U-Pb systematics in volcanic and plutonic rocks of the Krokskogen area: Resolving a 40 million years long evolution in the Oslo Rift

The development of the Oslo Rift started in the latest Carboniferous in the northern foreland of the Variscan Orogen. The initial, intensive, and mainly mafic magmatism at around 300 Ma, was followed by a period of some 40 m.y. of mafic and felsic volcanic and plutonic activity. The present study applied U-Pb in zircon on rocks of the Krokskogen area northwest of Oslo to resolve in greater detail the sequence of events in this evolution. A major challenge, however, has been the absence or paucity of zircon, and their low quality for dating, in some of the important units, especially the alkalic (latitic) rhomb porphyries. The earliest rhomb porphyry flow RP1 yields an age of 299.7 ± 0.4 Ma. It was thus coeval with the earliest basaltic and alkalic eruptions in the southern part of the Oslo Graben. Flows RP6 and RP7 yield overlapping ages of 285.5 ± 0.8 and 284.9 ± 0.5 Ma, and RP11 is 280.2 ± 0.5 Ma. The Bærum, Oppkuven and Heggelia caldera volcanoes, and the Slottet kjelsåsite pluton developed at 276.7 to 275.9 Ma while the Øyangen caldera is somewhat younger at 273-272 Ma. A kjelsåsite stock has an age of 265.4 ± 0.6 Ma, predating the 262.3 ± 0.5 Ma Blindern rhomb porphyry-like dyke, which is among the youngest magmatic expressions in the rift. The new data stress the importance of the magmatic burst at around 300 Ma, possibly plume related, along the entire Oslo Rift, and sharpens our understanding of the timing of the subsequent protracted sequence of magmatic events reflecting continuous heating and stress in the lithosphere, likely broadly controlled by processes accompanying the closing of Pangea.