Compositional Gradients and Gaps in High-silica Rhyolites of the Rattlesnake Tuff, Oregon

Abstract

The Rattlesnake Tuff of eastern Oregon comprises >99% of high-silica rhyolite glass shards and pumices representing ∼280 km3 of magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass shards cluster in five chemical groups that range in color from white to dark gray with increasing Fe concentration. Compositional clusters are 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 modal mineralogy, mineral composition, and partition coefficients. Partition 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 for the chemical variations among rhyolite compositions. On the other hand, ∼50% fractionation of observed phenocryst compositions in non-modal proportions agrees with chemical variations among rhyolite compositions. Such non-modal fractionation might occur along the roof and margins of a magma chamber and would yield compositions of removed solids ranging from syenitic to granitic. A differentiation sequence is proposed by which each more evolved composition is derived from the previous, less evolved liquid by fractionation and accumulation, occurring mainly along the roof of a slab-like magma chamber. As a layer of derivative magma reaches a critical thickness, a new layer is formed, generating a compositionally and density stratified magma chamber.