Date of Award
Master of Science
Geological and Atmospheric Sciences
Jacqueline E Reber
The Shonkin Sag laccolith (MT) is a differentiated mafic igneous body that concordantly intruded into undeformed Cretaceous sandstones at 50 Ma. Erosional processes have produced a cross-section-like exposure of the laccolith on the resulting cliff wall, revealing the base, roof, and eastern contact of the laccolith where it transitions to fringing sills. Previous researchers used this site to document fractionation and crystal settling and concluded that for a magma chamber the size of the Shonkin Sag laccolith to have formed and differentiated, it must have been emplaced in a single pulse. This conclusion contradicts more recent studies, which show that many plutons are emplaced in multiple small pulses over time.
By using a combination of six different methods, we show that the rapidly cooled margin of the laccolith and fringing sills preserves evidence of magma emplacement. Data from the eastern contact between the laccolith and fringing sills reveal evidence for internal contacts and large zones of solid-state deformation indicative of multiple emplacement pulses. Anisotropy of magnetic susceptibility, thin section analyses, image analyses, and thermal models support these observations. The combined data indicate that a minimum of seven pulses of magmatism must have occurred over ~3 years, while total cooling of the laccolith to the solidus would have occurred over ~21 years, during which the laccolith would have differentiated.
The detailed study of small intrusions such as the Shonkin Sag laccolith and its fringing sills provide insight into the emplacement processes of magma in active volcanic systems. These include the 1980 Mt. St. Helens, USA cryptodome and the rapidly emplaced laccolith during the 2011 eruption of Cordón Caulle, Chile, both of which caused extensive ground deformation over the course of a few weeks to months.
Claire Elizabeth Ruggles
Ruggles, Claire Elizabeth, "A multiple-pulse emplacement model for the Shonkin Sag laccolith, MT" (2020). Graduate Theses and Dissertations. 18212.