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Mafic dyke cutting a pinch and swell structured, D b boudinaged, pegmatitic granite dyke (4 in Table 1). The cutting relationships indicate cooling after D b. Photo by T. Koistinen.
Source publication
Pajunen, M., Hopgood, A., Huhma, H. & Koistinen, T. 2008. Integrated structural succession and age constraints on a Svecofennian key outcrop in Västerviken, southern Finland. Geological Survey of Finland, Special Paper 47, 161-184, 21 figures and 2 tables. The Västerviken outcrop in Karjalohja, southwestern Finland, represents an exam- ple of polyp...
Contexts in source publication
Context 1
... felsic gneiss is cut by intermediate to mafic dykes (5a in Table 1) up to 0.5 m wide (Figure 7) that are strongly deformed and altered by later events. Isoclinally folded felsic segregation veins (6 in Table 1) indicate intense shortening in the dykes (Figure 8). ...
Context 2
... in the least deformed parts near fold hinges, a vague ophitic structure can be identi- fied; one dyke shows plagioclase-porphyritic struc- ture (5b in Table 1). The dyke cuts sharply across the felsic gneiss and the early phase of felsic neosome and pegmatitic veining ( Figure 7); thus it post-dates the early migmatization pulses and boudinage of the felsic segregation veins. In the better-preserved parts the dyke is composed of hornblende, plagio- clase and quartz with minor opaque minerals. ...
Context 3
... later generation of more coarse-grained vein- ing (4 in Table 1) is also isoclinally folded and changes from trondhjemitic to granitic in composi- tion (Figure 7 and 10). No garnet was found in the veins. ...
Context 4
... early veins are isoclinally folded and attenuat- ed indicating intense flattening strain (Figure 9). The later, more coarse-grained granite veins (4 in Table 1) have spaced S b foliation lying parallel to the vein con- tacts ( Figure 10) and were deformed into boudins and pinch and swell structures (Figure 7). Garnet rims (II in Figures 6a and 6b) overgrow S b that is preserved as some M b biotite relics (Figure 6c), but S b is curved to surround the garnet core (I in Figure 6b). ...
Context 5
... e Axial plane crenulation cleavage -S d /S e . Some biotite growth into S e axial plane ( Figure 17). L e Intersection lineation between S d and S f parallel to hinges ( Figure 18). ...
Context 6
... E c mafic dykes (5a in Table 1) cut the foliation, trondhjemitic veining (3 in Table 1) and also split the pinch and swell structure of the early pegmatite dykes (4 in Table 1). Some faulting occurred during the dyke emplacement because of non-continuation of the pinch and swell structure on the opposite side of dyke ( Figure 7). Their sharp and originally planar contacts suggest intrusion into cooled, rigid crust indicating a stabilizing and cooling stage after the E b event. ...
Context 7
... spaced foliation ( Figure 15) in the ma- fic dykes is F e -folded ( Figure 15 and 16); under the microscope some remnants of small tight angular F d folds are crenulated by D e . Locally the hornblende- plagioclase assemblage of the dyke was altered to almost monomineralic coarser-grained (3 mm) bio- tite ( Figure 17). This type of alteration implies meta- somatic potassium enrichment during the develop- ment of the foliation. ...
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Citations
The present-day constitution of the c. 1.9-1.8 Ga old Svecofennian crust in southern Finland shows the result of a complex collision of arc terranes against the Archaean continent. The purpose of this study is to describe structural, metamorphic and magmatic succession in the southernmost part of the Svecofennian domain in Finland, in the Helsinki area. Correlations are also carried out with the Pori reference area in southwestern Finland. A new tectonic model on the evolution of the southern Svecofennian domain is presented. The interpretation is based on field analyses of structural successions and magmatic and metamorphic events, and on interpretation of the kinematics of the structures. The structural successions are linked to new TIMS and SIMS analyses performed on eight samples and to published age data. The Svecofennian structural succession in the study area is divided into deformation phases DA-D1; the post-Svecofennian events are combined as a Dp phase. Kinematics in the progressively-deforming transtensional or transpressional belts complicates analysis of the overprinting relations due to their simultaneous contractional and extensional structures. In this work the Svecofennian structural succession in the study area is related to major geotectonic Events 1-5, E1-E5, on the basis of their characteristic kinematics and tectono-metamorphic and magmatic features. Geotectonic event E1 includ es the DA deformation (corresponding to the D1 in the northern Svecofennian units) that deformed the oldest supracrustal sequences, including volcanic Series I. The horizontal DA structures formed during c. N-S shortening. Event 1 is suggested as an early major thrusting deformation at c. 1.9-1.88 Ga ago. The crust thickened and crustal slices from different depths, and geotectonic and depositional environments, were juxtaposed. Geotectonic event E2 inclu des the DB deformation (D2 in the north). It is related to a crustal extension that caused a strong vertical shortening and prograde low-pressure (andalusite-sillimanite type) metamorphism. Extension is related to an island arc collapse; this collapse did not occur in the Archaean domain. The younger volcanic Series II evolved during the collapse and is proposed to represent intra-arc volcanism. Large amounts of syn-/late DB intermediate igneous rocks were also emplaced. E2 shows diachronic evolution, getting younger towards south; the age of E2 in the south is c. 1.88-1.87 Ga. The E2 extension was rapidly followed by a N-S shortening event, E3, that formed the contractional DC structures (D3 in the north) in the south at c. 1.87 Ga; E3 was also a diachronic event. According to the structure and age correlations, the northern Svecofennian units already underwent E3 contraction during the southern E2 extensional stage. This supposes that the crust as a whole was under a compressional regime and that E2 was a southwards-migrating localized event. In the Archaean continent and close to its border zone, the escape of crustal blocks towards the NNW-NNE occurred; movements occurred along the old Archaean or pre-collisional Proterozoic N-S-trending shear zones. After E3 the Svecofennian and Archaean domains formed a coherent continent and, therefore, the later geotectonic events occurred in this new continental crust. Geotectonic event E4 deformed the new continental crust and is in the south predominantly connected to the evolution of the Southern Finland Granitoid Zone (SFGZ). The E3 N-S stress field rotated into a SW-NE orientation. The pre-SFGZ crust in the south is characterized by the DC+D dome-and-basin interferences formed at c. 1.87 Ga due to the SW-NE transpressional deformation DD (D4 in the north). At that time, in the northern units, the wide-scale oblique contractional folding, eastwards transport and clockwise rotation of crustal blocks occurred. The deformation pattern of crustal blocks, like the Pomarkku and Vaasa blocks during D4 (DD in the south) and the Central Finland block during DF at c. 1.86-1.85 Ga, as well as related shear zone patterns, are interpreted as a product of escape tectonics related to the SW-NE transpression. Simultaneously with the termination of the D4 deformation in the north at 1868 ± 3 Ma, evolution of the Southern Finland Granitoid Zone (SFGZ) was initiated by the early DE tonalites in the south. The SFGZ shows a very complex pattern of DE, DF, DG and DH structures that formed in kinematics representing different local stress fields. Intense intermediate to felsic DE magmatism occurred in the oblique SE-extensional zones between the active N-trending deformation zones, the Baltic Sea-Bothnian Bay Zone (BBZ), Riga Bay Karelia Zone (RKZ) and the Transscandinavian Igneous Belt (TIB). In the erosion level, progressive extensional evolution led to the development of the pull-part basins with supracrustal sequences exemplified by the Jokela supracrustal association. The peak of DE extensional evolution is represented by gabbro intrusions at c. 1.84 Ga; this mafic and intense intermediate magmatism may be related to the volcanism in the pull-apart basins. The Porkkala-Mäntsälä shear/fault zone and the wide-scale FG folding are results of a short-lasting N-S shortening DG at c. 1.85 Ga. In southern Finland the late E4 evolution, from c. 1.84 to 1.80 Ga, is characterized by SENW transpression during DH. Sinistral movements in the N-trending shear zones, like the Vuosaari-Korso shear/fault zone, oblique dextral movements in the E-W-trending Hyvinkää and Southern Finland shear/fault zones, oblique contractional DH folds and the oblique extensional or spot-like granites and diabase dykes resulted. The localized DE SE-NW extension in the SFGZ turned to SE-NW transpression due to rotation of the zone under an overall SW-NE transpression between the major N-trending deformation zones, the Baltic Sea-Bothnian Bay Zone (BBZ), Riga Bay-Karelia Zone (RKZ) and Transscandinavian Igneous Belt (TIB). The ev olution of the major N-trending D1 deformation zones, the BBZ, RKZ and TIB, already started during the DD deformation, but their latest phase, related here to the E5, deformed the Southern Finland Granitoid Zone (SFGZ) and the E-W-trending shear zones. The DG+H+I dome-and-basin interferences determining the present exposition of the geotectonic and metamorphic units were formed. The youngest dated Svecofennian rock in the study area is a <1.8 Ga old amphibolite facies diabase dyke, which confines the fade of the Svecofennian orogeny in southernmost Finland. The post-Svecofennian deformations and magmatic e vents, included here as a combined DP, are strongly controlled by the older Svecofennian structures. In the study area the c. 1.65 Ga Bodom and Obbnäs rapakivi granite magmas were channelled by the intersections of the major Svecofennian crustal shear structures, like the southern Finland, Porkkala-Mäntsälä and the DI shear zones/faults, and the low-angle structures like the SE/H foliation planes controlled their emplacement. On a wider scale, rapakivis are located in close association with the major Svecofennian DI deformation zones such as the Baltic Sea-Bothnian Bay Zone (BBZ) and Riga Bay-Karelia Zone (RKZ). Tectonic and kinematic analysis combined with the tectonically-fixed ag e data on magmatic events establishes that the Svecofennian orogeny was a continuous diachronic event getting younger towards the south/southwest. The Svecofennian island arc system collided obliquely against the Archaean continent simultaneously with the growth of an island arc. The Svecofennian arc components were accreted to the Archaean continent to form a new continental crust. The continent was deformed during the continued transpressional orogen into major crustal blocks that transported and rotated with simultaneously formed extensional zones - e.g. the pull-apart basins and granitoid zones - and strike slip terranes. The tectonic pattern is characteristic for terranes formed by escape tectonics. In summary, the tectonic evolution of the Svecofennia n orogeny is related to one oblique collision of the active, growing Svecofennian island arc system against the Archaean continent, and to subsequent transpressional continental deformation characterized by escape tectonics.
A study has been made to test the likelihood that migmatites cropping out in the Finnish Archipelago have undergone a different, similar, or identical, tectonic history to those in small, poorly exposed inland areas of the forested region further east in the vicinity of Orijärvi. The study is based on comparison of structural associations and has used the detailed small-scale structural succession already determined from the exceptionally well-exposed Proterozoic Svecofennian migmatites on outcrops in the Jussarö-Skåldö area of the Finnish Archipelago. Preliminary results show strong similarities between the Archipelago migmatite successions and those at Orijärvi, suggesting that they shared at least part of a common tectonic history. This indicates that further detailed structural examination would be rewarding and should confirm, or otherwise, that the regions shared a comparable tectonic history.
The isotope laboratory at GTK has contributed enormously to geological research in Finland ever since its inception in the early sixties. The main analytical methods used have been U-Pb, Sm-Nd, Rb-Sr, Pb-Pb and light stable isotopes, and with the recent LA-MC-ICPMS instrument installation, the repertoire is increasing. Isotope research has contributed to many joint projects over a wide range of topics including modelling, mapping, mineral exploration, investigations related to nuclear waste disposal, hydro-geology and GTK consortium mapping projects abroad. Co-operation with universities has been important and isotope geology has had a role in numerous Ph.D. theses in Finland. Building a full picture of the evolution of a piece of the Earth's crust requires a large amount of radiometric age and isotopic data that can only be supplied by a premiere isotope facility. Examples of the types of information the GTK isotope laboratory has produced include: 1. The oldest rocks so far discovered in Fennoscandia are the 3500 million years old Siurua gneisses, but signs of even older crust are evident in these rocks. 2. The main periods of crustal growth in Finland were related to collisional events at 2.8–2.7 Ga and ca. 1.9 Ga. Yet sediments produced over a wide region dur-ing the 1.9 Ga event contain abundant ca. 2.0 Ga zircons, for which there is no obvi-ous source, suggesting that a major block of still unlocated crust must have existed somewhere nearby and supplied abundant detritus to proximal ocean basins ca. 1.9 billion years ago. 3. Before the breakup of the ancient Archean continental core, sev-eral pulses of mafic magmatism have been recognized between 2.44 Ga and 2 Ga, and these intrusions have proven to be particularly important as they contain some of the major ore bodies in Finland. These and other important results are briefly described in this paper to illustrate the importance of isotope geology in deciphering the geological history of the Fennoscandian Shield, and Finland in particular.
