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.
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
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