PRESSURE–TEMPERATURE–TIME HISTORY OF BASEMENT AND ALLOCHTHONS, WESTERN GNEISS REGION, NORWAY

The Western Gneiss region (WGR) of Norway contains one of the largest known tracts of ultrahigh-pressure (UHP) rocks and is overlain by a sequence of oceanic and continental allochthons. Resolving the precise relationship between the UHP basement and the allochthons is central to understanding whether or how the emplacement of the allochthons caused the UHP metamorphism. To shed light on this issue we have determined the pressure–temperature–time history of the basement and the allochthons along a 160 x 100 km E–W transect from the core of the orogen to the foreland.

Thermobarometry of allochthon pelites (garnet + biotite ± kyanite ± staurolite) and garnet amphibolites shows consistent ~1.1 GPa pressures across the entire transect; this stands in contrast to earlier studies that implied a westward increasing PT gradient. Temperatures are high, ranging from 650–800 °C. Basement rocks record similar temperatures but lower pressures (0.6–0.7 GPa). In situ eclogites from both the allochthons and the basement yield minimum pressures of 1.4–1.5 GPa, and basement orthopyroxene eclogites record ~3 GPa, ~825 °C. Allochthon garnets display prograde zoning, whereas basement garnets are homogeneous. Both units demonstrate retrograde resorption of garnet, the effects of which were removed by recalculation utilizing the Mn spike at garnet rims. The Gibbs method of Spear was then used to model P–T paths in allochthon pelites. While the presence of in situ eclogites within the 1.1 GPa pelites suggests >0.4 GPa decompression, all modeling of allochthon garnets across the area indicates consistent heating and mild compression. No decompression is evident in the garnet zoning, and this we tentatively attribute to garnet resorption during decompression.

These observations suggest that: i) the eclogites formed in a relatively warm subduction zone; ii) the allochthon recrystallized as a subhorizontal sheet that stalled at lower crustal conditions (1.1 GPa) after exhumation from the subduction zone, and iii) the basement recrystallized at mid-crustal conditions (0.6 GPa) following a second stage of exhumation.