WHAT CAUSED THE ‘GRENVILLE CURSE’ OF DETRITAL ZIRCON STUDIES? POTENTIAL MECHANISMS DRIVING EARTH’S MOST ZIRCON-FERTILE MAGMATIC EVENT

The majority of granitic magmas generated during the Grenville Orogeny (1.2 – 1.0 Ga) are extremely zirconium rich (Zr contents often > 500 ppm) and thus typically contain very abundant, and often very large, zircon. No prior, nor any subsequent, orogenic cycle produced, on average, granitoids that are so zircon rich as those generated during the Grenville Orogeny. The extreme zircon fertility of Grenville granitoids resulted in a disproportionate amount of 1.2 to 1.0 Ga detrital zircon grains in the sedimentary record. Exceptional geochemical characteristics of any group of granitoids argues for exceptional crustal conditions for their generation. That Grenville-age zircons are so abundant in the world-wide detrital record, even in regions where remarkably little 1.2 – 1.0 Ga crust is exposed, attests to exceptional magmatic conditions in the late Mesoproterozoic. Four testable mechanisms could explain the cause of the very high concentration of Zr and other high field strength elements in Grenville age magmas: (1) an unusually high percentage of xenocrystic zircon; (2) unusually high magmatic temperatures allowing for considerable Zr incorporation prior to zircon saturation; (3) production of magmas from extremely trace element enriched sources; (4) massive continental crustal recycling via assimilation of significant amounts of sediment in subduction zones. Although it is not yet know if any of these mechanisms is dominant, Grenville magmatism is unique as it is the Earth’s most zircon-fertile magmatic event. Provenance studies that use the magnitude of detrital zircon age modes as a proxy for source contribution are likely to be overestimates for Grenville-aged zircons. Because the zircon fertility of magmas is so variable the abundance of detrital zircon of a specific age should be viewed with caution in terms of using it (1) as a proxy for the degree of melting associated with an orogeny, (2) for inferring that time interval must have been important for juvenile continental crustal generation, (3) as an accurate reflection of exposed crustal area.