Gesichtspunkte zur Stratigraphie des Archaikums im Grenzgebiet zwischen Västerbotten und Norrbotten, Nordschweden

Abstract

The paper is a reply to one of S. Gavelin (Geol. För. Förh. 80, 1958) in which he discusses Archean stratigraphy in Västerbotten and Norrbotten. Since publication both Gavelin and myself have carried out additional investigations.

New material is here presented in support of my previously published view (SGU Ser. C, 543, 1957) that the Revsund granite is younger than the Vargfors conglomerate. In addition, the conglomerates with granite pebbles at Duobblon and Ledfat are discussed on which A. Högbom (1937) and S. Gavelin (1955, 1958) based their earlier view that the Revsund granite was older than the Vargfors conglomerate.

S. Gavelin's microscopic examination of granite pebbles from Ledfat has shown that the petrography of these does not show any similarity with the Revsund granite. The coarse-grained granite of the conglomerate pebbles contains plagioclase but no microcline, while the coarse-grained Revsunds type is clearly a microcline granite. S. Gavelin describes his results on p. 742 in the present paper. From these results the main argument for the theory that the Revsund granite is earlier than the Vargfors conglomerate has disappeared.

The age relationships of the Revsund granite now become more uncertain. There is much to suggest that it is of Karelian age and thereby more closely related to the Sorsele, Arjeplog and Lina granites. This is supported by the results of age determinations (K/Ar method) carried out in recent years on Finnish rocks, as well as by the relationships of the Revsund granite to other rocks. In the present area the possibility of separating granites on the basis of absolute age methods carried out so far is questioned. The events of the Karelian orogeny have so strongly affected the area that even older rocks may assume “Karelian” values.

The supracrustal rocks of the Malanäs area correspond stratigraphically for the most part to the upper units of the Maurliden series (Petikträsk phyllites and Skogsheden volcanics) and the basal unit of the Elvaberg series (Mensträsk conglomerate). The Jörn granite in the Jörn massif does not plunge at a gentle angle concordantly beneath the rocks of the Maurliden series as previously stated. Instead it cuts off the Maurliden series succession and often has steep contacts.

The Elvaberg series along the Skellefte River and north of it lies strongly discordant on the Maurliden series. Southwards, away from the Arvidsjaur landmass, the angular discordance becomes less and gradually disappears altogether. Concordant relations then seem to prevail.

Coarsely porphyritic microcline-bearing granites of Revsund type in the central parts of the Jörn granite show transitions to even-grained granites which form the main part of the Jörn granite massif at Jörn. The coarsely porphyritic microcline-bearing granites there clearly belong to the Jörn granite suite.

At åselet north of Jörn, granite pebbles were found in the conglomerate which lies intraformationally in the widely distributed Elvaberg schists east of the Jörn granite massif. The matrix of the pebbles is schistose. The granite types of the pebbles lie within the Jörn granite's limits of variation. Massifs of Jörn granite are situated only a few kilometres west of the conglomerate localities. This is a direct proof that the Elvaberg schists are considerably younger than the Jörn granite and the porphyries of the Maurliden series which are intruded by this granite.