Abstract

Geothermometry from staurolite-grade pelitic assemblages may fail to reflect peak temperatures of metamorphism because of (1) prograde garnet-consuming reactions, and (2) diffirsional reequilibration during cooling in response to retrograde reactions. Garnet dissolution during staurolite formation leads to sharp compositional gradients within a few micrometers of garnet rims. These gradients may then be modified by diffusion on cooling and lead to anomalous temperature estimates calculated from thermobarometers involving garnet.

Numerical simulations of mineral glowth and diffusion were used to examine these effects for a prograde metamorphic sequence from the Salmon Hole Brook syncline, north-western New Hampshire. In higher grade assemblages, mineral textures indicate that staurolite grew by the consumption of garnet, and garnet-biotite temperature estimates fall in the range 515 ± 30°C. Interbedded layers containing garnet and biotite, but not staurolite, yield temperature estimates of 565 ± 25°C. In the eastern portion of the syncline, rocks containing kyanite + staurolite + garnet + biotite assemblages indicate temperatures of 550 ± 25°C. Hypothetical growth and reaction histories that adhere to the modal, mineralogical, and known geochronological constraints were developed using a thermodynamically rigorous treatment of mineral modes and compositions incorporating calculations for multicomponent diffusion in garnet. These models demonstrate that temperature estimates from staurolite schists may underestimate actual thermal maxima by more than 50°C. Garnet growth accompanying the reaction staurolite = kyanite + biotite + garnet can lead to higher absolute temperature estimates, but these may still be at least 50°C lower than actual peak temperatures. Simulations also demonstrate that Fe enrichment in biotite accompanying retrograde production of chlorite may lead to calculated temperatures that are 10–50°C higher than would occur where only diffusional exchange operated on cooling. Evidence from additional metamorphic terranes suggests that this reaction model has wide applicability.

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