QUANTITATIVE TEXTURAL ANALYSIS OF GIANT PLAGIOCLASE BASALT FLOWS IN THE STEENS BASALT, SE OREGON

The Steens Basalt in southeast Oregon is an early member of the Columbia River Basalt Group that initiated at ~17 Ma, was emplaced in ~300 ka, and is noted for conspicuous giant plagioclase (laths from 1-5 cm) basalt flows (GPB). The GPB have three distinct textures: “daisystone” radiating glomerocryts, sandwich-structured glomerocrysts, and single-isolated crystals. Lower Steens GPB commonly contain glomerocrystic textures, while upper Steens GPB are dominated by single-isolated crystals. We apply quantitative textural analysis and crystal size distributions (CSD) to the Steens GPB to decipher nucleation and growth histories of the plagioclase in these flows and elucidate the relationship between texture and composition of the magmas.

CSDs were calculated in CSDCorrections 1.60 using crystal intersection data measured in outcrop at thirty areas from six flows (two from lower Steens and four from upper Steens Basalt). The majority of the CSD patterns suggest crystal coarsening processes dominated within magma reservoirs, while some suggest crystal fractionation dominated. Most crystal coarsening occurs in the intermediate size range (~10 mm). Characteristic lengths of the plagioclase range from 1.87 to 8.97 mm. Crystal residence times range from 600 to 2800 years, assuming a 10^-10 mm/yr growth rate. Reservoir volume estimates range from 60 to 1900 km³. Comparison of calculated melt densities with applicable plagioclase densities (~An_75-50) suggests settling or stagnation of plagioclase in upper Steens melts and flotation of plagioclase in the lower Steens Basalt, albeit plagioclase in the lower Steens does occur in clusters with olivine.

We attribute the textural distinctions between lower and upper Steens GPBs to a difference in composition. Lower Steens Basalts have mostly ≤50 wt. % SiO₂ and olivine may serve as nucleation sites for plagioclase to create glomerocrystic textures. Upper Steens lavas range to basaltic andesite compositions achieved through fractionation with lesser assimilation; GPB accordingly have single isolated plagioclase that are denser than the melt. Further study will seek to pinpoint the location of coarsening within the system: does it occur deep in the crust, during ascent, at shallow levels in the crust, or possibly within insulated flows following eruption?