MAGMA ACCUMULATION AND EVOLUTION IN THE ANDEAN CENTRAL VOLCANIC ZONE: A MINERAL-SCALE PERSPECTIVE OF MAGMA HOMOGENIZATION

Upper crustal magma reservoirs control the surface expression of volcanic rocks and numerous other processes in continental crust. The Altiplano-Puna Magma Body (APMB) is the largest imaged magma reservoir on Earth, with an estimated volume of over 500,000 km³. Although the APMB is a first order geologic feature, its role in controlling magma composition and volcanic eruption style is poorly understood. Determining the size, shape, composition, crystallinity, and thermal history of large silicic magma bodies can provide insight for more accurate models of magma accumulation. Recent studies have attempted to quantify these variables and although our knowledge of large magmatic systems has greatly improved, we are still a long way from understanding the APMB and magmatism in the Central Andes. Compositional variation in single mineral populations within volcanic rocks record the magmatic conditions in which they nucleate, grow, and reside prior to eruption providing a time series record of the chemical and physical processes operating in the subvolcanic plumbing system.

Here, we present plagioclase mineral data combined with oxygen isotope values of mineral separates, and whole rock Sr isotope ratios from <1 Ma andesitic to dacitic composition lavas from four volcanoes in the Central Andes with geographic relationships to the APMB: Uturuncu, Ollagüe, Aucanquilcha, and Lascar. On an arc wide scale, lavas from these volcanoes display systematically higher K₂O contents, δ¹⁸O values, and ⁸⁷Sr/⁸⁶Sr ratios towards the interior of the APMB. These eastward changes are interpreted to reflect increasing interaction with the APMB. Variation in ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O_WR values and plagioclase compositions for each lava suite is limited (Lascar- δ¹⁸O_WR=7.4-9.0‰, ⁸⁷Sr/⁸⁶Sr=0.7057-0.7067; Aucanquilcha- δ¹⁸O_WR=8.2-9.2‰, ⁸⁷Sr/⁸⁶Sr=0.7058-0.7068; Ollagüe- δ¹⁸O_WR=7.9-9.4‰, ⁸⁷Sr/⁸⁶Sr=0.7063-0.7084) with the most variation occurring early in the volcanic history. Older Uturuncu lavas (δ¹⁸O_WR=8.7-11.4‰, ⁸⁷Sr/⁸⁶Sr=0.7101-0.7165) have bimodal mineral compositions, while plagioclase in the youngest lavas is a homogeneous hybrid. The most primitive compositions after homogenization tend to occur outside the boundary of the APMB indicating minimal influence of the APMB on magma composition at the arc-front.