Resolving the paradigm of the late Paleozoic-Triassic Chilean magmatism: Isotopic approach

Abstract

The Andean orogenic cycle and its subduction-related magmatism along the southwestern margin of South America began during the early Jurassic after an accretionary history throughout Paleozoic times. The Chilean and Argentinian Frontal Andes batholiths, together with the Coastal Batholith, represent most of the pre Andean orogenic cycle plutonism. However, how late Paleozoic-Triassic magmatism occurred along this margin and its transition to the Andean orogenic cycle still remains unclear. Here we present a geodynamic model using all the available published Lu-Hf and oxygen isotopic data ranging from latitudes 28 degrees to 40 degrees S, together with 5 new Hf-O data and U-Pb zircon ages from the Chilean Frontal Andes. Data indicate that subduction began at least in the latest early Carboniferous and was continuous throughout the late Paleozoic-Triassic period. Isotopic and geochronological results show a continuous magmatic trend, from high delta O-18 values (continental) to mantle like signatures, as the rocks get younger. Between latest early Carboniferous and earliest middle Permian, magmas formed in a subduction-related arc during the Gondwanide Orogeny. Later, throughout middle Permian to Triassic, magmatism occurred in a slab rollback extensional setting, triggered by low subducting plate velocities while Pangea was essentially in a static reference mode. There is no evidence for cessation of subduction during the Triassic and its renovation in the early Jurassic as previous work suggested. Therefore, we propose that Andean subduction has been a continuous tectonic process since Paleozoic times, whose initial. geodynamic evolution was directly related to the Gondwanide Orogeny as part of the Pangea Assembly. Slab rollback, as well as shallowing and steepening of the subduction angle were among the triggers for the change in the type of magmatism observed among these rocks.