The plutonic rocks of the Elbe valley-zone (Germany): evidence for the magmatic development from single-zircon evaporation and KAr age determinations

Abstract

Plutonic bodies of different geochemical characteristics and probably of different origin occur within the Elbe valley-zone (E-Germany): The complex of the so-called Meißen massif consists, from rim to core, of a nearly concentric sequence of diorite to monzodiorite, monzonite and some granite intrusions. Field-geological arguments point for an emplacement during the Variscan orogenic cycle. In contrast, due to the lack of geochronological data, the time of intrusion of the granodiorites from Dohna and Laas, situated in the southeast and northwest of the massif, is still under discussion. According to 207 Pb 206 Pb ages of ∼355±12 Ma, produced by the single-zircon evaporation technique (all errors are given at the 1σ level), the pyroxene-monzodiorite from Gröba represents an early stage of the Variscan plutonic activity. It is followed by different monzodiorites and monzonites of the Meißen massif. Inherited zircon relics of ∼ 399–429 Ma are comparable with similar zircon ages of some granulites and gneisses of the Saxothuringian Zone. Single-grain determinations on zircons of the main granodioritic to monzogranitic intrusive rocks of the Meißen massif gave only relic ages varying between 573 and 2222 Ma. These results are in agreement with the microscopic recognition of voluminous cores in the majority of the zircons. Based on KAr analyses, the cooling of the main granitic intrusion below 350±50°C took place at ∼326±6 Ma. 207 Pb 206 Pb isotope ratios, measured on 13 zircon grains, yield implications for a Cadomian emplacement of the Granodiorites from Dohna and Laas. 550 Ma are interpreted as the most reliable minimum age of their intrusion. In the case of Laas, a scattering of the data between 552 and 634 Ma may be related to minor inherited components, although no such indications were detected by microscopic investigations. However, a second group of zircons is characterized by the predominance of relictic domains showing strong crustal influences on these Cadomian intrusions.