HAFNIUM ISOTOPE EVIDENCE ON THE PROVENANCE OF ~1.1 GA DETRITAL ZIRCONS IN WESTERN NORTH AMERICA

“Grenville”-age (1.0 Ga to 1.3 Ga) detrital zircons are common in Neoproterozoic to Cambrian siliciclastic sedimentary rocks throughout western North America. These zircons are thought to have been fluvially transported across the continent from sources in Grenville bedrock along the eastern margin of Laurentia. To test this assertion, we obtained high precision (solution-based MC-ICPMS) Hf isotopic data from individual Grenville detrital zircons in Neoproterozoic to Cambrian sedimentary rocks in northern Sonora and southeastern California. The ~1.1 Ga detrital zircons in the Neoproterozoic El Arpa sandstone in the Caborca region have low measured ε_Hf (-22 to -26) that overlap the values determined for the local 1.1 Ga Aibo Granite (~-26). These zircons are likely of local origin. In contrast, ~1.1 Ga detrital zircons in “Unit 1” of the overlying Cambrian Puerto Blanco Formation have significantly higher ε_Hf (-15 to -20) for which no local Mesoproterozoic source is known. In the Death Valley region, detrital zircons in the correlative middle member of the Wood Canyon Fm. also have high ε_Hf (-10 to -18). The high ε_Hf zircons in both units correlates with their high whole rock ε_Nd(0) values of -5 to -10. Paleocurrent data suggest that both units were derived from sources to the east (in present coordinates) and the occurrence of high ε_Nd and high ε_Hf zircons in both imply a common source that was widely dispersed across the SW portions of Laurentia during the Cambrian. There is insufficient published Hf isotopic data from potential “Grenville” basement sources to fully refine the location of the source of these high ε_Hf zircons, although distal Grenville-age intrusive rocks in the Adirondacks can be ruled out due to their relatively low ε_Hf (Bickford et al., 2010). We are currently assessing whether more proximal Mesoproterozoic crustal sources in the southern Appalachians, north Texas, and within the mid-Cambrian Transcontinental Arch in New Mexico and/or Colorado could have been the primary sources of the high ε_Hf detrital zircons.