Periods of crustal reworking and of crustal growth can be distinguished within orogenic belts through Nd isotope studies of both igneous and sedimentary samples. The active continental margin of the Central Andes has been the site of magma genesis since at least 300 Ma.During the late Paleozoic, orogenesis was virtually an intracrustal reworking event, resulting in Late Proterozoic Nd model ages. During the Jurassic, a marked decrease in model age indicates a critical change in tectonic style and the onset of crustal growth. Younger model Nd ages are restricted to the 180-30 m.y. period of Andean tectonics consistent with active crustal growth through mantle input. Over the past 30 m.y., elevated model Nd ages from volcanics and sediments record a return to intracrustal processes in the Central Andes, synchronous with Tertiary crustal thickening. These results imply that crustal growth is not a continuous process at active continental margins and may progressively decelerate during orogenic evolution.

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