Mid-Cenozoic SHRIMP U-Pb detrital zircon ages from metasedimentary rocks in the north Patagonian Andes of Aysén, Chile

Abstract

Previously undated low-grade metamorphic rocks from the Puerto Cisnes-Queulat area (44 degrees 30' S) contain detrital zircons of mid-Oligocene age (ca. 28 Ma). Their outcrops represent the easternmost occurrence of the late Oligocene to early Miocene marine volcano-sedimentary Traiguen Formation; previous correlation with the Paleozoic metamorphic basement of this sector of the North Patagonian Andes is thus refuted. A similar age and provenance were obtained for a paraconglomerate bed of the La Junta Formation ca. 80 km to the north, which is thought to represent a high-energy lateral facies variation of the Traiguen Formation. Miocene plutonic rocks of the North Patagonian Batholith intruded these metasedimentary rocks, generating a contact metamorphic aureole that reaches biotite grade and overprints a previous metamorphic fabric probably formed during closure of the Traiguen Basin. Similar young ages for metamorphic rocks located immediately west of the Liqufile-Ofqui Fault Zone 300 km north, near Ayacara, suggest a regional pattern of earliest Neogene metamorphism and rapid exhumation in this segment of the Patagonian Andes.