Models of crustal anatexis in volcanic rifts: applications to southern Finland and the Oslo Graben, southeast Norway

Abstract

Crustal anatexis in volcanic rifts is discussed in terms of a 1-D numerical model incorporating lithospheric extension, emplacement of hot mafic magma at crustal levels, various crustal compositions and pre-rift conditions. Only the volume of melt generated by anatexis is considered; crustal melt segregation, transport and emplacement at higher levels are not included. We find that under a broad range of conditions, influx of mafic magma to the crust is required for substantial anatexis to take place, and once this stops, silicic magmatism dies out rapidly. In many rifts the observed change in type of magmatism from predominantly mafic to silicic during rifting is probably more due to a diminishing potential for mafic magmas to ascend though the crust than to an actual halt in mafic melt generation at depth. This has important implications where melt production rates are used to analyse the formation of volcanic rifts and margins. When influx of mafic magmas takes place over a finite period of time, they may be focused in the depth region where the potential for anatexis is largest, and in some cases more anatexis can occur than by instantaneous emplacement of the entire mafic body. Our results support the idea that the 1.6 Gyr old rapakivi granites in southern Finland were formed by anatexis of an intermediate to felsic Svecofennian crust. Provided a mantle thermal anomaly of +200–250 °C existed beneath Finland, the seismically observed ∼10 km thick magmatic underplated body could have produced the exposed silicic rocks which are on average 2–3 km thick. In the Oslo Graben in southeast Norway, where the thickness of magmatic underplating is also about 10 km, the amount of rock produced by anatexis is poorly constrained, but appears to be between 1 and 2 km. Provided the pre-rift crustal thickness in the graben was 35–40 km or more, and heat was transported to crustal levels more efficiently than is feasible by conventional stretching models, the model predicts melt volumes in agreement with these estimates. We propose that secondary convection within the narrow graben could have contributed to the high heat transport efficiency. Finally, a conspicuous feature of the southern Finland magmatic region as well as the Oslo Graben is the large discrepancy between stretching factors derived from analysing dyke intrusions and faulting (Finland: β∼1.001; Oslo Graben: β<1.05) and by using variations in crustal thickness (Finland: β∼2; Oslo Graben: β≥1.7–2). A possible explanation for this difference may be that when large volumes of mafic rocks cause crustal anatexis, the partially molten crust becomes very weak and acts as a barrier for stress propagation, effectively isolating the upper crust from further deformations. This could add to the thermal weakening of the ductile lower crust due to the influx of hot mafic magmas. This hypothesis could be tested by investigating a large number of magmatic rifts, combined with development of new rheological models of the lithosphere including the effects of underplating and anatexis.