Structures and PT determination of the Caledonian metamorphism of the lower part of the Planetfjella Group in the area around Mosseldalen, northern Ny Friesland, Svalbard

The archipelago of Svalbard is a key area for understanding the North Atlantic Caledonides. The pre-Devonian rocks of Svalbard are an assemblage of different terranes, probably juxtaposed by transpression during the Caledonidian orogenesis. Svalbard's Eastern Terrane consists of a thick (c. 18 km) succession, of Paleoproterozoic to Ordavician age, referred to as the Hecla Hoek sequence, and is divided into three parts; Lower, Middle and Upper. Within the upper part of the Lower Hecla Hoek lies the Planetfjella Group. It is interprented to be sequence of marine sediments, including turbidites. In northern Ny Friesland, in the area of Mosseldalen, the contacts between the Planetfjella Group and the underlying Harkerbreen and overlying Veteranen Groups are tectonic. The importance of these faults has been disputed. The pressure and temperature of metamorphism of impure pelites from near the base of the Planetfjella Group, have been calculated to 575-700°C and 8-11 kbar, using the Hodges and Spear (1982) garnet-biotite thermometer and the Hodges and Crowley (1985) geobarometer. These results are supported by petrographic studies. The Planetfjella Group (c. 4km thick) has been described to be overlain by c. 8 km of younger strata of Neoproterozoic and Early Paleozoic age. At the time of metamorphism (c. 420 Ma) the analysed specimens should therefore have been at a maximum of c. 12km depth. The P-T data presented here indicate that this succession is not adequate to account for the metamorphism; about another 10 km are needed. The rocks overlying the Planetfjella Group in the northern Ny Friesland, are not metamorphosed above lowermost greenschist facies, which corresponds to a stratigraphic overburden that cannot have exceeded 15km. This implies that at least 5km is missing between the base of the Planetfjella Group and the overlying Veteranen Group. The thinning is thought to be partly concentrated to a 100 m wide zone at the top of the Planetfjella Group, but also to be distributed within the latter.