It is widely held that a large composite terrane including the East Siberian shelf, Northern Chukotka and Arctic Alaska moved to its current location from the other side of the Mesozoic ocean in the course of the Amerasia basin opening. The western boundary of this terrane is critical for tectonic models of the Mesozoic Arctic. Consequently, the New Siberian Islands, which lie in the centre of this critical region, should help to define the position of this boundary. We have studied subvolcanic gabbroic intrusions on Bel'kov Island, the westernmost island in the New Siberian Archipelago. Their age is 252 ± 2 Ma (U–Pb zircon, secondary ion microprobe technique) and they represent mantle-derived tholeiitic magma modified by continental crust contamination. Their age and petrographic and geochemical features are identical to those of the Siberian traps, suggesting that in Early Mesozoic time the western New Siberian Islands were adjacent to northern Siberia (modern coordinates). This indicates that the large composite terrane that includes the western New Siberian Islands is not exotic to Siberia as is currently implied in some Arctic Mesozoic tectonic reconstructions.