Single crystal sanidine and biotite 40Ar/39Ar ages of the cerro galán intracaldera and extracaldera ignimbrite flowse.

MAHLBURG KAY, S., COIRA, B. AND SINGER, B.

San Carlos de Bariloche

Simposio; VI South American Symposium on Isotope Geology (SASIG).; 2008

INGEIS-CIG

Single crystal 40Ar/39Ar sanidine and biotite ages for pumice and whole rock samples from three localities in the Cerro Galán ignimbrite in the southern Puna of the central Andean plateau show that the giant Cerro Galán ignimbrite is constituted by a series of flows that had begun erupting by at least 2,13 Ma and continued to erupt until at least 2,06 Ma. The results of 40Ar/39Ar ages from pumice and whole rock samples from the same locality are virtually indistinguishable as are the weighted mean plateau and isochron ages. The dated samples are from an intracaldera pumice-rich flow in the southeastern part of the Cerro Galán caldera (ca. 2,12 Ma) and two relatively pumice-poor extensive extracaldera flows that erupted to the west (at ca. 2,09 Ma) and north (at ca. 2,06) of the caldera rim. Each of the flows has a coherent set of younger sanidine and older biotite ages that are characteristic of the flow and whose differences in age reflect variations in the thermal and temporal histories of the individual flows. The 40Ar/39Ar biotite ages are in the same range as the K/Ar biotite ages reported by Sparks et al. (1985) and confirm their conclusion that the biotite ages are older than the eruptive ages. For the dated ignimbrites, the discrepancies in the sanidine and biotite ages range from about 0,25 to 0, Ma.