The Central-Sudetic ophiolites (NE Bohemian Massif) and their geodynamic setting compared with Devonian ophiolites of the Variscan suture in Europe

Abstract

The Central-Sudetic Ophiolites (CSO) are located in the Sudetes constituting the NE fragment of the Bohemian Massif, one of Variscan basement outcrops in Central Europe. The CSO involve the Ślęża, Braszowice-Brzeźnica, Szklary and Nowa Ruda massifs that are dated at 404.8 ± 0.3 – 401.2 ± 0.3 Ma (Awdankiewicz et al., 2020). These massifs display highly depleted, harzburgite mantle sections containing gabbroic dykes and local occurrences of mostly isotropic, large gabbroic bodies as well as volcanic rocks. The ultramafic rocks locally show melt percolation-derived clinopyroxene-olivine aggregates and chromitites. The low REE composition and depletion in LREE relative to HREE of the clinopyroxene as well as the chromite Cr# and Mg# values point to phases formed from refractory melts occurring in the supra-subduction zone environment. The gabbroic bodies consist of differently evolved, mostly cumulate rocks, while the volcanic rocks form a relatively monotonous basalt sequence. The trace element compositions of both the plutonic and volcanic rocks display depleted N-MORB affinities, their derivation from a refractory mantle source is further reflected by depleted mantle-like Sr-Nd isotopic compositions. The ultramafic and mafic members of the CSO show greenschist- to lower amphibolite facies metamorphic overprints.The CSO represent an ancient supra-subduction-type oceanic lithosphere that formed in a slow- to intermediate spreading regime. The lithosphere of the CSO is heterogeneous and lacks the structure of a typical layered ophiolitic complex, but rather resembles that of slow spreading oceanic complexes with gabbroic bodies formed due to local magma injections. Melt percolation phases in ultramafic member as well as plutonic and volcanic rocks of the CSO display geochemical signatures accounting for their derivation from a refractory mantle source, typical of N-MORB-type melts depleted in supra-subduction zone settings but lacking subduction-related enrichment. These rocks of the CSO are believed to have formed in a mature, intra-oceanic back-arc basin. Chemical affinities between the CSO and other Devonian ophiolites belonging to the Middle Allochthon (for instance Careón ophiolite in the NW Iberian Massif, Spain or Tisoviţa Iuţi ophiolitic massif in Romania) confirm that a typical MORB-type lithosphere is absent in the European Variscides. Therefore, these ophiolites are thought to constitute fragments of lithosphere that were generated in supra-subduction-zone domains during the amalgamation of Pangea.Reference:Awdankiewicz, M., Kryza, R., Turniak, K., Ovtcharova, M., Schaltegger, U., 2020. The Central Sudetic Ophiolite (European Variscan Belt): precise U-Pb zircon dating and geotectonic implications. Geological Magazine 158, 555–556.