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, s… Show more
“…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).…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: 1029/2020tc006145mentioning
confidence: 95%
“…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.…”
Conflicting models of Rodinian rifting have been proposed to explain the recognized variation in the Neoproterozoic and early Cambrian tectonostratigraphic architecture of the western Laurentian margin. However, discrimination among rift models is hampered by limited exposure and metamorphism of the rocks. Southeastern Idaho preserves more than 6 km of Neoproterozoic and Cambrian strata. In contrast, along the inferred continuation of the margin in east central Idaho, correlative rocks are missing across the Lemhi arch. Our field mapping and U‐Pb dating studies, located approximately 50 km west of the Lemhi arch unconformity, focused on a succession of regionally extensive rocks that were previously assigned an Ordovician age. We show that ~1.5 km of strata here overlies a ~667 Ma reworked felsic tuff and was intruded by a 601 ± 27 Ma gabbro sill; we thus redesignate these rocks as Cryogenian and Ediacaran in age. These rocks are overlain by a ~1 km thick Ediacaran to middle Cambrian quartzite. Middle Ordovician quartzites overlie these middle Cambrian strata, indicating that though Neoproterozoic and lower Cambrian rocks are present west of the Lemhi arch, upper Cambrian and Lower Ordovician rocks are thin or absent. Comparison of this redesignated section to the closest correlative sections suggests an initial stage of symmetric rifting followed by later asymmetric rifting. We suggest that prerifting ~1,370 Ma magmatism within the Belt basin produced lithospheric rigidity that influenced the final stage of rifting and produced heterogeneity in the geometries of structural domains similar to those documented in other well‐defined, modern rift margins.
“…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).…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: 1029/2020tc006145mentioning
confidence: 95%
“…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.…”
Conflicting models of Rodinian rifting have been proposed to explain the recognized variation in the Neoproterozoic and early Cambrian tectonostratigraphic architecture of the western Laurentian margin. However, discrimination among rift models is hampered by limited exposure and metamorphism of the rocks. Southeastern Idaho preserves more than 6 km of Neoproterozoic and Cambrian strata. In contrast, along the inferred continuation of the margin in east central Idaho, correlative rocks are missing across the Lemhi arch. Our field mapping and U‐Pb dating studies, located approximately 50 km west of the Lemhi arch unconformity, focused on a succession of regionally extensive rocks that were previously assigned an Ordovician age. We show that ~1.5 km of strata here overlies a ~667 Ma reworked felsic tuff and was intruded by a 601 ± 27 Ma gabbro sill; we thus redesignate these rocks as Cryogenian and Ediacaran in age. These rocks are overlain by a ~1 km thick Ediacaran to middle Cambrian quartzite. Middle Ordovician quartzites overlie these middle Cambrian strata, indicating that though Neoproterozoic and lower Cambrian rocks are present west of the Lemhi arch, upper Cambrian and Lower Ordovician rocks are thin or absent. Comparison of this redesignated section to the closest correlative sections suggests an initial stage of symmetric rifting followed by later asymmetric rifting. We suggest that prerifting ~1,370 Ma magmatism within the Belt basin produced lithospheric rigidity that influenced the final stage of rifting and produced heterogeneity in the geometries of structural domains similar to those documented in other well‐defined, modern rift margins.
“…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 .…”
Fault‐controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including ‘deep‐seated’, ‘crustal’ or ‘basinal’ brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large‐scale dolostone bodies are fabric‐destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross‐cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the δ18Owater and 87Sr/86Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO2‐rich fluids. This study highlights that large‐scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.
“…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).…”
The Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.
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