Detrital zircon record and tectonic setting

Detrital zircon spectra re ect the tectonic setting of the basin in which they are deposited. Convergent plate margins are charac-terized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic settings contain greater proportions with older ages that re ect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sedi-ment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.INTRODUCTION Detrital zircons are a minor constituent of clastic sedimentary rocks, yet their physiochemical resilience and high concentrations of certain key trace elements means that they have become an important phase in sedimentary provenance analysis and in crustal evolution studies (e.g., Cawood et al., 2007b; Hawkesworth et al., 2010). Large numbers of in situ, high precision analyses of both igneous and detrital zircons are now available, and a striking feature of the zircon record is that it clusters into peaks of crystallization ages (Condie et al., 2009). Compilations of crys-tallization ages for detrital and igneous zircons show remarkably similar patterns of peaks and troughs, although with some variation in the rela-tive amplitude of the peaks (Condie et al., 2009). This coincidence sug-gests that the sedimentary record is a valid representation of the magmatic record (Hawkesworth et al., 2010).We establish that detrital zircon spectra have distinctive age distribu-tion patterns that re ect the tectonic setting of the basin in which they are deposited. These patterns are principally controlled by (i) the volumes of magma generated in each tectonic setting and their preservation poten-tial, (ii) the ease with which magmatic and detrital zircons of various ages and origins become incorporated into the sedimentary record, and (iii) the record of old zircons incorporated into the sediment. These in turn provide a framework that can be used to constrain the tectonic setting of sedimen-tary packages. This approach distinguishes between three tectonic settings (i.e., convergent, collisional, and extensional), and it is most sensitive when the depositional age of the sediment investigated is well constrained. Basin setting will evolve with tectonic regime; for example, arc-continent or continent-continent collision will result in the evolution of convergent and extensional basins into collisional foreland basins. Hence the three settings distinguished herein are end-members, and the zircon age patterns associ-ated with each show a spectrum of distributions that merge and overlap rather than de ne discrete elds. Discriminant plots developed for igneous rock geochemistry (e.g., Pearce and Cann, 1973) or sediment framework modes (e.g., Dickinson and Suczek, 1979) often have diffuse boundaries or overlap between elds, but remain important approaches in understanding and constraining tectonic setting. Equally important, exceptions to simple end-member classi cations can provide insight into subtleties of tectonic process, such as outlined below for Avalonia in eastern North America.