FROM SOURCE TO SINK, ZIRCON CAN STINK – OR HOW BILL THOMAS AND I STARTED OUR ADVENTURES WITH DETRITAL ZIRCON GEOCHRONOLOGY

It is commonly assumed that as a crustal source (volcanic arc, microcontinent, supercontinent, etc.) gets closer to a basin then the detritus from that source will reflect, more and more, that of the encroaching source. This led to my “brilliant” idea that perhaps we could nail down the timing of the approach of Gondwana to Laurentia for the beginning of the Alleghanian orogeny using detrital zircon ages. A characteristic of western Africa is its abundance of 2 Ga rocks and its essential absence in eastern Laurentia. Bill was the guru of Pennsylvanian-Permian sandstones in the southeastern U.S., and thus we thought this study would surely show an ever-increasing amount of 2 Ga detrital zircon, providing one of the most accurate pictures for the timing of Pangea formation and the start of the Alleghanian orogeny. Wrong! We found that from the oldest to youngest sandstones from Tennessee, and eventually much of the Appalachian foreland and hinterland, contained almost nothing but Grenville-age detrital zircon. It wasn’t until approximately Devonian time when a very minor amount of Taconian, Acadian/Neo-Acadian, or Alleghenian detrital zircon appears in successively younger strata. There was no Gondwanan signature at all. This led to us to try to explain the cause of the “Grenville Curse” as well as methods to try to get around it. The cause appears to be related to the incredibly high temperatures of much of the Grenvillian granitic magmatism (~ 900+ degrees!) and hence the ability to incorporate vast amounts of zirconium in the magmas, higher than any single age group of granitic magmas worldwide. The way around it seems to be to date detrital monazite, ‘double-dating’ of detrital zircon (combined U-Pb and [U-Th]/He dating), or using other detrital proxies. Both methods show the intense amount of sedimentary recycling (inferred by Bill and others , e.g., Eriksson et al. 2003, 2004), and that the Grenvillian crust is the ultimate source of the majority of detrital zircon, but not the most recent immediate source of sediment, which is what most provenance studies are actually focused on. The precise timing of the arrival of Gondwana in the late Paleozoic is still unknown. Sorry Bill, I guess I should have had a better tectonic idea about Gondwanan collision. But we both, and others, sure did learn a lot from my mistake!