Geodynamic controls at the southernmost Northern Andesmagmatic arc: geochemical and geophysical evidence

National audience; Understanding how magmatic arcs develop is crucial for the comprehension ofsubduction systems and the processes leading to continental growth and lithosphericrecycling. Chronologic and provenance from U-Pb single grain dating by LA-ICP-MS ofdetrital zircons obtained from the forearc basin paired with Hf and O isotopic data andtrace elements have provided indicators of the Late Cretaceous-Cenozoic evolution ofthe Ecuadorian magmatic arc. A major Late Cretaceous accretionary event (75-65 Ma)is marked by large isotopic variations (Hf>20 and 18O>8) attesting for intensemagmatism resulting from melting of lower and upper crust materials. Highlyfractionated signatures in trace elements and Eu/Eu* paired with 18O mantle-like andHf juvenile values prevailed up to 45 Ma, suggesting that the arc was most likelyemplaced in an over thickened crust. Subsequently, negative isotopic incursions from45 to 30 Ma are defined by mantle sources as well as drops in the U/Yb, Eu/Eu* andTh/U ratios laying rough indicators of a broad extensional magmatic arc. From 30 to10 Ma the arc evolved towards a bi-modal arc, with highly juvenile isotopical values inthe North and slightly juvenile values in the south. Temporal trends from detritalanalysis are further compared with: 1) in-situ zircon geochemical and isotopic data and2) geophysical data sets, mostly gravimetric, magnetometric and seismic reflectionrecords. The geophysical analysis provide a good template on the first-ordersegmentation of this part of the Andes, including deep anomalies related to ancientaccreted arcs and/or serpentinized mantle and sub-surface extensional tectonics. Theseresults give fresh perspectives on the long-term evolution of magmatic arcs in accretedoceanic terranes.