STRAIN PATTERNS, STOPING, AND SUBSIDENCE IN THE 7KB SAUSFJELLET PLUTON, NORTH-CENTRAL NORWAY

The Sausfjellet pluton is a 447 Ma asymmetrically zoned pluton that solidified at 7 kb after intruding complexly deformed, metasedimentary host rocks. The kidney-shaped pluton contains a locally well-layered north-central zone with cm to m-scale alternating dioritic and anorthositic layers that have a uniform NE strike. Oxygen isotope and major/trace element data indicate that the western zone of the pluton assimilated rocks of pelitic composition and that the western zone magmas have the same parental magma as the central layered zone. Magmatic foliations are well developed throughout most of the pluton and have a consistent NE strike except adjacent to the host rock contact. A moderately-developed, SW-plunging lineation occurs within the foliation plane. Magmatic foliations locally define penetrative “hypersolidus” shear zones that transpose and strongly attenuate anorthositic layers; magmatic foliations cut across the compositional zonation of the pluton. Together these relations indicate that the foliation patterns were produced after the majority of the chamber was constructed. Host rock contacts dip steeply inward. A ~1km-wide structural aureole is defined by changes in host-rock fabric orientation, development of diatexitic migmatite, and tightening of pre-existing folds. Host rock structures are locally concordant; however at the map scale a major contact between migmatite and carbonate rocks is dominantly discordant. Xenoliths of calc-silicate and diorite up to several hundred square meters occur within the pluton and locally disrupt layering. Elsewhere, magmatic foliations appear to be undeflected from their regional NE orientation by the presence of adjacent xenoliths. We envision chamber construction by ductile host-rock deformation and concurrent stoping. Formation of the layers occurred during or after the majority of stoping and assimilation, but prior to the final “strain event” that produced the NE-trending hypersolidus fabric. This event may have included SW-directed subsidence or foundering of the dense, cumulate and layered magmas into the western, fractionated zone as indicated by the SW-plunging magmatic lineations and back-injection of western-zone magmas into the layered zone.