Crustal growth along a long lived accretionary margin: Evidences from the proto-Andean batholiths of Peru

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Miskovic, Aleksandar; Schaltegger, Urs
Holding Date 21 September 2008

The results of a coupled laser ablation (MC) ICPMS U-Pb and Lu-Hf isotopic study on zircons from the Eastern Cordilleran intrusives of Peru reveal 1.15 Ga of magmatic activity along a 1200 km long segment of the western Amazonian margin that is largely dominated by mid-Phanerozoic plutonism related to the assembly and break up of Pangea. The Hf isotope systematics of magmatic zircons from batholiths along this unique segment of the proto-Andean margin are invariably characterised by a range in the initial 176Hf/177Hf compositions for a given intrusive event suggesting mixing of material derived from the Paleoproterozoic crustal substrate and variable Neoproterozoic to recent juvenile sources. However, the periods of well documented compressive tectonics correspond to negative mean εHfi values of -6.73, -2.43, -1.57 for the Ordovician Famatinian, Carboniferous-Permian and late Triassic Gondwanides respectively, suggesting the minimum crustal contribution between 74% and 44% by mass. The average initial Hf systematics from granitoids associated with intervals of regional extension such as the middle Neoproterozoic, Permian-Triassic and Cenozoic Andean back arc plutonism are consistently shifted toward the positive values (mean εHfi = -0.7 to + 8.0) indicating systematically larger inputs of juvenile magma (22% to 49%). In the absence of evidence for lateral accretion of exotic crust, the time integrated Hf record from the central proto-Andean margin of western Amazonia suggests crustal reworking as the dominant process during episodes of arc magmatism and implies that continental growth along non-collisional accretionary orogens, if any, takes place vertically via crustal underplating of isotopically juvenile, mantle derived magma during intervals of crustal attenuation.