The Geology and Petrology of Enigmatic Rhyolites at Graveyard and Gordon Buttes, Mount Hood Quadrangle, Oregon | Posted on:2015-04-11 | Degree:M.S | Type:Thesis | University:Portland State University | Candidate:Westby, Elizabeth G | Full Text:PDF | GTID:2470390017992309 | Subject:Geology | Abstract/Summary: | | Rhyolite lava flows are found at two dome complexes at Graveyard Butte and Gordon Butte, Mount Hood Quadrangle, Oregon. At Graveyard Butte, the White River has cut a winding canyon 150 m deep, exposing at its base, a 40-meter-thick outcrop of flow-banded rhyolite (73 wt.% SiO2, 3.67+/-0.01 Ma) that laterally extends along the canyon wall for about 1 km. Stratigraphically above the flow-banded rhyolite is locally-erupted iron-rich andesites (lava flows, agglutinate and other pyroclastic rocks as well as clastic debris), a rhyolitic ash-flow tuff (74 wt.% SiO2), and the 2.77+/-0.36 Ma tholeiitic basalt lava flows of Juniper Flat (Sherrod and Scott, 1995). Roughly 2 km downstream, a phenocryst-poor, maroon-colored rhyolite (3.65+/-0.01 Ma) is visible again, forming steep canyon walls for about 1.6 km. A compositionally similar silicic unit is found 18 km to the northwest of Graveyard Butte at Gordon Butte. Exposed units along Gordon Butte's Badger Creek (3.64+/-0.03 Ma) and the southeastern upper slopes of Gordon Butte include rhyolite flows (69.6-72.1 wt.% SiO2).;The rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are nearly chemically indistinguishable and both contrast with the younger rhyolitic ash-flow tuff at Graveyard Butte and lava flows on Gordon Butte's Upper Slopes. The rhyolites of Graveyard Butte and Badger Creek are richer in Nb and Zr (30--40 ppm and 487--530 ppm, respectively) than the younger rhyolitic tuff and Upper Slopes flows (13--19 ppm and 235--364 ppm, respectively) and share characteristics with A-type granitoids. The rhyolite lavas are porphyritic (~7%) with the porphyroclasts comprising primarily individual feldspars (250--500 microm in length) with ragged margins, oscillatory zoning and less commonly, spongy cores. Other phenocrystic phases include fayalitic olivine, Fe-rich clinopyroxene, and Fe-Ti oxides.;A-type-like incompatible trace-element-enriched compositions as well as mineralogical indicators suggest rhyolite lava flows at Graveyard Butte and Gordon Butte's Badger Creek are likely generated in an extensional tectonic setting. A possible geotectonic framework for generation of these rhyolite lavas is the northward propagating intra-arc rift of the Oregon Cascades. | Keywords/Search Tags: | Rhyolite, Butte, Graveyard, Gordon, Lava flows | | Related items |
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