Vergleichende gravimetrische, geomagnetische und geologische Untersuchungen im Bereich des Diatrems von Ebersbrunn und der Triasscholle bei Greiz
Final Report Abstract
The Ebersbrunn diatreme is a deeply eroded (>1 km) diatreme structure in western Saxony, Germany. At current erosion levels, this ultramafic to carbonatitic diatreme is about 2×1.5 km in map view, which makes it a large one. Based on shallow drill cores, the diatreme contains coarse unbedded volcaniclastic rocks with up to 80 % country rock fragments. The diatreme is characterised by positive and negative magnetic anomalies, which are controlled mostly by the changing proportions of magnetic minerals in the rocks. The magnetic minerals are themselves contained in the juvenile fraction. Rock magnetic studies on three drill cores, one from the magnetic low and two from magnetic highs, including bulk susceptibility and its anisotropy, temperature dependent susceptibility, various remanence measurements (natural remanent magnetisation, anhysteretic remanence and isothermal remanent magnetisation) and alternating field demagnetisation have been performed. Additionally scanning electron microscope imaging and energy-dispersive X-ray spectroscopy were performed to better characterise the magnetic minerals. Magnetisation is caused by titanomagnetite with slightly varying Ti contents within all cores. Samples from the positive magnetic anomalies have a relatively high degree of anisotropy, but no preferred orientation of one of the principal axes can be seen. The magnetic highs are caused by non-bedded volcaniclastic rocks comparatively rich in juvenile particles. The negative magnetic anomaly is associated with a compaction-like magnetic fabric but without macroscopically visible bedding. Hence, the magnetic low is caused by rocks with a lower content of juvenile material. To the authors’ knowledge, this is the first time rock magnetic methods have been applied to diatreme rocks. 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.
Publications
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Origin of magnetic anomalies in the Ebersbrunn diatreme – W Saxony, Germany, Bull Volcanol, 75:766
Schmidt, A., Nowaczyk, N., Kämpf, H., Schüller, I., Flechsig, C., Jahr, T
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(2014), The Triasscholle near Greiz, E Thuringia – a volcanic origin? Bull Volcanol, 76:806
Nickschick, T., Kämpf, H., Jahr, T.
