ORIGIN OF GIANT PLAGIOCLASE IN A LARGE-VOLUME BASALTIC MAGMA: ENTRAINMENT OF PARTIALLY-MELTED GABBROIC CUMULATE IN THE STEENS BASALT, SE OREGON, USA

Plagioclase phenocrysts in the Steens Basalt, the oldest and most mafic member of the Columbia River Basalt Group, can exceed 2 cm and can account for up to 40% modal abundance in some samples. Geochemical evidence for entrainment of gabbroic mush contributing to large and abundant plagioclase formation is preserved as core-to-rim changes in major element, trace element, and ⁸⁷Sr/⁸⁶Sr compositions that are revealed by in-situ analysis of plagioclase from 16 lava flows representing the range of whole rock (WR) compositions and plagioclase textures.

Over 200 individual lavas flows can be divided into the older, more mafic lower Steens (WR MgO ~4-12 wt.%) and the younger, more evolved upper Steens (WR MgO ~3-7.5 wt.%). Lower Steens lavas have lower WR incompatible trace element (e.g. Ba, La, Sr) concentrations and less radiogenic ⁸⁷Sr/⁸⁶Sr than the upper Steens, suggesting a recharge-dominated early system and an assimilation-dominated late system. Plagioclase, up to 40 modal %, dominates phyric samples. Plagioclase agglomerates (smaller laths ± olivine ± clinopyroxene) up to ~5 cm in diameter are primarily found in the lower Steens, whereas single isolated plagioclase up to 5 cm long are common in the upper Steens. Although a majority (72%) of plagioclase anorthite (An) contents are between 60 to 70, lower Steens crystals are typically more depleted in incompatible trace elements than upper Steens plagioclase. In most crystals, major and trace element zoning is modest; e.g., 84% of core-to-rim An differences <10_.

Phase equilibria modeling indicates plagioclase crystallization initiates at ~7 wt.% MgO, yet higher MgO lower Steens samples contain plagioclase. Addition of gabbroic cumulate mush (antecrysts ± melt) may have caused plagioclase saturation and contributed antecrystic plagioclase agglomerates to the lower Steens. For the more evolved upper Steens magmas, entrainment of gabbroic cumulate and higher rates of crustal assimilation may have enhanced plagioclase saturation. As a result, entrained and phenocrystic plagioclase may have grown via ripening, accounting for the large size of upper Steens plagioclase. Pending in-situ Sr isotope data will evaluate the balance of antecrystic entrainment vs. phenocrystic growth by assessing isotopic characteristics of plagioclase, groundmass, and whole-rock.