MAGMA MINGLING AS A TRIGGER OF EXPLOSIVE SILICIC VOLCANISM? A TEST OF MODELS INVOLVING THE INTERACTION OF MAFIC AND FELSIC MAGMAS IN THE EASTERN BEAR PAW MOUNTAINS, MONTANA

Models proposed by Sparks (1977), Murphy et al. (1999), Eichelberger (1980), and Feeley and Dungan (1997) cite extreme temperature contrast, volatile exsolution, and equilibrating densities as plausible mechanisms responsible for driving pluton scale convection of interacting felsic and mafic magmas. Models are tested in the southeastern Bearpaw Mountains, MT. where abundant small mafic inclusions are distributed throughout well exposed latite plutons. Latite co-exists with small (0.5-3cm) subspherical shonkinite inclusions showing many stages of disaggregation, cuspate margins, and an outward decrease in grain size. Pluton-scale convection is evidenced by an even distribution of inclusions, compositional homogeneity, amphibole phenocryst alignment, heterogenous reaction-rim thicknesses, and multiple reaction rims on amphiboles. Exsolved gas vesicles in inclusions of pyroclastic rocks, coeval shonkinite (~1100ºC) and latite (~750ºC) magmas, resorbed pyroxenes, and thin amphibole reaction rims around shonkinite inclusions all point to the injection of a hot, volatile-bearing shonkinite magma into a latite magma host. Shonkinite stringers trailing off inclusions of both tuff-breccia and plutonic samples point to a still-molten state of shonkinite magma during a rapid eruption. Biotite and amphibole grains lacking reaction rims, and skeletal plagioclase grains that grew rapidly around melt cores, are also present in erupted samples. Deer Butte and Suction Butte in the northern Rattlesnake Quadrangle are both interpreted to be latite magma chambers that had partially crystallized before being intruded by an initially hot, dense shonkinite magma. As the shonkinite magma began to release heat into the host latite and crystallize, volatile phases exsolved, lowering the density of the shonkinite magma. The inclusions then rose buoyantly in the magma. Continued transfer of heat and volatiles rapidly initiated pluton-scale convection, effectively disaggregating and distributing the shonkinite magma. Exsolved volatiles from the shonkinite magma sufficiently raised the pressure within the magma chamber to drive explosive eruption of pyroclastic tuff and tuff-breccia.