Archean–Proterozoic collision boundary in central Fennoscandia: Revisited

Abstract

Archean–Proterozoic (AP) collision boundary in central Fennoscandia is a complex collage of Archean basement rocks, its Paleoproterozoic supracrustal cover rocks, and Svecofennian (1.93–1.78Ga) supracrustal and plutonic rocks. The apparent steep nature and complexity of the AP boundary has led to contradictory tectonic models varying from arc-continent collision via back-arc closure to accretion of arc terranes along a strike-slip shear zone. We present new geochronology data from detrital zircons combined with whole-rock geochemical and Sm–Nd isotopic data on Paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature.The 2.06Ga Siilinjärvi volcanic complex and associated turbidites, with an inferred deposition age at 2.1–2.06Ga, are part of the cover sequence. The Siilinjärvi volcanic rocks derive from both plume (OIB) and sub-continental lithospheric mantle (SCLM) magma sources and are often contaminated by the Archean crust. The Sahara suite sedimentary cover rocks are derived from local 1.95–1.92Ga bimodal volcano-plutonic source, which source also explains the common age population found in younger sedimentary rocks. The Näläntöjärvi suite psammites, with maximum depositional ages of 1.97–1.93Ga, are continental slope to deep water sediments deposited at the continental margin. Contemporaneous mafic magmatism, seen as lavas and dykes, show a change from SCLM dominated (present east) to asthenospheric depleted mantle (DM) dominated (present west) melts. The interpretation of the Näläntöjärvi suite as cover rocks of the lower plate indicates that the Archean crust continues at least 50km more to the west than thought earlier. Parautochthonous sedimentary rocks from Salahmi, Eastern Kansanneva and Loutemäki with maximum depositional ages at 1.92–1.91Ga are inferred as collision-related foredeep deposits from local sources.Both the Lampaanjärvi suite and Pyhäjärvi suite sedimentary rocks have deposited at 1.89–1.88Ga or later, whereas the Lampaanjärvi suite rocks are dominated by a local source and the Pyhäjärvi suite rocks derive from a mixture of older sediments and 1.89–1.88Ga granitoids. At least two tectono-metamorphic stages separated with a major unconformity can be differentiated in the study area. The early deformation and metamorphic stage in the Näläntöjärvi suite has occurred before the intrusion of the late- to post-collisional pyroxene granitoids at 1.88Ga. The Pyhäjärvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875Ma and then subsequently deformed and metamorphosed at ≤1875Ma.We propose a tectonic model where the continental breakup occurred at 2.1–2.06Ga followed by a new spreading within the pre-existing passive margin at 1.98–1.93Ga. During initial continent-arc collision the subducted slab steepened strongly at high angle, subduction ceased rapidly, and finally detached and shallow slab breakoff occurred at 1.92–1.91Ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt. Mantle-crust decoupling of the Archean lithosphere occurred and a crocodile jaw formed, where the arc rocks are partly buried beneath the bounding Archean continent. Similar features have been recorded in other AP collision zones, which could indicate that the buoyant nature of the Archean lithosphere combined with denser Paleoproterozoic lithosphere may be a general cause for decoupling of Archean crust and mantle during arc-continent collision in Paleoproterozoic.