Detrital zircon ages: Improving interpretation via Nd and Hf isotopic data

Abstract

The bulk of sedimentary provenance studies use similarities in the detrital zircon age patterns or “barcodes” in sedimentary systems and potential source regions to make interpretations about palaeogeographic settings. While this “age-only” approach is generally considered to be effective, it is limiting because the timing of zircon growth events may not be unique to specific terrains and important rock forming events may be associated with little zircon growth. To a significant extent these limitations can be overcome by employing additional isotopic data sets such as Sm–Nd and Lu–Hf that provide information on the crustal evolution of the source region, and allow comparisons with sedimentary packages. As an example, the age spectra of detrital zircons in Palaeoproterozoic metasedimentary rocks in the eastern Gawler Craton in southern Australia are virtually identical to the dominant zircon growth timelines in adjacent older domains of the Gawler Craton, presenting a prima facie case that it was the source region. However, whole rock Nd isotopic data indicate that the pre-existing proto Gawler Craton was isotopically too crustally evolved ( ε Nd (1850 Ma ) − 10) to have supplied the bulk of the sediment to the relatively more juvenile metasedimentary units ( ε Nd (1850 Ma) − 1 to − 5). In addition, zircon Hf isotopic compositions from ca 2000 Ma detrital zircons in the metasedimentary rocks ( ε Hf (2000 Ma) + 2 to + 5) are significantly more juvenile than 2000 Ma rocks in the adjacent Gawler Craton ( ε Hf (2000 Ma) − 2 to − 5). The combination of bulk rock Nd and Hf zircon data suggest that the Gawler Craton is not a viable source region for the metasedimentary packages, despite the striking similarity between detrital zircon ages and zircon crystallisation events within the craton.