The late Palaeozoic Variscan Orogen forms the backbone of western and central Europe. It formed during the closure of the Rheic Ocean, when Laurussia and Gondwana collided. The complex structure of the Variscan Orogen is due the contrasting response of thick and thin crust to subduction and collision. Areas with thick crust largely consist of Cadomian basement that was stiffed by Cadomian magmatism (570-540 Ma), whereas areas with thin crust, separating these blocks, have developed during late Ordovician extension and rifting of northern peri-Gondwana. These rift basins have been filled with voluminous volcano-sedimentary deposits.

The Saxo-Thuringian Zone (Bohemian Massif) is an integral part of the Variscan orogen. With respect to Variscan deformation and metamorphism, the Saxo-Thuringian Zone is distinguished into an Autochthonous Domain, an Allochthonous Domain and a Wrench-and-Thrust Zone. Cadomian Basement and Palaeozoic sedimentary rocks of the Autochtonous Domain were not metamporphosed during the Variscan orogeny. In contrast, Palaeozoic sedimentary rocks of the Allochthonous Domain were subducted and experienced Variscan high-grade metamorphism. The Thrust-and-Wrench Zone is a late-Variscan shear zone along which rocks of the Allochthonous Domain habe been juxtaposed against rocks of the Autochthonous Domain.

The Palaeozoic volcano-sedimentary rocks deposited on the Gondwana shelf have received siliciclastic input from Gondwana alone, once the Rheic Ocean had opened. The sedimentary rocks record the entire history of the Rheic Ocean, starting with the rifting of peri-Gondwana preceding the separation of Avalonia, which eventually led to the formation of the Rheic Ocean; recording the transition from narrow to more open marine conditions; having a range of facies changes reflecting the deepening of the deposition area preceding the Variscan orogeny; and eventually recording the rapid filling of the deposition area during the Variscan orogeny. This sedimentation history also has resulted in systematic geochemical and isotopic changes in the siliciclastic sedimentary rocks through time, reflecting the combined effect of source character (the chemically deeply weathered source rocks were scarped off during the Hirnantian glaciation), transport, and deposition environment.

The metamorphic rocks of the Allochthonous Domain represent a pile of nappes that experienced contrasting Variscan metamorphism. Cadomian basement had been subducted to medium pressure conditions and is now exposed in gneiss domes of the Erzgebirge. The volcano-sedimentary cover had been subducted to experience a wide range of pressure and temperature conditions, reaching from very-low grade metamorphism to ultra-high pressure (Erzgebirge) and ultra-high temperature (Saxon Granulite Massif) metamorphism. These rocks all have been subducted beneath the Bohemian Massif, reaching peak metamorphic conditions at c. 340 Ma, and being thereafter rapidly uplifted and emplaced onto rocks of the Autochthonous Domain. Metamorphic rocks from an early subduction event, reaching peak metamorphic conditions at c. 370 Ma, are only locally preserved. Orogenic collapse eventually led in the Allochthonous Domain to voluminous, predominantly 327-318 Ma old granitic magmatism, which includes I, S, and A type rocks and in part is Sn and W specific.

The Saxo-Thuringian Zone is unusual as the metamorphic units of the Allochthonous Domain include the same lithological units that are exposed in the Autochthonous Domain. This particular feature allows studying element redistribution and associated isotopic fractionation of light elements during progressive metamorphism and devolatilization. Furthermore, the systematic change of geochemical and isotopic composition of the sedimentary rocks with stratigraphic position – in part having distinctive and “exotic” compositions – in the Autochthonous Domain allows these geochemical fingerprints to be used for correlation with other Gondwana-derived Variscan terranes and to distinguish them from terranes not sharing these geochemical fingerprints and, thus, providing a tracer to constrain the position of Variscan terranes relative to the Rheic suture.

Further literature:

 

Romer, R.L., Kirsch, M., and Kroner, U. (2011) Geochemical signature of Ordovician Mn-rich sedimentary rocks on the Avalonian shelf. Can. J. Earth Sci., 48, 703-718. doi: 10.1139/e10-092

Romer, R.L. and Hahne, K. (2010) Life of the Rheic Ocean: Scrolling through the shale record. Gondwana Res., 17: 236-253. doi:10.1016/j.gr.2009.09.004

Linnemann, U. & Romer, R.L. (2010, eds.) Pre-Mesozoic Geology of Saxo-Thuringia – From the Cadomian Active Margin to the Variscan Orogen. Schweizerbart, Stuttgart, 488 pp. ISBN 978-3-510-65259-4

Kroner, U., Hahn, T., Romer, R.L., and Linnemann, U. (2007) The Variscan orogeny in the Saxo-Thuringian zoneheterogenous overprint of Cadomian / Palaeozoic Peri-Gondwana Crust. In: Linnemann, U., Nance, D., Kraft, P., and Zulauf, G. (Eds.) The Evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision. Geol. Soc. Am. Spec. Pap. 423: 153-172. doi:10.1130/2007.2423(06)

Romer, R.L. and Rötzler, J. (2001) P-T-t evolution of ultrahigh-temperature granulites from the Saxon Granulite Massif, Germany. Part II: Geochronology. J. Petrol., 42: 2015-2032. doi:10.1093/petrology/42.11.2015

Rötzler, J. and Romer, R.L. (2001) P-T-t evolution of ultrahigh-temperature granulites from the Saxon Granulite Massif, Germany. Part I: Petrology. J. Petrol., 42: 1995-2013. doi:10.1093/petrology/42.11.1995

 

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