RECYCLED AND REUSED OR HOT AND BOTHERED? EXPLORING THE EXTREME ZIRCON FERTILITY OF GRENVILLE (1.2 – 1.0 GA) GRANITES

Granitic magmas generated during the Grenville Orogeny, i.e. a series of tectonic events occurring between 1.2 – 1.0 Ga, appear to be the most zirconium enriched compared to any other major granitic magmatic episode in Earth history. Thus, the ‘zircon fertility’ of Grenville granitoids is much higher than for most typical granitoids. This enhanced fertility is responsible for the enormous amount of ~ 1 Ga detrital zircon, found not only in proximity to exposed Grenville crust (i.e. eastern N. America) but as far away as NW Canada and southern California.

The cause(s) of the extreme Zr abundance is not yet known, however a number of competing hypotheses have been proposed. One suggestion is that many c. 1 Ga granitoids contain a significant degree of zircon xenocrysts, thus the high whole-rock Zr contents don’t reflect actual magmatic values. We have begun testing this idea by dating a large number of zircon crystals from selected high Zr granites. Preliminary data suggest the percent of xenocrysts is too low to explain observed Zr contents. An alternative hypothesis is that ~ 1 Ga plates were significantly thicker than modern ones, and that, combined with unusually rapid plate motion, resulted in massive amounts of crustal recycling. Very large degrees of sediment subduction during Grenville orogenesis would lead to magmas that were enriched in incompatible elements as well as having high δ¹⁸O. This hypothesis is being tested by determining whole-rock Nd isotopic composition with δ¹⁸O of zircon on high Zr granitoids. Negative ε_Nd values and high δ¹⁸O would be consistent with a significant recycled crustal component. A third possibility is the magmas generated during Grenville events were unusually hot allowing for a substantial amount of Zr to be incorporated into the magma prior to reaching zircon saturation. This hypothesis is being tested by comparing Ti-in-zircon abundance for a suite of high Zr granitoids compared to a suite of low to moderate Zr granitoids. These combined tests may establish if a single mechanism was the dominant cause of the world’s most zircon fertile episode.