Abstract

Quantitative modeling of crystallization histories using the program rhyolite-MELTS indicates that zircon crystallization in 1.0 to 1.2 Ga Grenville granitoids with Zr concentrations of 300–1200 ppm begins at 930–1000 °C, and always after onset of crystallization of most other phases (pyroxenes, feldspars, quartz, apatite, ilmenite). Zircon saturation temperatures, calculated from whole-rock compositions and Zr concentrations for modeled samples, range from 835 to 997 °C. Zircon in the two samples with the highest Zr (1201 and 829 ppm) analyzed by sensitive high-resolution ion microprobe (SHRIMP) yielded Ti contents of 10–77 ppm, corresponding to Ti-in-zircon temperatures as high as 1035 °C and 915 °C, respectively. These are among the highest Ti-in-zircon temperatures recorded in magmatic rocks. The modeling and SHRIMP data support the hypothesis that high-Zr Grenville granites are hot granites, and support tectonic models that invoke high-temperature (>900 °C) crustal conditions for generation of Grenville magmas.

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