Thermodynamic analysis of garnet growth zoning in eclogite facies granodiorite from M-Mucrone, Sesia Zone, Western Italian Alps

Samples of the metagranodiorite from M. Mucrone (Sesia zone, Western Alps) show pseudomorphic and coronitic textures where the igneous minerals were partially replaced by high-pressure metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K-feldspar, biotite and minor phases. During the eclogitic event the original plagioclase was fully replaced by zoisite, jadeite and quartz +/- K-feldspar pseudomorphic symplectites and the biotite was in part replaced by phengitic mica. Moreover, a composite corona often developed around the biotite. This corona consists of a layer of phengite I and garnet and, where the igneous biotite and feldspars were in contact, of an outer layer of phengite II intergrown with quartz. Biotite, phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic reactions. A numerical simulation of the observed garnet zoning is performed assuming that intercrystalline diffusion and plagioclase resorption were the slowest rate-determining processes during the prograde P-T path. The metamorphic paragenesis constrains the P-T path chosen in the simulation. The comparison between measured and calculated garnet zoning permits evaluation of the relative weights of interface kinetics, grain-boundary and lattice diffusion.