WAS UHP TECTONISM IN NORWAY CAUSED BY OPHIOLITE EMPLACEMENT?

Ophiolite emplacement onto the Baltica continental margin is inferred to have occurred in western Norway at ~425 Ma, soon before the 395–400 Ma regional ultrahigh-pressure metamorphism in the Western Gneiss Region. We undertook a thermobarometry and geochronology study in the Trondheim–Tännfors–Røros–Oppdal area to assess the relationship between these two events. A series of allochthonous nappes overlie the Baltica basement in our study area: the (upper) oceanic Köli, (middle) continental Gula, and (lower) continental Seve nappes. We calculated pressures and temperatures with Thermocalc using equilibria between garnet, biotite, muscovite, plagioclase, kyanite, staurolite, hornblende, and quartz, and determined PT paths by applying the Gibbs method of Spear et al.

The Seve nappe reached 40–50 km depth via heating and compression paths; the highest temperatures (800°C) were attained in the southwest. Eastern Gula nappe samples show early contact metamorphism at ~20 km depth, followed by compression to 30–35 km depth during heating to cooler peak temperatures (650°C) than the Seve nappe; a sillimanite overprint indicates exhumation to shallower depths while temperatures remained high. Central Gula and Köli nappe samples show heating and decompression from 35 km to 30 km depth. These calculations suggest that the Seve and eastern Gula nappes were buried together in the footwall of a west-dipping contractional fault. If the intraoceanic arc of the Köli nappe formed the hanging wall, as seems likely, the Köli arc must have been relatively young to produce the observed rather warm P/T gradient; this is consonant with ~440 Ma ages for Köli magmatism.

New ion microprobe Th/Pb ages indicate that monazite (re)crystallization in the Gula and Seve nappes began by 412 Ma. Combined with hornblende and mica Ar/Ar ages of ~416 ± 6 Ma (Dallmeyer et al., 1985), these results constrain the Barrovian metamorphism to have ended ~10 Myr after emplacement of the Köli ophiolite onto the Seve–Gula continental margin and ~15 Myr before the peak of the ultrahigh-pressure metamorphism.