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4. Locations

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Location No. 18

Outcrop is located at the east coast of Skälö. Approach via road from Västrum to Skälö, follow the road to the harbor of Baggeholmen.

R: 1549474 - H: 6385261
RT 90: N 57° 35' 24.2" - E 016° 38' 08.2"
WGS 84: N 57° 35' 23.1" - E 016° 37' 57.5"

Keywords: flecky gneisses, premetamorphic ductile shear zone, metatuffites


Description:

This location has two outcrops. At first some flecky gneisses (a) with a premetamorphic ductile shear zone. They are located along the road at the harbor and extend in east direction to the coast. There, in sharp contact to the flecky gneisses, some metatuffites (b) with excellent preserved graded bedding are exposed. A wall west of the flecky gneisses show some reddish metaquartzites, with channel pattern, of the Västervik Formation. a. The flecky gneisses are characterized as a medium gray colored, homogeneous or slightly bedded metasediment with dark gray spots (cf. fig. 4.18a). These spots cause a slight relief on the weathered surface. On careful observation a zonation of the flecks can be detected: A dark core (melanosome) surrounded by a lighter rim (leucosome). According Farrenkopf (2000), the dark core consists of xenocrysts of quartz, biotite and muscovite within an extremely fine-grained matrix consisting of serizite. It is surrounded by a light rim of quartz-serizite-sillimanite minerals. The main components of the matrix are quartz, microcline, serizite, muscovite and biotite.

The spots vary from round to elongated, even mylonitic stretched where the rock was involved in an ductile shear zone (cf. 4.18b-c). These different forms are used as a norm to define the degree of deformation. The flecky gneisses demonstrate an excellent example of synkinematic metamorphism. Due to the elongation of the flecks in NW-SE direction Westra et al. 1969 concluded that they where formed in the F2 stress-field. Farrenkopf (2000) has noticed high-grade metamorphic minerals such as sillimanite, andalusite and additional cordierite in the Mg-rich layers. Based on detailed microfabric studies he 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.

b. The metatuffites adjacent to the flecky gneisses are brown to gray in color and fine- to coarse-grained with preserved graded bedding. Quartz intruded the layered tuffites concordant and led to formation of the so-called fir tree fabrics (cf. fig. 4.18e-f).



Further Reading:

Dahl, O., 1972: Some Aspects on the Evolution of Fleck Gneisses in the Västervik Area. - GFF 94: pp 69 - 82.

Loberg, B., 1963: The Formation of a Flecky Gneiss and Similar Phenomena in Relation to the Migmatite and Vein Gneiss Problem. - GFF 85: pp 3 - 109.

Russell, R.V., 1969: Porphyroblastic Differentiation in Fleck Gneiss from Västervik. - GFF 91: pp 217 - 282.



Fig. 4.18a: Partial overview of the outcrop showing flecks from rounded to elongated shape due to the shear zone on the right side of the photo.
Fig. 4.18b & c: Close up view of rounded spots (b) and layering of the shear zone (c).
Fig. 4.18d: Close up view of a thin section showing the dark core and light rim of two flecks. (Photo: S. Farrenkopf, pers. comm.)
Fig. 4.18e & f: Close up views of the graded metatuffites. Quartz intrusions causing fire tree fabric. (Photo 4.18f: K. Wemmer, pers. comm.)


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Fig. 4.18a
Fig. 4.18a

Fig. 4.18b
Fig. 4.18b

Fig. 4.18c
Fig. 4.18c

Fig. 4.18d
Fig. 4.18d

Fig. 4.18e
Fig. 4.18e

Fig. 4.18f
Fig. 4.18f
 

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