GEOCHEMISTRY AND PETROGENESIS OF THE OWYHEE FRONT RHYOLITES, SOUTHWESTERN IDAHO

The Western Snake River Plain (WSRP) is a northwest-southeast trending extensional basin, ~ 60 km wide by 250 km long. Extension and magmatism initiated in the WSRP near contemporaneously in the mid Miocene, and coincides with the bulk of magmatism in Bruneau-Jarbidge (BJ) eruptive center along the Yellowstone/Snake River Plain hotspot, 50-100 km to the southwest of the WSRP. This study focuses on the petrogenesis of rhyolite lavas and pyroclastic deposits in the Owyhee Front (OF) region along the southwest margin of the WSRP. Ar-Ar dates on selected units within the OF return ages of 11.7 – 10.6 Ma. Lava flows are volumetrically dominant in the OF, but pyroclastic deposits varying from non-welded fall out deposits to densely welded rheomorphic tuffs occur within the region. OF rhyolites have major and trace element concentrations consistent with A-type silicic magmas, with SiO₂ contents ranging from 71-77%. High large-ion-lithophile-element concentrations preclude generation from deep crustal meta-igneous rocks which have experienced previous melt extraction. Pb isotope ratios range between 18.51 and 18.90 for 206Pb/204Pb, from 15.64 to 15.69 for 207Pb/204Pb, and from 38.88 to 39.53 for 208Pb/204Pb. The Sr isotope ratios range between 0.70568 and 0.71004. These Sr isotope ratios are similar to those of two granitic rocks which underlie the OF rhyolites, and have ratios of 0.70731 and 0.70756. These granites fall well within the range of values observed in the Idaho batholith, and do not preclude the batholith as source material for the OF rhyolites. The ignimbrite deposits generally have higher 206Pb/204Pb and lower 208Pb/204Pb ratios than the lava flow, which is consistent with a two stage process of magma generation involving sources with varying U/Th ratios. Rhyolites in the WSRP are chemically, petrographically, and morphologically, very similar to lavas in the B-J region, and only differ significantly in their aerial extent and δ¹⁸O_VSMOW values. Thus the OF rhyolites offer insight into the petrogenesis of voluminous rhyolite lavas and tuffs in an anorogenic tectonic environment.