THE EFFECT OF FLUID AND DEFORMATION ON ZONING AND INCLUSION PATTERNS IN POLY-METAMORPHIC GARNETS

Within the Bergen Arcs of W Norway, Caledonian eclogite facies assembl ages (T greater-than-or-equal-to 650-degrees-C, P greater-than-or-equa l-to 15 kbar) have formed from Grenvillian granulites (T = 800-900-deg rees-C, P greater-than-or-equal-to 10 kbar) along shear zones and flui d pathways. Garnets in the granulites (grtI: Pyr56-40 Alm45-25Gro19-14 ) are unzoned or display a weak (ca. 1 wt% FeO over 1000 mum) zoning. The eclogite facies rocks contain garnets inherited from their granuli te facies protoliths. These relict garnets have certain areas with com positions identical to the garnets in their nearby granulite, but can be crosscut by bands of a more Alm-rich composition (grtII: Pyr31-41Al m40-47Gro17-21) formed during the eclogite facies event. These bands, orientated preferentially parallel or perpendicular to the eclogite fo liation, may contain mineral filled veins or trails of eclogite-facies minerals (omphacite, amphibole, white mica, kyanite, quartz and dolom ite). Steep compositional gradients (up to 9 wt% FeO over 40 mum) sepa rate the two generations of garnets, indicating limited volume diffusi on. The bands are interpreted as fluid rich channels where element mob ility must have been infinitely greater than it was for the temperatur e controlled volume diffusion at mineral interfaces in the granulites. The re-equilibration of granulite facies garnets during the eclogite facies event must, therefore, be a function of fracture density (defor mation) and fluid availability. The results cast doubts on modern petr ological and geochronological methods that assume pure temperature con trolled chemical re-equilibration of garnets.