Year of Award


Document Type

Thesis - Campus Access Only

Degree Type

Master of Science (MS)

Degree Name


Department or School/College

Department of Geosciences

Committee Chair

Julie Baldwin

Commitee Members

Andrew Ware, Steven Sheriff


dike swarm


University of Montana


The East Fork Dike Swarm (EFDS) at Lost Trail Pass (LTP) is one of many dike swarms that lie within the North American Cordillera, a region that has been transformed through the complex interactions between contemporaneous tectonics and magmatism. Determining the age of the EFDS at LTP, and the LTP pluton into which the dikes are emplaced, further constrains the timing of regional extensional tectonics during the Eocene. Classifying variations in lithology and geochemistry in these rocks provides insight into the petrogenetic relationships between the rocks in this study and other regional intrusive and extrusive igneous features. Petrographic analysis was used to characterize variations in lithology within the dike swarm and pluton related to differences in cooling history and composition. X-ray fluorescence (XRF) whole rock geochemistry of the EFDS at LTP and the LTP pluton was compared to geochemical data from regional igneous features to determine the petrogenesis of the rocks of the dikes and pluton. 40Ar/39Ar thermochronology of biotite from 6 samples yielded cooling ages of 48 to 50 Ma for each sample. Petrographic analyses determined that the large range of cooling histories within the field area, evidenced by textures ranging from aphanitic to phaneritic, suggests partial crystallization within a magma chamber and rapid cooling upon shallow emplacement of magma. Additionally, geochemical results showed that there was a large degree of variation in compositions ranging from mafic to felsic, and variations in aluminum content from metaluminous to peraluminous. Based on these results it has been determined that the LTP pluton was emplaced shallowly, cooled quickly, and fractured while still warm at which time the EFDS at LTP was emplaced and experienced rapid cooling. The dikes and pluton in this study are petrogenetically related to the dikes swarms assessed by Simonsen (1997), and the Challis intrusive suites studied by Gaschnig et al. (2011), specifically the lithologically variable quartz monzodiorite suite. The EFDS at LTP and LTP pluton are likely sourced from reactivated arc magmas related to Sevier and Laramide orogenesis during the Cretaceous.

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