Devonian magmatism in the accretionary complex of southern Chile

Abstract

Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U-Pb dating of zircon, with Hf- and O-isotope analyses of selected grains. Ages of 384 +/- 3 and 382 +/- 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 +/- 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c. 330 - 385 Ma. Devonian ages of 391 +/- 10 and 374 +/- 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaiten (361 +/- 7 and 364 +/- 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian-Carboniferous times