AGES AND PETROGENESIS OF RHYOLITES AT TOWER MOUNTAIN CALDERA, EASTERN OREGON

Tower Mountain Caldera is the main feature of an Oligocene volcanic field located in the Umatilla National Forest, eastern Oregon. The formation of the caldera produced the Dale Tuff, which comprises the intra-caldera and outflow facies. Argon-argon dating places the age of the tuff at 32.66 ± 0.36 Ma. Post-caldera rhyolites erupted along apparent ring fractures and elsewhere. Radiometric U/Pb dating of zircons from three of these rhyolites yielded ages of 32.167 ± 0.020 Ma (#CH07a), 31.798 ± 0.012 Ma (#TM5), and 31.426 ± 0.016 Ma (#CH08a). All rhyolites at Tower Mountain range from low to high silica varieties. Among high silica rhyolites, there are two types. Some of the post-caldera rhyolites are chemically similar to the Dale Tuff, such as sample CH07a, and have compositions typical of rhyolites of calc-alkaline volcanic centers, while others are similar to A-type rhyolites (CH08a and TM5). The ages indicate that the calc-alkaline rhyolites were followed by the A-type rhyolites.

Tower Mountain is perfectly suited to investigate models of rhyolite petrogenesis as all of the important rock components for evaluating generation models are present in a single location; basalts, intermediate igneous rocks (which consist of older plutons and younger volcanic rocks, which are approximately coeval with rhyolites), metamorphic basement rocks of significant grade, and rhyolites of varying composition. The Tower Mountain Caldera has not been studied in detail, despite that existing rock exposures provide the aforementioned unusual combination of all important rock components for evaluating petrogenetic models of rhyolite formation, including fractional crystallization of basaltic melt with or without the assimilation of country rock, partial melting of crustal material with basalt as the heat source, and rhyolitic melt extraction from intermediate crystal mush. In evaluating the metamorphic rocks as potential rhyolite source material, melting experiments were performed and the resulting melts and residual minerals analyzed. In general, these experimentally produced melts are rhyolitic in composition. Trace element modeling based on observed residual mineralogy indicates that batch melting of a metamorphic protolith is a suitable mechanism for the generation of the TM rhyolites.