THE SUPERCONTINENT CYCLE REFLECTED IN THE AGE RECORD OF THE ARCHEAN SCLM (Invited Presentation)

The continental crust and sub-continental lithospheric mantle (SCLM) are linked such that the latter may act as a life raft for the former and, thus, both have co-evolved as part of the global supercontinent cycle. Testing models for the long-term evolution of the continental lithosphere requires resolving the long-term age record of the SCLM; however, this is complicated by the fact that obtaining reliable age constraints for rock-forming minerals in mantle rocks is difficult. Garnet Lu-Hf geochronology is ideally suited for this challenge as the technique typically enables robust P-T-t-D information for crustal rocks—despite extreme thermal histories—and may record petrochemical processes that the Sm-Nd chronometer no longer preserves. In this study, we used a refined Lu-Hf garnet chronology method to investigate the age record of pyrope exsolved from majorite in fragments of the Laurentian SCLM now exposed as orogenic peridotites in the Western Gneiss Complex of Norway.

The peridotite bodies mainly comprise (garnet-)dunite, which records decompression and melting from >350 km depth, and volumetrically minor fertile lithologies produced by melting and metasomatism. Pyrope exsolved from majorite in two dunite samples yielded identical c. 2.7 Ga internal isochrons results; these are the first Archean internal-isochrons obtained for mantle garnet and, together with mineral chemistry, indicate that these dates represent the timing of extraction of these rocks from the asthenosphere. The Neoarchean was a time in which there was voluminous crust formation, which may plausibly be linked to the deeply sourced mantle upwellings that led to the formation of these dunite samples. In contrast, the websterite- and clinopyroxenite samples yielded c. 1.35 Ga and c. 0.61 Ga internal isochron ages that fall within two supercontinent break-up intervals that affected the Laurentian and Baltic cratons and likely reflect secondary melting and metasomatism of the mantle in response to added heat to the base of the lithosphere. Together, these Lu-Hf results indicate that the Laurentian SCLM has remained petro-physically and chemically stagnant since its extraction in the Neoarchean except for discrete pulses in response to the supercontinent cycle.