Abstract

Calc-alkaline volcanic rocks ranging in age from Miocene to Recent are encountered in the Ojos del Salado area (27°0'S, 68°45'W). They lie at the southernmost end of the Central Volcanic Zone, coinciding with a major morphological, seismic and volcanic discontinuity. It has previously been suggested that this Ojos del Salado lineament may represent a continental extension of the chain of islands and seamounts referred to as the 'Easter Hot Line'. The volcanic rocks of the area have been sub-divided into five groups based on K–Ar dates. The first three groups, all of Miocene age, show a progressive younging eastwards with a concomitant increase in K2O. The formation of Wheelwright and other caldera structures appears to date from the close of this phase (8–6 Ma). During the Pliocene the Group 4 volcanoes were constructed mainly along a line trending N65°E but a number of other cones developed around Wheelwright caldera which was partially filled by their products. Clockwise rotation of the volcanic axis continued in Pleistocene–Recent times with the eruption of the Group 5 volcanoes of Ojos del Salado. Extensive pumice flows were associated with this new volcanic chain and there is generally a shift to more acidic compositions, especially dacite. Apart from the increase in K and related elements from west to east in Groups 1 to 3, there is no evidence for any systematic compositional change. In particular, the youngest group of volcanoes with an east–west trend is geochemically indistinguishable from its predecessors on the dominantly north–south axes. Trace element ratios show similar trends, the rare earth element patterns are LREE enriched and the 86Sr/87Sr initial ratios are all approximately 0.706. The Ojos del Salado volcanics are thus devoid of any compositional signature to link them with the Easter 'Hot Line'.

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