COMPOSITIONAL GRADIENTS AND GAPS IN HIGH-SILICA RHYOLITES OF THE RATTLESNAKE TUFF, OREGON

The Rattlesnake Tuff of eastern Oregon comprises >99% of high-silica r hyolite glass shards and pumices representing similar to 280 km(3) of magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass sh ards cluster in five chemical groups that range in color from white to dark gray with increasing Fe concentration. Compositional clusters ar e defined by Fe, Ti, LREE, Ba, Eu, Rb, Zr, Hf, Ta, and Th. Progressive changes with increasing degree of evolution of the magma occur in mod al mineralogy, mineral composition, and partition coefficients. Partit ion coefficients are reported for alkali feldspar, clinopyroxene, and titanomagnetite. Models of modal crystal fractionation, assimilation, successive partial melting, and mixing of end members cannot account f or the chemical variations among rhyolite compositions. On the other h and, similar to 50% fractionation of observed phenocryst compositions in non-modal proportions agrees with chemical variations among rhyolit e compositions. Such non-modal fractionation might occur along the roo f and margins of a magma chamber and would yield compositions of remov ed solids ranging from syenitic to granitic. A differentiation sequenc e is proposed by which each more evolved composition is derived from t he previous, less evolved liquid by fractionation and accumulation, oc curring mainly along the roof of a slab-like magma chamber. As a layer of derivative magma reaches a critical thickness, a new layer is form ed, generating a compositionally and density stratified magma chamber.