Petrogenesis of the eclogites from Soazza, Switzerland

Abstract

The metabasite bodies interbedded in the kyanite—staurolite-subfacies gneisses of the Adula Nappe in the vicinity of Soazza (Mesolcina Valley) preserve the eclogite assemblage in their cores and grade into common amphibolite towards their margins. Garnets vary irregularly in composition from sample to sample. Omphacites included in garnets have Jd 32, those occurring as large crystals Jd 45, but intergranular laths again Jd 35. They all display the ordered P2/n space group. Amphibole formed late in the eclogite assemblage is a pargasitic hornblende while those occurring in amphibolites are edenites or Mg-hornblendes. Garnet—clinopyroxene pairs equilibrated first at T = 675° C and P ≥ 15 kbar then at a constant T = 760° C under P conditions decreasing from 15 to 10 kbar; the amphibolite-facies assemblages equilibrated at ≈ 570° C, 6 kbar. Chemically, the eclogite relics plot within the field of “common” ( B-type) eclogites in high-grade metamorphic rocks. Magmatic trends are still indicated by their minor elements, and discrimination diagrams suggest that these eclogites were originally tholeiitic basalts of oceanic affinity. In spite of now being within a crustal sequence, the Soazza eclogites are believed to have been a part of an oceanic crust that underwent subduction and “Eo-alpine” metamorphism to be emplaced later, as disrupted solid wedges, during the Alpine tectonism and then recrystallized under Barrovian amphibolite-facies regional conditions during the “Lepontine” phase.