Skip Navigation LinksEXCURSION NO 19
Paleoproterozoic accretional area in southern Finland

Leaders: Mikko Nironen and Raimo Lahtinen (Geological Survey of Finland;
mikko.nironen@gtk.fi, raimo.lahtinen@gtk.fi), Annakaisa Korja and Yrjö Kähkönen (University of Helsinki, Finland; annakaisa.korja@helsinki.fi, yrjo.kahkonen@helsinki.fi)

The excursion will provide an opportunity to study a section in the 1.9-1.8 Ga Svecofennian accretionary orogen. The section consists of two accreting Palaeoproterozoic (~2.0 Ga) microcontinental units and intervening supracrustal sequences. The excursion will traverse perpendicular to the east-west trending main geological units of southern Finland, which are from north to south Central Finland Granitoid Complex, Tampere Belt, Pirkanmaa Belt, Häme Belt, and Uusimaa Belt.

The Central Finland Granitoid Complex is the core of the northern microcontinent. It consists mainly of "synkinematic" (1.89-1.88 Ga) granitoids and minor mafic rocks. It was formed as the result of thickening of the crust and mantle lithosphere during the accretion of the Paleoproterozoic continental block to the Archean craton in the northeast at 1.91 Ga. These rocks are crosscut by "postkinematic" (1.88-1.87 Ga) quartz monzonites, granites as well as gabbros and diabase dykes that were emplaced in a transtensional/extensional collapse stage of the thickened crust. Brittle shear zones developed in the late stages of the collapse.

The classic Tampere (schist) Belt has been an area of intensive studies since the times of J.J. Sederholm over 100 years ago. The 1.90-1.89 Ga medium- to high-K calc-alkaline and shoshonitic volcanic rocks represent a mature continental island arc or an active continental margin -type tectonic setting. These volcanic rocks are underlain by turbiditic graywackes and mudstones, and EMORB/WPB-affinity pillowed lavas, and overlain by mafic volcanic rocks, conglomerates and arenites. The sequence deposited at an active plate margin of the northern microcontinent.
The Pirkanmaa Belt consists mainly of migmatitic, terrigenous (mainly psammitic) sedimentary rocks and it is locally relatively rich in black shales/schists. Units of MORB- to WPB-like volcanic rocks and rare chert are interpreted as expressions of a marine/marginal basin. We propose that the Pirkanmaa Belt is a composite accretionary prism including early rift-stage sediments and volcanics (>1.91 Ga), fore-arc sediments (1.91-1.89 Ga) and accreted material (1.91-1.89 Ga). These rocks were squeezed together when the oceanic crust was consumed by double subduction at 1.89-1.88 Ga.

The Häme Belt consists of a ~1.89-1.88 Ga volcanic-sedimentary sequence that deposited close to the southern microcontinent. The volcanic rocks have been divided into older medium-K calc-alkaline arc-type basalts, andesites and rhyolites, and into younger more rift-affinity tholeiitic medium-K basalts. The supracrustal rocks are currently interpreted to have formed an in an evolved island arc that began to rift. The supracrustal rocks are crosscut by "lateorogenic" 1.83-1.82 Ga granites that are expressions of subhorizontal crustal-scale movements and associated magmatism.

The Uusimaa Belt supracrustal rocks consist of mudrocks, graywackes, volcanic rocks, sedimentary carbonates, and a few banded iron-formations and massive sulphide deposits. The volcanic rocks range from subduction-related rocks (1.90-1.88 Ga) to EMORB/WPB-like picrites and basalts. Also notable is the occurrence of mature quartzites and related pelites, which are part of an upper sedimentary succession. The volcanic rocks represent a variety of tectonic settings that range from volcanic arc and back-arc or intra-arc settings formed on or close to the southern microcontinent to evident within-plate settings. The close juxtaposition of continental-type, arc-type, oceanic-affinity, cratonic and continental-rift rocks in the Uusimaa and Häme belts is best explained as a tectonic collage of partly exotic crustal blocks that have been tectonically transported to their present locations.

In addition to geological studies and interpretations shortly described above, the tectonic scenario is supported by the results of deep seismic refraction (FENNIA-94) and reflection (FIRE1-2 -2002) profiles across the belts. The excursion provides a possibility to compare lithological and structural changes at surface with changes in reflection seismic patterns in the upper crust in a Paleoproterozoic orogen that cratonized already 1.80 Ga ago.
Excursion no. 19
START/END: Helsinki/
Helsinki, Finland
DURATION: 6 days
TIME: 30 July - 4 August