Emplacement conditions and wallrock reactions of pegmatitic granitoid dikes, Kopparnäs, SW Finland

The topic of this thesis is the emplacement conditions and wallrock reactions of pegmatitic granitoids dikes in Kopparnäs, Southwestern Finland. Two locations from Kopparnäs peninsula, where pegmatitic granitoid dikes intrude into mafic host rocks, were chosen for a detailed petrological study. In the first study location, the granitoid dikes intrude into a noritic host rock and in the second study location the dikes intrude into a migmatitic host rock. Analysis on previous studies, field observations and aerial pictures taken with a drone revealed that two generations of granitoid dikes were emplaced in quick succession during the formation of the Southern Finland Granitoid Zone at around 1.83 Ga. The first generation of dikes intruded into E-W trending shear zones. The second generation of dikes intruded into N-S trending shear zones that were formed under SE-NW transpression. Quantitative estimates of pressure and temperature conditions could not be made. The structural evidence points to an intrusion of hot magma into a cold host rock. This information combined with PT-information from previous studies implies a high temperature and low pressure environment. A study of the rock microstucture was done with optical and cathodoluminescence microscopes in order to describe melt-wall rock reactions. Mineral identification and a study of major element composition were done with the electron probe microanalyzer (EMPA). Trace element measurements were made on the laser ablation inductively coupled mass spectrometer (LA-ICP-MS) to find out what transpired chemically when the dikes were emplaced in the host rocks. There are clear sings of melt-wallrock interaction between the granitoid dikes and the mafic host rocks. It is possible that a drop in pressure caused expulsion of fluids from the dikes. Alteration was most extensive in first study location where amphibole devolatilization reactions in the host norite released fluids to the contact zone causing further alteration. The alteration zone around the contacts of the dikes and the migmatites is much narrower in the second study location, since no fluid was released from devolatilization reactions. Despite this, existence of myrmekite in the contact zone in the second location point to sodic-calcic auto-metasomatism. The alteration reactions had a big effect on the trace element compositions, creating some clear linear diffusion trends. Elements, such as zinc, gallium, niobium and tantalum were mobilized and diffused from the dikes to the host rocks. The most hydrothermally altered rocks in the first study location were enriched in easily soluble elements, such as barium and cesium, and depleted in rare earth elements.