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

Abstract

We report new petrographic and geochemical data on volcanic rocks erupted over the last 9 Ma ca. within El Tatio volcanic region (Western Cordillera – CVZ). They originated from compound volcanism alternating composite volcano activities, lava domes formation and minor low-mild explosive eruptions, whereas ignimbrite-like deposits outcropping in the region originated from external caldera system (Altiplano Puna Volcanic Complex). The volcanics – mostly erupted in the last 1 Ma - have composition ranging between calcalkaline (CA) to high-K calcalkaline (HKCA) basaltic andesite to rhyolite, but most of them are andesites and dacites. Petrographic features of the studied rocks can be frequently related to strong disequilibrium conditions in the crystallizing system: 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 observed. Most of these crystal disequilibrium features may be explained by convective self-mixing processes in magma reservoirs cooling from above and characterized by mafic magma batch at the base as a probable heat source, with minor role for magma mixing between compositionally different magmas. On the other hand, the high crystallization degree of the rocks together with evidence provided by geochemical data suggest that FC of recurrent mineral assemblages (plag.±pyrox±hornbl±bt) in closed magma chamber must be considered the most significant differentiation process for several volcanic apparatus forming El Tatio area. Although the FC holds a leading role, the correlation of geochemical and few available isotopic data also point to AFC processes as further important evolutionary mechanism. Isotopic trends and patterns of incompatible elements (i.e LILE enrichment, LILE/HFSE ratios) mirror the interaction of magmas with crustal contaminants occurring both in the mantle wedge (material transported via subduction) and during the ascent/ ponding of the parental magmas along the thick crust beneath the CVZ. In other words, starting from already modified parental magmas, further modification of variable type affected the magmas at higher level. Finally, we suggest that the plumbing systems (high level magma reservoirs) hosting the magmas of El Tatio region were characterized by multistage evolutionary processes consistent both with open- and closed-system regimes.