Metamorphic conditions of the Variscan collision inferred from metasomatic shear zones (Ile d'Yeu, Armorican Massif, France)
Résumé
The island of Ile d'Yeu is mostly made of moderately deformed granitoid orthogneisses of Cambrian age. A network of amphibolite-facies shear zones developed within the orthogneisses 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 them developed early during the prograde, compression-related part of the Variscan P-T evolution and remained active until the temperature peak. Within the shear zones, intense fluid circulation transformed the granitoids into "micaschists", which locally display interesting mineral parageneses including staurolite, cordierite, corundum, kyanite, sillimanite and andalusite. The paragenetic sequence has been interpreted in terms of a simple Barrovian clockwise P-T path (Semelin and Marchand 1984: C.R.A.S. 299, 633-638). Careful paragenetic analysis and reinterpretation of the crystallisation sequence of the Al2SiO5 polymorphs (Ky And Sil) leads us to propose a qualitatively different evolution. Based on phase diagrams calculated with THERMOCALC, we infer that the fluid-assisted syntectonic development of kyanite-bearing peak parageneses occurred at temperatures and pressures higher than 580 °C and 6 kbar, respectively. Pseudomorphic replacement of kyanite by andalusite and the development of cordierite coronas indicate decompression, possibly associated with cooling. Local development of corundum + cordierite symplectites around staurolite and kyanite suggests fluid-deficient conditions at this stage. Subsequent crystallisation of sillimanite implies renewed heating at low pressures, possibly accompanied by further infiltration of fluids. The cause of this heating is a matter for discussion. We conclude that the transformation of the granitoid orthogneisses into aluminous lithologies results from long-lasting, probably episodic fluid-rock interaction during a complex P-T path involving heating and compression, decompression and further heating.