MIOCENE RHYOLITES OF THE OWYHEE FRONT, WESTERN SNAKE RIVER PLAIN, IDAHO

The rhyolite units of the Owyhee Front (OF) are discontinuously exposed for more than 40 km, from southwest of Homedale to southwest of Murphy, Idaho, along the southwestern margin of the western Snake River Plain (WSRP). The WSRP is a complex extensional basin, in which northeast-southwest extension accompanied the development of the main Snake River Plain-Yellowstone hot-spot trend. Flows are often cut by normal faults, but evidence also suggests that units flowed over existing normal fault scarps. This indicates that the WSRP rhyolites began erupting a short time after the initiation of extension in the WSRP. Ar-Ar dates for selected units in the OF return ages between 11.7 and 10.6 Ma, and activity in the WSRP appears to have been concurrent to the bulk of volcanism in the Bruneau-Jarbidge (BJ) volcanic center to the southeast. Both rhyolite lava flows and ignimbrite deposits are present in the OF, with volumes ranging from ~ 2-20 km³. Pyroclastic deposits vary from non-welded fallout deposits to densely-welded, rheomorphic ignimbrites, and range in thickness from ~ 5 – 50 m. Lava flows range in thickness from ~ 25 – 200 m, and vary in aerial extent from ~ 5 – 200 km². Typically, units were erupted from vents along the southwestern margin of the WSRP and flowed down regional paleoslopes to the northeast. Often portions of the OF rhyolites appear to have encroached into water, suggesting the presence of a persistent, extensive lake system in the WSRP during OF activity. The OF rhyolites are sparsely to abundantly porphyritic (3-35% phenocrysts), with mineral assemblages including plagioclase + sanidine ± quartz ± pyroxenes + ilmenite + magnetite ± fayalite ± zircon ± apatite. These units represent high-temperature (800-950° C), anhydrous melts which were likely generated by shallow (<15 km) melting of granitic crust due to injection of mantle derived basaltic melts into the mid to upper crust during extension. Although the BJ and OF rhyolites are contemporaneous and have similar major- and trace-element geochemistry, morphology, and eruptive temperature; significant differences in magmatic oxygen isotope signatures in the WSRP (7-9‰) and BJ rhyolites (-1.4 – 3.8‰) indicate distinct petrogenetic histories for the two provinces.