STRUCTURE, LATTICE-PREFERRED ORIENTATIONS, AND METAMORPHISM OF THE BJØRKEDALEN PERIDOTITE, WESTERN NORWAY

The emplacement mechanism(s) of peridotites into orogenic belts is an important, but poorly understood, aspect of Earth’s geological processes with implications for crust–mantle interactions. These mechanisms include i) an upward intruding diapir; or ii) brittle- or ductile sinking intrusion. Many orogenic peridotites are relatively small and/or pervasively overprinted, thus determining their method of emplacement is challenging.

Meter- to km-scale, chlorite- and garnet-bearing peridotite bodies are widespread in the Scandinavian Caledonides, but their mechanism(s) of emplacement remain largely unclear. The Bjørkedalen Peridotite in the Western Gneiss Region of western Norway covers approximately 10 sq. km and is well exposed, easily accessible, and the tectonic framework is well established. It is mostly dunitic (olivine Fo~90 ± orthopyroxene) with minor anorthosite and eclogite. The peridotite lens occupies a regional synform in continental gneiss, and the contact between the peridotite and country rock is sharp and generally highly deformed. Samples were collected to illustrate the transition in microstructure and olivine LPO from the boundary to internal parts of the body. The samples range from highly serpentinized peridotites with a weak to absent foliation and lineation, to strongly foliated and lineated peridotites, to undeformed examples. Metamorphic overprinting associated with foliation includes chlorite and porphyroblastic tremolite and anthophyllite. The fabric becomes progressively more prominent and less serpentinized moving away from the boundary, but central parts are weakly foliated or undeformed. Olivine texture within the peridotite varies from mylonitic porphyroclastic texture near the margins into protogranular and equigranular texture inwards.

A mantle diapir model involves an upwelling plume intruding continental crust from below, but this seems unlikely due to the improbability of dense mantle rising into less dense crust. Ductile sinking inserts hot, dense mantle into crust subducted underneath. Current structural interpretations, however, indicate the Bjørkedalen peridotite was also overlain by crustal material. Elucidating the emplacement mechanisms of the Bjørkedalen locality and other orogenic peridotites in western Norway requires further detailed structural and metamorphic analysis.