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The Höhenberg sill in the Thuringian Forest belongs to the last stage of the late Variscan magmatism. The distribution of major and trace elements in the upper part of the dolerite intrusion shows the increasing degree of differentiation from the marginal facies, which crystallized first, to the central fazies (quartz dolerite). Trace element geochemistry and REE-data allow to compare the intrusion with continental flood basalts and to consider it an initial stage of continental flood basalt magmatism. Normalized element patterns, on the one hand, prove that the tholeiitic melt was derived by a small degree of partial melting from an enriched mantle source, which was part of a subcontinental lithosphere. On the other hand, it is also possible that the melt underwent crustal contamination. The ascending magma heated up the overlying parts of the crust until the generation of acid melts. Therefore the dolerite intrusion was associated with a small amount of rhyolitic effusions and shallow intrusions. The late Variscan molasse development in the Thuringian Forest was caused by NW-SE extension above a detachment zone dipping SE from the Mid-European suture, but it was strongly modified by elongation in SW-NE direction. | Thus, a long and flat, SW-NE striking basin was intersected by a NW-NE striking graben structure, including today's Thuringian Forest, and for a time also its southwestern and northeastern adjacent areas. First, in Gehren times, the SW-NE extension, associated with dextral strike slip movements, produced meridionally oriented volcanotectonic depressions within the graben which was filled with predominantly andesitic lavas. Afterwards, this graben split up into single tilted blocks that were simultaneously covered with clastic sediments. Then, during Oberhof times, rhyolitic magmas used a meridionally oriented detachment zone as their ascent paths. Lavas flowed out into NE to N direction and melts partly intruded close to the surface. In Rotterode and Tambach times, the extent of the detachment sidewards as well as into the depth caused the formation of the Rotterode-Tambach syncline. Extensional movements connected with lithospheric pull-apart movements induced an adiabatic melting in the upper mantle. At the beginning of Rotterode times (late Autunian), the ascending basaltic magma intruded into the western limb of the syncline forming the Höhenberg dolerite sill within the Oberhof beds. |
