Abstract

During the Pliocene and upper Miocene periods, uplift of the Puna block was associated with extensive ignimbritic eruptions covering much of the Andes of northern Chile, southern Peru, Bolivia and northern Argentina. Part of this field to the north of the Salar de Atacama is described here.

The pre-ignimbrite topography can be reconstructed from the distribution of individual sheets. The Puna block did not form the present imposing feature that it does now; much of its uplift took place during Pliocene time, but two pre-Miocene mountain blocks acted as barriers to pyroclastic flows. One barrier was the Cordillera Domeyko on the west side of the Salar de Atacama, the other was the Cordillera Media running down the middle of this part of the Atacama Desert.

Field characters are similar to those described from other ignimbrite fields. Lithic fragments are abundant and many of those were picked up by the flows from the underlying surface. Two types of surface pattern have been produced by jointing; one is a reticulate pattern of cooling joints, and the other is a pattern of parallel joints thought to be of tectonic origin. Arcuate ridges indicate direction of flow.

Below the Sifon ignimbrite (the lowest widespread sheet) are several ignimbrites of limited distribution, some of which are interbedded with sediments of the San Pedro and Loa Formations.

The distribution of the Sifon ignimbrite is controlled by the underlying topography. In the southern part it fills a pre-existing trough. Valleys in the mountains on the west side allowed some flows to escape from the trough. At its northern outcrop the ignimbrite forms a continuous sheet from the Puna, and evidence suggests that the pyroclastic flows had a minimum thickness of 125 m. Where fully developed there is a basal non-welded zone followed by a black vitric welded zone; above this is a welded crystallized zone grading up to felsitic tuff which becomes progressively less welded. Distribution of green and brown hornblende indicates that the initial temperature was less than about 800° C, but that in the crystallized zones heat of crystallization raised the temperature above this level. Chemical differences between the zones are small.

Deposits between the Sifon and Puripicar ignimbrites to the north consist of poorly welded ignimbrites (the Toconce ignimbrites), and a series of avalanche and nuée ardente deposits produced by the eruption of extrusive domes on the eastern side of the area. A widespread sheet of clastic sediments formed by erosion of the domes. In the southern part of the area are a number of sheets that include the Vilama ignimbrite.

The Puripicar ignimbrite has a volume of about 100 km3. Its emplacement was relatively unimpeded by topography, although some hills rise above its present surface. The sheet is folded, showing continuation of uplift of the Andes in this region. There is no vitric welded zone and in the lower densely welded zone the groundmass has undergone spherulitic crystallization. This zone grades up into one of partial welding in which there is a dusty devitrification and the flow has a non-welded vitric front. Again the chemical differences between zones are small.

The Chaxas ignimbrites which are in part younger than the Puripicar ignimbrite, surround and dip away from the extrusive dome of Chaxas; the younger ignimbrites in this succession were probably erupted from the vent through which the dome subsequently rose.

Rhyolitic ignimbrites are not well represented in this area but the Toconao ignimbrite at the southern end of the area is typical of Andean rhyolitic ignimbrites. Identical types are present in southern Peru, enabling comparisons to be made.

Although the actual sources of the larger ignimbrites in the area are not seen, extrusive domes within the ignimbrite succession mark eruption sites; these represent degassed magma from which the ignimbrites had been erupted previously. Because of the close association of ignimbrite eruptions, Andean uplift and consequent folding of the ignimbrites, volcano-tectonic depressions have not been identified.

First Page Preview

First page PDF preview
You do not currently have access to this article.