Integrated Nd isotopic and U-Pb detrital zircon systematics of clastic sedimentary rocks from the Slave Province, Canada: Evidence for extensive crustal reworking in the early- to mid-Archean

A combined U-Pb geochronology and Nd isotopic analysis of clastic metasedimentary rocks from the Archean Slave Province, Northwest Territories, Canada is presented. A series of clastic sedimentary rocks with deposition age of ~3.13 Ga to ~2.58 Ga was collected from the west-central Slave Province. These samples include conglomerates as well as finer sedimentary rocks such as greywacke, arkose and quartzite. Although it is generally agreed that the Nd model age (TDM) of clastic sedimentary rocks represents the average sedimentary provenance age, TDM of samples studied here is generally older than the average U-Pb age of detrital zircons extracted from the same sample. Rather, there seems to be a better ~1:1 relationship between the TDM and U-Pb age of the oldest zircon from each sample. A simple model relationship between U-Pb geochronology and TDM suggests that such correlation is a result of Archean crustal evolution of the Slave Province being dominated by crustal reworking (i.e. intracrustal processes) rather than addition of juvenile materials from the mantle. One exception is the period between ~3.1 and ~3.2 Ga, during which period a significant portion of detrital zircons have U-Pb ages exceeding the TDM, indicating a large input of juvenile material and/or decreasing level of crustal reworking. An alternative explanation for the U-Pb vs. TDM trend may be a mixing of older mafic juvenile crust (containing no zircon) and younger felsic juvenile crust. However, the general lack of extensive pre-2.8 Ga mafic crust in the Slave Province makes this scenario unlikely. The results of this study cannot constrain the change in the volume of continental crust at any given period of time, since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle. However, a dominance of crustal reworking during the early- to mid-Archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events. Since the majority of zircon may be lost during crustal reworking processes (unless they are preserved as an inherited core), absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early Earth.