Timescales for magma-storage and re-melting of mushes in the Gangdese active continental margin

• High spatial resolution laser ablation U-Pb zircon dating afforded by depth profiling indicates >50 Myr duration of batholith crystallization. • This duration, deduced from zircons from single samples, implies a history of mush crystallization followed by re-melting. • Preservation of zircons suggest mush temperatures remained below zircon saturation for long periods. • Long-lived mushes were sustained by a regional flare-up providing heat and H 2 O to the system. • Batholith intrusion also triggered the anatexis of its host rock that also lasted tens of million years. Accurate constraints on magma crystallization timescales at convergent margins are critical for understanding arc thermal evolution and crustal differentiation. Prolonged zircon crystallization beyond single magmatic pulse timescales challenge rapid magma cooling models, yet how the durations of crystallization are recorded by zircons remains unclear. To clarify this, we dated zircons from the lower crustal Gangdese arc including the Cretaceous Lilong batholith and its Jurassic tonalite host rocks, both remelted into migmatites. High-resolution depth-profile dating via laser ablation-inductively coupled plasma-mass spectrometry resolved multiple date domains within individual zircons from rims towards the core, with an average width of 12 μm, providing precise reconstruction of magmatic timescales. Combining dates with chemistry of different date domains, we found that the younger Lilong batholith experienced ∼40 Myr of mush crystallization and subsequent re-melting for ∼30 Myr, with a 10 Myr overlap. The older host Jurassic tonalite records ∼50 Myr long crystallization and ∼40 Myr of re-melting triggered by the Lilong batholith intrusion, with a 10 Myr hiatus. These findings record a repetitive process of intrusion and re-melting over 100 Myr, indicating deep arc roots can remain near solidus conditions in a mushy state for tens of millions of years, causing prolonged zircon crystallization recorded in individual rock samples. These mushy magma reservoirs may either crystallize or be re-melted by heat and aqueous fluids flow into the crust during regional magmatic flare-ups. Long-lived lower crustal mushes allow extraction of evolved melts from increasingly residual cumulates, and may occur throughout the lifespan of an arc driving crustal differentiation.