Emplacement of polygeneration pegmatites in relation to Sveco-Norwegian contractional tectonics: examples from southern Norway

Abstract

Recent studies have demonstrated that there is a strong link between sites of concentrated magmatism and crustal deformation zones. Magmatism is often described as being syn-kinematic where one or more increments of intrusion punctuate deformation with successive generations of injections being progressively deformed. Pegmatite formation in the Mesoproterozoic of south Norway has been considered as post-kinematic in nature relative to Sveconorwegian (Grenvillian) deformation (1.13 Ga to ∼0.85 Ga) during accretion of the SW margin of Baltica. This paper presents structural observations from mapping of some pegmatites in the Bamble Terrane of south Norway demonstrating that the pegmatites are structurally related to Sveconorwegian fold geometries associated with peak metamorphism at approximately 1.14 Ga and are kinematically related to overthrust geometries associated with the initial overthrusting phase of the Porsgrunn-Kristiansand Fault when the Bamble Terrane docked with the underlying Telemark Terrane. Local fold geometries suggest a strain regime where the maximum contractional axis is sub-horizontal and oriented NW-SE consistent with the regional kinematics. The pegmatites display a systematic deformation pattern consistent with deformation in the isoclinal fold limbs. Pegmatites are also deformed into asymmetric antiformal folds above reverse-shear structures and are also cut by these structures thus showing a thrust geometry. The undeformed pegmatites are emplaced in sub-horizontal fractures and suggests sub-horizontal contractional axis related to overthrusting which is identical to the strain regime of the folding. The evidence suggests that the pegmatites are syn-tectonic and were injected into reverse shear-related fractures. Deformation was progressive and incremental with longer periods of ductile deformation at low strain rate punctuated by shorter periods of fracturing and pegmatite injection at high strain rate. This work therefore implies an intimate spatial and temporal relationship between deformation and magmatism during crustal accretion on the western margin of Fennoscandia.