INFLUENCES OF REACTION HISTORY AND CHEMICAL DIFFUSION ON P-T CALCULATIONS FOR STAUROLITE SCHISTS FROM THE LITTLETON FORMATION, NORTHWESTERNNEW-HAMPSHIRE

Geothermometry from staurolite-grade pelitic assemblages may fail to r eflect peak temperatures of metamorphism because of (1) prograde garne t-consuming reactions, and (2) diffusional reequilibration during cool ing in response to retrograde reactions. Garnet dissolution during sta urolite formation leads to sharp compositional gradients within a few micrometers of garnet rims. These gradients may then be modified by di ffusion on cooling and lead to anomalous temperature estimates calcula ted from thermobarometers involving garnet. Numerical simulations of m ineral growth and diffusion were used to examine these effects for a p rograde metamorphic sequence from the Salmon Hole Brook syncline, nort h-western New Hampshire. In higher grade assemblages, mineral textures indicate that staurolite grew by the consumption of garnet, and garne t-biotite temperature estimates fall in the range 515 +/- 30-degrees-C . Interbedded layers containing garnet and biotite, but not staurolite , yield temperature estimates of 565 +/- 25-degrees-C. In the eastern portion of the syncline, rocks containing kyanite + staurolite + garne t + biotite assemblages indicate temperatures of 550 +/- 25-degrees-C. Hypothetical growth and reaction histories that adhere to the modal, mineralogical, and known geochronological constraints were developed u sing a thermodynamically rigorous treatment of mineral modes and compo sitions incorporating calculations for multicomponent diffusion in gar net. These models demonstrate that temperature estimates from stauroli te schists may underestimate actual thermal maxima by more than 50-deg rees-C. Garnet growth accompanying the reaction staurolite = kyanite biotite + garnet can lead to higher absolute temperature estimates, b ut these may still be at least 50-degrees-C lower than actual peak tem peratures. Simulations also demonstrate that Fe enrichment in biotite accompanying retrograde production of chlorite may lead to calculated temperatures that are 10-50-degrees-C higher than would occur where on ly diffusional exchange operated on cooling. Evidence from additional metamorphic terranes suggests that this reaction model has wide applic ability.