Edenite/tremolite and edenite/magnesio‐hornblende in equilibrium with plagioclase, chlorite, epidote, quartz and vapour involve several types of reactions for which KD can be related to T and P. Thermodynamic calculation of these equilibria leads to isopleth systems. Given knowledge of the progressive changes of end‐member activities in zoned Ca–Mg amphiboles (based on microprobe analyses), it is possible to construct precise pressure–temperature–time paths (P–T–t paths) which have been followed by metabasites during polyphase metamorphism. When applied to basic rocks from the River Vilaine area, this method allows us to construct a P–T–t path that can be compared directly to the P–T–t path constructed from interbedded acid rocks (aluminous micaschists) in the same structural unit. Through time, both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the L–S fabric system of Flinn. This allows us to construct a P–T–t deformation path for this structural unit. These paths are interpreted in terms of an under/overthrusting continental collision belt (the Hercynian belt), and represent an illustration of the time delay caused by stacking of more than two crustal units.

Microprobe analysis of the continuous chemical evolution of coexisting biotite-garnet and biotite-garnet-staurolite has been undertaken from interbedded micaschists of the volcanodetritalgroupof the Vilaine. A thermobarometric study using pertinent mineralogical equilibria reveals a complex P-T evolution, continuous throughout time, from high pressure, medium temperature (kyanite zone) to medium pressure, high temperaturc (sillimanite zone), then low pressurc, medium temperature (andalusite zone). The T, P, fHIO and XHZO variations have been calculated from coexisting biotite-garnet pairs, and from the equilibria: paragonite (in white mica) + quartz 2 albite (in plagioclase) + Al silicate + H,O; and, 3 anorthite 2 grossular + 2 Alsilicate + quartz. The P-T evolution is correlated with the continuous change in composition of minerals (using P-X,, and T-X,, diagrams) and with the evolution of assemblages. This continuous P-T-time evolution, correlated with the successive formation of S,-S, foliations, allows us to propose a P-T-time-deformation path for the micaschists and to relate the growth ofits mineral components to tectonic processes.

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Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France) Abstract On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock d 18 O SMOW values of the metabasites show limited variations (10.4±12.0½) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue-and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP±LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing Al IV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, Al IV and Al VI . Empirically calibrated thermobarometers allowed P±T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400±500 C/10±12 kbar was followed by a retrograde P±T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600 C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP±LT rocks escaped from an early erosion by a moderate second burial event and explains the longlasting slow uplift with low average cooling rates.Keywords P±T paths´High-pressure metamorphismÁ mphibole mineral chemistry´Amphibole thermobarometry´Oxygen isotopes´Uplift´South Armorican Domain´Variscan orogeny B. Schulz ( ) )