A clinopyroxene–basalt geothermobarometry perspective of Columbia Plateau (NW‐USA) Miocene magmatism

The origin of NW‐USA Columbia River Basalt Group Miocene magmatism and its relation to tectonism has been widely debated and is still open to study. We investigated the pre‐eruptive evolution of the magmas, to constrain pressures and temperatures of the ascending magmas, and plumbing conditions. We determined major element concentrations of 17–6 Ma tholeiites, and applied clinopyroxene – liquid geothermobarometry to calculate pre‐eruptive pressures and temperatures. These ranged from 0 to 0.66 GPa and 1120 to 1222 °C, respectively, defining two age‐related parallel trends in a P–T diagram. This indicates a consistent crustal evolution of the magmas, and records at least two distinct initial temperatures. Using clinopyroxene interdiffusion coefficients we estimated magma ascent speeds ≥ 0.6 km yr −1 . Possible geological explanations for the calculated parameters are: lower‐crust magma chamber processes; magmatism and tectonism feed‐back consistent with an extensional environment.