Dissertation

Distribution, Geochronology, and Petrogenesis of the Picture Gorge Basalt with Special Focus on Petrological Relationships to the Main Columbia River Basalt Group

Cahoon, Emily Bogdan

2020

Overview

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 plausibleThis 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.

• Publisher: ProQuest Dissertations & Theses
• Subject: Geochemistry / Geology / Petrology
• ISBN: 9798557026970