Supra-subduction zone ophiolites retain hydrogen generation potential after 300 Myr

Arc-continent collisional orogenesis in the SW Pacific and the nature, source and correlation of emplaced ophiolitic nappe components

Final emplacement of the mid-Jurassic and mid-Cretaceous supra-subduction zone (SSZ) ophiolites onto adjacent continental areas in the Mediterranean region is synchronous with reductions in the rate of motion between Africa and stable Europe. The Apennine-Ligurian-Alpine ophiolites lack SSZ chemistry, are mid-ocean ridge basalt (MORB)-like, range in age fromc.169 to 148 Ma, and were emplaced in late Cretaceous and Cenozoic time. The Hellenic-Dinaric SSZ ophiolites include some MORB, ranging from 173 to 168 Ma, and were emplaced, eroded, and covered by younger sediments byc.140 Ma. The creation of the Apennine-Ligurian-Alpine and Hellenic-Dinaric suites is attributed to the motion of Adria, which formed a promontory on Africa, or essentially moved with it, as the central Atlantic opened. Extension to the west of Adria gave rise to the Ligurian Sea, generating MORB crust; to the east, a pre-existing Triassic ocean was subducted, with rollback creating Jurassic SSZ ophiolites that were emplaced onto adjacent continental margins in the later stages of convergence. The two episodes of slower motions between Africa and Europe are attributed to two episodes of attempted subduction of a passive continental margin. This speculation suggests that emplacement of some SSZ ophiolites may exert a significant control on oceanic spreading patterns.

An extensive suite of hydrothermally altered basalts, gabbros, and plagiogranites was recovered from the trench‐facing slope of the Tonga forearc. The tectonic setting, lithology, and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone (SSZ) ophiolites. Petrography, mineral chemistry, and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern SSZ setting. Seawater‐derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures >800°C. As the plutonic sequence cooled, amphibole progressively replaced the mafic phases, followed by chlorite and epidote below ∼550°C. Basalts record peak alteration temperatures up to 773°C; however, most were altered at lower temperatures typical of mid‐ocean ridge (MOR) volcanic sequences [Kelman, 1998]. Epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water‐rock ratios. Pervasively altered gabbros and basalts display evidence of late cataclastic deformation and/or contain veins that is likely related to a later tectonic event such as trench rollback. The range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both SSZ ophiolites and MORs. However, the degree of alteration in basalts and the presence of epidosites in the Tonga collection are most similar to alteration characteristics in SSZ ophiolites. The initiation of high‐temperature brittle deformation in the absence of ductile deformation suggests that the Tonga forearc crust was constructed in a magma‐rich environment, similar to fast spreading MORs.

The Mirdita-Subpelagonian ophiolites of the Albanide-Hellenide orogen are parts of a continuous belt extending from the former Yugoslavia to Greece, and share common geological, litho-stratigraphical, geochemical, and metallogenic features. In the Albanian sector, two distinct ophiolitic belts can be clearly identified: the Western Belt, mainly composed of mid-ocean ridge (MORB) ophiolites, and the Eastern Belt characterized by supra-subduction zone (SSZ) ophiolites with prevalent island arc tholeiitic (IAT) and minor boninitic affinity. In the easternmost border of the Western Belt (Central Mirdita), a transitional zone with MORB/IAT intermediate basalts and boninitic dykes also occur. In the Greek sector, a definite distinction into two ophiolitic belts cannot be made, and MORB-type ophiolites (western type) are subordinate, being represented only by the intrusive and lower volcanic sequences of the Pindos Massif. By contrast, SSZ- ophiolites (eastern type) are predominant and well-represented by the IAT and boninitic sequences of the Vourinos Massif, as well as by MORB/IAT intermediate basaltic-andesitic suites and boninites of the upper part of the Pindos volcanic sequence. Petrological and geochemical modelling suggest that the different Albanide -Hellenide ophiolitic sequences originated from distinctly different parental magmas by partial melting of mantle sources progressively depleted by previous melt extractions. MORB may have derived from 10 - 20% partial melting of an undepleted lherzolitic source, while MORB/IAT intermediate basalts may have generated by ca. 10% of H2O-assisted partial melting of a cpx-poor lherzolite that had previously experienced MORB extraction. IAT magmas and boninites may, in turn, have derived from 10 – 20 % and ca. 30% partial melting of the same source, variably enriched by subduction-derived fluids and related incompatible elements. The favoured tectono-magmatic model for the genesis of the Albanide-Hellenide ophiolites implies a low plate-convergence velocity with: 1) intra-oceanic subduction within a pristine MORB lithosphere, resulting in SSZ magmatism with IAT affinity, and generation of a nascent arc by nearly open-system supply of undifferentiated basalts (sheeted dyke complexes); 2) progressive slab sinking and retreat coupled with mantle diapirism and extension from the arc axis to the forearc region, with generation of boninites and/or very low-Ti tholeiites from depleted sub-arc sources, leaving highly depleted harzburgitic residua; 3) contemporaneous generation at the spreading axis of IAT/MORB intermediate basalts resulting from the interference of MORB-source diapirs with suprasubduction mantle sources; 4) convergence processes leading to obduction of large and relatively intact lithospheric sections of SSZ ophiolites onto the Pelagonian continental margin, often with the interposition of metamorphic soles. The latter have prevalent MORB affinity and represent relics of the pristine MORB lithosphere overthrust by the still hot ophiolitic slab.

Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode

The geologic and alteration characteristics of 11 supra-subduction zone (SSZ) ophiolites have been compiled in order to assess the variability of hydrothermal alteration patterns. All SSZ ophiolites show evidence for high-temperature hydrothermal discharge in the form of polymetallic sulfide deposits and/or metalliferous sedimentary rocks. Metamorphic zona-tions are commonly subparallel to the igneous stratigraphy, and metamorphic grade increases with depth, but varies laterally in response to local magmatic and tectonic activity. The degree of alteration is more pervasive in sheeted-dike complexes than in volcanic or plutonic sequences. The greatest variability in the style of alteration is in volcanic sequences, where alteration occurred at low-temperature to greenschist-facies conditions. The prevalence of high porosity and permeability within volcanic sequences in modern oceanic crust, which maintains low temperatures, indicates that special conditions are required to develop zeolite- or greenschist-facies zones away from sites of hydrothermal discharge. Recent study of sediment-covered ridges show that early burial of oceanic crust, at or near a spreading center, does not lead to the widespread development of high-temperature alteration zones in the uppermost volcanic rocks. Alteration patterns in the Izu-Bonin and Tonga forearcs show many similarities to SSZ ophiolites interpreted to have formed in a forearc, such as the presence of epidosites and the nature of alteration within plutonic sequences. However, modern forearc volcanic sequences only record low-temperature alteration. Much more detailed documentation of most ophiolites is required in order to fully understand the effects of geologic setting on the evolution of hydrothermal systems.

Here, we investigate early Permian adakites from Inner Mongolia, North China. The adakites are found along the Hegenshan suture zone, were emplaced into the late Carboniferous Meilaotewula suprasubduction zone (SSZ) ophiolite and consist mainly of fine‐ to medium‐grained granodiorites. U–Pb zircon dating reveals that the Meilaotewula adakites crystallized at 292 Ma. The adakites belong to the low‐K tholeiitic and medium‐K calc‐alkaline series. They are characterized by high SiO2 (65.20–70.05 wt.%), Al2O3 (16.46–19.27 wt.%), and Sr (343–666 ppm); low MgO (1.26–1.95 wt.%), Yb (0.65–1.16 ppm), and Y (8.08–10.6 ppm) contents; and Na2O/K2O ratios (3.26–10.73). They are relatively enriched in large‐ion lithophile elements, such as K, Rb, and Sr, and depleted in high‐field‐strength elements, such as Nb, Ta, Zr, Ti, and P, and have low total rare‐earth element (REE) contents (27.73–49.63 ppm), with distinct REE fractionation (chondrite‐normalized La/Yb of 3.05–8.88) and no pronounced negative Eu anomalies. The rocks have relatively low initial 87Sr/86Sr values (0.70285–0.70326), high εNd(t) values (+8.8 to +10.8), and relatively high zircon εHf(t) values (11.9 to 15.9), indicating that the magma was derived from young juvenile oceanic crust derived from depleted mantle, similar to adakitic rocks formed by partial melting of subducted oceanic crust. The relatively high Mg# of the adakites indicates that the melts interacted with mantle peridotite during ascent. The adakites, together with Meilaotewula ophiolite (308 Ma), may have formed during the early stages of intra‐oceanic subduction, demonstrating that subduction initiation in the southeastern Palaeo‐Asian Ocean occurred during the late Carboniferous to early Permian.

Petrogenesis of arc-related peridotite hosted chromitite deposits in Sikhoran-Soghan mantle section, South Iran: Evidence for proto-forearc spreading to boninitic stages

Submarine arc volcanism in the southern Mariana Arc as an ophiolite analogue

Petrogenesis of boninitic lavas from the Troodos Ophiolite, and comparison with Izu–Bonin–Mariana fore-arc crust

Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth’s layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone (SSZ) ophiolites and mantle-wedge-type (MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism, deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism (including silicatic melts, carbonatitic melts and silicate-bearing C-H-O fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism, deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.

Suprasubduction zone ophiolite formation along the periphery of Mesozoic Gondwana

Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore‐arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off‐ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far‐field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

This study presents a comparative microstructural and mineralogical characterization of peridotite samples collected from two distinct layered mafic–ultramafic intrusions: the Stillwater Complex, Montana, and the Yellow Dog Peridotite of the Marquette Range, Michigan. A multi-method approach was employed, integrating thin-section petrography, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), mercury intrusion porosimetry (MIP) and Brunauer–Emmett–Teller (BET) specific surface area analysis to quantify discrepancies between two peridotite deposits. Petrographic analysis reveals substantial textural variations between the two peridotite suites, with Stillwater samples exhibiting relatively coarse-grained pyroxenes, while pyroxenes in Yellow Dog display finer grained and more advanced serpentinized textures. SEM images taken under various magnifications show differences in surface structure thus indicating greater diversity of clinopyroxenes and orthopyroxenes in peridotites from Yellow Dog site while rock samples from display much more uniform sequence of orthopyroxenes. Despite both rocks are classified as peridotites, chromite flakes (determined by SEM-EDS) are present on rock samples from Montana and might act as a serpentinization catalyst. Porosity and surface area analyses highlight critical differences in reactive surface properties. BET measurements indicate that Yellow Dog peridotites possess higher (7.8895 m2/g vs 0.1917 m2/g) specific surface areas compared to Stillwater peridotites, suggesting greater reactivity and H2 generation potential. Complementary MIP results demonstrate that the Montana samples contain a more complex pore network, characterized by higher cumulative pore volume and greater hysteresis between intrusion and extrusion runs. Although those features are expected to enhance reaction kinetics during serpentinization, they can also hamper hydrogen transport from the reaction site to the production well due to limited flow and higher adsorption potential in Michigan's samples. Collectively, the data indicates that mineralogical and microstructural variations strongly influence hydrogen generation in peridotites. We formulate a table that demonstrates how characterization data of different types can be combined to rank sites in terms of their hydrogen generation and deliverability potential. The Yellow Dog peridotites, with their enhanced surface area and pore connectivity, may provide a more favorable substrate for H2 release during serpentinization, whereas Stillwater peridotites exhibit limited reactive surface exposure. However, in terms of hydrogen deliverability to the production well, Stillwater peridotites may be more favorable than Yellow Dog peridotites. This comparative study underscores the importance of coupling mineralogical characterization with quantitative microstructural analysis to evaluate the hydrogen generation potential of ultramafic lithologies in diverse geologic settings.

Subduction initiation is recorded by upper plate magmatism and lower plate metamorphism, that is, supra‐subduction zone (SSZ) ophiolite–metamorphic sole pair. Here, we report geochemical and geochronological data as well as P–T calculations of amphibolites (metamorphic sole) and hornblende gabbros (SSZ ophiolite) from the Saga ophiolitic mélange in Tibetan Plateau. Amphibolites show trace element contents compatible with normal‐mid‐ocean ridge basalt (N‐MORB), indicating that the protolith of amphibolite formed in a MOR setting. Instead, hornblende gabbros show significant high field strength elements (HFSEs) negative anomalies, enriched large ion lithophile elements (LILEs) and high zircon εHf(t) values, suggesting they formed by fluid‐induced partial melting of a depleted mantle. Thermobarometry and phase equilibrium modelling suggest two stages of metamorphism for garnet–clinopyroxene amphibolites: (I) a peak metamorphic stage (~1.9 GPa and 1000°C) and (II) a retrograde metamorphic stage (1.1–1.6 GPa and 800–1000°C). Zircon U–Pb ages of amphibolite and hornblende gabbro are 128.8 ± 5.1 Ma and 128.1 ± 1.5 Ma, respectively, suggesting subduction initiation within the eastern Neo‐Tethys occurred no later than 128 Ma and SSZ ophiolite formed at ~128 Ma. Apatite U–Pb ages of amphibolite and hornblende gabbro are 121.8 ± 2.1 Ma and 117.5 ± 4.5 Ma, respectively. Titanite U–Pb age of amphibolite is 122.2 ± 1.5 Ma. Overall, our data suggest that the metamorphic sole and SSZ ophiolite were exhumed since 128–118 Ma, and finally exhumed into the ophiolitic mélange.