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The Llangorse volcanic field : volcanology and mantle petrology Harder, Margaret Christine

Abstract

The Llangorse volcanic field (LVF) comprises Miocene to Holocene mafic alkaline volcanic rocks near Llangorse Mountain, in northwest British Columbia. Volcanic remnants of alkali olivine basalt, basanite, and nephelinite are exposed in the LVF. The largest of these volcanic remnants is the Llangorse Mountain locality, a 100 m thick basanite lava flow overlying debris flow sediments and containing peridotite xenoliths. Lavas at this locality are interpreted as a valley-filling flow that was substantially overthickened by ponding against a valley-wide barrier. The characteristics of the debris flow sediments and their temporal relationship with the overlying lavas suggest that they may derive from lahars or outburst floods related to melting of a snow pack or ice cap during the eruption. Palagonitized glass clasts in the debris flow sediments provide ancillary evidence of post-depositional conditions in the sediments and suggest that they were water-saturated when deposited and were heated to at least 80-120°C by the overlying flow. Peridotite xenoliths collected from the lava flow and underlying debris flow sediments at Llangorse Mountain are the basis for a detailed study of the mantle lithosphere beneath this portion of the Cordillera. The xenoliths are spinel lherzolites and spinel harzburgites. Twopyroxene thermometry is applied to 44 xenoliths, recovering minimum temperatures of 800- 850°C and maximum temperatures of 1050-1100°C for this xenolith suite. The minimum xenolith temperatures are taken to represent the maximum MOHO temperature, and the maximum temperatures are taken to approximate temperatures near the lithosphere / asthenosphere boundary. These data are combined with measured heat flow data in the northern Canadian Cordillera to produce a set of model geotherms, which are used to constrain the thickness of the mantle lithosphere in this area. These model geotherms indicate the mantle lithosphere in the northern Canadian Cordillera to be 18-39 km thick, corresponding to a depth to the lithosphere / asthenosphere boundary of 54 - 75 km. The implications of these geotherms are explored, defining three potential source regions for lavas in the LVF: a) the amphibole-bearing lithospheric mantle; b) the spinel-bearing base of the mantle lithosphere or upper asthenosphere; c) the deep, garnet-bearing asthenosphere. Variation in peridotite chemistry, mineralogy, and physical properties with depth are used to determine that the mantle lithosphere in the northern Canadian Cordillera is heterogeneous, with distinct layers of foliated, lherzolitic mantle versus non-foliated, harzburgitic mantle.

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