New time constraints for the age of metamorphism at the

Abstract

Fission-track (FT) analysis was performed on ten samples of detrital zircons from several pre-Late Jurassic metamorphic accretionary complexes at the ancestral Pacific margin of Gondwana in southern Chile (42-52°S) previously dated by U-Pb SHRIMP. Post-metamorphic zircon FT cooling ages combined with published U-Pb provenance ages allow the estimation of the following maximum and minimum ages of deposition and metamorphism: the Eastern Andes metamorphic complex (EAMC), 364 to 250 Ma (Late Devonian to Permian/Triassic boundary); the Duque de York flysch of the Madre de Dios accretionary complex (MDAC), 234 to 195 Ma (Middle Triassic to Early Jurassic); and the Chonos/Chiloé metamorphic complex (CMC), 213 to 198 Ma (Late Triassic to Early Jurassic). The implications of these results are (1) they verify that deposition, accretion and metamorphism of most of the EAMC took place well before continentally derived flysch in the complexes west of the Patagonian batholith (MDAC and CMC) was deposited, accreted, and metamorphosed, supporting either a punctuated evolution of accretion along the ancestral Pacific Gondwana margin or the existence two distinct basement terranes and (2) that the continentally derived flysch of the CMC and MDAC share an equivalent depositional and metamorphic history indicative of widespread accretion in Late Triassic to Early Jurassic times. Mixed post-metamorphic zircon FT single grain ages and an apatite FT age from the island of Chiloé (43°S) reveal a previously unknown Late Cretaceous (109-60 Ma) phase of reheating in these rocks, most likely linked to plutonic activity of this age described in other parts of the CMC.