Abstract

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 upper crust. Numerical models indicate that coalescence of thermal aureoles from multiple felsic intrusions can produce regionally extensive LPM where the abundance of intrusions exceeds ∼50%. 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.

A simulation based on the distribution and U-Pb geochronometry of the composite Sierra Nevada batholith compares favorably with the observed distribution of metamorphic grades and temperature histories; coeval medium-pressure facies-series metamorphism to the east of the batholith requires a different mechanism. Predicted surface heat flow is qualitatively similar in magnitude and distribution to that measured in modern arcs. Sequential intrusions can produce brief (<1 m.y.) high-grade events after several million years of lower-grade metamorphic conditions, providing a mechanism for hornfelsic textures to overprint schistose textures formed during the same event. Alternative mechanisms for LPM include (1) rapid uplift during extension or following crustal thickening and (2) deep heat sources (including magmas). These mechanisms are inconsistent with the geologic record in areas such as the western United States, where large near-surface gradients, lack of requisite major (>10 km) uplift, and evidence for moderate over-all paleothermal gradients require high-level advection of heat. This model of relatively cool crust punctuated by local, short-lived thermal events differs significantly from the image of a broad, uniform thermal maximum that might be drawn from the spatial distribution of LPM; it has implications for crustal deformation igneous petrogenesis, and the interpretation of high metamorphic gradients in Archean terranes.

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