U–Pb zircon ages and structural development of metagranitoids of the Teplá crystalline complex: evidence for pervasive Cambrian plutonism within the Bohemian massif (Czech Republic)

Abstract

U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Tepla Barrandian unit (Tepla crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Tepla orthogneiss define an upper intercept age of 513 +7/–6 Ma. The 207Pb/206Pb ages of 516±10 and 511±10 Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Tepla Barrandian unit (Domažlice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Tepla orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Tepla orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.