Vestige of an Early Cambrian incipient oceanic crust incorporated in the Variscan orogen: Letovice Complex, Bohemian Massif

The Letovice Complex, composed of metamorphosed enriched mid-ocean ridge basalts with trondhjemitic sheets, as well as gabbroic and ultrabasic bodies, is exposed within an important Variscan suture separating the Moldanubian Domain (to the west) from the Brunovistulicum (to the east) in the eastern part of the Bohemian Massif, Czech Republic. A polyphase tectonic evolution of the study area was connected with underthrusting and subsequent exhumation of the former oceanic realm, which resulted in three deformation phases (D 1 –D 3 ). New U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircons from amphibolite and trondhjemite yielded statistically identical concordia ages of 530 ± 6 Ma and 529 ± 7 Ma (2σ), respectively. These are interpreted as dating intrusions of the Early Cambrian protoliths of the studied rocks. Based on this age as well as whole-rock geochemical and Nd isotopic data, the Letovice Complex is interpreted as a vestige of an incipient oceanic basin developing on attenuated crust. The most likely geotectonic setting was a post-Cadomian extensional regime at the northern margin of Gondwana that marked the onset of its break-up. Supplementary material: Photomicrographs of microstructures, sample locations, descriptions of analytical techniques, and selected electron microprobe analyses of the main rock-forming minerals are available at http://www.geolsoc.org.uk/SUP18424.