P-T paths from garnet zoning: a new technique for deciphering tectonic processes in crystalline terranes.

A procedure for calculating quantitative rock P-T paths from garnet zoning involves formulating the P and T of the rock as a function of n composition parameters, where n is the thermodynamic variance of the assemblage. Application of the technique to samples from diverse tectonic settings has shown that the calculated P-T path is a sensitive monitor of tectonic processes. Terrains where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the 'prograde' P-T path and cooling during decompression in the 'retrograde' path. In nappe terrains where hot rocks are emplaced over cool rocks, the upper plate shows cooling during compression, whereas the lower plate shows heating during decompression.-L.H.