THE ZIRCON RECORD OF ANDESITIC MAGMATISM AT GOAT ROCKS VOLCANIC COMPLEX, SOUTHERN WASHINGTON CASCADES

A long term record of magmatic activity beneath the Goat Rocks volcanic complex is preserved by zircon crystal cargoes with a range of ages that are older than the erupted lavas. Goat Rocks was active between ~3 Ma and 100 ka, based on mapping, ⁴⁰Ar/³⁹Ar ages, and U-Pb zircon rim ages. Volcanism occurred in four major eruptive stages: Tieton Peak (~3 Ma to 2.6 Ma), Bear Creek Mountain (~1.7 to 1.3 Ma), Lake Creek (1.1 to 0.6 Ma), and Old Snowy Mountain (0.4 to 0.1 Ma). We focus on zircons with ages that fall within the latter two stages, as we have few data from the older two.

Typically, zircon rims yield ages consistent with the argon age of the host groundmass, but crystal interiors are often 10⁵-10⁶ years older, which we interpret as recycling of cognate plutonic roots. Much older zircons we interpret as xenocrysts, with ages between 7.5 and 177 Ma. Bulk compositions of magmas from Lake Creek to Old Snowy Mountain stage generally decrease in K₂O, increase slightly in Al₂O₃, and decrease in Zr, Y, and Ce concentrations with time. During Old Snowy Mountain stage, bulk compositions jump to elevated Sr/Y. Zircons exhibit a broader range in Y and Ce concentrations after the approximate onset of Old Snowy Mountain stage (~0.5-0.4 Ma). The youngest zircons (<0.2 Ma) from the Clear Fork andesite stand out from the suite with a broader and overall higher range in Eu/Eu* (0.2 to 0.8, compared to a range of 0.1 to 0.4 in older zircons), consistent with less residual plagioclase. Hf concentrations increase slightly between 1.2 Ma and 0.4 Ma, then decrease slightly in zircons <0.2 Ma. Ti-48 concentrations more dramatically mirror this trend, decreasing from a broad range of 15 to 40 ppm between ~1.2-1.1 Ma, to a much narrower range of 2 to 8 ppm between 0.7 and 0.4 Ma, and increase slightly to a range of 2 to 16 ppm for zircons with age <0.2 Ma. Corresponding Ti-48 temperatures suggest a 200-250°C range within any 50-kyr window, but overall decrease from as high as 983°C to about 700°C between 1.2 and 0.4 Ma, increasing again to a range of about 700° to 875°C after 0.2 Ma.

Overall, zircons crystallized in a cooler and more evolved environment with time, but the most recent eruption, the Clear Fork andesite, appears to tap a rejuvenated (hotter and less evolved) system.