New chemical data on the clinopyroxene-garnet pair in the Alpe Arami eclogite, Central Alps, Switzerland

In the Gorduno area, near Bellinzona, Central Alps, well-preserved eclogites crop out only at the margin of the Alpe Arami garnet peridotite massif. These bodies of eclogite have a thermal evolution that differs from that of the peridotite. The bulk composition and clinopyroxene-garnet trace element distribution have been studied in a sample of eclogite from a lens occurring at the rim of the peridotite. It was found that: 1) the trace-element abundance in the garnet and clinopyroxene may be strongly influenced by the major-element composition, 2) their distribution within both minerals is inhomogeneous. In particular, their highly variable contents highlight the complex evolution of the Alpe Aramai eclogite and emphasize the existence, at least for some elements and on a very local scale, of chemical disequilibrium, confirmed indirectly by a significant difference in delta (super 18) O between clinopyroxene and garnet.The low temperature of equilibration estimated for this eclogite records the lack of complete equilibration between clinopyroxene and garnet with changing P-T conditions during the various metamorphic events.