Caledonian ages in Variscan rocks: Rb-Sr and Sm-Nd isotopic variations in dioritic intrusives from the northwestern Bohemian Massif, West Germany

Abstract

Dioritic intrusives and granites from the northwestern Bohemian Massif, West Germany were analyzed for the chemical and isotopic composition of Rb-Sr and Sm-Nd. The Marktredwitz intrusive suite and the Tirschenreuth-Mähring sill intrusions yield apparent Rb-Sr whole-rock isochron ages of 468 ± 9 Ma and 470 ± 33 Ma, respectively, while the Reuth-Erbendorf granodiorites display a scatter with an apparent age of 545 ± 16 Ma. The Rb-Sr whole-rock ages conflict with geological evidence: the diorites intruded metamorphic country rocks which experienced an Early Devonian metamorphism. Contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement. The intrusion ages of two porphyritic granites from this area have been dated at 319 Ma and 311 Ma. The diorites display a variation in ϵ Nd (320 Ma) ranging from −1.0 to −4.4, whereas the two granites yield homogeneous initial ϵ Nd values of −3.7 and −6.6. The variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites. The mixing partners are a presumably mantle-derived magma and crustal components of varying composition. The isotope systematics of the diorites and the concordance of their Rb-Sr whole-rock age values with Caledonian rock-forming events in Central Europe point to a pre-intrusive Caledonian event inherited in these rocks. We suggest that the diorites were formed by multistage anatectic processes.