Wagnerite in a cordierite-gedrite gneiss - witness of a long-term fluid-rock interaction in Variscan lower crust (Ile d'Yeu, Armorican massif, France)
Résumé
Wagnerite, a relatively rare F-bearing phosphate occurs within a cordierite-orthoamphibole gneiss on the Ile d'Yeu island (Variscan Armorican Massif, France). Most of the island is built up by a moderately deformed granitoid orthogneisses of presumably Cadomian age. A network of amphibolite-facies shear-zones developed within the orthogneiss during the Variscan deformation. Based on the geometry and geochemistry of the shear-zones, Sassier et al. (2006: Int. J. Earth Sci. 95, 2-18) argued that some of the shear zones developed early during the prograde part of the Variscan clockwise PT evolution and remained active at least till the temperature peak. Associated intense fluid circulation locally transformed the granitoids into kyanite-bearing "micaschists". A layer of a spectacular cordierite-gedrite gneiss occurs within the orthogneiss, close to a major shear-zone. Progressive transition from the orthogneiss into the Crd-Ged gneiss is documented at the outcrop scale. Numerous gedrite-filled veins suggest that this transformation is related to fluid circulation. The gneiss is coarse-grained, built up of albite, cordierite, gedrite, biotite and chlorite. Sheaves of gedrite display a weak preferred orientation parallel to the orthogneiss foliation. Anhedral relics of staurolite are systematically surrounded by cordierite. Wagnerite forms subhedral crystals, up to several mm large, in textural equilibrium with all the phases excepting staurolite. Wagnerite Fe/(Fe+Mg) ratio decreases rimwards (0.16-0.11) and displays a negative correlation with the F/(F+OH) ratio (0.75-1.00), probably attributable to the F-Fe avoidance. Based on phase diagrams calculated with THERMOCALC, we infer that the wagnerite-bearing Crd-Oam-Bt-Chl paragenesis equilibrated at 500-600°C, and pressures lower than 4 kbar, whereas the presence of staurolite implies similar temperatures, but pressures higher than 4 kbar. We conclude that the transformation of the common granitoid orthogneiss into a relatively unusual Al- and Mg-rich, K- and Ca-poor wagnerite-bearing Crd-Ged gneiss results from long-lasting fluid-rock interaction during decompression at —500-600°C. Together with previous results, this emphasizes the essential role played by fluids during most of the PT evolution of this part of the Variscan belt.