Precambrian and Caledonian history of Svalbard’s West Ny Friesland Terrane

Sammanfattning: This thesis deals mainly with age-investigations of metasedimentary and magmatic rocks of Ny Friesland, located in the northeastern Svalbard. However, the provenance of quartz-porphyry clasts in a Devonian conglomerate have also been investigated in northwestern Svalbard. This information is important for reconstructions of plate movements in the high Arctic during the Caledonian orogeny and earlier tectono-thermal events. It is also important for understanding the regional geological processes in the Svalbard Caledonides. The Svalbard Caledonides is composed of three terranes, the Northwestern, Southestern and the Eastern terranes, which are separated by large transcurrent faults. In the Eastern Terrane, the rocks of western Ny Friesland are dominated by a major antiformal thrust-stack, including four nappes. They are thrust together and folded during the Caledonian orogeny. The schists and gneisses are generally metamorphosed in amphibolite facies. Each thrust sheet contains granitic-gneisses of c. 1750 Ma age, which are overlain by metasediments. In this thesis, detrital zircons from metasediments have been dated and show that the metasediments in each nappe contain zircons of equal or younger age than the granitic-gneisses. It is therefore concluded that the metasediments are younger than the granitic gneisses. The detrital zircon ages from all the three upper nappes fall generally in three age groups, 1700-1750, 1850-2050, and 2500-2800 Ma. The lowest nappe contains slightly different age-populations of 1200-1300, 1650-1720 and 2500-2700 Ma. These sediments have probably been deposited in the Neoproterozoic but could be as young as Ordovician. In one of the thrust-sheets a quartz-monzonite have been dated to 2709 ±28 Ma and show that the basement complex also includes some older rocks. The rocks of western Ny Friesland differ greatly from those further to the east in Ny Friesland and on Nordaustlandet, which is dominated by Grenville-age metasediments and granites. The data obtained on western Ny Friesland support previous suggestions that the Eastern Terrane can be divided into two parts, the West Ny Friesland Terrane and the Nordaustlandet Terrane. The pre-Caledonian position of the West Ny Friesland Terrane is suggested to be close to Northeastern Greenland based on the remarkable similarities of the rocks and the ages. In the Northwestern Terrane, close to the mountain Lilljeborgfjellet, large quartz-porphyry clasts in a Devonian conglomerate have been dated to c. 1740 Ma, an age not previously determined in the Northwestern Terrane. It is concluded that the transportation cannot exceed a few kilometres; affinity to the West Ny Friesland Terrane is suggested where ages are similar. It is proposed that the terrane boundary between the West Ny Friesland Terrane and the Northwestern Terrane may be located along the Breibogen-Bockfjorden Fault instead of the Billefjorden Fault which previously has been suggested to be the main terrane boundary on Svalbard.