Heat and fluid flow in contact metamorphic aureoles with layered and transient permeability, with application to the Notch Peak aureole, Utah

Abstract

We have investigated the control of layered and transient permeability structures on fluid flow and thermal evolution in contact metamorphic aureoles using two‐dimensional numerical modeling with petrological and geochemical constraints from the Notch Peak aureole, Utah. The model includes interbedded aquitard and aquifer lithologies on the scale of formations and transient enhancement of permeability due to devolatilization reactions. The results show that a layered permeability structure causes focusing of fluids into aquifer layers, resulting in flow pattern that is strongly time‐dependent and significantly different from the predicted simple convection cells in aureoles with homogeneous permeability. The model predicts temporal development of three distinctive flow regimes: (1) early radial down‐temperature flow due to release of magmatic fluid, (2) small flat convection cells near the pluton‐aquifer contacts during peak metamorphism, (3) late unidirectional down‐temperature flow in the upper aureole and up‐temperature flow in the middle to lower aureole. When the bulk permeability of host rocks is >10−16 m2, heat advection by flowing fluids is significant. At lower bulk permeability, heat conduction dominates, and the effects of detailed permeability structure on the thermal field become negligible. Metamorphic reactions enhance permeability within the inner aureole, localizing fluid convection and increasing fluid flux. The observed spatial distribution of mineral assemblages and oxygen isotopic shifts in the upper part of the Notch Peak aureole is consistent with the predicted time‐integrated fluid flux, which over duration of metamorphism is dominated by the down‐temperature flow of a large amount of water from the pluton into the wall rocks.