Giant plagioclase in the Steens Basalt, southeast Oregon, USA: Tracking the balance of differentiation processes in continental flood basalt evolution

Abstract

Giant plagioclase basalts (GPBs), flows that contain abundant and conspicuous plagioclase crystals ≥1 cm in length, exist at many continental flood basalt provinces, and their presence can provide a window into the crustal differentiation processes and storage conditions that produced the magmas prior to eruption. GPB flows occur in the Steens Basalt of southeast Oregon, USA, one of the earliest members of the Columbia River Flood Basalt Group, but their origin in this province has not been studied in detail. We conducted quantitative textural analysis utilizing crystal size distributions (CSDs) at the outcrop and thin section scale, as well as in situ plagioclase trace element and isotopic compositional analysis, in order to elucidate the processes responsible for generating two end-member textural types of GPB found in the Steens Basalt: daisy stone glomerocryst clusters composed of radiating plagioclase laths and single isolated plagioclase. CSD patterns suggest a changing balance of multiple differentiation processes affected the magmas that created each GPB textural type. In the daisy stone plagioclase, crystals rarely exhibit resorption textures, trace element abundances are comparatively low with minimal variability, and Sr isotopes are commonly in equilibrium with groundmass. In the single isolated plagioclase, resorption textures are common, trace element abundances are generally higher and display within crystal variation, and Sr isotopes are typically not in equilibrium with groundmass. These distinctive characteristics require separate mechanisms to generate each end-member GPB texture. Daisy stone GPBs form by growth of large crystals following mafic recharge events that completely resorb entrained cumulate plagioclase. A large amount of plagioclase may then grow with minimal cooling following recharge due to saturation in plagioclase components. In contrast, single isolated GPBs are the result of partial resorption of both entrained and resident plagioclase following less voluminous and less primitive recharge events; regrowth then occurs on the partially resorbed cores when cooling resumes. Both GPB types are formed by thermal cycling, but each texture reflects the differentiation processes that were prevalent during different eruptive stages of the Steens Basalt, as the balance between recharge, crystal fractionation, and assimilation varied in time.