301 Geochronology (Posters)

Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Eocene to Oligocene continental arc migration in Ecuador and the origin of temporal trends in zircon Lu-Hf radiogenic isotopes

Paper number: 301-9
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

The modern continental arc system in Ecuador is well understood, but less is known about arc systems in the past, hindering our understanding of the tectonic evolution of the northern Andes. The Macuchi arc, located in the Western Cordillera of Ecuador, was thought to be the main continental arc system during the Eocene, but new zircon U-Pb LA-ICPMS dates from intrusive arc rocks in central Ecuador indicate a more complex history.  During the mid-Eocene, the footprint of arc magmatism broadened and expanded into the Inter-Andean Valley and Eastern Cordillera.  During the late Eocene to Oligocene, the arc migrated back toward the trench at ~4 km/Myr, which is interpreted to be due to steepening of the subduction angle and/or slab rollback.  New zircon Lu-Hf LA-ICPMS isotopic data and a compilation of previously published radiogenic isotope data show a clear spatial isotopic trend in Ecuador.  Igneous rocks erupted or emplaced in the Western Cordillera, in Caribbean plateau basement, have strongly juvenile isotopic compositions that approach depleted mantle values.  Magmatic rocks located farther from the trench have more evolved compositions.  This spatial isotopic trend can be observed in all Cenozoic igneous rocks regardless of specific age, including in the Quaternary to recent volcanic arc. Temporal trends in zircon εHf data from modern rivers and sedimentary deposits reflect the migration of the arc through the spatial isotopic trend.  Zircon εHf values exhibit a broader range of values and an overall shift to more evolved composition when the arc broadened into the Inter-Andean Valley.  As the arc migrated back toward the trench and back into oceanic plateau basement, zircon εHf values shifted to more juvenile compositions.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413067

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