Exploration and Investigation of High-Level Radon Medicinal Springs in the Crystalline Units: Lugicum

Radioactive (radon) groundwaters are highly valued among mineral waters for their healing effects. Between 2005 and 2015, a large exploratory event for prospecting and documenting radon water springs took place in the crystalline area of Lugicum (Bohemian Massif) under Czech–Polish cooperation. For these purposes, an exploration method was developed as a combination of GIS (ArcMap 9.1–10.2) area preparation followed by field radiohydrogeochemical mapping at a scale of 1:10,000. The gamma indication method was optimized and used for the selection of water samples. A total of 2354 water sources were examined. Radon activity concentrations were measured at 660 sources found throughout the territory. Of those, 111 sources exhibited 222Rn activity above 1500 Bq/L and, thus, were categorized as sources of mineral radioactive waters according to Czech legislation. The highest 222Rn activity was found in the Michael spring near Nové Město pod Smrkem (up to 6237 Bq/L 222Rn). Many discovered sources with high balneological potential are significant and, therefore, are quickly becoming popular among the public.

Experimental evidence on the origin of Ca-rich carbonated melts formed by interaction between sedimentary limestones and mantle-derived ultrabasic magmas

In this experimental study, we documented the formation of strongly ultrabasic and ultracalcic melts through the interaction of melilititic and basanitic melts with calcite. Three strongly to moderately SiO2-undersaturated volcanic rocks from the Bohemian Massif (central Europe) were mixed with 10, 30, and 50 wt% CaCO3 and melted at 1100, 1200, and 1300 °C at 2 kbar to evaluate the maximum amount of carbonate that can be assimilated by natural ultrabasic melts at shallow depths. Experiments revealed a surprisingly complete dissolution of the CaCO3, only rarely reaching carbonate saturation, with typical liquidus phases represented by olivine, spinel, melilite, and clinopyroxene. Only in the runs with the most SiO2-undersaturated compositions did abundant monticellite form instead of clinopyroxene. For all starting mixtures, strongly ultrabasic (SiO2 down to 15.6 wt%), lime-rich (CaO up to 43.6 wt%), ultracalcic (CaO/Al2O3 up to ~27) melt compositions were produced at 1200 and 1300 °C, with up to ~25 wt% dissolved CO2. When present, quenched olivine showed much higher forsterite content (Fo95–97) than olivine in the natural samples (Fo79–85). The two major results of this study are (1) silicate-carbonatite melt compositions do not necessarily imply the existence of carbonatitic components in the mantle, because they are also produced during limestone assimilation, and (2) Fo-rich olivines cannot be used to infer any primitive character of the melt nor high potential temperature (Tp).

Timing and kinematics of the Variscan orogenic cycle at the Moldanubian periphery of the central Bohemian Massif

The high-grade complexes along the northern Moldanubian periphery of the central Bohemian Massif provide an outstanding structural record of all episodes of the Variscan collisional evolution. Kinematics and timing of orogenic processes have been examined by structural and microstructural study of middle and lower crustal rocks combined with xenotime and monazite geochronology. Four distinct tectonic events have been identified in the studied units. A first relict sub-horizontal fabric S1 associated with the HP/HT metamorphism is developed only in the lower crustal rocks and was related to back-arc extension or lower crustal flow in a supra-subduction domain. This fabric was at c. 340 Ma completely reworked to the sub-vertical foliation S2 by the major collisional thickening leading to the lower and middle crust juxtaposition. Thereafter, the extensional collapse of thickened orogenic system caused strong refolding to the HT sub-horizontal fabric at c. 325 Ma. The region was subsequently affected by the NNE–SSW oriented horizontal shortening related to the dextral shearing and clockwise rotation of crustal blocks adjacent to the large scale dextral shear zone, the Elbe Zone. It led to the fragmentation and reorientation of the Moldanubian margin to the current position.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5708800.v1

Late to post-Variscan basement segmentation and differential exhumation along the SW Bohemian Massif, central Europe

The exposed Variscan basement in central Europe is well-known for its complex structural and lithological architecture resulting from multiple deformation phases. We studied the southwestern margin of the Bohemian Massif, which is characterized by major and long-lived shear zones, such as the Pfahl and Danube shear zones, extending over > 100 km and initiated during Variscan tectonics. We integrated Bouguer gravity anomaly and lidar topographic data analyses and combined our results with available data and observations from low-temperature thermochronology, metamorphic grades, and the exposed granite inventory to detect patterns of basement block segmentation and differential exhumation. Three NW–SE-striking basement blocks are bordered by the Runding, Pfahl, and Danube shear zones from the northeast to the southwest. Basement block boundaries are indicated by abrupt changes in measured gravity patterns and metamorphic grades. By applying high-pass filters to gravity data in combination with lineament analysis, we identified a new NNW–SSE-striking tectonic structure (Cham Fault), which further segments known basement blocks. Basement blocks that are segmented by the Cham Fault differ in the abundance and spatial distribution of exposed late Variscan granites and are further characterized by variations in apparent thermochronological age data. Based on our observations and analyses, a differential exhumation and tectonic tilt model is proposed to explain the juxtaposition of different crustal levels. Block segmentation along the NW–SE-striking Pfahl and Runding shear zones most likely occurred prior, during, and after late orogenic granite emplacement at ca. 320 ± 10 Ma, as some of the granites are cross-cut by the shear zones, while others utilized these structures during magma ascent and emplacement. In contrast, activity and block segmentation along the Cham Fault occurred after granite emplacement as the fault sharply truncates the granite inventory. Our study provides evidence of intense and continuous fault activities during late and post-orogenic times and highlights the importance of tectonic structures in the exhumation and juxtaposition of different crustal levels and the creation of complex lithological patterns in orogenic terrains.