Characterization and interpretation of P-T paths with multiple thermal peaks

This paper is included in the Special Publication entitled 'What drives metamorphism and metamorphic reactions?' edited by P.J. Treloar and P.J. O'Brien. Using pressure-temperature (P-T) paths to decipher sequences of tectonic events in the evolution of contractional orogens is difficult because in most metamorphic terrains only a limited part of the path can be inferred from petrologic evidence. Perturbations such as thermal pulses may not be recorded if they occurred over very short geological time scales. Deviations from standard P-T path slopes (e.g. pressure and temperature increase followed by decompression and cooling) are seldom recognized although they may reflect major crustal processes that affect the thermal structure of continents during active tectonism. In particular, recognition of complex P-T trajectories with multiple thermal peaks is essential for understanding tectonic, metamorphic and magmatic processes that occur during unroofing of metamorphic terrains. Using the example of the Nigde metamorphic core complex (Turkey) and other similar terrains, we propose that P-T paths with initial Barrovian trajectories followed by one or more low-pressure- high-temperature thermal spikes (increase in temperature at nearly constant pressure) may be characteristic of metamorphic core complexes. These temperature spikes are commonly associated with granitic magmatism that results when metasedimentary rocks are buried and heated during regional metamorphism. Additional thermal contributions such as enhanced mantle heat flow or mafic magmatism are not required to explain the thermal spikes.