Geochronology and Nd-Sr systematics of Lusatian granitoids: significance for the evolution of the Variscan orogen in east-central Europe
AbstractA variety of pre-Variscan granitoids and two Variscan monzogranites occurring in the central and western parts of the Lusatian Granodiorite Complex (LGC), Saxonia were dated by the single zircon evaporation method, complemented by whole rock Nd isotopic data and Rb-Sr whole rock and mineral ages. The virtually undeformed pre-Variscan granitoids constitute a genetically related, mostly peraluminous magmatic suite, ranging in composition from two-mica granodiorite, muscovitebearing biotite quartz diorite (tonalite) and granodiorite to biotite granodiorite and monozogranite.
207Pb/206Pb isotopic ratios derived from the evaporation of single zircons separated from 13 samples representing the above rock types display complex spectra which document significant involvement of late Archaean to late Proterozoic continental crust in the generation of the granitoid melts. Mean 207/Pb/206Pb ages for zircons considered to reflect the time of igneous emplacement range between 542 ± 9 and 587 ± 17 Ma, typical of the Cadomian event elsewhere in Europe, whereas zircon xenocrysts yielded ages between 706 ± 13 and 2932 ± Ma. Detrital zircons from greywackes intruded by the granitoids and found as xenoliths in them provided ages between 1136 ± 22 and 2 574 ± Ma. Rb-Sr whole rock data display good to reasonable linear arrays that, with one exception, correspond to the emplacement ages established for the zircons. Two post-tectonic Variscan monzogranites yielded identical 207/Pb/206Pb single zircon ages of 304 ± 14 Ma and record the end of Variscan granitoid activity in the LGC.The variations in Nd and Sr isotopic data of the Cadomian granitoids are consistent with an origin through the melting and mixing of Archean to early Proterozoic crust with variable proportions of mantle-derived, juvenile magmas. Such mixing may have occurred at the base of an active continental margin or in an intraplate setting through plume-related magmatic underplating. The LGC is interpreted here as a Cadomian (Pan-African) terrane distinct from adjacent Variscan and pre-Variscan domains, the origin of which remains obscure and which probably became involved in Palaeozoic terrane accretion late in the Variscan event.