Graduate Thesis Or Dissertation
 

Deep crustal and mantle inputs to North Sister Volcano, Oregon High Cascade Range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p2677047h

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  • The Central Oregon High Cascade Range is an anomalously mafic segment of the Cascade Arc due to ongoing intra-arc extension, which allows most magmas to traverse the crust without stalling and evolving to more evolved compositions. North Sister, a composite volcano in this arc segment, has produced a seemingly monotonous basaltic andesite (52.5-54.0% SiO₂) that is depleted in incompatible elements from [aprroximately] 320 ka until 15 ka. This dissertation addresses the volcanic development of North Sister Volcano and deep crustal processes that produced its basaltic andesite. Stratigraphic variation signaling changes in eruptive style, unconformable contacts, ⁴⁰Ar/³⁹ dating, and geochemical correlations divide North Sister's eruptive history into four stages: (1) the early shield, (2) the subglacial stage, (3) the upper shield stage, and (4) the stratocone stage. Lastly, the north-striking, >1 11-km Matthieu Lakes Fissure (MLF) transected North Sister in three splays. North Sister's eruptive stages correspond to four compositional groups that record a general decrease in compatible elements such as Ni (112 to 35 ppm), while incompatible elements are constant or generally decrease (e.g. Ba 302 to 247 ppm) through time. Isotopic variations at North Sister are small, but systematic; Sr and Nd isotopic ratios become more mantle-like with time. Petrologic modeling places the generation of North Sister's basaltic andesite in the deep crust by a) deciphering upper crustal re-equilibration; b) mixing a primitive, low K tholeiite (LKT) with a silicic, Al-rich partial melt of 5-30 million year old crust; and c) high recharge rates by mantle-derived LKTs and interaction with a refractory crust. The Sr and Nd isotopic ratios of primitive basalts from the Cascade Range define four segments of the arc that reflect separate isotopic reservoirs and mantle melting regimes and correlate with major crustal domains. These segments are 1) north, Mt. Meager to Glacier Peak; 2) Columbia, Mt. Rainier to Mt. Jefferson; 3) central, the North Sister to Medicine Lake, and 4) south, Mt. Shasta to Mt. Lassen. The central segment has Basin and Range in the backarc and is the most restricted in bulk and isotopic composition.
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Thumbnail Title Date Uploaded Visibility Actions
file details North Sister Geologic Map 1_12000.pdf 10/20/2017 Public
file details APPENDIX 3 40Ar-39Ar.zip 10/20/2017 Public
file details APPENDIX 2 geochem.xls 10/20/2017 Public
file details APPENDICES.doc 10/20/2017 Public
file details SchmidtMariekE2006.pdf 10/20/2017 Public