Diversification of Archean tonalite-trondhjemite-granodiorite suites in a mushy middle crust
Bibliographic record
Abstract
Abstract Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant component of Earth's first continents, but their origins are debated. The trace element concentrations of these rocks are conventionally linked to their source depths and inferred geodynamic settings with the implicit assumption that TTG compositions are source-controlled. Alternatively, their variable compositions may be caused by fractional crystallization in TTG plutons after emplacement and less clearly linked to source depth. Most TTGs in Archean mid-crustal exposures are the dominant component of igneous gray gneiss complexes; the processes that influence the evolution of TTG magmas in this setting are poorly understood. We present a petrological–geochemical model that explains the coexistence of TTGs in the middle crust with low-pressure and high-pressure geochemical trends, irrespective of tectonic setting or depth of the TTG source. We propose that mid-crustal TTGs were long-lived crystal mushes with compositions controlled by the separation of early-crystallizing plagioclase and melt. Using phase equilibrium modeling, we demonstrate that a suite of TTGs from the southern Superior Province in Canada represents variably plagioclase-rich and melt-rich fractions from a common parent magma. The behavior of plagioclase may have a strong influence on the geochemical trends of TTGs, including the degree of rare earth element fractionation. Our results suggest that trace element compositions of TTGs may not primarily reflect the depth of the source and cannot be used alone to infer Archean geodynamic settings.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.012 | 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".