Five lithological groups overlain by Recent deposits are recognised. Otaian (Oligocene) sandstones and mudstones with associated andesitic agglomerates overlie argillaceous limestones of probable Bortonian (Eocene) age. These strata, with gently undulating or horizontal attitudes lap on to highly-sheared, siliceous claystones resting on a basement of deformed Mesozoic greywackes and argillites which contain interbedded spilites, chert bodies and ores of manganese, iron and copper. The interrelations and origin of the spilitic lavas, chert bodies and copper are discussed. The basement rocks have attained a grade of regional metamorphism equivalent to the chlorite-l zone of the Otago Schists. The sequence is intruded and overlain locally by Pliocene Basalts.
Rona Island consists of Lewisian quartzofeldspathic gneisses and amphibolites that show the effects of polyphase deformation. The age of the gneisses, from a six point Rb-Sr whole rock isochron, with λ 87 Rb 1.39 × 10 −11 yr −1, is 2790 ± 210 m.y. This is not significantly different from the age of 2710 ± 20 m.y. determined from zircon fractions from a single block of the gneiss. These results are interpreted as reflecting amphibolite facies metamorphism and gneiss formation, associated with the first recognized fold phase, during the Scourian orogenic episode A Rb-Sr age for unaltered muscovite books from a pegmatite emplaced in the hinge zone of a fold of the third deformational phase is 1740 ± 10m.y. and a Rb-Sr whole rock isochron for a later granite sheet, emplaced between the fifth and sixth fold phases is 1680 ± 170 m.y. These igneous events, and the fold phases between them, belong to the later part of the Laxfordian orogenic episode. No isotopic disturbance of the Rb-Sr whole rock systems of the analysed gneisses has taken place during the Laxfordian episode and no isotopic events between the c 2700 m.y. and c 1700 m.y. episodes have been found in the quartzo, feldspathic rocks. K-Ar ages on the muscovite samples suggest minor opening of minerals to argon loss sometime after 1740 m.y.
In spite of the structural complexity of polyphase deformed gneisses and migmatites and the absence of distinctive marker horizons, these rocks are amenable to structural investigation and interpretation with resolution of their tectonic history possible, particularly in regions of reasonable exposure. Although methods such as those employing interpretation of stereo-plots can seldom be used, direct observation of refold relationships on the outcrop using all available structural, igneous and metamorphic features will ultimately lead to the resolution of the deformational history. This is achieved by determining the relationships of fold sets observed at the various localities examined throughout the field of investigation and then combining these by correlation according to basic stratigraphical principles. The effects of weak deformation normally not discernible in complicated successions may be detected by the simple form of folded planar and linear structures in igneous masses emplaced in complex structures at intermediate stages in the deformational history.Synthesis of the total succession and determination of the overall sequence of deformation in particularly complex terranes is made easier if at least one distinctive set of structures can be recognised throughout the area. These key structures have the effect of subdividing the long succession into two in which refold relationships are generally more easily determined. An example of the method is given using photographs from the migmatite terrane in southern Finland.
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.
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