A PETROGRAPHIC RECORD OF OPEN AND CLOSED SYSTEM MAGMATIC PROCESSES IN THE SUNFLOWER FLAT RHYODACITE AT LASSEN VOLCANIC NATIONAL PARK

This study describes the textures and mineral assemblages of tephra and lava samples from the Sunflower Flat lava dome complex in Lassen Volcanic National Park. Lava samples contain ~30% phenocrysts suspended in a vitrophyric and vesicular groundmass with flow-aligned acicular plagioclase, hornblende, CPX, and OPX microlites. Lava phenocrysts include plagioclase, hornblende, biotite, CPX, OPX, quartz, and Fe-Ti oxides. Plagioclase phenocrysts occur as 4-10 mm crystals either as individuals or as glomerocrysts. Almost all plagioclase phenocrysts possess at least one disequilibria texture, including coarsely sieved interiors, dusty sieved rims, resorbed margins, and complex compositional zoning patterns with intracrystalline disconformities marked by resorbed dissolution zones overlain by subsequently formed growth zones. In contrast, plagioclase microphenocrysts are generally euhedral and unzoned. Hornblende usually occurs as 1-3mm phenocrysts and microphenocrysts that lack reaction rims but may also occur as small inclusions within plagioclase cores. Biotite occurs most frequently as resorbed crystals surrounded by reaction rims of hornblende, pyroxenes, plagioclase, and Fe-Ti oxides. CPX and OPX are usually 0.5-2 mm in diameter with compositionally zoned interiors that occur individually and in aggregates with plagioclase and Fe-Ti oxides. Rare quenched magmatic inclusions have ~40% vesicles and are dominated by 0.5-2 mm plagioclase with less CPX, OPX, hornblende, and Fe-Ti oxides. Embayed quartz accounts for <1% of the lavas. Lavas and tephra have similar mineral assemblages, but tephras are more vesicular (~60%), appear to contain more quartz, and have biotite that is generally larger and enclosed by thinner and finer grained reaction rims. Mineral and textural observations, as well as the presence of sparse and disaggregated quenched magmatic inclusions, are consistent with both open and closed system magmatic processes at Sunflower Flat.