FISSION-TRACK DATING OF 15 MACUSANITE GLASS PEBBLES FROM THE MACUSANIVOLCANIC FIELD (SE PERU)

We dated, by fission tracks, 15 Macusanites from four sites located in side or in the vicinity of the Macusani ignimbritic field. Eight of th ese obsidian pebbles are from the new site of Chilcuno Grande, the rem aining seven from the three sites of Caluyo Mayo, Chilcuno Chico and S amillia. 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 availa ble plateau ages suggests that Macusanites formed both during the main activity of the Macusani volcanic field (7 +/- 1 Ma ago) and late epi sodes (5.7-5.3 and 4.8-4.3 Ma ago).