THE RATTLESNAKE TUFF, OREGON: AN EXCEPTIONAL RECORD OF A VOLUMINOUS HIGH-SILICA RHYOLITE SYSTEM

Voluminous phenocryst-poor silicic magma stored shallowly below the surface are likely prevented from solidifying before erupting as voluminous ignimbrites through heat released from basaltic magma, in turn accumulated somewhere beneath the silicic reservoir. The long-proclaimed notion that the more voluminous the rhyolitic eruptive products the less likely it is for the eruption to have tapped also into the mafic part of the silicic system is generally upheld although there are exceptions. One of which is the 7 Ma Rattlesnake Tuff from eastern Oregon. It represents a magma system from which voluminous (~300 km³), crystal-poor (~1%) high-silica rhyolites but also abundant evidence for the intermediate to basaltic underpinning erupted forming a single cooling unit. In addition, a coeval snapshot of the upper crust is provided by syn-eruptive metamorphic crustal xenoliths. High-silica rhyolite pumices and glass shards cluster in distinct compositional groups and these correlate with progressive changes in modal mineralogy, mineral chemistry, and partition coefficients with increased degree of differentiation. The diversity of high-silica rhyolites is best explained by progressive fractionation processes starting with the least-evolved rhyolite which in turn was likely generated by dehydration melting of a basaltic protolith evidenced by high Ba/Rb and low La/Yb in least-evolved high-silica rhyolites in relation to metamorphic xenoliths or regional primitive basalts. Dacite pumices and a variety of cognate mafic inclusions reveal the nature of the mafic root zone. Dacite magma formed at the interface between high-silica rhyolites and underlying basaltic andesite magma while basaltic andesite magma evolved from regional primitive high-alumina olivine tholeiite (HAOT) lavas for which evidence is found in form of fresh Fo80 olivines. The Rattlesnake Tuff system offers through its completeness of eruptive products and their chemical relationships an exceptional opportunity to address several points recently discussed regarding generation of silicic magmas and their dynamics as well as possible feedback mechanisms between silicic caps and mafic underpinnings.