The age and origin of the South Mountain Batholith (Nova Scotia, Canada) as constrained by zircon U–Pb geochronology, geochemistry, and O–Hf isotopes
Bibliographic record
Abstract
The South Mountain Batholith (SMB; Nova Scotia, Canada) is the largest composite batholith exposed in the Appalachians and lies entirely within the most outboard Meguma terrane. In situ and CA–TIMS U–Pb dating and in situ isotopes (Lu–Hf, O) and geochemistry for zircon from all phases of the SMB constrain its source as well as its evolution. CA–ID–TIMS for zircon yields emplacement (autocryst) ages, indicating a transition from granodiorite (378.7 ± 1.2 to 375.4 ± 0.8 Ma) to leucogranite (375.4 to 371.8 ± 0.8 Ma) over several million years. Furthermore, in situ SHRIMP, LA–MC–ICP–MS, and SIMS analyses of distinct zircon domains reveal: (1) abundant ancient xenocrysts (∼420 Ma to 2.2 Ga); (2) antecryst ages ca. 3–15 million years older than SMB emplacement; (3) autocryst δ 18 O values between +7.3‰ and +9.1‰ (V-SMOW); (4) similar isotopes, REE signatures, and derived fO 2 values among antecrysts and autocrysts; and (5) ε Hf values from the 371.8 ± 0.8 Ma Davis Lake Pluton (DLP) autocrysts that are higher (+1.74 to +4.38) than the rest of the SMB (−2.99 to +1.68). Collectively, these data suggest a protracted magmatic evolution for the SMB with melt generation and assembly from ∼390 to 370 Ma via melting of a metasomatized mantle source followed by contamination, first from the structurally underlying Avalonian terrane and later by metasedimentary wall rocks of the Meguma terrane. The most southwesterly part of the SMB (i.e., DLP) represents a petrogenetically distinct magmatic phase that underwent less overall contamination than the rest of the SMB.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.003 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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".