U PB GEOCHRONOLOGY AND SIGNIFICANCE OF LATE PERMIAN IGNIMBRITES IN NORTHERN CHILE/

U/Pb zircon ages of four ignimbrite samples from a Late Paleozoic volc aniclastic succession in northern Chile are presented. The 2 km thick succession consists of the Peine Fm. and the overlying Cas Fm., which crop out at the eastern margin of the Salar de Atacama near the villag e of Peine. One sample was collected from the oldest ignimbrite of the succession (Lower Member of the Peine Fm.); the other three samples w ere collected from the Middle Member of the: Cas Fm. and its assumed e quivalents. The Pb-206/U-238 ages of all samples overlap within their analytical uncertainties, which results in a best estimate of 248 +/- 3 Ma (Late Permian: Harland, 1990) for deposition of the entire succes sion. The age for the stratigraphically older sample is statistically indistinguishable from the other three samples, so that the deposition al interval could have been less than 1 million years; maximum errors in the data allow that the Peine and Cas Fms, could have formed during a time span of 6 Ma. The data provide tight constraints on the age of these units, part of which formed in the Lanquir Caldera Complex. The y also have implications for the stratigraphy and paleoclimatic models of the Southern Central Andes during the Late Permian. Although the z ircon fractions analyzed here document Late Permian ages, they show va rying amounts of inheritance of early Paleoproterozoic (to Late Archea n?) zircons and also indicate the incorporation of ca. 2.5 Ga initial common lead to varying degrees. This and results from other studies in the area lead us to infer the existence, at least during the Paleozoi c, of Precambrian basement located below the Salar de Atacama area. Co pyright 1996 Elsevier Science Ltd & Earth Sciences & Resources Institu te