Geochemistry and stratigraphy of a Maar complex in the Imnaha basalt, OR

David Wesley Unruh

Large Igneous Provinces (LIP) represent the largest volcanic eruptions in the geologic record and have significant impacts on crustal growth and extinction intervals. Continental flood basalts are the best studied and understood type of LIP. Most continental flood basalts include an interval of explosive deposits which, until recently, were ignored in favor of better preserved basalt flows which tend to occur later in the sequences. Whether these deposits are due to inherently high volatile contents of LIP magmas or incidental interaction between ascending magma and the environment in which they erupt is an evolving area of study. The Columbia River Basalt Group (CRBG) is the youngest and best-studied LIP on Earth. This study characterizes deposits formed during a maar-type eruption at Lookout Mountain in northeastern Oregon in the Imnaha Basalt early in the eruption of the CRBG. Two discrete explosive intervals are identified from two separate basaltic tuff deposits at Lookout Mountain. The first eruption was shallow and occurred close to the surface as evidenced v by the lack of basement lithic material, fluidized juvenile clasts, and low S content in lapilli groundmass glasses (<255 ppm). A second explosive interval occurred after an interval of effusive eruption. A significant proportion of basement clasts and higher S concentrations in lapilli groundmass glass (<800 ppm) indicate this eruption occurred deeper below the surface than the first eruption. Olivine-hosted melt inclusions in both tuffs record a maximum S content of ~1200 ppm. Using the S saturation model D-Compress, calculations indicate melt inclusions formed in crystals at ~3.3 Kbar. Major and trace element data indicate that all basalt units at Lookout Mountain share the same source. Olivine-liquid thermometry and clinopyroxene-liquid thermobarometry indicates crystals and melt equilibrated at 1150±43 °C and 3.11±0.28 Kbar. Data gathered in this study indicate the volcaniclastic deposits at Lookout Mountain are part of a maar-type eruption due to the incidental interaction between ascending magma and groundwater rather than intrinsically high volatile content of the parental magma. This finding supports existing evidence about explosive basaltic eruptions in LIP occurring due to interactions between ascending magma and the environment in which it erupts.

Geochemistry and stratigraphy of a Maar complex in the Imnaha basalt, OR

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