Geodynamic evolution of the first basaltic volcanism (B1) in the Oslo Rift (Southeast Norway)

The Oslo Rift represents the northernmost part of the “The Skagerrak-Centered Large Igneous Province” (SCLIP) characterized by the initial phase of mafic volcanism (“B1” basalts) around 300 Ma. We present a rift-scale geochemical overview of the B1 lavas and discuss their origin and evolution. The B1 phase comprises lava piles reaching up to 1500 m thickness, emplaced in a short time (1–2 m.y.). The lavas are strongly alkaline in the south (Brunlanes melilitites) and decrease in alkalinity towards the north (Krokskogen tholeiites). A strong regional dependence is also evident in the trace element and radiogenic isotope geochemistry. The initial radiogenic isotope compositions of the B1 lavas imply that their formation involved at least four mantle sources. One source was unradiogenic in Sr–Nd isotopes (LoNd type) and gave rise to the southernmost, strongly alkaline Brunlanes series. This component has been interpreted as plume-derived. An alternative possibility is that it was located in metasomatized domains within the lower lithosphere, produced by low-Sr-Nd isotope melts/fluids at some stage just before, or during, the earliest SCLIP phase. We regard the plume model to be most likely. The second mantle source had HIMU-like Sr-Nd-Pb isotopes and was enriched in incompatible trace elements. The third source had the Sr-Nd-Pb isotope characteristics of enriched mantle (EM-type) and was less enriched in incompatible trace elements than the HIMU-like source. We propose that the lower lithosphere beneath the Skien-Vestfold-Jeløya area consisted of garnet peridotite with EM-type Sr-Nd-Pb isotope characteristics that at some stage, long before the SCLIP event, was metasomatized by melts/fluids forming veins strongly enriched in incompatible trace elements and with HIMU-like isotopes. The formation of the strongly to mildly alkaline Skien B1 series involved all these three mantle sources, whereas the mildly alkaline Vestfold and Jeløya B1 basalts were formed by mixing between HIMU-like and EM-type melts. The strongly to mildly alkaline B1 melts formed at ≥140 km depth before, or during, the earliest stages of stretching and thermal erosion of the lithosphere. The northernmost B1 series, the Krokskogen tholeiites, probably formed by partial melting of lithospheric mantle rocks (the fourth mantle source) relatively depleted in incompatible trace elements, that had not been affected by metasomatic processes. The Krokskogen tholeiites appear to have formed at lower pressures, possibly implying a short duration for the thermal erosion of the lithosphere. Reconstructing the petrogenetic evolution of the B1 lavas is important because CO2 degassing during this basaltic phase of the SCLIP may have impacted the global climate around the Carboniferous-Permian transition and further into the Permian.