EXHUMATION HISTORY OF AN ULTRAHIGH-PRESSURE TERRANE

We studied the P–T–t and deformation histories of rocks extending east of the Norwegian ultrahigh-pressure (UHP) terrane to gain insight into the tectonic processes of UHP exhumation. The 220 x 100 km study area comprises both Baltica basement and allochthons emplaced during the Caledonian Orogeny. Mineral stretching lineations plunge gently E or NE. Electron backscatter diffraction of quartzites reveals basal and prism slip in the <a> direction during coaxial to top-W noncoaxial, greenschist- to amphibolite-facies deformation.

Thermobarometry of the allochthons reveals uniform peak temperature conditions (650–750 °C; ~11 kb) across the area; basement rocks record similar peak temperatures but lower pressures (6–8 kb). Eclogites occur as boudins and pods in all units and are typically retrogressed. Bimineralic eclogites yield peak temperatures of ~600–650 °C and minimum pressures of 11–17 kb. Orthopyroxene eclogites record UHP conditions of ~830 °C and 28–29 kb; polycrystalline quartz pseudomorphs after coesite have been identified in an eclogite ~50 km east of previously cited coesite-pseudomorph localities.

U/Pb dating of eclogite zircons by secondary ion microprobe (SIMS) reveals the extent of the Caledonian metamorphism. Allochthon eclogites yield chiefly Caledonian ages with a suggestion of Precambrian inheritance. Basement eclogites yield mainly Precambrian ages with only a hint of Caledonian metamorphism. Th/Pb dating of monazite by SIMS emphasizes this age difference: basement monazites record Precambrian ages, while allochthon monazites record a late-stage, low-temperature event at ~375 Ma. ⁴⁰Ar³⁹Ar muscovite exhumation ages decrease westward from ~395 to ~385 Ma. Considerably older ages of ~405 and ~418 Ma in the allochthons to the east may document cooling after emplacement.

The Norwegian UHP terrane was exhumed in two stages. During the first stage of exhumation, the UHP rocks stalled in the lower crust and re-equilibrated at 650–750 °C. The regional extent of this prograde overprint suggests the subducted allochthons acted coherently during this stage; the basement was likely juxtaposed with the allochthon after this stage. During the second stage of exhumation, rocks were exhumed to the surface by predominantly top-W extension.