Metamorphic constraints on the Caledonian Upper Allochthon of Central Norway: the Gula Complex staurolite–garnet–kyanite mica schist

Abstract Petrological studies of staurolite–garnet–kyanite–biotite schist and garnet–muscovite schist of the Gula Complex, central Norway, provide constraints on metamorphic evolution during Scandian continent–continent collision, burial and exhumation of the Caledonian Upper Allochthon. The biotite schist contains conspicuous porphyroblasts of Fe-rich staurolite, garnet and kyanite, set in a fine-grained, well-foliated matrix of biotite, quartz, minor plagioclase and muscovite. The muscovite schist is fine- to medium-grained with a muscovite–quartz-dominated matrix, including garnet, biotite, minor plagioclase and clinozoisite. Pressure–temperature ( P – T ) modelling based on thermobarometric calculations and construction of P – T pseudo-sections illustrate that metamorphism reached 680 °C with pressures estimated up to 1.01±0.11 GPa. Retrogression and decompression are constrained by secondary mineral reactions: local replacement of kyanite to fibrous sillimanite indicates decompression below 0.7 GPa. Growth of foliation-parallel chlorite reflects cooling below 640 °C and the chlorite formation proceeded during cooling and decompression towards 550 °C and 0.4 GPa. The metamorphism is associated with a strong north–south-trending regional foliation, and retrogression and decompression apparently continued within the same strain regime. The P – T modelling shows that even small variations in whole-rock chemistry and P–T conditions can explain heterogeneity and significant shifts in mineralogy and modal concentration of the index minerals of metapelites.