A 2023 synthesis proposed that the Neo-Tethys spreading ridge was subducted northward beneath the southern Eurasian margin at roughly 100–90 Ma, producing an east–west belt of adakitic intrusions from the Gangdese batholith of southern Tibet to the South China block, a coeval north–south compression event, and a lateral contrast in porphyry Cu fertility. Five linked questions remain open: the precise geometry of the subducted ridge (trench-parallel versus oblique, and the role of transform-fault offsets); the cause of the inferred east–west difference in Neo-Tethys oxidation state and its exact impact on the oxygen fugacity of the resulting magmas; why no large porphyry Cu deposit has been found in the Gangdese segment despite abundant adakitic magmatism; whether the ~100 Ma crustal thickening of the Lhasa terrane records this ridge subduction rather than Bangong–Nujiang Ocean subduction; and the construction of a comprehensive, quantitative thermomechanical model of the episode integrating magmatic, metamorphic, and structural data. These five components remain open.

Background

The Neo-Tethys was the vast ocean that closed during the Mesozoic and Cenozoic along the Yarlung Zangbo (Indus–Yarlung) suture as India approached Asia; its oceanic lithosphere was consumed beneath the southern margin of Eurasia, above which grew the Lhasa terrane (a continental block now forming southern Tibet) and the Gangdese batholith, a chain of Cretaceous-to-Eocene granitic intrusions that forms an Andean-style magmatic arc. North of the Lhasa terrane, the Bangong–Nujiang suture records the earlier closure of the Meso-Tethyan Bangong–Nujiang Ocean against the Qiangtang terrane; subduction associated with that closure is a competing candidate for driving Cretaceous deformation and thickening of the Lhasa terrane.

Adakites are silica-rich magmas with high Sr, low Y and heavy rare-earth contents (high Sr/Y), originally attributed to partial melting of young, hot subducted oceanic crust, although the same chemical signature can also be produced by melting of thickened lower continental crust — an ambiguity central to the debates below. Ridge subduction occurs when the spreading ridge of an oceanic plate enters a subduction zone; the young, hot, buoyant slab and the slab window (a gap left in the downgoing plate where ridge lithosphere once was) allow asthenospheric upwelling, producing high-temperature magmatism and metamorphism — for example charnockites and granulite-facies rocks (granitic and crustal rocks recrystallized at very high temperatures) — together with slab-melt-type adakitic magmas and compression of the overriding plate, because a buoyant ridge resists subduction.

Zhang et al. (2023, Ore Geology Reviews 154:105302) compiled published zircon U–Pb ages (dating of crystallization using the radioactive decay of uranium to lead in the mineral zircon) and whole-rock geochemistry to identify an east–west belt of Late Cretaceous (~100–80 Ma) adakitic intrusions along the southern Eurasian margin, from the Gangdese batholith (Milin, Zedong, Nimu, Caina) to the South China block (e.g., the Zijinshan district), coinciding with a roughly north–south compressional event recorded in both regions at ~110–90 Ma. Citing plate-kinematic reconstructions, they proposed that the Neo-Tethys spreading ridge was subducted northward beneath Eurasia at ~100–90 Ma, generating the belt. Because ridge subduction is spatially and genetically associated with giant porphyry Cu deposits worldwide (porphyry deposits are large intrusion-related copper deposits formed from oxidized, water-rich magmas), the research group's published model (Sun et al. 2018; Zhang et al. 2023) further infers a metallogenic chain: the eastern sector of the Neo-Tethys was relatively oxidized, feeding fertile magmas beneath South China (where large ~108–100 Ma porphyry Cu–Au deposits such as Zijinshan occur), whereas the western sector carried more organic-rich reducing sediments — plausibly linked to Cretaceous oceanic anoxic events, i.e., episodes of widespread marine oxygen depletion and organic-carbon burial — damping the oxygen fugacity of Gangdese magmas (oxygen fugacity, commonly expressed as ΔFMQ relative to the fayalite–magnetite–quartz redox buffer, measures a magma's oxidation state) and explaining why no large porphyry Cu deposit is known there despite abundant adakites.

The hypothesis is contested. The ~81 Ma Laimailang adakitic rocks of the southern Gangdese have been derived from melting of thickened lower crust rather than slab melting (Li et al. 2022); Late Cretaceous (90–65 Ma) deformation of the Gangdese belt and a coeval magmatic lull have been attributed to flat-slab subduction of the Neotethyan slab (Ma et al. 2023); ~90 Ma high-pressure metamorphic rocks show the southern Lhasa crust thickened to about 60 km (Zhang et al. 2022); and ca. 90 Ma high-Sr/Y intrusions of the western central Lhasa terrane have been explained by regional thickening plus multipoint lower-crustal delamination (foundering of dense lower-crustal material into the mantle) rather than ridge subduction (Wang et al. 2024). For South China, Pacific/Izanagi-plate dynamics offer a competing frame for the Cretaceous events (Sun & Li 2023). The Bangong–Nujiang interpretation of the ~100 Ma Lhasa thickening remains an untested alternative, and the geometry, dynamics, and metallogenic consequences of the proposed ridge subduction remain open (see Problem Statement).

Problem Statement

Under the overarching objective of establishing what actually happened along the southern Eurasian margin during the proposed Cretaceous Neo-Tethys ridge-subduction episode — testing the ridge-subduction hypothesis against the competing Bangong–Nujiang and flat-slab interpretations — the following five linked questions, left open by the research program of Zhang et al. (2023) and its precursor Sun et al. (2018), remain to be answered:

(i) What was the precise geometry of the subducted Neo-Tethys ridge — trench-parallel or oblique to the ~100–90 Ma trench — and how did transform-fault offsets along the ridge influence the magmatic, metamorphic, and structural record of the overriding plate?

(ii) What caused the inferred differential oxidation state between the eastern and western parts of the Neo-Tethys Ocean, and what exactly was its impact on the oxygen fugacity of the resulting magmas along the adakitic belt?

(iii) Why have large porphyry Cu deposits not yet been discovered in the Gangdese segment despite its abundant adakitic magmatism — intrinsic magma redox or fertility, post-subduction tectonic disruption, erosion level, or some additional control?

(iv) Does the ~100 Ma crustal thickening of the Lhasa terrane reflect Neo-Tethys ridge subduction, as the hypothesis requires, or subduction of the Bangong–Nujiang Ocean to the north?

(v) What would a comprehensive, quantitative thermomechanical model of the proposed, roughly trench-parallel ridge subduction and its effects on the overriding plate — integrating the magmatic, metamorphic, and structural data — imply for the dynamics and metallogenic consequences of the episode, and are those implications consistent with the observations? The ridge geometry such a model assumes remains subject to question (i).

Together, these five questions address the research program's stated aim of understanding the dynamics and metallogenic consequences of Neo-Tethys ridge subduction. The five questions are linked, and the case for or against ridge subduction rests on the (i)–(iv) evidence taken together. Because the hypothesis is contested (see Background), work on these questions may support ridge subduction as the driver of the adakitic belt, the ~110–90 Ma north–south compression, and the ~100 Ma crustal thickening, or may instead favor one of the competing interpretations.

References

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