Zircon provenance of SW Caledonian phyllites reveals a distant Timanian sediment source
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Cambrian to middle Ordovician schists and phyllites in southwestern Baltica, now exposed in the (par-)autochthon to Lower Allochthon nappes of the Scandinavian Caledonides in southern Norway, contain previously unrecognized far-travelled detrital zircons with ages in the intervals 0.47–0.8 and 1.85–3.2 Ga and εHf in the range −27 to +18. These ages are assigned to Timanian and Fennoscandian Shield sources respectively and contrast with the locally derived detritus with zircon ages of c. 0.9–1.8 Ga and εHf values c. −13 to +10. The far-travelled zircons provide evidence that a steady, long-haul, source-to-sink drainage system existed from the northeastern fringe of Baltica to the SW passive margin across the whole palaeocontinent (c. 2000 km) since c. 521 Ma, and that the Timanian orogen shed detritus across large distances towards both its foreland (Baltica) and hinterland (Arctica). There are several arguments against an Avalonian source for these zircons. Recycling of the detrital zircon from the Cambrian to middle Ordovician sediments is probably responsible for the presence of Cryogenian to Middle Ordovician zircon ages in younger sedimentary sequences of southwestern Baltica. The development of an ophiolitic mélange associated with Ordovician phyllites underlying the Jotun Nappe Complex seems to mark the change to an active continental margin in the middle Ordovician, heralding the Caledonian orogeny. The study demonstrates that detrital zircon-poor fine-grained (siltstone to mudstone) sediments are an extremely valuable indicator for distal sources and favourably complement zircon-rich coarser sandstone in provenance analysis.