MODELING METAMORPHISM IN THE VARISCIDES OF SOUTHERN BRITTANY - A QUANTITATIVE APPROACH USING PSEUDOSECTIONS

Previous studies of subsolidus to suprasolidus metapelitic rocks from southern Brittany have used reaction microstructures and conventional thermobarometry to suggest the Variscan lower crust followed a clockwise P-T evolution. We construct equilibrium phase diagrams (pseudosections) for the MnNCKFMASH system and use average P-T calculations to investigate in more detail the metamorphic evolution of these rocks. For migmatites, phase relations predicted based on the superimposition of the P-T path inferred from microstructural relations among mineral phases onto pseudosections calculated for an average metapelite composition, contoured for proportions of L and Grt, are broadly consistent with those inferred from petrography. The sequential occurrence of Ky, Ky + St and Sil suggests a prograde evolution to P > 8 kbar at 625ºC decreasing to P around 6 kbar at 650ºC, and followed by increasing P and T to the metamorphic peak. A major melting step occurred at 750ºC and 9 kbar by incongruent breakdown of Ms. At the metamorphic peak of 8 kbar and 800ºC, ~25 mol % L and >20 mol % Grt are predicted from volatile phase-absent melting that also consumed Bt.

The retrograde evolution began with decompression and cooling, allowing crystallization of melt and replacement of Grt by Bt + Sil; this was followed by a near-isothermal decompression segment from around 6 kbar to 4 kbar. The decompression segment was associated with widespread development of Crd associated with a second episode of melt generation, consistent with phase relations calculated for a melt-depleted, low-Mn effective bulk composition. Microstructural relations in upper amphibolite facies metapelites from the unit structurally overlying the migmatites suggest retrograde development of St, And and WM in these rocks. Based on a pseudosection contoured for the quantity of H₂O required to saturate the assemblage at P-T, we suggest that an influx of an H₂O-rich volatile phase is required, which we infer to have been derived from crystallising melt in underlying migmatitic and granitic rocks. The P-T-X evolution of a segment of Variscan lower crust is determined quantitatively using pseudosections in a model system in combination with thermobarometry, which enables better constraints than before to be placed on tectonic models for the Variscan evolution of Southern Brittany.