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The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Paleozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 +/- 3 Ma (2 sigma) and 397 +/- 3 Ma (2 sigma) for two cale-alkaline metarhyolite samples indicate are volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 +/- 2 Ma (2 sigma). Geochemical and Nd isotopic data for low-grade malic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc-back-arc setting. In addition, there are high- and low-grade felsic metaigneous rocks with subduction-related chemical signatures and heterogeneous Nd isotopic compositions indicating an origin from mixed sources, including juvenile and Precambrian crustal components. The field observations combined with the geochemical and isotopic data suggest that the Tseel terrane represents a segment of a mature Japan-type are associated with a juvenile back-arc basin.