Petrogenesis of Early Neogene Magmatism in the Northern Puna; Implications for Magma Genesis and Crustal Processes in the Central Andean Plateau
Bibliographic record
Abstract
New compositional data and petrogenetic models are presented for pre-Upper Miocene volcanism in the northern Puna of Argentina (22°S–24°S). Two phases of volcanism produced small dome complexes of mainly silicic andesite to low-SiO2 rhyolite. The Upper Oligocene–Lower Miocene phase (UOLM, 20–17 Ma), produced two distinct groups of rocks. The UOLM-1 group is metaluminous and mainly andesitic, with isotopic compositions like those of the recent arc (87Sr/86SrT ∼0·706; εNdT −3). The UOLM-2 group is more silicic and peraluminous, and has isotopic compositions indicating a substantial crustal contribution (87Sr/86SrT ∼0·713; εNdT −8). The Mid-Miocene phase (MM: 15–12 Ma) produced rocks similar in composition to those of the UOLM-2 group (87Sr/86SrT ∼0·710; εNdT −7) but with higher incompatible element contents. Ratios of Ba/Nb and Zr/Nb in the UOLM group rocks are uniform and similar to those of the current arc, whereas the ratios in MM centres show a mixed arc and back-arc affinity. This suggests that the westward shift in the arc began in the northern Puna in the mid-Miocene. Neither the exposed Palaeozoic felsic basement nor the lower-crustal granulites known from xenolith suites are compositionally suitable as protoliths for the UOLM and MM magmas. The preferred petrogenetic model for the magmas involves hybridization of a depleted arc basalt with partial melts of the felsic basement. Geochemical modelling and thermal arguments rule out magma mixing as the process of hybridization. Successful assimilation–fractional crystallization (AFC) solutions indicate an increase in crustal assimilation from 15–25% in UOLM-1, to 40–60% in the case of UOLM-2 and MM group rocks. Assuming the same end-member compositions, the modelling suggests genesis of the MM magmas at higher pressure than the UOLM-2 centres (≥10 kbar vs 7 kbar), which may reflect the influence of crustal thickening in the plateau region by the mid-Miocene. The felsic dome complexes of this study are compositionally similar to the large-volume, caldera-sourced felsic ignimbrites that dominated volcanism in the region from 10 to 2 Ma and our results suggest that there is no fundamental difference in magma genesis between them. The differences in the volumes and the mode of eruption reflect changes in the stress and thermal regime with time.
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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 | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".