Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

The Sausfjellet pluton (~447 Ma) is an asymmetrically zoned (W to E) dioritic pluton emplaced at ~7 kbar into Late Proterozoic migmatitic gneiss and marble, which are all part of the Helgeland Nappe Complex of the Uppermost Allochthon. The contact aureole is defined by a km-wide strain gradient with deformation increasing toward the host/pluton contact. Foliations within the contact aureole dip steeply towards the pluton. Kinematic indicators show pluton-up sense of displacement. These observations suggest downward return flow of the wall rocks during magma emplacement. The pluton is characterized by a well-developed NE-striking, moderately NW-dipping magmatic foliation with mineral lineations that plunge shallowly to moderately to the SW. In the north-central portion of the pluton, this fabric is parallel to cm-scale schlieren banding and modal layering defined by alternating plagioclase- and pyroxene-rich layers. Layers display hypersolidus deformation in the form of hook folds, boudinage, and shear bands. Foliation and layering are synformally-folded in the northwestern part of the pluton about a SW axis. Intensity of igneous layering and magmatic foliations, and xenolith abundance appear to increase towards the north-central part of the pluton. Xenoliths consist of calc-silicate and quartzo-feldspathic rocks with contacts that are commonly concordant to magmatic foliations. Additional xenoliths located in the central and eastern portions of the pluton are blocks (up to 200 m long) that are identical to the Akset-Drevli pluton, which crops out north of Sausfjellet. Magmatic foliations and layers are locally deformed and/or truncated in the hypersolidus state by the Akset-Drevli blocks, whereas other exposures show these fabrics draping around the blocks. We conclude that (1) at middle crustal levels brittle and ductile processes accommodate magma emplacement; (2) synchronous stoping and fabric development occurred over much of the pluton's history; (3) layering and foliations in the central part of the pluton may reflect a hypersolidus high strain zone.