Lake Mien — An Astrobleme or a Volcano-Tectonic Structure

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Abstract
Lake Mien forms a depression in the acid Precambrian bedrock in southeastern Sweden. According to the first interpretation, the brecciated and lava-like rocks found in the glacial deposits south of Lake Mien were believed to be of volcanic origin and were described as rhyolites. Recehtly, a meteoritic origin has been proposed from identification of coesite and shock-wave deformation lamellae in quartz grains in the rhyolitic rocks. In the present paper some preliminary results of field investigations in the Mien area are discussed, which in some respects do not support the meteoritic theory. Furthermore, these results establish that the peculiar rocks, described earlier as rhyolites, are not typical volcanic rocks. More detailed investigations, especially of a geophysical and petrographical character, are necessary, however, to draw a final conclusion about the origin of the Mien structure.