GRENVILLE ZIRCON FERTILITY, BABY BOOM, AND BABY BOOM ECHO: NATURAL BIAS IN THE DETRITAL ZIRCON RECORD

Grenville-aged (~1150-1050 Ma) granitoid plutons and gneissic equivalents of eastern Laurentia are as much as 20 times more Zr-rich, and significantly more fertile for crystallizing zircon, compared to Paleozoic Appalachian granitoids of non-Laurentian terranes. Erosion of Grenville sources (e.g., Blue Ridge terranes) will generate proportionately greater amounts (number and/or size) of detrital zircon, but essentially the same amount of other clastic detritus (primarily quartz), compared to an equal volume of less fertile magmatic sources (e.g., Carolina terrane). The latter are rendered nearly undetectable by standard detrital zircon provenance methods (SHRIMP or LA-ICP-MS analysis of magmatic cores of > 100 micron grains). Grenville modes dominate detrital zircon age distributions for Appalachian Neoproterozoic-Cambrian rift basins (e.g., Unicoi Fm.), foreland syn-orogenic clastic sequences (Taconian, Acadian, and Alleghanian), metasedimentary terranes (Inner Piedmont), and modern Appalachian drainage systems (e.g., James, New Rivers). The dominance is an artifact of the Grenville zircon ‘baby boom' that echoed through successive orogenies and denudation events. Grenvillian zircon fertility biased the detrital sedimentary record as a result of: (1) zircon durability and insolubility in aqueous fluids means detrital zircons eroded from Grenville basement terranes are recycled during repeated orogenesis; (2) inertness of zircon below upper amphibolite facies (onset of anatexis), and high Zr resulting from concentration of detrital zircon in sedimentary protoliths, means dominantly metasedimentary terranes will fail to crystallize sufficient new zircon corresponding in age to the time of accretion of those terranes to Laurentia. Zircon growth under anatectic conditions generates new zircon mostly as overgrowths on detrital magmatic Grenville cores; overgrowths are often too thin to analyze by ion or laser beam. Metasedimentary terranes will therefore be severely underrepresented in detrital zircon age distributions. The natural bias in the detrital zircon record resulting from zircon petrogenesis further compounds bias related to sampling and microanalysis protocols that typically select for large zircons and involve ion probe or laser ablation analysis of magmatic cores.