Melt extraction in a transcrustal magmatic system at Cerro Uturuncu, Bolivia: insights from zircon and plagioclase

The eruptive history of Cerro Uturuncu in the rear arc of the Andean Central Volcanic Zone is defined by six eruptive series spanning over 1 m.y. The main cone-building phase occurred between 595-220 ka erupting over 150 chemically unique dacite lavas and domes. Activity has continued at Uturuncu through persistent degassing and uplift centered beneath the volcano-related to fluid exchange between the highly ductile Altiplano Puna Magma Body (APMB) and a shallow hydrothermal system. Despite several recent studies on the eruptive history, petrology, and magma plumbing system, many aspects of the shallow magmatic system's longevity and prevailing physiochemical conditions still need to be answered. Here we present new 40Ar/39Ar eruption ages, zircon U-Pb crystallization ages and trace element contents, and plagioclase textures, major- and trace element contents which reveal continuous and overlapping crystallization histories among individual eruptions and within an eruptive stage. Zircon records storage at near solidus to sub-solidus conditions after crystallization with significant rim-to-core age variation suggesting prolonged storage. This contrasts with shorter Sr-in-plagioclase diffusion timescales suggesting eruption occurs within several hundred years of crystallization. This discrepancy results from melt separation from the crystal-rich APMB incorporating zircon, whereas plagioclase crystallizes shortly before eruption in the shallow plumbing system. The incorporation, dissolution, and recrystallization of zircon and plagioclase are repeated several times before eruption in different locations in the magmatic system leading to blended zircon ages and plagioclase with multiple dissolution surfaces and core-to-rim trace element variation. We suggest the assembly of Uturuncu dacites involve mixing mid-crustal felsic mush bodies with varying volumes of more primitive magmas. The result is homogeneous hybridized magmas with distinct plagioclase phenocryst populations, short plagioclase residence times, and zircons with inherited cores. Magma recharge causes melt remobilization in the APMB and crystal hybridization over timescales ranging from near instantaneous to millennia preserving a complex record of pre-eruptive processes.

For more information about Dr. Michelfelder's research, please see their Research Gate page: https://www.researchgate.net/profile/Gary-Michelfelder