The Mongolia is a key part of the Central Asian Orogenic Belt (CAOB) of East Asia. The Ulaanbaatar terrane forms part of the COAB, and lies within the Hentey sub-basin of the larger Hangay-Hentey basin. The terrane is mainly composed of Silurian–Carboniferous sediments. Whole-rock XRF analyses were made of a suite of turbidite sandstones and mudrocks to determine their provenance signatures. Geochemical classification and Index of Compositional Variability (ICV) values show the sediments are immature, as also indicated by moderate SiO2 contents (all <70 wt%). Most trace elements (As, Ga, Cu, La, Nb, Ni, Pb, Rb, Sc, Th, V, Y and Zn) show positive correlation with increasing Al2O3 and lower abundances in sandstones than in mudrocks, reflecting hydrodynamic separation of quartz and clays. Source composition ranged from intermediate (dacite) to felsic (rhyolite– rhyodacite and plutonic equivalents), as indicated by major element abundances and immobile trace element ratios (Ce/Sc, Ti/Zr, Th/Sc, Zr/Sc). Maximum Chemical Index of Alteration (CIA) values are very low (CIA=65) indicative of minimal source weathering. Deposition occurred at a subduction margin setting, probably an evolved continental island arc, in either a fore-arc or back-arc environment. The Ulaanbaatar terrane is compositionally similar to the Tsetserleg terrane in the Hangay sub-basin, although chemical variability is less and CIA values are a little lower. This similarity suggests a common source for both terranes.