THE RELATION OF MULTI-MINERAL CRYSTAL SIZE DISTRIBUTIONS TO RATES OF TEMPERATURE CHANGE DURING METAMORPHISM

Metamorphic episodes commonly form sequences of minerals that are related to different stages of the thermal history of a rock. The crystal size and spatial distributions of each of these minerals are strongly influenced by the rates of temperature change during their reaction history. For example, an upper amphibolite facies metapelite experiencing regional-contact metamorphism in the middle crust will typically cross garnet, staurolite and aluminum-silicate forming reactions that produce textures which provide information about the heating rates during different parts of a rock’s history. In this environment the size and spatial distribution of garnets are most strongly affected by dT/dt within the temperature interval of ~500-550 °C, staurolite distributions are most influenced by dT/dt in the temperature interval of ~550-590 °C and aluminum silicate distributions are most influenced by dT/dt at temperatures above 590 °C.

To investigate the effect of thermal history on the size and spatial distributions of garnet, staurolite and sillimanite, textures in rocks near tabular plutons in the middle crust were calculated using combined thermal/kinetic models. The thermal model provides T-P-t information around the pluton that is used as input to kinetic models of metamorphic reactions, which simulate mineral nucleation and growth at various positions around the pluton. The country rock permeability, pluton depth, pluton thickness, and geothermal gradient were varied to provide different types of thermal histories for the nucleation and growth models.

Environments that experience rapid heating and cooling (conductive heating; t<50,000yr) produce rocks with many small crystals; the garnet, staurolite and sillimanite will be approximately the same size (diameters <0.5 mm). Environments that experience slower heating and cooling (advective heating; t~100,000-500,000 yr) produce rocks with fewer and larger crystals (diameters >0.5 mm). Environments that experience two thermal peaks, one related to rapid heating & cooling due to early conductive heat transport and a second longer lived thermal event due to advective heat transport, have many small garnets produced by the conductive thermal history and larger staurolite and sillimanite, which grew during the advective portion of the thermal history.