Paleomagnetism and 40Ar/39Ar chronology of ignimbrites and lava flows, Central Volcanic Zone, Northern Chile

Abstract

New 40Ar/39Ar ages and paleomagnetic data are presented from a wide range of ignimbrites and lava flows from the region east of the Atacama Desert in the northern Chilean part of Central Volcanic Zone of the Andes. Eight dacitic to rhyolitic ignimbrites, 12 lava flows and one shallow intrusion were dated by the 40Ar/39Ar incremental heating method, and are the first 40Ar/39Ar ages reported for these units. Ignimbrites range in age from 4.2 Ma to 0.5 Ma, with the ~100 km3 Puripicar ignimbrite dated at 4.18 ± 0.02 Ma, the ~2500 km3 Atana ignimbrite, one of the largest ignimbrites in the world, at 3.99 ± 0.02 Ma, and the Purico ignimbrite at 0.99 ± 0.03 Ma. The new 40Ar/39Ar ages fall within the uncertainty limits of published K-Ar dates for the Purico, Patao and Puripicar tuffs but are different at the 95% confidence level for Atana and Talabre tuffs. Isochron ages from the 12 lava flows range from 3.5 Ma to 0.1 Ma, with 11 of the flows younger than 2.5 Ma. One lava sample and the shallow intrusion are from the Plio-Pleistocene magnetite-rich El Laco volcanic complex, and give ages of 2.18 ± 0.03 Ma and 1.83 ± 0.09 Ma respectively. Paleomagnetic directions from 49 sites (23 ignimbrites and 26 lava flows) provide a mean direction of I = −43.8°, D = 352.7° with α95 = 10.0°, and a corresponding mean pole position of −81.1° and 179.1°. Normal directions are predominantly from lava flows and have steeper than expected inclinations (ΔI = 15.3°) whereas reverse sites are mostly ignimbrites and yield directions close to the geo-axial dipole field (ΔI = −3.6°). Paleosecular variation studies on a selection of 39 sites yield an angular standard deviation of site poles about the mean pole of 18.5°, considerably larger than predicted by current models of magnetic field variation with latitude.