Geodynamic controls at the southernmost Northern Andes magmatic arc: geochemical and geophysical evidence
Abstract
Understanding how magmatic arcs develop is crucial for the comprehension of
subduction systems and the processes leading to continental growth and lithospheric
recycling. Chronologic and provenance from U-Pb single grain dating by LA-ICP-MS of
detrital zircons obtained from the forearc basin paired with Hf and O isotopic data and
trace elements have provided indicators of the Late Cretaceous-Cenozoic evolution of
the 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 intense
magmatism resulting from melting of lower and upper crust materials. Highly
fractionated 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 likely
emplaced in an over thickened crust. Subsequently, negative isotopic incursions from
45 to 30 Ma are defined by mantle sources as well as drops in the U/Yb, Eu/Eu* and
Th/U ratios laying rough indicators of a broad extensional magmatic arc. From 30 to
10 Ma the arc evolved towards a bi-modal arc, with highly juvenile isotopical values in
the North and slightly juvenile values in the south. Temporal trends from detrital
analysis are further compared with: 1) in-situ zircon geochemical and isotopic data and
2) geophysical data sets, mostly gravimetric, magnetometric and seismic reflection
records. The geophysical analysis provide a good template on the first-order
segmentation of this part of the Andes, including deep anomalies related to ancient
accreted arcs and/or serpentinized mantle and sub-surface extensional tectonics. These
results give fresh perspectives on the long-term evolution of magmatic arcs in accreted
oceanic terranes.