Cambrian Zawidów granodiorites in the Cadomian Lusatian Massif (Central European Variscides): what do the SHRIMP zircon ages mean?

Abstract

The Lusatian Massif in the Central European Variscides, composed of upper Neoproterozoic (c. 570 Ma) greywacke intruded by c. 540 Ma old Cambrian granodiorites (and the somewhat younger Zawidow granodiorites), constitutes a fragment of the Cadomian basement of the Saxo-Thuringian Zone. The Lusatian Massif adheres on the east to the Karkonosze-Izera Massif composed of the c. 500 Ma Izera/Rumburk granites related to the Cambro-Ordovician rifting of the Cadomian basement, and narrow belts of micaschists. Trace-element and Sm-Nd isotope data suggest that the source rocks for the Lusatian greywacke, the Zawidow granodiorite and the Izera/Rumburk granite could have been similar, though not the same. The new SHRIMP U-Pb zircon data for the Zawidow granodiorite reveal, apart from the expected c. 540 Ma ages, three other zircon age populations of around 630 Ma, 600 Ma and 510 Ma, the latest being evidently younger than 540 Ma that was considered as the age of the granodiorite magma emplacement. Zircons with 206Pb/238U ages around 510 Ma, and somewhat older (up to c. 538 Ma), have also been reported from the Izera granites. This could mean that the granitic plutonism related to the Cadomian orogenic cycle and the Cambro-Ordovician rifting triggered two or more magmatic pulses during at least c. 30 My. During the prolonged period of igneous activity, the plate-tectonic environment at the Gondwana margin changed from collisional (Cadomian Orogeny), to initial rifting (Cambro-Ordovician).