EVIDENCE FOR LATE-STAGE HEATING DURING EXHUMATION OF ECLOGITE-FACIES METAPELITES FROM ALP DE CONFIN, ADULA NAPPE, SWITZERLAND

Metapelites from Alp de Confin, Switzerland provide evidence of late-stage heating during exhumation of the Adula Nappe. Alp de Confin lies within the structurally upper portion of the Adula Nappe and contains a 30m x 50m paragneiss outcrop that preserves eclogite-facies metamorphism in a wide-range of pelitic bulk compositions. All of the samples from the outcrop contain garnet + paragonite + phengite + quartz and can be broken into two groups based on the presence or absence of kyanite. No plagioclase, biotite, staurolite, or chlorite has been observed in any of the samples. A bimodal garnet population is present in all of the samples with the smaller population matching compositionally to the final layer of growth on the larger garnet population. Large garnets in the kyanite-bearing samples have euhedral zoning patterns with four distinct compositional regions. The third, high-calcium zone has been correlated with the eclogite-facies event and the fourth compositional region is therefore linked to garnet growth during exhumation. Pseudosection modeling of garnet modes and compositions requires a late-stage heating event during decompression to produce the large garnet rims and small-sized garnet population that are observed. This supports the complex exhumation path for the Adula Nappe proposed by Engi et al. (2001, 2004) and is not consistent with more traditional isothermal decompression paths proposed for most eclogite-facies terranes.