The timing of Dun Mountain Ophiolite emplacement via Rb-Sr isotope dating of metasomatic reactions along the basal Livingstone Fault in New Zealand
Bibliographic record
Abstract
Juxtaposition of oceanic and continental lithosphere along terrane boundary faults is an important tectonic process that can occur during closure of an ocean basin; however, the timing of faulting can be difficult to constrain. Here, we show that a spectacular exposure of the basal fault (Livingstone Fault) to the Dun Mountain Ophiolite in New Zealand may be dated using 87Sr/86Sr isotopes. At this boundary, quartzofeldspathic schist is faulted against the ultramafic base (peridotites and serpentinites) of the ophiolite and has resulted in metasomatic alteration of the schist, driven by the significant geochemical contrast between the contrasting rock types. We show that metasomatic alteration of the schist resulted in near complete removal of Rb due to the loss of mica, an increased modal abundance of metasomatic actinolite and appearance of metasomatic garnet and hedenbergite. Because Rb was removed from the metasomatized schist, its 87Sr/86Sr composition was essentially frozen at the time of metasomatism, while the 87Sr/86Sr composition of unaltered schist evolved due to the radioactive decay of 87Rb. Back calculating the present day 87Sr/86Sr composition of the unaltered schist to the frozen 87Sr/86Sr composition of the metasomatized schist yields a date of 170 Ma + 5 Ma. This date is broadly consistent with geological reconstructions of the Triassic-Jurassic Zealandia margin and provides a minimum age constraint on the timing of juxtaposition of the Dun Mountain Ophiolite against the crustal rocks and therefore the closure of the vast ocean basin along the eastern margin of Gondwana. Similar metasomatic reactions are described in similar settings elsewhere and so this method may be applied outside of this example.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".