K-feldspar-quartz and K-feldspar-plagioclase phase boundary interactions in garnet-orthopyroxene gneiss's from the Val Strona di Omegna, Ivrea-Verbano Zone, northern Italy

A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron microprobe analysis (E MPA)] and microstructural data transmission electron microscopy (TEM) has b een made of K-feldspar micro-veins along quartz-plagioclase phase and plagi oclase-plagioclase grain boundaries in granulite facies, orthopyroxene-garn et-bearing gneiss's (700-825 degreesC, 6-8 kbar) from the Val Strona di Ome gna, Ivrea-Verbano Zone, northern Italy. The K-feldspar micro-veins are com monly associated with quartz and plagioclase and are not found in quartz ab sent regions of the thin section. This association appears to represent a l ocalised reaction texture resulting from a common high grade dehydration re action, namely: amphibole + quartz orthopyroxene + clinopyroxene + plagiocl ase + K-feldspar + H2O, which occurred during the granulite facies metamorp hism of these rocks. There are a number of lines of evidence for this. Thes e include abundant Ti-rich biotite, which was apparently stable during gran ulite facies metamorphism, and total lack of amphibole, which apparently wa s not. Disorder between Al and Si in the K-feldspar indicates crystallisati on at temperatures >500 degreesC. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldsparplagioclase interface could only have f ormed at temperatures > 500-600 degreesC. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioc lase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. R elatively high dislocation densities (> 2 x 10(9) to 3 x 10(9)/cm(2)) in th e K-feldspar suggest plastic deformation at temperatures >500 degreesC. We propose that this plastic deformation is linked with the extensional tecton ic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures wi thin 20-30 degreesC of the peak metamorphic temperatures estimated for the sample (770 degreesC). Contact between these K-feldspar micro-veins and Fe- MS silicate minerals, such as garnet, orthopyroxene, clinopyroxene or bioti te along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea t hat these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulit e facies metamorphism of the metabasite layers in an extensional tectonic e nvironment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clin opyroxene, plagioclase, K-feldspar and H2O. It is proposed that the dehydra tion of the metabasite layers to an orthopyroxene-garnetbearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphi sm was a metasomatic event initiated by the presence of a highgrade, low H2 O activity fluid (most likely a NaCl-KCl supercritical brine), related to t he magmatic underplating event responsible for the Mafic Formation; and tha t this dehydration event did not involve partial melting.