The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

Fairchild, Luke M.; Swanson‐Hysell, Nicholas L.; Ramezani, Jahandar; Sprain, Courtney J.; Bowring, Samuel A.

doi:10.1130/abs/2016am-283146

Cited by 9 publications

(16 citation statements)

References 43 publications

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“…As in our results, their data revealed a distinct mid‐temperature component with a shallow upward inclination and a high‐temperature component with a near horizontal inclination. A progression from horizontal to upward inclinations is consistent with the expected change through time if the movement along the Keweenawan Track persisted past the end of rift magmatism (Fairchild et al, ; Swanson‐Hysell et al, ) and is consistent with a later age of remanence acquisition for the mid‐temperature component. While the inclination of the mid‐temperature and high‐temperature components are indistinguishable between our data and that of Henry et al (), the declinations are different such that their declinations are ~24° more northerly than those obtained for BRa.…”

Section: Paleomagnetic Resultssupporting

confidence: 69%

“…Abundant fine‐grained red siltstones within the Freda Formation have a well‐behaved magnetic remanence dominated by hematite (Henry et al, ). A maximum age constraint on the Freda Formation of 1085.57 ± 0.25/1.3 Ma (2 σ analytical/analytical + tracer + decay constant uncertainty; Fairchild et al, ) is provided by an U‐Pb date of a lava flow within the underlying Copper Harbor Conglomerate. Minor volcanics within the Freda Formation on the Keweenaw Peninsula are unlikely to be substantially younger than the youngest dated volcanics within the Midcontinent basin (1083.52 ± 0.23/1.2 Ma from the Michipicoten Island Formation; Fairchild et al, ).…”

Section: Introductionmentioning

confidence: 99%

“…A maximum age constraint on the Freda Formation of 1085.57 ± 0.25/1.3 Ma (2 σ analytical/analytical + tracer + decay constant uncertainty; Fairchild et al, ) is provided by an U‐Pb date of a lava flow within the underlying Copper Harbor Conglomerate. Minor volcanics within the Freda Formation on the Keweenaw Peninsula are unlikely to be substantially younger than the youngest dated volcanics within the Midcontinent basin (1083.52 ± 0.23/1.2 Ma from the Michipicoten Island Formation; Fairchild et al, ). An age of circa 1080 Ma for the basal 500 m of the Freda Formation is consistent with modeling of postrift thermal subsidence (Hutchinson et al, ).…”

Section: Introductionmentioning

confidence: 99%

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Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

2019

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The magnetization of hematite-bearing sedimentary rocks provides critical records of geomagnetic reversals and paleogeography. However, the timing of hematite remanent magnetization acquisition is typically difficult to constrain. While detrital hematite in sediment can lead to a primary depositional remanent magnetization, alteration of minerals through interaction with oxygen can lead to the postdepositional formation of hematite. In this study, we use exceptionally preserved fluvial sediments within the 1.1-billion-year-old Freda Formation to gain insight into the timing of hematite remanence acquisition and its magnetic properties. This deposit contains siltstone intraclasts that were eroded from a coexisting lithofacies and redeposited within channel sandstone. Thermal demagnetization, petrography, and rock magnetic experiments on these clasts reveal two generations of hematite. One population of hematite demagnetized at the highest unblocking temperatures and records directions that rotated along with the clasts. This component is a primary detrital remanent magnetization. The other component is removed at lower unblocking temperatures and has a consistent direction throughout the intraclasts. This component is held by finer-grained hematite that grew and acquired a chemical remanent magnetization following deposition resulting in a population that includes superparamagnetic nanoparticles in addition to remanence-carrying grains. The data support the interpretation that magnetizations of hematite-bearing sedimentary rocks held by >400-nm grains that unblock close to the Néel temperature are more likely to record magnetization from the time of deposition. This primary magnetization can be successfully isolated from cooccurring authigenic hematite through high-resolution thermal demagnetization.

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Section: Paleomagnetic Resultssupporting

confidence: 69%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

2019

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“…Locally, on the Keweenaw Peninsula, lava flows of the Lake Shore Traps erupted within the Copper Harbor Conglomerate and an andesitic lava within these flows has a U-Pb date of 1,085.57 ± 0.25/1.3 Ma (Fig. 1) (10). The Copper Harbor Conglomerate fines upward and is conformable with the overlying shales, siltstones, and sandstones of the Nonesuch Formation, which are the focus of this study.…”

Section: Paleolake Nonesuchmentioning

confidence: 87%

Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy

Slotznick

Swanson‐Hysell

Sperling

2018

Proc. Natl. Acad. Sci. U.S.A.

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Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (FeHR/FeT) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.

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“…Paleomagnetic, geochronological, and geochemical analyses of the MCR rocks have proved useful for deciphering the evolution and geodynamical context of the rift and for constructing the late Mesoproterozoic apparent polar wander path (APWP) for Laurentia (Halls & Pesonen, ; Hnat et al, ; Hollings et al, ; Kulakov et al, ; Palmer & Davis, ; Swanson‐Hysell et al, ). However, while the main and final stages of MCR have been represented by several high‐quality paleomagnetic data sets (Fairchild et al, ; Kulakov et al, ; Kulakov et al, ; Swanson‐Hysell et al, , ; Tauxe & Kodama, ), the paleomagnetic data for the precursor stage remain limited.…”

Section: Introductionmentioning

confidence: 99%

Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

et al. 2018

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We present new paleomagnetic and geochemical data from a suite of the ~1144‐Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4°, I = 72.1°, α95 = 5.5°, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4°, I = 65.5°, α95 = 8.3°, N = 8) previously reported for the nearly coeval ~1142‐Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt‐type mantle sources, consistent with the mantle plume hypothesis for the formation of the ~1.1‐Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1°, I = 70.2°, α95 = 4.5°, N = 27) corresponds to a paleomagnetic pole at Plat = 55.8°N, Plong = 220.0°E (A95 = 7.3°). The new pole merits the highest classification on the Q‐scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high‐quality data from the ~1108‐Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 ± 1.4 cm/year, comparable with the present‐day velocities of continental plates, before switching to extremely rapid motion between ~1108 and ~1099 Ma.

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The End of Midcontinent Rift Magmatism and the Paleogeography of Laurentia

Cited by 9 publications

References 43 publications

Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy

Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

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