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Thickness and composition of the Bonanza Arc crustal section, Vancouver Island, Canada

Dante Canil, Jolene Styan, Jeff Larocque, Emilie Bonnet and Jeff Kyba
Thickness and composition of the Bonanza Arc crustal section, Vancouver Island, Canada
Geological Society of America Bulletin (July 2010) 122 (7-8): 1094-1105

Abstract

The composition and origin of continental crust is in part constrained by evidence preserved in exhumed arc crustal sections. The Bonanza arc on Vancouver Island, Canada, is a Jurassic-aged arc crust section exhumed in the Eocene. We use Al-in-hornblende geobarometry of felsic plutons in the Bonanza arc to assess the depths of crystallization of its exposed arc crust. Plutons of the unfoliated Island Plutonic Suite are emplaced as sheets at depths of 2-10 km within a stratigraphy of Devonian to Triassic supracrustal rocks. The West Coast Complex is more generally strained, and intrudes below the pre-Jurassic supracrustals at depths of 10-18 km. Previous work shows that rare ultramafic units within the West Coast Complex crystallized at depths of 14-26 km. The Bonanza arc crustal section has a present structural thickness of at least 15 km. The average composition for each arc component (volcanic and plutonics) when apportioned into the crustal section derived by stratigraphy and hornblende barometry, shows the bulk composition of the entire arc is basaltic andesite (56% SiO (sub 2) , Mg/Mg+Fe (Mg50), not unlike some estimates for bulk continental crust. Although only vestiges of ultramafic rocks occur in the Bonanza arc today, a significant component (6-20 km) of this rock type must have been present in the section for the bulk arc to have been a melt in equilibrium with mantle olivine, and may have been lost by foundering or tectonic thinning.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 122
Serial Issue: 7-8
Title: Thickness and composition of the Bonanza Arc crustal section, Vancouver Island, Canada
Affiliation: University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada
Pages: 1094-1105
Published: 201007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 54
Accession Number: 2010-050188
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2010046
Illustration Description: illus. incl. sects., 3 tables, geol. sketch maps
N48°30'00" - N51°00'00", W128°30'00" - W123°30'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201027
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