barrett and Friedrichsen 1982), pointing out the lack of subordinate influence of subducted sediments or of crustal contamination. In Ecuador, no highly radiogenic end-member continental crust exists under the frontal arc, and contamination by this material is limited to a few percent under the Eastern Cordillera (Samaniego et al 2002(Samaniego et al , 2005Bourdon et al 2002b (1) Mindanao, Luzon and Batan islands (Philippines, Sajona et al 1993Sajona et al , 1996Sajona et al , 2000Prouteau et al 2000); (2) Sarawak and Kalimantan islands (Indonesia, Prouteau et al 1996Prouteau et al , 2001; (3): the Quimsacocha volcanic center (southern Ecuador; Beate et al 2001); (4) glassy adakitic inclusions from Bataan (Philippines, Schiano et al 1995); (5) results from experimental partial fusion of a basalt with/without interaction with peridotites (Rapp and Watson 1995;Rapp et al 1999); and (6) the average adakite composition from Martin (1999) only small amounts of crustal contamination might have affected the Mojanda-Fuya Fuya volcanic complex, and this process is unable to account for the Mojanda and Fuya Fuya geochemical differences.…”