and AR. McBirney 1 A. Nicolas 2 and 1Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA 2Laboratoire de Tectonphysique, Universite de Montpellier II, Place E Bataillon, 34095 Montpellier, France
Re-examination of textural and structural features of the Skaergaard Intrusion has clarified the process responsible for various forms of layering. Two general types are distinguished, one produced by magmatic flow and another by non-dynamic processes, such as compaction and varied nucleation and growth of crystals. Our concern here is mainly with layering of the first type. Magmatic flow results in contrasting layers and strong foliation accompanied by a linear orientation of elongated minerals. It is associated with slumping and cross-bedding in a zone near the margins of the intrusion. Rocks in the interior of the Layered Series are foliated and commonly layered but have little or no lineation; their layering cannot be due solely to deposition from currents. The principal structural elements produced by dynamic processes in the magmatic state-foliation, lineation, shear zones, folds, and current structures-define parallel belts, mainly around the western and northern margins. Their distribution suggests that the strongest component of flow was for the western side and that its strength decreased toward the east. Shear flow trajectories deduced from lineations and kinematic indicators confirm this asymmetry and point to mass movement toward the southeastern part of the floor.
: layering; magmatic flow; Kaergaard
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