ZIRCON SATURATION IN FE-RICH A-TYPE RHYOLITES OF THE JOHN DAY FORMATION

Products of early Oligocene volcanism in central Oregon display a chemical shift from the calc-alkaline intermediate rocks of the Clarno Formation (54 Ma – 33 Ma) to the bimodal assemblage of the John Day Formation (39 Ma – 20 Ma). The western facies of the John Day Formation contains several rhyolitic ash flow tuffs (members A-I) near Ashwood and Antelope as well as several age equivalent units to the south towards Prineville. Whole rock data of these age equivalent rhyolitic ash-flows indicate A-type affinities previously unidentified in the John Day Formation.

The rhyolitic ash-flows have experienced significant post-eruptive alteration and alkali loss, and are now metaluminous to peraluminous. Two groups of rhyolites are recognized, based on distinct Fe contents. The units of the low-Fe group are suggested products of the Crooked River caldera, while units of the high-Fe group are from a separate unknown older vent. The high-Fe group (FeO ~4.0 ± 1.0 wt %) has higher, more consistent Zr/Hf and Zr/Th ratios and heavy rare earth element abundances than the low-Fe group (FeO ~2.0 ± 1.0 wt %), suggesting little to no zircon fractionation during petrogenesis.

Zircon saturation of rhyolitic liquids depends largely on molar (Na₂O+K₂O)/Al [1]. Thus, peraluminous to metaluminous melts are saturated in zircon at low Zr concentrations, with consequent effects on those elements strongly compatible into zircon during subsequent fractionation. Peralkaline rhyolites, however, are typically not saturated in zircon. The evidence against significant zircon fractionation among the high-Fe John Day rhyolites therefore indicates that they were peralkaline when erupted.