CRUSTAL GROWTH AT DEPTH: TRACING THE EVOLUTION OF SUBDUCTION RELATED MAGMAS THROUGH MULTIPLE STAGES OF MAGMATISM USING HF ISOTOPES AND TEXTURAL RELATIONS, CHELAN MIGMATITE COMPLEX, NORTH CASCADES, WASHINGTON, USA

The Chelan Migmatite Complex (CMC), located in central Washington, exposes evidence of crustal growth at depth (~30km) associated with the evolution of subduction-related magmas in a continental arc setting. Zircon U-Pb ages, Hf isotopes, trace element signatures, and a detailed roadcut map record multiple stages of magmatism and partial melting. The oldest zircons record Jurassic-Triassic (~160-250 Ma) ages and εHf values (10-13) are consistent with juvenile arc-generated magmas forming the earliest crust in this area. Field relations and widespread inherited zircons demonstrate that this Jurassic-Triassic crust was remelted in the Cretaceous (~90-110 Ma) in conjunction with multiple pulses of mafic magmas. Around 100 Ma a pulse of magma (~57 wt.% SiO₂) records εHf values clustered between 2-6. The drop in εHf and lack of correlation between εHf and whole-rock SiO₂ suggests the low εHf signal is likely from enriched mantle, rather than a result of crustal assimilation. The texturally youngest samples cut most leucosomes, are unfoliated, have higher εHf (6-9), and record ages between 85-90 Ma, suggesting a shift back to a more juvenile magma source. Based on these findings, the multi-stage history of crustal growth in the CMC involved recycling of older crust and magma mixing with heterogeneous mantle input over a >30 Myr period. The specific cause of the heterogeneity in the mantle is unclear, but the pulse of low εHf magmas may indicate the arrival/increase of cratonic-derived sediments at the trench or changes in subduction conditions that enhanced sediment-derived input to the mantle wedge. Further work should look for the regional extent of this signal to evaluate the role of changes to the tectonic scenario and/or sediment source in driving the Cretaceous crustal growth pulse.