Abstract:
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 preeruptive
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 feedback
consistent with an extensional environment.
