H₂O-fluxed remelting of arc roots drives continent maturation and the Daly Gap

Granitic plutons are key components of the continental crust, resulting from the interplay between fractional crystallization (FC) of mantle melts and crustal anatexis. Field and geochemical evidence suggest that remelting of arc rocks in convergent margins, formed through FC of primitive arc magmas, plays a crucial role in granite formation. The chemical and temporal framework of deep crustal remelting, however, remains poorly constrained. Here, we show that voluminous granitic rocks, exposed as a ~ 1200 km² batholith in the Gangdese arc, Tibet, originated from a ca. 20 Myr-long H₂O-fluxed remelting or defrosting of newly crystallized mafic arc roots at ~12 kbar. Thermodynamic modeling indicates that this occurred at low temperatures (<850 °C), with 4-10 wt.% H₂O, generating large melt fractions and voluminous granitic melts. Crucially, remelting in a H₂O-rich environment does not produce voluminous residues, thus relaxing the volume constraints related to FC models. Remelting also explains the scarcity of intermediate arc rocks, known as the Daly Gap. Our findings demonstrate that aqueous fluids inflow into continental arcs triggers substantial lower crustal remelting, driving intra-crustal differentiation. This crustal maturation process and the genesis of the Daly Gap are thus intrinsic to subduction zones.