PSEUDOSECTION MODELING OF HIGH PRESSURE METAPELITES IN THE NCKFMASH SYSTEM: COMPARISON TO SAMPLES FROM ALP DE CONFIN, CENTRAL ALPS

The calculation of mineral compositional and modal changes along a metamorphic pressure-temperature path is a critical step prior to the 3D nucleation and growth modeling of Carlson et al. (1995; 2002) and Foster (2003). Pseudosections determined using two different methods (THERIAK-DOMINO, de Capitani, 1994; and PERPLEX, Connolly, 1990; Connolly and Petrini, 2002) for a metapelite bulk composition along a decompression path from eclogite to amphibolite facies are compared to specimens from Alp de Confin in the central Alps. PERPLEX used the Holland and Powell (1998) database and both the Berman (1998) and Holland and Powell (1998) datasets were used in separate calculations for DOMINO.

Pseudosections calculated by each method predict different reactions will occur along the decompression path as well as at what temperatures and pressures the reaction will occur. Models using PERPLEX suggest the dissolution of garnet and the breakdown of paragonite to form plagioclase, the growth of biotite at the expense of phengite, and the dissolution of garnet to form chlorite for a decompression path from the eclogite-facies kyanite + garnet + phengite + paragonite + quartz assemblage. DOMINO predicts the formation of pargasite or plagioclase at the expense of garnet and paragonite for the Berman (1998) dataset, but decompression reactions forming melt, chlorite, talc, or biotite for the Holland and Powell (1998) dataset.

A metapelite outcrop at Alp de Confin in the Adula Nappe, Switzerland preserves a decompression reaction from eclogite facies at the center to amphibolite-facies assemblages near the rim of the outcrop. The eclogite-facies conditions are preserved are a garnet + kyanite + paragonite + phengite + quartz assemblage. Amphibolite facies samples consist of: garnet + paragonite + phengite + quartz with no staurolite, biotite, plagioclase, talc, pargasite or chlorite. None of the modeled assemblages perfectly matches the natural samples. Posisble explanations for this discrepancy include reactions with an external fluid phase, heating along the decompression path, or imperfect white mica solution models at high pressures.