THE IMPORTANCE OF LAWSONITE METASOMATISM IN BLOCK-IN-MATRIX STRUCTURES: FIELD AND GEOCHEMICAL FINGERPRINTS FROM ALPINE CORSICA

Fluid-rock interaction at High-Pressure (HP) has been shown to represent a fundamental process for recycling elements such as REE and LILE during subduction and a source for arc magmatism. Chaotic units, including block-in-matrix structures, are a perfect environment for metasomatism and are the locus of intense chemical exchange between contrasting lithologies and circulating fluids at HP (e.g. Bredding et al., 2004, Geology 32, 1041-1044).

We present spectacular examples of lawsonite-bearing metasomatism affecting contacts between metaophiolite blocks (mafic, ultramafic or ophicalcite) and the surrounding metapelites in blueschist-facies terranes of Alpine Corsica (France), a classical type-locality for well-preserved lawsonite. Lawsonite metasomatites, often lawsonitite, form at the expense of metapelitic protolith, and range in size from a few cm to about 25 m depending on the size of the “allochthonous” body. Lawsonite forms mm-size idiomorphic blasts that include trails of the protolith’s fabric, most notably carbonaceous material (CM) and acicular rutile. Lawsonite compositional zoning and bulk rock geochemistry show a systematic REE and LILE (most notably Ca and Sr) enrichment in lawsonitites compared to the metapelite protolith, thus indicating strong variation in the bulk rock composition during lawsonite growth. Different geochemical patterns characterize retrogressed lawsonitites, where REE and LILE are strongly depleted compared not only to the lawsonite metasomatite, but also to the metapelitic protolith. We conclude that lawsonite metasomatites formed by fluid-rock interaction represent an important reservoir for (C)OH volatiles and elements like REE and LILE and for their recycling at greater depth at the lawsonite breakdown, and a possible source of these elements for arc magmatism. Block-in-matrix structures, which are common in many orogenic belts (e.g. Corsica, Franciscan complex, Syros, New Caledonia), represent the perfect environment for widespread metasomatic lawsonite precipitation, and suggest its relevance at global scale.