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3. The Geology of the Västervik Area

3.2 Structural Development and Metamorphism

The oldest rocks within the Västervik Area are the metasediments with a maximum age of about 2.12 - 1.87 Ga, given by the youngest population of detrial zircons and a minimum age of about 1.91 - 1.85 Ga, given by the oldest granite intrusions (see above). Only a narrow time frame can be assumed for deposition of these sediments. They were deposited in a large shallow basin over an unknown basement and are intruded by the Loftahammar granites in the north and northeast, the Småland granites in the south and southwest and several generations of mafic rocks. During the development of the TIB the sedimentary rocks have experienced at least two phases of folding (F1: NW-SE and F2: NE-SW) which resulted in a complex inter-ference fold pattern. The major striking trend of NW-SE, which can be traced throughout the whole Västervik area, is assigned to the second phase of deformation (Westra et al. 1969).

Field observations show that folding mainly took place before the intrusion of the granitoids, as fold structures are cut by intrusives and folded xenoliths can be found in the granitoids. Also, related ductile shear zones are in parts overprinted by contact metamorphism in the vicinity of these granitoids (e.g. Farrenkopf 2000). Moreover, it is assumed that locally, rotation of fold structures happened through forceful intrusions of granitoids. Evidence of intensive post-granitic tectonism can also be observed within the Västervik area. Conspicuous off-sets in granitoids like the Götemar massif (see above) testify that some brittle and ductile shear zones have been developed after 1.4 Ga.


Fig 3.2  

The rocks in the Västervik area have experienced both regional and contact metamorphism associated with granitoid intrusions. Both are classified as low-pressure-high-temperature-type (greenshist to amphibolite facies) metamorphism (cf. fig 3.2 & 3.3) which proceeded statically in many regions. Thus, in these regions sedimentary structures are well preserved, and hence, can be used to reconstruct fold structures upon identification of hanging walls and footwalls (e.g. Gavelin 1984), in connection with primary magmatic structures within the metabasite flows of the Västervik Formation.

The flecky gneisses in the southeastern part of Skälö demonstrate an example of synkinematic metamorphism. Elongation of the flecks in NW-SE direction indicates that they where formed in the F2 stress-field (Westra et al. 1969). The flecky gneisses contain high-grade metamorphic minerals such as sillimanite, andalusite and additional cordierite in the Mg-rich layers (Farrenkopf 2000). Based on detailed microfabric studies Farrenkopf (2000) concluded, that the flecky gneisses have partly been involved in a ductile mylonitic shear zone close to the peak temperature of metamorphism (650°C), followed by a retrograde overprinting of minerals and texture under K-rich fluids and static conditions.


Fig 3.3  

Sillimanite is also found in several migmatites which lead to the suggestion that the peak of metamorphism and migmatization occurred concurrently (Westra et al. 1969). In many regions, depending on substantial constitutions, granitic heating reached anatectic overprinting of the metasediments with suitable compositions and led to the formation of migmatites. Partly they contain relicts of former contact metamorphism, e.g. the xenoliths of flecky gneisses in the migmatites at Blankaholm. Migmatites show a wide spectra of fabrics, depending on their primary composition and layering, the degree of anatectic melt formation and the intensity of synmigmatic deformation, So-called injection migmatites represent a special kind of migmatites (e.g. Szagun 1997), also known as veined gneisses. They form lit-par-lit intrusions of granitic melts into layered epiclastic sediments. It is supposed that they originate either from granitic magma (artenites) or mobilized material from pre-existing rocks (venites). In the case of artenites steeply inclined metasediments, as observed in large parts of the Västervik area, seem to be favored for such intrusions. In today's surface level the proportion of migmatites increases northwards. This would consequently mean that, regarding the granite genesis and level of intrusion respectively, we would have exposed a deeper crustal level in the north as compared to the south (Hansen et al. 1998).

Many granitoids show different macrofabrics of mingling with mafic injections. During injection in an early state of crystallization of the granitic magma the mingling of both magmas is more homogeneous producing isolated mafic enclaves of different sizes. At a higher state of crystallization of the granitic magma mafic dikes are formed which are disrupted again by the still mobile granitic magma. If mafic magma intrudes into completely crystallized granitoids common mafic dikes are developed with a sharp contact to the wall rocks. All forms of magma mingling can be observed within the Västervik area, they are, however, in parts restricted to certain generations of granitoids. Another phenomenon within the Småland granites of the Västervik area is characterized as a heterogeneous mixing of an alkali feldspar granite and a granodiorite, best visible along the road between Västrum and Skaftet. Within the granodiorite biotite often accumulates in irregular alignment causing the look of flow fabric.

Authors often distinguished between older and younger granitoids by means of the presence of a foliation. The Loftahammar granitoids are characterized as pre- to synkinematic and their conspicuous schistosity is ascribed to their involvement in deformation. The Småland granitoids in contrast were described as only very weak schistose, since they mostly were not affected by deformation. The tendency of foliation may also be regarded as flow structures caused by compositional differences. This can be observed within the biotite-rich border areas of the Småland granites, where schistosity is more pronounced. Thus it may be doubtful to use foliation as a criteria for relative age relations among the granites (Hansen et al. 1998). A safer method for identifying their age relationships should be based on indicators like the sequences of intrusion. For example, the Loftahammar granites are intruded by the mafic dikes which in turn are partly intruded by the Småland granites (Kresten 1972).

The most prominent fault zone in the Västervik area is the approximately 10 km wide Loftahammar-Linköping Deformation Zone (LLDZ). It separates the TIB in the south from the Svecofenian Domain in the north and strikes nearly NW-SE with a dextral sense of shear. According to Beunk & Page (2001), this brittle-ductile shear zone was active around 1.8 - 1.78 Ga and affected the surrounding rocks in 10-15 km wide border zones. Many of the granites within this border zone have developed porphyroblastic growth of, mainly, K-feldspar phenocrysts, e.g. at Hallmare, or have turned into mylonites, e.g. at Bjursund's Campsite.

Hydrothermal superimposition has occurred within restricted areas and in different variations. Fe-Oxide impregnation on grain boundaries and microcracks is responsible for the intense red colors in several granitoids. Quartz-plagioclase veins north to northeast of Västervik show macro- and microfabrics of soda metasomatism. In comparison to the host rock, plagioclase is more abundant in the veins which leads to the conclusion that K and Fe were leached out and substituted by Na and Ca (Elbers & Hoeve 1971, Hoeve 1978). Pegmatites and aplites are found frequently within the Västervik area. Often tourmaline accumulates in pegmatites. There seem to exist two varieties of pegmatites, those with and those without tourmalines (A. Vollbrecht, pers. comm.). Pegmatites and aplites are thought to represent residual melt of magma. Pegmatoid liquids are often rich in water and other volatiles. These volatiles cause a delayed cooling of the magma, which allows the growing of large crystals. In the Västervik area the pegmatite and aplite veins occur in all rock types but mainly in granitoids. They often do not show a definite strike direction but pegmatites seem to have a slightly preferred NW-SE to E-W direction, whereas aplites vary from NW-SE to NE-SW.

Sedimentary dikes within granites indicate a late phase of extension while the granitoids were covered with soft sediments, probably during the early Paleozoic. Some of these veins display a distinct zonation giving evidence of a multiphase development. The sedimentary dikes in the Västervik area show a trend of N-S and NE-SW.

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