Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longav|¤ Volcano (36 28S; Andean Southern Volcanic Zone, Central Chile) CAROLINA RODRI ¤ GUEZ 1,2 , DANIEL SELLE ¤ S 1,2 , MICHAEL DUNGAN 1 , CHARLES LANGMUIR 2 AND WILLIAM LEEMAN 3 1 UNIVERSITE ¤ DE GENE ' VE, DE ¤ PARTEMENT DE MINE ¤ RALOGIE, 13 RUE DES MARAICHERS, 1205 GENEVA, SWITZERLAND 2 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, HARVARD UNIVERSITY, 20 OXFORD STREET, CAMBRIDGE, MA 02138, USA 3 NATIONAL SCIENCE FOUNDATION, 4201 WILSON BLVD., ARLINGTON, VA 22230, USA RECEIVED SEPTEMBER 22, 2006; ACCEPTEDJULY 31, 2007 ADVANCE ACCESS PUBLICATION OCTOBER 8, 2007 The mid-Holocene eruptive products of Nevado de Longav|¤ volcano (3628S, Chile) are the only reported occurrence ofadakitic volcanic rocks in the QuaternaryAndean SouthernVolcanic Zone (33^468S). Dacites of this volcano are chemically distinct from other evolved magmas of the region in that they have high La/Yb (15^20) and Sr/Y (60^90) ratios and systematically lower incompatible element contents. An origin by partial melting of high-pressure crustal sources seems unlikely from isotopic and trace element considerations. Mafic enclaves quenched into one of the dacites, on the other hand, constitute plausible parental magmas. Dacites and mafic enclaves share several characteristics such as mineral chemistry, whole-rock isotope and trace element ratios, highly oxidizing conditions (NNO þ1 5 to 4 NNO þ 2, where NNO is the nickel^nickel oxide buffer), and elevated boron contents. A two-stage mass-balance crystal fractiona- tion model that matches both major and trace elements is proposed to explain magmatic evolution from the least evolved mafic enclave to the dacites. Amphibole is the main ferromagnesian phase in both stages of this model, in agreement with the mineralogy of the magmas. We also describe cumulate-textured xenoliths that corre- spond very closely to the solid assemblages predicted by the model. We conclude that Nevado de Longav|¤ adakitic dacites are the products of polybaric fractional crystallization from exceptionally water-rich parent magmas. These basaltic magmas are inferred to be related to an exceptionally high, but transient input of slab-derived fluids released from serpentinite bodies hosted in the oceanic Mocha Fracture Zone, which projects beneath Nevado de Longav|¤. Fractional crystallization that is modally dominated by amphibole, with very minor garnet extraction, is a mechanism for generating adakitic magmas in cold subduction zones where a high flux of slab-derived fluids is present. KEY WORDS: adakite; amphibole; Andes; differentiation; Southern Volcanic Zone INTRODUCTION Most of the magmatism at convergent plate margins origi- nates in the high-temperature region of the asthenospheric mantle wedge as a consequence of fluxing by fluids liberated from subducted oceanic crust. Erupted magmas derived directly by melting of the oceanic crust are rare for a number of reasons. Foremost among these is that most thermal models require subduction of late Miocene or younger oceanic lithosphere to reach the temperature of the wet basaltic solidus at depths of 90^120 km (e.g. Peacock & Wang, 1999). By defining the geochemical characteristics of arc magmas presumed to be products of slab melting (elevated La/Yb, elevated Sr and Al 2 O 3 contents, and high Sr/Y) and then giving this magma type the name ‘adakite’ (following Kay, 1978), Defant & Drummond (1990) have created a serious nomenclatural *Corresponding author. Present address: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA. E-mail: dselles@fas.harvard.edu ß The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 48 NUMBER 11 PAGES 2033^2061 2007 doi:10.1093/petrology/egm049 Downloaded from https://academic.oup.com/petrology/article/48/11/2033/1563926 by guest on 04 January 2023