Evolution of Silicic Magma Chambers and their Relationship to Basaltic Volcanism

Abstract

Silicic volcanism is commonly preceded by eruption of basalt. Similarly, the earliest phases of granitic plutonic complexes are often gabbro or diorite. Basalt magma must play a role in the initiation of a large silicic magma system, but three lines of evidence suggest that basalt magma also enters silicic chambers and influences their further evolution. These are: contemporaneous basalt vents flank silicic volcanic centers; thermal models of silicic bodies suggest that their heat must be replenished to maintain them in the upper crust for their observed life span; and petrologic data indicate that mafic clots and cognate xenoliths common in granodiorite and andesite represent basalt magma quenched within active silicic magma chambers. Phase assemblages in volcanic rocks and bulk composition of volcanic and plutonic rocks in suites of this type show that much of the variation in erupted or crystallized end products is due to this interaction of basaltic and rhyolitic magmas. It is proposed that basalt magma from the upper mantle provides heat for generating rhyolitic melt in the lower crust, and that the resulting rhyolite magma body continues to receive injections of basalt as it rises through the crust. Implications of this model are discussed.