Magmatic history of granite-derived mylonites from the southern Desna Unit (Silesicum, Czech Republic)

摘要

In the southern part of the Hruby Jesenik Mts. (Silesicum, Bohemian Massif) granitic orthogneisses and quartz-feldspars mylonites occur that were variously deformed, metamorphosed and imbricated with the overlaying Devonian volcano-sedimentary complex during the Variscan orogeny. Based on combined mapping, petrologic and geochemical studies, three main rock groups are distinguished. The fairly primitive nature of the tonalite suite is shown by low ~(87)Sr/~(86)Sr_(550) ratios (0.7034-0.7038) and positive epsilon_(Nd)~(550) values (+3.8 and +3.1). Isotopic composition of the metagranite suite is similar (epsilon_(Nd)~(550) = +1.9 to +2.9). Both suites probably belong to a single Cadomian calc-alkaline tonalite-granite association, which can be correlated with acid-intermediate plutonic rocks of the eastern Brunovistulicum (Slavkov Terrane). All these units are interpreted as representing dismembered fragments of the same continental margin magmatic arc system. The leucogranite suite is most likely early Variscan in age. The initial Nd isotopic composition (epsilon_(Nd)~(330) = +0.8 and +2.3) is consistent with its formation by a Variscan remelting of the metaigneous Cadomian crust represented by the rocks of the tonalite-granite association. The within-plate granite affinity of the leucogranite suite is most likely related to the break-up of the Brunovistulicum during the Variscan orogeny.