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Abstract: Magmatic zircons from the basal part of an arc terrane in the accretionary complex NE of the Baydrag microcontinental block in central Mongolia were dated at 544±7Ma and reflect arc magmatism at the Ediacaran/Cambrian boundary. Detrital zircon ages for clastic metasediments of the adjacent Dzag zone suggest a depositional age <445±6Ma and suggest that the zircons were not derived from the arc terrane which was probably not exposed in the Late Ordovician. The detrital age pattern is similar to the overall pattern for Mongolian detrital and xenocrystic zircons and suggests significant input from a Grenville-age source that could either be the Tarim craton or microcontinental blocks within the central Asian collage of arcs and microcontinents. [Copyright &y& Elsevier]