Intra-oceanic Subduction Zone Research Articles

Age and composition of the Rebang Co and Julu ophiolites, central Tibet: implications for the evolution of the Bangong Meso-Tethys

Subduction of the Bangong Meso-Tethys and collision between the Lhasa and Qiangtang blocks were important for the growth of Tibetan crust, as well as the development of super-large porphyry copper deposits in central Tibet. However, the initiation and closure timing, and nature and structure of the Bangong Meso-Tethys are still poorly constrained. Petrology, geochronology, and geochemistry of the Rebang Co and Julu ophiolites in the western part of the suture zone are used to constrain the tectonic history and structure of the Bangong Meso-Tethys. These two suites consist of ultramafic rocks, gabbro and diabase dikes, pillow basalts, and radiolarian cherts. Zircon U–Pb dating of zircon grains of gabbros yields well-defined weighted mean ages of 161.5 ± 1.5 Ma and 103.8 ± 3.9 Ma for the Rebang Co and Julu suites, respectively. Both of ophiolitic suites have mid-oceanic ridge basalt and island-arc tholeiite affinities, similar to back-arc basin crust. We suggest these suites formed above an intra-oceanic subduction zone at least during Middle Jurassic through mid-Cretaceous times. Along with coeval subduction beneath the southern Qiangtang margin, as well as fore-arc basin development, the Bangong Meso-Tethys has a complicated structure and history. Clearly, subduction in the Bangong Meso-Tethys was still active during mid-Cretaceous time and the Meso-Tethys did not close until Late Cretaceous time.

Jurassic ophiolite formation and emplacement as backstop to a subduction-accretion complex in northeast Turkey, the Refahiye ophiolite, and relation to the Balkan ophiolites

The eastern Mediterranean region within the Tethyan realm shows a high concentration of ophiolites with contrasting times of formation and emplacement along the belt: In the Balkans, the ophiolites formed during the early to medial Jurassic, and were obducted during the late Jurassic, whereas in Turkey and farther east, structurally intact Jurassic ophiolites are rare and Jurassic ophiolite obduction is unknown. Here we report a structurally intact, large ophiolite body of early Jurassic age from NE Turkey, the Refahiye ophiolite, located close to the suture zone between the Eastern Pontides and the Menderes-Taurus block. The Refahiye ophiolite forms an outcrop belt, 175 km long and 20 km wide, and is tectonically bound by the late Cretaceous ophiolitic melange to the south, and by the North Anatolian Transform Fault against the Triassic low-grade metamorphic rocks to the north. Early to medial Jurassic very low- to low-grade metamorphic rocks, interpreted as intraoceanic subduction-accretion complexes, occur either beneath the ophiolite or as thrust slices within it. The ophiolite body within the studied section is made up of mantle peridotite (clinopyroxene-bearing harzburgite and minor dunite) crosscut by up to 20 cm thick veins of clinopyroxenite and later dikes/pods/stocks of gabbro ranging in size from 2 m to several hundreds of meters. The gabbro is represented by two distinct types: (i) cumulate gabbro, and (ii) non-cumulate gabbro with locally well-developed igneous foliation. Within the non-cumulate gabbro or enclosing peridotite, there are up to 5 m and 50 cm-thick veins of trondhjemite and pegmatitic gabbro, respectively. LA-ICP-MS dating on zircons from two trondhjemite samples yielded weighted mean ages of ∼184 ± 4 Ma and 178 ± 4 Ma (2σ), respectively, suggesting formation during early Jurassic time. Formation in a suprasubduction-zone forearc setting is inferred from (i) wide-ranging pyroxene and spinel compositions in the peridotites as documented in most suprasubduction-zone ophiolites, (ii) arc tholeiitic signature of the non-cumulate gabbros, and (iii) association of the ophiolite with the coeval subduction-accretion complexes. Emplacement of a trapped forearc ophiolite above its own subduction-accretion complex as a backstop is proposed based on a series of field relationships such as (i) intimate association of the unsubducted suprasubduction-zone ophiolite with coeval accretionary complexes, (ii) absence of unambiguous relationship to the southern Atlantic-type continental margin, and (iii) absence of any stratigraphic indications for the ophiolite obduction in the southern Atlantic-type continental margin during Jurassic time. This is a clear difference from the Jurassic ophiolites in the Balkans that were obducted over the Atlantic-type continental margin. This difference in mode of emplacement is most probably related to the greater distance of the intra-oceanic subduction zone to the Atlantic-type continental margin than it was in the Balkans, which is commensurate with the greater width of the Tethys in the east during Jurassic time.

U–Pb zircon ages for Yarlung Tsangpo suture zone ophiolites, southwestern Tibet and their tectonic implications

Ophiolite complexes preserved along the Yarlung Tsangpo suture zone (YTSZ) and obducted onto the northern continental margin of India in southern Tibet represent the remnants of the once extensive Permian–Mesozoic Neo-Tethyan Ocean that separated India from Asia. Complete ophiolite successions are preserved near Xigaze, whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks. U–Pb zircon LA-MC-ICP-MS geochronology on two gabbro samples from the Luobusa ophiolite yielded concordant data with mean 206Pb/238U ages of 149.9±1.4 (2σ) Ma and 150.0±5.0Ma. These ages are in contrast to a younger age of 131.8±1.0Ma obtained from a pegmatitic gabbro in Xigaze. Five U–Pb zircon TIMS ages from gabbroic samples in the western portion of the ophiolite belt reveal that the Dangxiong ophiolite formed between 126.7±0.4Ma and 123.4±0.8Ma. Zircons from the Jungbwa ophiolite have similar ages of 123.4±0.8Ma and 123.9±0.9Ma. A single zircon analysed from a gabbro in Kiogar has an age of 159.7±0.5Ma. Geochronological data reported here show YTSZ ophiolites formed in association with intra-oceanic subduction zone systems and are related a significant tectonic episode within the Tethyan Ocean during Late Jurassic to Early to mid Cretaceous times.

Timing of deformational events in the Río San Juan complex: Implications for the tectonic controls on the exhumation of high-P rocks in the northern Caribbean subduction–accretionary prism

An integrated structural, petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high-P rocks of the Río San Juan complex in the northern Caribbean subduction–accretionary wedge. In the main structural units of the complex, microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite-facies conditions and during the main retrogressive event toward the low-P amphibolite or blueschist/greenschist-facies conditions.U–Pb SHRIMP dating on zircon rims (71.3±0.7Ma) coupled with 40Ar–39Ar analyses on phengite (~70–69Ma) in felsic sills placed temporal constraints on the exhumation of the Jagua Clara serpentinite-matrix mélange during the blueschist-facies stage at the early Maastrichtian. In the Cuaba unit, U–Pb TIMS zircon ages of 89.7±0.1Ma and 90.1±0.2Ma obtained for the crystallization of tonalitic/trondhjemitic melts in the lower Guaconejo and upper Jobito subunits, respectively, are similar. These ages coupled with a U–Pb SHRIMP zircon age of 87±1.8Ma obtained in a garnet amphibolite and a group of older 40Ar–39Ar cooling ages on calcic amphibole constrain the exhumation of the Guaconejo subunit from the high-P stage to the low-P stage at the ~90–83Ma time interval. Further, the age data collectively supports a genetic relationship between the distributed extensional ductile shearing, the related decompression and the local partial anatexis in the subunit, at least from the Turonian–Coniacian boundary to the early Campanian. A group of younger 40Ar–39Ar ages obtained in the mylonitized amphibolites of the basal Jobito detachment zone indicates late ductile deformation and exhumation/cooling in the late Campanian to Maastrichtian (~75–70Ma). Therefore, structural and age data established deformation partitioning and reworking of retrograde fabrics during ~20Ma in the Cuaba unit.The different exhumation rates obtained for the Jagua Clara mélange can be explained by uplift in two contrasting tectonic settings: a first stage of slow exhumation (1.4mm/yr) in the subduction channel, largely lower than plate velocities, and a second stage of relatively fast exhumation (7.6mm/yr) up to the surface. Therefore, the exhumation was temporally discontinuous and the velocity increase at ~70Ma probably was triggered in response to the entrance of buoyant material in the subduction zone, such as a Caribeana continental ribbon or the distal part of Yucatán–Bahamas continental margin. In contrast, the exhumation path of the Guaconejo subunit is composed of a first segment from the baric peak to the low-P amphibolite stage (at 84–83Ma) with an exhumation rate of 7.2mm/yr, a second segment around the closure temperature of calcic-amphibole (at 82–70Ma) with a rate of 0.4mm/yr, and a third segment to the surface exposure (at 60Ma) with a rate of 1.8mm/yr. These velocity differences can be correlated with the P–T path proposed for the exhumation of the subunit, with an initial isothermal decompression from the peak in the high-P amphibolite to the eclogite-facies produced by distributed extensional shearing, followed by a relatively slow cooling at low-P in the newly acquired structural position, and the final tectonics mainly partitioned in the late Jobito detachment zone. The initial fast exhumation can be related to a major modification in the convergence conditions across the intra-oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore–onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged fromthe conversions of teleseismic body-waves and the locations of earthquakes appearswith kinks which increase the dip to 10–20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: “supraslab” earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and “deep flat-thrust” earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations are not readily available either for the large crustal thickness of this shallow water marine upper plate, except when remarking its likeness to oceanic plateaus formed above hotspots. The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose regions of stick-slip behavior on the interplate among stable-gliding areas.

Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting

An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5Ga granite pluton and cross-cut by undeformed 2.5Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic–ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc–forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc–continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.

Sedimentation and volcanism in the Panamanian Cretaceous intra-oceanic arc and fore-arc: New insights from the Azuero peninsula (SW Panama)

Abstract The Azuero Peninsula, located in SW Panama, is a region characterized by a long-lived intra-oceanic subduction zone. Volcanism began in Late Cretaceous time, as the result of subduction of the Farallon plate beneath the Caribbean plate. Usually, ancient volcanic arcs related to intra-oceanic subduction zones are not preserved, because they are in areas with difficult access or covered by modern volcanic arc material. However, on the Azuero peninsula, a complete section of the volcanic arc together with arc basement rocks provides the opportunity to study the sedimentation and volcanism in the initial stages of volcanic arc development. The lithostratigraphic unit which records fore-arc evolution is the “Río Quema” Formation (RQF), a volcanic apron composed of volcanic and volcaniclastic sedimentary rocks interbedded with hemipelagic limestones, submarine dacite lava domes, and intruded by basaltic-andesitic dikes. The “Río Quema” Formation, interpreted as a fore-arc basin infilling sequence, lies discordantly on top of arc basement rocks. The exceptionally well exposed arc basement, fore-arc basin, volcanic arc rocks and arc-related intrusive rocks provide an unusual opportunity to study the relationship between volcanism, sedimentation and magmatism during the arc development, with the objective to reconstruct its evolution. The “Río Quema” Formation can be divided into three groups: 1) proximal apron, a sequence dominated by lava flows, interbedded with breccias, mass flows and channel fill, all intruded by basaltic dikes. The rocks represent the nearest materials to the volcanic source, reflecting a coarse sediment supply. This depositional environment is similar to gravel-rich fan deltas and submarine ramps; 2) medial apron, characterized by a volcanosedimentary succession dominated by andesitic lava flows, polymictic volcanic conglomerates and crystal-rich sandstones with minor pelagic sediments and turbidites. These rocks were deposited from high-density turbidity currents and debris flows, directly derived from erupted material and gravitational collapse of an unstable volcanic edifice or volcaniclastic apron; 3) distal apron, a thick succession of sandy to muddy volcaniclastic rocks, interbedded with pelagic limestones and minor andesitic lavas, intruded by dacite domes and by basaltic to andesitic dikes. Bedforms and fossils suggest a quiet, relatively deep-water environment characterized by settling of clay and silt (claystone, siltstone) and by dilute turbidity currents of reworked volcaniclastic detritus. The timing of the initial stages of the volcanic arc has been constrained through a biostratigraphic study, using planktonic foraminifera and radiolarian species. The fossil assemblage indicates that the age of the “Río Quema” Formation ranges from Late Campanian to Maastrichtian, providing a good constraint for the development of the volcanic arc and volcaniclastic apron, during the initial stages of an intra-oceanic subduction zone.

Late Palaeozoic–Cenozoic tectonic development of carbonate platform, margin and oceanic units in the Eastern Taurides, Turkey

Abstract Continental margin-type, ophiolitic and mélange units are exposed throughout central eastern Turkey (e.g. Gürün, Hekimhan and Pınarbaşı areas). These restore as a north-verging Triassic-rifted continental margin that underwent Jurassic–Early Cretaceous passive margin subsidence. Chemically ‘enriched’ basaltic lavas of seamount type are interbedded with and overlain by Middle Jurassic–Early Cretaceous ribbon cherts. Ophiolitic rocks (e.g. Pınarbaşı, Dağlıca, Kuluncak, Hekimhan, Divriği) formed by spreading above a Late Cretaceous northwards-dipping intra-oceanic subduction zone. Emplacement of continental margin units, mélanges and ophiolites onto the East Tauride platform was driven by trench-margin collision during latest Cretaceous. The northern part of the East Tauride neritic carbonate platform detached and overthrust the continent to the south (Malatya Metamorphics) which was deeply underthrust, metamorphosed at least to greenschist facies and exhumed by latest Cretaceous. Collision-related Mid-Eocene southwards thrusting strongly affected the western part of the region (e.g. Pınarbaşı), whereas areas further east (e.g. Darende, Hekimhan, Divriği, Sivas) mainly experienced folding. Taking account of the regional tectonic setting, we infer that the Gürün platform, with its distinctive unbroken up to Lutetian-aged succession, represents a small exotic terrane that was translated from a relatively southerly (‘internal’) part of the Tauride platform (Geyik Dağ), related to strike-slip displacement (syn/post-Eocene to pre-Pliocene). Supplementary material: Two supplementary tables giving the GPS coordinates of the samples of basic intrusive and extrusive igneous rocks that were analyzed during this work are available at www.geolsoc.org.uk/SUP18569

Contrasting exhumation P–T paths followed by high-P rocks in the northern Caribbean subduction–accretionary complex: Insights from the structural geology, microtextures and equilibrium assemblage diagrams

The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction–accretionary complex built during convergence between the Caribbean island arc and the North America continental margin. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined structural geology, microtextural relations, multi-equilibrium calculations and thermodynamical modelling, together with published isotopic ages, allow reconstructing the metamorphic P–T–t paths of each nappe/unit and their links to the structural evolution. In all units of the complex, three major stages (M1 to M3) in the tectonothermal evolution have been distinguished. The M1 stage corresponds to the prograde evolution towards the pressure-peak of metamorphism under blueschist or eclogitic-facies conditions. The M2 stage is related to the main retrogressive event and is characterized by the S2–L2 fabric development in all lithologies and at all scales. The M3 stage represents continuous exhumation from ductile to ductile–brittle deformation regimes. However, the shape of the retrograde P–T path, the age of the exhumation-related D2 structures and the tectonic significance of D2 deformation are different in each structural unit.In the upper structural levels of the Jagua Clara serpentinitic-matrix mélange, the counter-clockwise P–T path of the eclogite blocks is typical of rocks exhumed in the early stages of intra-oceanic subduction zones. The clockwise P–T path obtained for the lower Cuaba unit is characterized by a strong isothermal decompression from the garnet-epidote amphibolite and eclogite-facies pressure-peak. This P–T evolution can be explained by rapid exhumation caused by extensional tectonics, in relation to a major modification of convergence conditions across the subduction zone. The P–T path also explains local syn-M2 partial melting processes, because it crosses the wet solidus for IAT mafic compositions. The P–T path obtained for the high-P Guineal Schists, with exhumation trajectory following the burial trajectory, can be related to exhumation during active subduction. This exhumation was most likely driven by a combination of underthrusting of tectonic units and erosion processes.Available geochronological data and T–t/P–t estimates reveal a Late Campanian to Maastrichtian retrograde M2 metamorphism in the lower structural units of the complex during a consistent D2 top-to-the-NE/ENE tectonic transport. A similar tectonic transport has also been recognized in the metasedimentary nappes of the Samaná complex during Eocene to earliest Miocene. These relations indicate a northeastward progradation of deformation during the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction–accretionary complex.

U–Pb and Sm–Nd geochronology of the Kızıldağ (Hatay, Turkey) ophiolite: implications for the timing and duration of suprasubduction zone type oceanic crust formation in the southern Neotethys

AbstractThe Kızıldağ (Hatay) ophiolite in Turkey represents remnants of the southern Neotethyan ocean and is characterized by a complete ocean lithospheric section. It formed in a fore-arc setting above a N-dipping intraoceanic subduction zone, and represents the undeformed, more northerly part of the same thrust sheet that also forms the Baer–Bassit ophiolite to the south. The ophiolite was emplaced southwards from the southerly Neotethyan ocean in Maastrichtian time. U–Pb and Sm–Nd dates are used to constrain the crystallization age and duration of magmatic activity of the Kızıldağ ophiolite. U–Pb dating yielded ages of 91.7 ± 1.9 Ma for a plagiogranite and 91.6 ± 3.8 Ma for a cumulate gabbro. The cumulate gabbro also yielded a Sm–Nd isochron age of 95.3 ± 6.9 Ma. The measured ages suggest that the oceanic crust of the Kızıldağ ophiolite formed in a maximum time period of 6 Ma, and that the plagiogranite may have formed later than the gabbroic section. The U–Pb zircon ages from the Kızıldağ ophiolite and the cooling age of a metamorphic sole beneath the Baer–Bassit ophiolite are indistinguishable within the analytical uncertainties. This indicates the presence of young and hot oceanic lithosphere at the time of intraoceanic subduction/thrusting in the southern Neotethys. The U–Pb zircon ages from the Kızıldağ, the Troodos and the Semail ophiolites overlap within analytical uncertainties, suggesting that these ophiolites are contemporaneous and genetically and tectonically related within the same Late Cretaceous southern Neotethyan ocean.

New occurrences of jadeitite, jadeite quartzite and jadeite-lawsonite quartzite in the Dominican Republic, Hispaniola: petrological and geochronological overview

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Rio San Juan Complex (RSJC) of the northern Dominican Republic in serpentinite melanges associated with a former intra-oceanic subduction zone. Allochthonous blocks in lag deposits developed on the melange outcrops or boulders in river beds are common. A very unusual feature for the RSJC is the occurrence of concordant layers and discordant veins of cm to dm thickness in blocks of jadeite±lawsonite- or omphacite-garnet-bearing blueschist of the melange. Two suites of jadeite-rich rocks can be recognized. The first is represented by quartz-free jadeitite s.str . (>90 vol% jadeite) found so far only as blocks and boulders. The second suite comprises quartz-bearing jadeitite s.str . grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). The second suite is found both as blocks and boulders as well as layers and veins in blueschist blocks. One single occurrence of a cross-cutting omphacitite vein in blueschist has also been observed. Additional important phases so far found in both suites are omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have only been identified in quartz-free jadeitite s.str .; almandine-rich garnet has so far been observed only in JLQ. The two suites of jadeite-bearing rocks occur in various shades of green, are fine- to coarse-grained, and usually equigranular. Mineral distribution is commonly homogeneous, but may be patchy in JLQ, giving this rock type a distinctly mottled appearance. Cathodoluminescence (CL) images show oscillatory zoning patterns in jadeite, zircon, apatite and calcite; this is evidence for crystallization from an aqueous fluid under open-system conditions. Zircons separated from a sample of quartz-free jadeitite s.str . contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, indicating coeval zircon growth. The cores of the zircons yield ages of 114.9 ± 2.9 Ma, thus defining a crystallization age close to the initiation of subduction in the Rio San Juan Complex, when “warm” geotherms of ≈15°/km prevailed. These ages are in contrast with the crystallization ages of the blueschists hosting the second, quartz-bearing suite of jadeite-rich rocks. These range from 80 to 62 Ma, towards the end of subduction-zone activity at ≈55 Ma and “cool” geotherms of 8–9 °C/km. For the younger quartz-bearing suite, the combination of phengite compositions with the available P-T-t paths of the host blueschists suggests crystallization temperatures of ≈ 350 to ≈ 500 °C at minimum pressures of 15–16 kbar. The P-T conditions for the older quartz-free suite are more difficult to constrain, but the combination of phengite compositions with the prevailing geotherms in the young and warm subduction zone suggest minimum conditions of at least 500 °C and 11 kbar. However, temperatures and pressures as high as 600 °C and 15 kbar, as documented for jadeitites of similar age in the same subduction zone exposed in neighbouring eastern Cuba, are possible. Jadeitites and jadeite-rich rocks of the RSJC are thus interpreted to have crystallized over a time-span of ≥ 60 Myr at initial temperatures of at least 500 °C, later evolving down to 350 °C in a single, thermally self-organizing, cooling subduction zone. The P-T conditions suggested for the younger quartz-bearing suite correlate well with those of jadeitite formation in Guatemala south of the Motagua Fault Zone, the only other occurrence world-wide where jadeitite with both lawsonite and quartz appears to be common. Further evidence is needed to corroborate that the older quartz-free suite represents another example of rare high-temperature jadeitite as documented in Cuba.

Intra-Panthalassa Ocean subduction zones revealed by fossil arcs and mantle structure

The vast Panthalassa Ocean once surrounded the supercontinent Pangaea. Subduction has since consumed most of the oceanic plates that formed the ocean floor, so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean’s history since the Cretaceous period 1,2 , and the Triassic‐Jurassic plate tectonic evolution of the Panthalassa Ocean remains largely unresolved 3,4 . Geological clues come from extinct intra-oceanic volcanic arcs that formed above ancient subduction zones, but have since been accreted to the North American and Asian continental margins 4 . Here we compile data on the composition, the timing of formation and accretion, and the present-day locations of these volcanic arcs and show that intra-oceanic subduction zones must have once been situated in a central Panthalassa location in our plate tectonic reconstructions 5‐7 . To constrain the palaeoposition of the extinct arcs, we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic-wave tomographic models 8,9 . We suggest that a series of subduction zones, together called Telkhinia, may have defined two separate palaeo-oceanic plate systems— the Pontus and Thalassa oceans. Our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the Telkhinia subduction zones and Panthalassa Ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics. The spatial uncertainty in reconstructions of oceanic basins increases the further we go back in time, because oceanic plates subduct. Plate reconstructions for the TriassicJurassic history of the Panthalassa Ocean, once surrounding the Pangaea supercontinent, are based on the premise that the Pacific plate formedintheMiddleJurassicasatriangle 2 ,originatingfromatriple junction between the Farallon, Phoenix and Izanagi plates (Fig. 1). The preserved area of Jurassic palaeoplates represents only a small portion (<10%) of the oceanic crust of the Jurassic Panthalassa Ocean. The remaining part, along with pre-Jurassic oceanic plate configurations, is entirely unconstrained. However, the geology of the circum-Panthalassa continental margins provides evidence that early Mesozoic subduction-related volcanism occurred in the Panthalassa Ocean 4,10 . On subduction, continental fragments, volcanic arcs, ophiolites and ocean-floor sediments collided with or accreted to the continental margins of, in particular, western North America, far-east Asia and Japan. Marine microfossils in these sediments and palaeomagnetic data constrain their age and palaeolatitude, but their palaeolongitudes remain undetermined. Palaeogeographic reconstructions therefore show uncorrelated intra-Panthalassa exotic terranes essentially

Origin and significance of the ophiolitic high-P mélanges in the northern Caribbean convergent margin: Insights from the geochemistry and large-scale structure of the Río San Juan metamorphic complex

The Río San Juan metamorphic complex exposes a segment of a high-pressure accretionary wedge, built during Late Cretaceous intra-oceanic subduction below the Caribbean island-arc. Detailed mapping, large-scale structural analysis, U–Pb/Ar–Ar geochronology, in situ trace element analyses of clinopyroxene, and bulk-rock geochemical data show that the northern half of the complex is composed of arc- and oceanic-derived units, whose large-scale internal structure consists of an imbricate stack of mainly high-P rocks. In the lower structural levels, the Gaspar Hernández serpentinized peridotite–tectonite is composed of massive, serpentinized harzburgite that grades into surrounding sheared serpentinite or gouge. Gabbroic rocks and dolerite sills with N-MORB geochemical signatures are sparse and altered to greenschists facies. U–Pb data indicate a Lower Cretaceous magmatic age (136.40±0.32Ma). These rocks are interpreted as ophiolitic fragments of the proto-Caribbean Ocean. In the intermediate Jagua Clara serpentinite-matrix mélange, the massive serpentinite with relict peridotite textures was overprinted by schistose and sheared serpentinite, and serpentinite gouge, recording various degrees and conditions of internal deformation. Serpentinite schistosity surfaces warp around mélange hard blocks (<1m to 2.5km). Blocks are massive serpentinized peridotite and exotic high-P crustal rocks. The protoliths of the mafic metaigneous rocks originated from arc-like and non-arc-like magmas. The Jagua Clara serpentinite-matrix mélange represents the deep subduction channel, formed during intra-oceanic subduction. In the upper structural levels, the Morrito unit is composed of high-P Puerca Gorda and Guineal Schists. Protoliths are basic-intermediate volcanic rocks with IAT, low-Ti IAT and boninitic geochemical signatures, similar in composition to the Lower Cretaceous Caribbean island-arc. The Morrito basal fault-zone thrusted the Puerca Gorda Schists onto the Jagua Clara mélange. The geochemical characteristics of many mafic metaigneous rocks found within the Jagua Clara mélange, and particularly in the Morrito unit, indicate that they occupied a supra-subduction zone position at some time in their history and, therefore, the mélange reflect the position of the Late Cretaceous intra-oceanic subduction zone. The presence in the mélange of exotic blocks plucked from both the upper plate (Caribbean island-arc) and the lower plate (proto-Caribbean Ocean) suggests that the arc crust was eroded at its base by tectonic processes in the Late Cretaceous. Regional structural data indicate that the Morrito basal fault-zone now forms the suture between the Caribbean island-arc terranes and the accreted units of the paleo-continental margin of North America.

Petrology and geochemistry of peridotites in the Zhongba ophiolite, Yarlung Zangbo Suture Zone: Implications for the Early Cretaceous intra-oceanic subduction zone within the Neo-Tethys

The Zhongba ophiolite is located in the western part of the Yarlung Zangbo Suture Zone (YZSZ) separating Eurasia to the north from the Indian plate to the south. This ophiolite comprises a well-preserved mantle sequence dominated by harzburgites with minor dunites. Highly depleted modal, mineral and bulk rock compositions of the harzburgites indicate that they are residues after moderate to high degrees of partial melting (13–24%) mainly in the spinel-stability field. These rocks display typical U-shaped chondrite-normalized Rare Earth Element (REE) patterns and fractionated chondrite-normalized Platinum Group Element (PGE) patterns. These characteristics, in combination with their hybrid mineral and whole-rock compositions intermediate between those of abyssal and forearc peridotites, indicate melt-rock interaction resulting in the selective enrichment of LREE and Pd. We propose a two-stage model to explain the generation of the Zhongba harzburgites: 1) original generation from a MORB-source upper mantle, and 2) subsequent trapping as part of a mantle wedge above a subduction zone. Comparable observations from the ophiolitic massifs along the whole YZSZ allow us to propose that a ca. 2500-km long complex subduction system was active between India and the Lhasa terrane, Burma, and the Karakoram microcontinent within the Neo-Tethys during the Early Cretaceous, similar to the modern active intra-oceanic subduction systems in the Western Pacific.

A Devonian to Carboniferous intra-oceanic subduction system in Western Junggar, NW China

The Devonian and Carboniferous rock assemblages in Western Junggar, southern Altaids, include ophiolitic mélanges and coherent units. At Baijiantan, Yeyagou and Dagun Devonian gabbros and one group of volcanic rocks (type I lava) generally crop out as blocks in a matrix of ultramafic rocks and/or mudstone. They have a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Another group of volcanic rocks (type II lava) from Baijiantan, Yeyagou, Western Karamay and Maliya includes alkaline basalt, basanite, andesite, dacite and trachyte that contain microphenocrysts of clinopyroxene–plagioclase, amygdales filled with chalcedony and calcite, and accessory pyrite and ilmenite. The type II lavas are further subdivided into five sub-types: (1) Type II1 lava is strongly enriched in LREE with incompatible trace element ratios similar to oceanic island basalt (OIB), suggesting derivation from enriched mantle. (2) Type II2 lava and tuff are slightly LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment indicating that they were generated in a supra-subduction zone (SSZ). (3) Type II3 lava is also moderately enriched in LREE suggesting derivation from a more-enriched MORB mantle. (4) Type II4 lava is enriched in LREE and has high Nb/Yb values close to those of OIB. It is extensively depleted in Nb suggesting a source containing a mixture of enriched and SSZ mantle. (5) Type II5 lava has MORB-like REE patterns, and Nb/Yb and Th/Yb ratios. Based on field structural data, and the contemporaneity of the Baogutu adakite, a high-Mg diorite dike and the Miaoergou charnockite, these enriched and depleted magmatic features suggest generation in an intra-oceanic subduction zone influenced by ridge-trench interaction. The Altaids in Western Junggar are characterized by multiple intra-oceanic subduction systems, which may have continued to operate to the late Carboniferous.

Field and geochemical characteristics of Mesoarchean to Neoarchean volcanic rocks in the Storø greenstone belt, SW Greenland: Evidence for accretion of intra-oceanic volcanic arcs

The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Sm cn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Yb cn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Sm cn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.

New occurrence of UHP eclogites in Limousin (French Massif Central): Age, tectonic setting and fluid–rock interactions

Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure–temperature conditions around 660 °C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite–eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb–Ta anomalies, εNd: + 6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe–Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr–Nd isotopic composition ( 87Sr/ 86Sr: 0.706; εNd: − 3 to − 5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid–rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr–Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 ± 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U–Pb isotopic system at 382 ± 7 Ma is related to an anatectic high pressure–medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia–South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean–continent subduction during Early Devonian.

Oceanization of the northern Neotethys: Geochemical evidence from ophiolitic melange basalts within the İzmir–Ankara suture belt, NW Turkey

The remnants of the Neotethyan Izmir–Ankara Ocean, the main branch of Neotethys in the eastern Mediterranean are represented by the Dagküplü Melange Complex in Central Sakarya, NW Turkey. It comprises several blocks or tectonic slices of pillow lavas, some of which include mudstones and radiolarian cherts as intra-pillow-fillings or interlayers. In the Igdecik area, a huge basaltic block has been studied in detail. Geochemical data reveal three distinct basalt types separated by sheared contacts. The first of these groups is an enriched mid-oceanic ridge basalt (E-MORB) type which is enriched in the most incompatible trace elements relative to normal MORB (N-NORB) in addition to having heavy rare earth elements (HREE) depletion, suggesting the influence of residual garnet in their mantle source region. The second is back-arc basin basalt (BABB) type with relatively depleted trace element compositions with respect to N-MORB together with a negative Nb anomaly, suggesting generation above an intra-oceanic subduction zone where partial melts are derived from a depleted (MORB-like) mantle. The final group is island-arc tholeiite (IAT) type, displaying the most depleted trace element abundances among the studied groups in addition to marked Nb depletion, reflecting intra-oceanic supra-subduction zone (SSZ) signatures similar to the BABB-type but requiring a depleted mantle source which has experienced a previous melt extraction. Combined with a previously ascribed Late Triassic age of Tekin et al. (2002) (221 Ma, Late Carnian; based on the radiolarian fauna found in a chert layer alternating with mudstones), the associated basalts with E-MORB-type geochemical signatures, suggest formation of oceanic crust as early as Late Carnian. This age is the oldest thus far obtained from the basalts of the Izmir–Ankara Ocean. This new data provides constraints on tectonic models for the opening the Izmir–Ankara Ocean and its relationship to other branches of the Neotethyan ocean in the Eastern Mediterranean area.

Geochemistry and tectonic evolution of the Neoproterozoic incipient arc–forearc crust in the Fawakhir area, Central Eastern Desert of Egypt

The geodynamic origin of the Neoproterozoic ophiolites of the Arabian-Nubian Shield exposed in the Eastern Desert of Egypt remains controversial. In this study, we present new geochemical and field data from the Fawakhir ophiolite and from some mélange blocks along the Qift-Qusier Road in order to constraint the tectonic evolution of this part of the Central Eastern Desert. The Fawakhir ophiolite contains most lithological units of a Penrose-type ophiolite sequence, and includes ultramafic rocks in the west overlain by isotropic gabbro, sheeted dikes and pillow basalt in the east. These ophiolitic units are enriched in LREE (light rare earth elements) and LILE (large ion lithophile elements) but depleted in high field strength elements (La/Sm cn = 0.40–1.22, Th/Nb pm = 1.7–10.9, La/Nb pm = 1.4–6.6). Their magmas appear to have been derived from a depleted (N-MORB-like) mantle source, and their geochemical characteristics are comparable to those of the Izu-Bonin-Mariana forearc oceanic crust formed during the initiation of an intra-oceanic subduction zone. Pillow lava blocks in the eastern mélange have geochemical signatures similar to those of oceanic crust generated in back-arc basins. The decrease in the magnitude of mantle depletion and the change of the geochemical signature along the Qift-Qusier Road from a forearc in the west to a back-arc in the east suggest the formation of the Fawakhir intra-oceanic arc system over an east-dipping subduction zone. With continued subduction and arc migration, this intra-oceanic arc system finally collided with the passive margin of the West Gondwana (the Saharan craton), resulting in the accretion of the Fawakhir arc–forearc units. Following its tectonic accretion onto the West Gondwana continental margin, the Fawakhir ophiolite was intruded by calc-alkaline dikes, whose magmas were derived partly from partial melting of the sub-continental lithospheric mantle. These dikes have geochemical characteristics similar to those of modern active continental margin (Andean-type) rocks (La/Sm cn = 2.13–2.48, Gd/Yb cn = 2.04–4.25, Th/Nb pm = 3.2–5.8, La/Nb pm = 2.5–4.9), suggesting that the West Gondwana passive margin (Atlantic-type) was converted to an Andean-type margin subsequent to the arc-continent collision. The inferred conversion of the Atlantic-type margin to an Andean-type margin resulted from the collision-induced reversal of the subduction direction.

Comparison of gravity anomaly between mature and immature intra-oceanic subduction zones in the western Pacific

The western Pacific hosts major subduction systems such as Izu–Bonin–Mariana and Tonga–Kermadec, but also less conspicuous systems such as Yap, Mussau and Hjort trenches which constitute the young, incomplete, or ultraslow-member in the evolutionary spectrum of subduction zones. We used satellite-derived gravity data to compare well-developed and immature subduction systems. It is shown that at spatial resolution > 10–20 km or so, the satellite data have accuracy comparable to ship-board gravity measurements over intra-oceanic subduction zones. In the isostatic residual gravity anomaly map, the width of non-isostatically-compensated region of the mature subduction zones is much wider than that of immature ones. More importantly, when the gravitational attraction due to seafloor is removed, a large difference exists between the mature and immature subduction zones in the overriding plate side. Mature subduction zones exhibit broad low gravity anomalies of ~ 200–250 mGal centered at distances of 150–200 km from the trench which are not found over immature subduction zones. The cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property. We discuss the following four causes: (1) serpentinization of the upper mantle beneath the forearc; (2) presence of partial melt in the mantle wedge caused by release of volatiles from the slab, frictional heating and distributed by mantle circulation; (3) difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle; and (4) anomalous thickness of the arc not explained by isostasy. Our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears ~ 150–200 km from the trench. Although the extent and distribution of partial melt within the mantle wedge remain in question, to our best estimate, partial melting contributes little (< 50 mGal) to the total negative gravity anomaly. The difference in density structure reflecting temperature difference can only explain less than half of the low gravity anomaly. The sinking of lighter crustal material produces a large negative anomaly in the forearc but its location does not match the observed gravity anomaly. It appears that one cannot explain the total difference in gravity anomaly without invoking anomalous thickness of the arc. Although we could not identify the sole or combination of factors that give rise to the low gravity anomaly in mature subduction zones, the comparison of gravity anomalies between mature and immature subduction zones is likely to provide an important constraint for understanding the evolution and structure of subduction zones as more complementary evidences become available.