JURASSIC TO MIOCENE MAGMATIC EVOLUTION OF THE NORTHERN ANDES: A HAFNIUM ISOTOPE PERSPECTIVE

The Northern Andes represent a complex segment of the Andean orogen where accretionary processes overlapped with deformation and exhumation along the ocean-continent convergent plate boundary. Late Cretaceous accretion of oceanic crust to continental South America impacted the composition and location of the magmatic arc. Detrital zircon U-Pb geochronological results on 35 samples from Cretaceous to Miocene sedimentary rocks in the forearc, Interandean Valley, and Oriente foreland basin of Ecuador characterizes key phases of magmatic activity from 180 to 10 Ma, including peaks at ca. 180, 155, 125, 75, 60, 45, 30, and 15 Ma. The temporal and geochemical evolution of this Mesozoic-Cenozoic magmatism can be derived from hafnium isotopes, which provide a proxy for crustal evolution through time. Here, new hafnium isotope data for 152 detrital zircons are combined with published bedrock and detrital hafnium data from Colombia and Ecuador. We identify key relationships between magmatic age and hafnium values, which are compared to the reconstructed position of the magmatic arc through time. From ca. 190-115 εHf values become progressively more juvenile, and arc activity migrated westward, consistent with upper-plate extension. At ~73 Ma, coeval with posited accretion of oceanic crust and the onset of Andean shortening in Ecuador, there is a rapid shift from juvenile (εHf= +13) to evolved (εHf= -5) isotopic signatures, suggesting the assimilation of continental crust. From ca. 40-20 Ma, there is a shift to juvenile εHf values (ranging from +4 to +15) and a corresponding arc retreat which we interpret to represent a magmatic flareup linked to slab rollback. Finally, from ca. 20 Ma onward, εHf decreased as the magmatic arc migrated eastward, likely reflecting slab shallowing, and associated crustal recycling. Together these results demonstrate a fluctuating arc system that has migrated over ~200 km, variably intruded both accreted oceanic and continental crust, and significantly shifted in isotopic composition through time.