A Geotectonic Model of the Svecofennidic Orogeny

Abstract

The nowadays generally accepted history of the Archaean Svecofennidic orogeny seems to fit very well into a cyclic pattern with four cycles. Each cycle begins with the eruption of acid rocks and ends with basic ones. As an explanation of this cyclic development mobilisation, or melting in the border region between the sial and the underlying basic layer or mantle is proposed. The acid mobilised rock or magma, being lighter than the underlying basic one, prevents the latter from moving upwards in the crust until almost all acid magma has left the magma chamber. The result of this is the observed sequence of eruption beginning with acid rocks and ending with basic ones. The strong movements during the first folding phase of the orogeny, however, mix to some extent the acid and basic magmas forming the calc-alkaline suite of eruptive rocks. This cycle begins with basic intrusions and is followed by acid ones because the basic magmas move faster than the acid ones owing to their lower viscosity. At the end of this cycle, however, basic magmas again intrude the crust in the form of dikes.

The mobilised rocks or magmas moved also laterally from the space below the sinking geosyncline causing an upward bulging of the crust, the erosion of which supplied sedimentary material for filling the geosyncline. The deep parts of these rising areas or blocks became rich in granites of different types and generations and they are nowadays exposed as large granitic areas around which the geosynclinal gneiss and schist zones curve.