Graduate Thesis Or Dissertation
 

Langer_Vera W._1992.pdf

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  • Steens Mountain in SE Oregon is part of the northern Basin and Range Province, and represents a horst tilted about 100 to the west that is bounded to the east by a high angle, NNE trending normal fault. The minimum displacement is about 1200 m. Volcanic rocks, exposed along the eastern escarpment, range from basalt to high-silica rhyolite. By establishing a stratigraphy and determining chemical variations in the volcanic rock sequence underlying Steens Mountain Basalt, it was possible to evaluate the origin of this volcanism and the role of crustal contamination, explain its chemical variation with time, and relate the chemical evolution to extensional processes. The necessary chemical data were obtained by X-ray fluorescence, electron microprobe, and instrumental neutron activation analyses; field mapping provided information about stratigraphy and structure. The oldest units are lake sediments which are overlain by a volcanic rock sequence that proceeded from early rhyolitic tuffs to rhyolitic lava flows, dacites, andesites, and culminated with the eruption of Steens Mountain Basalt (S MB). All stratigraphic units are tilted to the NW or SW to various degrees. Early sediments and rhyolitic tuffs are tilted as much as 25°; whereas younger andesites (15° tilt), and SMB (5-10° tilt) overlie the older units unconformably. The stratigraphy and mapped unconformities indicate that active, volcanic episodes alternated with episodes of erosion and block tilting. Tilting events are thought to be connected to extensional tectonics, and the maximum extension for the Steens Mountain area is estimated to be 30%. Relationships between basalt, andesites, dacites, and rhyolites of the Steens Mountain volcanic suite are complex. Crystal fractionation alone cannot relate the different rocks to each other. Trace element chemistry of intermediate compositions requires fractional crystsllization plus large degrees of assimilation and magma mixing. Magma mixing is also suggested by different plagioclase populations and zoningpatterns in plagioclase and dinopyroxene phenocrysts from intermediate rock compositions. The role of assimilation is greater in the andesites and that of mixing in the dacites. The silicic mixing partner has to be extremely low in rare earth elements. Rhyolites are not directly related to the rest of the suite, but are related to each other by crystal fractionation. They most likely evolved from a partial melt of depleted crust (probably lower crust) by fractionation. Assimilation coupled with fractionation and simple magma mixing are typical magma processes for major metaluminous volcanic suites in the Basin and Range and elsewhere. These processes decrease the density of stagnated basaltic melts and subsequently increase their buoyancy. Extension triggers the ascent of mafic magma, but small amounts of extension alone do not increase the mean crustal density enough for basalts to rise to the surface. Increasing the crustal density by injection of mafic plutons and extension is able to produce a shift in volcanism from intermediate to more basaltic compositions as seen at Steens Mountain, where the influence of crustal material decreases upwards in the volcanic sequence.
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