Chronology of the Saxothuringian subduction in the West Sudetes (Bohemian Massif, Czech Republic and Poland)
Abstract
Isotopic dating of monazite and garnet from high-pressure metamorphic rocks exposed in the northern part of the Saxothuringian palaeo-suture in the Bohemian Massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the Devonian-Carboniferous subduction-accretionary complex. Mafic blueschists of the middle unit yielded a Lu-Hf garnet age of 363.9 +/- 1.3 Ma.
The blueschists occur within high-pressure, garnet-free phyllites. Monazite extracted from this rock type yielded a U-Pb isotope dilution thermal ionization mass spectrometry age of 336.5 +/- 0.5 Ma.
Garnet-bearing micaschist of the lower unit contains monazite with a U-Pb secondary ion mass spectrometry age of 341 +/- 3 Ma, consistent with Lu-Hf garnet-whole-rock ages of 344.5 +/- 1.3 and 342 +/- 7 Ma obtained from the same rock type. Existing tectonic models of the Bohemian Massif, and particularly of its northern part, assume that the period of oceanic subduction was terminated at c. 380-375 Ma by the arrival of an attenuated Saxothuringian continental crust, which was partly subducted and partly relaminated underneath the overriding Tepla-Barrandian Domain.
However, our data, as well as data from mafic high-pressure rocks in the southern part of the Saxothuringian Domain, suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the Saxothuringian oceanic domain. After its subduction and relamination, the oceanic subduction was re-established and terminated by continental subduction and later collision of the Saxothuringian passive margin at c. 345-335 Ma.