Thermal aureole around the Joshua Flat – Beer Creek Pluton (California) requires multiple magma pulses: constraints from thermobarometry, infra-red spectroscopy and numerical modelling

Abstract

Geothermobarometry on rocks from the Joshua Flat – Beer Creek Pluton and its aureole was combined with fl uid investigations and numerical modelling. The pluton is composed of distinct bodies of diorites, monzonites and granodiorites. The granodiorite intruded the still partially molten monzonite. This led to reheating of the pluton and its aureole. Hornblende thermobaro metry gives temperatures of ca. 700 °C and pressures of about 2 kbar. In the contact aureole three metamorphic zones were distinguished with decreasing distance from the pluton: the andalusite-cordierite, sillimanite, and cordierite-K-feldspar zones, respectively. Leucocratic veins together with reaction fabrics between biotite and K-feldspar document dehydration melting due to biotite breakdown in the innermost aureole. This is supported by Na-in-cordierite thermometry and infra-red spectroscopy measurements at cordierite from metapelites. The latter showed relatively low contents of channel volatiles indicating that fl uids did not play a major role during the prograde contact metamorphic evolution of the pluton’s thermal aureole and that heat transport in the aureole was conductive. Numerical modelling of the Tt path of the pluton/wall rock system resulted in temperatures, which are close to those determined by geothermometry when assuming reheating of the system by a second intrusion. Moreover, modelling suggests a conductive nature of heat fl ow around the pluton.