EQUILIBRIUM AND DISEQUILIBRIUM REACTIONS INVOLVING STAUROLITE, KYANITE, SILLIMANITE, AND CORDIERITE – INDUCED BY RAPID DECOMPRESSION AND DIFFERENTIAL FABRIC DEVELOPMENT: CENTRAL LARAMIE MOUNTAINS, SE WYOMING

Archean metapelitic units of the Elmers Rock Greenstone Belt contain mineral assemblages of staurolite, garnet, cordierite, kyanite, sillimanite, biotite, plagioclase, quartz, +/- muscovite. Regional and microstructural analysis in conjunction with P-T calculations indicate that the rocks were subjected to relatively high-pressure metamorphism, M2 (10.5kbar at 670°C), during E-W Paleoproterozoic D2 shortening (the Trans-Hudson Orogeny?). This was followed by nearly isothermal decompression, M3 (4.5-5kbar at 635°C), during NW-SE Paleoproterozoic D3 shortening during the Medicine Bow Orogeny.

The metapelites preserve evidence of equilibrium decompression reactions along spaced S3 foliation planes and D3 strain shadows adjacent to garnet porphyroblasts; however, disequilibrium reaction textures occur in areas away from obvious D3 textures. Sillimanite (fibrolite) occurs in kyanite-bearing rocks in D3 strain shadows and along S3 foliation planes, and sillimanite plus biotite along S3 has locally reacted to form cordierite. In one area of intense F3 folding, kyanite is locally replaced by staurolite via the decompression reaction: grt+bt+ky+H2O=st. Disequilibrium reaction textures involving the replacement of kyanite and staurolite by cordierite occur in microlithons away from the S3 foliation and D3 strain shadows. These textures are the result of reactions that are inconsistent with thermodynamically predicted petrogenetic topologies and can only be explained by reaction overstepping. In these areas, staurolite is partially replaced by cordierite, ilmenite and biotite, and kyanite plus biotite are replaced by cordierite in an overstepped analog to the equilibrium breakdown of sillimanite and biotite to cordierite along S3 foliation planes. The overstepped cordierite-forming reactions in the presence of equilibrium decompression reactions suggests that the disequilibria is localized in areas of the samples that are relatively unaffected by D3 fabrics. Phase compositions away from the S3 foliation planes are relatively homogenous within and between samples and preserve peak M2 compositions. Garnets as much as 1 cm in diameter were completely homogenized during M2 and contain only thin cooling diffusion rims generated during M3.