Ti-in-zircon thermometry and crystallization modeling support hot Grenville granite hypothesis

• Published in: Geology (Boulder) 2014-03, Vol.42 (3), p.267-270
• Main Authors: Moecher, D. P, McDowell, S. M, Samson, S. D, Miller, C. F
• Format: Article
• Language: English
• Subjects: Appalachians; concentration; Crystallization; Geochemistry; geologic thermometry; gneisses; Granite; granites; Grenvillian Orogeny; igneous and metamorphic rocks; Igneous rocks; Magma; Mathematical models; Mesoproterozoic; metals; metamorphic rocks; nesosilicates; North America; orthogneiss; orthosilicates; P-T conditions; Petrology; Phase transitions; plutonic rocks; Precambrian; Proterozoic; Quartz; rock, sediment, soil; Shrimps; silicates; Tectonics; temperature; Temperature effects; Titanium; trace elements; upper Precambrian; Zircon; zircon group; Zirconium
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G35156.1

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.