ZIRCON-BASED PROVENANCE STUDIES: GOING ALL THE WAY REQUIRES MORE THAN JUST A DATE

U-Pb dating of detrital zircon has become the method of choice for extracting regional thermotectonic history and the potential source terranes of siliciclastic sediments. Recent advances in microanalytical methods (SIMS and LA-ICP/MS) have greatly accelerated the acquisition of U-Pb dates, which in turn has led to an explosion of zircon-based provenance studies. With such rapid analysis (approximate ages can be determined in less than a minute) a far greater number of detrital zircon ages can be measured for a given sample than was feasible in the past. Unfortunately, this ease of analysis has begun to foster a mind set that detrital zircon studies are “quick and dirty.” This has led to a disturbing de-emphasis on characterizing each detrital crystal prior to age analysis. The hope appears to be that with enough analyses the entire zircon age spectrum will be captured and the geologic history of the source region will be unraveled without the need for time-consuming imaging of the grains.

Our data show that detrital zircons hold a wealth of untapped information, obtainable only through detailed examination of each grain. We demonstrate the extreme value of cathodolumiscence imaging (CL) in identifying multiple growth zones within single grains that provided distinguishable ages from the main portion of the crystal. Ages from all growth zones provide a more complete fingerprint of the source terrane. We present U-Pb analyses of detrital zircons in modern alluvium collected from the French Broad River and a number of its tributaries in North Carolina and Tennessee. The French Broad drains the following potential zircon donors: variably metamorphosed Grenville ortho- and paragneissic basement; Neoproterozoic Ocoee metasediments; and Paleozoic plutons, all potentially deformed and/or metamorphosed during Taconian, Neo-Acadian, and Alleghanian orogenesis.

In addition to demonstrating the utility of CL imaging, we present the surprising longitudinal variation of detrital zircon-age populations as different source lithologies are drained. These data from modern drainage systems have important implications for sandstone provenance studies where the primary goal is to identify the source region of the ancient sediment.