INSIGHT INTO ZIRCON CRYSTALLIZATION FROM MODELING AND INCLUSION RELATIONS IN HIGH-ZR GRENVILLE GRANITES: IMPLICATIONS FOR ZIRCON THERMOMETERS

Zircon is arguably the most powerful geo-/chemochronometer for studies of crustal evolution. Zircon also has potential for granite thermometry via zircon saturation (T_Zc) and Ti-in-zircon (T_Ti) thermometry, providing insight into ‘hot vs. cold’ granites. As with all geothermometers, geologic factors may limit their utility. Most calc-alkaline granitoids and silicic volcanics have bulk rock [Zr] of 50-200 ppm; Harrison et al. (2007) discuss the meaning of T_Zc and T_Ti in such ‘low’ Zr magmas. We evaluate the meaning of temperatures returned by both thermometers in ‘low’ (110 ppm; T_Zc = 760), ‘moderate’ (280 ppm; T_Zc = 820^oC), ‘high’ (540 ppm, T_Zc = 870^oC), and ‘very high’ (1650 ppm, T_Zc = 980^oC) Zr granites exemplified by Grenville granites from eastern Laurentia by modeling granite crystallization histories using rhyolite-MELTS and examination of zircon inclusion assemblages. Modeling permits monitoring the modal phase assemblage and melt composition with decreasing melt fraction and temperature. The liquidus (T_L) for granitoids with 63 to 71 % SiO₂ studied here ranges from 1030-1130^oC (500 MPa, 1 wt. % H₂O). Primary predictions from modeling are: zircon crystallization (T_xln) begins at higher T than that for the bulk rock T_Zc (ΔT = 110^o, 130^o, 70 ^o, 29^o). In all runs, Opx or Pigeonite is the liquidus phase, always followed by a ternary feldspar, then Afs or Qtz, Ilm (+/- Mag), and finally Ap. Zircon crystallization does not begin until a temperature (T_xln) well below the liquidus (125-175 ^oC) but above the T_Zc, and relatively late in the modal phase crystallization sequence (melt fractions = 0.23-0.70). Depending on zircon crystallization rate, the T_Ti thermometer should return a range of T reflecting the interval over which zircon crystallizes, assuming other equilibrium constraints are met. The upper limit on the T_Ti range should be T_xln, which ranges from 870-1010^oC but in all cases is well below T_L. The highest T_Ti values should be found in the highest [Zr] granitoids, such as those comprising Grenville terranes in eastern Laurentia. However, in most low to moderate [Zr] granitoids the T_Ti thermometer should yield temperatures that correspond to late stages of crystallization of magmas. Zircon in low to moderate Zr granites containing inclusions of Qtz, Afs, Bt, Hbl or Ap requires relatively late crystallization of zircon.