Paper No. 79-12
Presentation Time: 11:25 AM
CREATING THE CANADIAN SHIELD AND THE ‘GREATEST’ UNCONFORMITY: ENHANCED CRUSTAL EROSION DURING RODINIA BREAKUP AND SNOWBALL EARTH GLACIATIONS
Central tenets of the snowball Earth hypothesis are that mantle upwelling and widespread igneous magmatism during equatorial supercontinent breakup led to increased silicate weathering, CO2 consumption, and biosphere feedbacks that drove Neoproterozoic low-latitude glaciation1-4. The synchronicity of these events is key to this hypothesis, yet the timing of significant Canadian Shield planation and weathering of Precambrian cratonic rocks remains an important unresolved question. Stratigraphic evidence for Neoproterozoic continental denudation is lacking because Paleozoic sedimentary rocks unconformably overlie Archean basement – a gap of >2 billion years coinciding with the Great Unconformity. New and published apatite U-Pb, 40Ar/39Ar K-feldspar MDD, apatite fission-track, and zircon/apatite (U-Th)/He data from ≥1.8 Ga crystalline basement rocks across the Canadian Shield (e.g. W. Superior Province, ON & Baffin Island) constrain temperatures below ~450ºC and collectively address the broad time-temperature range required for constraining significant exhumation over such protracted histories. Modelled thermal histories indicate that parts of the Laurentian interior were at temperatures of ~150-200ºC at ca. 1.1-0.95 Ga due to minor Grenvillian burial5 and/or residence at crustal depths of up to ~7 km. Cooling ensued ca. 0.90-0.70 Ga, coincident with initial Rodinia breakup. Major cooling and exhumation began after ca. 0.7 Ga and concluded when rocks were at near-surface conditions during Paleozoic platform sedimentation. Our interpretations agree with seawater 87Sr/86Sr compilations4 that advocate a shift from mantle (hydrothermal/submarine) to continental flux during this time interval. Further support comes from Archean detrital zircons in Cambrian sandstones from the Laurentian rift-margin6-8 and interior basins9. The observed regional cooling signal indicates large-magnitude unroofing that directly opposes inferred long-term cratonic stability and suggests a large area of the Canadian Shield was exhumed in the Neoproterozoic. This supports a model where renewed Neoproterozoic crustal weathering set the conditions for snowball Earth.
References:  Hoffman & Schrag 2002;  Goddéris et al. 2003;  Donnadieu et al. 2004;  Cox et al. 2016;  Rainbird et al. 1992;  Cawood et al. 2007;  Matthews et al. 2017;  McMechan et al. 2017;  Lovell & Bowen 2013