Compositionally variable and distinct rhyolites during the 300 kyr prolific, explosive and effusive volcanism at the Mahogany Mountain Three Fingers rhyolite field, eastern Oregon.

The Mahogany MountainThree Fingers rhyolite field (MMTFrf), encompassing ~900 km2, is one of the largest mid-Miocene rhyolite centers associated with the Columbia River flood basalt volcanism. A minimum of 10, mostly high-silica rhyolite magmas erupted in short succession over a ~300 kyr time span from 16.02 to 15.71 Ma to produce an intricate assembly for flows, domes and pyroclastic deposits including the several hundred-meter-thick intra-caldera tuff of Leslie Gulch that resulted largely from phreatomagmatic activity. Rhyolite magma units are distinguished by a combination of composition, mineralogy, and age. In general, rhyolites range from less fractionated with high Ba (1500 to 2000 ppm) to low Ba (<300 ppm), from less to more incompatible element enriched with, e.g., Nb ranging from 20 to 60 ppm, and from rhyolites with low-18O to normal-18O signatures. Only general time-composition trends can be established due to overlap in 40Ar/39Ar ages despite 2 errors on the order of 50 kyr. First erupting rhyolites carry the low-18O signature, have high Ba, and are the most trace-element enriched, A-type magmas. Following shortly, rhyolites with high Ba, less A-type like, and normal-18O signatures began erupting at Mahogany Mountain along the southern periphery at 15.820.05 Ma. All rhyolites (with one possible exception) that erupted after 15.8 Ma are low Ba and have normal-18O signatures but vary considerably in their degree of HFSE element enrichment. We interpret the early low-18O rhyolites to be generated from partial melting of relatively shallow crust that experienced prior hydrothermal alteration facilitated by early Oregon-Idaho graben development. This may correlate with development of a lacustrine environment that could have driven phreatomagmatic activity to generate the tuff of Leslie Gulch. More southernly erupting high Ba and later erupting low Ba rhyolites everywhere with normal-18O signatures are possibly derived from deeper sources of the underlying Olds Ferry terrane. Overall, the MMTFrf highlights the production of strongly variable and distinct rhyolite batches at one prolific rhyolite center over the short period of 300 kyr. Rhyolite eruptions recommenced in the SW of the field with the eruption of the 14.94 Ma Birch Creek rhyolite, and the 14.42 Ma McCain Creek rhyolite.