A. Zeh et Il. Millar, Metamorphic evolution of garnet-epidote-biotite gneiss from the Moine Supergroup, Scotland, and geotectonic implications, J PETROLOGY, 42(3), 2001, pp. 529-554
Metapelitic gneisses from the Glenfinnan Group of the Moine Supergroup, Sco
tland, contain sparse large and numerous small garnets, associated with com
plex zoned epidote and plagioclase in a biotite matrix. The large garnets s
how four zones (AI-AIV), whereas the small garnets show three or fewer Zone
s, indicating successive garnet nucleation with increasing nucleation densi
ties. Garnet zones AI and AIV grew under static conditions, whereas the for
mation of AII and Am was accompanied by deformation. Garnet zones AI and AI
I were formed in the assemblage (all + biotite + epidote + plagioclase + qu
artz + fluid + apatite) garnet + chlorite + muscovite +/- ilmenite +/- sphe
ne +/- magnetite; zone Bm in the assemblage garnet + muscovite + sphene +/-
magnetite; and zone AIV in, the assemblage garnet + sphene +/- ilmenite. T
he chemical zonation and microstructures of garnet A indicate two important
discontinuities; one at the transition between garnet zones BI and AII, an
d a second between zones AII and AIII, which correlate with complex zonatio
n shown bg epidote and plagioclase. These discontinuities may result from p
olymetamorphic garnet growth during different orogenic cycles affecting the
Moine Supergroup. Geothermobarometric calculations and Gibbs method modell
ing provide evidence that garnet zone AI grew rapidly during heating from a
bout 550 to 560 degrees C at pressures of about about 4-6 kbar. in contrast
, the formation of zone AII was accompanied by nearly isothermal compressio
n from 6 to 8.5 kbar (560 575 degrees C), indicating crustal stacking. Afte
r a certain period of cooling, garnet zone AIII grew during renewed heating
at P-T conditions of about 640 degreesC and pressures between 5 and 9 kbar
. Growth of garnet AIV was accompanied by further temperature mse, reaching
maximum conditions of about 670 degrees C at 5 kbar.