CRYSTALLIZATION MODELING AND TI-IN-ZIRCON THERMOMETRY SUPPORT THE “HOT” GRENVILLE GRANITE HYPOTHESIS

The Grenville is claimed to have been one of Earth’s largest and hottest orogens. Any viable model for generating extensive, hot conditions in the lithosphere requires constraints from rocks that are the product of these purportedly high-temperature crustal settings. Grenville granites are unusually Zr-rich (500-1800 ppm Zr), generally lack inherited zircon, and return high zircon saturation temperatures (850-1000^oC) implying the Grenville magmas are exceptionally hot. We test the “hot Grenville” granite hypothesis and the utility of high-Zr granites as probes of deep crustal melting regimes using a combination of (1) Ti-in-zircon thermometry and (2) quantitative modeling of the modal phase crystallization history of variable-Zr granites using the rhyolite-MELTS program. Hot (~850-1050^oC), high-Zr granites should contain zircon with high Ti contents (10-80 ppm Zr). Quantitative modeling of crystallization histories indicates that zircon saturation (i.e., onset of zircon crystallization) in relatively dry granitic magmas with Zr ranging from 322 to 1650 ppm, typical of Grenville granites, begins at 950-1050 ^oC, and always after the onset of crystallization of most other phases from the melt (Opx, Plag, Afs, Qtz, Ilm, Ap). This prediction is consistent with the presence in some zircon grains in modeled samples of inclusions of Afs, Qtz, and Ap. Zircon in two high-Zr Grenville granites (1201 and 829 ppm Zr) analyzed by SHRIMP yield Ti contents of 10-77 ppm, and apparent Ti-in-zircon temperatures of 1035-850 and 915-780 ^oC, respectively (assuming the same silica and titania activities for all analyses). These are among the highest Ti-in-zircon temperatures recorded in magmatic rocks, and demonstrate that meaningful Ti-in-zircon temperatures can be retrieved in suitable samples. The pattern of zircon rims (visible from CL imaging) having higher Ti than cores, and the exceptionally slow diffusion of Ti in zircon even at such high temperatures, requires two periods of zircon growth under different, albeit high temperature magmatic regimes. The modeling and SHRIMP data support the hypothesis that high-Zr Grenville granites are indeed ‘hot’ granites, and support tectonic models that invoke high temperature (900-1050 ^oC) lower crustal conditions for generation of Grenville magmas.