TESTING THE “HOT GRANITE” HYPOTHESIS FOR HIGH-ZR GRENVILLE GRANITES FROM EASTERN LAURENTIA USING ZIRCON THERMOMETERS

The Grenville orogeny was a protracted (ca. 300 m.y.) series of magmatic-metamorphic events that contributed to the modification of the Laurentian margin in the late Mesoproterozoic. Grenville granitoids throughout eastern Laurentia possess Zr concentrations (300 – 2,200 ppm) that are much higher than any age group or tectonic setting for granite production in Earth history, resulting in exceptionally “zircon-fertile” granites (Moecher and Samson, 2006), which generated a pulse of detrital zircon that has been recycled across Laurentia through the Phanerozoic. Here we explore one possible mechanism of Grenville magmagenesis, i.e. the “hot granite” hypothesis, and use high-Zr granites as sensors of potential “hot zones” of magma generation. Cathodoluminescence imaging is used to assess the presence of an inherited zircon component (“hot” granites should not have xenocrystic zircon); calculate zircon saturation temperature from whole rock compositions (T_Zc should be high compared to collisional/arc granites); perform quantitative modeling of zircon crystallization history for granitoids using rhyolite-MELTS to obtain T_xln (zircon should appear early in the crystallization history for hot granites); and measure Ti contents of zircon via SHRIMP (Ti-in-zircon geothermometry [T_Ti] should return apparent temperatures of > 850 °C). A representative suite of samples from the Blue Ridge (n = 7) range from 320 to 2,210 ppm Zr, and all lack zircon xenocrysts. Lowest Zr samples contain zircon with Qtz, Bt, Kfs, and/or Ap inclusions, but zircon in the highest [Zr] samples lack inclusions. T_Zc ranges from 830 to 1000°C and T_xln ranges from 930 to 1060°C. As [Zr] increases, T_Zc, T_sat, and T_Ti all converge supporting the “hot” granite hypothesis. Based on variation in [Ti] (10 to 80 ppm), zircon crystallizes over a range of temperature that in the highest [Zr] samples begins at ~1000°C. These results serve as constraints on mechanisms (plumes? underplating? thermal lids? high magma flux?) that could potentially generate the unique lithospheric conditions that led to widespread hot granite production at a unique time in Earth history. Questions remaining include: what mechanisms result in variable [Zr] among granites? Can granites be generated in hot zones but have low [Zr]? How would the latter granites be detected?