Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities
Bibliographic record
Abstract
Abstract 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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".