MICRO-SCALE ELEMENT MIGRATION DURING ECLOGITISATION IN THE BERGEN ARCS (NORWAY): A CASE STUDY ON THE ROLE OF FLUIDS AND DEFORMATION

Abstract

Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindås Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogite- and amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another.