The tenuous geological record of Precambrian oceanic spreading centers; revisiting the Paleoproterozoic Purtuniq ophiolite sheeted dike complex, Nunavik, Canada
Bibliographic record
Abstract
Sheeted dike complexes (SDC) are primary records of oceanic spreading centers, where dikes feed fissural volcanism above axial magma chambers. Their recognition provides unequivocal constraints on oceanic crust accretion at divergent plate boundaries through Earth's history. However, the identification of Precambrian SDCs has often hinged on field observations while their cogeneticity and coevality with the overlying basalt and underlying gabbro remain unaddressed. Here, we examine the SDC of the Watts Group (2.0 Ga), located in the Ungava Orogen (Canada) and interpreted as the oldest unequivocal remnant of a mid-ocean ridge, the ‘Purtuniq ophiolite’. The dominant calc-alkaline affinity of the mafic dikes contrasts with the tholeiitic host gabbro and basalt, ruling out cogeneticity. SHRIMP U Pb dating of zircon from a dike yields an older Archean population from inherited cores (2981 ± 19 Ma to 2663 ± 17 Ma) overgrown by metamorphic rims (1815 ± 11 Ma). The presence of Archean inherited grains in the dikes is consistent with geochemical indicators of contamination by a felsic component. We conclude that the dikes do not represent a SDC but rather an isolated, petrogenetically unrelated dike array intruding the Watts Group. The ‘Purtuniq Ophiolite’ is thus only comprised of basalt and gabbro that are likely part of the Minto-Povungnituk Large Igneous Province (LIP) and does not represent a remnant of oceanic crust formed at a spreading center. A review of 19 possible cases of pre-850 Ma SDC-bearing ophiolites shows there is no definitive evidence for old SDCs formed in a modern-style oceanic spreading center. • SDCs record oceanic spreading centers and are cogenetic and coeval with underlying gabbro and overlying basalt. • The oldest (2.0 Ga) and most widely accepted case of seafloor spreading, the ‘Purtuniq Ophiolite’ or Watts Group, is revealed to be an isolated, petrogenetically unrelated dike array. • A review of 19 cases predating 850 million years finds no definitive evidence for oceanic spreading centers, challenging their representation in Earth's early crustal history.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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 teacher head, 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".