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 inter-grown 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.