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Field relations and petrology of the Rainbow Range shield volcano, west-central British Columbia Bevier, Mary Lou


The Rainbow Range is a Late Miocene shield volcano (30 km diameter, 370 km³) whose stratiform flanks surround a complex central vent zone. Over a time span of 1-2 m.y., extrusion of highly fluid comendites and comenditic trachytes, along with minor mugearites and hawaiites, built up the gently sloping flanks. The viscosity of the peralkaline lavas was so low that their eruption produced a shield volcano rather than a composite cone. Comenditic trachytes (65.5 percent S10₂) are the lowest flows exposed on the north flank of the Rainbow Range. Chemical traits include high Na20 + K/jO (11 percent), moderately high AI₂O₃ (15 percent), low total iron as Fe₂0₃ (5 percent), and high Ba (300-1000 ppm). Thin flows of mugearite (54.9 percent Si0₂) rest on the comenditic trachytes. Comendites (68.7 percent Si02) uncon-formably overlie the mugearites and account for at least 75 percent of the volume of flows within the flank zone. These lavas are distinguished by lower AI2O3 (13 percent), higher total iron as Fe203 (7 percent), and extremely depleted Sr (1-10 ppm) and Ba (10-100 ppm). The termination of flank volcanic activity is recorded by the eruption of capping flows and related feeder dikes of hawaiite (50.1 percent Si0₂). Comenditic trachytes contain phenocrysts of anorthoclase (Or₂₅_₂₇), heden-bergite, and iron-titanium oxides in a groundmass of alkali feldspar, quartz, acmite, iron-titanium oxides, aenigmatite, and arfvedsonite. Comendites bear the phenocryst assemblage sanidine (Or₃₄_₃₇) + hedenbergite + fayalite + arfvedsonite set in a pilotaxitic groundmass of alkali feldspar, quartz, acmite, iron-titanium oxides, aenigmatite, and arfvedsonite. Continuous variation in major and trace element trends and feldspar compositions suggests that the hawaiite-mugearite-comenditic trachyte-comendite suite was derived from an alkali basalt parent, tapped several times as it underwent prolonged fractional crystallization in an intracrustal magma chamber. A best-fit mathematical model for the origin of the suite involves step-wise derivation of the lavas.in the order hawaiite > mugearite ? comenditic trachyte > comendite, with the main phases precipitating out in the order olivine, clinopyroxene, plagioclase, iron-titanium oxide, and alkali feldspar. Strontium isotopic evidence indicates that the peralkaline lavas were erupted soon after differentiation. The Rainbow Range and other peralkaline and alkaline volcanic centers of the Anahim volcanic belt are coeval.with calc-alkaline volcanic centers of the Pemberton volcanic belt. Together these belts outline the orientation and extent of the subducted Juan de Fuca plate during Late Miocene time. Volcanic activity in the Anahim belt may be related to a) an "edge effect" of the subducted Juan de Fuca plate, b) movement of the North American plate over a mantle hot spot at a rate of 2-3 cm/year, or c) an east-west trending rift zone.

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