FISSION-TRACK ANALYSIS: A KEY TOOL FOR DISCERNING SEDIMENT PROVENANCE

From its inception fission-track (FT) analysis has been recognised as an ideal tool for detrital studies as the method is reliant on data gathered from single grains. Common applications include constraining the age of biostratigraphically barren sediments and characterisation of source terrain in terms of post-metamorphic cooling (exhumation) history. Most detrital FT studies use zircon and have, over the last decade, been directed at monitoring source exhumation in young orogenic belts where sediment routing systems are known and the time-lag between exhumation and deposition is short. This information defines when an exhumational steady-state was first reached and is diagnostic of maturity within the orogenic system. However the success of zircon FT or any other detrital geochronological method depends on correct interpretation of the significance of measured ages. The presence of polycyclic grains and/or material from unrelated sources can give a misleading provenance picture. For zircon FT data there is an additional issue as to whether measured ages record source formation or exhumation (a similar issue exists for zircon U-Pb dating where it is unclear whether measured ages relate directly to source terrain lithologies or simply reflect inherited ages a consequence of polycyclic history). The real benefit of zircon FT data can only be realised if the true nature of a measured age is known by combining both U-Pb and FT methods in a single study. Zircon FT igneous formation ages are identified by identical U-Pb ages, whilst a diverse range of U-Pb ages can be unified (in terms of source cooling history) if they share the same FT cooling age. Case studies from SE-Asia and North Atlantic basins are used to demonstrate the benefits of FT provenance data and highlight the regional nature of post-metamorphic cooling which leaves a more uniform age signature in basin sediments compared to U-Pb data which generally give more diverse single grain ages.