The volcanic rocks of the Länkipohja-Jämsä area in the southern part of the Central Finland Granitoid Complex

• Main Author: Heikura, A. (Antti)
• Format: Dissertation
• Language: English
• Published: University of Oulu 2017
• Subjects: Geology
• Online Access: http://urn.fi/URN:NBN:fi:oulu-201704251579; http://nbn-resolving.de/urn:nbn:fi:oulu-201704251579

The Länkipohja-Jämsä area is situated in the southern part of the Central Finland Granitoid Complex and consists of granitic plutonic rocks and supracrustal rocks including intermediate volcanic rocks and volcanogenic sediments accompanied with less abundant mafic and felsic variants. It is a part of the eastern extension of the well-studied Tampere schist belt located between the Central Finland Granitoid Complex to the north and Pirkanmaa migmatite belt to the south. To the east of the study area is the Makkola area, also geologically related to the Tampere schist belt. The deformational events in these supracrustal areas are related to the collision of Central and Southern Svecofennia at 1.88–1.87 Ga. The purpose of this Master’s project was to study the volcanic and volcanogenic rocks of the study area extending over an area of 30 x 20 km and assess their tectonic setting and genesis. The study is part of the Gold Potential Mapping Project of the Geological Survey of Finland (GTK). The data include more than 1300 field observations, 80 thin sections and 85 geochemical analyses. Some older field observations and geological and aeromagnetic maps were also utilized. The occurrences of volcanic rocks have been divided into five subareas: Partala, Kakaristo, Mustajärvi, Ouninpohja, and Länkipohja. The volcanic rocks have mainly intermediate, calc-alkaline to shoshonitic composition, they are enriched in light rare earth elements, depleted in high-field strength elements, and have trace element features that plot them in arc volcanic and active continental margin settings in tectonic discrimination diagrams. The volcanic rocks of Länkipohja and Partala areas are more acidic and alkaline than other volcanic rocks of this study. Otherwise, no remarkable differences in geology or geochemistry were found between the subareas of the study area and the Makkola area to the east and the Tampere schist belt to the west. It is concluded that the whole geological system has formed in a continental arc setting. Furthermore, the presence of a subduction component is inferred from large LILE/HFSE ratio. Despite some signs of hydrothermal alteration, the ore potential of the area is low, disregarding orogenic gold and quartz vein related gold deposits.