Early Permian to Late Triassic batholiths of the Chilean Frontal Cordillera (28°–31°S): SHRIMP U–Pb zircon ages and Lu–Hf and O isotope systematics

Abstract

One of the major geological units of the Main Andean Range (Frontal Cordillera) of north–central Chile is a group of composite and heterochronous late Paleozoic–early Mesozoic batholiths that extends for 500 km roughly NS along from 26° to 31°S. Ten new SHRIMP zircon crystallization ages together with 11 recently published U–Pb zircon ages by other authors indicate an episodic intrusion history which can be divided in 4 groups: Mississippian (earliest Carboniferous; 330–326 Ma), Cisuralian (earliest Permian; 301–284 Ma), latest Permian to Middle Triassic (264–242 Ma) and Late Triassic (225–215 Ma). Volcanic rocks in the area span a similar time. Lu–Hf and O isotopic systematics in zircon grains from eight of the plutonic rocks indicate the magma source areas have contributed variable amounts of crustal and mantle components. Zircon δ18O values evolve fromcrustal values(+7‰) in the earliest Permian intrusives tomantle values in the latest Permian to Upper Triassic, including evidence for likely hydrothermal alteration of the source (+4‰). Zircon εHf values vary in a good linear correlation with the δ18O isotopes, from −6 to 0 in rocks older than 270 Ma increasing to + 2 to +7 from Lower to Upper Triassic (250 to 215 Ma). The petrogenetic constraints indicated by these values, suggest that the influence of magma sources varied with time from predominantly crustal to mantle like. In accord with the regional tectonic models, the earliest Permian rocks were generated in a subduction-related magmatic arc, which varied towards an extension-related environment in the latest Permian and Triassic.