Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Václav Kachlík

Relationship between Moldanubicum, the Kutna Hora Crystalline Unit and Bohemicum (Central Bohemia, Czech Republic): A result of the polyphase Variscan nappe tectonics

Journal of the Czech Geological Society, volume 44 (1999), issue 3-4, 201 - 291


  Abstract

The Moldanubian Zone, forming a highly metamorphosed root of the Variscan orogenic belt in Central Europe, is surrounded by several different lithotectonic units within the Bohemian Massif. These units mostly show lower metamorphic grade and different P-T histories. The relationships of these units to the Moldanubicum s. s. have been explained in different ways, but especially the relationships with the Bohemicum and the lowermost part of the Kutna Hora Crystalline Unit (KHCU) remained unclear. Boundaries between the above mentioned units are exposed in the Sazava River valley area, 50 km ESE of Prague, which allows to study lithological, structural and metamorphic interrelationships of these units in detail. The following lithotectonic units of different lithologies and P-T paths can be distinguished from the bottom to the top of the structural succession: Moldanubian Variegated Group (MVG), Micaschist Zone (MSZ), Kourim orthogneiss nappe (both usually assigned to KHCU), Gfohl Unit (GU) and strongly sheared granitoids of CBP with preserved relics of metasedimentary and metavolcanic rocks of the Bohemicum. All units (except for the Kourim Nappe) are dominated by psammitic to pelitic lithologies. The present geological configuration of the nappe sequence is a result of polyphase westerly-orientated shearing of deep crustal slices with incorporated mantle material. This process started on a major shear interface located between GU and extremely ductile Kourim orthogneisses. Sm-Nd garnet cooling ages of mantle peridotites and eclogites (Beard et al. 1991, Bruckner et al. 1991) suggest that exhumation of deep crustal and mantle rocks occurred between 370 and 340 Ma. When deep crustal nappes reached middle crustal levels (in the amphibolite facies field), strain partitioned into structurally lower unit (Micaschist Zone) and the whole nappe sequence was thrust over the paraautochthonous MVG. During this thrusting event, rocks of GU followed a retrograde P-T path in the kyanite stability field. Rocks of GU cooled down below the closing temperature of the Ar-Ar system on micas at ca. 340 Ma (Maluski, pers. comm), while rocks of MSZ and MVG underwent prograde metamorphism, partly comparable in both units. Peak metamorphic conditions reached sillimanite field in both units. Clockwise part of the P-T path can be interpreted by underthusting during emplacement of higher nappes. The partly syntectonic emplacement of the Central Bohemian Pluton (350-330 Ma) postdates the main thrusting event at the Moldanubian-KHCU boundary. During this separate phase, rocks of CBP and associated roof pendants were thrust in a dextral transpressive ramp regime over the above mentioned stack of deep crustal nappes. Subsequent decompression probably associated with a slight increase in temperature represents tectonic uplift in later phases penecontemporaneous with the emplacement of the Central Moldanubian Pluton (328-305 Ma). The greenschist facies retrogression and mylonitization in the Rataje Zone is associated with the strike-slip deformation operating at the interface between MSZ and MVG

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ISSN: 1802-6222

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