Two-dimensional study of a layered intrusion — the Hyllingen Series, Norway

Abstract

AbstractThe Hyllingen Series, comprising the southern part of the 160 km2 Caledonian, synorogenic, layered, mafic Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, crystallized from a basaltic parent magma at 5–6 kb. Well developed modal layering strikes directly towards the magma chamber wall at the southern margin. The lithologies of inclusions match those of the adjacent country rock envelope. Eleven sample profiles (274 samples; over 1000 mineral analyses) allow the Hyllingen Series to be subdivided into four stages which generally decrease in thickness from north to south. A sporadically developed Stage I (< 100 m thick) of mostly unlayered ferrodiorites with relatively evolved compositions occurs at the base. Stage II (400–1500m) consists of layered, broadly ferrodioritic rocks. Modal layering is undisturbed by numerous, plate-like, metabasaltic inclusions which occupy about 22% of Stage II. Cryptic variation is slight except at the top where more evolved compositions are developed. Stage III (350–550 m) has fewer inclusions and is characterized by a gradual regression to more primitive compositions. Stage IV (800–2300 m) shows normal fractionation patterns, and a few minor reversals, with extremely evolved rocks in contact with the roof. Occasional metapelitic, platy inclusions occur near the southern margin.The rocks gradually become more evolved along the strike of modal layering towards the southern margin. Olivine and plagioclase vary systematically over about 7 km from Fo23:An46 to Fo7:An35 along the Stage II/III boundary and from Fo75:An63 to Fo13:An42 along the Stage III/IV boundary. While the apparent angle of discordance between modal and cryptic layering is usually less than 20°, in Stage IV near the wall of the magma chamber they are highly discordant (approaching 90°) over a thickness of about 900 m.In situ crystallization along an inclined floor took place from magma which became stratified by double-diffusive convection during Stage II. Modal layering developed concordant with the crystallization front while cryptic layering developed essentially parallel to the liquid stratification. Gradual influx of dense hot primitive magma caused elevation of the stratified magma column in Stage III. During this uplift, progressively more primitive liquid came into contact with earlier crystalline products along the inclined crystallization front. Crystallization during uplift of the magma column gave rise to the gradual regression of Stage III. Highly discordant modal/cryptic layering relations in Stage IV require that the magma became zoned with a horizontal, as well as a vertical, component. This may have occurred in response to lateral cooling establishing horizontal thermal gradients within individual magma layers, the resulting tendency towards density increase being compensated by material diffusion to more evolved compositions.The original roof during the early period of magma chamber evolution is possibly among the inclusions in Stage II. Repeated magma addition during Stage II resulted in the top magma layer having an extremely evolved composition. During the later stages this buoyant, evolved liquid ultimately crystallized to produce the quartz-bearing syenite in contact with the roof.