EVALUATING ZIRCON THERMOMETRY USING HIGH-ZR GRENVILLE GRANITOIDS

High-Zr granitoids produced during the Grenville orogeny provide a unique opportunity to test the limitations of the Ti-in-zircon and zircon saturation thermometers for determining initial melt temperatures. Zirconium concentrations up to 1900 ppm (whole-rock) are reported for xenocryst-free Grenville granitoids in the Virginia Blue Ridge and New Jersey Highlands (Moecher and Samson, 2006). In melts containing such high-Zr values, zircon likely reaches saturation early in the crystallization sequence. Thus, temperatures obtained from the zircon saturation thermometer may provide insight into near-liquidus conditions. However, the lowest whole-rock Zr value which provides this information is not known. To establish this, we have collected samples from six Grenville plutons from the French Broad massif in western North Carolina whose whole-rock Zr concentrations range from 146-573 ppm. Corresponding zircon saturation temperatures range from 650-810 °C. Since the high-Zr granitoids previously described by Moecher and Samson (2006) were produced during the Grenville orogeny, we considered it important to target plutons with the full range of Zr concentrations of similar age and tectonic setting.

Preliminary SHRIMP-RG zircon U/Pb ages indicate these samples do not contain a xenocrystic component. However, to fully test the degree of xenocrysts a larger number of U/Pb dates from the same suite of samples will be determined to test these initial results. Using SHRIMP-RG, we measured Ti and select REE concentrations on the same spot from which our U/Pb dates were determined. REE patterns aid in identifying the accidental analysis of micro-inclusions not visible in CL or BSE. Ti concentrations for all analyzed samples range from 2-35 ppm, (n=146). Preliminary temperature estimates range from 650-1000 °C, for silica and titania activities of 1 and 0.7, respectively. These are considered minimum estimates because for every 0.1 decrease in titania activity, there is an associated ~15 °C increase in temperature (Ferry and Watson, 2007). The calculated temperatures will be compared with modeled temperatures using whole-rock chemistry combined with rhyolite-MELTS modeling (Gualda et al., 2012).