CRYSTAL SIZE DISTRIBUTIONS OF TEXTURALLY VARIED GIANT PLAGIOCLASE BASALTS IN THE STEENS BASALT, SOUTHEAST OREGON: FROM OUTCROP TO MICRO SCALE

The Steens Basalt in SE Oregon is part of the Columbia River Basalt Group that initiated at ~17 Ma. An informal distinction between lower A, lower B, and upper Steens stages is based on petrologic and geochemical changes that discern variable roles of recharge, fractional crystallization, and assimilation (RAFC) throughout the life of magmatism. Like many worldwide flood basalt provinces, the Steens contains Giant Plagioclase Basalts (GPB), which are common in all 3 stages. Plagioclase megacrysts in the GPB are 1-7 cm long and comprise up to 40% of the mode. GPB textures vary between two distinct endmembers: 1) “daisystone” radiating glomerocrysts that are dominant in the lower B stage, and 2) single isolated (SI) plagioclase laths that are dominant in the lower A and upper stages. Previous work indicates that most of the giant plagioclase in the lower B stage are in equilibrium with groundmass plagioclase, whereas disequilibrium is common in the upper stage.

Crystal size distributions (CSDs) were calculated from plagioclase measurements at the outcrop and micro scale for multiple flows from each stage (10 in total). There is no distinction in CSD patterns between textural endmembers, and CSDs are similar across the 3 stages. Each flow has CSD patterns that suggest steady state crystallization, but the bulk of CSDs from all flows indicate crystallization hiatuses, periods of crystal coarsening, and/or cumulate entrainment. Calculated residence times assuming a growth rate of 10^-10 mm/yr are distinct between stages and textural endmembers: SI GPB (most in lower A/upper) have residence times up to ~5000 yrs, whereas daisystone GPB (most in lower B) have shorter residence times, up to ~2000 yrs. Results for daisystone GPB are consistent with frequent and voluminous recharge that may resorb plagioclase in fractionating magma reservoirs, leaving resulting magma super saturated in plagioclase components, leading to growth of large crystals over shorter timescales. Results for SI GPB are consistent with less voluminous and infrequent recharge or assimilation, causing partial resorption of entrained plagioclase, followed by crystal regrowth over cores. A changing balance of RAFC in each stage is consistent with previous interpretations for evolution of the magmatic system from compositional/petrographic data and modeling.