Abstract The 160 km2 Fongen-Hyllingen Complex, situated 60 km southeast of Trondheim, Norway, is a synorogenic, layered mafic intrusion of Caledonian age. Lateral correlation along 40 km of strike length through ca. 6 km of layered cumulates has allowed reconstruction of the form and evolution of the magma chamber. The chamber has the shape of a double-bottomed bowl with an asymmetrical, wide lip which extends towards the south. The northern (Fongen) part of the intrusion occupies the deepest part of the chamber. The southern (Hyllingen) part occupies the lip-like protrusion. Initial 87Sr/86Sr ratios (Sr0) in cumulates range from 0.70308 to 0.70535 and reflect mixing between uncontaminated replenishing magma (with ca. Sr0 0.70308) and resident magma contaminated by partial melts of metapelitic country rocks. Mineral chemistry and Sr0 show a good correlation through most of the layered series, with more evolved mineral compositions having more contaminated isotopic signatures. Assimilation and fractional crystallization therefore were linked during much of the magma chamber evolution. The intrusion, which has a dioritic bulk composition, is divided into four evolutionary stages based on mineral chemistry. Stage I comprises a basal reversal. Stage II has fairly constant compositions in any profile normal to modal layering and is characterized by the presence of numerous, large, raft-like, dominantly metabasaltic, inclusions. Stage III consists of a major compositional regression, ending with the most primitive assemblages in the intrusion (with olivine Fo75, plagioclase An63). Stage IV comprises a sequence showing strong normal fractionation, ending with quartz-bearing syenites with low-temperature end-member mineral compositions at the roof. A major feature in the Hyllingen Series is the presence of systematic lateral compositional variations in mineral chemistry, with more evolved compositions along the strike of modal layering approaching the southern margin. These discordant relations between modal and cryptic layering developed as a result of the crystallization of compositionally zoned magma along an inclined floor. The upper part of the magma chamber became zoned with respect to Sr-isotopes and chemical composition. The lower part remained isotopically homogeneous but chemically zoned until progressive mixing took place with new, uncontaminated magma during the formation of Stage III. Extensive compositional zoning developed both from the roof down (by the generation of roof melts) and from the floor up (by repeated, quiescent emplacement of dense magma along the floor and by mixing as a result of new, dense magma fountaining into buoyant, resident magma).