U-Pb zircon ages of Permian magmatism in the Vestfold lava plateau and Ramnes caldera, Oslo Graben

The Vestf old lava plateau, in the southern segment of the Oslo Graben, is composed of some 200 members (flow complexes) of rhomb porphyry, basalt and trachyte, with a combined estimated thickness of ca. 3 km. Three trachytes dated using the zircon U-Pb method yield ages of 286.8 +/- 0.6 Ma for trachyte T1, 282.2 +/- 0.4 Ma for T4 and 282.5 +/- 1.2 Ma for Ts (Sverkholt-type trachyte). The Ramnes caldera developed in the late stages of the Vestf old volcanic evolution and dating provides ages of 280.5 +/- 1.3 Ma for an ignimbrite, 279.7 +/- 1.0 for a monzonite and 279.5 +/- 0.4 Ma for a syenite. The Ramnes volcano was thus almost coeval to the Skrim complex to the west but 4 to 8 million years older than similar volcanoes in the Krokskogen area to the north. The present results are consistent with the deductions obtained from various lines of geological evidence and provide robust reference pointsforthe intermediate stages, i.e. the climactic rhomb porphyry stage and the central volcano and caldera stage, in the evolution of the rift.