Early Proterozoic continental tholeiites from western Bergslagen, Central Sweden, III. Geodynamic inferences

Abstract

Abstract The 1.88 Ga old within-plate continental tholeiites belong to the mainly metavolcanic, bimodal Bergslagen Supracrustal Series (BSS) in the Svecofennian domain of the Baltic shield. The BSS evolved in an intracontinental rift. Despite only moderate lithospheric extension (β The tholeiites are the (near-) surface expression of a 10–20 km thick magmatic crustal underplate that probably underlies most of the SW Svecofennian domain and caused the Svecofennian crust to be three-layered, thick, but yet isostatically stable. Underplating strengthened the lithosphere sufficiently to control the time of cratonization. Geodynamic models for the origin of the BSS consistent with our interpretations are gravitational collapse of an overthickened continental accreted terrane margin, or back-arc processes related to a marginal volcanic arc 250 km further south. A tentative reconstruction of relative plate motions during Svecofennian time indicates predominantly longitudinal motion of the plates in question and complies with the relatively slow latitudinal drift of the shield at the time. Recent ion-microprobe U-Pb ages of Proterozoic zircons in Svecofennian metasediments imply about 0.2 Ga of pre-Svecofennian crustal evolution, while the Nd model-ages of the most primitive metatholeiites may be indicative of initial mantle differentiation as early as 2.4 Ga BP, accompanying protocrust formation.