GEOCHEMISTRY AND EVOLUTION OF THE BLOODGOOD CANYON TUFF, MOGOLLON-DATIL VOLCANIC FIELD, NEW MEXICO, USA: THE EVOLUTION OF CRYSTAL SIZES IN AN OPEN MAGMATIC SYSTEM

The Bloodgood Canyon tuff is a rhyolitic ignimbrite erupted at 28 Ma in southwestern New Mexico, USA. Three samples representing the stratigraphic section were studied in an effort to better understand the pre- and syn-eruptive history of the magma storage system and place constraints on the timescales of its existence. The samples contain phenocrysts of K-spar>Plag>Quartz>Biotite>Titanite, few lithic fragments, and pumice. Here we present bulk-rock geochemistry and crystal size distributions (CSDs) for crystals ranging from ~10 to 2000 µm for three mineral phases: plagioclase, quartz and K-feldspar.

Quartz phenocrysts are usually euhedral but may be broken up or resorbed. Phenocrysts of quartz are typically smaller than the K-feldspars that frequent the samples (~100 µm versus 1000-2000 µm K-feldspars). The CSD slope for this population is a linear trend that steepens until it nears the 1000 µm scale. The plagioclase phenocrysts occur in two phases: one as larger, sieved and resorbed phenocrysts, and second as smaller euhedral twinned phenocrysts. The CSD slope for plagioclase is a concave up trend, with few phenocrysts between 100 and 1000 µm, and a higher population of <100 µm phenocrysts than >1000 µm. K-feldspar phenocrysts are typically sanidine and occur as large, sieved phenocrysts, large, unsieved phenocrysts, or small, non-twinned euhedral phenocrysts. It is very uncommon to see a twinned K-feldspar phenocryst in these samples. The CSD slope for K-feldspar is a parabola because of the equal distribution of <100 µm and >1000 µm phenocrysts.

We interpret these crystal populations as resulting from a shift in crystallization regime. We suggest that the CSD slopes describe a pre-eruptive quartz + K-feldspar growth dominated regime and a plagioclase unstable regime. This was followed by a population that grew during nucleation at the onset of decompression at the initiation of eruption represented by the steeper slope. Initial crystal growth may coincide with eruption and collapse of the Gila Cliff Dwellings caldera 4 million years before the eruption of the BCT as indicated by the larger resolved crystal phases. Remobilization of the magma mush may have destabilized the magma chamber and allowed new phenocryst populations to grow for thousands of years before eruption.