Magma evolution in the upper part (Stage IV) of the Fongen-Hyllingen Layered Intrusion, central Norway

Abstract

The upper part (Stage IV) of the Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, comprises a ∼ 1800 m thick sequence of cumulates showing a strong normal fractionation trend from olivine gabbros at the bottom through diorites to quartz-bearing syenites at the roof. Mineral compositions show extreme ranges: olivine Fo74-0; Ca-rich pyroxene Mg#79-0; plagioclase An60-0. The cumulate sequence has been divided into five intervals based on cumulus assemblages. The summation method has been used to calculate successive magma compositions for these intervals based on whole-rock major element chemical analyses of 49 samples. The most primitive calculated magma composition has 51.7% SiO2, 5.0% MgO, 5.3% total alkalis and a Mg-number of 47.5; it lies near the base of the basaltic trachyandesite field in the Total Alkali Silica (TAS) diagram, very close to the dividing line between tholeiitic and alkaline suites. This magma evolved through the trachyandesite field into the trachyte field. The most evolved magma composition has ∼ 11.4% alkalies; alkali enrichment is not due to assimilation of metapelites. There was very little iron-enrichment during fractionation and magma density decreased continuously, enabling repeated influxes of new magma to pond and to elevate the resident melt, facilitating magma stratification. The parental magma may have resided in a lower crustal chamber where it differentiated before repeatedly supplying the overlying Fongen-Hyllingen chamber.