Boron isotope insights into the origin of subduction signatures in continent-continent collision zone volcanism

•First δ11B ratios for young post-collisional volcanic rocks gives δ11B of −5 to +2‰.•Sr-Nd-B isotopes are distinct from both arcs and ocean ridges/islands.•Boron inherited from previous subduction and stored in amphibole for >40 Myr.•The slab component is dominated by sediment melts, rather than aqueous fluids.