Evidence for different processes of magma evolution in El Tatio volcanic region (22°.16 to 22°.30 S - Central Volcanic Zone, Andes)

We report new petrographic and geochemical data on volcanic rocks emplaced fromMiocene to late Peistocene in El Tatio volcanic region (Central Volcanic Zone, Andes). They originated from dominantly effusive volcanism and minor low-mild explosive eruptions, giving rise to several stratovolcanoes and lava domes belonging to the Western Cordillera volcanic chain, in alternance with intense ignimbrite-type explosive activity from external caldera systems in the Altiplano Puna Volcanic Complex. El Tatio volcanics are mostly erupted in the last 1 Ma and display compositions ranging from calcalkaline (CA) to high-K calcalkaline (HKCA) basaltic andesite to rhyolite, with prevalence of andesites and dacites, which bear a tipycal subdution-related signature. Petrographic features of studied De Astis et al. 2 rocks - deeply resorbed and rounded mineral phases, reaction rims, skeletal habits, large ranges of mineral compositions with direct and reverse zoning, oxidations and uralitizations phenomena - are generally related to strong disequilibrium conditions in the crystallizing system. Based on our data, these disequilibrium features are better explained by convective self-mixing processes in magma reservoirs cooling from above and heated by mafic magma batch at camera bottom, rather than by magma mixing between compositionally different magmas. The high crystallization degree of rocks, together with evidence provided by geochemical data, suggests that fractional crystallization (FC) of recurrent mineral assemblages (plag.±pyrox±hornbl±bt) is the most significant differentiation process for several magmas from El Tatio suites. In addition, AFC (Assimilation plus FC) is another important evolutionary mechanism able to explain some trends correlating the few available isotopic data and some geochemical indexes. Isotopic trends and patterns of incompatible elements (i.e., LILE enrichment, LILE/HFSE ratios) mirror the addition, via subduction, of different amounts of crustal material. Magma modification can occur either in the mantle wedge (fluids and melts from the slab) or during the ascent/ponding of parental magmas within the thick crust of Central Andes. Finally, we suggest that the high level magma reservoirs hosting magmas of El Tatio region are characterized by multistage evolutionary processes consistent both with open- and closed-system regimes.