ZIRCON AS A PROXY FOR “TAKING THE TEMPERATURE” OF GRANITES: AN EXAMPLE USING ZIRCON THERMOMETRY APPLIED TO GRENVILLIAN MID-CRUSTAL MAGMAS IN THE BLUE RIDGE PROVINCE, VIRGINIA

The Grenville orogeny is regarded as a protracted (ca. 100 m.y.) series of “hot” magmatic-metamorphic events that contributed to the growth of the Laurentian margin in the late Mesoproterozoic. The Zr-rich granites that have been shown to characterize the Grenville in eastern Laurentia result in exceptionally zircon-fertile granites (Moecher and Samson, 2006). Grenville granitoids throughout Laurentia contain remarkably high Zr contents (300 – 1900 ppm), much higher than any age group or tectonic setting for granite production, they lack xenocrysts, and become zircon saturated at high temperatures (850 – 1000 °C), all of which are unusual for felsic magmas. Here we will test the “hot Grenville granites” hypothesis and use of high-Zr granitoids as sensors of potential zones of crustal magma generation by employing U-Pb geochronology and cathodoluminescence (CL) imaging to assess the presence of an inherited zircon component (“hot” granites should not have xenocrystic zircon); perform quantitative modeling of zircon crystallization history for granitoids with varying Zr concentrations using rhyolite-MELTS (zircon should appear early in the crystallization history for hot granites); and measure Ti contents of zircon (Ti-in-zircon geothermometry should return temperatures of ca. 850 – 1000 °C). SIMS U-Pb zircon ages for two samples from the Virginia Blue Ridge that do not contain xenocrysts are 1168 ± 25 Ma and 1050 ± 13 Ma. These samples contain 2209 ppm and 918 ppm Zr, respectively. A third sample from the Hudson – New Jersey Highlands (Mt. Eve granite), has been dated at 1014 ± 11 Ma (1238 ppm Zr) and likewise does not contain a detectable xenocrystic component. We predict that these samples will contain high-Ti concentrations (20 – 80 ppm) and will produce crystallization histories that range over higher temperatures than their colder, low-Zr counterparts (650 – 750 °C). This combined approach will contribute to the understanding of zircon’s utility and limitations as a proxy in granite petrogenesis, and serve as constraints on thermal models that produced the uncommon lithospheric conditions that led to widespread hot granite production at a unique period in Earth history.