INTERDEPENDENCE OF DEFORMATION, FLUID INFILTRATION AND REACTION PROGRESS RECORDED IN ECLOGITIC METAGRANITOIDS (SESIA ZONE, WESTERN ALPS)

The effects of high-strain deformation and fluid infiltration during A lpine eclogite facies metamorphism have been studied across ductile sh ear zones in relatively undeformed metagranitoids at Monte Mucrone (Se sia Zone, Western Alps, Italy). Microfabrics together with bulk rock a nd stable isotope data indicate that the mineralogical and chemical va riations are related to the degree of deformation, rather than to chan ges in P-T conditions or tectonic position. Transformation of meta-qua rtz diorite to recrystallized eclogitic mylonites involved the breakdo wn of biotite and plagioclase and required the influx of H2O. Bulk-roc k geochemical data show that ductile deformation to form eclogitic myl onites involved an increase in volume with a weight percent gain in H2 O and Si and variable loss of K, Na, Ca and Al. DeltaO-18 changes syst ematically across ductile shear zones into the undeformed country rock s. Constant values in shear zone centres indicate advection parallel t o the shear zone and within 10 cm of the mylonites. A dominant compone nt of diffusive oxygen exchange perpendicular to the shear zones produ ced isotopic fronts, evident from a gradual increase in deltaO-18 valu es to the reference values of the country rocks. The degree of isotopi c shift within the shear zones reflects increasing deformation and deg ree of reaction progress. Multiple phases of Alpine deformation and mi neral growth are recognized in the Monte Mucrone metagranitoids, and i n some cases, eclogite facies shear zones were reactivated under green schist facies conditions. The results of this study suggest that high- strain deformation provided pathways for both synkinematic and post-ki nematic metamorphic fluids which were necessary for complete reactions . Relict igneous fabrics, as well as the presence Of corona textures a round biotite and pseudomorphs after primary igneous plagioclase in th e least deformed rocks, indicate a paucity of hydrous fluids and suppo rt the conclusion that fluid movement was channelled rather than perva sive.