Reshuffling the Columbia River Basalt chronology—Picture Gorge Basalt, the earliest- and longest-erupting formation

Anthony A.P Koppers,Daniel P Miggins,Emily B Cahoon,Martin J Streck

doi:10.1130/g47122.1

Anthony A.P Koppers, Daniel P Miggins + Show 2 more

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https://doi.org/10.1130/g47122.1

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Journal: Geology	Publication Date: Jan 31, 2020
Citations: 19	License type: cc-by

Affiliation: Oregon State University, Portland State University

Abstract

Abstract The Columbia River Basalt Group (CRBG) is the world’s youngest continental flood basalt province, presumably sourced from the deep-seated plume that currently resides underneath Yellowstone National Park in the northwestern United States. The earliest-erupted basalts from this province aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. We explore the Picture Gorge Basalt (PGB) formation of the CRBG, and discuss the location and geochemical significance in a temporal context of early CRBG magmatism. We report new ARGUS-VI multicollector 40Ar/39Ar incremental heating ages from known PGB localities and additional outcrops that we can geochemically classify as PGB. These 40Ar/39Ar ages range between 17.23 ± 0.04 Ma and 16.06 ± 0.14 Ma, indicating that PGB erupted earlier and for longer than other CRBG main-phase units. These ages illustrate that volcanism initiated over a broad area in the center of the province, and the geochemistry of these early lavas reflects a mantle source that is distinct both spatially and temporally. Combining ages with the strongest arc-like (but depleted) geochemical signal of PGB among CRBG units indicates that the shallowest metasomatized backarc-like mantle was tapped first and concurrently, with later units (Steens and Imnaha Basalts) showing increased influence of a plume-like source.

Highlights

The Columbia River Basalt Group (CRBG)of the Pacific Northwest of the United States is the world’s youngest flood basalt and has played an important role in understanding the dynamics of large igneous provinces (LIPs)
We sampled along stratigraphic sections of the known outcrop area of Picture Gorge Basalt (PGB), age-equivalent basalts that are adjacent to the known outcrop area, and sections that were previously correlated with other CRBG units such as Steens or Imnaha Basalts at Malheur Gorge (Hooper et al, 2002; Camp et al, 2003, 2013)
There, the previous CRBG stratigraphy included Steens, Imnaha, and Grande Ronde Basalts (Hooper et al, 2002; Camp et al, 2003); we identify PGB flows, which are capped by Grande Ronde Basalt

Summary

Introduction

The Columbia River Basalt Group (CRBG)of the Pacific Northwest of the United States is the world’s youngest flood basalt and has played an important role in understanding the dynamics of large igneous provinces (LIPs). Flood basalts are a type of LIP representing the most voluminous periods of volcanic activity on Earth, commonly coinciding with times of environmental crisis. While flood basalt provinces can be active for millions of years, the majority of lava erupts during the first million years, or “main phase” of activity (Coffin and Eldholm, 1994). This main-phase period is thought to represent impingement of the mantle plume head on the lithosphere (Ernst et al, 2005). Continental flood basalt provinces are composed of pyroclastic rocks, lava flows, dikes, and sills, and cover extensive areas >100,000 km (Coffin and Eldholm, 1994; Ernst et al, 2005). The location of the mantle plume and its temporal development are evaluated based on age and distribution patterns of lavas and dike swarms

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