ND, PB, AND SR ISOTOPE COMPOSITION OF JUVENILE MAGMATISM IN THE MESOZOIC LARGE MAGMATIC PROVINCE OF NORTHERN CHILE (18–27°S): INDICATIONS FOR A UNIFORM SUBARC MANTLE

Abstract

The Jurassic to Early Cretaceous magmatic arc of the Andes in northern Chile was a site of major additions of juvenile magmas from the subarc mantle to the continental crust. The combined effect of extension and a near stationary position of the Jurassic to lower Cretaceous arc favoured the emplacement and preservation of juvenile magmatic rocks on a large vertical and horizontal scale. Chemical and Sr, Nd, and Pb isotopic compositions of mainly mafic to intermediate volcanic and intrusive rock units coherently indicate the generation of the magmas in a subduction regime and the dominance of a depleted subarc mantle source over contributions of the ambient Palaeozoic crust. The isotopic composition of the Jurassic (206Pb/204Pb: ~ 18.2; 207Pb/204Pb: ~ 15.55; 143Nd/144Nd: ~ 0.51277; 87Sr/86Sr: ~ 0.703–0.704) and Present (206Pb/204Pb: ~ 18.5; 207Pb/204Pb: ~ 15.57; 143Nd/144Nd: ~ 0.51288; 87Sr/86Sr: ~ 0.703–0.704) depleted subarc mantle beneath the Central and Southern Andes (18°–40°S) was likely uniform over the entire region. Small differences of isotope ratios between Jurassic and Cenozoic to Recent of subarc mantle-derived could be explained by radiogenic growth in a still uniform mantle source.