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

In the area of Gorduno, near Bellinzona, Central Alps, Switzerland, well-preserved eclogites crop out only at the margin of the garnet peridotite massif of Alpe Arami (AA). These bodies of eclogite were not derived from cognate mafic layers of mantle origin, and their thermal evolution is different from that inferred for the peridotite. Some questions are still open about the P–T evolution of AA eclogites, and the data given in literature cover a broad range of variation. We have investigated the bulk composition and the clinopyroxene–garnet trace-element distribution in a sample of eclogite from a lens occurring at the rim of the peridotite body. We report new data on trace elements in the bulk rock, obtained by ICP–MS, as well as the results of SIMS analyses on REE, Ti, V, Cr, Y, Sr, Zr, Sc carried out on the two major rock-forming minerals, i.e., clinopyroxene and garnet. We also provide AAS data for Co, Ni and Cu in purified clinopyroxene and garnet concentrates, and O isotope ratios both on the bulk rock and on clinopyroxene and garnet. We found that: i) the trace-element abundance in garnet and clinopyroxene may be strongly influenced by the major-element composition; ii) their distribution within both mineral phases is inhomogeneous. In particular, their highly variable contents highlight the complex evolution of the AA eclogite and emphasize the existence, at least for some elements and at a very local scale, of chemical disequilibrium, also confirmed, in an independent way, by a significant difference in 18 O between clinopyroxene and garnet. Therefore, the low temperature of equilibration estimated in eclogite sample 70–AM–10 is not an artifact of the computational strategy adopted, but is the record of the lack of complete equilibration between clinopyroxene and garnet with changing P–T conditions during the various metamorphic events. …………………………………