Focused pulses of regional metamorphism

Growing evidence is emerging to support the idea that metamorphism, even in a regional context, may be punctuated – or dominated – by relatively short pulses of heating, fluid flow, and/or mineral growth. Here, we describe data from two Barrovian metamorphic terranes which test this idea.
In the Barrovian zones of Scotland, garnet Sm/Nd geochronology from the garnet and sillimanite zones yield the same peak metamorphic ages [1] (~465 Ma). The age is similar to the age of crystallization of large igneous bodies in the area. The contemporaneity of peak ages is explained by an efficient, advective component of heating, perhaps mediated by synchronous fluid flow [2]. The duration of this regionwide pulse of peak metamorphism is constrained by new Srin-apatite diffusion modeling. Apatite grains have detrital cores and metamorphic overgrowths and are included within porphyroblasts (e.g. garnet, staurolite). Modeling of intragrain diffusion of Sr constrains the duration of peak metamorphism to <250 kyr for garnet through staurolite zone samples. Garnet multi-component diffusion modeling from the sillimanite zone corroborates this brief pulse duration.
The Wepawaug Schist of Connecticut USA, also yields contemporaneous peak-T garnet Sm/Nd ages from different grades across the terrane (~380 Ma). This age matches a population of texturally young zircons associated with igneous intrusions in these rocks. Garnet cores from the kyanite zone, which have growth textures indicative of extremely rapid growth [3], have been dated by Sm/Nd at 388.6 Ma. This age is matched by another population of zircons also associated with igneous intrusions. This earlier prograde growth event may be related to another pulse of metamorphic growth, brought on by magmatic fluid and heat.
Brief pulses of metamorphic heating and mineral reactions, perhaps catalyzed by the introduction of fluids, may be superimposed on regional scale conductive heating at tectonic rates. Such short pulses could help explain the discrepancy between rapid lab-based reaction kinetics and much slower time-integrated field-based reaction kinetics [4].