Residence of REE in Low-Temperature Eclogites from Ligurian Alps, North Western Italy: The Role of Accessory Minerals

Abstract

The ophiolites of Ligurian Alps were subjected to pervasive re-equilibration under low-temperature eclogite facies conditions, as a result of a subduction event probably of Cretaceous-Eocene age (Ernst, 1976). Eclogitized Fe-Ti-gabbros developed the bimineralic assemblage of omphacite and Fe-rich garnet, with coronitic to mylonitic textures caused by stress gradient during synmetamorphic deformation. The involvement of fluids during eclogite facies metamorphism, probably related to the breakdown of hydrous minerals developed during the preceding ocean-floor metamorphism, is testified by the local occurrence of garnet (__. omphacite __+ rutile)veins. In coronitic eclogites, high pressure clinopyroxenes have different REE compositions which reflect the original distribution of the major igneous minerals (Messiga et al., 1994). Clinopyroxenes from mylonitic eclogites show homogeneous REE compositions, due to the enlargement of reaction domains into the shear zones. The eclogitic clinopyroxenes, as well as garnets, invariably show a marked LREEdepletion (Fig. 1), stronger than that of igneous diopside relics from coronitic eclogites which have LaN/SmN ~ 0.2. In order to unravel the REE redistribution during the eclogite facies metamorphism, accessory apatites and allanites from an eclogite sample were analyzed using a Cameca IMS 4F ion microprobe. modal percentage is lower than 1%. Other accessory phases are quartz and Fe-sulphides. Apatites display high REE contents (~ 103 x C1), their pattern is characterized by slight MREE-enrichment and negative Eu anomaly (Fig. 1). Allanites have extremely high amounts o f L R E E (,~ 105 x CI; Fig. 1), with a slight Nd-