WAS UHP TECTONISM IN NORWAY CAUSED BY OPHIOLITE EMPLACEMENT?
The Seve nappe reached 4050 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 3035 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 SeveGula continental margin and ~15 Myr before the peak of the ultrahigh-pressure metamorphism.