Comparison of lithology and tectonometamorphic evolution of units at the northern margin of the Moldanubian Zone: implications for geodynamic evolution in the northeastern part of the Bohemian Massif

Abstract

*Corresponding author Multidisciplinary research evaluates structural, metamorphic and petrochemical data of selected rock types in different units located in the northeastern part of the Bohemian Massif, Czech Republic: (1) the Stražek Unit in the northeastern part of the Moldanubian Zone, (2) the Svratka and Kutna Hora units correlated with the Orlice–Sněžnik Unit in Western Sudetes as well as (3) the Policka, Hlinsko and Zabřeh units belonging to the Tepla–Barrandian Zone. Petrochemical data for metasediments of the Policka, Hlinsko and Zabřeh units are mutually comparable and confirm a lithologi cal affinity to the upper-crustal Tepla–Barrandian Zone. The FeO t /MnO ratios in metasediments of the Stražek Unit and the Svratka Unit indicate differences in the origin of sedimentary protolith rocks. Relict pre-Variscan structures, including extensive migmatization, and high-grade mineral assemblages with peak metamorphic pressures of ~1.4 GPa in skarn bodies, are preserved in the Svratka Unit. The evidence for Palaeo-Variscan (390–355 Ma) HP and UHP events, recorded in the high-grade Kutna Hora and Orlice–Sněžnik units, was observed neither in the NE part of the Moldanubian Zone (Stražek Unit) nor the Tepla–Barrandian Zone (Policka and Zabřeh units). The Variscan orogenic event imprinted in the Svratka, Policka and Zabřeh units was the right-lateral slip along WNW–ESE trending shear zones. This deformation was accompanied by metamorphism at ~580–650 °C and ~0.5–0.7 GPa (352–343 Ma) and intrusion of numerous small bodies of syn-deformation calc-alkaline granitoids in the Policka and Zabřeh units. The younger metamorphic fabrics in the northeastern part of the Moldanubian Zone reflect a fast exhumation of deep-seated high-grade complexes at ~340 Ma, which was constrained by post-tectonic emplacement of durbachites at ~339 Ma. Metamorphic development in felsic granulites of the Stražek Unit, metamorphosed c. 340 Ma ago at 850 °C and 1.8 GPa, was followed by decompression to T ≅ 790 °C and P = 1.3 GPa and finally T = 700 °C and P = ~0.4 GPa. In contrast, Běst vina granulite in the Kutna Hora Unit, with the ~360 Ma high-grade metamorphism at 800–920 °C and 1.8–2.1 GPa, is free of such a HT–LP overprint. Thus the data indicate that the Svratka and Kutna Hora units, exhibiting numerous mutual differences, should not be considered as belonging to the Moldanubian Zone as they evolved as independent entities. The geochemical data on garnet–clinopyroxene skarns from the Moldanubian Zone, the Svratka and Kutna Hora units do not provide mutually distinguishing features. This is largely due to a very wide compositional variation in rocks interpreted as metamorphosed exhalite with detrital material admixture. Skarns from the Svratka Unit preserve clinopyroxenes with elevated jadeite component (0.5–24 mol. %) and prograde compositional zoning in garnet – features obliterated in samples from the Moldanubian Zone and the Kutna Hora Unit.