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The Shonkin Sag laccolith (Montana) is a differentiated mafic hypabyssal intrusion that was concordantly emplaced into undeformed sandstones. Erosional processes have produced a cross-section-like exposure of the laccolith, revealing its base, roof, and eastern contact, where it transitions to fringing sills. We present evidence for a multi-pulse emplacement model for the Shonkin Sag laccolith by showing that the rapidly cooled margin of the laccolith and fringing sills preserve features formed during emplacement. These features include internal contacts associated with variably deformed zones at the margin of the laccolith and normal and thrust faults within the surrounding sedimentary rocks. Bulk magnetic susceptibility measurements, anisotropy of magnetic susceptibility and thin section analyses support evidence for strain accumulation in pulses. Thermal models suggest that the pulses emplaced over ~3 years, while total cooling and differentiation of the laccolith to the solidus occurred over ~21 years. From these results, a three-stage emplacement model was determined. The model provides insight into ground deformation and associated hazards caused by the rapid emplacement of shallow magma bodies similar in size to the Shonkin Sag laccolith, such as the 1980 Mt. St. Helens cryptodome and the 2011 Cordon Caulle laccolith. |
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