New zircon U-Pb ages from Pasalanmäki, Eastern Finland and their implications for the geological development of the area

Abstract

The Geological Survey of Finland conducted bedrock mapping of area surrounding the Kotalahti mine in 1990s in Leppävirta, Eastern Finland, as a part of Ni-Cu potential evaluation within the Raahe–Ladoga shear zone. However, a large part of so called hornblende-gneisses remained poorly studied. This study provides new age data from these gneisses and granitoids occurring in Pasalanmäki in Leppävirta. These gneisses have been tentatively interpreted as part of the older Svecofennian magmatism (1.93 – 1.91 Ga), even though no age data has been previously collected. This study provides a brief summary of the different rock types of the area, their mutual relationships on the field and single grain U-Pb data collected from them. In total, seven age samples were collected from granites, granodiorites, gneisses, and diorites.

Based on the age data, no Archean rocks are present in the area. The individual spot ages for all samples scatter and yield ages spanning from 1.94 to 1.84 Ga, preventing calculation of meaningful concordia ages, or even 207Pb/206Pb ages. The scatter is attributed to significant metamorphic lead loss during post-crystallisation metamorphism and deformation.

Using the field observations together with the collected age data and the knowledge of surrounding rock units, the rocks at Pasalanmäki appear to belong to the older Svecofennian magmatism.