Finnmarkian Monazite EMP ages in the Central belt of the Seve Nappe, north Jämtland/south Västerbotten, Sweden

Abstract

In this Msc thesis an age dating project is described on high grade gneisses of the Central Belt of the Seve Nappe Complex, Central Sweden. The age dating is done with an Electron Microprobe applied to monazites. The units dated are the Lillfjället gneiss, Avardo gneiss and the Svartsjöbäcken schist (Western Belt) located in the upper and central parts of the Seve Nappe Complex, Swedish Caledonides, Jämtland, Sweden. The Lillfjället gneiss has a T0 age of 497.9 +/- 20 Ma, the Avardo gneiss 502.3 +/- 20 Ma and the Svartsjöbäcken schist 491.2 +/- 22 Ma. All Electron Micro Probe (EMP) monazite ages are interpreted to be the result of the same tectonometamorphic event; the Finnmarkian. During the chemical age dating a standard monazite was used with known age of 1125 Ma. The measurements gave a mean apparent age for the standard monazite of 1132.7 Ma +/- 10 Myr. In addition the PT-conditions are determined for the Lillfjället gneiss. This is done by thermobarometry and with the computer program Domino. Calculated PT conditions are 750 – 800 ºC and 9 – 10 kbar for the Lillfjället gneiss. The calculated metamorphic conditions and age data are used to construct a geodynamic model for the area. This model explains the regional tectonometamorphic setting between the Finnmarkian (~500 Ma) and the Scandian (~420 Ma) orogeny resulting in two collisional events with corresponding subduction and eduction in the surrounding area.