Archaean-Proterozoic transition: geochemistry, provenance and tectonic setting of metasedimentary rocks in central Fennoscandian Shield, Finland

The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover. A cryptic suture separating these areas and another tentative suture dividing the Svecofennian into central and southern parts have been proposed. The chemical composition of sedimentary rocks (N = 300) within the study area, including the effects of palaeoweathering, hydraulic sorting, depositional environment and post-depositional processes, have been studied in order to delineate sediment source components. The main proposed source components for the Archaean sedimentary rocks are weathered 3.0-3.2 Ga greenstone + granite +/- TTG and local 2.7 Ca sources. Autochthonous 2.2-1.9 Ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol (2.2-235 Ca), sedimentary rocks derived from the palaeosol, and mafic dykes and plateau volcanics (mainly 2.2-2.1 Ga) although in places locally derived non-weathered Archaean sources dominated. Archaean crust and 2.0-1.92 low-K bimodal rocks from a primitive island are are the proposed source for the allochthonous Western Kaleva cover rocks. These formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone. The central Svecofennian sedimentary rocks can be divided into local are-derived rocks (less than or equal to 1.89 Ga) and older (greater than or equal to 1.91 Ca) rocks from a mixture of Western Kaleva sources and a 1.91-2.0 Ca mature island arc/active continental margin source. Rifting followed by increased subsidence during initial collision in the NE and subsequent are reversal caused rapid erosion from the mountain belt, exposing diverse source compositions as seen in the large variation of Th/Sc (2-0.5), and deposition into an oblique hinterland basin further developing into a subduction related foredeep. Mature greywackes from the southern Svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline-affinity complexes and Archaean rocks. Less mature rocks also occur and had sources dominated either by island arc/active continental margin rocks or local picritic rocks. In the sedimentary record the Archaean-Proterozoic transition up to 2.1 Ga was dominated by input of mainly mafic plateau-type volcanic contribution to the Archaean detritus. Palaeoproterozoic sediments having a crustal component (less than or equal to 2.1 Ga) show higher Th/Sc, Th/Cr, and lower Sm/Nd and Eu/Eu* relative to the Archaean rocks but locally low Th/Cr ratios complicate the situation. Ba depletion relative to K, Rb and Th is a characteristic feature of the sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the elastic record during 2.3-1.9 Ga and further recycled back to the mantle forming a subduction component and an enriched mantle component. Ba depletion seems to have been especially characteristic of chemical weathering during 2.35-2.2 Ga under CO2-rich and low-O-2 atmosphere. Whether this strong Ba depletion is characteristic of the Archaean-Proterozoic transition and quiet supercontinent stages in general remains to be determined. (C) 2000 Elsevier Science B.V. All rights reserved.