ZIRCON INHERITANCE IN LYON MOUNTAIN GRANITE: EVIDENCE FROM ZIRCON SATURATION TEMPERATURES

Aleinikoff and Walsh (2015) have proposed that the time of crystallization of the Lyon Mountain granite (LMG) is given by the ca. 1170 Ma U-Pb ages of cores in zircons and that ca. 1050 Ma overgrowths on these grains, previously interpreted to be of magmatic origin, are in fact metamorphic. Other than interpretation of the nature of the overgrowths, a critical factor is whether magmas that crystallized to form the Lyon Mountain might be expected to preserve older, xenocrystic zircons. To this end we have calculated zircon saturation temperatures for six samples of LMG, following the method of Hanchar and Watson (2003). Largely because of the high SiO₂ and alkaline nature of these rocks, zircon saturation temperatures are high, ranging from 969 ^oC to 819^oC, but five are greater than 865^oC and four are greater than 900^oC. Estimates of the crystallization temperature of LMG, based on oxygen isotopic data, are on the order of 700^oC. Thus the magmas were likely well below the zircon saturation temperature, and, although some resorption of xenocrystic zircons might occur, it is likely they would be preserved. This observation is consistent with the interpretation of McLelland et al. (2001) that the somewhat corroded ca. 1170 Ma cores observed in LMG zircons are xenocrysts, and consequently support the contention that the surrounding ca. 1050 Ma portions of the zircons are of magmatic origin.