POTENTIAL IMPACT OF DIFFERENTIAL ZIRCON FERTILITY OF GRANITOID SOURCE ROCKS ON CORDILLERAN DETRITAL ZIRCON POPULATIONS

Populations of Grenville-age (0.95-1.35 Ga) detrital zircons form prominent U-Pb age peaks in many Cordilleran sedimentary assemblages of varied depositional age even though the Grenville orogen occupies <10% of Laurentian Precambrian basement and is confined to the southeastern fringe of Laurentia. One factor influencing the inordinately high content of Grenville zircons in diverse sedimentary assemblages of North America may be the anomalously high Zr content of Grenville plutons (Moecher and Samson, 2006). A compilation of 1290 whole-rock Zr analyses (ppm) for 67 North American suites of granitoid rocks (including associated diorite and syenite but not gabbro or anorthosite) reveals systematic variations in Zr content and inferred zircon content (quantitative estimates of percentage zircon are not generally available for any rocks). Some Zr enters the lattices of igneous pyriboles but most Zr in magmas probably contributes to the formation of magmatic zircon. Subduction-related granitoid suites of multiple ages (Archean, Paleoproterozoic, accreted peri-Gondwanan Neoproterozoic, Appalachian Paleozoic, Cordilleran Mesozoic) have a mean Zr content of 150 ppm, with a typical range for individual samples of 50-250 ppm at 1ó, and are assigned a zircon fertility factor of 1.0. By contrast, Grenville granitoid suites associated with crustal collision have a mean Zr content of 525 ppm, with a typical range for individual samples of 275-775 ppm at 1ó, and are accordingly assigned a zircon fertility factor of 3.5. Anorogenic plutons of the transcontinental Mesoproterozoic belt (1490-1370 Ma), and of Appalachian pre-Iapetan and pre-Atlantic rift belts, have an intermediate mean Zr content of 375 ppm, with a typical range for individual samples of 250-500 ppm, and are assigned a zircon fertility factor of 2.5. If zircon content is proportional to Zr content in granitoid rocks, erosion of the same volume of source rock would produce age peaks in populations of detrital zircons that are 3.5 times as large for Grenville derivation as for derivation from arc-related plutons of any age, and peaks 2.5 times as large as the latter for derivation from anorogenic sources. Volumetric provenance interpretations based on age populations of detrital zircons should allow for differential zircon fertility of source granitoids.