Petrography of pre-Tertiary rocks of the Blue Mountains, Umatilla County, northeast Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7d278z347

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  • Nine small Mesozoic plutons in TJmatilla County, northeast Oregon, crop out within an area of 58 sq km in the Blue Mountains. The plutons are divided into an earlier series of probable Permian-Triassic age and a later intrusive series of probable Late Jurassic to Middle Cretaceous age. The earlier rocks show effects of regional metamorphism whereas the later plutons are unmetamorphosed. The five units of the earlier intrusive series probably are Early to Middle Triassic in age. They correlate in age and general petrologic type with the Canyon Mountain ophiolite complex. Rock types, whcch may vary within individual plutons, are serpentinized peridotite, quartz-hornblende metagabbro, hornblende metatonalite, and hornblende metatrondhjemite The elongation of several plutons suggests that emplacement was guided by northeast-trending zones of weakness. All plutons were intruded as magmas except the peridotite which was probably tectonically emplaced. Intrusion of these units into the country rock produced a hornblende gneiss and thermal metamorphism up to the sillimanite-cordieritemuscovite-almandine subfacies of Abukuma-type facies series metamorphism. Permian-Triassic regional metamorphism of greenschist grade produced a pronounced northeast foliation in the older series of plutons and associated country rocks. The remaining four plutons are included within a later intrusive episode and are probably Late Jurassic to Middle Cretaceous in age. Age and petrologic type are typical of the Wallowa and Bald Mountain intrusive series. Rock types, in the order of intrusion, are pyroxene-hornblende melagabbronorite; quartz-pyroxene-hornblende gabbronorite; biotite -bearing, quartz-hornblende gabbro; hornblende trondhjemite, and granodiorite. Country rocks are metavolcanic dacitic and andesitic flows and pelitic metasedimentary schists, interbedded on a large scale. They presumably correlate with portions of the Elkhorn Ridge Argillite and/or the Clover Creek Greenstone on the basis of lithologic similarities. Staurolite, not found in pre-Tertiary rocks elsewhere in northeast Oregon, is common in the metasediment A series of interbedded sediments and flows unconformably overlies the pre-Tertiary rocks. The sediment ranges from carbonaceous shale to arkose. Abundant leaf fossils indicate an early or middle Eocene age. Foreset beds, angular grains, and the occurrence of staurolite suggest that the sediment is locally derived from outcrops to the southeast and deposited under fluvial conditions. Most flows are andesitic and commonly contain 30 to 40 percent large phenocrysts of piagioclase, mica, quartz, and hornblende, Post-Eocene deformation has tilted the sediments eastward as much as 37 degrees. Columbia River Basalt unconformably overlies the tilted Eocene strata. Only Yakima-type basalt was found. One prominent flow contains small phenocrysts of plagioclase and is sufficiently distinctive to serve as a marker unit. Gentle northeast-trending folds and small, inconspicuous northwest faults are the chief structural characteristics of the basalt. Quaternary sediments include the loessial Palouse Formation, reworked deposits of Mount Mazama ash, and stream gravels. Chemical analyses of the major oxides, trace elements, and rare earths show consistently low amounts of Ti02 and K2O and depletion in the light rare earths. These chemical data combined with petrologic similarities to known ophiolite complexes suggest that the Permian-Triassic (?) units in the thesis area represent a dismembered ophiolite assemblage. The sialic metasediment suggests a continental provenance; the source of the metavolcanic unit is possibly from an island arc. Together they suggest deposition in a Permian-Triassic back-arc environment.
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