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Abstract
Isotopic data on the Proterozoic Svecofennian bedrock of south-central Sweden are reviewed. Interpretation of the results by various radiometric methods establishes the ages of the syn-orogenic and late-orogenic igneous events to be 1.89–1.85 Ga and 1.80–1.78 Ga, respectively. Early volcanism is possibly 15–20 Ma older than the syn-orogenic intrusions at 1.87–1.85 Ga. During the time-span from 1.85 to 1.80 Ga deformational and metamorphic processes initiated anatectic melting producing veined gneisses and migmatites. Rocks formed during this time interval have not been dated. The late-orogenic granites and associated pegmatites have ages in a narrow interval of 1.80–1.78 Ga.
The origin of the Svecofennian rocks is discussed on the basis of Sm-Nd data. The sedimentary rocks have a considerable portion of older crustal material but in all the igneous rocks a mantle-derived component dominates. Genetic models imply that the late-orogenic igneous rocks crystallized from mobilized granitic material formed by partial melting of Svecofennian metasedimentary rocks. These models are not supported by the ENd(T) values.
Closure of the K—Ar and Rb—Sr systems in minerals and rocks was related to cooling and uplift of the orogenic belt. At 1.65 Ga, the present bedrock surface is estimated to have been at a depth of 8 km with a temperature below 280°C (estimated blocking temperature for Ar diffusion from biotite). With continued uplift the deformation character of the bedrock changed from ductile to brittle. The oldest fractures are reflected by 1.53 Ga old dolerite dykes. Uplift continued from 1.65 Ga to 1.22 Ga when the Jotnian sediments were deposited on the eroded surface of Svecofennian rocks. This surface coincides approximately with the present-day level of erosion. The youngest intrusions indicating tensional fracturing in south-central Sweden were emplaced at 0.85–1.0 Ga in a N-S belt along the eastern side of the Protogine zone.
