Fission-track dating of 15 macusanite glass pebbles from the Macusani volcanic field (SE Peru)

We dated, by fission tracks, 15 Macusanites from four sites located inside or in the vicinity of the Macusani ignimbritic field. Eight of these obsidian pebbles are from the new site of Chilcuno Grande, the remaining seven from the three sites of Caluyo Mayo, Chilcuno Chico and Samillia. The apparent fission-track ages of these obsidians range from about 4.6 to 3.2 Ma. As in all samples, fossil tracks show evidences of annealing; the apparent ages are only lower estimates of Macusanite formation time. Fission-track plateau ages could be determined in 12 samples. They range between 7.9 and 4.8 Ma. The distribution of available plateau ages suggests that Macusanites formed both during the main activity of the Macusani volcanic field (7 ± 1 MA ago) and late episodes (5.7−5.3 and 4.8−4.3 Ma ago).