Garnet-chloritoid-kyanite metapelites from the Raspas Complex (SW Ecuador): a key eclogite-facies assemblage
Gabriele, Piercarlo; Ballèvre, Michel; Jaillard, Etienne; Hernandez, Jean
European Journal of Mineralogy Volume 15 Number 6 (2004), p. 977 - 989
published: Jan 5, 2004
ArtNo. ESP147051506005, Price: 29.00 €
The Raspas Complex (Ecuador) contains one of the few eclogitic bodies in the northern Andes. It consists of metaperidotites, eclogites, and metapelites. The latter display three assemblages: (i) garnet + chloritoid + kyanite, (ii) garnet + chloritoid and (iii) garnet + chlorite, in all cases with quartz and muscovite in addition. The growth of these assemblages was coeval with the main ductile deformation, and was followed by minor reequilibration (chlorite growth in garnet + chloritoid samples and chloritoid + quartz aggregates replacing garnet and kyanite in garnet + chloritoid + kyanite samples). Detailed microprobe analyses show increasing magnesian compositions for garnet (from core to rim) and chloritoid (inclusions within garnet compared to matrix grains) in kyanite-bearing samples. The above data are interpreted in the framework of the KFMASH system. Reaction progress along the divariant reaction Cld = Grt + Ky explains the change in chemistry of coexisting phases. The divariant Grt-CldKy assemblage has a narrow stability field, and the P-T conditions are estimated at about 20 kbar, 550-600°C. Decompression, recorded by chloritoid-quartz pseudomorphs of garnet, probably occurred as temperature decreased.