A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades) |
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Authors: | Mathias Dessimoz Othmar Müntener and Peter Ulmer |
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Institution: | (1) Insitut de Min?ralogie et G?ochimie, Universit? de Lausanne, 1015 Dorigny, Switzerland;(2) Department of Earth Sciences, ETH Zurich, Clausiusstrasse, 25, 8092 Zurich, Switzerland |
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Abstract: | Amphibole fractionation in the deep roots of subduction-related 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 U.S. forms the lowest level of a 40-km 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 ∼1.0 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 tonalite. |
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