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4. Locations

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Location No. 13

Approach outcrop via E33 from Gunnebo to Ankarsrum immediately west of lake Långsjön. Parking on a lane entry. This outcrop has many details to detect.

R: 1528671 - H: 6396910
RT 90: N 57° 41' 47.2" - E 016° 17' 20.9"
WGS 84: N 57° 41' 46.1" - E 016° 17' 10.3"

Keywords: granite, granodiorite, mylonite, ductile shear zone


Description:

The main point of interest in this outcrop is a ductile shear zone which separates a porphyritic granite from a granodiorite. It strikes nearly in a N-S direction and has a maximum extension of 1.5 – 2 m (cf. fig. 4.13a).

The porphyric granite east of the shear zone is red-gray in color with the main components consisting of quartz, K-feldspars and plagioclase, as well as hornblende and biotite. The extremely coarse-grained K-feldspar causes the porphyric fabric. The granodiorite generally is a medium- to dark-gray colored, fine- to medium-grained rock and is located west of the shear zone. Only close to the shear zone does the granodiorite show an area of porphyroblastic growth of K-feldspar clasts. According to Ondrasina (2003), the granodiorite contains a proportion of mafic minerals, like hornblende and biotite, of about 25 vol.%. Leucocratic components are mainly quartz, plagioclase and in parts K-feldspar. Some quartz grains occur as blue quartz (cf. fig. 4.13e).

Macroscopically the transition between the porphyric granite and the shear zone is more striking than between the latter and the granodiorite. The ductilely sheared area is quite narrow, but the surrounding rocks are affected up to several meter to both sides with smooth transitions. The mylonitic area is in dark color and consists of interbedded light-, medium-, dark-brown, red and green colored bands (cf. fig. 4.13b-c). The mylonite layers are generally finer-grained than the surrounding rocks, even of ultramylonitic shape in the center. Fig. 4.13f shows a partial view of a thin section from epidote- and chlorite-rich mylonitic layers. These layers occur as a coating on several joints. To the W’ of the mylonite zone porphyroblastic growth of K-feldspars occurred quite similar to location no. 04 at Hallmare (cf. fig 4.13d and loc. 04).



Fig. 4.13a: Partial overview of the outcrop showing the ductile shear zone.
Fig. 4.13b: Close up view of fig. 4.13a and the sharp contact between the shear zone and the porphyric granite.
Fig. 4.13c: Close up view of mylonitic layers. (Photo: K. Wemmer, pers. comm.)
Fig. 4.13d: Partial view of porphyroblastic feldspar growth and an epidote layer. (Photo: K. Wemmer, pers. comm.)
Fig. 4.13e: Close up view of blue quartz within the granodiorite. (Photo: K. Wemmer, pers. comm.)
Fig. 4.13f: Partial view of thin section from the epidote- and chlorite-rich mylonite area. (Photo: A. Vollbrecht, pers. comm.)


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Fig. 4.13a
Fig. 4.13a

Fig. 4.13b
Fig. 4.13b

Fig. 4.13c
Fig. 4.13c

Fig. 4.13d
Fig. 4.13d

Fig. 4.13e
Fig. 4.13e

Fig. 4.13f
Fig. 4.13f
 

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