Sill intrusions in the Oslo Rift were pulsed: New evidence from CA-ID-TIMS U-Pb geochronology.

Intrusive networks in continental rifts provide key constraints on the depth, lifespan, and organisation of magmatic plumbing systems. In the Oslo Rift, maenaite (microsyenite) and camptonite sills have long been interpreted as the earliest magmatic products and linked to specific early stress regimes, largely on the basis of Rb–Sr whole-rock and mineral ages of ~304–294 Ma (e.g. Sundvoll et al., 1992). Within this framework, the coexistence of felsic maenaite and phenocryst-rich camptonites has been used to infer tectonically controlled emplacement during a transition from compressional to extensional conditions at the onset of rifting (Larsen et al., 2008). However, U–Pb geochronology indicates prolonged intrusive magmatism in the Oslo Rift.

Here we present new high-precision U–Pb zircon CA-ID-TIMS ages from maenaite sills across the Oslo Rift. Maenaite sills at Jevnaker, central Oslo, and Slemmestad yield ages of 280–278 Ma. A younger maenaite sill at Byrud Emerald Mines yields ~271 Ma, and a trachyte sill in Alnabru yields ~265 Ma. These data define two principal sill-emplacement pulses at 282–278 Ma and ~273–270 Ma, followed by a later phase of intrusions at ~265 Ma, documenting a pulsed magma emplacement throughout most of the lifespan of the magmatic province.

The ~280 Ma pulse coincides with late plateau to early caldera-stage magmatism, including rhomb porphyry no. 11 (RP11), the Skrim Plutonic Complex, the B2 basalt, and the Ramnes Caldera (Corfu et al., 2024). The younger ~273–270 Ma pulse overlaps central volcano–caldera systems such as Drammen and Nittedal, broadly consistent with the stage-based evolution of the Oslo Rift outlined by Larsen et al. (2008). The revised chronology therefore removes the temporal basis for interpreting the maenaite sills as purely a product of an early, distinct tectonic regime. Instead, linking sill emplacement to more mature stages of rift evolution in an extensional to transtensional setting, when magma transport was apparently organised by mature plumbing systems also feeding central volcanoes.

Petrological observations support this interpretation. Although only maenaites are dated here, they occur together with camptonites, sometimes observed in the same sill, indicating a close relationship. The camptonites commonly contain very high proportions of amphibole and clinopyroxene phenocrysts and display cumulate textures, consistent with repeated recharge of deeper magma reservoirs. Preliminary thermobarometric calculations show amphibole and clinopyroxene crystallization in magma chambers at 20-30 km depth, in line with a model suggesting the presence of mafic cumulates remaining in the deeper crust, as indicated in geophysical data (Neumann et al., 1992).  

Together, these results show that Oslo Rift sill emplacement records deep-rooted, long-lived magmatic systems pulsing throughout much of the lifespan of the volcanic province, providing new insight into how mantle and deep crustal processes govern magmatism in intracontinental rifts.