Metamorphic evolution of garnet-epidote-biotite gneiss from the Moine Supergroup, Scotland, and geotectonic implications
Metapelitic gneisses from the Glenfinnan Group of the Moine Supergroup, Scotland, contain sparse large and numerous small garnets, associated with complex zoned epidote and plagioclase in a biotite matrix. The large garnets show four zones (AI–AIV), whereas the small garnets show three or fewer zones, indicating successive garnet nucleation with increasing nucleation densities. Garnet zones AI and AIV grew under static conditions, whereas the formation of AII and AIII was accompanied by deformation. Garnet zones AI and AII were formed in the assemblage (all + biotite + epidote + plagioclase + quartz + fluid + apatite) garnet + chlorite + muscovite ± ilmenite ± sphene ± magnetite; zone AIII in the assemblage garnet + muscovite + sphene ± magnetite; and zone AIV in the assemblage garnet + sphene ± ilmenite. The chemical zonation and microstructures of garnet A indicate two important discontinuities; one at the transition between garnet zones AI and AII, and a second between zones AII and AIII, which correlate with complex zonation shown by epidote and plagioclase. These discontinuities may result from polymetamorphic garnet growth during different orogenic cycles affecting the Moine Supergroup. Geothermobarometric calculations and Gibbs method modelling provide evidence that garnet zone AI grew rapidly during heating from about 550 to 560°C at pressures of about 4–6 kbar. In contrast, the formation of zone AII was accompanied by nearly isothermal compression from 6 to 8·5 kbar (560 575°C), indicating crustal stacking. After a certain period of cooling, garnet zone AIII grew during renewed heating at P–T conditions of about 640°C and pressures between 5 and 9 kbar. Growth of garnet AIV was accompanied by further temperature rise, reaching maximum conditions of about 670°C at 5 kbar.