THE RELATION OF POLYMETAMORPHIC TEXTURES TO TRANSIENT THERMAL REGIMES IN REGIONAL-CONTACT TERRANES WITH MULTIPLE INTRUSIONS

Thermal regimes in regional-contact metamorphic terranes associated with multiple intrusions commonly produce a wide variety of polymetamorphic textures that provide a record of the complex temperature-time (T-t) histories in these environments. Three-dimensional heat and mass transport modeling coupled with modeling of reaction kinetics show that the spatial distributions of temperature, corresponding isograds, and reaction textures are strongly influenced by permeability of the country rock as well as the arrangement of plutons.

In situations where low permeability prevents advection, heat is transported by conduction, producing short lived (10-20 kyr) thermal pulses that restrict metamorphism to country rocks close to the plutons, a characteristic typical of contact metamorphism. Kinetic models of mineral reactions with these T-t histories produce typical contact textures (large numbers of small crystals due to high nucleation rates and short growth times.) In contrast, situations with higher permeability that allow fluid flow to advectively transport heat produce much more long-lived (several 100kyr), widespread, and complex thermal events. Fluid circulation may result in broad metamorphic zones that loosely conform to intrusion boundaries, a characteristic typical of regional-contact metamorphism. Kinetic models of mineral reactions in this setting produce textures that are typical of regional-contact metamorphism (mm to cm sized porphyroblasts, due to lower nucleation rates and longer growth times.) Distribution of isograds and mineral textures suggest that the thermal regime of regional-contact metamorphism in northwest Maine was dominated by isotropic advective flow.

Because of the long-range transport of heat in systems dominated by advection, models with multiple intrusions similar to those in northwest Maine produce spatially complex, transient thermal regimes. Some areas have a single thermal event while others experience multiple events. Thus, there are a wide variety of nucleation, growth,+/- dissolution, +/-re-nucleation and growth patterns that can be related to the local thermal history by kinetic models of mineral reactions. These patterns are used to decipher the spatial distribution of thermal events and infer heating rates during polymetamorphism in NW Maine.