RE-OS GEOCHRONOLOGY OF THE NEOPROTEROZOIC – CAMBRIAN DALRADIAN SUPERGROUP OF SCOTLAND AND IRELAND: IMPLICATIONS FOR NEOPROTEROZOIC STRATIGRAPHY, GLACIATIONS AND RE-OS SYSTEMATICS

The Dalradian Supergroup of Scotland and Ireland is a thick (up to 25 km) metasedimentary succession that records deposition on the eastern margin of Laurentia during the late Proterozoic. The distinctive Port Askaig Formation of the Argyll Group is a glaciogenic unit consisting of >900 m of diamictites interbedded with conglomerate, sandstone and mudstone. The paucity of fossils and horizons suitable for U-Pb geochronology in the Dalradian Supergroup severely limits our chronostratigraphic understanding and hampers correlation of the Dalradian Supergroup with better constrained Proterozoic successions.

New Re-Os geochronology for the Ballachulish Slate Formation of the Dalradian Supergroup yields a depositional age of 659.6 ± 9.6 Ma (Model 1 age). The Re-Os age represents a maximum age for the glaciogenic Port Askaig Formation and suggests deposition of the Argyll Group lasted less than 60 Ma. This study also represents the first successful application of the Re-Os geochronometer in sedimentary rocks with low Re and Os abundances (<1 ppb and <50 ppt, respectively).

The Re-Os geochronology refutes previous chemostratigraphic and lithostratigraphic studies, which correlated the Port Askaig Formation with a series of middle Cryogenian (ca. 715 Ma) glacials. Thus, the Re-Os geochronology implies that the Port Askaig Formation is correlative with the Sturtian (ca. 650 Ma) glaciations of Australia.

Furthermore, the Re-Os geochronology enhances our understanding of the tectonic evolution of the Dalradian basin and helps refine Neoproterozoic palaeogeographies related to the formation and breakup of the Rodinia supercontinent.

Initial Os isotope data from the Ballachulish Slate Formation coupled with data from Australia reveals a radiogenic Os isotope composition (~0.8 to 1.0) for seawater during the late Neoproterozoic, which is comparable to that of seawater today. This indicates that the contribution of radiogenic Os from riverine and weathering of continental crustal material dominated over the influx of unradiogenic Os from hydrothermal alteration of oceanic crust and peridotites.