V33E-03
Late Pleistocene pluton formation during the waning stages of the Altiplano-Puna ignimbrite flare-up

Wednesday, 16 December 2015: 14:10
308 (Moscone South)
Casey R Tierney, Northern Arizona University, Flagstaff, AZ, United States, Axel K Schmitt, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, Oscar M Lovera, University of California Los Angeles, Earth and Space Sciences, Los Angeles, CA, United States and Shanaka L de Silva, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States
Abstract:
The latest phase of volcanism during the late Mio- to Pleistocene ignimbrite flare-up in the Altiplano-Puna Volcanic Complex consists of a series of late Pleistocene lava domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) which are distributed over a roughly elliptical area of ~2,000 km2. With a collective volume of ~40 km3, these represent a waning of magmatic activity from the peak of the flare-up.

Zircon chronochemistry and zircon age spectra were modeled using a finite-difference thermal- and mass-balance model for open-system magma evolution (recharge-assimilation-fractional crystallization; RAFC) where zircon crystallization is treated in accord with experimentally calibrated zircon saturation. Zircon rims and interiors were dated (U-Th, U-Pb) for a total of 252 crystals distributed equally over the domes, with a ~50% subset also analyzed for trace elements. The domes share remarkably consistent zircon crystallization histories: the youngest zircon rim ages (ca. 83 to 104 ka) are near the 40Ar/39Ar eruption ages (ca. 87 to 120 ka; sanidine and biotite), but a significant population of rim ages predates eruption, often ranging to secular equilibrium. U-Pb interior ages of crystals with secular equilibrium rim ages date to ca. 2.0 Ma, and a single core of Chascon to 3.5 Ma. Ti-in-zircon, backed by indirectly T-dependent compositional parameters, show that zircon crystallized at steady-state in a magma reservoir with essentially insignificant (<10-4 °C/a) secular cooling.

Most zircon rims postdate peak flare-up, but nevertheless record a protracted and continuous crystallization history. To match the record of zircon production, time-integrated recharge rates of ~1 ×10-3 km3/a are required. At lower rates, older zircon ages are predicted for the dominant zircon rim age population than observed. At higher rates zircon would crystallize late, resulting in a lack of antecrysts. Intermediate recharge rates that best match the data call for at least 600 km3 of plutons underpinning each dome. The combined intrusive volume of 3,000 km3 results in a high intrusive to extrusive ratio of 75:1. This suggests that plutonic addition to the uppermost crust can be significant during the waning stages of high-flux events, although volcanic output is diminished.

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