The geology of the east central desolation butte quadrangle, Grant County, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j67317983

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  • Pre-Tertiary metamorphic rocks, Jurassic granitic intrusions, and Eocene basalts are exposed along the North Fork of the John Day River at its confluence with Granite Creek. Geochemical and textural evidence suggest greenschist-metamorphosed, strongly sheared, volcanogenic rocks originated in an island-arc environment. These greenstones were apparently intruded during the Late Permian by a silicic pluton that is similarly metamorphosed and brecciated. South of this arc terrane, tectonically disrupted ophiolitic rocks are exposed. This east-west-trending belt of melange contains blocks of chert, metagabbro and metabasalt in a serpentinite matrix. Titanaugite indicates the original basalt may have been alkalic. Paleozoic or Triassic Elkhorn Ridge Argillite underlies much of the thesis area and consists mostly of contorted chert and argillite. Graywackes, greenstones and limestones are intercalated with Elkhorn Ridge Argillite. Regional metamorphism is lower greenschist facies. Two relatively fresh granitic stocks may be satellites of the Upper Jurassic Bald Mountain batholith exposed nine kilometers to the east. An intrusive sequence ranging from mafic quartz diorite to granite comprises the larger stock, exposed along Granite Creek. This pluton contains mostly quartz diorite and tonalite. A 0.5 kilometer wide stock of porphyritic tonalite intrudes argillite on the north side of the North Fork, John Day River canyon. Mineral assemblages in the contact metamorphic aureoles around the two stocks are characteristic of hornblende hornfels facies. Tertiary dark gray basalt overlies the Mesozoic and Paleozoic rocks at a profound unconformity. Geochemistry suggests the olivine-bearing, vesicular basalt is equivalent to the Clarno Formation exposed farther to the west.
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