Magmatic textures and whole-rock major element analyses of metaluminous to mildly peralkaline alkali feldspar syenite and granite in the late Palaeozoic Oslo Rift in S.E. Norway, suggest that most of these rocks formed from a mixture of liquids and cumulus minerals, mainly alkali feldspar, and, in some cases, ternary anorthoclase. A likely scenario is that the syenitic to granitic plutons were emplaced as slurries of crystal-laden melts, which continued to accumulate feldspar ± quartz once emplaced at their final level. Interstitial and miarolitic mineral assemblages with arfvedsonite, aegirine, astrophyllite, elpidite and other alkali-rich minerals formed from trapped, highly evolved residual melts. Energy-constrained modelling of the magmatic evolution shows that a mildly alkaline, mafic parental magma, appropriate for basalts and intermediate magmatic rocks in the Oslo Rift, can differentiate to peralkaline, syenitic residual compositions close to silica saturation by fractional crystallization only in a narrow pressure interval (4.5 to 5.0 kbar), at fO2 between ca. QFM-1 and QFM + 1, with low initial water content. When emplaced into the shallow crust, such melts will deposit alkali feldspar cumulates with the composition and mineralogy of mildly peralkaline syenite. Peralkaline granitic residual liquids can be formed by further fractionation of residual melts in this system at lower pressure (2–3 kbar) and will eventually deposit alkali feldspar – quartz cumulates. The residual melts are too strongly peralkaline to account for observed plutonic rock compositions, but they are close to a suite of accompanying peralkaline trachytic to rhyolitic dykes. Similar accumulation processes may be important for syenitic rocks enriched in alkali feldspar and depleted in nominally incompatible trace elements worldwide.