Scotland–Finland Precambrian correlations

Abstract

Isotopic age determinations indicate the generally co-eval nature of major episodes of earth history shown by the Precambrian rocks of Scotland and Finland, viz. (1) the c. 2 800-2 600 m.y. Scourian and Presvecokarelidic episodes, (2) the c. 2 200 m.y. Inverian episode and the c. 2 300 m.y. episode in Finland, (3) the c. 2.1 b.y. deposition of the Loch Maree Group in Scotland and the Jatulian quartzite, Karelian schists and related geosynclinal deposits in Finland, and (4) the c. 1 975-1 700 m.y. Laxfordian and Svecokarelian episodes. The existence of spatially connected belts in Precambrian times is suggested by the correspondence in both the sequence of development and the orientations of structural elements resulting from polyphase deformation in the successively formed Precambrian orogenic belts. This matching of complex structural frameworks implies that, before the initiation of the British-Scandinavian Caledonides, the Baltic Shield extended westward to include northwestern Britain and the continental shelf of the eastern North Atlantic Ocean. The Caledonian belt would then have developed on a plate of continental crust, or between the separated parts of a rifted continental crustal segment.

Rb–Sr muscovite age of a pegmatite near Sivakkavaara, Finland

“…However, the post F 3 pegmatites at Sivakkavaara are considerably older than pegmatites emplaced 1720 ± 10 m.y. ago into a corresponding position in the structural sequence of the Laxfordian orogenic episode of Scotland (Bowes 1975;Lyon et al 1973; age reduced by one per cent, see van Breemen et al 1974). Whether this age difference is evidence of diachronism or separate development must await further study, as must detailed knowledge of the ages through the complete deformational sequence of the Svecokarelian episode in Finland.…”

“…In places it has been possible to relate the successive emplacement of igneous masses to the deformational sequence. This, together with the dating of these masses, raises the possibility of determining whether apparently corresponding deformational phases, correlated on structural grounds (Hopgood and Bowes, 1972;Bowes, 1975) from different regions, were co-evel or whether the deformational history showed marked diachronism. In addition to elucidating crustal history, such information may provide a basis for correlating across major tectonic or lithological boundaries, or younger orogenic belts such as the Caledonides.…”

Structural evolution of an early Proterozoic strata-bound Cu-Co-Zn deposit, Outokumpu, Finland

“…In other zones they have two major axial trends (NNE and NW), are apparently conjugate, and are deformed by structures of the third deformational phase. They are of rare occurrence in those regions where the deformational sequence in this part of the Svecokarelides was first established (Gillen & MacDonald 1970;Gaal et al 1975;Bowes 1975Bowes , 1976a or are only represented there locally by a few small folds or by a very weakly expressed cleavage or a crenulation. As (1) they were not recognised until after the regionally expressed F 1; F 2 , F 3 , F 4 etc.…”

“…Post-D 5 deformation includes very open, upright folds and kink-bands (F 6 ) with axial planar fractures (S 6 ) and near N-S axial trend (Bowes 1975(Bowes , 1976avan Breemen & Bowes 1977). In addition vertical or near-vertical healed fractures with N-S traces which have been observed at many outcrops are considered to be expressions of the D 6 deformational phase.…”

Structural development of migmatites near Skåldö, southwest Finland

“…al. 1975;Bowes 1975; in press). However, from the detailed illustrations and descriptions of Sederholm (1967) it is evident that the migmatites also have undergone popyphase deformation and multiple igneous injection.…”