| dc.description.abstract | The Scandinavian Caledonides consists of a series of Nappe Complexes that were transported in early Paleozoic times, from west to east over the Baltic Shield. This Allochthonous Nappe Complex is subdivided from bottom to top into Lower, Middle, Upper and Uppermost Allochthons. The investigated metamorphic complex, called the Seve Nappe Complex (SNC) in Sweden, defines the lower part of the Upper Allochthon in the counties of N. Jämtland and S. Västerbotten. Here the SNC is subdivided into three NS running belts, from top to bottom, called the Western, Central and Eastern Belts. The metamorphic grade (=temp; °C) of the SNC increases from the structural top (Western Belt; lower amphibolites facies) downwards (Central Belt; upper amphibolite- to granulite facies) and than decreases again (Eastern Belt; lower amphibolite- upper greenschist facies). However, although from north to south the metamorphic temperature, defined by mineral assemblages in the Central Belt of the SNC, remains (approximately) the same, this is not the case with associated metamorphic pressures. Associated metamorphic pressures (i.e. depths) in the Central Belt vary orders of magnitude when northern, central and southern sequents are mutually compared. In contrast, characteristic metamorphic mineral assemblages in the over- and underlying Western and Eastern Belts do not reveal this particularity. This change in metamorphic facies, from north to south is exclusively restricted to the Central Belt of the SNC and has never been explained before. The aim of the research project is to date the characteristic mineral assemblages in northern (Marsfjället gneiss) and central (Avardo gneiss) domains of the Central Belt using the chemical age dating EMP technique applied to monazites from samples collected in the summer of 2009 and to determine the formal metamorphic PT conditions of these rocks using the theoretical program Theriak-Domino applied to bulk rock compositions obtained by XRF. The EMP monazite ages of monazite grains included in peak metamorphic mineral assemblages, combined with detailed geothermobarometric work, is used to evaluate existing geodynamic models that can explain the variation in metamorphic facies. The age (T(0)) that has been found for the northern part of the Central Belt of the SNC is 497.3 +/- 3 Ma. The HP rocks in the central part are slightly younger, T(0) = 490.6 +/- 3 Ma. These ages are interpreted as Finnmarkian. The accompanied PT conditions for the rocks of the northern part, obtained by the program Theriak-Domino, are between ~12-17 kbar and ~550-800 oC. This is interpreted as a single stage of continental subduction followed by eduction in the SNC of N. Jämtland and S. Västerbotten. Renewed thrusting occurred during Scandian. | |
