ORIGIN OF COMPOSITIONAL ZONING IN RHYOLITIC IGNIMBRITES BY CUMULATE MELTING: A PETROLOGIC COMPARISON OF 'BISHOP TYPE' AND 'SNAKE RIVER TYPE' RHYOLITES

In search of a mechanism to explain compositional zoning in large bodies of silicic magma, we review petrologic features of caldera-forming rhyolitic ignimbrites (10 - 1,000 cu. km) of low to moderate (0 - 20%) crystallinity. First, high-temperature ignimbrites of the ‘Snake River’ type typically lack compositional zoning. Second, chemical variations in zoned ignimbrites with compositions near the granite minimum (e.g. Bishop and Bandelier tuffs) are fully consistent with mineral/melt partitioning predicted from the observed phenocryst assemblage, and are inconsistent with mixing with more mafic magma. Third, internal isotopic variations are weak or absent in both types of rhyolite, even in systems where associated volcanic rocks exhibit wide isotopic variation and strong contrasts exist between the isotopic compositions of mantle and crust. An exception to this general rule occurs in 'Bishop-type' systems where Sr has been depleted to subchondritic concentrations and is exceptionally sensitive to open-system processes. Fourth, both types of ignimbrite commonly contain crystal aggregates interpreted as fragments of cumulate mush. In zoned systems, these aggregates exhibit evidence for partial resorption of early-formed crystals. We infer that chemical zoning is a near closed-system process, and propose that it arises through melting of cumulate mush beneath a crystal-poor body of melt due to heating by invading mafic or intermediate magma. If the crystal mush is fusible (e.g. sanidine + quartz), part of it melts to yield mobile, water-poor rhyolite which pools at the interface between the mush and the overlying rhyolitic liquid. This new melt has a cumulate composition and is thus less evolved than the original melt lens. The result is a chemically zoned crystal-poor rhyolitic magma produced with negligible mass transfer from the invading magma. Fusibility of the cumulate is key to the process; high-temperature 'Snake River' type rhyolites are not zoned because their cumulates are dominated by pyroxene and plagioclase. Previous models of compositional zoning have envisaged a pot of silicic magma undergoing slow cooling towards thermal senescence. We propose that zoning records a history of thermal rejuvenation in which any one recharge event has the potential to be a caldera-forming eruption trigger.