Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

Abstract

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical 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, geochemical, 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), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene-and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., e Nd = ?1.5 to ?6, 87 Sr/ 86 Sr = 0.7040-0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/Yb N = 5.7-34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower-to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower-to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt-mantle interactions. The isotopic signatures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ''normal'' calc-alkaline magmatic rocks of Ecuador suggesting that the source of the magma did not change 123Contrib Mineral Petrol (2009) 1...

Vulcanismo De Afinidad Adaquítica en El Miembro Inferior De La Formación Combia (Mioceno Tardío) Al Sur De La Subcuenca De Amaga, Noroccidente De Colombia

“…Bourdon et al, 2002;Castillo, 2006) incluyen fenómenos de desgarre de la placa subducente, o subducción plana. Otras posibilidades mencionan fusión de la cuña mantélica por los fluidos de la placa subducente, o fusión parcial de corteza inferior máfica (Chiaradia et al, 2009). Pero algunos de estos mismos procesos petrogenéticos son invocados para explicar los patrones geoquímicos de los volcanes del sur de Colombia, cuyos productos en general no tienen afinidad adaquítica (e.g., Droux and Delaloye, 1996;Calvache and Williams, 1997;Marín-Cerón et al, 2010).…”

Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, northeast Iran

“…The compositional variations of the amphibole reflect changes in T , P , pH 2 O, f O 2 and melt composition (e.g., Rooney et al, 2010); intense fractionation of amphibole will increase the Sr / Y ratios and decrease the Y concentrations in the resulting melt (Foley et al, 2013), driving compositions to that of typical adakite magmas. Crystal clots composed of amphibole have been suggested to represent clusters of phenocrysts (Garcia and Jacobson, 1979), the products of amphibole breakdown (Stewart, 1975), cooler wall-rock material (Humphreys et al, 2009), disrupted cumulates or crystal mush zones (e.g., Seeman, 2000; Chiaradia et al, 2011), or xenoliths (Chiaradia et al, 2009). Our investigation shows that the mineral compositions and the crystallization conditions (T , P and f O 2 ) of the crystal clots overlap with the phenocryst assemblage of the host-rock, implying that the clots in the present system are clusters of microphenocrysts that crystallized from the magma chamber and thus represent the semi-solidified portions of a dynamic plumbing system.…”

Deciphering petrogenic processes using Pb isotope ratios from time-series samples at Bezymianny and Klyuchevskoy volcanoes, Central Kamchatka Depression

“…8), so Kamen compositions likely do not contribute to Bezymianny magmas. The presence of two mid-crustal xenoliths (01BZT09b and 01BZT09c) within Bezymianny samples lends support to the hypothesis of middle or lower-crustal assimilation, a common process in arc magmas discussed extensively since Bowen (1928) and Daly (1933), and quantitatively explored by a number of papers (e.g., Taylor 1980;DePaolo 1981;Hildreth and Moorbath 1988;Reiners et al 1995;Chiaradia et al 2009;Dufek and Bergantz 2005). N-MORB normalized rare earth element patterns of the xenoliths are flat and depleted relative to all other samples of Bezymianny and N-MORB (Fig.…”