The effect of fluid and deformation on zoning and inclusion patterns in poly-metamorphic garnets |
| |
| Authors: | Muriel Erambert Håkon Austrheim |
| |
| Institution: | (1) Mineralogisk-Geologisk Museum, Sarsgate 1, N-0562 Oslo, Norway |
| |
| Abstract: | Within the Bergen Arcs of W Norway, Caledonian eclogite facies assemblages (T 650°C, P15 kbar) have formed from Grenvillian granulites (T= 800–900°C, P10 kbar) along shear zones and fluid pathways. Garnets in the granulites (grtI: Pyr56–40 Alm45–25Gro19–14) are unzoned or display a weak (ca. 1 wt% FeO over 1000 m) zoning. The eclogite facies rocks contain garnets inherited from their granulite facies protoliths. These relict garnets have certain areas with compositions identical to the garnets in their nearby granulite, but can be crosscut by bands of a more Almrich composition (grtII: Pyr31–41Alm40–47Gro17–21) formed during the eclogite facies event. These bands, orientated preferentially parallel or perpendicular to the eclogite foliation, may contain mineral filled veins or trails of eclogite-facies minerals (omphacite, amphibole, white mica, kyanite, quartz and dolomite). Steep compositional gradients (up to 9 wt% FeO over 40 m) separate the two generations of garnets, indicating limited volume diffusion. The bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature 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 (deformation) and fluid availability. The results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re-equilibration of garnets. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
