Subglacial, phonolitic volcanism at Hoodoo Mountain volcano, northern Canadian Cordillera

Abstract

Hoodoo Mountain volcano (HMV) is a Qua- ternary phonolitic volcano situated on the north side of the Iskut River, in the Coast Mountains of northwestern British Columbia, Canada. Its activity spans the last 100,000 years, it may have erupted as recently as 9 ka, and it encompasses a volume of approximately 17 km3. Throughout its history, much of the volcanic activity has been directly affected by glaciers, which accounts for its striking morphology and volcanic deposits. The physical development of the volcano includes at least six stages: (1) subglacial eruptions at about 85 ka; (2) ice-confined eruptions at about 80 ka; (3) subaerial(?) pyroclastic eruptions between about 80 and 54 ka; (4) subaerial effu- sive eruptions at about 54 ka; (5) subglacial eruptions between 54 and 30 ka; and (6) subaerial, post-glacial eruptions at about 9 ka. The chemical and mineralogical compositions of HMV lavas remained limited through- out the 100,000-year history of the HMV. All samples are phonolite or trachyte with (micro-) phenocrysts of al- kali-feldspar, clinopyroxene, and magnetite. Samples are almost exclusively nepheline and acmite normative. Compared with primitive mantle values, samples from HMV are 10-100 times enriched in rare earth element concentrations and have moderate negative Eu anoma- lies. Field observations, petrological data sets, and ther- modynamic modeling support derivation of the phonolite magmas at HMV from alkali olivine basalt by a combi- nation of fractional crystallization and crustal assimila- tion at mid-crustal pressures. The attendant changes in magma density related to differentiation permitted the continued buoyant ascent and eruption of phonolitic magmas derived from basaltic parental magmas that stalled in the crust. In this environment, the slight chang- es in lithostatic pressure accompanying fluctuations in Cordilleran ice sheets could amplify volcanism by "gla- cial pumping" of the magma-charged crustal lithosphere.