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| Abstract |
- 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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| Digitization Specifications |
- PDF derivative scanned at 300 ppi (256 B&W, 24-bit Color), using Capture Perfect 3.0.82, on a Canon DR-9080C. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
Plate: Master file scanned at 600 dpi, 24-bit color on a Paradigm ImagePRO GxT 42 HD (OEM version of ColortracSmartLF Bx 42). Image manipulated by SmartLF1.3.05.
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| Additional Information |
- description.provenance : Submitted by Digital Production (digitalproduc@gmail.com) on 2009-09-02T15:38:11Z
No. of bitstreams: 5
SchmidtMariekE2006.pdf: 3185156 bytes, checksum: bf0f2cc74404deef0f1b0d54902f4f45 (MD5)
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APPENDIX 3 40Ar-39Ar.zip: 728416 bytes, checksum: b6120307133c24038e00c394e7d9ea47 (MD5)
North Sister Geologic Map 1_12000.pdf: 123171474 bytes, checksum: 26481e148557e1536bc3f07afc6b2cea (MD5)
- description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2009-09-02T17:31:43Z (GMT) No. of bitstreams: 5
SchmidtMariekE2006.pdf: 3185156 bytes, checksum: bf0f2cc74404deef0f1b0d54902f4f45 (MD5)
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APPENDIX 3 40Ar-39Ar.zip: 728416 bytes, checksum: b6120307133c24038e00c394e7d9ea47 (MD5)
North Sister Geologic Map 1_12000.pdf: 123171474 bytes, checksum: 26481e148557e1536bc3f07afc6b2cea (MD5)
- description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2009-09-02T17:42:15Z (GMT) No. of bitstreams: 5
SchmidtMariekE2006.pdf: 3185156 bytes, checksum: bf0f2cc74404deef0f1b0d54902f4f45 (MD5)
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APPENDIX 3 40Ar-39Ar.zip: 728416 bytes, checksum: b6120307133c24038e00c394e7d9ea47 (MD5)
North Sister Geologic Map 1_12000.pdf: 123171474 bytes, checksum: 26481e148557e1536bc3f07afc6b2cea (MD5)
- description.provenance : Made available in DSpace on 2009-09-02T17:42:15Z (GMT). No. of bitstreams: 5
SchmidtMariekE2006.pdf: 3185156 bytes, checksum: bf0f2cc74404deef0f1b0d54902f4f45 (MD5)
APPENDICES.doc: 347648 bytes, checksum: d6150a077d56d269e5c41e14013e8410 (MD5)
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APPENDIX 3 40Ar-39Ar.zip: 728416 bytes, checksum: b6120307133c24038e00c394e7d9ea47 (MD5)
North Sister Geologic Map 1_12000.pdf: 123171474 bytes, checksum: 26481e148557e1536bc3f07afc6b2cea (MD5)
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