Testing for Rapid Thermal Pulses in the Crust by Modeling Garnet Growth-Diffusion-Resorption Profiles in a UHT Metamorphic 'Hot Spot', New Hampshire, USA

Abstract

A series of localized high-temperature granulite-facies domains ('hot spots') are present within the regional (10-100 km2 scale) amphibolite-facies rocks of the Central Maine Terrane in central New Hampshire (NH), USA. Based on the spatial coincidence of a thermal anomaly and contours of depressed δ18O values centered on networks of quartz-graphite and pegmatitic veins in the vicinity of Bristol, NH, it was proposed in an earlier study that large-scale ascending hot fluid focused through a vein network drove heating and introduced isotopically distinct fluids. The thermal anomaly could be preserved only if the timescales of heating were extremely short, otherwise conduction would smooth the field temperature gradient. Herein, we conduct a petrological test to estimate the peak temperatures and the durations of metamorphism across the Bristol region, using pseudosection analysis as well as forward modeling of garnet growth-diffusion-resorption profiles. This region attained granulite-facies conditions in the sillimanite-K-feldspar-cordierite zone over a larger area than previously mapped. Cordierite is variably present, which reflects bulk compositional controls on its stability as well as its destruction during retrogression. The forward modeling reveals protracted (5-8 Myr) granulite-facies conditions of 0·5-0·6 GPa and ∼750-820°C, and an overall counterclockwise P-T path. Furthermore, a short-lived thermal anomaly or 'spike' (>100°C, ∼0·15 Myr) is superimposed on the granulite-facies core, reaching ultrahigh-temperature (UHT) conditions >900°C, much higher than previously recognized in the area. The short timescale is fully consistent with the localized radius of the thermal anomaly of ∼1·5 km. Subsequently, the area underwent variably developed amphibolite-facies retrogression at ∼650°C and 0·4-0·5 GPa, accompanied by fluid infiltration, garnet breakdown, and muscovite growth. The transient thermal spike and the counterclockwise P-T path indicate that heat transfer could not have been solely the result of internal heating of overthickened crust. We posit that external heat fluxes driven by Acadian plutonism, in addition to heat generation in crust enriched in heat-producing elements, led to the granulite-facies metamorphism. Magmatic loading in the crust, potentially in response to an elevated basal heat flux during the Acadian orogeny, can account for the counterclockwise P-T path. Heat transported advectively by channelized flow of magma or magma evolving hydrothermal fluids is the most likely cause of the transient and local UHT thermal anomaly. The results show that UHT metamorphic events can be extremely brief.