QUANTIFYING MAGMA ASCENT RATES AND CRUSTAL CONTAMINATION TIMESCALES IN A CONTINENTAL MAGMATIC SYSTEM: A CASE STUDY OF THE BLOCK MOUNTAIN LAVA FLOW, SW MONTANA

The ascent of mantle-derived magmas through the continental crust and subsequent magma-crust interactions are commonly preserved in continental magmatic rocks. Here we use in situ crystal chemistry, thermobarometry, and geospeedometry to reconstruct the pressure-temperature-time pathway associated with an Eocene extensional magmatic event in SW Montana. The ~45 Ma Block Mountain basaltic-andesite (~56-58 wt.% SiO₂) contains three texturally and compositionally distinct types of clinopyroxene that equilibrated in distinct P-T environments. Large (100-500 μm), subhedral clinopyroxene phenocrysts (En₄₈Wo₄₁Fs₁₁) characterized by relatively high Mg# (~82), Al₂O₃ (~5.7 wt.%) and Cr₂O₃ (~0.29 wt.%), are in chemical equilibrium with the bulk-rock basaltic-andesite, and equilibrated at high temperatures and pressures (~1210-1220 °C and 610-940 MPa) consistent with crystallization within the upper mantle. Smaller (<100 μm), euderal to subhedral, clinopyroxene microphenocrysts (En₄₆Wo₃₇Fs₁₇) with notably lower Mg# (~73), Al₂O₃ (~2.5 wt.%), and Cr₂O₃ (~0.20 wt.%) make up ~40% of the basaltic-andesite groundmass (GM), and appear to have crystallized over a larger, lower range of temperatures and pressures (1166-1180 ° C and 380-860 MPa), presumably during ascent through the lower crust (~31-14 km). An additional population of clinopyroxene microphenocrysts (<100 μm; En₄₄Wo₄₀Fs₁₅) is found exclusively in the diffusion coronas (clinopyroxene-glass reaction rims) surrounding resorbed quartz crystals. These microphenocrysts have Mg#s (~74) similar to microphenocrysts in the GM but have significantly lower Al₂O₃ (~0.45 wt.%) and Cr₂O₃ (b.d.l.) concentrations, and crystallized at significantly lower temperatures and pressures (1006-1101 ° C and 140-300 MPa). Corona textures and glass compositions (~73 wt.% SiO₂) indicate that the microphenocrysts formed during the peritectic reaction of quartz and the ascending mafic magma in the upper crust (5-11 km). We utilize crystal growth and diffusion-based element modeling in the groundmass and corona glass to determine (1) the rate at which the mafic magma ascended from the upper mantle to the upper crust (groundmass clinopyroxene), and (2) the timescale over which ascending mafic magma interacted with the upper crust prior to eruption.