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"
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 dont 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.
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.)