Erupted silicic cumulates in large ignimbrites

If chemical diversity in igneous rocks is dominated by crystal-liquid separation in open-system magma reservoirs, a significant number of crystal accumulation zones must be preserved in the crust and upper mantle. Such cumulates are conspicuous in mafic lithologies (MOR, layered mafic intrusions, lower crustal arc sections), but have rarely been described and/or are controversial in the silicic upper crust. Although it is possible to recognize signs of crystal accumulations in plutonic exposures, the fact that these batholiths are typically: 1) at least several millions of years old, 2) multi-stage, 3) deformed and 4) biased towards the youngest intrusive episodes, some ambiguity remains in how to interpret geochemical and textural observations. We have chosen to explore large zoned ignimbrites, which represent an instantaneous evacuation of an upper crustal magma reservoir, to isolate potential crystal accumulation zones. Late-erupted, crystal-rich scoria with unusual chemistries (e.g., high Ba, Zr, Eu/Eu*) have been found in multiple examples of these zoned ignimbrites around the world, including the 900+ km3 Ammonia Tanks and Carpenter Ridge Tuffs, both erupted during the Tertiary magmatic flare-up in the Western USA. As already suggested for the 7700 BP Crater Lake ignimbrite, such crystal-rich scoria have mineralogical and geochemical characteristics that are most convincingly explained by accumulation of low temperature minerals as highly-evolved melt escapes upward and pools at the top of large crystalline mushes. To account for the eruption of such crystal-rich zones (technically uneruptible with >50vol% crystals), some melting of low-temperature mineral phases is required; evidence for resorption textures in sanidine and quartz is commonplace in these scoria. The presence of mafic enclaves and/or mingling textures in such scoria indicate that recharge from below ultimately drove melting of part of the mineral assemblage within the cumulate rootzone, while simultaneously triggering eruption of the voluminous crystal-poor cap.