MORE THAN DATES: PROVENANCE OF DETRITAL ZIRCON BY EXCIMER LASER ABLATION ICP-MS

Excimer laser ablation (ELA) ICP-MS permits accurate and precise in situ (15-85 µm spot) measurement of U-Th-Pb isotopes and selected major and trace elements in 20-30 sample zircon grains per analytical hour. We have used these capabilities in studies of modern and ancient detrital zircons from a variety of geologic settings including: rivers in North and South America, Neogene volcanics in the Asia-Pacific region, Neoproterozoic through late Paleozoic sandstones of the Appalachians, and Proterozoic conglomerate and Archean quartzite in Brazil. By analyzing large numbers of grains (minimum n=60, typical n=100) for both their age and chemical composition, we have been able to place tighter constraints on the provenance of these siliciclastic materials. We report on some of the highlights of our work to date in southeastern North America.

In the central and southern Appalachians, ~80% of detrital zircons from modern river sands and Neoproterozoic through late Paleozoic sandstones yield Grenville ages, recording continuous magmatism from 1250 Ma to 950 Ma with a peak at 1050-1010 Ma. This continuum of ages contrasts with previous geochronological studies of Grenville crystalline rocks that have identified at least 3 discrete events at 1190-1140 Ma, 1080-1020 Ma and 1000-980 Ma. The similarity of detrital zircon age distributions in river sands and sandstones indicate that Grenville-age detritus has dominated the sedimentary mass in eastern North America for the past 600-1000 million years. Detrital zircons of Paleozoic age in these same samples exhibit age peaks at 420-500 Ma and 350-380 Ma corresponding with the Penobscot-Taconic and Acadian orogenies. Interestingly, many of the Acadian-age zircons have Th/U < 0.1, implying a metamorphic rather than an igneous origin. Only 2% of detrital zircons in the Savannah River have Alleghenian ages (250-330 Ma) which is unexpected considering the widespread occurrence of late Paleozoic plutons in the southern Appalachians. Equally surprising is the rarity (1 out of 116) of Archean-age zircons in the Ohio River given the availability of glacially deposited sediment derived from sources to the north that presumably included the Superior Province.