Hydrothermal alteration and melting of the crust during the Columbia River Basalt–Snake River Plain transition and the origin of low-δ18O rhyolites of the central Snake River Plain

Abstract

We present compelling isotopic evidence from ~15 Ma rhyolites that erupted coeval with the Columbia River Basalts in southwest Idaho's J-P Desert and the Jarbidge Mountains of northern Nevada at that suggests that the Yellowstone mantle plume caused hydrothermal alteration and remelting of diverse compositions of shallow crust in the area where they erupted. These rhyolites also constitute the earliest known Miocene volcanism in the vicinity of the Bruneau–Jarbidge and Twin Falls (BJTF) volcanic complexes, a major center of voluminous (103–104km3) low-δ18O rhyolitic volcanism that was previously defined as being active from 13 to 6Ma. The Jarbidge Rhyolite has above-mantle δ18O (δ18O of+7.9‰ SMOW) and extremely unradiogenic εHf (−34.7) and εNd (−24.0). By contrast, the J-P Desert units are lower in δ18O (+4.5 to 5.8‰), and have more moderately unradiogenic whole-rock εHf (−20.3 to −8.9) and εNd (−13.4 to −7.7). The J-P Desert rhyolites are geochemically and petrologically similar to the younger rhyolites of the BJTF center (the one exception being their high δ18O values), suggesting a common origin for J-P Desert and BJTF rhyolites. The presence of low-δ18O values and unradiogenic Nd and Hf isotopic compositions, both of which differ greatly from the composition of a mantle differentiate, indicate that some of these melts may be 50% or more melted crust by volume. Individual J-P Desert units have isotopically diverse zircons, with one lava containing zircons ranging from −0.6‰ to +6.5‰ in δ18O and from −29.5 to −2.8 in εHf. Despite this diversity, zircons all have Miocene U/Pb ages. The range of zircon compositions fingerprints the diversity of their source melts, which in turn allow us to determine the compositions of two crustal end-members which melted to form these rhyolites. These end-members are: 1) Archean basement with normal to high-δ18O and unradiogenic εHf and 2) hydrothermally altered, shallow, young crust with low-δ18O (0–1‰) and more radiogenic εHf. We suggest that the shallow crust's low-δ18O composition is the result of hydrothermal alteration which was driven by a combination of normal faulting and high heat fluxes from intruding Yellowstone plume-derived basalts shortly prior to the onset of silicic magmatism. Furthermore, zircon diversity in the J-P Desert units suggests rapid assembly of zircon-bearing melts of varying isotopic composition prior to eruption, creating well-mixed magmas with heterogeneous zircons. We suggest that this hydrothermal priming of the crust followed by remelting upon further heating may be a common feature of intraplate mantle plume volcanism worldwide.