Fission-track dating of granitic rocks from the Eastern Cordillera of Peru: evidence for Late Jurassic and Cenozoic cooling

Abstract

Apatite and zircon fission-track ages of three samples of the Huachon granite and one sample of the Quiparacra granite, from the Eastern Cordillera of the Andes of central Peru, have been determined. Zircons from the Huachon granite yielded two groups of ages. An older group, made up of zircons with the lowest uranium content, gave an age of c . 270 Ma. The zircons with high uranium content from this granite gave ages of c . 160 Ma. The zircon ages from the Huachon granite show no correlation with elevation. The age of 270 Ma is interpreted as the age of probable emplacement; the 160 Ma age is evidence of a mild cooling (≥200°C) event that ended in late Mid-Jurassic time. This cooling correlates, in central Peru, with thick conglomerates in the Sub-Andes and strong exhumation in the Eastern Cordillera during Bathonian to Malm times. Zircons from the Quiparaca granite, which is a component of a composite batholith of possible Precambrian to Tertiary age, show ages of c. 30 Ma which are interpreted as the emplacement age. These zircons were totally annealed prior to the cooling in the Oligocene. This required temperatures, >200°C. Apatite fission-track dating of the Huachon granite yielded only Neogene ages which reflect the total annealing of the tracks present in the apatite before exhumation began c. 22 Ma. Apatite ages show a correlation with elevation and indicate two periods of cooling during the Neogene at c. 21 Ma and between 12 Ma and the Present. These young ages suggest approximately 4 to 6 km of erosion in the Eastern Cordillera of central Peru in the past 30 Ma. These amounts are consistent with known geological data and fission-track ages obtained from other parts of the eastern Andes.