A case for hornblende dominated fractionation of a

Authors: Mathias Dessimoz Othmar Müntener Peter Ulmer

Publish Date: 2011/10/04

Volume: 163, Issue: 4, Pages: 567-589

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Abstract

Amphibole fractionation in the deep roots of subductionrelated magmatic arcs is a fundamental process for the generation of the continental crust Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes The Chelan Complex in the western US forms the lowest level of a 40km thick exposed crustal section of the North Cascades and is composed of olivine websterite pyroxenite hornblendite and dominantly by hornblende gabbro and tonalite Magmatic breccias comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes Phase equilibria textural observations and mineral chemistry yield emplacement pressures of ∼10 GPa followed by isobaric cooling to 700°C The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3 of olivine websterite 12 of pyroxene hornblendite 33 of pyroxene hornblendite 19 of gabbros 15 of diorite and 2 tonalite Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process Crustal assimilation during fractionation is thus not required to generate peraluminous tonaliteThis study was funded by the Swiss National Foundation grant 200021–210054 Installation of the Element XR facility at UNIL was supported by SNF grant 206021117405 We would like to thank Laurent Nicod for thin sections preparation JeanClaude Lavanchy for XRF measurements Alexey Ulianov for the time invested to optimize the LAICPMS instrument James Allibon and Franois Bussy for granting time access to the microprobe Olivier Bachmann and Georges Bergantz Seattle USA for hospitality the Public Utility District of the Chelan county especially Rob Campbell Chelan USA and Eric Degman Wenatchee USA allowing the visit of the amazing Chelan gorge without flooding us Finally we thank G Bergantz and D Canil for constructive reviews

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