Petrology and geochemistry of plagiogranite and related basic rocks of the Canyon Mountain ophiolite complex, Oregon
Gerlach, David Christian
Lallemant, Hans G. Avé
Master of Arts
The Canyon Mountain complex, located in the Blue Mountains of eastern Oregon, is part of a belt of Paleozoic and Mesozoic rocks believed to represent former island arc and oceanic crust. Stratigraphic relations and radiometric dating have confirmed the age of the complex to be Permian. The Canyon Mountain complex is similar in many ways to other typical ophiolites, although some problematical differences are apparent. Sheeted basalt dikes normal to cumulate layering, and pillow basalts are absent. Basaltic intrusions in the upper portion of the complex cure mostly in the form of sills. A large volume of plagiogranite is present at Canyon Mountain as well. Plagiogranites are concentrated along the contact of high-level hornblende gabbros and diabase with the overlying keratophyre host rocks. Diabase and basalt both predate and intrude the plagiogranites. The field relations, and major- and trace element chemistry support a model by which the plagiogranites were derived by lowpressure partial melting of hydrothermally-altered basic rocks. It is also possible that some of the plagiogranites could be late-stage differentiated liquids. The major and trace element chemistry of the gabbros, diabases, and basalts afford evidence that the ophiolite complex was an open system during much of its development. The evidence does not support a simple model of fractional crystallization of a basaltic magma, but repeated influxes of new basic magmas, each possibly produced by partial melting of a mantle source already depleted in LIL elements. Different mantle source lithologies or more complex melting models may be necessary to explain the comparatively greater depletion in both LREE and HREE in certain samples of basalts. The Canyon Mountain ophiolite was probably created as a result of short-lived magmatism representing incipient spreading in a marginal or back-arc basin. The keratophyres may represent volcanic rocks derived from an adjacent island arc; these may be correlative with arc volcanics of Lower Permian age in the Seven Devils terrane to the north, in which case the Central Melange terrane enclosing the Canyon Mountain complex is also exotic. These terranes, and possibly the Huntington Arc terrane as well, were created at a great distance from the North American continent and were accreted in the Late Jurassic based on structural, stratigraphic, and paleomagnetic data.