Pairing Geochemistry with Geophysical Models to Constrain the Petrogenesis of Age-Progressive Rhyolites from the High Lava Plains and Northwest Basin and Range, Oregon

Over the past 12 Ma, the High Lava Plains (HLP) and northwestern Basin and Range (NWBR) of Oregon have been adjacent areas of bimodal, basalt-rhyolite volcanism. Rhyolitic volcanism in both provinces is age-progressive, tracking to the west-northwest, and is temporally decoupled from the tholeiitic basalts which show no such spatial relationship. The rhyolites are temporally linked but geochemically distinct in some major and trace elements and isotopic (Sr, Nd, Pb) compositions indicating that rhyolites in this region can have a different origins. These differences are caused primarily by the amount of mantle-derived basalt and state of transtension in the crust. We speculate that the basaltic flux is produced by asthenospheric flow which is caused by the steepening and retreating Cascadia slab and is the only geodynamic mantle feature required to produce the age-progressive silicic volcanism in the HLP and NWBR. The HLP has experienced less extension and a significantly greater flux of mantle-derived basalts, based on the volumes of eruptive products and crustal densities derived from seismic refraction models. Increased crustal densities suggest that basaltic magmas stall in the mid to lower crust (intraplate). Other workers have shown that low silica rhyolites can be produced by partial melts of mafic protoliths, which in this case are likely recently emplaced mantle-derived basaltic magmas in mid to lower crustal "hot zones". These low silica rhyolites coalesce and buoyantly rise into the upper crust where they differentiate into high silica rhyolites. Likewise, basaltic andesites form by differentiation of basalts in upper crustal regions. Rare mixes of basaltic andesite and rhyolite produce intermediate (andesite and dacite) compositions in this upper-crustal region. A few rhyolites show evidence of minor assimilation of earlier Cenozoic volcanic products but anatexis of older, accreted terranes and associated plutons is precluded. Mantle-sourced magma flux progressively modified the crust and gravitationally stratified it into seismically identifiable lower and upper crustal seismic reflectors. In the NWBR, there is greater extension and a lower overall flux of mantle-derived basaltic magma into the crust. Due to the extension, basaltic magmas are less likely to stall in the crust and produce large partial melt zones. There are significantly greater percentages of intermediate compositions in the NWBR and based on major and trace element modeling, these cannot be produced by fractional crystallization or mixing alone. Small crustal reprocessing zones which combine fractional crystallization, mixing and recharge created the intermediate compositions. These same processes likely produced some of the rhyolites while others were created by partially melting the lower crust, similar to conditions in the HLP. In this region, there is less modification of the crust and less segregation into dense lower crustal and less dense upper crustal regions. These models are internally consistent and adhere to the temporal, geochemical, petrological, and geophysical constraints on the HLP and NWBR system.