ERUPTION HISTORY OF THE COLUMBIA RIVER BASALT GROUP CONSTRAINED BY HIGH-PRECISION U-Pb AND 40Ar/39Ar GEOCHRONOLOGY (Invited Presentation)

Large igneous province volcanism of the Columbia River Basalt Group (CRBG) has been suggested to play a causal role in elevated global temperatures and atmospheric carbon dioxide levels of the Miocene Climate Optimum (MCO). However, assessing the connection between volcanism and warming is dependent upon an accurate and precise chronology for the timing and duration of CRBG emplacement. Building on our previous work (Kasbohm & Schoene, 2018), we present fifteen new high-precision ages, using CA-ID-TIMS U-Pb on zircon and multi-collector ⁴⁰Ar/³⁹Ar on basaltic groundmass, to provide a detailed dual-chronometer timeline for CRBG eruptions. We also present new preliminary U-Pb zircon dates for the Lower Steens Basalt, providing new constraints on the earliest emplacement of the CRBG. We use both sets of new ages and precise stratigraphic positions of our samples in an integrated Markov Chain Monte Carlo model to calculate average long-term emplacement rates for main-phase CRBG volcanism of 0.2-0.9 km³/a, with a high likelihood of one prominent hiatus of 60-120 kyr duration occurring after main-phase emplacement. We analyzed trace elements and hafnium isotopes of each dated zircon from CRBG interbeds. The compositions are consistent with both Cascades subduction volcanism and evolved syn-CRBG volcanism proximal to the depositional area. Our age model also yields ages for all magnetic field reversals during the main phase of CRBG emplacement, which can be used to improve calibrations of Miocene paleoclimate records. We find that main-phase CRBG emplacement is coincident with the greatest sustained warmth of the MCO in astronomically-tuned records. Our work shows the power of using both U-Pb and ⁴⁰Ar/³⁹Ar geochronology in an integrated stratigraphic context to assess data reliability and develop the most robust age model possible for large igneous province emplacement.