Generation, transport, and accumulation of granitic melts: A study on Svecofennian migmatites and leucogranites in southern Finland

This thesis investigates partial melting processes, products, source rocks, and melt transport systems that resulted in the late Svecofennian (1.85–1.79 Ga) migmatites and leucogranites in southern Finland. Drawing on geochronological and whole-rock geochemical data, field observations, and thermodynamic modelling, the thesis presents a model for the formation of migmatites and leucogranites in southern Finland. The thesis also explores what leucosome size distribution in a migmatite can reveal about the melt transport mechanisms in a migmatite area.

The results show that the late Svecofennian migmatites and leucogranites in southernmost Finland are distinct from their coeval counterparts elsewhere in the Southern Finland Subprovince. Many of the differences can be explained by different melting mechanisms, as migmatites in southernmost Finland were formed through fluid-fluxed melting – that is, in the presence of water that was not bound into the crystal lattices but occurred as a free phase. Dehydration melting, where hydrous minerals release the water necessary for partial melting as the metamorphic grade rises, is more prevalent elsewhere in Southern Finland, as well as globally. The presence of water in southernmost Finland affected the migmatite morphology so that the parts solidified from melt and those that remained solid throughout the partial melting are not as clearly distinguishable as in dehydration melting migmatites. Because excess water also lowers melting temperatures and increases melt productivity, more melts could be produced. Certain elements and elemental ratios in the melt products are also different between the two styles of partial melting. Finally, the presence of water led to melts from different protoliths pooling into melt sinks, where they interacted with each other and their surroundings.

Contrasting southernmost Finland with the rest of the Southern Finland Subprovince emphasizes that partial melting should not be assumed to occur uniformly within the same orogen. The local bedrock features, such as its composition and ability to transport fluids, melts, and magmas, are of great importance.