Geochronology and geochemistry of Cenozoic magmatic intrusions in the north-western Ecuadorian Andes: the role of crustal thickness

The Ecuadorian Cenozoic arc developed upon autochthonous continental and allochtonous accreted oceanic terranes. It provides a unique opportunity to explore the processes governing arc magmatism and crustal evolution. Using a multi-proxy approach, combining zircon petrochronology (U-Pb geochronology, trace element geochemistry, and isotopic analysis) with whole-rock geochemistry, we trace the tectono-magmatic evolution of the northernmost segment of this arc.

Our results define two distinct magmatic episodes: ~41–16 Ma and ~14–7 Ma. The older episode comprises tonalitic rocks exhibiting zircon δ¹⁸O (6.4 – 3‰) and εHf values (+17 – +12), as well as trace element ratios, indicating derivation from juvenile sources. Magma genesis during this period is believed to occurred within the amphibole stability field, in a moderately thick crust (~35 km). In contrast, the younger episode is dominated by granodioritic rocks derived from more enriched reservoirs (δ¹⁸O: 8.2 – 5.8‰ and εHf: +13 – +7). Zircon and whole-rock trace element and isotopic data suggest magma genesis in the garnet stability field, within a thickened crust (~60 km).

The transition to a thicker crust and enriched sources occurred around 14 Ma, coinciding with the tectonic reorganization associated with the arrival of the young (and buoyant) Nazca plate at the South American margin. This event likely induced a shallower subduction angle, increased compressional stresses, and facilitated melting of an evolved oceanic crust.

These findings highlight the dynamic interplay between tectonics, crustal processes, and magmatic evolution in shaping Cordilleran arcs. They also demonstrate the efficacy of zircon petrochronology as a tool for resolving crustal-scale processes, providing insights into the mechanisms driving continental growth and orogenesis.