Thermal state of the upper mantle beneath the Northern Cordilleran Volcanic Province (NCVP), British Columbia, Canada

Abstract

Data are presented for a suite of peridotitic mantle-derived xenoliths collected from basanite lavas within the Llangorse volcanic field, northwest British Columbia. The xenoliths comprise spinel lherzolite and subordinate spinel harzburgite. Two-pyroxene thermometry based on the Brey and Köhler (1990) [Brey, G.P., Köhler, T., 1990. Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology, 31, 1353–1378.] calibration was applied to 44 xenolith samples. The resulting geothermometry define minimum (800–850 °C) and maximum (1050–1100 °C) temperatures of equilibration for the xenolith suite which are estimates of thermal conditions in the underlying lithospheric mantle. We take the minimum temperatures as indicative of the maximum MOHO temperature; the maximum xenolith temperatures provide a minimum temperature for the transition from lithospheric to asthenospheric mantle. The geothermometry data are combined with published heat flow data to produce a set of model geotherms for this portion of the northern Cordillera. The model geotherms constrain the thickness of the mantle lithosphere in the northern Canadian Cordillera to between 16 and 30 km, corresponding to depths to the lithosphere/asthenosphere boundary of 52–66 km. We show that this model is consistent with an underlying convecting asthenospheric mantle with an average temperature and viscosity of 1210–1250 °C and 10 19.4 Pa s, respectively. We conclude by exploring the implications this model has for the source regions of alkaline magmas erupted over this portion of the northern Cordilleran volcanic province.