Cambro–Ordovician anatexis and magmatic recycling at the thinned Gondwana margin: new constraints from the Kouřim Unit, Bohemian Massif

The Kouřim Unit represents one of the largest pre-Variscan metaigneous complexes in the Bohemian Massif and a geochronological, whole-rock geochemical and Sr–Nd isotopic study was conducted in order to better understand the magmatic evolution of the Early Palaeozoic Gondwana margin. Five orthogneisses give U–Pb zircon ages ranging from 492 ± 4 Ma to 484 ± 2 Ma. Two leucogranites give U–Pb zircon ages of 500 ± 4 Ma and 485 ± 2 Ma, interpreted to be inherited from their orthogneiss host rock. Two samples from the metasedimentary host rock are dominated by Neoproterozoic–Cambrian detrital zircons. The abundance of zircon inheritance in the orthogneisses and whole-rock Sr–Nd isotopic composition imply an origin from relatively matured continental crustal material. The subalkaline, subaluminous–slightly peraluminous and high-K calc-alkaline arc-like geochemical signature of the orthogneisses is interpreted as inherited from the recycled Cadomian metasedimentary source and both the magmatic and metasedimentary rocks are correlated with similar occurrences in the adjacent Moldanubicum and Teplá–Barrandian Unit. The Late Cambrian–Early Ordovician magmatic activity is linked to crustal anatexis, which was likely initiated by thermal and gravitational relaxation of the thickened Cadomian arc-type crust, followed by lithospheric thinning assisted by far-field forces. The extensional event led to the formation of a passive margin associated with the opening of the Rheic Ocean.