Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle
along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic
rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with
rather uniform initial 143Nd/144Nd (∼0.5127–0.5128) and 87Sr/86Sr (∼0.7032–0.7040). The initial 206Pb/204Pb ratios are variable
(18.5–19.7) at uniform 207Pb/204Pb ratios (15.60±0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks
shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due
to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic
active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late
Palaeozoic age) intra-plate magmatic rocks (143Nd/144Nd∼0.5123, 87Sr/86Sr∼0.704, 206Pb/204Pb∼17.5–18.5, and
207Pb/204Pb∼15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.
Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the
composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene)
intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted
mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle
did not contribute to Mesozoic to Cenozoic arc magmatism.
© 2007 Elsevier B.V. All rights reserved.
(2007): Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin