Formation of high-silica adakites and their relationship with slab break-off: Implications for generating fertile Cu-Au-Mo porphyry systems
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Bibliographic record
Abstract
• Geochemical characteristics of adakites, evaluation of the genesis of adakites. • Formation of high-silica adakites and their relationship with slab break-off. • Implications for generating fertile Cu-Au-Mo porphyry systems. • Utilizing trace elements from the GEOROC database to investigate the contrasting slab subduction versus slab break-off mechanisms. In recent years, the characteristics and sources of fertile adakites has received considerable attention. As well, most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation, evolving to slab rollback, then slab break-off after collision (i.e. late- to post-collisional slab failure (arc-like magmatism) and transpression) has gained more recognition, although their relationship to each other has yet to be explored. The geochemical characteristics imply that adakites/adakite-like, in particular high-silica adakites (HSA), can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction, lithosphere thickening, and then failure (all late to post collisional), while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series (ADR) to form intraoceanic island arc to intracontinental margin arc systems, before the collisional stage. HSAs are characterized by high-silica (SiO 2 > 67 wt.%), Al 2 O 3 > 15 wt.%, Sr > 300 ppm, Y<20 ppm, Yb < 1.8 ppm, and Nb ≤ 10 ppm, and MgO < 3 wt.%, with high Sr/Y (>50), and La/Yb (>10). Some specific geochemical features, such as high Mg# (ave 0.51), Ni (ave 924 ppm), and Cr (ave 36 ppm), in HSAs are typical, in contrast to calc-alkaline arcs, although both groups display similar but less pronounced negative anomalies of Nb, Ta, and Ti in primitive mantle-normalized trace element spider diagram profiles. These unique geochemical features are likely ascribed to the involvement of garnet, hornblende, and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization (AFC) within the mantle and crustal during ascent in a transpressional collisional environment. Hypotheses for origin of HSA derivative from melting in convergent margins from young, hot oceanic plates subducting into the mantle is applicable to only some adakitic systems. The difference in geochemical characteristics of adakites compared to ADR, such as relative higher MgO, Cr, Cu, and Ni, are due to their slab source, as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle; altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements, as well as sulfates and sulfides. HSA magmas related to slab failure have special geochemical properties, such as Sr/Y > 20, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Gd/Yb > 2, and Sm/Yb > 2.5. Slightly higher Nb + Ta is due to high T melting of rutile. Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure (roll back to break-off). High T - P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H 2 O, SO 2 , HCl, with chalcophile metals that remain incompatible at higher f O 2 (low f H 2 ); these situations happen in late- to post-collisional settings where the subducting oceanic crust experienced slab failure, resulting in advective heat addition to the system from upwelling asthenosphere. In such a slab failure setting, transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust. When oxidized slab melts interact with the subduction-modified lithospheric mantle, the resulting magmas stay oxidized, potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.
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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 | 0.001 |
| 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) | 0.000 | 0.000 |
Machine scores (provisional)
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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 it