Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: New zircon U-Pb age and whole-rock Nd- and Hf-isotope and lithogeochemical results from the Kechika group, Yukon, Canada

Abstract

Post-breakup magmatic rocks are recognized features of modern and ancient passive margin successions around the globe, but their timing and significance to non-plume-related rift evolution is generally uncertain. Along the Cordilleran margin of western North America, several competing rift models have been proposed to explain the origins of post-breakup igneous rocks that crop out from Yukon to Nevada. New zircon U-Pb age and whole-rock geochemical studies were conducted on the lower Paleozoic Kechika group, south-central Yukon, to test these rift models and constrain the timing, mantle source, and tectonic setting of post-breakup magmatism in the Canadian Cordillera. The Kechika group contains vent-proximal facies and sediment-sill complexes within the Cassiar platform, a linear paleogeographic high that developed outboard of continental shelf and trough basins. Chemical abrasion (CA-TIMS) U-Pb dates indicate that Kechika group mafic rocks were generated during the late Cambrian (488–483 Ma) and Early Ordovician (473 Ma). Whole-rock trace-element and Nd- and Hf-isotope results are consistent with the low-degree partial melting of an enriched lithospheric mantle source during margin-scale extension. Equivalent continental shelf and trough rocks along western North America are spatially associated with transfer-transform zones and faults that were episodically reactivated during Cordilleran rift evolution. Post-breakup rocks emplaced along the magma-poor North Atlantic margins, including those near the Orphan Knoll and Galicia Bank continental ribbons, are proposed modern analogues for the Kechika group. This scenario calls for the release of in-plane tensile stresses and off-axis, post-breakup magmatism along the nascent plate boundary prior to the onset of seafloor spreading.

Neoproterozoic Windermere Supergroup Near Bayhorse, Idaho: Late‐Stage Rodinian Rifting Was Deflected West Around the Belt Basin

“…In their study focused on rift-related Neoproterozoic and Cambrian rocks exposed in northern Utah and southeastern Idaho, Yonkee et al (2014) advocated for a shift from pure-shear extension during early rifting followed by a second stage of depth-dependent extension. Recent work on similar rocks along the western Laurentian margin (e.g., Beranek, 2017;Campbell et al, 2019;Moynihan et al, 2019) has also suggested a broad analog of Rodinian rifting to better-constrained models of rifting described for opening of the Atlantic Ocean. However, an incomplete and fragmented geologic record of these events along the projected continuation Plot is based on the cross-correlation metric; the axes are arbitrarily calculated values to illustrate similarity of each data set in 2-D space (Saylor & Sundell, 2016).…”

“…Following Beranek (2017) and Campbell et al (2019), we suggest that the second phase of predominantly Ediacaran and early Cambrian rifting (~600-520 Ma), volcanism, and transition to drift along the Cordilleran margin (Yonkee et al, 2014) consisted of depth-dependent and asymmetric crustal thinning during formation of the necking domain ( Figure 14d) and eventual lithospheric breakup (Figure 14e). During this interval, we identified significant stratigraphic differences from the newly documented sections in central Idaho and those of southeastern Idaho, including likely discontinuous sedimentation and less subsidence, as well as exhumation in east central Idaho.…”

“…For example, workers have proposed that segmentation and asymmetry of the margin across an ancient northeast striking fault along the modern eastern Snake River Plain (Figure 1) separated rift domains formed above oppositely dipping lithosphere‐scale detachment faults (“upper and lower plates;” Link, Todt, et al, 2017; Lund, 2008; Lund et al, 2010). Others (e.g., Beranek, 2017; Campbell et al, 2019) have identified first‐order similarities of the western North American rift margin with the hyperextended, magma‐poor margins of Newfoundland and Iberia.…”

Evaluating new fault‐controlled hydrothermal dolomitization models: Insights from the Cambrian Dolomite, Western Canadian Sedimentary Basin

“…It is possible that CO 2 was sourced by: (i) degassing during volcanism related to the Terreneuvian to Series 2 Cambrian rifting; or (ii) metamorphism of deeply buried carbonate and clastic strata due to high geothermal gradient. Several studies have reported the presence of mafic and tholeiitic volcanism during the same period as rifting in Western Canada (Stewart, ; Cecile et al ., ; Beranek, ; Campbell et al ., ) and it is possible that CO 2 generated during volcanism could have reacted with the deep‐seated serpentinite to liberate large amount of magnesium. Sandiford et al .…”

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

“…Neoproterozoic and early Paleozoic alkalic plutonic suites in northcentral Idaho have been dated at 665-650 Ma (Lund et al, 2010) and 500-485 Ma (Evans and Zartman, 1988;Lund et al, 2010), and similar age igneous rocks are present along most of the western miogeocline boundary (Lund et al, 2010). Recent investigation of mafic rocks in the southern Yukon Territory yielded zircon ages of 488-473 Ma (Campbell et al, 2019).…”