Petrological and geochemical constraints on the origin of adakites in the Garibaldi Volcanic Complex, southwestern British Columbia, Canada

Abstract

The Garibaldi Volcanic Complex (GVC) is located in southwestern British Columbia and comprises two related but distinct volcanic fields: the Garibaldi Lake and the Mount Garibaldi volcanic fields. The rocks of the GVC range from basalt to rhyolite, and analyses of samples from both fields distinguish these as adakites. The GVC magmas have high Sr/Y, Mg#, and Al2O3; low K2O/Na2O; and fractionated rare earth element compositions. Models of adakite genesis fall into two main groups: slab melting and non-slab melting. Adakites generated by slab melting commonly occur from young subducting crust (≤25 Ma) and are felsic partial melts of the subducting slab that interact with the mantle wedge during ascent. Non-slab melting models vary widely and include basalt fractionation, assimilation, fractional crystallization processes and partial melting of mafic lower crust. Data from the GVC are too limited to fully elucidate the mechanisms of adakite genesis; however, the petrographical and geochemical characteristics of the GVC rocks in this study do not refute an origin by slab partial melts. Variations in trace elements that reflect non-adakitic values (e.g., low La, low Cr) are likely the result of magma mixing at shallow depths within the magma reservoirs of each center, for which there is mineralogical and textural evidence. The adakite rocks of the GVC share geochemical traits akin to both low-SiO2 adakite (LSA) and high-SiO2 adakite (HSA) groups, though additional data are needed to investigate whether LSA- or HSA-type dominates within the GVC, and by extension, which should be the preferred model of adakite genesis.