Geological investigations in the Bamble sector of the Fennoscandian shield, S. Norway, no. II. Metasediments and metapyroclastics (?) within the Precambrian metamorphic suite of the S. Norwegian Skaergaard

Metasediments and possible metapyroclastic breccias exhibiting well-preserved primary features, despite metamorphism of upper amphibolite facies grade and some degree of metamorphic differentiation, are described from the Precambrian Bamble series exposed in the skaergaard of S. Norway. A unique sequence of 2000+ ft. of metapyroclastic (?) breccias intruded by orthoamphibolites is described from Rösholmen. These meta-breccias, of trondhjemitic affinity, are interbedded with and grade along strike into a metasedimentary series of sillimanite quartzites and quartz-sillimanite-muscovite schists. The island of Arö and its neighbouring islets contain a metamorphosed, sillimanite-rich sequence of interbedded quartz arenites, quartzpebble orthoconglomerates and quartz-pebble paraconglomerates. The sillimanite- bearing, quartzose meta-arenites are characterized by cross-stratification and a few primary, recumbent, sedimentary fold structures. The pelitic intercalations of sillimanite-biotite schist in the sequence are B-tectonites, devoid of any palimpsest sedimentary structures. The origin of nodular, sillimanite-bearing metamorphic rocks in the Bamble series is reviewed in the light of the almost ubiquitous quartz-sillimanite segregations in the Arö metasediments. A brief review of local tectonics is presented and a tentative palaeocurrent azimuth, derived by unrolling the fold-structures is recorded. The local palaeocurrents depositing the Bamble metasediments appear to have flowed from the NW. Recent geochronological data suggest that most of the metasediments in the Bamble district were derived by erosion of a metamorphic- igneous suite which crystallized around 1450 m.y. ago.