DETRITAL ZIRCON GEOCHRONOLOGY IN GANDERIAN AND AVALONIAN TERRANES OF CAPE BRETON ISLAND AND SOUTHERN NEW BRUNSWICK, CANADA: CHALLENGES AND OPPORTUNITIES
Bibliographic record
Abstract
The definition of Ganderian and Avalonian terranes in the northern Appalachian orogen is based on a long history of inquiry on multiple fronts. Differences in magmatic and metamorphic events and in isotopic characteristics have been used most commonly to separate terranes from each other, and to subdivide them internally. However, detrital zircon geochronology has made increasingly important contributions in recent years and has provided information regarding source areas for Ganderian and Avalonian terranes during their pre-Appalachian evolution. This work is hampered by difficulties in interpreting the provenance of Meso- to Neoproterozoic zircon grains typically interpreted as derived from the West African and/or Amazonian cratons. For example, 1.4 – 1.0 Ga zircon grains can be derived from a number of long-lived magmatic systems in arcs and continental settings associated with the Grenville orogen. The well-documented preservation bias for Grenville-aged zircon grains makes this problem even more difficult to address. In syn- and post-collisional strata all along the Appalachian orogen, distinctions among detrital zircon populations become even more problematic because of the potential Laurentian contribution, which mirrors many of the Meso- to Neoproterozoic source areas in the Amazonian and West African cratons. These younger strata also commonly contain large populations of magmatic zircon grains derived from arc and other magmatic activity during the Ordovician through Devonian accretionary and collisional phases of the Appalachian orogen, making it even more difficult to see the Precambrian contributions and to interpret source areas from small populations of grains. On-going work in Precambrian rocks of the Ganderian Bras d’Or and Brookville terranes in Nova Scotia and New Brunswick has shown that these terranes preserve rocks from a former passive margin of Gondwana, arc rocks formed in Pan-African subduction zones, and rocks formed during the initial stages of rifting of Ganderia from Gondwana. The dominant contributions of zircon grains are from the Amazonian craton, with signatures distinct from those typical of Avalonian rocks of the same age range. In post-collisional strata from the same terranes, consistent distinctions are much more difficult to observe.
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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 | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.006 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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".