Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany--a Complementary Pseudosection Approach

Abstract

Previous studies of metapelitic rocks from the core of the southern Brittany metamorphic belt suggest a complex clockwise P–T evolution. We use pseudosections calculated for an average subaluminous metapelite composition in the MnNCKFMASH system and average P–T calculations to investigate in more detail the metamorphic evolution of these rocks. For migmatites, sequential occurrence of kyanite, kyanite + staurolite and sillimanite suggests that a prograde evolution to P > 8 kbar at T ≈ 625°C was followed by a P decrease to around 6 kbar at 650°C. Subsequent heating and burial led to melting as a result of the incongruent breakdown of first muscovite then biotite; at the metamorphic peak (P ≈ 8 kbar, T ≈ 800°C), around 25 mol % melt and >20 mol % garnet are predicted. The retrograde evolution began with conductive cooling, allowing crystallization of melt and retrograde replacement of garnet by reaction to biotite and sillimanite. Subsequent near-isothermal decompression from around P ≈ 6 kbar to P ≈ 4 kbar was associated with widespread development of cordierite and a second episode of melt generation. Microstructural relations in upper amphibolite-facies metapelites from the unit structurally overlying the migmatites are consistent with growth of staurolite, andalusite and white mica along the retrograde path. On the basis of phase relations that account for the quantity of H2O required to saturate a given assemblage, an influx of an H2O-rich volatile phase is implied, which we infer to have been derived from crystallizing melt in underlying granites. The close correspondence of predictions from pseudosections to thermobarometric results is encouraging, and the complementary approach used has allowed tighter constraints than before to be placed on the P–T–X evolution of this sector of the Variscides.