Structural development of migmatites near Skåldö, southwest Finland

Abstract

The migmatites of the Skåldö area show the effects of an extensive polyphase deformational history at relatively deep crustal levels during the Svecokarelian episode. Despite the complexity of the fold patterns it is possible to establish a deformational sequence into which the many phases of igneous emplacement in the rocks can be placed. There are at least seven successive fold sets and a number of phases of both basic and acidic igneous activity as well as the effects of metamorphism which reached a peak, under amphibolite facies conditions, early in the deformational history. Taken together the various features comprise a complex framework which can be used as a basis for comparison and correlation with the products of the Svecokarelian orogenic episode in other parts of the Baltic Shield. The characteristics of these migmatites in this classic area, viz. their »mixed» aspect as described by J. J. Sederholm, is shown to result from their extensive deformational, metamorphic and igneous history in which tectonic and igneous activity played prominent roles.

Strongly Peraluminous Granites across the Archean–Proterozoic Transition

“…However, in southernmost Finland a later generation of peraluminous granitic melts intruded between 1Á84 and 1Á81 Ga, forming a 500 km long and 100 km wide belt of intrusions (Huhma, 1986;Vaasjoki & Sakko, 1988;Suominen, 1991;Ehlers et al, 1993;Vä isä nen et al, 2000). These granites are associated with granulite metamorphism and extensive migmatization (Sederholm, 1926;Hopgood et al, 1976;Edelman & Jaanus-Jä rkkä lä , 1983;Korsman et al, 1984) in what is known as the late Svecofennian granite-migmatite (LSGM) zone. The country rocks include pelitic and psammitic garnet-cordierite 6 spinel gneisses with intercalations of volcanic rocks and tonalitic sheets (Vä isä nen & Hö lttä , 1999).…”

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

“…The growth of the continental crust began in the eastern part of the Baltic Shield in eastern and northern Finland, in Soviet Karelia and on the Kola peninsula, where evidence of a late Archean cycle (2500--2800 Ma) with greenstone belts and granitoids is widespread (BowEs, 1976;LOBACH-ZHUCHENKO et al, 1976;BLAIS et al, 1978;GALL et el., 1978;SILVENNOINEN et al, 1980, andSIMO-NEN', 1980). There is also some indication of an early Archean cycle older than 2800 Ma forming, at least partly, an ensialic basement for the greenstone belts (CA.~L et al, 1978, andKRONER et al, 1981).…”

“…Modern structural studies in the Baltic Shield have demonstrated that the Svecokarelan orogeny evolved in several subsequent deformational phases (GAKL & ]=~AUHAMAKI, 1971;GAs 1972;GAAL et al, 1975;BOWES, 1976;HOPGOOD et al, 1976;STKLHSS, 1976;EHLERS, 1978;CAMPBELL, 1980, andVERFIOEF &DIETVORST, 1980). Each major deformation phase produced, under certain metamorphic conditions, its own fold set and fracture system in connection with plutonic rock associations.…”

Deformational behaviour of migmatites and problems of structural analysis in migmatite terrains

“…Migmatites formed by metamorphic or metasomatic processes may respond very differently from those formed by anatexis or intrusion. Campbell (1980) and Hopgood (1980;Hopgood, Bowes & Addison, 1976, and references therein) have recently attempted to relate the development of anatectic and intrusive migmatites to structural events in complexly deformed terrains. In failing to consider the mechanical implications of partial melting doubt is cast on their structural analysis.…”