TRACKING LOWER CRUSTAL PROCESSES THROUGH HIGH-PRECISION ZIRCON U-PB, HF ISOTOPE ANALYSES IN SUCCESSIVE SUPER-ERUPTIONS (SOCORRO CALDERA COMPLEX, USA)

Large, rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, and partial melting and differentiation within the middle and lower crust drive geochemical and isotopic evolution. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine magmatic rates as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry of four ignimbrites from the nested caldera complex near Socorro, within the Mogollon-Datil volcanic field.

The Mogollon-Datil volcanic field in New Mexico erupted large volume, caldera-forming ignimbrites from roughly 36 to 24 Ma, and the Socorro caldera cluster was active for part of this period. In agreement with past ⁴⁰Ar-³⁹Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hell’s Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra become more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hell’s Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections. In contrast, the Hf isotope record is dominated by autocrystic zircon, which we interpret to be?? domains due to the limitations of isotope mixing, and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision data sets to advance volcanic models.