Crustal growth along a non-collisional cratonic margin: A Lu–Hf isotopic survey of the Eastern Cordilleran granitoids of Peru

An extensive in situ Lu–Hf isotopic study of zircon from the Eastern Cordilleran batholiths of Peru using laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICPMS) reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto-Andean margin of central Western Amazonia since > 1.15 Ga. The Hf isotope systematics are characterised by a range in initial 176Hf/177Hf compositions for a given intrusive event suggesting mixing of material derived from the Paleoproterozoic crustal substrate and variable addition of juvenile sources from Neoproterozoic to Cenozoic time. Intrusives associated with phases of regional compressive tectonism correspond to mean initial εHf values of − 6.73, − 2.43, − 1.57 for the Ordovician (Famatinian), Carboniferous–Permian and Late Triassic respectively, suggesting a minimum crustal contribution between 40% and 74% by mass. The average initial Hf systematics from granitoids associated with periods of regional extension such as the middle Neoproterozoic, Permian–Triassic and Cenozoic Andean back arc plutonism are consistently shifted towards positive values (mean initial εHf = − 0.7 to + 8.0) indicating systematically larger inputs of juvenile magma (25% to 38% of ancient crust by mass). In the absence of evidence for significant lateral accretion of exotic crust, the time integrated Hf record from the central proto-Andean margin of western Amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism, implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile, mantle derived melts during intervals of crustal extension.