TAKING THE TEMPERATURE OF GRENVILLIAN GRANITIC MAGMAS – HOW HOT IS HOT?

Estimating the temperature of granitic magmas is a fundamental aspect of understanding igneous petrogenesis but one that is fraught with difficulty. There are a variety of methods used to try to constrain magmatic temperatures, such as using the zirconium concentration of granitoids along with their major element chemistry (the Zr saturation temperature pioneered by Watson and Harrison). Well understood metamorphic reactions can be used to place some constraints on the crustal temperatures of a region. The oxygen isotopic composition of two minerals (e.g. quartz and magnetite) in an undisturbed igneous rock can shed light on probable temperatures. More recently, the use of Ti-in-zircon has been used to estimate magmatic temperatures. However, employing any paleothermometer requires a series of assumptions. How representative of the chemistry of a granitoid is actually related to the initial composition of the magma? Are the measured oxygen isotopic composition of two minerals original or are have they been altered post crystallization? When did the zircon crystallize in the magma – near the liquidus temperature or closer to the solidus temperature? And what was the silica and titania activity of the magma during zircon crystallization? Using different parameters can have a profound effect on the calculation of magmatic temperature. What if the titania activity is 1.0 (the early view of the Ti-in-zircon thermometer), or if 0.6, or if 0.3? The calculated temperature can vary significantly with these different values.

The Grenville orogenic events produced massive amounts of zircon that have dominated the Paleozoic through Modern sedimentary systems in Eastern North America. We will present examples of how Grenvillian magmatic temperatures appear to be exceptionally high, allowing for the dissolution of significant amounts of Zr into the magmas. The calculation of these temperatures, however, depend on the parameters chosen. We also present our “best practice” for making these temperature estimates. While imperfect, we argue that many calculated Grenvillian granitic magmas appear to be far hotter than most granitic magmas that formed before and after the Grenvillian orogenic cycle – i.e., the Grenville events may indeed have seen the hottest granitic magmas ever recorded in Earth history.