Supra-subduction Zone Ophiolites Research Articles

Timing of the subduction initiation and ophiolite emplacement of the inner Tauride Ocean: Insight from the Pınarbaşı ophiolite in Central Türkiye

Late Cretaceous ophiolites are widespread in southern Türkiye, including the Tauride Belt Ophiolites and the Central Anatolian Ophiolites. Here, we report new geological, geochemical, geochronological, and zircon isotopic data from ophiolitic rocks of the Pınarbaşı ophiolite (Eastern Tauride Belt). The Pınarbaşı ophiolite forms incomplete sequences, with its mantle section dominated by serpentinized harzburgite and chromitites, while the crustal section comprises ultramafic and mafic cumulates. Whole-rock and mineral chemistry data suggest a supra-subduction zone origin. LA-ICP-MS zircon U-Pb age of 88.2 ± 0.4 Ma (MSWD = 0.84) was obtained from a plagiogranite dyke cutting serpentinized peridotite of the Pınarbaşı ophiolite. Another, U-Pb zircon crystallization age of 96.8 ± 0.6 Ma (MSWD = 1.53) was obtained from a plagiogranite dyke within the mélange of the Pınarbaşı ophiolite. Hafnium isotope compositions of these zircons (εHf(t) = +26.4 to +3.1) suggest a depleted to slightly enriched mantle source. The formation ages of the crustal rocks (gabbro, plagiogranite, and dolerite) of the Cretaceous ophiolites in the Tauride Belt from this and previous ophiolite occurrences, suggest that supra-subduction zone ophiolites in the Inner Tauride Ocean formed during the period ∼101–87 Ma. On the other hand, the peak metamorphism of the metamorphic sole rocks is reported to have occurred around ∼104 Ma. According to all available petrogenetic and geochronological data, it is considered that the supra-subduction zone crust developed shortly (∼ 3–4 Myr) after the initiation of intra-oceanic subduction and this lasted ∼10–15 Myr.

Forearc Variability and the Geochemical Diversity of Suprasubduction Zone Ophiolites: Insights From the Leka Ophiolite Complex, Norway

AbstractNew whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the “Leka sector” of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere.

Chemical constraints and tectonic setting of the Ginebra ophiolite complex

The occurrence and types of ophiolites in the Central Cordillera of the Colombian Andes have not been identified in terms of their tectonic settings and geochemical features. Therefore, the study of the tectonic setting and subduction zones in the ophiolites’ magmatic evolution becomes a significant issue. The Ginebra Ophiolitic Complex (GOC) is located on the western flank of the Central Cordillera in the Valle del Cauca Department, southwest of Colombia, assigned to the Lower Cretaceous period, as indicated by the contact of the GOC with the Buga Batholith. The GOC is formed by three main lithologies: amphibolite, gabbro, and ultramafic rocks (pyroxenite and peridotite), according to Ossa (2007). The rocks in the Puente Piedra section, located in the northeast of Ginebra municipality, are part of the gabbroic cumulates within the ophiolitic sequence. This sequence comprises interspersed layers of gabbroic cumulates, gabbrodiorites, and diorites; interpreted as a result of crystal accumulation and fractional crystallization processes. The mafic rocks of the GOC are sub-alkaline and correspond to the low potassium (K2O wt% 0.03-0.06) tholeiitic series. Geochemically, they have SiO2 wt% ranging from 49 to 61 and an aluminum oxide saturation index of approximately ~0.4 and ~0.8, indicating a metaluminous type. The geochemistry of the studied rocks from Puente Piedra section indicates that the GOC formed through fractional crystallization and accumulation processes from a single magma source. Cluster analysis, used to compare the geochemistry of GOC rocks and the Amaime Complex basalts, suggests a similar magma source, possibly linked to multiple recharge events that underwent fractional crystallization, melt extraction, and accumulation processes. The geochemical parameters are indicative of a suprasubduction zone ophiolite, characterized by a low potassium tholeiitic series affinity, TiO2 values typically <1.2 wt%, Th enrichment typical of subduction zones, high Pb content, and low values of Ti, Y, Yb, Ta, Nb, Zr, and Hf.

Thrust-related metamorphism in Carboniferous slates of southern Patagonia (South America): the fate of forearc successions

The Devonian to early Carboniferous western margin of Patagonia (South America) includes a NW–SE-trending magmatic arc associated with a NE-dipping palaeo-subduction zone. Along the Andean region of southern Patagonia, the Eastern Andean Metamorphic Complex developed in a forearc position and consists of a succession of very low- to low-grade metaturbidite–metabasic rocks emplaced from the Devonian to Carboniferous. There are significant uncertainties surrounding this metamorphic complex, mainly related to the tectonosedimentary setting of the basin and the subsequent conditions of deformation and metamorphism, which hinder our understanding of the orogenic architecture. To reveal the links between tectonics and metamorphism, we conducted a structural analysis and sampled metapelites to measure the illite crystallinity along a regional structural cross-section in the Eastern Andean Metamorphic Complex. Our analysis reveals broadly lower to upper anchizonal metamorphism roughly synchronous with deformation along northwards-verging thrusts. These findings support the development of a forearc hyperextended basin that was subsequently closed during the Gondwanide Orogeny (late Paleozoic), a model that reconciles previous proposals suggesting passive margin v. back-arc basin models. This closure led to the emplacement of supra-subduction zone ophiolites and turbidites over the continent through the landwards migration of brittle–ductile reverse shear zones. Supplementary material: Measurements of the Kübler index are available at https://doi.org/10.6084/m9.figshare.c.7160849 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Coeval formation and exhumation of metamorphic sole and ophiolite in the Saga ophiolitic mélange: Insights into subduction initiation of the Neo‐Tethys

AbstractSubduction 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.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

Heavy mineral indicators of ridge subduction beneath the Bainaimiao arc in the southeastern Central Asian Orogenic Belt

As one of the major subduction-accretionary systems in the southeastern Central Asian Orogenic Belt, the Bainaimiao arc presents a valuable opportunity for investigating the subduction–accretion process of accretionary orogens. It is located adjacent to the northern margin of the North China Craton and developed arc-related deposits during the Silurian–Devonian period. In this paper, we employed heavy mineral geochemistry and U–Pb ages of zircon and rutile, to determine the provenance of the Silurian–Devonian deposits and reconstruct the paleogeography of the source area. The stratigraphic units involved include the Xuniwusu Formation of Early Silurian and the Xibiehe Formation of Early Devonian.Both the Xuniwusu and Xibiehe formations exhibit heavy mineral assemblages dominated by zircons and tourmalines. Nevertheless, the Xibiehe Formation contains Cr-spinels and garnets which are absent in the Xuniwusu Formation. Geochemical analyses of the detrital Cr-spinels exhibit characteristics of a supra-subduction zone ophiolite. The garnets originated from middle- to high-grade metamorphic rocks, as well as mafic and felsic rocks. The tourmalines in both formations primarily indicate a metasedimentary rock source, with a small percentage of Li-poor granitoid species in the Xuniwusu Formation. The U–Pb age spectra of both detrital zircon and rutile in the Xuniwusu Formation show multiple peaks. The age spectra consist of Precambrian ages, indicative of a recycled origin, as well as Paleozoic ages, suggesting a magmatic origin. In contrast, the U–Pb age spectra of zircon and rutile in the Xibiehe Formation display a unimodal age peak at ca. 455 and 490 Ma, respectively. These differences suggest a provenance shift from the Xuniwusu to Xibiehe formations. The Xuniwusu Formation was sourced from the Bainaimiao arc itself, whereas the Xibiehe Formation has a mixed origin of the Bainaimiao arc and Ondor Sum accretionary complex. The provenance change correlates with the unconformity between the two formations, suggesting a forearc uplift event. In combination with the previous research on the Tulinkai ophiolite, it can be inferred that the forearc uplift was triggered by ridge subduction of the Paleo-Asian Ocean plate at ca. 450–430 Ma. This research provided a new perspective on the ridge subduction of the oceanic slab in fossil orogens.

Intra−Neo-Tethyan subduction initiation inferred from the Indawgyi mafic rocks in the Central Ophiolite Belt, Myanmar

Geological evidence has demonstrated the presence of an intra−Neo-Tethyan subduction system during the Cretaceous. However, when and how this intra-oceanic subduction was initiated, especially for the eastern Neo-Tethys, are still not well constrained. Here we present geochemical and geochronological analyses of the Indawgyi mafic rocks from the Central Ophiolite Belt in the West Burma Block (Myanmar), which record early forearc spreading during the intra−Neo-Tethyan subduction initiation. Zircon U-Pb ages of gabbros indicate the ophiolitic crust formation at ca. 120 Ma. Gabbros show mid-oceanic-ridge basalt−like rare earth element patterns and depleted Sr-Nd-Pb isotopic compositions with negative anomalies of high field strength elements (e.g., Nb, Ta, Zr, and Hf), similar to forearc basalt characteristics. Basalts show more slab-derived component signatures than the gabbros and represent mantle wedge magmas most likely formed between forearc spreading and arc maturation. These data, together with regional geological records and geophysical observations, suggest that the Indawgyi gabbros were derived from an intra−Neo-Tethyan forearc setting during the early stage of subduction initiation. Considering the timing of supra-subduction zone ophiolites and metamorphic sole in the Indo-Burma Range, we propose that spontaneous subduction initiation and sinking of the eastern Neo-Tethyan lithosphere during the Early Cretaceous (ca. 120 Ma) led to formation of the Indawgyi forearc crust, whereas subsequent mature subduction resulted in the Middle Cretaceous (ca. 108‒90 Ma) arc magmatism in the West Burma Block. These findings confirm the double-subduction model of the Neo-Tethys Ocean and shed new light on the intra−Neo-Tethyan subduction initiation.

Cadomia origins: paired Ediacaran ophiolites from the Iberian Massif, the opening and closure record of peri-Gondwanan basins

Abstract The recent discovery of Ediacaran ophiolites in the SW Iberian Massif has made it possible to pinpoint the evolution of the Cadomian basement of Europe. The Calzadilla and Mérida ophiolites (gabbroic protoliths dated at c. 600 and 594 Ma, respectively) have geochemical characteristics typical of supra-subduction zone ophiolites. They are interpreted as originating during the initial opening of a forearc basin with boninitic magmatism (Calzadilla), followed by the formation of a back-arc basin with arc-tholeiites (Mérida). Widening of the back-arc led to the rifting and drifting of a section of the active continental margin (Cadomia). Closure of these oceanic domains initiated rapid contraction, culminating in the collision of Cadomia with Gondwana ( c. 590–540 Ma). The application of a PANALESIS model to this palaeogeographic setting confirms the plausibility of Cadomian rifting and the likely opening of broad oceanic domains. It also confirms the final collision of Cadomia with Gondwana, although the synthetic and regional data disagree in the precise chronology of the convergence and collision of Cadomia with the West Africa Craton. This work shows that the evolution of the Cadomian basement is much more complex than traditionally considered.

Neotethyan Ankara Melange, central Turkey: Formation by accretion of seamounts and supra-subduction zone ophiolites in an oceanic fore-arc setting

The Neotethyan Ankara Melange is a global reference for oceanic accretionary processes preserved in a collisional orogen. Here we show, based on eight representative outcrops around Ankara, that the melange encompasses stratigraphically coherent volcanic-sedimentary successions of mainly Late Jurassic-Early Cretaceous age. Our results indicate that the melange has a relatively organised tectonostratigraphy, in contrast to previous interpretations that emphasised its chaotic nature. Coherent volcanic-sedimentary successions of Late Jurassic-Early Cretaceous age are interpreted as flank facies of a large oceanic seamount (probably plume related) and its capping carbonate platform (atoll), including reef-and back-reef facies. Remnants of other seamounts are dated as Late Triassic-Early Jurassic and Late Cretaceous. Variably dismembered supra-subduction zone ophiolites of latest Early Jurassic-Middle Jurassic age, including boninites, formed in an oceanic fore-arc setting, similar to that of the Cenozoic Izu-Bonin fore arc, NW Pacific. The melange in the Ankara region partly accreted during the Early Cretaceous (pre-Albian) in response to collision of the inferred large seamount with the oceanic (ophiolitic) fore arc. During the collision which involved subduction-erosion slices of proximal–distal seamount flank lithologies accreted sequentially, whereas the seamount core subducted. Also during the collision, the distal outer edge of the ophiolitic fore arc, mainly mantle harzburgite, detached from the over-riding lithosphere and was incorporated into the melange. Structural evidence emphasises the importance of tectonic processes (rather than olistostromes) in the melange accretion. The accretionary prism in the outcrops studied was covered by arc-related sediments and volcanics during the Late Cretaceous and deformed by collision-related processes during latest Cretaceous-Paleogene.

Do microcontinents nucleate subduction initiation?

Abstract Subduction initiation is a pivotal process in plate tectonics. Models of subduction initiation include the collapse of passive margins, oceanic transform faults, inversion of oceanic core complexes, and ridge failure but have ignored the potential effects of continental crust relicts within the oceanic crust. In this paper, we explore the role of microcontinents on subduction initiation through two-dimensional thermo-mechanical numerical modeling. We consider three scenarios with variable ages of oceanic crust surrounding the microcontinent and parametrically examine the microcontinent characteristics (size, crustal thickness, thermal gradient, and rheology), oceanic plate age, and convergence rates. Results suggest that moderate-size (≥300 km) microcontinents can nucleate subduction initiation at the junction between continental and oceanic plates. A large part of the microcontinent would be dragged into the subduction zone, and the subsequent asthenosphere upwellings would incorporate part of the microcontinent. Our numerical models add a new hypothetical scenario for subduction initiation, especially for those places where a young and buoyant plate subducts beneath an older and denser oceanic plate. Moreover, they can explain the origin of exotic crust materials and ultrahigh-pressure minerals in supra–subduction zone ophiolites.

Crustal structure of the Western Azuero Peninsula, Panama: Insights into the structure of accretionary complexes and forearc ophiolites

ABSTRACT Detailed geologic mapping (639 field stations in ~700 km2) and a ~50 km-long gravity survey (142 stations) in the western Azuero Peninsula revealed two faulted and folded slivers of oceanic crust attached to the trailing edge of the Caribbean Large Igneous Plateau (CLIP). Our new data, along with published geochronology, allowed us to reconstruct the Cretaceous forearc configuration of the trailing edge of the CLIP prior to seamount collision, ophiolite accretion, and whole-margin deformation. The ophiolite in western Azuero is composed of two tectonic slivers arranged in south-verging, imbricated thrust faults that stack a ~73 Ma pillow, flow, and picritic basalt and black chert, together with a ~ 89–93 Ma and older basalt flows and capping red chert sequences. Accretion of these slivers to form a supra-subduction zone ophiolite resulted from the middle Eocene collision and accretion of Galapagos seamounts against the trailing edge of the CLIP. Accreted seamounts are arranged in a north-verging antiformal stack duplex, and below the thrust sheets. Change in kinematics after fission of the Cocos-Nazca Plate during early Miocene times prompted the propagation of the Azuero-Sona fault zone flower structure, favouring the preservation of these slivers of oceanic crust.

Ordovician Japan: geotectonic setting and palaeogeography

Abstract Ordovician Japan formed a mature arc-trench system developed along the palaeo-Pacific (Panthalassa) margin of the Greater South China (GSC) continental block. GSC consists of South China, East China Sea, SW–NE Japan and the Khanka–Jiamusi–Bureya megablock in the Far East; Paleozoic GSC was thus, in total, twice as large as the South China components by themselves (Yangtze and Cathaysia). The Ordovician crust of Proto-Japan comprised coeval arc-related rocks, such as granitoids, supra-subduction zone ophiolites and fore-arc basin strata, although most of them were considerably fragmented. The Ordovician and middle–late Paleozoic fossils from Japan are highly limited but suggest that Proto-Japan was positioned in the low-latitude domains probably of the palaeo-Pacific Ocean in connection to Paleo-Tethys. GSC became separated from Rodinia in the Neoproterozoic, and its Proto-Japan segment evolved as a collision-free subduction margin for nearly 500 myr since the mid-Cambrian. The GSC framework provides critical constraints to the palaeogeographical reconstruction of circum-Pacific continental blocks. First, the Cambro-Ordovician GSC should have been isolated from Australia/India/East Antarctica that formed East Gondwana by a relatively wide ocean domain for keeping ‘subduction potential’. Second, the Cathaysian margin of GSC should have faced to an extensive ocean without major continents since the Cambrian. The palaeo-Pacific is the only possible candidate for this.

Plate tectonics in action in the Mesoarchean: Implication from the Olondo greenstone belt on the Aldan Shield of Siberian Craton

The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥30% in volume, the OGB is unique among other greenstone belts in the world. In this study, we present the most up-to-date geochemical and isotopic data for the ultramafic-mafic rocks of the OGB, in order to better constrain their mantle sources and the plate tectonic process involved in the formation of OGB at ca. 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, and are highly refractory as residual mantle phase as indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs for highly siderophile elements (HSE). Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of late metasomatism in their mantle source. Rhenium-Osmium isotopic compositions of these dunites yield mantle model age (TMA) of 2960–3020 Ma, comparable to the formation age of the OGB at ca. 3 Ga. Together, the data suggest that, unlike mantle cumulate origin for most of the Archean ultramafic rocks, the OGB dunites were mantle residuals after a high degree of partial melting (>30%), which subsequently interacted with the subduction-related melt/fluid. On the other hand, the OGB mafic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the ultramafic residuals that reinforce a subduction-related regime as their formation setting, despite extra mid-ocean ridge and plume settings. Tholeiitic basalts yield variable REE patterns from depleted, chondritic, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to mafic rocks in typical supra-subduction zone (SSZ) ophiolites. Such mafic rocks with combined lower εNd(t) and negative Nb-Ta anomalies were most likely the result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requiring a mantle plume to transport and melt at such a depth. The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time.

Evidence for early Pennsylvanian subduction initiation in the Mongol–Okhotsk Ocean from the Adaatsag ophiolite (Mongolia)

The Late Paleozoic–Mesozoic Mongol–Okhotsk Ocean was the latest ocean basin witnessing the final amalgamation of East Asia. However, the kinematic evolution and geodynamics of Mongol–Okhotsk's subduction initiation remain enigmatic mainly due to the lack of appropriate studies of its ophiolitic records. Here, we report for the first time a subduction initiation record from the Adaatsag ophiolite in the central Mongolia. This ophiolite presents a magmatic sequence akin to that in the Izu–Bonin–Mariana forearc, considered archetypical of subduction initiation. The gabbroic and basaltic rocks of the ophiolite display geochemical signatures similar to the forearc basalts. Zircon UPb geochronology of the gabbro shows a mean age of 319 ± 2 Ma, which we interpret as crystallization age. The high-Mg diorite intruding the gabbros of the ophiolite displays a transitional affinity between sanukite and boninite. Zircon UPb geochronology of the high-Mg diorite shows a mean age of 317 ± 2 Ma. Together with the neighbor Khuhu Davaa suprasubduction zone ophiolite (∼321 Ma), we interpret that the subduction of the southwestern Mongol–Okhotsk Ocean initiated during the Early Pennsylvanian (∼320 Ma). The structurally underlying gneissic granite (∼534 Ma) indicates the Adaatsag ophiolite was emplaced southward onto the Ereendavaa cratonic terrane, and the overlying molassic sequences (youngest detrital zircon peak age of ∼252 Ma) constrain this emplacement occurred before ∼252 Ma. The older (499–870 Ma) inherited zircons of the ophiolites and negative Hf isotope data of the high-Mg diorite indicate that a continental fragment existed in the newly formed intra-oceanic arc, which probably facilitated the Early Pennsylvanian subduction initiation in the Mongol–Okhotsk Ocean.

Mineralogy and geochemistry of serpentinized peridotites of the Northern Nubian Shield: The origin of compositionally banded olivine and implications for Neoproterozoic supra-subduction zone metasomatism

A sequence of Neoproterozoic ultramafic rocks — serpentinized peridotite, serpentinite, and clinopyroxenite — outcrop in Wadi El-Mireiwa in the south Eastern Desert of Egypt. They represent the mantle and lowermost crustal section of a fragmented ophiolite, emplaced by thrusting above metasedimentary rocks and an island arc assemblage and later intruded by a granite mass. In samples of the serpentinized peridotite, back-scattered electron images of olivine reveal banded zoning, defined by straight and parallel Mg-enriched (Fo ∼ 96) lamellae within typically magnesian host crystals (Fo ∼ 89). The presence of clinopyroxene, the relatively low whole-rock Mg# (∼88), the low NiO content of olivine (0.1–0.32 wt%), and the relatively high TiO2 content of fresh Cr-spinel (∼0.3 wt%) are all consistent with a cumulate origin for the serpentinized peridotite and serpentinites. On the other hand, some associated serpentinite has higher whole-rock Mg# (92) and low TiO2 in Cr-spinel (∼0.01 wt%), consistent with a residual origin by high-degree melt extraction in the fore-arc region of a supra-subduction zone (SSZ) environment. The formation of Mg-enriched bands in olivine is attributed to the enhancement of metasomatic Mg-Fe exchange with high-Mg/Fe2+ fluids along crystallographically-oriented subgrain boundaries produced by high strain-rate dislocation creep. Such high-Mg/Fe2+ fluid metasomatism also explains reverse zoning of MgO in Cr-spinel, whereas the low whole-rock concentrations of REE and other trace elements exclude a significant role for melt-rock interaction. The evidence for high-Mg/Fe2+ fluid metasomatism in this suite indicates that SSZ fluids and high fore-arc strain rates affect not only the mantle wedge but also overlying ultramafic cumulates. The oxygen fugacity [ΔlogƒO2(FMQ)] values calculated from unmetasomatized (+2.48 to +2.67) and metasomatized (+3.75 to +4.53) samples reflect the oxidized nature of Neoproterozoic SSZ magma and the even more oxidizing character of the metasomatic fluids. Banded zoning has also been recorded in olivine from Mariana fore-arc peridotite and Phanerozoic SSZ ophiolites; the Neoproterozoic Wadi El-Mireiwa case shows that rapid mantle flow has been a persistent feature of the tectonic environment where fore-arc ophiolites develop.

Early–Middle Jurassic metamorphic and non-metamorphic supra-subduction zone ophiolite fragments in a Late Cretaceous ophiolitic mélange (northern Turkey): implications for long-lived and supra-subduction zone ophiolite formation

Early–Middle Jurassic metamorphic and non-metamorphic supra-subduction zone ophiolite fragments in a Late Cretaceous ophiolitic mélange (northern Turkey): implications for long-lived and supra-subduction zone ophiolite formation

Mineralogy and PGE geochemistry of chromitites and peridotites of the sapat complex in the indus suture zone, northern Pakistan: implications for magmatic processes in the supra-subduction zone

ABSTRACT The Sapat Complex in Northern Pakistan contains remnants of the northern Neo-Tethys Ocean, presently exposed along the Indus Suture Zone. The mantle peridotites of the Sapat Complex include harzburgites, dunites, and subordinate lherzolites. Harzburgites are the dominant peridotite variety over dunites. The dunites are hosted by harzburgites and occur exclusively as ‘envelopes’ surrounding chromitite pods. The podiform chromitites show disseminated, banded, and massive textures. Chromitites exhibit variable Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], which range from 0.76 to 0.77, and from 0.64 to 0.66, respectively, while TiO2 contents are <0.2 wt.%. These features perhaps reflect crystallization of the chromian spinel from a boninitic magma. Similarly, chromian spinel in peridotites manifest a wide range of Cr# and Mg#, from 0.49 to 0.83 and 0.41 to 0.57, respectively, and are characterized by very low TiO2 values, averaging at 0.1 wt.%. Chromian spinel of chromitites and peridotites of the Sapat Complex have also very low Fe3+# (<0.01), which indicate their crystallization under low oxygen fugacities. The platinum group elements (PGE) distributions show high (Os + Ir + Ru)/(Rh + Pt + Pd), very low Pd/Ir values, and are defined by a prominently fractionated chondritic normalized PGE pattern, hence, this deposit is a typical example of an ophiolitic chromitite. The studied peridotites are highly depleted in PGE compared to chondritic values. The PdN/IrN values, averaging to 1.5 in dunites are unfractionated, while PGE spidergrams of harzburgites and lherzolites depict minor positive slopes, a minor positive Ru anomaly, and have average PdN/IrN values of 2.3 and 2.4, respectively. Furthermore, the harzburgites, dunites, and lherzolites display generally flat chondritic and primitive mantle normalized PGE patterns, and therefore, are nearly identical to highly depleted mantle peridotites. The mineralogical and PGE geochemical imprints of Sapat Complex chromitites and peridotites establish a strong affinity to supra-subduction zone ophiolites. Moreover, calculated parental melts of the chromitites and various geochemical discrimination diagrams elucidate that the chromitites were derived from boninitic magma produced by melting of depleted mantle peridotites in an oceanic arc, characterized by low oxygen fugacity, similar to a supra-subduction zone tectonic setting. This research highlights the use of mineralogy and geochemical compositions of chromitites and peridotites to reveal deep magmatic processes in a supra-subduction zone environment.

Petrogenesis of mafic rocks from the Xigaze ophiolite, Southern Tibet: Insights into forearc extension induced by Neotethyan rollback

The Xigaze ophiolite in the central Yarlung Zangbo suture zone (southern Tibet) provides evidence of the tectonic processes that formed the Neotethyan Ocean between the Indian and Eurasian plates. In this study, we present zircon U Pb geochronological, mineralogical, and geochemical data for mafic rocks from the Xigaze ophiolite to determine their petrogenesis and tectonic environment. The crustal section of the Xigaze ophiolite includes mainly gabbro lenses, amphibolite dikes, dolerite sills or dikes, and basaltic lavas. The absence of a well-developed gabbroic crust and sheeted dike complexes in the ophiolite indicates slow to ultraslow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, which is not atypical for layered ophiolites. Zircons from the amphibolite and dolerite dike reveal crystallization ages of 125.0 ± 1.8 Ma to 123.8 ± 1.3 Ma, respectively. Geochemically, all mafic rocks have mid-ocean ridge basalt (MORB)-like affinities with enrichment in large ion lithophile elements (LILEs, U) relative to high field strength elements (HFSEs, Nb, Ta and Ti) and light rare earth elements (LREEs). They also exhibit high εNd(t) values ranging from +8.4 to +8.8 at 206 Pb/ 204 Pb = 16.75–17.64. Moreover, chromite and clinopyroxene grains from the mafic rocks have a wide compositional spectrum and are mainly plotted in the supra-subduction zone (SSZ) fields. These features indicate that MORB-like magmatism was triggered by decompression melting of the upwelling asthenospheric mantle with minor mass transfer from the subducting slab. The Xigaze ophiolite lacks boninite, high‑magnesium andesites, island arc tholeiites, and calc-alkaline lavas, which indicates that it is incompatible with the complete Izu-Bonin-Mariana forearc sequence. Combined with previous studies, we speculate that the Xigaze ophiolite represents an ancient SSZ-type oceanic lithosphere formed in the Gangdese forearc during the Early Cretaceous, and the upper plate extension is linked to the southward retreat and rollback of the Neotethyan slab. Thus, the Xigaze and other Neotethyan SSZ ophiolites with no systematic change in composition with stratigraphic age provide new insights into the diversity of melt evolution in the forearc crust. • Amphibolite and dolerite dikes in the Xigaze ophiolite were formed during the Early Cretaceous. • The mafic rocks were derived from upwelling asthenosphere with minor slab input. • The Xigaze ophiolite originated from the Gangdese forearc setting. • The SIR model appears likely to be oversimplified for variable forearc ophiolites.

Supra-subduction zone ophiolite generated by the initial subduction of an Early Paleozoic island arc system abutting the northern North China Craton: Evidence from meta-igneous rocks

To constrain the evolution of an Early Paleozoic island arc belt abutting the North China Craton (NCC), this study presents new geochronological, geochemical, and Sr–Nd–Hf–O–Pb isotopic data for igneous rocks from the central part of Jilin Province within the southeastern part of the Central Asian Orogenic Belt. The igneous complex in this area hosts both mid-ocean ridge basalt (MORB)- and arc-type rocks that yield zircon 206Pb/238U ages of ca. 492 and 488–459 Ma, respectively. The MORB-type units in this area include metacumulate gabbros, metadiabases, and metabasalts that have normal MORB (N-MORB)-like compositions with (La/Yb)N values of 0.29–0.99, Th/Nb values of 0.07–0.52, whole rock εNd(t) values from +4.32 to +7.29, and initial 208Pb/204Pb values of 38.12–38.20. These compositions are indicative of derivation from the depleted asthenospheric mantle. The arc-type igneous rocks consist of amphibolite, epidiorite, and metagabbro with major- and trace-element and isotopic compositions that are similar to island arc tholeiite to calc-alkaline rocks, with (La/Yb)N values of 1.11–4.26, Th/Nb values of 0.11–2.25, zircon εHf(t) values from +6.03 to +15.30, zircon δ18O values from +4.32 to +5.1, whole-rock εNd(t) values from +2.09 to +7.25, and initial 208Pb/204Pb values of 38.09–38.11. These compositions are indicative of derivation from partial melting of the hydrous depleted mantle wedge material within an intra-oceanic arc setting. The MORB-type rocks formed within a forearc region of the SSZ ophiolite fragment, whereas the arc-type rocks record the evolution of this region from immature to mature island arc tectono-magmatic settings. Combining these new data with the results of previous research suggests that initial subduction of the Paleo-Asian oceanic plate abutting the northern NCC began during the latest Cambrian.