The Holterkollen complex contains the following rock types (oldest to youngest): biotite granite with quartz porphyry phases, syentitic ring dikes, and hornblende granite dikes. Chemical data plots form similar and overlapping trends. Trend surface maps plotting chemical variations within the pluton show a series of 'highs' and 'lows' that cross-cut rock type boundaries. Quartz porphyry zones within the pluton were apparently formed by sudden volatile loss whereas the chemical variation pattern is the result of multiple magma injection. The dike rocks appear to have formed from one fractionation series and the plutonic rocks another. Their parent magmas were probably produced by immiscible separation into a felsic and mafic fraction of an original, mantle derived, basaltic melt.
Petrology of the Holterkollen plutonic complex, Oslo Region, Norway