Inherited, enriched, heated, or recycled? Examining potential causes of Earth's most zircon fertile magmatic episode

Abstract

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 zircon. The extreme zircon fertility of Grenville granitoids appears to be unique in the geological record – no major magmatic event before or after produced on average such zircon-rich granitoids. Five potential mechanisms could explain the cause of the very high concentration of Zr (and other high field strength elements) in Grenville age magmas: an unusually high percentage of xenocrystic zircon; production of magmas from extremely trace element enriched sources; enrichment of crustal sources by moderate-temperature melting prior to Grenville magmatism; unusually high magmatic temperatures allowing for considerable Zr incorporation from source to magma prior to zircon saturation; massive continental crustal recycling via assimilation of significant amounts of sediment in subduction zones. Current data are not sufficiently abundant to completely dismiss any of these potential mechanisms. However, the notion that most high-Zr Grenville granitoids are solely the result of a very significant proportion of xenocrystic zircon is viewed as unlikely due to the limited number of xenocrystic ages that have been determined in conventional UPb dating studies as well as the large number of zircon crystals (N ≥ 100) analyzed in this study of three extremely high-Zr granitoids (Zr range = 504–1306 ppm) that failed to demonstrate evidence of significant xenocrystic components. Massive degrees of crustal recycling via sediment subduction is also viewed as an unlikely dominant mechanism as it does not appear that the oxygen isotope composition of Grenville-age zircon is unique. Enrichment mechanisms of potential source regions, such as metasomatizing of the mantle or low-temperature melting of lower crust prior to Grenville magmatism, are viewed as more plausible mechanisms to produce high-Zr magmas. The occurrence of very high magmatic temperatures of at least some high-Zr granitoids, based on modeling and Ti-in-zircon thermometry, also appears to be critical in helping to explain the exceptional zircon fertility of 1.2–1.0 Ga granitoids in North America.Because Grenville magmas had such high Zr contents they produced intermediate-felsic plutons with a much higher zircon content compared to typical granitoids. This resulted in a disproportionate amount of ~ 1 Ga detrital zircon in the sedimentary record. This superabundance of detrital grains can cause biases in provenance studies and crustal growth models that use the magnitude of detrital zircon age modes as a proxy for source contribution or amount of new crustal material generated.