Sequential injections of mafic magma into a crystallizing magma chamber - a case study from the Lulea area, northern Sweden

During the emplacement and crystallization of a magmatic pluton, the physic al properties of the melt will change with cooling. If magma of a more mafi c composition is sequentially injected into more felsic magma before and du ring crystallization, the result will depend on when the mafic component wa s introduced. The interplay between changing viscosity, crystal content and temperature of the coeval magmas is critical for the finally developed str uctures. Patterns observed at a locality c. 4 km SW of the town of Lulea in northern Sweden have been interpreted in this context. At this locality, a coarse-grained gabbroid is cut by several dykes of both mafic and, in a la ter stage, felsic compositions. The dykes contain round to angular fragment s of host material and the contacts between the dykes and the host vary fro m sharp to diffuse. Mafic microgranular enclaves in the gabbroid host are a lso common structures, which evidences the contrasting rheological behaviou r of the different magmas.