CRUSTAL MELTING AND MAGMA MIXING IN THE TOSEN FJORD REGION, NORTH CENTRAL NORWAY

A zone of migmatite cut by felsic and mafic dikes is spectacularly exposed in a one-kilometer-long road cut near Tosen Fjord in North Central Norway. This migmatite is part of a regional zone of migmatization in the Helgeland Nappe Complex that occurred prior to 440 Ma. Relative ages of the different rock types can be interpreted from the cross-cutting relationships present at this outcrop. The oldest rocks are migmatites with adjacent two-mica granites, and pyroxene-rich gabbros. These rocks appear to have been stretched and boudined during a deformational event, and the migmatite boudin necks are filled in with a garnet and alkali feldspar-rich diorite. The gabbro boudin necks are filled in with pegmatitic comb-layered material, which is interpreted to be leucosome from a deeper migmatization event. The migmatites are cut by granodiorite/diorite net-veined dikes, which in many cases are also deformed. One large deformed dike contains five different rock types, granitic to dioritic in composition, which show evidence of both mechanical and chemical mixing. Large alkali feldspar phenocrysts are often found in diorite adjacent to contact granite (granite with large alkali feldspar phenocrysts). Occasionally, rapakivi textures occur. In one area, a dioritic dike cuts the migmatite, but is in turn cut by migmatite leucosome. The granodiorite/diorite dikes are cut by granitic dikes with mafic enclaves. These dikes are also deformed, and in some cases the enclaves appear to be concentrated on one side of the dike. A post-deformational stage of dioritic diking is present that crosscuts all rock types. The rocks in this road cut can yield insight into processes that occur at mid-crustal levels. The above-stated field relationships and observations suggest that magma mixing and mingling occurred during a partial melting event in this area of present-day Norway.