T162 Granulite Terranes and Evolution of Continental Lower Crust: Insights from the Canadian Shield and Beyond II

Wednesday, 28 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

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

Paper number: T162-7
Program: Technical Sessions
Time: 6:55 PM – 7:05 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-359119

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