Fission-track and K-Ar ages of "macusanite" obsidian glasses, (SE Peru): Geodynamic implications

Eight Macusanite pebbles from three fluvio-glacial sedimentary sites, at Caluyo Mayo, Chilcuno Chico and Samillia, were dated by the Fission Track (FT) method. In all these volcanic glasses, the etched spontaneous fission tracks were observed to have, on the average, shorter diameters than induced tracks i.e., they are partially annealed. As a consequence, their apparent FT ages, in the range from 2.9 to 4.7 Ma, are only lower estimates of their eruption ages. FT plateau-ages could be measured on seven Macusanites. Interpreted as formation ages, they range between 4.8 and 6.6 Ma. Four of these samples were also dated by K-Ar, with ages distributed between 5.44 and 5.72 Ma; for each of these samples, the FT plateau-ages and K-Ar ages are in agreement to within 15%, suggesting that no significant radiogenic 40Ar losses took place since the time eruption. From our results and previously determined FT plateau-ages, it appears that the Macusanite-producing volcanism occurred between 4.8 and 6.9 Ma, possibly in the form of discrete events. It would essentially post-date the igneous activity responsible of most of the ignimbritic infilling of the Macusani Basin, which occurred from 6.7 to 8 Ma ago. Thus, the main volcanic activity in the Macusani ignimbritic field appears to have have been contemporaneous with the F5 tectonic pulse of the Central Andes. © 1992.