A REFINED APPROACH FOR USING DETRITAL ZIRCON TO INTERPRET THE MAXIMUM DEPOSITIONAL AGE OF CLASTIC ROCKS: USING LA-ICPMS AND CA-IDTIMS IN TANDEM

U-Pb detrital zircon geochronology is a robust and valuable technique for investigating the sedimentary provenance and maximum depositional age (MDA) of clastic rocks. While detrital zircon has become widely used, no consensus has been reached for interpreting the MDA from a given dataset. Several approaches have been suggested and used in the literature with varying degrees of success. In general, the use of multiple youngest grain ages or the youngest mode of ages has been suggested to provide more reliable results compared to the use of the single, youngest detrital zircon. In this study, we use detrital zircon data from Triassic-Jurassic strata (the Boulder Creek Beds) from the Jackson Mountains (NW Nevada) to argue that LA-ICPMS used in tandem with CA-IDTIMS provides a more accurate and precise measure of MDA in clastic rocks. All analyzed subunits from the Boulder Creek Beds contain a dominant Late Triassic to Early Jurassic mode with tails defined by grains with apparent dates as much as 13-17 Ma younger than the dominant mode. When several of those youngest grains are analyzed using the more precise CA-IDTIMS, the resulting ages are up to ~17 My older and generally overlap with the primary youngest mode of LA-ICPMS data. This relationship suggests that portions of those youngest grains have experienced varying degrees of Pb loss, that when chemically abraded in concentrated HF, the portion of the grains with the loss is removed. Therefore, an interpretation of MDA using those youngest grains analyzed by LA-ICPMS would be biased by as much as 17 My younger than the true geological age. While in some instances this may not be a significant issue, in the Jackson Mountains this change in age relationships would drastically alter the geologic history and timing of structural events in the region. These new data from the Jackson Mountains highlight the need for exercising caution in the interpretation of MDA using detrital zircon. We strongly discourage the use of the youngest detrital zircon age and/or several ages from LA-ICPMS data when investigating the MDA due to problems associated with Pb loss. When solely using LA-ICPMS, these data support the use of the youngest mode in the interpretation of the MDA. However, if possible, the tandem use of LA-ICPMS and CA-IDTIMS is a more robust tool to investigate the MDA of sedimentary rocks.