The Cadomian Orogen: Neoproterozoic to Early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton—Constraints from U–Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany)

Abstract

The Cadomian Orogen in the NE Bohemian and the northern Armorican Massifs shows a distinct orogenic zoning from recent NW to SE consisting of (i) an outboard sitting continental crustal unit comprising Neoproterozoic rocks associated with c. 2.0Ga old Icartian Basement, (ii) a magmatic arc and a back-arc basin, (iii) a foreland or retro-arc basin, and (iv) the passive margin of the back-arc basin. New U–Pb zircon ages of detrital zircon of Neoproterozoic to Fortunian siliciclastics from the Schwarzburg Antiform in the Saxo-Thuringian Zone (NE Bohemian Massif) identify the West African Craton as the hinterland for the Cadomian Orogen as demonstrated by zircon populations dated at 1.8–2.2, 2.5–2.7, 3.0–3.1, and 3.4–3.5Ga. The dominant zircon population (c. 50–70% in each sample) is derived from a Cadomian magmatic arc in a time slice of c. 570–750Ma. The magmatic activity of the Cadomian arc stopped at c. 570Ma. Closure of the back-arc basin by arc-continent collision occurred between c. 570 and 542Ma under the formation of a foreland (retro-arc) basin. A short-living remnant basin existed between c.542 and 540Ma. Granitoid plutonism at 539–540Ma documents the final pulse of the Cadomian Orogeny. Hf isotope compositions, calculated ɛHfi values and TDM model ages for detrital and magmatic zircon show that during the c. 180Ma long Cadomian magmatic arc activity juvenile arc magmas were contaminated by recycling of Eburnian and Archaean crust. Mixing with an evolved continental crust is always present. The inferred geotectonic setting is a continental magmatic arc during the Neoproterozoic developed on a stretched Archaean and Palaeoproterozoic (Eburnian) crust. In the West African crustal evolution it can be demonstrated that during Eburnian orogenic processes (c. 1.8–2.2Ga) in most cases a 2.5–3.4Ga old basement became reworked. Archaean 2.5–2.9Ga magmas remelted a 3.0–3.4.Ga crust. Zircon grains with an age of 3.0–3.1 and 3.4Ga are derived from juvenile magmas. Two zircon grains dated at 2779±22 and 3542±28Ma imply reworking of pre-existing Eoarchean to Hadean crust and show TDM model ages of 3.98 and 4.29Ga, respectively.