The Turku granulite terrain of southern Finland is located within the south-west corner of Svecofennian Schist Belt, part of the Scandinavian Svecofennian Orogenic Belt. The field area is a representative slice of an island arc system which collided against the northern Baltic Shield at 1890 Ma. This resulted in amphibolite-granulite facies metamorphism in the time interval 1890-1870 Ma when two phases of compression (D1 and D2) deformed the Turku rocks into recumbent isoclinical folds, with axial traces striking NW/SE. A second thermal pulse within the Turku terrain was accompanied by a gentler period of compression (D3) and rotation that produced folds with axial traces orientated NE/SW. Maximum pressure and temperature estimates from three garnet/cordierite barometers, five Fe-Mg exchange thermometers and an internally consistent thermodynamic dataset, are 750°C and 5-5.5 kbar. Under these P/T conditions the pelitic supracrustals partially melted, producing felsic segregation and a restite rich in garnet, cordierite and biotite. Two different generations of garnet and cordierite are linked to D2 and D3. The melting started late D2 and was continuous through early/late D3, during which most of the melting occurred. The leucosomes that were produced throughout this period ranged from syenogranites to granodiorites based on their normative quartz and feldspar components. They all have A/CNK values of >1.10 and thus are peraluminous in composition in accordance with melts produced experimentally within pelitic systems. Fe-Mg contents of leucosomes range from 0.31 to 12 wt %, above 3 wt % the leucosomes are also enriched in Zr and Th. This suggests either entrainment of restitic material during inefficient melt segregation, or that leucosomes are themselves restitic after further melt migration i.e. they are enriched in 'cumulate' phases. The composition of the leucosomes and the presence of garnet and cordierite poikiloblasts suggests that melting proceeded via incongruent melting reactions involving the breakdown of biotite and sillimanite to produce garnet, cordierite, K-feldspar and melt. As volatiles liberated through the breakdown of biotite will partition directly into melt and cordierite, the total volatile contents and the XCO<sub>2</sub> (CO<sub>2</sub>/CO<sub>2</sub>+H<sub>2</sub>O) of cordierite were analysed to distinguish between fluid-present or fluid absent conditions and to allow calculation of fluid species activities.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:663298 |
| Date | January 2000 |
| Creators | Voase, Amanda |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11508 |
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