Subduction Accretion Complex Research Articles

High-pressure complexes of Central Asian Fold Belt: geologic setting, geochemistry, and geodynamic implications

Blueschists are constituents of subduction-accretion complexes, where they occur either as separate tectonic slabs, slices, and lenses, or as exotic blocks in a melange. Often associated with ophiolites, blueschists are commonly metamorphosed members of ophiolite sections. Along with ophiolite, a typical component of high-pressure belts is alkaline ocean-island basalt comprehensively studied in the Uimon zone of Gorny Altai and Chara zone of Eastern Kazakhstan. This suggests active plume magmatism in the Early Paleozoic evolution of the Paleoasian Ocean. As inferred from sutures with high-pressure rocks, different parts of Paleoasian Ocean were closing successively.

Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas

We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2σ) laser step-heating ⁴⁰Ar/³⁹Ar plateau ages for phengite from quartzite mylonites from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone. We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred.

Cretaceous and Triassic subduction-accretion, high-pressure-low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

AbstractThe 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub‐greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote‐amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high‐pressure zone III to lower grade zones II and I to the west and east. The P–T conditions estimated from zoned amphibole indicate an anticlockwise P–T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high‐pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.

Lead isotope variations across terrane boundaries of the Tien Shan and Chinese Altay

The Altaid orogen was formed by aggregation of Paleozoic subduction–accretion complexes and Precambrian basement blocks between the Late Proterozoic and the Early Mesozoic. Because the Altaids are the site of abundant granitic plutonism and host some of the largest gold deposits in the world, understanding their formation has important implications on the comprehension of Phanerozoic crustal growth and metallogeny. In this study, we present the first extensive lead isotope data on magmatic and metasedimentary rocks as well as ore deposits of the southern part of the Altaids, including the Tien Shan (Tianshan) and southern Altay (Altai) orogenic belts. Our results show that each terrane investigated within the Tien Shan and southern Altay is characterized by a distinct Pb isotope signature and that there is a SW–NE Pb isotope gradient suggesting a progressive transition from a continental crust environment in the West (the Kyzylkum and Kokshaal segments of the Southern Tien Shan) to an almost 100% juvenile (MORB-type mantle-derived) crust environment in the East (Altay). The Pb isotope signatures of the studied ore deposits follow closely those of magmatic and metasedimentary rocks of the host terranes, thus supporting the validity of lead isotopes to discriminate terranes. Whereas this apparently suggests that no unique reservoir has been responsible for the huge gold concentration in this region, masking of a preferential Pb-poor Au-bearing reservoir by mixing with Pb-rich crustal reservoirs during the mineralizing events cannot be excluded.

Late Cretaceous-Early Cenozoic tectonic evolution of the Eurasian active margin in the Central and Eastern Pontides, northern Turkey

Abstract The Izmir-Ankara-Erzincan suture zone (IAESZ) in the Central and the Eastern Pontides comprises a stack of thrust sheets of mainly Late Cretaceous-Early Cenozoic age that are restored as: (1) a subduction-accretion complex; (2) a continental-margin magmatic arc, plus an associated forearc basin; (3) a back-arc basin and its mainly sedimentary fill. Northward thrusting affected all of the Late Cretaceous units during latest Cretaceous (Campanian-Maastrichtian) time. This was followed by regional southward thrusting to form the present thrust stack during Mid-Eocene time. Alternative tectonic models are considered in the light of sedimentary, igneous geochemical and structural evidence, and global comparisons. We infer that the Northern Neotethys was subducted northwards beneath the Eurasian active margin during the Late Cretaceous. Subduction was associated with the genesis of a magmatic arc and a related forearc basin. The subduction zone retreated oceanwards, associated with the opening of a back-arc basin along the Eurasian margin, floored by oceanic crust and overlain by mixed terrigenous and volcaniclastic deep-marine sediments. Ophiolite genesis in a continental margin back-arc setting is suggested by the presence of screens of basement-type metamorphic rocks within an ophiolite-related sheeted dyke complex in the Eastern Pontides. During the latest Cretaceous closure of the inferred back-arc basin resulted in northward emplacement of ophiolitic and related units onto the Eurasian margin, as well exposed in the Central Pontides. In addition, accretionary mélange, volcanic arc, forearc and ophiolitic units were emplaced southwards onto the Tauride continent, represented by the Munzur platform in the Eastern Pontides, also during latest Cretaceous time. This incipient (‘soft’) collision was followed by widespread Paleocene-Early Eocene deposition of Nummulitic shelf carbonates and coarse clastic sediments on deformed and emplaced accretionary mélange, arc and ophiolitic units. Final closure (‘hard collision’) of the Northern Neotethys occurred during the Mid-Eocene, resulting in large-scale southward imbrication, together with northward backthrusting in some areas. Suture tightening and Plio-Quaternary strike-slip ensued.

Rift-Drift, Seafloor Spreading, and Subduction Tectonics of Albanian Ophiolites

Albanian ophiolites in the Dinaride-Albanide-Hellenide mountain system in the Balkan Peninsula represent oceanic remnants of the Mesozoic Pindos-Mirdita basin and structurally overlie the peripheral tectonic units, composed mainly of volcanosedimentary rift assemblages and conjugate passive margin sequences. These tectonic units and the ophiolites collectively record the rift-drift, seafloor spreading, and subduction evolution of the Pindos-Mirdita basin, which developed as a Tethyan seaway between the Apulian and Korabi-Pelagonian microcontinents. The peripheral tectonic units on the west include, from west to east, the Apulian platform (Sazani, Ionian, and Kruja zones), Apulian passive margin sequences (Krasta-Cukali Zone), volcanosedimentary unit (Triassic-Upper Jurassic), subophiolitic mélange, and metamorphic assemblages; the Korabi-Pelagonian platform and its passive margin sequences bound the ophiolites on the east. Metamorphic assemblages include amphibolite, micaschist, and marble that generally display inverted field gradients, analogous to metamorphic soles beneath other ophiolites in the eastern Mediterranean region. Some metamorphic assemblages with cooling ages >169 Ma in southern Albania may represent, instead, dynamothermally metamorphosed volcanosedimentary rocks developed during advanced stages of continental rifting, prior to the onset of seafloor spreading. The subophiolitic mélange contains blocks and clasts of material derived from ophiolites, metamorphic units, platform carbonates, and passive margin sequences, and represents a subduction-accretion complex formed during initial displacement and tectonic emplacement of the ophiolites. The supra-opholitic mélange and the overlying flysch deposits (Tithonian-Valanginian) and neritic limestones (Barramian-Upper Cretaceous) unconformably rest on the ophiolites and the platform carbonates, and constrain the timing of ophiolite emplacement onto the Korabi-Pelagonian continental margin as the latest Jurassic-Early Cretaceous. The Pindos-Mirdita basin initially developed as a broad transtentional, intra-continental rift system with highly oblique, ultraslow rifting and spreading. Ophiolites in the Mirdita region show a transition from MORB to IAT and boninitic affinities from west to east and structurally upwards, indicating magma genesis from highly depleted, refractory mantle sources increasingly affected by subduction-derived fluids. These spatial, temporal, and chemostratigraphic relations point to a suprasubduction zone origin of the eastern ophiolites above a west-dipping, intra-oceanic subduction zone, whose rollback resulted in lithospheric extension and the formation of a boninitic "proto-arc" in the upper plate. This subduction-accretion system was terminated by collision of the Korabi-Pelagonian margin with the trench that facilitated eastward emplacement of the ophiolites. Subsequent oblique collision of Apulia with Eurasia during the Maastrichtian-Paleogene transported the Albanian ophiolites westward onto the Apulian margin. The bidivergent emplacement tectonics of the Albanian ophiolites is hence a result of two separate collisional events at different times.

Petrology of the metamorphic basement of the Tisza Block at the Jánoshalma High, S Hungary

The Variscan metamorphic basement of the Great Hungarian Plain (Tisza Block, E Hungary) consists of deeply buried crystalline highs comprising several types of gneiss and metabasic rocks. A common gneiss type is an orthogneiss variety that can be followed across the basement from Jánoshalma northeastward within the Tisza Block. We believe the Jánoshalma High to be a uniform orthogneiss block of peraluminous composition; this is suggested by the presence of idiomorphic accessory phases (apatite needles, zircon), polygonal feldspar texture, occurrence of rock bodies of exotic origin, xenoliths (amphibolite, eclogite) and xenocrysts (feldspar megacrysts, amphibole, garnet) of varying composition and metamorphic evolution. The peculiar features of the orthogneiss body are indicative of processes that may have taken place in an ancient Alaskan-type orogenic belt (subduction-accretionary complex). In these situations, because of the continuous ample sediment supply, the accretionary prism retreats oceanward, forming structures of increasing steepness with depth, accreting against the continental margin. Thereafter, this causes oceanward retreat of the subduction trench as well. For this reason mantle-derived magmas do not penetrate the continental margin but the accretionary prism itself, resulting in the melting of its material. In the case of the Jánoshalma High, following the emplacement of the onetime granodioritic body at shallower crustal levels, it metamorphosed via to a two-stage overprint. An early high-temperature (HT) and a succeeding medium-temperature (MT) event formed orthogneiss from the ancient intrusive rock. This is the first detailed investigation of the Jánoshalma high that has been published.

Tracing the geometry of an active fault using remote sensing and digital elevation model: Ganos segment, North Anatolian Fault zone, Turkey

The surface expression of the North Anatolian Fault between the Sea of Marmara and the Gulf of Saros, Turkey was determined using Landsat 5 Thematic Mapper (TM) and Système Probatoire de l'Observation de la Terre (SPOT) high resolution visible (HRV) Panchromatic images and a digital elevation model (DEM) based on 1 : 25 000 scale topographic maps. The remotely sensed data had been enhanced to reveal linear details. Between Gaziköy (Marmara Shore) and Saros two small releasing steps were identified, one to the east of Mürseli village and the other between Mürseli and Yayaköy villages. Hitherto, a single straight fault line had been drawn between Gaziköy and Saros on the geological map of this region. Between the Yeniköy and Sofular villages, the fault strike bends ∼7° further to the SE. The course of the fault between the Sea of Marmara and the Gulf of Saros is, therefore, more complicated than formerly believed and this is probably because the fault has utilized the pre-existing structures of the Neo-Tethyan subduction–accretion complex.

The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland

The Caledonian geology of western Ireland records the collision of two arc complexes with the Laurentian Margin during the closure of the Iapetus Ocean. An earlier complex collided with this hitherto passive margin in the mid-Ordovician during the Grampian Orogeny. Subsequently, arc magmatism developed along the Laurentian margin and continued until the late Silurian collision between Laurentian and Avalonia. The Ordovician volcanic and sedimentary rocks comprising the South Connemara Group lie along the Southern Uplands Fault, the terrane boundary separating these two arc complexes. Palaeontological dating indicates an Arenig-Llanvirn age for part of this complex (Williams, Armstrong and Harper, 1988), making it contemporaneous with the earlier arcs. However, most authors correlate this complex with the northern belt of the Southern Uplands (Morris, 1983; Williams, D.M., 1984. The stratigraphy and sedimentology of the Ordovician Party Group, south-eastern Murrisk, Ireland. Geological Journal, 19, 173–186; Williams et al., 1988), associated with post-Grampian subduction of north directed polarity. We present new field evidence that the South Connemara Group is tectonically disrupted by bedding parallel shear zones and that contacts previously interpreted as conformable are marked by units of tectonic mélange. We present structural and provenance arguments consistent with the mélanges forming above a north-dipping subduction zone after 463Ma. This Group is reinterpreted as occurring within a subduction–accretion complex that was generated by the accretion of early Ordovician mafic seamounts into a post-Grampian trench, thus reconciling the age of the Group with its generally accepted tectonic setting. We discuss the regional significance of this finding with respect to the Caledonide-Appalachian orogeny and argue that this is the site along which the Iapetus Ocean closed.

Tectonic evolution of the western Kunlun orogenic belt, western China

The western Kunlun orogenic belt in western China evolved through the development of a large subduction–accretionary complex, including flysch sediments and granitic plutons, and by collision of three terranes, namely the North and South Kunlun and Karakorum–Qiangtang blocks from the early Paleozoic to the early Mesozoic. North-dipping subduction of the Paleo–Tethys ocean beneath the Kunlun terranes, which may have commenced in the Cambrian, produced an early Paleozoic Andean-type magmatic arc on the South Kunlun, and a marginal back arc basin, represented by the early Paleozoic Oytag–Kudi ophiolite belt, between the North and South Kunlun. A northward subduction zone consumed the basin and the young, hot upper plate lithosphere was obducted southward onto the South Kunlun following closure of the basin. This resulted in collision of the North and South Kunlun in the early Devonian. Continuous northward subduction of Paleo–Tethys resulted in the development of a Carboniferous–Triassic magmatic arc, and a back arc rifting sequence composed of the Carboniferous to Permian carbonates and clastic sediments on the North and South Kunlun terranes. The Paleo–Tethys ocean finally closed in the late Triassic–early Jurassic, when the Kunlun and Karakorum–Qiangtang blocks were accreted, with the Kara–Kunlun accretionary prism marking their suture zone.

Arc to craton provenance switching in a Late Palaeozoic subduction complex, Wandilla and Shoalwater terranes, New England Fold Belt, eastern Australia

The Wandilla and Shoalwater terranes of the northern New England Fold Belt are accretionary subduction complexes formed during the Carboniferous at the convergent plate boundary that extended along the eastern edge of Palaeozoic Gondwana. Sandstones from the Wandilla terrane are quartz‐poor and quartz‐intermediate lithic or feldspathic volcanic sandstones that were derived principally from the associated magmatic arc, whereas those of the Shoalwater terrane are quartz‐rich and were sourced from a cratonic region dominated by low‐grade metamorphic and granitic rocks.The location of the Shoalwater source cannot be determined unambiguously but probably was situated to the north with detritus supplied longitudinally to the convergent plate boundary. At that time arc‐derived sediment was confined to the forearc basin behind an outer arc ridge, the physiographic manifestation of the accreted rocks of the Wandilla terrane. The tectonic situation may have been similar to that of the present‐day Lesser Antilles subduction system where quartzose sediment carried down the Orinoco River is transported northward and incorporated into the Barbados Ridge subduction complex.

Petrology of metabasites from the Terekta Complex as a constituent of ancient accretionary prism of Gorny Altai

The blueschist/greenshist Terekta Complex is the only blueschist locality known in the Russian Altai. The Terekta metabasites contain Na and Na–Ca amphibole, actinolite, phengite, epidote, albite, quartz, calcite, magnetite (or hematite). Depending on the amphibole composition, these rocks were subdivided into blueschist, transitional blueschist/greenschist and greenschist. Both blueschists and transitional blueschist/greenschists (glaucophane-bearing and winchite–actinolite schists) have compositions similar to alkaline basalts of oceanic islands, whereas the greenschists correspond to ocean-floor tholeiitic basalts, or MORB. Available geothermobarometry yielded the following estimates of metamorphic conditions: T=350–400 °C and P=6–8 kbar. The different mineral assemblages of the metabasites are believed to be a result of their different lithologies. The presence of matabasalts with ocean island basalt and MORB affinity, as well as the occurrence of layered metachert, marble, metagraywacke, and plates of serpentinized dunites, pyroxenites indicate that the complex was very likely a subduction-accretionary complex. The complex contains rocks of accretionary wedge, and fragments of oceanic crust which are regarded to be a remnant of an Early Paleozoic subduction zone in the Russian Altai.

Ordovician‐Silurian accretion tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia contains a remnant ocean basin with Cambrian igneous basement of intra‐arc and backarc boninitic to tholeiitic volcanics and island‐arc calc‐alkaline volcanics overlain by ?Upper Cambrian to dominantly Ordovician siliciclastic turbidite successions derived from Gondwana and deposited in a huge submarine turbidite fan(s). Much of the remnant ocean basin is preserved in accretionary subduction complexes that are characterised by abundant coherent successions with relatively sparse mélanges. The lack of chaotic rocks reflects the accretion of thick (2–5 km) siliciclastic turbidite successions and low rates of underthrusting. Three previously recognised accretionary subduction complexes are located in western Victoria (Stawell and Bendigo Zones), eastern Victoria (Tabberabbera Zone) and along the eastern coastline of southeast Australia (Narooma subduction complex). In addition, a short‐lived west‐dipping subduction zone is proposed to account for the areally restricted Howqua River Zone along the eastern margin of the Melbourne Zone in eastern Victoria. The Howqua River Zone contains gently dipping mélanges and subduction‐related blueschist fragments. The subduction complex in the Tabberabbera Zone is considered to extend throughout the remnant ocean basin succession of the Wagga‐Omeo Zone of the central Lachlan Fold Belt. Apart from the Howqua River Zone, these subduction complexes are an end‐member in the spectrum of accretionary complexes that contrast with more chaotic assemblages as preserved in southwest Japan.

Multiple Accretionary Orogenesis and Episodic Growth of Continents: Insights from the Western Kunlun Range, Central Asia

The Western Kunlun Range in northwestern Tibet records a history of enlargement of the Central Asian crust. In the Early Ordovician a south-dipping intra-oceanic subduction-accretion complex was accreted to the Tarim block, forming an Andean-type margin in the Late Ordovician. This margin grew southward (present-day coordinates) in the Early Silurian, with an accretionary complex on its southern side against which the Kudi gneiss-schist complex was attached. Clastic sediments continued to infill forearc/intra-arc basins on the top of the arc-accretionary complexes during Devonian-Carboniferous time. In the Early to Mid-Carboniferous, tectonic and magmatic quiescence is indicated by an absence of volcanic and plutonic rocks in the Kudi area, whereas subduction was still active in the western part of the range in the Gaz area near the present-day Pamir syntaxis. A Late Carboniferous-Permian to Early Mesozoic accretionary arc and forearc developed in front of the Andean-type margin, indicating that north-dipping subduction was renewed in the Late Carboniferous. Final collision of the Karakoram-North Qiangtang block to the western Kunlun orogenic collage by the Mid-Jurassic resulted in closure of Paleo-Tethys. The accretion and development of the Western Kunlun orogen contributed huge quantities of material for the crustal growth of Asia.

Proterozoic ophiolites of the Arabian Shield and their significance in Precambrian tectonics

Abstract Neoproterozoic ophiolites, ranging in age from c. 870 Ma to c. 627 Ma, occur in several discrete suture and/or fault zones within the Arabian Shield and display a record of riftdrift, sea-floor spreading and collision tectonics during the evolution of the East African Orogen. The ophiolites within the Yanbu and Bir Umq suture zones in the west are among the oldest (870–740 Ma) in the Shield, locally show a Penrose-tyep complete pseudostratigraphy, and have chemical compositions typical of modern forearc oceanic crust. They are spatially associated with coeval and younger volcanic arc assemblages and were incorporated into the Arabian Shield during a series of collisional events that amalgamated these ensimatic arc terranes. The ophiolites of the Hulayfah-Ruwah suture zone in the central Arabian Shield are coeval with and/or slightly younger ( c. 843–821 Ma) than the ophiolites in the west and probably developed in a rifted ensimatic arc system that evolved as a volcanic archipelago near the Afif continental plate. Younger ophiolites ( c. 694 Ma) of the Halaban and Al Amar suture zones in the eastern Arabian Shield were incorporated into a subduction-accretion complex that evolved at the Andean-type active margin along the eastern edge (in present coordinate system) of the Afif continental plate. The Halaban suture zone ophiolites represent forearc oceanic crust, whereas the Al Amar suture zone ophiolites are scraped-off fragments of Mozambique ocean floor, seamounts and/or ocean island(s); the Abt Schist between them corresponds to a Franciscan-type accretionary prism of the ‘Halaban’ subduction zone. The incorporation of these ophiolites and the continental plates (Afif and Ar Rayn) into the Arabian Shield during 640–620 Ma marks a major shift in the direction of convergence (from northerly to westerly) during the assembly of the Shield and distinct episodes of continental collisions during closure of the Mozambique Ocean. The ophiolites of the Nabitah-Hamdah fault zone within the Asir terrane are the youngest ( c. 627 Ma) in the Shield, post-collisional in origin, display mid-ocean ridge basalt chemical affinity, and represent Ligurian-type oceanic crust developed in an intracontinental pararift zone. The ophiolite tectonics of the Arabian Shield indicates an eastward progression of continental growth through time as the East African Orogen was built during the late Neoproterozoic, following the breakup of Rodinia.

Assembly of Archean cratonic mantle lithosphere and crust: plume–arc interaction in the Abitibi–Wawa subduction–accretion complex

Recent thermodynamic models suggest that direct interaction between mantle plumes and island arcs will enhance long-term arc buoyancy and contribute disproportionately to the crustal record. However, crustal growth models have also proposed a range of differing mechanisms for Archean crust formation that emphasize specific rock types, such as tonalite–trondhjemite–granodiorite (TTG) plutons or high magnesian andesites. Studies in the Abitibi–Wawa subprovince, allow these proposals to be evaluated in the context of the world's largest greenstone belt. Crustal growth in the southern Superior Province was the product of subduction–accretion tectonics enhanced and modified by mantle plume processes, particularly mantle plume–island arc interaction. High Archean geothermal gradients promoted volumetrically minor slab melts from the earliest phases of the Abitibi–Wawa arc, resulting in the adakite–high magnesium andesite–Niobium–enriched basalt association. However, recent flat subduction models for the formation of adakites also provide important insights into the generation of syn-tectonic Archean TTG batholiths that were probably derived from subducted, rather than accreted, oceanic crust. The distribution of Niobium enriched basalts (NEB) in the southern Superior Province may reflect plume controlled shallow subduction beneath the Abitibi belt that limited melting depths within a restricted asthensopheric mantle tongue. A well-constrained tectonic history, the mantle source requirements of successively-formed components of the Abitibi–Wawa crust, and detailed seismic interpretations of crustal architecture all preclude the presence of an autochthonous mantle lithospheric root beneath the Abitibi–Wawa arc. Instead, the late diapiric ascent of buoyant refractory plume residue and subducted oceanic crust resulted in the coupling of the mantle root and arc crust 10's of million years following batholith emplacement. High-velocity material identified at the base of the crust and centered beneath the Abitibi–Pontiac suture zone is probably Archean aged rather than Proterozoic. It corresponds to minor melts generated during ascent of the plume residue diapir and underplated prior to and during formation of the >200 km thick Abitibi–Wawa continental mantle lithosphere root.

Carboniferous-Triassic subduction and accretion in the western Kunlun, China: Implications for the collisional and accretionary tectonics of the northern Tibetan Plateau

A newly defined, 250 km by 500 km, Carboniferous-Triassic subduction-accretion complex, the Mazar accretionary prism in the western Kunlun, comprises two subduction complexes and a forearc-basin succession. (1) The Bazar Dara subduction complex contains imbricated blocks of sandstone, arenite, limestone, and metavolcanic rocks in a matrix of weakly metamorphosed Triassic deep-sea turbidites. The metavolcanic rocks include basalt, diabase, spilite, and andesitic porphyry. Trace element geochemistry shows that pillow and amygdaloidal basalts are oceanic-island tholeiites. This zone has Ordovician to Permian fossils and is situated on the older, more highly deformed and metamorphosed side of the prism adjacent to the Sailiyak magmatic arc. (2) The Heweitan subduction complex is composed of blocks of limestone, turbidite, and radiolarite in a slate-phyllite matrix intercalated with calc-alkalic volcanic rocks. This complex has Permian to Triassic fossils and is situated on the younger, less deformed, and metamorphosed side of the prism adjacent to the suture zone. (3) The Qitai forearc basins are infilled with turbidites (Late Triassic) intercalated with carbonates; these rocks overlie the accretionary prism. The accretion-related structure is dominated by large-scale northeast-dipping thrusts and subvertical cleavage stitched by 215‐190 Ma granites. There is an overall decrease in metamorphic grade and deformation intensity from the arc to the suture zone across the Bazar Dara and Heweitan subduction complexes. The Mazar accretionary prism formed by subduction-accretion processes during closure of the Paleotethyan Ocean and the final docking of the Gondwanan Karakoram-Qiangtang block to the Cathaysian (Eurasian) Kunlun block.

Geodynamic processes, continental growth, and mantle evolution recorded in late Archean greenstone belts of the southern Superior Province, Canada

An episodic continental growth model, by lateral spread of subduction–accretion complexes, is proposed for the late Archean southern Superior Province, based on structural relationships and high precision trace element data for the 2.7 Ga Wawa and Abitibi subprovinces. Two principal volcanic associations are recognized: (1) tholeiitic basalt–komatiite; and (2) tholeiitic to calc-alkaline bimodal basalt–rhyolite. Tholeiitic basalts of the former association are characterized by near-flat REE patterns and complex Th–U–Nb–LREE systematics; rare transitional to alkaline basalts and Al-depleted komatiites have fractionated REE patterns and OIB-like trace element signatures. This is interpreted as representing an oceanic plateau derived from a heterogeneous mantle plume. The latter bimodal association has fractionated REE patterns and negative Nb, Ta, P, and Ti anomalies typical of arc magmas. Turbidites plot on mixing hyperbolae between mafic and felsic end members. This association is interpreted as an arc–trench system. Tonalite plutons derived from partial melting of subducted oceanic slabs intrude the subduction–accretion complex as the magmatic arc axis migrated towards the trench. In the Abitibi belt, boninite-series low-Ti tholeiite flows stratigraphically intercalated with the basalt–komatiite association signifying coeval eruption of plume and arc lavas. Plateau tholeiites are characterized by Nb/U, Th/U, Nb/Th, Nb/La, Th/La, and La/Sm pm ratios that span the primitive mantle values. Arc associations have Nb/U, Nb/Th, and Nb/La ratios that are systematically lower than the primitive mantle ratios, but have La/Sm pm ratios that are systematically higher than the primitive mantle ratio. The variability of these ratios in oceanic plateau basalts is interpreted in terms of recycling variable quantities of oceanic crust processed through a subduction zone (high Nb/U, Nb/Th), and complementary arc crust (low Nb/U, Nb/Th) into the source of the plumes. In the Wawa and Abitibi greenstone belts of the southern Superior Province, eruption of voluminous ocean plateau and island arc volcanic sequences, deposition of kilometer-thick siliciclastic trench turbidites, and intrusion of the supracrustal units by syn- to post-kinematic TTG plutons occurred in the relatively short period of time of 2.75–2.65 Ga. All these events were accompanied by contemporaneous intense poly-phase deformation, regional greenschist to amphibolite facies metamorphism, and terrane accretion along a north–northwest-dipping subduction zone. Collectively, these events are interpreted as a part of a late Archean supercontinent cycle, involving accretion of oceanic plateaus, island arcs, continental fragments, closure of ocean basins, rifting of magmatic arcs, and plume arc interactions.

The Loch Maree Group: Palaeoproterozoic subduction–accretion complex in the Lewisian of NW Scotland

The Lewisian complex of northwest (NW) Scotland has long been correlated with intercontinental Palaeoproterozoic belts of the North Atlantic region but uncertainty about the age and origin of the supracrustal rocks of the Loch Maree Group (LMG) and the apparent lack of subduction-related intrusive rocks have precluded interpretations of a similar tectonic setting for the Lewisian. We present integrated field, geochemical and geochronological data that resolve both issues and are consistent with an intercontinental setting. The LMG is made up of two components, one oceanic (plateau basalts or primitive arcs, plus associated abyssal sediments, ferruginous hydrothermal deposits, and platform carbonates) and the other continental (deltaic flysch, greywacke shale). The metasediments have geochemical characteristics that imply a source outside the Archaean gneisses of the Lewisian, an interpretation that agrees with the detrital zircon populations (from the Flowerdale schists) that have a significant 2.2–2.0-Ga component. The Ard gneiss, formerly regarded by some as a tectonic sliver of basement, is a strongly foliated granodiorite that occurs in sheets intrusive into the LMG, and has given a UPb crystallisation age of 1903±3 Ma, consistent with its syntectonic relationship with the major D1/D2 phase of Proterozoic deformation. The gneiss has a rather primitive geochemistry, which implies that it was not generated by melting of the local metasediments but was derived by partial melting of a more mafic source. The most likely model is that the LMG evolved as an accretionary complex, modern parallels of which can be found in the Shimanto belt in Japan, Rhodope in north Greece and Colombia and the Caribbean. The various elements of the complex became tectonically intermixed and subject to extreme deformation during accretion to the overriding Lewisian continent. Eventual relaxation and exhumation of the accretionary complex may have resulted in the generation of the Ard gneiss (possibly by melting of the underplated oceanic plateau) followed by collision with the continental crust of the lower plate. The younger D3 phase of the Palaeoproterozoic deformation sequence was coincident with the emplacement of the Tollie pegmatites at 1.7 Ga, c 200 m. years after the main collisional event, and may be related to a younger accretionary event (Labradorian?).