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4. Locations

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Location No. 12

Outcrop along the eastern side of the road Ankarsrum - Hummelstad, ca. 500 m north of Västantorp.

R: 1529440 - H: 6402830
RT 90: N 57° 44' 14.2" - E 016° 22' 31.8"
WGS 84: N 57° 44' 13.2" - E 016° 23' 21.3"

Keywords: metarhyolite


Description:

This outcrop shows a metarhyolite which occurs to the east and west of lake Långsjön. The metarhyolite is consistently red colored with phenocrysts of quartz and K-feldspars and/or plagioclase in a fine-grained matrix. In some areas the rhyolite is slightly foliated and quartz veins of different size cut these rocks throughout the whole area. Their preferred strike direction seem to be NW-SE and NE-SW. In the north and south the rhyolite is framed by a porphyric granite. The contact between both is always sharp and the rhyolite shows a darker colored rim at these contact zones. It is discussed whether the rhyolites source from the same magma as the surrounding granites. A relation to the intrusion of the granites is not clarified yet. Maybe it can be considered as a reaction rim (S. Hoffmann and T. Seidel, pers. comm.). Gavelin (1984) has drawn a map of the metavolcanic area E’ of Långsjön (cf. Gavelin 1984, p 47).

On macroscopic examination distinction between K-feldspar and plagioclase turns out to be quite difficult. In hand specimen the feldspars don’t show distinct twinning (cf. fig. 4.12b). In thin sections phenocrysts are mainly quartz grains within the rhyolite (cf. fig. 4.12c) and plagioclases within the contact area of the surrounding granite (cf. fig. 4.12d). Detailed results can not be given at this time, for this area is still under examination by several students from the Göttingen study group.



Further Reading:

Gavelin, S., 1984. The Västervik Area in South-Eastern Sweden: Studies in Proterozoic Sedimentation, High-Grade Metamorphism and Granitization, SGU, Ba 32, 1-172.



Fig. 4.12a: Partial view of a quartz vein striking through the foliated rhyolite. (Photo: S. Hoffmann, pers. comm.)
Fig. 4.12b: Close up view of a rhyolite sample with phenocrysts of quartz and K-feldspars within a fine-grained matrix. (Photo: S. Hoffmann, pers. comm.)
Fig. 4.12c: Partial view of a thin section showing quartz grains within the fine-grained matrix of the rhyolite. (Photo: T. Seidel, pers. comm.)
Fig. 4.12d: Partial view of a thin section showing plagioclases within the fine-grained matrix of the rhyolite from the contact area. (Photo: T. Seidel, pers. comm.)


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Fig. 4.12a
Fig. 4.12a

Fig. 4.12b
Fig. 4.12b

Fig. 4.12c
Fig. 4.12c

Fig. 4.12d
Fig. 4.12d
 

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