Zircon inheritance in diatexite granodiorites and its consequence on geochronology—a case study in Lusatia and the Erzgebirge (Saxo-Thuringia, eastern Germany)

Abstract

We used detailed investigations of zircon morphology and internal structures for the interpretation of zircon age data of diatexite migmatites. The fine- and medium-grained varieties of Cadomian granodiorites from Lusatia and the Erzgebirge are interpreted on the base of their geological, geochemical and isotope characteristics as well as zircon populations as migmatites, where no segregation of melt from residuum occurred. Whereas, coarse-grained varieties contain a large proportion of new-formed zircons with a limited age range (540–530 Ma), most zircons in medium- and especially fine-grained granodiorites seem to be inherited from their source rock with only a small proportion of new zircon material in form of rare single grains or overgrowths. Therefore, we consider mean evaporation zircon ages from these rocks as “mixed ages”. Consequently, the formerly inferred age span of magmatic activity (from 587±17 to 542±9 Ma) and the interpretation of three successive magmatic episodes (Kröner et al. [Geol. Rundsch. 83 (1994) 357] reported values of ∼585, ∼563 and ∼542 Ma) have to be reconsidered as a mixture of two successive magmatic events: (i) the late Cadomian event (about 540–530 Ma) that formed the granodiorites; and (ii) a former magmatic event (about 580–570 Ma) in the source indicated by inherited zircons.