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.

中文翻译：

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智利北部中央火山带火山岩和熔岩流的古地磁和 40Ar/39Ar 年代学

摘要 从安第斯山脉中央火山带智利北部阿塔卡马沙漠以东地区的广泛范围的熔凝灰岩和熔岩流中提供了新的 40Ar/39Ar 年龄和古地磁数据。通过 40Ar/39Ar 增量加热法测定了 8 个英安质至流纹质凝灰岩、12 个熔岩流和一个浅层侵入体的年龄，并且是这些单元报告的第一个 40Ar/39Ar 年龄。Ignimbrite 的年龄范围从 4.2 Ma 到 0.5 Ma，其中 ~100 km3 Puripicar ignimbrite 的年龄为 4.18 ± 0.02 Ma，~2500 km3 Atana ignimbrite，世界上最大的 ignimbrite 之一，年龄为 3.99 ± 0.02 Ma，和 Purico Ma，在 0.99 ± 0.03 Ma 处点燃。新的 40Ar/39Ar 年龄在 Purico 已公布的 K-Ar 日期的不确定性范围内，Patao 和 Puripicar 凝灰岩，但 Atana 和 Talabre 凝灰岩的 95% 置信水平不同。12 条熔岩流的等时线年龄范围从 3.5 Ma 到 0.1 Ma，其中 11 条熔岩流的年龄小于 2.5 Ma。一个熔岩样本和浅层侵入体来自上新世-更新世富含磁铁矿的 El Laco 火山复合体，年龄分别为 2.18 ± 0.03 Ma 和 1.83 ± 0.09 Ma。来自 49 个地点（23 个熔结凝灰岩和 26 个熔岩流）的古地磁方向提供的平均方向为 I = -43.8°、D = 352.7°，α95 = 10.0°，相应的平均极位为 -81.1° 和 179.1°。法线方向主要来自熔岩流，并且具有比预期更陡的倾角（ΔI = 15.3°），而反向位置主要是熔结岩和靠近地轴偶极场的屈服方向（ΔI = -3.6°）。