UNIVERSITÀ DI PISA
Corso di Laurea Scienze Geologiche
Anno Accademico 2013/2014
Candidate: Beani Massimo
Title of thesis
The lavas of the Plio-Quaternary mafic monogenetic volcanic centres at 25°S and 67°W in the Central Andes (Salar de Arizaro, NW Argentina)
Supervisor: Dott.ssa Gioncada Anna
Assistant Supervisor: Professor Mazzuoli Roberto Abstract
The subject of this thesis are the lava flows coming from some monogenetic volcanic centers of Plio-Quaternary age, positioned on the east side of the active vulcanic arc in the Central Andes, in a region enclosed by the arc itself and the Salar de Arizaro (Figure 1) between 24.50° and 25.30°
latitude south and 67.60° and 68.50° longitude west.
Figure 1 A. Central Andes between 15° and 27° latitude south. The picture shows the main morpho-structural units that characterize this andean sector and magmatism of the back-arc on the east side of the Western Cordillera (from Mazzuoli et al. 2008, modified). B. Satellite imagery of the same region.
The area object of the study is at the margin of the Puna Plateau, whose backarc volcanism is identified by a series of large strato-volcanoes arisen between Early Miocene and Pleistocene, several ignimbrite blankets and many monogenetic scoria and lava centres. Many of these are scattered in the region close to the Salar de Antofalla.
The volcanic monogenetic centers object of the study are characterized by a mainly effusive activity (Figure 2 and 3), seldom in association with small projection of scoriae; these lava are basaltic andesites and andesites.
Figure 2 Satellite image from Google Earth where the lava cast emanated by a monogenetic center close to Salar de Arizaro is clearly visible. On the back, largest stratovolcanoes in the Antofalla region.
Main purpose of this thesis is to give a petrographic and geochemical characterization of the various lava from Salar de Arizaro, in order to find the characteristics of the source and to compare with the rocks of the mafic monogenetic centers of the close region of the Salar of Antofalla, in the back-arc, using recent data present in literature (Risse et al. 2013).
Figure 3 Picture showing lava emanated by monogenetic centers on the front, while, on the back, stratovolcano Aracar which arises in the central andean cordillera arc.
In the Antofalla area, in fact, those authors, on a geochemical base, identified two different kind of lava with different trace element characteristics defined as “calcalkaline” and “intraplate”, with
transitional terms. In the Salar de Arizaro, instead, the magmas erupted are “calcalkaline” and transitional.
The area distribution and the quantity of lava erupted by these volcanoes have been analyzed, comparing extension and type of lava from both areas, using satellite imagery by Google Earth software.
The genesis of these magmas has been investigated too, by comparing these lavas from a chemical standpoint with those from the volcanic arc erupted by the stratovolcanoes, and from the backarc, erupted by the monogenetic centres, in order to understand the influence of the subduction of the pacific plate under the south american continent and of the delamination mechanism, which has been proposed for this backarc region.
Reference
Ducea, M. N., Seclaman, A.C., Murray, K.E., Jianu, D., Schoenbohm, L.M.: Mantle-drip
magmatism beneath the Altiplano-Puna plateau, central Andes, Geology, v. 41, doi:10.1130/
G34509.1 , 2013.
Kay, R.W. & Kay, S. M. (1993). Delamination and delamination magmatism.Tectonophysics219, p.
177-189
Kay, S.M., Coira, B., and Viramonte, J., 1994, Young mafic back-arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna plateau, central Andes:
Journal of Geophysical Research, v. 99, p. 24,323–24,339
Mazzuoli R. et al., 2008, Miocene magmatism and tectonics of the easternmost sector Calama–
Olacapato–El Toro fault system in Central Andes at ~Insights into the evolution of the Eastern Cordillera: GSA Bulletin; v. 120; no. 11/12; p. 1493–1517.
Risse A., R. B. Trumbull,S.M.Kay ,B.Coira, R. L. Romer, 2013: Multi-stage Evolution of Late Neogene Mantle-derived Magmas from the Central Andes Back-arc in the Southern Puna Plateau of Argentina, Journal of Petrology, pp. 1-33
