LONG-TERM DEFORMATION AND FLUID-ENHANCED MASS-TRANSPORT IN A VARISCANPERIDOTITE SHEAR ZONE IN THE IVREA ZONE, NORTHERN ITALY - A MICROTEXTURAL, PETROLOGICAL AND GEOCHEMICAL STUDY OF A REACTIVATED SHEAR ZONE

The microtextural, petrological and geochemical study of a ductile she ar zone in the phlogopite peridotite of Finero/Ivrea Zone (northern It aly) reveals the long-term deformation of this zone. The zone is divid ed into a protomylonitic and an ultramylonitic part. Both parts reflec t different periods of deformation, although the orientation of the mi neral lineations does not change. In the coarse-grained part (period 1 ) the deformation started under granulite facies conditions (about 775 degrees C). Olivine, ortho- and clinopyroxene and phlogopite recrysta llized dynamically. In the ultramylonitic part relies of the granulite facies event and evidence for a continuous or two-stage deformation h istory under amphibolite facies (minimum 640 degrees C) to upper green schist facies conditions (maximum 520 degrees C) are preserved (period 2). Amphibolite facies conditions are indicated by olivine recrystall ization, the monoclinization of orthopyroxene porphyroclasts and the r ecrystallization and chemical changes of clinopyroxene. The greenschis t facies final stage of period 2 is characterized by decreased X(CO2) and the syntectonic formation of antigorite, tremolite and phlogopite at the expense of recrystallized and porphyroclastic olivine and pyrox ene. Between both deformation periods a short break in deformation con tinuity is probable. Continuous deformation or reactivation in shear z ones of the Ivrea Zone has not been described so far. During the granu lite facies shearing, small amounts of channelized fluid flow led to a slight mass transfer. The shear zone shows a moderate enrichment of N a, Ba, Cu, Cs, H2O and CO2 and a small loss of P. The mass balance of the ultramylonite indicates a significant increase in mass transport. A mass gain can be inferred for H, Na, K, Ba, Al, Ti, P, S, Cs, Sr, Rb , C, Zn, Zr: S, Sc, light rare earth elements, Nb, Cl and Au. The zone is depleted in Ca, Cu, Co, F and Ni. Si, Mg, Cr, Mn, Y, Nb and V are constant within analytical error. Deformation and fluid infiltration l ed to a change in Volume which increases during the granulite facies e vent by 5.7% and during the lower temperature phase by 3.3%. The calcu lated fluid to rock ratios by standard equations results in unrealisti cally high values. For the interpretation of highly deformed rocks wit h drastic grain size reduction it is therefore necessary to consider t he enhanced diffusion, which is mainly controlled by the increased gra in boundary surface.