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4. Locations

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Location No. 15

Road from Västrum to Skaftet. Park at Västrum’s church at the exit of Västrum.

R: 1545925 - H: 6392835
RT 90: N 57° 39' 30.4" - E 016° 34' 39.8"
WGS 84: N 57° 39' 29.3" - E 016° 34' 29.1"

Keywords: heterogeneous magma mingling


Description:

The outcrop is located at the roadcut close to Västrum’s church. The exposed rock is characterized by a heterogeneous mingling of a leucocratic and a granodioritic granite type (cf. fig. 4.15a-c) which was called Skaftet Granite by Frei (1997). This special granite type can also be found at several other locations between the villages Västrum and Skaftet, e.g. at the bus stop Bäckemåla (R: 1546523 - H: 6392314) and at an outcrop in Skaftet (R: 1546940 - H: 6391670).

The leucocratic granite is medium- to coarse- and irregular-grained. It is light red in color. Essential minerals are quartz, alkali feldspar and plagioclase giving a alkali feldspar granitic to granitic composition (cf. Streckeisen diagram, appendix fig. 3, field 2 and 3). According to Frei (1997), mafic minerals only occur as a minor component (biotite, chlorite) with a maximum of 5 vol.%. Biotite is often accumulated in nests (cf. fig. 4.15e).

The dark colored variety is a granite-granodiorite type (cf. Streckeisen diagram, appendix fig. 3, field 3 and 4). It is also medium- to coarse- but regular-grained and gray-red in color. In thin section the main components are plagioclase, K-feldspar, and quartz. The amount of mafic components, i.e., biotite, titanite (sphene) and amphibole with 25–30 vol.% is significantly higher than in the leucocratic type (cf. fig. 4.15e). Feldspars cause a relief on the weathered surface. Biotite sometimes occurs in an irregular alignment and may be interpreted as a flow fabric.


Study Questions:
  • Discuss the source of different magma types and mingling processes.


Fig. 4.15a: Partial view of the outcrop at Västrum's church shows the heterogeneous mixing of leucocratic granite and granodiorite.
Fig. 4.15b: Partial view of the outcrop at Västrum's church shows another part of the heterogeneous mixing of leucocratic granite and granodiorite. (Photo: K. Wemmer, pers. comm.)
Fig. 4.15c: Close up view on the heterogeneous mixing with schlieren and flamy fabric.
Fig. 4.15d: Partial view of thin section from the leucogranite. 2.5 times magnified. (Photo: A. Vollbrecht, pers. comm.)
Fig. 4.15e: Partial view of thin section from the granodiorite. 2.5 times magnified. (Photo: A. Vollbrecht, pers. comm.)


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Fig. 4.15a
Fig. 4.15a

Fig. 4.15b
Fig. 4.15b

Fig. 4.15c
Fig. 4.15c

Fig. 4.15d
Fig. 4.15d

Fig. 4.15e
Fig. 4.15e
 

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