U-Pb zircon chronostratigraphy of early-Pliocene ignimbrites from La Pacana, north Chile: implications for the formation of stratified magma chambers

High spatial resolution U-Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent U-238/Pb-206 ages of 4.11 +/- 0.20 Ma and 4.65 +/- 0.13 Ma (2sigma), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U-Pb ages closely agree with the eruption age of Atana previously determined by K-Ar dating (similar to 4.0 +/- 0.1 Ma) and do not support long (> I Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent U-238/Pb-206 zircon ages of similar to 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano-Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented. (C) 2002 Elsevier Science B.V. All rights reserved.