Hot Grenville Granites: Evidence from Zirconium and Zircon Systematics, with Implications for Grenville Crustal Evolution

English Abstract:

Viable models for generating “large hot orogens” require constraints from rocks that are products of the purportedly high-temperature crustal settings. Grenville granites are unusually Zr-rich (500-1800 ppm), lack inherited zircon, and return high zircon saturation temperatures (850-1000^oC) implying the Grenville magmas were 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). Modeling indicates that zircon saturation (i.e., onset of zircon crystallization) in relatively dry granitic magmas with Zr ranging from 300 to 1200 ppm, typical of Grenville granites, begins at 930-1000 ^oC, and almost always after the onset of crystallization of Opx, Plag, Afs, Qtz, Ilm, and Ap. This prediction is consistent with Afs, Qtz, and Ap inclusions in zircon. 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. 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. Our 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.