Time scales and mechanisms of growth of active margins of Gondwana: A model based on detrital zircon ages from the Neoproterozoic to Cambrian Blovice accretionary complex,Bohemian Massif |
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| Institution: | 1. Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague 12843, Czech Republic;2. Institute of Geosciences, Goethe University, Altenhöferallee 1, Frankfurt am Main 60438, Germany;1. Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Wroc?aw INGPAN, Podwale St. 50-449, Wroc?aw, Poland;2. Centre of Isotopic Research, All-Russian Geological Research Institute, Sredny prospect 74, 199 106 St. Petersburg, Russia;1. Institute of Geological Sciences, University of Wroclaw, 50-204 Wroclaw, Pl. Borna 9, Poland;2. John de Laeter Centre for Isotope Research, Curtin University, GPO Box U1987 Perth, Western Australia 6845, Australia |
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| Abstract: | New U–Pb detrital zircon ages from (meta-)graywackes of the Blovice accretionary complex, Bohemian Massif, provide an intriguing record of expansion of the northern active margin of Gondwana during late Neoproterozoic and Cambrian. The late Neoproterozoic (meta-)graywackes typically contain a smaller proportion of Archean and Paleoproterozoic zircons and show a 1.6–1.0 Ga age gap and a prominent late Cryogenian to early Ediacaran age peak. The respective zircon age spectra match those described from other correlative Cadomian terranes with a West African provenance. On the other hand, some samples were dominated by Cambrian zircons with concordia ages as young as 499 Ma. The age spectra obtained from these samples mostly reflect input from juvenile volcanic arcs whereas the late Cambrian samples are interpreted as representing relics of forearc basins that overlay the accretionary wedge.The new U–Pb zircon ages suggest that the Cadomian orogeny, at least in the Bohemian Massif, was not restricted to the Neoproterozoic but should be rather viewed as a continuum of multiple accretion, deformation, magmatic and basin development events governed by oceanic subduction until late Cambrian times. Our new U–Pb ages also indicate that the Cadomian margin was largely non-accretionary since its initiation at ~ 650–635 Ma and that most of the material accreted during a short time span at around 527 Ma, closely followed by a major pulse of pluton emplacement. Based on the new detrital zircon ages, we argue for an unsteady, cyclic evolution of the Cadomian active margin which had much in common with modern Andean and Cordilleran continental-margin arc systems. The newly recognized episodic magmatic arc activity is interpreted as linked to increased erosion–deposition–accretion events, perhaps driven by feedbacks among the changing subducted slab angle, overriding plate deformation, surface erosion, and gravitational foundering of arc roots. These Cadomian active-margin processes were terminated by slab break-off and/or slab rollback and by a switch from convergent to divergent plate motions related to opening of the Rheic Ocean at around 490–480 Ma.The proposed tectonic evolution of the Teplá–Barrandian unit is rather similar to that of the Ossa Morena Zone in Iberia but shows significant differences to that of the North Armorican Massif and Saxothuringian unit in Western and Central Europe. This suggests that the Cadomian orogenic zoning was complexly disrupted during early Ordovician opening of the Rheic Ocean and Late Paleozoic Variscan orogeny so that the originally outboard tectonic elements are now in the Variscan orogen's interior and vice versa. |
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