Petrogenesis and Tectonic Evolution of Cenozoic Magmatism in the Southernmost Northern Andes: Insights From Zircon Petrochronology

This study integrates new zircon U–Pb ages, zircon εHf (t) –δ 18 O isotopic data, and whole‐rock geochemistry to refine the temporal and spatial framework of magmatism in the southern Northern Andes (SNA) from ∼65 to 6 Ma. The San Lucas, Catamayo, Portachuela, and Gonzanamá batholiths and magmatic suites, along with Late Miocene plutons, display emplacement patterns strongly controlled by pre‐existing crustal architecture. Early San Lucas magmatism (65–63 Ma) most‐likely intruded into thick continental crust (∼60 km) and exhibited enriched isotopic signatures (εHf (t) : −5 to +5 and δ 18 O: 6‰–12‰), while later pulses (54–49 Ma) intruded through thinner crust (∼30 km) and exhibited less enriched compositions (εHf (t) : +4.3 to +6.5 and δ 18 O: 5‰–6.5‰). West of the Las Aradas Fault, the Catamayo (36–32 Ma) and Gonzanamá (21–17 Ma) magmatic suites display juvenile signatures (εHf (t) : +7 to +14 and δ 18 O: 3.7‰–6‰), reflecting their emplacement into a thin oceanic‐type basement. East of the fault, the early pulse of the Catamayo suite (39 Ma) intruded thicker crust (∼60 km) and recorded more evolved signatures (εHf (t) : +4.7 to +7). Magmatism in the Portachuela (∼32–16 Ma) derives from enriched mantle sources, possibly influenced by limited sediment subduction (εHf (t) : −1.3 to +6.7 and δ 18 O: 3.5‰–6.7‰). Some Late Miocene plutons (13–6 Ma) exhibit adakitic signatures as well as high‐pressure fractionation and isotopic enrichment (εHf (t) : +0.7 to +7 and δ 18 O: ∼4‰ to 6.7‰) that are consistent with crustal thickening. Collectively, these results demonstrate that SNA magmatism reflects two contrasting reservoirs across the Las Aradas Fault, underscoring strong spatial controls and challenging models of large‐scale arc migration by highlighting the roles of mantle input, basement heterogeneity, and variable crustal thickness.