NEW U-PB CONSTRAINTS ON GUNBARREL VOLCANISM AND THE BREAK-UP OF RODINIA: THE 775 MA LITTLE DAL BASALTS

The Little Dal Basalts are a marker unit within the Neoproterozoic stratigraphy of the Mackenzie Mountains, yet they have not previously been dated directly. We present a CA-ID-TIMS ²⁰⁶Pb/²³⁸U zircon age of 774.93 ± 0.54 Ma (2σ) for the eruption of the Little Dal Basalts, an event that preceded fluctuations in the Earth’s climate that led to multiple, potentially global, glacial events. This date provides a minimum age for the Mackenzie Mountain Supergroup and a maximum age for the Coates Lake Group. A direct geochronological link has been established between the Little Dal Basalts and the ~780 Ma Gunbarrel magmatic event which confirms previous geochemical and paleomagnetic correlations. The Gunbarrel event signalled the break-up of Rodinia and owing to the high precision of our age, we suggest that this occurred at ~775 Ma as opposed to ~780 Ma.

Zircon Hf data from the Little Dal Basalts strongly implicate the mantle as a significant component of the source for Gunbarrel magmas. A mantle origin is consistent with the notion of a super-plume beneath Rodinia prior to the break-up of the supercontinent. A xenocrystic zircon from the Little Dal Basalts returned a ²⁰⁷Pb/²⁰⁶Pb age of 1565 ± 1 Ma (2σ) which may provide evidence for the age and make-up of the source area or crust through which the magma passed. We demonstrate that the Gunbarrel magmatic event did have a volcanic component and therefore it is possible that the weathering of Gunbarrel volcanic rocks and drawdown of CO₂ may have influenced global climate leading up to the Sturtian glaciation associated with Snowball Earth. Eruption and weathering of Gunbarrel volcanic rocks may have occurred in other areas, perhaps on other continents, and the existence of this volcanic marker provides a means for stratigraphic correlation across Rodinia. The potentially widespread nature of this marker and high-precision U-Pb geochronology make Gunbarrel volcanic rocks a good tool for global correlation and definition of Tonian and Cryogenian Neoproterozoic strata.