Structural, metamorphic and igneous history of migmatites in the deep levels of a wrench fault regime, Savonranta, eastern Finland

Abstract

The migmatite complex of the Savonranta district of eastern Finland represents the combined effects of polyphase deformation, polymetamorphism and the emplacement of a series of dioritic, grandioritic, granitic and pegmatitic masses. The most significant feature in terms of both regional structure and control of neosome emplacement is a large wrench fault zone along which there has been a minimum of 30 km displacement. Its development was controlled by a stress system related to plate movement in the early-middle Proterozoic Svecokarelian orogenic belt.Successive neosome development has been related to the extensive structural and metamorphic history of the amphibolite facies palaeosome mica schists which has been deduced from refolding and cross-cutting relationships. Rb–Sr whole-rock isotopic studies of a porphyritic granite, whose emplacement was controlled by the wrench fault regime, indicate an age of crystallisation of 1786 ± 80 m.y. The initial 87Sr/86Sr ratio of 0·7053 ± 01 shows the source region for the granite magma had a previous crustal history.

Metamorphic development of cordierite-bearing layered schist and mica schist in the vicinity of Savonranta, eastern Finland

“…18). They show the effects of a polydeformational and polymetamorphic history developed in conjunction with the repeated injection of quartzofeldspathic and granitic neosome material (Halden 1982 a). Individual features of the deformational sequence displayed in the mica schist palaeosome include two early phases of isoclinal fold development (F, and F 2 ), followed by a phase of wrench fault movement associated with the development of small asymmetrical folds (F 2C ).…”

“…1;Bowes 1976;Koistinen 1981), and used by Halden (1982 a) in describing the inter-relationship between folds (FL .... F laxe ), and their associated axial planar fabrics (S, .... S LAXE ) developed in the mica schists in the vicinity of Savonranta. Pre-D[ refers to a deformational phase that preceded the D, deformational phase; this includes major thrust movements that resulted in the tectonic interdigitation of rocks from different crustal levels and different crustal segments (c.f; Koistinen 1981;Park and Bowes 1983).…”

“…A NW-SE-trending ductile shear zone along which there has been dextral displacement bisects the Savonranta district (Gaal 1972;Halden 1982 a). To the SW of this zone, areas of dilatency have acted as the sites of emplacement for a porphyritic granite, dated at c. 1780 m.y.…”

“…Campbell et al 1979;Koistinen 1981;Treloar et al 1981;Halden 1982 a;Park 1983 a, b) means that the concept of a general increase in metamorphic grade from E to W in central Finland (Huhma 1975) needs amplification and integration with studies of mineral reactions and successive mineral growths throughout the region. This can be achieved by linking changes in P-T conditions with changes in structural features from district to district and so establishing phase by phase regional variations through the polymetamorphic sequence on the basis of correlation of coevally developed minerals.…”

“…Evidence relating to the structural history and times of igneous emplacement are given in Halden (1982 a). Assessment of metamorphic conditions involved the collection of 300 specimens, most oriented, and the particular detailed study of 60 thin sections, selected from the collected suite as being most relevant and amenable to the style of approach; they provided the critical evidence for microfabric and microprobe studies.…”