A sulphide-bearing orthopyroxenite layer in the Bjerkreim–Sokndal Intrusion, Norway: implications for processes during magma-chamber replenishment

Abstract

The Bjerkreim–Sokndal Layered Intrusion contains an up to 3-m-thick layer of sulphide-bearing orthopyroxenite or melanorite that can be followed for about 30 km along the boundary between megacyclic units II and III. The layer of orthopyroxenite is developed within a sequence of ilmenite leuconorites and defines the base of zone IIIa. The leuconorites are generally succeeded by ilmenite–magnetite leucotroctolite, which defines the base of zone IIIb. The orthopyroxenite layer has an initial Sr-isotopic ratio similar to the leucotroctolites (ca. 0.7051) and lower than the enveloping ilmenite leuconorites of zones IIc and IIIa (ca. 0.7054). Variations of tetrahedrally coordinated Al in orthopyroxene and elevated Cr/TiO 2 in the orthopyroxenite layer and the leucotroctolite imply that the orthopyroxenite is genetically more related to the leucotroctolite than the enveloping leuconorites. On the basis of field observations and analytical results, we conclude that the orthopyroxenite crystallised as a result of mixing between relatively primitive magma and differentiated resident magma, both of which were sulphide-saturated, during chamber replenishment. The injected magma formed a buoyant plume that spread out laterally at its level of neutral buoyancy within the compositionally zoned resident magma, some distance above the magma-chamber floor. Mixing in the plume resulted in a hybrid saturated in orthopyroxene, ilmenite and sulphide melt. Batches of relatively dense magma containing crystals of orthopyroxene and ilmenite and droplets of sulphide melt sank to the floor of the chamber from the hybrid magma layer and formed a layer of orthopyroxenite on the magma-chamber floor. Removal of orthopyroxene from the hybrid melt resulted in the sporadic crystallisation of plagioclase at the orthopyroxene–plagioclase cotectic. Plagioclase joined orthopyroxene and sulphide droplets in the dense melt batches which sank to the floor, resulting in the local development of melanorite instead of orthopyroxenite. Crystallisation of ilmenite leuconorite from the resident magma below the hybrid magma layer resumed after formation of the orthopyroxenite layer. This continued as the hybrid layer thickened and eventually came into contact with the chamber floor when leucotroctolites of zone IIIb started to crystallise on the deeper parts of the floor. Elsewhere, lower-temperature cumulates formed on elevated portions of the floor from magma higher in the stratified column. The sulphide-bearing orthopyroxenite layer is not associated with PGE mineralisation because both the replenishing and differentiated resident magmas had lost PGE through earlier sulphide saturation. Zone IIIb is followed by a sequence of lithologies that reflect progressive fractional crystallisation with the appearance of cumulus orthopyroxene at the expense of olivine (zone IIIc), and the successive entries of magnetite (zone IIId) and clinopyroxene plus apatite (zone IIIe). A new major influx of magma took place at this time to produce megacyclic unit IV.