The bedrock of Central Finland has a research history extending well back into the 19th century, and the overall evolution of the area is thus relatively well understood. Nevertheless, new data from previously poorly studied areas or the revisiting of areas studied decades ago bring new pieces to the Paleoproterozoic Svecofennian puzzle. Recently published studies from areas surrounding the city of Jyväskylä are examples of such pieces. Better understanding of this area, which is the culmination point of several major geological units, on the southeastern boundary of the Central Finland Granitoid Complex (CFGC) is important for understanding the geological evolution of the whole southern half of Finland.
The new data confirm earlier tentative suggestions that units belonging to the older Svecofennian magmatic phase (1930–1910 Ma) along the boundary with the Archean of East Finland (Karelia Province) extend 100 km further to the southeast than previously known. This older magmatic phase ended when the island arc collided with the Archean craton. Following this collision, the protoliths of the paragneiss units present in the area were deposited as greywackes along a passive continental margin. These units cannot be distinguished on a geochemical basis and also display similar detrital zircon patterns, defining ~1.92 Ga as the maximum depositional age for most of the samples. This makes the reliable separation of different units difficult, and it is thus likely that the current unit division mainly reflects differences in post-depositional evolution, not in the depositional setting or timing. The volcanic mafic and ultramafic units occurring as interlayers in different paragneiss units represent extensional phases in the development of the depositional basin(s). Ultramafic members are magnesium-rich picrites with highly variable trace-element patterns, whereas the mafic members display chemical compositions more typical of modern-day mid-ocean ridge volcanic rocks.
Subduction from the southwest beneath modern-day Central Finland commenced just after 1900 Ma, marked by the onset of arc-type calc-alkaline volcanism. Based on their age (1895–1875 Ma) and composition, these volcanic units in the area can be regarded as the eastern continuation of the classical Tampere group 200 km further west. The voluminous granitoid magmatism took place in two stages, at ca. 1895 and 1885–1875 Ma, and is coeval with less voluminous mafic to ultramafic plutonic activity. The bulk of the granitoids forming the CFGC crystallized during the younger phase and can be divided into separate units on a compositional basis. The calc-alkaline granitoids display evolution towards more felsic compositions. This probably reflects changes in the pooling level of granitoid magmas due to erosion and uplift, which allowed only the most evolved compositions to reach the present-day erosion surface during the last stages of this magmatic activity. The younger calc-alkaline magmas overlap in age with compositionally A-type granitoids that form a bimodal suite with more mafic rocks: diorites and gabbros. These lower crustal melts rose only locally to the present erosion level in a transtensional setting through crustal-scale shear zones.
The highest ore potential in the study area is for Vammala-type magmatic Ni and Cu ores related to ultramafic–mafic intrusions. The potential for orogenic gold ores is related to the crustal-scale Leivonmäki shear zone trending northeast–southwest and transecting the whole study area. This dextral shear zone is interpreted as a conjugate of the Raahe–Ladoga shear zone. Certain indications for relatively late hydrothermal Zn–Pb mineralisations and porphyry-type Cu mineralisations were also identified.
Mikkola, P., Hölttä, P. & Käpyaho, A. (eds) 2018. Development of the Paleoproterozoic Svecofennian orogeny: new constraints from the southeastern boundary of the Central Finland Granitoid Complex. Geological Survey of Finland, Bulletin 407, 221 p. Available at: https://doi.org/10.30440/bt407
Teksti: Perttu Mikkola
Perttu Mikkola has over 20 years of experience of studying bedrock in East and Central Finland, in addition to various assignments related to bedrock data management. He works as a geologist in the Ore and Industrial Minerals unit in the Geological Survey of Finland’s Kuopio office.