Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longaví Volcano (36·2°S; Andean Southern Volcanic Zone, Central Chile)

Abstract

SUMMARY AND IMPLICATIONS The two Holocene dacitic episodes at NLV produced themost evolved magma compositions that are preservedat this volcano, although occurrences of amphibole-richcumulate xenoliths in earlier main-cone lavas indicatethat magmas lying along similar liquid lines of descentwere being generated at depth by the NLV magmaticsystem prior to the Holocene (Selle¤s, 2006). The erupteddacites have adakitic signatures, high B and fluid-mobile/immobile trace element ratios, and amphibole-rich pheno-cryst assemblages, and they lack clinopyroxene. Quenchedmafic enclaves in dacitic rocks represent inputs of unu-sually water-rich magmas, which also have relatively lowconcentrations of most incompatible elements, high B,high mobile/immobile trace element ratios, and a highmodal proportion of amphibole phenocrysts.These simila-rities and their mingled association in the Lomas Limpiasdacite and Castillo Andesite strongly support a closegenetic link in the form of a parental role for the enclavemagmas.A polybaric, two-stage crystal extraction modelconstrained by experimentally determined phase relationsand natural mineral compositions reproduces whole-rockmajor and trace element trends and is a logical conse-quence of differentiation of these extremely water-richmagmas. Anomalously low incompatible trace elementconcentrations in dacites (except forboron) are not consis-tent with significant upper crustal assimilation, but theyare easily reconciled with volumetrically important frac-tionation of amphibole, which in turn is supported by thepresence of amphibole-rich cumulate xenoliths. The traceelement abundances in the early, high-pressure stage ofevolution have been adjusted by the fractionation of 1 5%garnet. Although modal garnet is not observed, experi-mental studies (Mu«ntener et al., 2001; Ulmer & Mu«ntener,2005; Alonso-Pe¤rez, 2006) have shown that it could bestable inwet mantle-derived melts at pressures correspond-ing to inferred lower crustal pressuresbelow NLV.The lackof erupted primitive melts precludes rigorous modeling ofthe early, deep fractionation, which is inferred to be thekey to the adakitic signature of the Holocene NLVmagmas. The deep hot zone model proposed by Annenet al. (2006) provides an adequate thermo-mechanicalframework for generation of magmas derived by fractiona-tion of high-pressure assemblages from hydrous, mantle-derived magmas.A crystal fractionation origin for adakite-like rocks hasbeen proposed in earlier studies for other arc settings,notably in the geodynamically complex Philippine arc(Castillo et al., 1999; Prouteau & Scaillet, 2003; Solidumet al., 2003; Castillo & Newhall, 2004; Macpherson et al.,2006), but also in the Ecuadorian Andes (Chiaradia et al.,2004) and the Archean of South Africa (Kleinhanns et al.,2003). All of these studies concur that amphibole plays apreponderant role in the generation of the adakitic signa-ture (with or without garnet), as well as limiting theamount of plagioclase being fractionated. Previous studiescall upon elevated magmatic water contents to explain theparticipation of amphibole-rich assemblages, althoughthe reasons why elevated water fluxes occurred at a givenplace and time remain largely unexplored. We proposethat this unique occurrence of adakitic magmas in theQuaternaryAndean SVZ is ultimately related to the posi-tion of Nevado de Longav|¤ above the down-dip projectionof the oceanic Mocha Fracture Zone, which could trans-port large amounts of water hosted in serpentinite bodies(e.g. Bonatti & Crane, 1984; Ulmer & Trommdorff, 1995;Kerrick, 2002; Macleod et al., 2002; Ranero et al., 2003;Ranero & Sallare's, 2004).