International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Cambrian-Ordovician metaigneous rocks associated with Cadomian fragments in the West-Carpathian basement dated by SHRIMP on zircons: a record from the Gondwana active margin setting

Published: Feb 2008

Pages: 3 - 18


Abstract: The Central West-Carpathian basement, included in Cretaceous-Early Tertiary structures, shows remnants of pre-Alpine, mainly Variscan tectonics. The paper documents in-situ U-Pb zircon (SHRIMP) ages of a major Cambrian to Ordovician magmatic event, recorded in the West-Carpathian basement. Cambrian to Ordovician, less Early Silurian concordia magmatic ages, with older 525–470 Ma and younger 480–440 Ma age intervals were dated in layered amphibolites and granitic orthogneisses of the tectono-stratigraphically Upper Variscan structural Unit. The zircon cores, especially in orthogneisses, often show Cadomian, mainly magmatic source (protolith) ages, spanning 638–549 Ma, with a concordia age at 617±11 Ma. Older Proterozoic to Archean protolith ages (2000 up to 3400 Ma) are much less common. Cadomian ages highly predominate in kyanite-garnet orthogneisses, tectonically incorporated into the medium-grade metasediments of the Middle Variscan structural Unit. Kyanite-garnet orthogneisses show idiomorphic, magmatically zoned zircons with ages spanning 649–554 Ma, with concordia ages at 607±10 and 558±7 Ma, and metamorphic ages between 540–530 Ma. These ages corroborate the protolith ages of hosting metasediments with a lower intercept around 500 Ma. Early Ordovician ages were also found from the calc-alkaline metavolcanics (482±6 Ma, metarhyolite, or 476±7 Ma, metadacite) of low-grade volcano-sedimentary complexes in the Lower Variscan structural Unit. Long-term magmatic/volcanic activity due to melting of the crust and subcrustal lithosphere is coeval with the rifting of Cadomian crust and evolution of the Gondwana northern active continental margin. The ages of 430–380 Ma of layered amphibolites and orthogneisses reflect the Eo-Variscan subduction/collision metamorphic event in the Prototethyan realm, coeval with the collision of the eastern Avalonian-Cadomian/western Hunic terranes, as part of the Armorican microplate, with Laurussia/Avalonia. The collapse of the Eo-Variscan collisionally thickened crust led to partial melting within the Upper Variscan structural Unit, generating 370–340 Ma old zircons in dated metaigneous rocks.

Keywords: Proterozoic/Archean, Paleozoic, Cadomian, Gondwana, West-Carpathian basement, SHRIMP zircon dating

Download PDF document