The Metamorphic History ofthe Helags Mountain Area,Scandinavian Caledonides

Sammanfattning:

The Scandinavian Caledonides formed as a result of collision between the continents Baltica and Laurentia, in Silurian and Early Devonian time. The evolution of the orogen has been a topic of research since before the turn of the last century. However, there are still uncertainties regarding the character and timing of the orogenic processes involved in the formation of the Caledonian orogen. Identification and study of high-pressure terranes are a key to understanding the processes involved, and such terrains are found in Jämtland, central Sweden. The most well-known location is Mt. Åreskutan. This study focuses on the Helags Mountain, a locality potentially equivalent to Mt. Åreskutan. It has combined structural and mineralogical studies, pressure and temperature esti-mates, and monazite geochronology, in an attempt to obtain an overview of the metamorphic his-tory.The Helags Mt. geology, as on Åreskutan, is dominated by a klippe of high grade gneisses, overlying lower grade schists and amphibolites, both typical of the middle and lower part of the Seve Nappe Complex in the Swedish Caledonides. The gneisses are dominantly felsic and contain garnet. Two episodes of garnet growth, likely separated in time, are observed in the gneisses. The first episode probably took place in the presence of melt, as is evident from the presence of inclusion of so called nanogranites. This is further supported, but not fully confirmed, by observed homo-genization of the garnet core chemistry. Such processes take place at high temperature, above 700°C. Pressure estimates are less well defined and indicate about 1 GPa during this first garnet growth event. This event may be related to the observed migmatisation. The second garnet growth episode took place at lower pressure and temperature conditions, and similarities with garnet observed in studies elsewhere indicate a connection with shearing and emplacement of the Middle Seve unit. However, no garnets were observed in the studied shear zone, and it is with the available data not possible to confirm a relation to a specific event. Monazite geochronology has contributed Caledonian ages (400-480 Ma) but has not yielded any precise results with regard to the timing of the migmatisation and thrusting.