ZIRCON AS A PROXY FOR "TAKING THE TEMPERATURE" OF GRANITES: AN EXAMPLE USING ZIRCON THERMOMETRY APPLIED TO GRENVILLIAN MID-CRUSTAL MAGMAS IN THE BLUE RIDGE PROVINCE, VIRGINIA

• Main Author: Burk, Samantha Rae
• Format: Others
• Published: UKnowledge 2017
• Subjects: Blue Ridge Province; Grenville Basement; Ti-in-zircon; Geothermometry; Zircon Saturation; Geochemistry; Geology
• Online Access: http://uknowledge.uky.edu/ees_etds/46; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1053&context=ees_etds

The Grenville orogeny was a protracted (~200 m.y.) series of “hot” magmatic- metamorphic events that contributed to the growth of the Laurentian margin in the late Mesoproterozoic. The granites contain remarkably high Zr content, lack xenocrysts, and become zircon saturated at high temperatures, which are all unusual for felsic magmas. The “hot Grenville granite” hypothesis is tested and use of high-Zr granitoids as potential zones of crustal magma generation through: U-Pb geochronology and cathodoluminescence imaging to assess inherited zircon components; quantitative modeling of zircon crystallization history using rhyolite-MELTS; and Ti-in-zircon thermometry. U-Pb zircon ages for two samples from the Virginia Blue Ridge are 1168 ± 25 Ma (2209 ppm Zr; Tzr = 1032°C) and 1050 ± 13 Ma (918 ppm Zr; Tzr = 898°C). A sample from the NJ – Hudson Highlands has been dated at 1018 ± 11 Ma (1238 ppm Zr; Tzr = 960°C). These samples produce crystallization histories that range over higher temperatures (983–1060°C) than colder, low-Zr counterparts and contain Ti concentrations of 2 to 86 ppm. This analytical approach will further the understanding of zircon’s utility and limitations as a proxy in granite petrogenesis, and constrain thermal models that produced uncommon lithospheric conditions that led to widespread hot granite production at a unique period in Earth history.