Igneous Petrology of the Synorogenic Fongen-Hyllingen Layered Basic Complex, South-Central Scandinavian Caledonides

The 160 km² Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo_86·2 to Fo_0·2 with a hiatus between about Fo₇₁and Fo₆₁. Plagioclase ranges from An_79·5 to An_1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo_2.4En_66.8Fs_30.8-Wo_2·0En_17·0Fs_81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs₆₇ and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo_44·7En_43·8Fs_11·5-Wo_47·0En₀Fs_53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo₅₅ and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing P_H2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after ‘removal’of the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material.