Magmatism and the development of low-pressure metamorphic belts: implications from the western United States and thermal modeling

Two-dimensional numerical modeling and geological and geophysical constraints from ancient and modern magmatic arcs demonstrate that magmatic heat advection is sufficient to produce low-pressure metamorphic belts in many areas, and that it is apparently necessary in some areas. In the western United States and other areas, regionally extensive low-pressure facies-series metamorphism (LPM) occurs where intrusions form >~50% of the uppercrust. This effect does not depend strongly on the rate of emplacement: LPM results even with complete cooling between intrusions. Models with geologically reasonable emplacement rates show that in an active magmatic arc, temperatures are near metamorphic maxima for only a small fraction of the time. Arc magmatism cannot sustain widespread thermal gradients of the magnitude indicated by the final distribution of LPM, a result consistent with heat-flow data in active arcs. Low-pressure metamorphic belts can thus develop through numerous local, short-lived metamorphic events while most of the crust remains considerably cooler. Metamorphic maxima largely depend on the biggest nearby intrusion; emplacement rates and other heat sources affect mainly the magnitude, not the distribution of metamorphism. -from Authors