UBC Theses and Dissertations
The Coquihalla volcanic complex, Southernwestern British Columbia Berman, Robert G.
The Coquihalla Volcanic Complex consists of calc-alkaline acid to intermediate extrusive and intrusive rocks which have an areal extent of roughly 30 km2, near Hope, British Columbia. The oldest and most voluminous members of the complex are rhyclitic pyroclastic rocks (variably welded lithic-crystal lapilli tuff, vitric tuff, and crystal-lithic lapilli tuff) , that have an overall thickness of approximately 1600 m. Later igneous activity produced numerous andesite to dacite domes, dykes, and sills- A late stage diorite to quartz-diorite stock forms the core of Coquihalla Mountain. Most pyroclastic rocks rest unconformably en the Jurassic to Cretaceous Eagle pluton. Monolithologic avalanche breccias formed in the southern portion of the map area, where pyroclastic recks were deposited against a fault scarp with uplifted Lower Cretaceous Pasayten Group rocks. In the southeastern part of the area, monolithologic avalanche breccias formed in response to tilting and uplift of the underlying Eagle pluton as the basin subsided. All tuffaceous rocks are characterized by vitroclastic textures, and contain phenocrysts of plagicclase (An 40-20), biotite, quartz, and minor potassium feldspar and titanomagnetite. Andesites are porphyritic with phenocrysts of plagioclase (An 76-30), calcic augite, magnesic- to tschermakitic hornblende, and titanomagnetite. Glomeroporphyritic clots consist of plagioclase, aluminous augite, and titancmagnetite. Porphyritic dacites contain phenocrysts of plagioclase(An 60-35), hornblende, titanomagnetite, and minor apatite. The diorite stock consists of orthopyroxene^ clinopyroxene, plagioclase, titanomagnetite, and ilmenite, with interstitial quartz and potassium feldspar. Three K-Ar dates average 21.4±0.7 Ma, and are concordant with a Eb-Sr isochron (22.3±4 Ma with initial 87Sr/86Sr = 0.70370±0.00008) based on seven whole rock sauries which span the entire compositional range of the suite. These results indicate that the Coquihalla Volcanic Complex is coeval with calc-alkaline centres in the Pemberton Volcanic Belt. The whole rock compositions of members of the Coquihalla Volcanic Complex show a range in silica contents from 54 to 76 weight per cent (volatile-free). In relation to increasing silica content, chemical variations within the suite are characterized by enrichment of K₂0, Na₂0, Rb, and Nb, and depletion in Al₂0₃, Ti0₂/ MgO, MnO, CaO, P₂O₅, Cr, Ni, V, and Sr. The elements Ba, Ce, Nd, and Zr show enrichment throughout most of the suite, but depletion in the most felsic members. Interpretation of chemical variations of whole rocks and constituent phenocrysts suggests that the chemical diversity of the suite is governed by fractional crystallization. The results of quantitative major and trace element modelling indicate that 1) hornblende dacites can be derived from basaltic-andesites by 50% crystallization of a mixture of plagioclase, hornblende, clinopyroxene, titanomagnetite, and apatite, and 2) rhyolites can be derived from dacites by roughly 45% crystallization of a mixture of plagioclase, hornblende, biotite, titanomagnetite, and apatite. Basaltic andesite compositions are consistent with derivation from basaltic liquids (modified by olivine fractionation) that are produced by partial melting of hydrous mantle peridotite above the subducted Juan de Fuca plate.
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