The Plio-Pleistocene Llancanelo Volcanic Field, together with the nearby Payun Matru
Field, comprises at least 800 scoria cones and voluminous lava fields that cover an
extensive area behind the Andean volcanic arc. Beside the scoria cones in the Llancanelo
Field, at least six volcanoes show evidence for explosive eruptions involving magma-water
interaction. These are unusual in the context of the semi-arid climate of the eastern
Andean ranges. The volcanic structures consist of phreatomagmatic-derived tuff rings
and tuff cones of olivine basalt composition. Malacara and Jarilloso tuff cones were
produced by fallout of a range of dry to wet tephra. The Malacara cone shows more
evidence for a predominance of wet-emplaced units, with a steep slump-slope characterized
by many soft-sediment deformation structures, such as: undulating bedding planes,
truncated beds and water escape features. The Piedras Blancas and Carapacho tuff rings
resulted from explosive eruptions with deeper explosion loci. These cones are hence
dominated by lapilli tuff and tuff units, emplaced mainly by wet and/or dry pyroclastic
surges. Carapacho is the only centre that appears to have started with phreatomagmatic
eruptions, with lowermost tephra being rich in non-volcanic country rocks. The presence
of deformed beds with impact sags, slumping textures, asymmetrical ripples, dunes,
cross- and planar lamination, syn-volcanic faulting and accretionary lapilli beds
indicate an eruption scenario dominated by excessive water in the transportational
and depositional regime. This subordinate phreatomagmatism in the Llancanelo Volcanic
Field suggests presence of ground and/or shallow surface water during some of the
eruptions. Each of the tuff rings and cones are underlain by thick, fractured multiple
older lava units. These broken basalts are inferred to be the horizons where rising
magma interacted with groundwater. The strong palagonitization at each of the phreatomagmatic
cones formed hard beds, resistant to erosion, and therefore the volcanic landforms
are well-preserved. (C) 2007 Elsevier B.V. All rights reserved.
