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Eclogitic garnet amphibolite samples from the Southern Steep Belt of the Central Alps show evidence of several stages of metamorphic evolution and exhumation. A method for unravelling this evolution is presented and applied to these samples. It involves a combination of detailed petrographic analysis and microchemical characterization with quantitative models of the thermodynamically stable phase relations for specific compositional domains of each sample. Preserved mineral relics and textural evidence are compared to model predictions to identify the important irreversible reactions. The interpretation of the exhumation history is thus based on the consistency of a wide spectrum of observations with predicted phase diagrams, leading to robust reconstruction of a pressure–temperature (P–T) path even where the mineralogical relics in samples are insufficient, due to retrogression, to warrant application of multi-equilibrium thermobarometric techniques. The formation of compositionally different domains in the metabasalt samples studied is attributed to prograde growth of porphyroblasts (e.g. garnet, plagioclase, zoisite) in the matrix, implying substantial metamorphic differentiation at the scale of a few millimetres. Chemical interaction among different domains during the subsequent P–T evolution is shown to have been very limited. This led to different reaction sequences during exhumation, in which relics preserved in different domains reflect a range of continually changing P–T conditions. For samples from a single outcrop, we deduce a Barrovian prograde path to eclogite facies (23 ± 3 kbar, 750 ± 50°C), followed by (rapid) decompression to 8 ± 1 kbar and 675 ± 25°C, and a final heating phase at similar pressures reaching 750 ± 40°C. This evolution is attributed to the Alpine cycle involving subduction→collision and slab breakoff→extrusion of tectonic fragments that make up the Southern Steep Belt of the Central Alps. Copyright © 2000 John Wiley & Sons, Ltd.

Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c. 50×100 km). P–T 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 °C along the limit of amphibolite grade metamorphism in the north and west, to c. 675 °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 °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.