First Advisor

Martin J. Streck

Term of Graduation

Fall 2020

Date of Publication

8-21-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Earth, Environment, & Society

Department

Earth, Environment, & Society

Language

English

Subjects

Flood basalts -- Oregon, Geological time, Geochronometry, Volcanism -- Oregon, Geochemistry -- Oregon

Physical Description

1 online resource (xiii, 181 pages)

Abstract

The Columbia River Basalt Group (CRBG) is the youngest and volumetrically smallest continental flood basalt exposed across the Pacific Northwest, USA. Similar to other large igneous provinces, the majority of material erupted during the initial 1 million years of activity, these lavas are subdivided into four main-phase units. The Picture Gorge Basalt (PGB) is the only main-phase unit of the CRBG whose age is not precisely known and understanding of PGB petrogenesis is largely based on a limited number of samples when compared with other main-phase units. It is suggested that a time gap of over 0.5 myr existed between eruptions of Steens Basalt and PGB (Wolff et al., 2013), however my initial results suggest otherwise as I have identified PGB interstratified between Steens Basalt flows in added extent near the Malheur Gorge.

Age constraint is critical to developing a petrologic and eruptive model that supports the geochemical data. Geochronological work via 40Ar-39Ar analysis constrains the timing of PGB eruptions and demonstrates that these lavas were the earliest and had the longest eruptive interval. Eruptions of PGB lavas span approximately 1.4 million years, potentially occurring as two discrete pulses. The first pulse occurred as the initial pulse of CRBG eruptive activity, and the second erupted contemporaneous with the Grande Ronde Basalt. This corroborates previous field observations of PGB and Grande Ronde Basalt lavas being interstratified, as the youngest PGB age ranges down to 15.76 ± 0.11 Ma. The spatial distribution of PGB lavas were thought to be relatively confined to the type locality of Picture Gorge in eastern Oregon. However newly identified basaltic lavas and dikes geochemically correlative to the PGB indicates this CRBG unit covers a larger areal extent than previously recognized. Combining stratigraphic correlation and geochemical similarities with 40Ar/39Ar geochronology, I am able to extrapolate the spatial extent of early and later PGB eruptions to newly correlated PGB lava flows and dikes. With the oldest age of 17.23 ± 0.04 Ma, the PGB erupted earlier and longer than other CRBG main-phase unit, and CRBG volcanism initiated over a broad region that includes Picture Gorge.

Geochemical and isotopic signatures observed in the PGB suggest contributions from two primitive (Mg# >58) compositional groups. Based on MELTS modeling, these basaltic compositions produce two different evolutionary patterns. The geochemical diversity of the PGB however cannot be attributed exclusively to fractionation of either composition, and PGB magmas were likely modified by crustal contamination. The wide distribution of PGB volcanism was likely the result of PGB lavas traveling further than previously recognized, although local tapping of a similar mantle to produce PGB-like lavas is also plausible

This research identifies three primary hypotheses, 1. Lava flows and dikes of the PGB were emplaced earlier than previously recognized and have a longer eruptive duration based on observed stratigraphic relationships, 2. Newly identified exposures of volcanic material geochemically correlated to PGB suggests this CRBG unit erupted across a wide spatial footprint of eastern Oregon, and therefore reflects a larger volume of this continental flood basalt, and 3. There is more than one significant contributing mantle component which yields PGB, and isotopic differences suggest that contributing components were not all depleted. The broader significance of this work expands the initial magmatic footprint for CRBG eruptions and highlights two temporal pulses of eruptive activity in PGB volcanism, also demonstrated by all CRBG ages. The revised distribution area of PGB increases the total eruptive volume of this continental flood basalt and when coupled with ages illustrates a clearer picture of spatial and temporal relationships to other main-phase CRBG units. Geochemical signatures in PGB lavas indicate at least two mantle components which reflect fluctuations in their contributions through time.

Rights

© 2020 Emily Bogdan Cahoon

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Comments

Appendices A-J are included below as supplemental files.

Persistent Identifier

https://archives.pdx.edu/ds/psu/34457

767938_supp_44144A80-E11E-11EA-83C4-ADC5466BB44D.xlsx (125 kB)
Appendix_A XRF_ICPMS Geochemical and Sample Location Data

767938_supp_51A14702-E11E-11EA-80F5-AEC5466BB44D.pdf (7254 kB)
Appendix_B_ArAr Age Spectra_Plateaus InverseIsochrons

767938_supp_65E2C858-E11E-11EA-824B-B0C5466BB44D (2).xls (69 kB)
Appendix C_ArArCALC_SUMMARY

767938_supp_81CA672E-E11E-11EA-B0A5-B4C5466BB44D.pdf (2324 kB)
Appendix_D_MajorTraceElementalComparisons_BaileyWolff

767938_supp_B12123AA-E11E-11EA-B6D1-C2C5466BB44D.pdf (4169 kB)
Appendix_E_MajorTraceElements_vs_MgNumberPGB

767938_supp_B824F280-E11E-11EA-AD8D-C4C5466BB44D.xlsx (13 kB)
Appendix_F_SrNdHfPb_IsotopicUncertaintyandIsobaricInterferences

767938_supp_C85A75C6-E11E-11EA-8A5C-C8C5466BB44D.xlsx (14 kB)
Appendix_G_OXYGEN ISOTOPE DATA

767938_supp_CFCC7322-E11E-11EA-AB0B-C9C5466BB44D.xlsx (1975 kB)
Appendix_H_MELTS Summary Table and Trace Element Calc

767938_supp_D9DA65D6-E11E-11EA-8F70-CAC5466BB44D.docx (15 kB)
Appendix_I_PCA Code

767938_supp_EF55FD44-E11E-11EA-AD14-CEC5466BB44D.pdf (572 kB)
Appendix_J_OnumaPlots

Available for download on Saturday, August 21, 2021

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