Metamorphic field gradients in the Central Alps

Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c. 50 x 100 km). PT were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature. Only fluid-conserved equilibria were used to determine P-T. The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors. The results are presented in contour maps. Temperature increases from 500 to 550 degrees C along the limit of amphibolite grade metamorphism in the north and west, to c. 675 degrees C toward the south at the Insubric line near the town of Bellinzona. Maximum recorded pressures of c. 7 kbar are in a central region c. 20 km north of the Insubric line, and decrease both to the north (5.5 kbar) and south (4.5 kbar). The P-T results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40 degrees C) of the kyanite-sillimanite phase boundary. Comparison of P-T conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source. Examination of the two-dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses affected the Central Alps during the Tertiary. In the second, heat affected only the southern parts of the area and overprinted the previously established P-T gradients.