Ar-40/Ar-39 isotopic evidence for mid-devonian post-metamorphic pegmatite emplacement in the Marianske Lazne Complex, Bohemian Massif, Central European Hercynides

Large (2 - 3 cm) crystals of ferroan pargasite in a narrow pegmatite vein that cross-cuts amphibolite in the Marianske Lazne Complex yield Ar-40/Ar-39 regression ages of 378.7 +/- 3.5 and 379.5 +/- 4.3 Ma: these date the time of cooling through 550-500degreesC soon after emplacement and crystallization. With this and associated veins also cutting flexural-type folds and a fracture cleavage, both tectonothermal activity in, and uplift of the host metamorphic assemblage into the upper crust must have taken place by, or near, a time corresponding to the Eifelian-Givetian boundary (mid-Devonian). The age of pegmatite formation corresponds, within errors, with the time of closure of the Sm-Nd isotopic system in eclogites elsewhere in the domain (377 +/- 7 Ma). This points to the crustal uplift having been rapid. Isotopic evidence also indicates a considerable time gap between the tectonothermal - igneous activity so prominently expressed in the Marianske Lazne Complex and the superimposed effects of early Carboniferous tectonothermal activity.