Tracking the Growth of Arc Batholiths: A Zircon O–Hf Isotope Approach from the Bega Batholith, Southeastern Australia
Bibliographic record
Abstract
Abstract Subduction-related magmatic arcs are considered principal sites of continental growth, but the origin of the voluminous felsic (granitic) igneous rocks emplaced in these settings is uncertain. Probing the magmatic history of these granites can provide insight into the magma plumbing systems responsible for differentiation and, potentially, generation of continental crust. Seminal studies suggest derivation of the Lachlan Fold Belt I-type granites in eastern Australia by partial melting of infracrustal sources, yet these same granites show field and chemical evidence for interaction between mantle-derived magmas and older source components, obfuscating the link between I-type granite petrogenesis and crustal growth. To evaluate these competing petrogenetic scenarios, we integrate U–Pb (zircon) geochronology, zircon εHf–δ18O, and bulk rock Nd-Sr-O isotopic and geochemical data to explore the formation of the ‘Cordilleran-style’ Bega Batholith, the largest I-type batholith in the Lachlan Fold Belt. Secular compositional trends are identified where the granites transition from isotope signatures characteristic of melting old continental sources in the west, to more mantle-like isotope compositions in the east. These compositional shifts, and correlated zircon εHf–δ18O arrays, are consistent with open-system magma generation and mantle-derived magmatic input, where the composition of the mantle-derived component changed, and the proportion of supracrustal source material diminished, with ongoing arc extension and oceanward migration of the subduction zone. Isotopic mass balance, incorporating constraints from whole rock Nd data, suggests a cumulative mantle input of ~85–111 × 103 km3 representing up to 75% of the Bega Batholith by volume at a magma production rate of ~60 km3 km−1 Ma−1 and indicating considerable addition of juvenile crust along the eastern margin of Australia between c. 420 and 385 Ma. Our results highlight the key role of crust–mantle interaction in the petrogenesis of I-type granites in extensional back-arcs, and that these environments represent important sites of Phanerozoic continental growth.
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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 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".