Résumé
Subduction zones, such as the Andean convergent margin, form large recycling systems and are the main producers of new continental crust. The subduction of continental material through either tectonic subduction erosion or sediment cycling is an important component when estimating the mass flux through an active margin over time. Subduction erosion in particular has been highlighted as an important process along certain parts of the Andean continental margin, especially to the west of the Central Andes. It has been suggested that the rate of subduction erosion along the north-central Andean margin has varied over time with constant, fast rates between 150 and 20 Ma and slower rates during the Neogene. A near constant rate of subduction erosion between 150 and 20 Ma suggests a continuous supply of continental material into the subduction zone, during this time. We have combined high resolution, U-Pb dating, isotope geochemistry, and major and trace element analyses, from a suite of magmatic rocks sampled from the southern Central Andes (28 degrees to 32 degrees S), in order to investigate source contamination by subducted continental material between the late Cretaceous and the late Miocene. The results of new, in-situ U-Pb dating of magmatic zircons by SIMS place further constraints on the presence of a widespread period of reduced arc magmatism in the southern Central Andes during the late Eocene to late Oligocene (35 - 26 Ma). Oxygen and hafnium isotopic analysis, of the same magmatic zircons (by SIMS and LA-ICPMS respectively), reveal variations in delta (super 18) O (sub (VSMOW)) of between 9.78 + or - 0.21ppm and 3.59 + or - 0.21ppm and in epsilon Hf of between 10.4 + or - 1.3 and -4.4 + or - 0.8. This new data, when combined with the results of whole rock, major and trace element analysis, provides increased constraint on the amount of continental material being subducted to depth and recycled to the continental crust via arc magmatism during this key time in the evolution of the Andes.