The “Triasscholle” near Greiz,Germany—a volcanic origin? |
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| Authors: | T Nickschick H Kämpf T Jahr |
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| Institution: | 1. German Research Centre for Geosciences, GFZ Potsdam, Section 4.3 Telegrafenberg, 14473, Potsdam, Germany
2. Institute for Geosciences, Friedrich-Schiller-Universit?t, Burgweg 11, 07749, Jena, Germany
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| Abstract: | Near the city of Greiz in Eastern Thuringia, Germany, there is a stratigraphically non-coherent breccia encased in shales from the Lower Carboniferous that includes larger blocks of Triassic sediments, the so-called Triasscholle. The origin of this breccia has previously been interpreted tectonically. The geophysical methods of geomagnetics and gravimetry have recently been applied to the area. We found that the formation is characterised by a lack of a magnetic anomaly, but it has a distinct, spatially small gravity anomaly of approximately ?2 mGal and whose shape hints at a structure about twice the previously suggested size. Most of the geological information available is derived from older drill cores located in the southwest part near the previously assumed margin. Scanning electron microscope (SEM) investigations on drill core samples from the polymict breccia at different depths show a cataclastic deformation of carbonate grains in the upper parts, while we find hypidiomorphic and idiomorphic zoned carbonate grains featuring a dolomitic/calcitic composition within the core as well as rims of ankeritic composition hinting at intense hydrothermal influence. The breccia itself contains granitoidic xenoliths from greater depths, which indicate upward transport processes. This and plastic deformation below 95m depth, both on macroscopic and microscopic scales, indicates an anomalous increase of heat with depth and material transport from depth. Judging from the gravimetric minimum and the SEM investigations on the breccia samples, we assume the Triasscholle near Greiz to be a deeply eroded maar-diatreme volcano. From pollen analysis results on the matrix, the formation can be considered Santonian or younger in age (≤85 Ma). Most interesting are the alteration of the mainly carbonatic components of the breccia, the deep erosion level of the diatreme and the multidisciplinary approach towards the reinterpretation of this formerly misinterpreted structure. |
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