COMBINING ZONING PATTERNS AND BIMODAL CRYSTAL SIZE DISTRIBUTIONS OF GARNETS WITHIN ECLOGITE-FACIES METAPELITES FROM ALP DE CONFIN, ADULA NAPPE, SWITZERLAND TO CONSTRAIN P-T PATHS

Garnet zoning and crystal size distributions in eclogite-facies metapelites from Alp de Confin, Adula Nappe have been studied to determine a complex pressure-temperature history. By employing both pseudosection and quanititative modeling techniques, a more precise understanding of the changes in pressure, temperature, and fluid can be derived.

Garnet zoning in the metapelites fall into one of two patterns: kyanite-bearing samples that contain several euhedral compositional domains and kyanite-absent samples, which preserve bell-shaped zoning profiles. The zoning within the kyanite-bearing samples is sharply delineated by variations in calcium and consistently defines five domains: zone 1, Grs_0.05-0.08; zone 2, Grs_0.01-0.02; zone 3, Grs_0.09-0.15; zone 4, Grs_0.02-0.04; and zone 5, Grs_0.04-0.05. Zone one and two are interpreted via pseudosection modeling to have formed prior to the Alpine eclogite-facies high pressure event, zone three during the highest pressure stage, and zone four and five during exhumation of the nappe.

Large garnets are produced when nucleation and transport rates allow significant garnet growth to occur after only a few nuclei have formed in the rock. This process can be due to: slow overstepping of the garnet forming reactions, which produce low nucleation rates, and relatively long periods of time to transport constituents to the growing garnet by diffusion along grain boundaries; the presence of relatively few sites that have low activation energies for nucleation; or enhanced grain boundary transport due to fluid flow, presence of melt, or high temperatures. Since the large garnet population retains evidence of zoning from metamorphic events prior to the eclogite-facies event, high diffusion rates due to high temperature, and possibly high fluid flow, are indicated. Abundant small garnets are produced when nucleation and transport rates allow many nuclei to form before significant garnet growth occurs. These conditions are caused by rapid overstepping of a garnet forming reaction or suppression of transport rates due to dry grain boundaries. To determine whether either one or another or a combination of both conditions produced the smaller population of garnets, the quantitative modeling techniques of Foster and Dutrow (2003) will be applied.