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 p artially replaced by high-pressure metamorphic assemblages. The original ma gmatic paragenesis consisted of quartz, plagioclase, K-feldspar, biotite an d minor phases. During the eclogitic event the original plagioclase was ful ly replaced by zoisite, jadeite and quartz +/- K-feldspar pseudomorphic sym plectites and the biotite was in part replaced by phengitic mica. Moreover, a composite corona often developed around the biotite. This corona consist s of a layer of phengite I and garnet and, where the igneous biotite and fe ldspars were in contact, of an outer layer of phengite II intergrown with q uartz. Biotite, phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic react ions. A numerical simulation of the observed garnet zoning is performed ass uming that intercrystalline diffusion and plagioclase resorption were the s lowest rate-determining processes during the prograde P-T path. The metamor phic paragenesis constrains the P-T path chosen in the simulation. The comp arison between measured and calculated garnet zoning permits evaluation of the relative weights of interface kinetics, grain-boundary and lattice diff usion.