Castle Rocks : a late miocene eruptive center at the north end of Green Ridge, Jefferson County, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vd66w4747

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  • Castle Rocks is 12 miles east of Mt. Jefferson, in the central Cascades of Oregon. The area contains volcanic rocks ranging from mid-Miocene to Holocene in age. These rocks record alternating periods of tholeiitic and calc-alkaline volcanism. The oldest rocks in the area range from basaltic andesite lava flows to fayalite-bearing dacite breccias, and are best exposed near the confluence of the Metolius and Whitewater rivers. Petrographic and compositional evidence indicates that the dacites are highly differentiated products of a tholeiitic fractionation trend. The dacites were apparently derived from basalt by the fractionation of olivine, clinopyroxene and plagioclase. The most prominent feature of the thesis area is the remnant of a calc-alkaline composite volcano of late Miocene age. The center of the volcano is marked by a pyroxene andesite plug near the north end of Green Ridge. This "Castle Rocks Volcano" produced andesitic lava flows and pyroclastic rocks characterized by abundant hornblende phenocrysts. Compositional trends are consistent with fractionation of hornblende, clinopyroxene and possibly plagioclase. The Castle Rocks Volcano is unconformably overlain by ash-flow tuffs and basaltic andesite lavas of the Deschutes Formation. The unconformity represents a period of change from compressional to extensional tectonics. Deschutes Formation rocks show tholeiitic compositional trends, apparently produced by the fractionation of olivine, clinopyroxene and plagioclase. North-south normal faulting on the Green Ridge fault system terminated deposition of the Deschutes Formation at about 5.4 Ma. Three parallel faults in the thesis area are spaced about 0.5 miles apart. Maximum displacement is about 2200 ft, in the southern part of the thesis area, and decreases to a negligible amount at the confluence of the Metolius and Whitewater rivers. High Cascade rocks representing two magma types erupted after the episode of normal faulting. Subduction-related, calc-alkaline basaltic andesites are confined to the area west of the Green Ridge fault scarp. They show compostional patterns very similar to those of the Castle Rocks Andesites, but are less phyric and lack hornblende. Diktytaxitic basalts spread east to the north of the Green Ridge fault scarp to form Metolius Bench. Younger diktytaxitic basalts flowed down the canyon of the Metolius River. The diktytaxitic basalts are unrelated to the High Cascade basaltic andesites. Instead, they are similar to extension-related high alumina olivine tholeiites of the northwestern Great Basin.
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