A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

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 w...

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Bibliographic Details
Published in:Contributions to mineralogy and petrology Vol. 163; no. 4; pp. 567 - 589
Main Authors: Dessimoz, Mathias, Müntener, Othmar, Ulmer, Peter
Format: Journal Article
Language:English
Published: Berlin/Heidelberg Springer-Verlag 01-04-2012
Springer Nature B.V
Subjects:
Continental crust
Crystallography
Earth and Environmental Science
Earth Sciences
Fractionation
Geochemistry
Geology
High pressure
Igneous rocks
Magma
Mineral Resources
Mineralogy
Original Paper
Petrology
Trace elements
Chelan Complex
Volcanic arc
Amphibole fractionation
Continental crust
North Cascades
Adakite
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Summary: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.
ISSN:0010-7999
1432-0967
DOI:10.1007/s00410-011-0685-5