AN ION MICROPROBE STUDY OF TRACE-ELEMENT COMPOSITION OF CLINOPYROXENES FROM BLUESCHIST AND ECLOGITIZED FE-TI-GABBROS, LIGURIAN ALPS, NORTHWESTERN ITALY - SOME PETROLOGIC CONSIDERATIONS

Detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized Fe-Ti-gabbros from Ligurian Alps (northwestern Italy). Ion microprobe analyses have also been carried out on garnets from eclogitized Fe-Ti-gabbros. The aim is to understand the relations between metamorphic reactions and trace element compositions of clinopyroxenes in high pressure/low temperature terranes. Trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists. Metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases. In particular, the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral, such as relatively high Sc, Y, and HREE contents. The trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites. This reflects the more limited major element equilibration of blueschists relative to eclogites. In deformed eclogites, the clinopyroxenes lack the geochemical signatures of the original igneous minerals, most probably due to the enlargement of reaction domains caused by synmetamorphic deformation. Trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites, chemical equilibrium is locally approached, and, therefore, the trace element composition of clinopyroxenes is controlled by the coexisting minerals. For instance, clinopyroxenes in eclogites are strongly HREE- and LREE-depleted (i.e., with bell-shaped pattern) due to the equilibration with garnet and allanite, respectively. The partition coefficients for REEs between omphacite and garnet are markedly lower than those reported in the literature. On the basis of crystal-chemical considerations, we suggest that this is probably due to lower temperature conditions during formation.