Geochemistry of Radiolarian Cherts from a Late Devonian Continental Margin Basin, Loei Fold Belt, Indo-China Terrane

More than 42 species of well-preserved Upper Devonian radiolarians have been obtained from cherts and siliceous shales in four sections (Chom Noi, Phu Kham Phe (P), Phu Kham Phe (C) and Pha Samyod) from the NE Thailand sector of the Loei fold belt, Indochina terrane. They include Ceratoikiscum sp. cf. planistellare Foreman, Helenifore laticlavium Nazarov & Ormiston, Astroentactinia sp. cf. paronae (Hinde), Stigmosphaerostylus herculea (Foreman), S. pusilla (Hinde), S. cf. vulgaris (Won), Trilonche davidi (Hinde), T. echinata (Hinde), T. elegans Hinde, T. hindea (Hinde), T. palimbola (Foreman), T. vetusta Hinde, Polyentactinia polygonia Foreman, Spongentactinella sp. cf. windjanensis Nazarov, Archocyrtium sp.?, Palaeoscenidium sp., Triaenosphaera sp., and others. Radiolarian faunas from these sections indicate a range from lower Frasnian to Famennian. Analyses of rare earth, trace and major elements suggest that the Upper Devonian chert and siliceous successions from Loei were deposited in a continental margin environment near to a volcanic arc. These successions contain distinctive weak negative Ce anomalies with strong positive Eu anomalies which differ from Upper Devonian cherts reported from northern Thailand, the Truong Son fold belt (Laos) and from South China. Negative Ce anomalies and low to moderate Lan/Cen ratios imply proximity between the Chom Noi and Phu Kham Phe (C), which were deposited in more distal parts of the depositional basin than the Pha Samyod and Phu Kham Phe (P) samples. Most of the Pha Samyod and Phu Kham Phe (P) samples exhibit high continental-derived trace element levels including Cr, Zr, Hf, Rb, Th and in part V, which supports close proximity to terrigenous sources. Paleozoic stratigraphic sequences of the Loei fold belt and geochemical results suggest that the Upper Devonian deep marine sequences in the Loei fold belt were deposited within a rifted, continental-margin basin, possibly a back-arc basin and not in a large oceanic basin as has been previously suggested.