Studies in the structure and metamorphic petrology of the Eo-Cambrian rocks of Eastern Seiland, North Norway

Abstract

The rocks exposed on the eastern part of the island of Seiland are the metasedimentary envelope to the large basic and ultrabasic plutons that make up the western part of the island and the neighbouring island of Stjernoy. These metasediments have been intensely deformed and metamorphosed, thus all sedimentary structures have been obliterated. The metasedimentary sequence is, therefore, a structural one. The lowest group is the psammitic Komagnes Group. This is followed by the Eidvageid Schist Group which is followed by the relatively thin psammitic Trollvann Group. Structurally above this is the pelitic Olderbugten Group and finally the dominantly psammitic Olderfjord Group. Broadly speaking two major fold-forming deformations have been recognised; F.1 and F.2. The latter part of the first phase, which was responsible for tight isoclinal folds and a penetrative schistosity, was accompanied by intrusion of sheets of basic material parallel to the axial planes of the early folds. During the static interval separating the two deformations the maximum grade of metamorphism was achieved, this, however, varies across the area and a sequence of metamorphic isograds have been recognised. The highest grade occurs in the west and is marked by a kyanite-sillimanite porphyroblastesis, migmatisation and intrusion of adamellitic sheets. The lowest grade in the east is characterised by albite, biotite, epidote, hornblende assemblages in the psammites of the Lower Komagnes Group. These isograds reflect a contemporaneous lateral change in metamorphic grade. Following the development of these high grade assemblages in the west, there was a phase of intense flattening leading to the development of mylonitic textures in the rocks. It is suggested that this deformation phase is related to a rising basic asthenolith. The majority of folds on the area are attributed to F.2.By the onset of F.2. the metamorphic grade appears to have waned to sub-garnet grade conditions. The folds have a very variable style. In the east, they have intensely attenuated long-limbs with a number of vertically-stacked folds in the short-limbs. In the west, the limbs are of more equal length. This change in style is related to the different states of competence of the rocks at the onset of F.2. A ubiquitous feature of F.2. folds on both the major and minor scale, is the curvature of their axial-lines. In the east a number of oblique boudins have been recorded which post-date F.2. They are closely associated with rotated tension-gashes and monoclinal folds. It is suggested that all these structures were formed in response to a progressive deformation sequence. This sequence was also responsible for the development of the late Caledonian thrusts on the mainland. In the north of the area there is a pronounced swing in strike. A number of open folds are associated with this swing. These folds have been designated F.3. Monoclinal warps have been recorded in the east of the area. Their temporal relationship to the F.3. folds in the north is not known. The final phase of movement in the area led to the development of joints, faults and locally kink-folds. The prevailing metamorphic grade during this phase of deformation, and indeed all the phases subsequent to F.2., was the Quartz-Albite-Muscovite-Chlorite Sub-facies of the Greenschist Facies.