Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name


Department or School/College

Department of Geosciences

Committee Chair

Julia A. Baldwin


Cordilleran magmatism, pluton, stock, subduction


University of Montana


This study addresses the petrogenesis of a plutonic complex in a continental magmatic arc setting by examining the Skookum Butte stock (SBS), a satellite pluton of the Idaho-Bitterroot batholith (IBB). The SBS is part of widespread middle to Cretaceous to Eocene magmatism that occurred in the Cordilleran Interior and is dominated by biotite-muscovite granite/granodiorite and biotite-hornblende granite/granodiorite, to subordinate quartz monzodiorite, and monzogabbro. Both field evidence and geochemical data indicate that SBS magmas were generated from at least two distinct sources: a primitive source of mafic to intermediate composition and a more evolved felsic source. The mafic primitive members of the SBS have a significantly lower silica content averaging 59 wt % and are metaluminous. The granitic main-phase melt of the SBS is felsic (63.99-73.82 wt % SiO2) and slightly peraluminous (alumina saturation index = 1.04). There are two distinct felsic members represented by biotite-muscovite and biotite-hornblende granitic rocks that possibly reflect multiple magmatic episodes or differences in magma fractionation. The SBS as well as other Cordilleran Interior granitoids are distinct in mineralogy and geochemistry from granitoids generated in typical subduction-zone settings and these differences indicate a significant amount of crustal contamination. The SBS is lithologically and geochemically similar to other intrusive members of the IBB and these similarities have been determined to reflect melting of common crustal source materials, namely Proterozoic metasedimentary and basement rocks. These results suggest that first stage mafic melts of the SBS were most likely derived in a subduction-zone environment while late stage felsic and largely contaminated melts occurred in response to crustal thickening. Distinct from main-phase subduction related melts of the IBB, the SBS is assumed to have formed as an early stage magma that underwent lesser degrees of partial melting and crustal contamination

Plate1_SBSGeologicMap.pdf (3028 kB)
Plate 1: Geologic Map of the Skookum Butte Stock

Plate2SBSLithologyChart.pdf (47 kB)
Plate 2: Lithology Chart



© Copyright 2008 Connie Lynn Brown