Eleventh Annual V. M. Goldschmidt Conference (2001) 3615.pdf

Phonolite-producing AFC processes at Hoodoo Mountain volcano, northern Canadian Cordillera. B. R. Ed- wards Department of Geology, Grand Valley State University, Allendale, MI 49401-9403 ([email protected]) J. K. Russell, Igneous Petrology Laboratory, Department of Earth & Ocean Sciences, University of , Vancouver, B.C. V6T 1Z4 ([email protected])

Field observations, petrological data sets, and thermo- involving alkali olivine and minerals representa- dynamic modeling support derivation of the tive of possible mid-crustal rocks, using MELTS [1], at Hoodoo Mountain volcano (HMV) from demonstrates that assimilation of K-rich crustal materi- alkali olivine basalt by a combination of fractional als is the most efficient process, thermodynamically crystallization and crustal assimilation (AFC) at pres- and geochemically, for producing large volumes of sures coincident with mid-crustal depths. Hoodoo phonolite . Constraints from kinetically con- Mountain is a Quarternary volcano of within the trolled models for AFC scenarios demonstrate that Northern Cordilleran volcanic province (NCVP) of rates of reaction are sufficient to produce the necessary northwestern British Columbia, Canada, and is one of geochemical differentiation within the 100 k.y. lifespan three large, peralkaline volcanoes within the NCVP. of Hoodoo Mountain. The attendant changes in magma Field evidence for AFC processes at HMV includes the density related to differentiation represent a viable presence of partly melted syenitic xenoliths in a basal- means of stimulating continued transport and eruption tic volcano adjacent to HMV, and the relatively com- of parental magmas that are stalled in the . In this mon occurrence of melted xenoliths in other nearby environment the slight changes in lithostatic pressure, basaltic volcanic centers [2]. Hoodoo Mountain com- for example those accompanying fluctuations in Cor- prises approximately 17 km3 of lavas, which have dilleran ice sheets, could amplify volcanism by “gla- maintained a nearly uniform chemical and mineralogi- cial” pumping of the magma-charged crustal litho- cal composition, straddling the phonolite- sphere. The common occurrence of crustal xenoliths at boundary, throughout the 100 k.y. history of HMV. All the other two Stikine subprovince volcanic centers is samples have (micro-) phenocrysts of alkali-feldspar, consistent with AFC processes playing an important clinopyroxene, and magnetite and are almost exclu- role in chemical modification of the mid- to upper crust sively nepheline and acmite normative. Compared to in the Canadian Cordillera. primitive mantle values, samples from HMV are 10- 200 times enriched in rare earth element concentrations References: [1] Russell JK, Hauksdottir S (In press) with moderate negative Eu anomalies; intra-volcano Can. Min. [2] Ghiorso MS, Sack RO (1995). Contrib. REE variations over 80 k.y. of eruptions are minor. Min. Pet. 119: 197-212 Thermodynamic modeling of likely AFC scenarios