Date of Award
Doctor of Philosophy (PhD)
Field and laboratory investigations were employed to detmine the mode of emplacement and a petrogenetic model for three igneous localities in Crook County, Wyoming: the Devils Tower, the Missouri Buttes, and the Barlow Canyon area.
X-ray fluorescence, microprobe data, and optimal analyses iden tify the Missouri Buttes rock as foid-bearing alkali trachyte and anal cime phonolite and the Devils Tower and Barlow Canyon rocks as analcime phonolite.
Associated alloclastic breccia with a crystal-charged volcanic glass matrix, surrounding depressions representing collapse of igneous material back into the vent, striking similarity to known volcante necks, and the occurrence of other extrusive volcan1S11\ in the vicinity lead to the conclusion that the Devils Tower and the Missouri Buttes are the erosional remnants of volcanic necks,
The Barlow Canyon intrusion is a small laccolith which caused additional folding of an Early Cretaceous dome.
Analcime constitutes 10% to 30% of the phonolite in Devils Tower and the Missouri Buttes, and up to 70% of the Barlow Canyon phonolite Calculated analcime unit cell dimensions are a0 =13.736 ± 003A, and reflect a silica-undersaturated composition. Analcime microphenocrysts and some groundlmass analcime are interpreted as primary igneous phases. The remainder of the analcime was magmatically derived from hydrous, sodium-rich fluids. The analcime-liquid stability field indicates a crystallization depth of 18 km to 43 ·km and a temperature of 600° 640°c.
Microprobe and x-ray fluorescence data plot along the trachyte phonolite trend and show a differentiation pattern from Missouri Buttes· trachyte to Missouri Buttes and Devils Tower phonolite to Barlow canyon phonolite, Differentiation occurred by fractional crystallization through the mechanisms of flotation and flow differentiation,
The rocks in all three localities are intratelluric material which in the Missouri Buttes and Devils Tower has been rapidly pro pelled to the surface by H20 and CO2 pressure.
Halvorson, Don L., "Geology and petrology of the Devils Tower, Missouri Buttes, and Barlow Canyon area, Crook County, Wyoming" (1980). Theses and Dissertations. 119.