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3. The Geology of the Västervik Area

Geologically the Västervik Area is located within the southeastern part of the Transscandinavian Igneous Belt (TIB) adjacent to the Svecofennian Domain to the North. This area mainly consists of early Proterozoic (2.3 - 1.7 Ga) metasediments and metavolcanites, which are described as the Västervik formation by some authors like Gavelin (1984) or Kresten (1986). Several granitoids and basic dikes intruded this formation at different ages causing contact metamorphism or migmatization of varying intensities (Kresten 1972).

3.1 Lithological Units
3.1.1 Granitoids

Granites and Granodiorites

The granites of the Västervik area can be divided into older and younger granites. The older granites are the so-called Loftahammar granites and the younger ones are called Småland granites. Both granitoids frame the metasediments or appear as intrusive bodies within these areas. The Loftahammar granites have their major extension to the northeast of Västervik (cf. fig. 3.1). In relation to the Svecofennian orogeny the Loftahammar granites, aged 1.91 - 1.85 Ga, are classified as pre- to synkinematic (primorogenic). The older granitoids typically are coarse-grained, porphyritic rocks with conspicuous schistosity, visible through a preferred orientation of mica. These granitoids show I-type character (Kresten 1986) and contain xenoliths of metasediments. They are frequently cut by several generations of metabasite dikes (cf. Metabasites and Metamorphic Evolution). Within the Loftahammar massif the granites are gneissic to blastomylonitic with a granitic composition. Northwest and southeast to southwest of Västervik the granitoids are commonly developed as granodiorites (cf. fig. 3.1 and loc. 9). The granodiorites typically are gray to reddish gray, medium and even-grained or slightly porphyritic rocks. Xenoliths of metasediments and metabasites occur more often than in other intrusions. Continuous transitions between granites and granodiorites appear to exist, or the granodiorite is intruded by the older granite which would allow the interpretation that the granodiorites are slightly older than the granites (Kresten 1986).


Fig 3.1  

The Småland granites have their major extension to the south and west of the Central Granodiorite Belt (cf. fig. 3.1 and loc. 13 - 15). Generally they are massive with no or only very weak schistosity. Småland granites are classified as S-type granites. With an age of 1.7 - 1.65 Ga Kresten (1986) regarded the Småland granites predominantly as post-kinematic (post-orogenic). U-Pb ages after Mansfeld (1991) resulted in 1.84 - 1.76 Ga and according to this the Småland granites could also be classified as late orogenic. Some generations of the above mentioned metabasite dikes are brecciated by these younger granites. There also exist transitional forms along contact zones between metasediments and granitoids. These transitions are relatively local in extent. They occur as migmatites or veined gneisses with lit-par-lit texture as well as metasedimentary xenoliths in granites. Compared to xenoliths in the Loftahammar granites they occur rather seldom (Gavelin 1984).

The Götemar granites in the southernmost part of the Västervik area represent another age group of younger granitoids (cf. fig. 3.1 & loc. 20). With an age of about 1.38 Ga, they are considered to be post-kinematic (anorogenic). This granite massif is rather circular with a diameter of about 5 km. Götemar granites are highly evolved alkali-rich granites with high fluorine contents. Three major varieties of coarse-, medium- and fine-grained character are developed (Kresten 1986). The Götemar granitoid is divided by a major N-S striking fault with 200 m lateral displacement in a sinistral direction and a ca. 500 m uplift of the western part. This adds up to a minimum depth of at least 500 m for the Götemar massif.

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3.1.2 Metamorphic rocks

Metasediments

The metasediments are the oldest rocks within the Västervik Area. Age determinations by Sultan et al. (2004) resulted in four age groups of 3.63 Ga, 3.03 - 2.94 Ga, 2.72 - 2.69 Ga and 2.12 - 1.87 Ga with a 75/25% proportion of early proterozoic and Archaean zircon ages. These age groups may represent major crust forming events. The metasediments reach an estimated depth of approximately 5000 meters (Kresten 1986) which probably do not correspond to true depth. During metamorphism these rocks have experienced folding and emplacement through granitoids. Clear evidence about the true thickness seems to be impossible to ascertain. According to Gavelin (1984), the Västervik formation can be divided into four main groups.

a. Quartzites/Micaquartzites. The quartzites around Västervik have been taken as typical representatives of the Västervik formation. They are quantitatively the most important metasediments. The quartzites are even-grained and homogeneous (cf. loc. 5). Besides quartz the rock contains potash feldspar and rarely plagioclase. Muscovite and biotite occur in varying amounts. Within these rocks, sedimentary structures like cross-bedding and others can be found in various forms.

b. Red (pink) metaarkoses. These rocks contain a large amount of red potash feldspar (caused by a fine coating of iron oxides along grain boundaries or microcracks) but less biotite, muscovite and chlorite than in the micaquartzites. The red metasediments only appear within restricted areas (e.g. Björnsholm).

c. Gray metasediments. Generally these rocks are fine-grained and only local in extent (e.g. north of Lake Rummen or around Henriksnäs). The main mineral is quartz. Feldspars and micas are additional constituents. Graded bedding can be found within these metasediments.

d. Red-gray metasediments. Though this name summarizes a wide variation of rocks, the alternation of reddish and grayish beds is characteristic to all of them. Both the reddish and grayish beds are rich in quartz and potash feldspar but the grayish contains more biotite. The beds mostly vary between 3 and 20 cm in thickness. Red-gray metasediments can be found around Vimmerby and Gunnebo.

Detailed studies on sedimentary structures by Gavelin & Russel (1967) resulted in the conclusion that the metasediments in the Västervik area (mainly Group a) represent a delta-tidal flat environment (Mississippi-type). Where granitoids intruded the metasediments contact metamorphism and migmatization can be observed (cf. Structural Development & Metamorphism).


Metavolcanites
Within the southern part of the Västervik area metavolcanites are rather widely spread, but due to metamorphic overprinting the differences between epi- and pyroclastic sediments are often barely visible to the naked eye. This tour contains two locations with metavolcanites, first a small appearance of metatuffites (cf. loc. 18b) and second a metarhyolite (cf. loc. 12).


Metabasites
The metabasites described here mainly appear as amphibolites and rarely as metagabbros or metadiorites. The amphibolites are dark, sometimes nearly black, dense and fine grained rocks. They mostly form flows, sills or dikes with a thickness of several dm to m intruding sediments and older granitoids (cf. loc. 9). Metagabbros or metadiorites appear in shades of gray due to their coarser grain size and generally build massive intrusions with a thickness up to 100 m and more (cf. loc. 8). The main constituents are plagioclase, green hornblende and in varying amounts biotite and quartz (Kresten 1972, Gavelin 1984). At another location the metabasites appear as a basic flow (cf. loc. 17) Kresten (1972) recognized seven generations of metabasites and described their age relations. According to his studies they can be assigned to three orogenic phases. Generation I and II are classified as prekinematic, while generation III - V are assumed to occur during emplacement of the older granitoids. Generation VI and VII of the metabasites, as the youngest, are dated post-kinematic and equal to the younger granites.


Gneisses & Migmatites
Within this guide we distinguish between flecky gneisses, veined gneisses and migmatites. Generally, the expression gneiss is only a structural description. The terms flecky gneiss (from the Swedish fläckgnejs) and veined gneiss are not common to everyone. Flecky gneisses are rocks related to contact metamorphism, similar to knotenschiefer. According to its name the most obvious characteristic feature of flecky gneisses are dark-gray spots in a lighter colored homogeneous or slightly bedded metasediment. Gavelin (1984) has noticed a zonation of the flecks: A dark core (melanosome) consisting of biotite and quartz surrounded by a light rim (leucosome) containing quartz and feldspar. Unlike this description the location chosen here does not show this phenomenon that obviously (cf. loc. 18a), but with careful observation it is possible to detect evidences of rims at several spots. Flecky gneisses result from differentiation through metasomatism and recrystallization as a pre-stage of porphyroblastic crystallization. A detailed description of the mineralogy and evolution of flecky gneisses in this area is given by Loberg (1963), Russell (1969) and Dahl (1972).

The description veined gneisses might be a little confusing, because it does not stand for rocks commonly known as gneisses, but for a special kind of migmatites. Characteristic features of veined gneisses are, in contrast to flecky gneisses, veins of leucosome intruding the metasediment either concordant or discordant. Such veins are supposed to come either from granitic magma (artenites) or mobilized material from pre-existing rocks (venites). For the Västervik area it is assumed that they mainly represent venites (Gavelin 1984). In some mapping reports of the Göttingen study group the term "injection migmatites" is used (e.g. Szagun 1997). The northernmost two locations (cf. loc. 1 & loc. 2) show two different types of veined gneisses. Compared to the so-called veined gneisses, the rocks at the bay of Blankaholm (cf. loc. 19) look quite different and can more obviously be described as common migmatites due to their visible flow-folding and the lack of primary layering.

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3.1.3 Mylonites

Mylonites are associated with shear zones, produced by ductile deformation in connection with crystal plasticity and dynamic recrystallization (cf. appendix fig. 4). Mylonites are characterized by strong foliation and a grain size that is smaller than that of the host rock from which they were formed.

Two out of three chosen outcrops are located within a large scale deformation zone north of Västervik called Loftahammar-Linköping Deformation Zone (LLDZ), which has been studied in detail by Beunk (2001). The first location at the shoreline of Bjursund's campsite shows strongly folded veins in a fine grained matrix (cf. loc. 3). The second one, north of Hallmare, is a blastomylonite with reddish feldspar blasts in a dark fine grained matrix (cf. loc. 4). The third location southwest of Västervik is a narrow shear zone within a porphyric granitic rock (cf. loc. 13).

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3.1.4 Dikes and Veins

The rocks in the Västervik area are frequently cut by different kinds of dikes and veins. Most frequent are aplites, pegmatites and mafic dikes, whereas quartz veins and sedimentary dikes don't appear that often.

Aplites are fine grained, light colored (usually white or pinkish) dikes with an average thickness from 5 to 20 cm. Their main components are quartz and feldspar. Generally there is no visible zonation but a sharp contact to the host rock. They probably represent a residual fraction of silica-rich magma.

Pegmatites are exceptionally coarse grained (often megacrysts) dikes with an average thickness of several dm. Their essential minerals are quartz and feldspar, mica and biotite are subordinated. These dikes often show a typical zonation with quartz in the center and a rim of feldspars (cf. loc. 11). Often tourmaline accumulates in pegmatite veins. Pegmatites are also thought to represent residual melt of a magma, but those 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 pegmatites and aplites occur in all rock types but mainly in the Västervik granitoids. Pegmatites and aplites 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. Intersecting dikes can be observed often.

Mafic dikes are dark and fine grained. They can be considered as the dike form of the metabasites. Kresten (1972) described seven generations of metabasites and their age relations.

Within the Västervik Area quartz veins are generally only a few cm thick and are thought to result from hydrothermal fluids and volatiles. Sedimentary dikes are rarely spread and only appear in the Västervik granitoids. Their striking direction is either N-S or NE-SW (Jacke & Vollbrecht 2004) (cf. loc. 6 & 16). Sedimentary dikes indicate late extension while the granitoids were covered with sediments, probably within the early Paleozoic.

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