Constraints on the early metamorphic evolution of Broken Hill, Australia, from <i>in situ</i> U‐Pb dating and REE geochemistry of monazite
Bibliographic record
Abstract
Abstract The Broken Hill Pb‐Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U‐Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub‐solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X‐ray maps, SHRIMP U‐Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c. 1630 Ma and c. 1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre‐heated by magmatic underplating at c. 1657 Ma.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 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".