Ilgachuz Range

Lying between the Rainbow Range and Itcha Mountains in the central Anahim Belt, the Ilgachuz Range is a moderately dissected shield volcano. The earliest rocks are rhyolitic and trachytic flows, breccias and intrusions with microphenocrysts of alkali feldspar and sparse ferrohedenbergite in a groundmass of alkali feldspar, aenigmatite, sodic pyroxene and arfvedsonite. The following 'Early dome-forming assemblage' (Souther, 1984, p. 2) comprises a number of cooling units, plugs and domes of comenditic rhyolite with groundmass aegirine, arfvedsonite and aenigmatite. The following 'Ilgachuz comendite' is the most voluminous single unit in the northern Ilgachuz Range and consists of numerous overlapping domes, flows, dykes, plugs and breccias. It contains 10-25% alkali feldspar, interstitial quartz and poikilitic arfvedsonite, aenigmatite and aegirine, and sparse opaques. A caldera then developed and filled with boulder and block deposits, vitric tuffs, and an uppermost feldsparphyric trachyte. The final 'Shield-forming assemblages' (Souther, 1984, p. 7) comprise flows of basalt and comendite and various types of basaltic pyroclastic rocks. Rock analyses are presented and discussed by Souther (1984, p. 8).