Geology of the Øyungen intrusion and surrounding rocks: constraining the setting of emplacement

Detailed mapping of a syn-orogenic mafic intrusion and the surrounding rocks provides supporting evidence for a recent re-interpretation of regional stratigraphy (Sturt et al. 1991). The Øyungen intrusion is a monotonous sheeted body of uralitised gabbro sitting in the Åsli formation of the Gula Group (Nilsen & Wolff 1989). The new tectono-stratigraphy places a first- order unconformity beneath the Åsli formation (Bjerkgård & Bjørlykke 1994), separating rocks which experienced an Ordovocoan orogenic event (Heidal series) from Ordovician sediments (Sel series). This unconformity has been identified in the Øyungen area, lying to the west of the intrusion. The boundary between the Åsli formation and Fundsjø group in this area is not a thrust (c.f.Nilsen and Wolff 1989) but is primary (as in Bjerkgård & Bjørlykke 1994). The Øyungen intrusion lies within metasediments of the Sel series and has sheeted contact and contains many sheeted xenoliths, all parallel to structures in the country rocks. Evidence from contact metamorphic porphyro- blasts and xenoliths suggest the Sel rocks were undeformed sediments prior to emplacement; bedding is preserved in many places. Weak fabrics in xenoliths and contact rocks suggest deformation due to flattening during emplacement. The main foliation in the metasediments is parallel to bedding and wraps contact metamorphic minerals. Locally this fabric appears to have been active at higher temperatures, that is, overlapping with the presence of a thermal aureole near the gabbro. Later folding is seen in the metasediments but not in the gabbro. Variation in the orientation of the gabbro contact is believed to be primary and related to the sheeting mechanism of emplacement. A wide contact metamorphic aureole is recognized: granulitic assemblages (cordierite + orthopyroxene) are seen in a narrow contact zone and in xenoliths. Migmatic rocks and ar

Download publication

Available:
NGU-biblioteket
Series:
NGU-rapport
Series nr:
96.113
Page number:
35
ISSN:
0800-3416
Document type:
Rapport