Differentiation of calc-alkaline magmas: Evidence from the Coquihalla Volcanic Complex, British Columbia

Abstract

The Coquihalla Volcanic Complex is part of the Miocene, calc-alkaline Pemberton Volcanic Belt, whose formation in southwestern British Columbia is associated with subduction of the Juan de Fuca plate. The complex consists of voluminous rhyolitic ash-flow tuffs cross-cut by hypabyssal andesitic to dacitic intrusive bodies and a diorite stock. Whole-rock compositions of members of the Coquihalla Volcanic Complex show a range in silica from 54 to 76 wt.% (volatile-free). Uniform initial strontium isotopic ratios (0.7037 ± 0.00008) argue against derivation of magmas by crustal melting or mixing of basaltic magmas with crustal material. Trace element concentrations of whole-rocks cannot be modelled by varying degrees of partial melting of mantle material. Compositional variations among whole-rocks and constituent phenocrysts form the basis for major element mass-balance calculations which 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. Mineral/liquid distribution coefficients for eight trace elements and the observed mineral assemblages (plagioclase, clinopyroxene, hornblende, titanomagnetite, biotite, apatite, and zircon) have been compiled in an appendix. Large uncertainties in these values have led to a reformulation of the Rayleigh fractionation model so that an internally consistent set of distribution coefficients has been calculated for the proposed differentiation sequence.