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