Thermal development of low‐pressure metamorphic belts: Results from two‐dimensional numerical models

Abstract

We have used two‐dimensional numerical models and analytical solutions to heat flow equations to investigate the mechanisms and the spatial and temporal development of regionally extensive low‐pressure facies‐series metamorphism (LPM). Two‐dimensional models are necessary for describing the evolution of isotherms in the crust where lateral heat flow and local time‐dependent heat sources are important. The models demonstrate that felsic intrusions can produce regionally extensive LPM where their abundance through time in the upper crust exceeds approximately 50%. Evenly spaced intrusions elevate vertical metamorphic gradients in regions between intrusions by ∼5° to 10°C km−1 over background values at ∼40% abundance and by ∼15° to >30°C km−1 at ∼70% abundance. Other mechanisms, such as intrusion of mafic magmas in the lower crust, crustal extension, or aqueous fluid advection, cannot independently produce maximum temperatures in such regions. If coeval with intrusion, these other processes will contribute to maximum temperatures; however, the distribution of isotherms and the timing of metamorphism in the upper crust are governed by the shallow intrusions. Intrusion width, abundance, and composition, and timing of emplacement of nearby intrusions have the dominant influences on maximum temperatures between intrusions. For 10‐km‐wide intrusions, abundances necessary for production of regionally extensive LPM are ∼15% less for gabbros than for granites and ∼20% less when intrusions are emplaced simultaneously than when complete cooling occurs between intrusive events. Intrusion shape, height, convection (magmatic and hydrothermal), and recharge (open system magma chambers) have a significantly less effect on thermal evolution. The total area affected by intrusions depends on their final distribution, not the magmatic flux. In the development of low‐pressure metamorphic terranes with abundant intrusions, temperatures in the upper crust are significantly elevated only near recently emplaced intrusions. Consequently, the final distribution of metamorphic grades has little resemblance to the distribution of isotherms at any time—a fundamental difference with LPM in regions lacking abundant shallow intrusions, where metamorphism likely occurs concurrently over large regions. This model of the development of widespread LPM by the juxtaposition through time of local, short‐lived thermal maxima should be considered when interpreting temperature‐dependent phenomena (for example, deformation) in terranes with abundant intrusions.