BIMODAL GARNET NUCLEATION AND GROWTH PATTERNS IN METAPELITES FROM ALP DE CONFIN, ADULA NAPPE, SWITZERLAND

A bimodal garnet size distribution within metapelites from Alp de Confin, Adula Nappe (Switzerland) represents two different nucleation and growth episodes. The larger population of garnets has five compositional zones: the first two zones may represent pre-Alpine events; the third zone, which contains the highest-Ca concentration, probably crystallized during the Alpine eclogite-facies event; the fourth and fifth zones are assumed to have formed during the exhumation of the nappe. Compositionally, the smaller garnets match with the fourth and fifth zones of the large garnet population.

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: 1) 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; 2) the presence of relatively few sites that have low activation energies for nucleation; or 3) 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 Alpine 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.