The hornblende-plagioclase hornfels from the contact aureole of the Tanvald granite, northern Bohemia - the raw material for Neolithic tools
Bulletin of Geosciences, volume 79, issue 1;
pages: 63 - 70;
Received 5 December 2002;
Accepted in revised form 5 May 2003;
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A hornblende-plagioclase hornfels is described from a new exposure near the village of Jeřmanice, where it is present as a 30-35 cm thick layer within spotted phyllites at the exocontact of the Krkonoše-Jizera granite massif in northern Bohemia. The hornfels layer seems to be part of the Poniklá Group of Ordovician -Upper Silurian age. It corresponds to the metabasites used in the production of Neolithic artifacts (e.g., the Jistebsko archeological site near Jablobec nad Nisou), which were hitherto known only as loose fragments from localities on the lower slopes to the south of the granite ridge between Liberec and Tanvald. New data show that this rock is situated inside the contact aureole of the Tanvald alkali-feldspar granite, its remarkable mechanical strength being due to the effects of contact metamorphism. The hornfels corresponds compositionally to tholeiitic basalt. Its major minerals are amphiboles ranging in composition from the predominate magnesiohornblende to subordinate actinolite, calcic plagioclase, and ilmenite. This rock represents neither a dike nor a lava sheet, but rather a sill or a tuff layer modified by regional and thermal metamorphic reconstruction and tectonic deformation.