Rivera M., Martin H., Le Pennec Jean-Luc, Thouret J. C., Gourgaud A., Gerbe M. C. (2017). Petro-geochemical constraints on the source and evolution of magmas at El Misti volcano (Peru). Lithos, 268, p. 240-259. ISSN 0024-4937.
Titre du document
Petro-geochemical constraints on the source and evolution of magmas at El Misti volcano (Peru)
Rivera M., Martin H., Le Pennec Jean-Luc, Thouret J. C., Gourgaud A., Gerbe M. C.
Source
Lithos, 2017,
268, p. 240-259 ISSN 0024-4937
El Misti volcano, a large and hazardous edifice of the Andean Central Volcanic Zone (CVZ) of southern Peru, consists of four main growth stages. Misti 1 (>112 ka) is an old stratovolcano partly concealed by two younger stratocones (Misti 2, 112-40 ka; Misti 3, 38-11 ka), capped in turn by a recent summit cone (Misti 4, <11 ka). In order to gain insights into magma composition controls on eruptive behaviour through time at El Misti, we have conducted a petrological and geochemical study of selected rock samples from the main growth stages of the volcano. Whole rock compositions range from andesite to rhyolite and belong to a medium to high-K calk alkaline magmatic suite. El Misti samples are characterised by high large-ion lithophile elements, but low concentrations of high field strength elements, and heavy rare earth elements, consistent with a subduction zone setting. The Sr-87/Sr-86 (0.70715-0.70882) and Nd-143/Nd-144 (0.511983-0.512277) isotope ratios suggest that magma composition is significantly affected by contamination and/or assimilation processes during their evolution, likely due to the presence of thick (65-70 km) continental crust beneath the CVZ in southern Peru. Geochemical evidence indicates that magmatic evolution is mostly controlled by Assimilation-Fractional Crystallisation (AFC) mechanisms. Modelling reveals a mass-assimilated/mass-fractionated ratio (p) <= 2.2, which suggests an assimilated crust fraction below 14 wt.% on average. Our isotopic data clearly identify the Proterozoic "Charcani gneiss" basement as the main contaminant. Both contamination and assimilation processes peaked at similar to 30 wt.%, during the Misti 3 stage when rhyolites were generated. We ascribe the general depletion in HREE and Y and elevated La/Yb and Sr/Y ratios in El Misti samples to the enrichment of the mantle wedge source of the parental magmas by a felsic melt of adakitic composition and hydrous fluids. Our work highlights that El Misti's magmatic system has remained relatively homogeneous since at least 0.12 Ma, with a marked influence of the contaminating crust in the Late Pleistocene Misti 3 stage, which resulted in highly explosive eruptions. Andesitic-dacitic compositions are dominant in the Holocene and historical Misti 4 stage, and are expected for future volcanic events at El Misti.
Plan de classement
Géologie et formations superficielles [064]
;
Géophysique interne [066]
