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Single zircon ages and whole-rock Nd isotopic systematics of early palaeozoic granitoid gneissess from the Czech and Polish Sudetes “jizerske hory, krkonoše mountains and orlice-sněžnik complex”

Abstract:

Granitoid orthogneisses make up the pre-dominant rock type in the West Sudetes from Jizerske hory in the NW to the Orlicke hory and Sněžnik Mountains in the SE. These generally strongly foliated gneisses are calc-alkaline in composition and display trace element characteristics suggesting generation in a volcanic arc setting. Single zircon ages reflecting the time of emplacement of the gneiss protoliths define a relatively narrow Cambro-Ordovician range between 502 and 515 Ma. This is similar to previously reported zircon ages from the Czech and Polish West Sudetes and documents an important and regionally extensive post-Cadomian magmatic event that we relate to continental arc magmatism on the margin of Avalonia that developed during closure of the Tornquist Ocean. An age of 492 Ma for a microgranite dyke cutting deformed and metamorphosed orthogneisses in the Orlicke hory shows the main deformation to be early Paleozoic. Zircon xenocryst minimum ages range between 546 and 2070 Ma and show maxima in the Cadomian/Pan-African “550-850 Ma” and Grenvillian “1000-1300 Ma” time brackets. The Grenvillian event is also evident from Nd mean crustal residence ages that vary between 1.34 and 1.87 Ga. From these data we suggest that the pre-Variscan granitoid gneisses of the Czech West Sudetes were largely generated by melting of a predominantly Grenville-age basement that was part of the Northern margin of Gondwana and may have been related to Greenville-age basement now identified in Northern South America.