PERMIAN LOW-POTASSIUM RHYOLITE IN HELLS CANYON, EASTERN OREGON AND WESTERN IDAHO: IMPLICATIONS FOR EARLY VOLCANISM IN INTRA-OCEANIC ISLAND ARCS

Low-Potassium rhyolites are common in oldest stratified sequences of island arcs. Permian rhyolites in Hells Canyon, within the Wallowa Terrane of the accreted Blue Mountains island arc, are characteristic. They occur as dikes and small irregular intrusions and are associated with quartz-rich tuffs, tuff breccias, and epiclastic rocks of the Hunsaker Creek and Windy Ridge formations. These are the oldest stratified sequences in the Wallowa Terrane. In places, the rhyolites are concentrated and host massive sulfide deposits. Compositions of the Permian rocks are in stark contrast to compositions of rocks in the unconformably overlying Middle and Upper Triassic Wild Sheep Creek Formation that are mostly mafic to intermediate tholeiitic and calc-alkalic lavas and associated clastic rocks. The Permian rocks overlie a partly older (Late Pennsylvanian?) "basement" complex of amphibolite, trondhjemite, diorite, gabbro, diabase, and rhyolite. Rhyolite dikes in the "basement" unit probably served as feeders to the rhyolites and tuffs of the Permian sequence. Quartz and albite phenocrysts are set in a fine-grained groundmass of quartz and albite with secondary minerals of epidote, chlorite, and calcite. No potassium-bearing minerals have been reported from thin section studies. The potassium-poor and silica-rich rhyolites have high sodium contents. Many contain no calcium at all. The primary source of sodium is not known, but it may have been derived either from concentrated sea water that circulated through the volcanic pile and shallow magma chambers during the arc's early growth or added during later metamorphism. Similar rhyolites are recognized in the Eocene rocks of the Komandorsky and Tonga Islands within the oldest known stratified rock units of those arcs. Low-potassium rhyolites in basal sequences of intra-oceanic island arcs may be more common that previously recognized and, therefore, very characteristic of volcanic rocks that are erupted during an arc's early igneous evolution.