THE THERMAL EVOLUTION OF A REGIONAL-CONTACT TERRANE IN NORTHWEST MAINE

Three dimensional heat and mass transport modeling of a regional-contact terrane associated with multiple intrusions in northwest Maine show that metamorphic features observed in the field can be produced by advective heat transport due to fluid flow around the intrusions. The temporal evolution of the advective flow results in spatially complex transient thermal regimes; some localities are affected by only a single thermal event while others experience multiple heating and cooling events that cause several periods of mineral growth and dissolution. We have recognized six different patterns of mineral growth and dissolution recorded by textures in rocks from the area that appear to mimic time-temperature histories produced by the models.

To further constrain heating rates, the fluid saturated effective diffusion coefficients from Carlson (2010) are used to calculate characteristic lengths of diffusion{ l = 4(Dt)^0.5 } over the temperature-time intervals predicted by the thermal models. The variable l is the distance away from the porphyroblast center where the matrix has begun to re-equilibrate with the crystal, D is the effective intergranular diffusion coefficient and t is time. In rocks that are well equilibrated at the end of nucleation (i.e. all metastable assemblages have reacted), the spacing between porphyroblast centers can be used with effective intergranular diffusion coefficients to estimate the time span of nucleation. Preliminary results have identified two distinct types of staurolite nucleation: one where nucleation occurred over a relatively short time span (~ 20kyr?) and one where the nucleation interval was about an order of magnitude longer (~ 200kyr?). Because the staurolite-forming reaction appears to be the same in both cases, the shorter interval was probably produced by a more rapid temperature increase than the temperature increase that produced the longer nucleation interval. Further analyses will permit quantitative dT/dt estimates from this data.