FIRST TEPHROSTRATIGRAPHICAL STUDY OF THE ICALMA–GALLETUE REGION (CHILE, 38° S –71° W) – TWO PUMICE LAYERS MARKING STRONGLY THE HOLOCENE LANDSCAPE

This study, part of the S.S.T.C. project EV/10/12B ‘A continuous Holocene record of E.N.S.O. (“El Niño Southern Oscillation”) variability in Southern Chile’, outlines the tephrostratigraphy of six outcrops and two peat bogs in Icalma and Galletue lakes’ watersheds (Chile, 38° S –71° W) where several tephra layers have been detected and analysed. Two important pumice layers are unequivocally discernible in this landscape and have been studied using several techniques: polarising microscope, electron microprobe and isopach mapping. Plagioclase (andesine), orthopyroxene (hypersthene), clinopyroxenes (diopside and Ca–rich augite), titanomagnetite and fluorapatite are common to both pumice layers. Moreover, the lower pumice fall has also two distinct olivine populations, Fo32-35 and Fo82-85, one of which (Fo82-85) did not crystallise in equilibrium with the magmatic liquid. Glass shards from the upper fall–out deposit have a K–rhyolitic composition while glass shards from the lower one are rhyodacitic. Mineralogical differences and isopach’s mapping allow us to propose different origins for the two pumice layers and to make correlations between the outcrops and peat deposits. The lower tephra erupted from Llaima volcano during it’s second eruptive phase in the first half of the Holocene (Naranjo & Moreno, 1991), while the upper one, on the NW flank of Alpehué Pumice (Naranjo et al., 1993), erupted from Caldera Sollipulli at about 2900 y.B.P. Analyses of other thinner tephras complete this work and allow us to propose the first complete Holocene tephrostratigraphy of Icalma-Galletue area.

References:

NARANJO, J. A. (1991). Actvidad explosiva postglacial en el Volcán Llaima, Andes del Sur (38°45'S). Revista Geológica de Chile 18, 69-80. NARANJO, J. A., MORENO, H., EMPARAN, C., and MURPHY, M. (1993). Volcanismo explosivo reciente en la caldera del volcán Sollipulli, Andes del Sur (39°S). Revista Geológica de Chile 20, 167-191.