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
De. Harlov et R. Wirth, 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, CONTR MIN P, 140(2), 2000, pp. 148-162
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.