Petrological analyses and Lu-Hf geochronology of subducted continental and oceanic crust from different locations in the upper and lower plate of the Caledonian Orogen

Abstract

In this thesis I conducted Lu-Hf analyses on samples from locations all over the Caledonian Orogen. I combined these ages with petrological analyses in order to help to unravel some of the open questions concerning the development of the Caledonian continental collision.<br /> I dated three meta-mafic / meta-ultramafic samples from the Northeast Greenland Eclogite Province but only one of the eclogites gave a precise isochron age of 357.8 ± 1.7 Ma. I interpret this age as dating a point shortly after the pressure-peak. This sample shows textural evidence of UHP metamorphism, which is in accordance with a c. 360 Ma age from another UHP rock from further northeast. The other eclogite and the ultramafic sample experienced probably only HP metamorphism and yielded errorchrons which are caused by a scatter of the garnet separates due to protracted garnet growth and partial re-equilibration of garnets during high temperature at peak-pressure conditions. Nevertheless, a geological meaningful age of c. 400 Ma age can be estimated for high-pressure metamorphism of those samples which is in accordance with previous dating for HP rocks from the Northeast Greenland Eclogite province. These results support the idea that UHP metamorphism in the Northeast Greenland Eclogite Province is c. 360 Ma, while HP metamorphism is c. 400 Ma. In addition this study showed that UHP metamorphism is not restricted to the Eastern Block of the Northeast Greenland Eclogite Province, but also occurs in the Central Block. The two main problems when explaining these ages is that the Northeast Greenland Eclogite Province is interpreted to be the Laurentian continental margin (i.e. upper plate) and that UHP metamorphism occurred after the main collisional phase between Baltica and Laurentia. Based on my analyses I support a tectonic model in which HP metamorphism at 400 Ma was caused by crustal thickening during collision between Baltica and Laurentia and UHP metamorphism at c. 360 Ma was caused by intra-continental subduction within<br /> Laurentia. <br /> I analyzed an eclogite and a blueschist from the Vestgötabreen Complex in western Svalbard. For the blueschist I obtained an age of 459.3 ± 5.4 Ma, which I interpret as a HP garnet-growth-related age. The garnets in the eclogite show a more complex metamorphic history. They show two growth phases with a discrete jump in composition between core and rim. Lu-Hf geochronology gave an errorchron of 466.3 ± 5.6 Ma which I interpret as a mixed age between core and rim that is biased towards the rim age. The core probably grew during early prograde metamorphism, while the rim grew during blueschist-facies metamorphism after c. 458 Ma, which is the youngest age obtained from the eclogite with Lu-Hf geochronology. This age fit into a model, where the Richarddalen Complex and Berzeliuseggene Unit belong to a continent together with the Pearya Terrane which is now found on Ellesmere Island in the Canadian Arctic and the Vestgötabreen Complex together with Nordenskiöld Land would be part of the Iapetus Ocean that was subducted before the collision. This could have happened either north of Laurentia or (not including the Perya Terrane) further south in connection with the Taconian/Grampian Arc.<br /> The main part of this thesis is devoted to rocks of the Seve Nappe Complex in Scandinavia, which is thought to be part of the outer margin of the Baltican continent. I dated rock from Norrbotten, Jämtland and the Lindas Nappe. The former two indisputably belong to the Seve Nappe Complex, while the affiliation of the Lindas nappe is still controversial. High-pressure rocks in the main part of the Seve Nappe Complex occur in the Tsäkokk Lens (Norrbotten), the Vaimok Lens (Norrbotten), Northern Jämtland and Central Jämtland (from north to south). They are dated at c. 484-487 Ma, c. 480 Ma, c. 460 Ma, and c. 445-450 Ma, respectively. The conditions in the subduction zone did not change during that time interval, as peak PT conditions from all locations fall on a geotherm of 7-9 °C/km. This subduction took place before the main collision of Baltica and Laurentia and was probably related to the collision of Baltica with an island arc. As the ages seem to get progressively younger from north to south, the arc-contintent collision probably slowly migrated south by 9-13 km/Ma. More high precision dating of rocks from locations between Norrbotten and Jämtland is needed to further verify this idea. The role of the Lindas Nappe in this tectonic scenario is still unresolved.