Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling

Abstract

This paper presents a compilation of 67 published and unpublished radiometric age determinations by the Rb-Sr method on magmatic rocks in the continental Oslo Rift in southeast Norway. The data include most major rock types and structural units in the subaerial part of the rift. The implications of these data with respect to the tectonomagmatic evolution of the rift, and processes in the mantle and crust, are discussed. The Oslo Rift was active for about 60 million years. The earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre-rift Lower Paleozoic sediments at about 300 Ma. Extrusion of plateau lavas of basaltic and intermediate composition (rhomb-porphyries) started about 295 Ma and lasted to about 275 Ma, with largest magma volumes and highest extrusion rate from 295 to 285 Ma in the Vestfold Graben Segment (GS) (one flow per 250,000 years and 0.30 km 3 per 10 3 years). The main faulting activity and graben formation occurred during this period. The plateau volcanism was succeeded by a stage of bimodal central volcanoes, most of which later underwent caldera collapses. Ring-dykes and central intrusions in these calderas give ages in the range 268-266 Ma in the Vestfold and 266-243 Ma in the Akershus GS. Large composite intrusive complexes were emplaced in the periods 27–268 and 273-241 Ma in the Vestfold and Akershus GS, respectively. A rate of propagation of the rift from SSW towards the NNE has been estimated to 1–2 cm y −1. On the basis of 87Sr 86Sr initial ratios and type of magmatic products, we estimate magmas to have been emplaced into the crust only during the first 20–25 Ma of the magmatic period. Some of the magmatism appears to be initiated by movement along Precambrian fault zones which were reactivated during the Oslo rifting event.