Batholith Maturation as Inferred From Zircons From the Aucanquilcha Volcanic Cluster, Northern Chile

New in situ (SHRIMP-RG) U/Pb and trace element data from zircons from the Aucanquilcha Volcanic Cluster (AVC), in northern Chile, help constrain the evolution of the magmatic underpinnings of a long-lived (11 Ma- present), intermediate volcanic complex through progressive stages of development. Large age spectra (<2.5 my) are observed in the samples from the beginning and waning stages of the AVC, times characterized by low eruption rates, while small age spectra (<500 ky) are typical of samples from the middle stage of the AVC, a time characterized by high eruptive output. Samples with large age spectra likely represent a cooler magmatic setting where discrete zircon age domains are preserved. Samples with smaller age spectra represent a hotter stage of the magma system during which crystals were largely equilibrated with ~cogenetic magmatic activity. Crystallization temperatures for AVC zircons, calculated using the Ti-in-zircon thermometer, range from 689 to 911 °C, with ~80 °C variation in a single sample being typical. Samples from the beginning of the eruptive flare-up (with zircon crystallization Ts of < 911 °C) are hotter than the rest of the samples (Ts of < 815 °C), suggesting a thermal driver for the eruptive increase. Temperature spectra from AVC zircons likely reflect variable zircon saturation histories typical of mafic to intermediate arc magmas. Crystal recycling enhances this effect by mixing together populations of zircons that crystallized from magmas that experienced different (if only slightly) fractionation histories. Such variation is apparently not expected in granites, where magmas are interpreted to have been generated at eutectic melt conditions and thus represent a highly regulated, repeatable process (Harrison et al., 2007). A composite view of all zircon spot ages and the cumulative volume vs. time plot of the AVC show that zircon crystallization and eruptive episodes are temporally correlative, suggesting that the timing of plutonic and volcanic events correspond. The oldest (antecrystic) grains analyzed correspond to the earliest eruptions. Also, no zircons analyzed hail from a roughly two million year eruptive gap from ~8 Ma to ~6 Ma. Samples from the last stage of AVC volcanism contain antecrysts that date back to the period of high eruptive output-- but not older--suggesting a system-wide eradication of zircons during the thermal maturation of the AVC. Alternatively, older sectors of the evolving AVC batholith are less likely to be sampled with progressive mush development as the magmatism becomes more centralized in the complex. This apparent masking of magmatic activity in the plutonic setting via thermal overprinting-with only small, likely marginal zones (or even sparse antecrystic grains) as remnant physical documents of the true life spans-may be a common occurrence and thus promote underestimates of the life spans of large, silicic to intermediate plutonic igneous systems. The effect would be more pronounced with intermediate systems, as these magmas are hot and more likely to dissolve extant zircon upon entry into the batholith.