North-dipping Subduction Zone Research Articles

The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys

In the Ladakh–Zanskar area, two groups of ophiolites document closure of the Neo-Tethys Ocean during India–Asia convergence. They are localized north of the Indus Suture Zone and in South Ladakh, respectively. The South Ladakh group includes the Spontang ophiolite, with both crustal and mantle units, the Nidar ophiolite, with mainly crustal units, and the small Karzog ophiolite, with abundant chromitites and gabbros. The mafic rocks of the three South Ladakh ophiolites are tholeiites with similar compositions, displaying slight LREE depletion relative to HREE and enrichment in LILE relative to REE and HFSE. They are also characterized by a low Y content (<20 ppm) and a Ti/V ratio between 10 and 20. Such chemical features and similarity imply that these ophiolites were derived from the same N-MORB-like mantle source slightly enriched in LILE during a metasomatic event in a supra-subduction zone setting. We suggest the South Ladakh ophiolites are relics of an intra-oceanic arc. According to 39Ar/ 40Ar dating on amphiboles from both Spontang and Nidar mafic rocks, this arc developed at around 110–130 Ma. The associated ultramafic rocks show a decoupling between LREE and LILE (both enriched relative to abyssal peridotites) and HREE (which decrease from lherzolites to harzburgites). This contrasting behavior reflects the depletion of residues during partial melting and subsequent metasomatism in a supra-subduction zone setting. REE modeling indicates that the Spontang and Nidar lherzolites could not have been the source of the mafic rocks because their partial melting would have produced magmas more depleted in LREE. The South Ladakh ophiolites are clearly distinct from the northern ophiolites and require the existence of another north-dipping subduction zone. Thus, the closure of the Neo-Tethys was not accommodated by a single but by two subduction zones in the Ladakh–Zanskar area and from Oman to Tibet, respectively.

Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: Termination of the central Asian orogenic belt

The Solonker suture records the termination of the central Asian Orogenic Belt (CAOB). However, tectonic development of the Solonker suture is poorly understood. We report new field data for the Ondor Sum melange in the Ulan valley, and present a new evaluation of the orogenic belt extending from the southern Mongolia cratonic boundary to the north China craton within the context of a new geological framework and tectonic model, which incorporates relevant data from the literature. The southern accretionary zone between the north China craton and the Solonker suture is characterized by the Mid‐Ordovician‐Early Silurian Ulan island arc‐Ondor Sum subduction‐accretion complex and the Bainaimiao arc. This zone was consolidated by the Carboniferous‐Permian when it evolved into an Andean‐type magmatic margin above a south dipping subduction zone. The northern accretionary zone north of the Solonker suture extends southward from a Devonian to Carboniferous active continental margin, through the Hegenshan ophiolite‐arc accretionary complex to the Late Carboniferous Baolidao arc associated with some accreted Precambrian blocks. This northern zone had consolidated by the Permian when it developed into an Andean‐type magmatic margin above a north dipping subduction zone. Final subduction of the central Asian ocean caused the two opposing active continental margins to collide, leading to formation of the Solonker suture in the end‐Permian. Predominant northward subduction during final formation of the suture gave rise in the upper northern plate to a large‐scale, postcollisional, south directed thrust and fold belt in the Triassic‐Jurassic. In summary, the CAOB underwent three final stages of tectonic development: early Japanese‐type accretion, Andean‐type magmatism, and Himalayan‐type collision.

Northward subduction of the Indian Plate beneath the Kohistan island arc, Pakistan Himalaya: new evidence from isotopic data

The intra-oceanic Kohistan island arc was initiated during the Cretaceous as a result of northward movement of the Indian Plate. The conventional assertion that the arc was erected above a north-dipping subduction zone has been challenged by an alternative hypothesis, which suggests that the arc was built above a south-dipping subduction zone. This hypothesis is based partly on the apparent presence of a ‘Dupal’ signature in mafic rocks of the arc, and partly on the occurrence of boninites exposed in the northern part of the arc. The ‘Dupal’ signature, with enhanced levels of radiogenic lead, is found only in rocks extracted from the mantle in equatorial regions. However, new radiogenic isotope data, presented here, suggest that the isotopic signature of the mafic volcanic rocks of the juvenile arc, rather than being ‘Dupal’-type, is a function of fluids derived from subduction and dehydration of sea-floor sediments. Boninitic volcanic rocks in the Kohistan arc form a volumetrically minor part of a compositionally diverse sequence in the northern part of the arc. Boninites and high-Mg andesites are found mostly, but not exclusively, in the fore-arc region, and evidence is accumulating that these may also be erupted into back-arc regions. Sedimentological and geochemical data suggest the Kohistan boninites to have been erupted into a short-lived back-arc basin. Both the eruption of boninites into the back-arc and the lack of a ‘Dupal’ signature would be consistent with northward subduction beneath Kohistan, although the latitude at which the arc was initiated remains unresolved.

The Beyşehir–Hoyran–Hadim Nappes: genesis and emplacement of Mesozoic marginal and oceanic units of the northern Neotethys in southern Turkey

The Beyşehir–Hoyran–Hadim Nappes crop out over 700 km from NW to SE. Above a regionally autochthonous Tauride carbonate platform the Beyşehir–Hoyran Nappes begin with a thrust sheet ( c . 400 m) of mainly redeposited carbonates, quartzose sandstones and mudstones of Mid–Late Triassic age, interpreted as a proximal slope–base-of-slope succession. Above is a thrust sheet ( c . 1 km) of Middle–Upper Triassic intermediate–acidic extrusive rocks, volcaniclastic rocks and minor pelagic carbonates, interpreted as a continental rift. Thin (&lt;100 m) Upper Triassic–Upper Cretaceous pelagic carbonate and radiolarian chert lie depositionally above. The uppermost thrust sheet comprises broken formation and mélange, including Jurassic shallow-water carbonate, radiolarian chert and Upper Cretaceous pelagic limestone. Zones of tectonic–sedimentary mélange separate higher units. The Beyşehir–Hoyran Nappes document Triassic rifting and Jurassic–Cretaceous passive margin subsidence bordering the Northern Neotethys. A harzburgitic ophiolite probably formed above a north-dipping subduction zone within the ocean basin. The ophiolite was emplaced southwards onto the northern margin of the Tauride platform in latest Cretaceous time. The nappe pile and underlying platform (Hadim Nappe) were thrust further south in Late Eocene time. Assuming in-sequence thrusting, the Beyşehir–Hoyran Nappes restore to a location north of a Neotethyan spreading axis. More probably, they originated near the south margin of the Northern Neotethys, but reached their position by out-of-sequence thrusting. Formation within a southerly strand of the Northern Neotethys (Inner Tauride ocean) is more probable than within the main Northern Neotethys further north.

Archean continental assembly in the southeastern Superior Province of Canada

Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium‐ to high‐grade Opatica plutonic gneiss belt, the low‐grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65‐km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, whieh are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large‐scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5–8.2 kms−1, are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism.

Geochemical and isotopic constraints on subduction polarity, magma sources, and palaeogeography of the Kohistan intra-oceanic arc, northern Pakistan Himalaya

Geochemical and isotopic data are presented for 18 representative samples from the intraoceanic phase of the Kohistan arc. A restricted range of initial 87 Sr/ 86 Sr (0.7036–0.7066) and ºNd (+2.8 to +7.4) along with measured 206 Pb/ 204 Pb (18.0–18.6) are consistent with formation of the arc complex in an intra-oceanic setting. The isotopic data demonstrate the involvement of enriched, DUPAL-type mantle, suggesting that the Kohistan arc formed at or south of the present equator. Subduction polarity inferred from geochemical and isotopic data indicate that the Chalt Volcanics and Kamila Amphibolites represent a forearc and backarc basin sequence, respectively. These inferences are most simply resolved with a tectonic model whereby the intra-oceanic Kohistan arc evolved over a south-dipping subduction zone, implying that Kohistan and India moved northwards on the same plate, although separated, during much of Cretaceous time. Collision of Kohistan with the Karakorum caused a new, north-dipping subduction zone to form on the south side of Kohistan, leading to collision with India in early Tertiary time.

Core complex development in central Anatolia, Turkey

Tectonic models for the central Anatolian segment of the Alpine-Himalayan orogen have not considered this region as part of the Aegean extensional province. We have found evidence, however, for Oligocene-Miocene extension and exhumation of midcrustal rocks in a metamorphic dome (Nigde massif) that is structurally and petrologically similar to Miocene core complexes >500 km to the west. Strong correspondence in the timing and kinematic development of the Menderes (western Turkey) and Nigde core complexes suggests that extensional tectonics and exhumation of mid- to lower-crustal rocks affected much of the eastern Mediterranean region. In the Nigde massif, supracrustal rocks were buried to depths of 16‐20 km at high temperatures (>700 °C) during contraction associated with closure of Neo-Tethyan seaways in late Mesozoic‐early Cenozoic time. Following cooling and decompression to <600 °C and <10 km, metasedimentary rocks underwent a second heating event at low pressures during Miocene magmatism that postdated much of the unroofing of the massif. Development of the Nigde core complex was related to exhumation of thickened and thermally weakened continental crust in the upper plate of a north-dipping subduction zone. The partial subduction of the Tauride carbonate platform in the Eocene resulted in choking of the subduction zone, followed by isostatic rebound and exhumation of the buoyant platform. This in turn caused a rapid emergence of the upper plate, resulting in erosion of upper-crustal rocks and exhumation of midcrustal rocks along the northern edge of the Inner Tauride suture zone. Although northern regions of central Anatolia contain the same protoliths as the Nigde massif and underwent extensive Tertiary magmatism, they did not undergo similar extension, suggesting that core complex development in south-central Turkey was controlled by the location of thickened and thermally weakened crust adjacent to a suture zone.

Pliocene-Pleistocene asymmetric unroofing of the Irian fold belt, Irian Jaya, Indonesia: Apatite fission-track thermochronology

The Central Range of New Guinea is an orogenic belt up to 100 km wide and more than 1300 km long with numerous peaks over 3 km high. The orogen was generated since the middle Miocene by the collisional underthrusting of the northern passive margin of the Australian continent into a north-dipping subduction zone beneath the Melanesian island arc. South of Puncak Jaya, the highest peak in New Guinea, Pliocene–Pleistocene uplift and unroofing have exposed Tertiary carbonates and Mesozoic clastic rocks of a rift and passive-margin sequence above unmetamorphosed Paleozoic strata and greenschist-facies slates probably of Precambrian age. Eight magmatic apatite samples from Pliocene intrusions of intermediate composition in the Gunung Bijih (Ertsberg) mining district at the crest of the Central Range in Irian Jaya, Indonesia, yield pooled fission-track ages ranging between 3.7 ± 0.9 and 2.0 ± 0.3 Ma (±1σ). Long etchable mean track lengths (>14 µm) and narrow track length distributions indicate rapid cooling, as expected for shallow-level intrusions. These track-length data and the observation that the Grasberg pluton was emplaced into its own volcanic cover indicate that <2 km of material has been eroded from the top of this part of the range since the Pliocene. Seven detrital apatite samples from the Triassic-Jurassic Tipuma and the Carboniferous–Permian Aiduna Formations and apatite from two igneous dikes exposed halfway down the southern slope of the range yield pooled fission-track ages ranging between 2.7 ± 0.7 and 2.0 ± 0.5 Ma (±1σ). Etchable mean track lengths of ≈9–12 µm and wide track length distributions indicate slower cooling compared to the Pliocene intrusions. Complete resetting of provenance fission-track ages in detrital apatite requires burial deeper than ≈4 km. Unroofing rates are ≈1.7 km/m.y. on the southern flank of the range but >0.7 km/m.y. at the crest. These fission-track data combined with local and regional geologic relationships indicate that unroofing of the Irian fold belt occurred at least two and a half times faster, and perhaps more than five times faster, on the lower slope than at the crest. This difference is probably due to regional weather patterns that result in up to ≈ 11 m/yr of orographically induced precipitation on the southern slope of the Irian Central Range but only ≈3 m/yr near the highest elevations.

Early Paleozoic Island arc accretion to the North China craton and the Shang Dan fault zone: A major paleoplate boundary in eastern Asia

The early Paleozoic tectonic evolution of part of the Qinling‐Dabie‐Shandong orogenic belt is reconstructed on the basis of the results of field mapping, geochemical, and geochronological studies. The Qinling orogenic belt separates the Archean‐Proterozoic North China craton (NCC) from the South China craton (SCC, also Yangtze craton). The study area is located about 90 km southwest of the city of Xi'an. Three lithologically distinctive zones, separated from each other by fault zones, can be distinguished. Passive continental margin deposits of the NCC (zone 1) have εNd(t) values between −6.16 and −12.54 and TDM ages between 1.29 and 2.20 Ga. They are separated from an early Paleozoic island arc sequence (zone 2) by the sinistral Chenche fault. Mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late Proterozoic time. Similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc. Accretion was accompanied by a strong deformation event between early Ordovician and late Silurian time. Magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short‐lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the NCC in late Silurian to early Devonian time. The εNd(t) values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1.78 to −1.05. Mafic to intermediate intrusions in the southern part of zone 2 have positive εNd(t) values from 1.28 to 2.04 as well as low 87Sr/86Sr(i) values between 0.705 and 0.706 indicating an increasing mantle component toward the south. Low‐grade metamorphic pelites are separated from the island arc sequence of zone 2 by a ≤2 km wide Shang Dan shear zone. Isotopic studies show strong similarities between a granite sample from zone 3 and samples from the SCC. 87Sr/86Sr(i) values increase from 0.709 to 0.713, whereas εNd(t) values decrease from −1.90 to −5.34 in a southern direction. The shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the NCC and the SCC. The results presented in this paper document a previously unknown early Paleozoic tectonic evolution of the Qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern Asia.

A Middle Silurian-Early Devonian Magmatic Arc in the Qinling Mountains of Central China

An early to middle Paleozoic magmatic arc was discovered in the Qinling orogenic belt of central China that marks the boundary between the North China craton and the South China craton. The arc was built on the deformed southern passive continental margin of the North China craton, which has been modified by the accretion of an oceanic island arc in early Paleozoic time. U/Pb analyses of zircons from granites indicate short-lived arc activity from Middle Silurian to Early Devonian time. Trace element analyses as well as REE analyses of I-type granites are consistent with the interpretation that the magmatic arc developed above a north-dipping subduction zone. Termination of magmatic activity was probably caused by the initiation of collision between the North China craton and the South China craton. This interpretation is consistent with faunal data and places new constraints on the Paleozoic tectonic evolution of the Qinling belt.

A magmatic belt within the Neo-Tethyan suture zone and its role in the tectonic evolution of northern Turkey

The Sakarya and Kırşehir continental fragments of Northern Turkey were separated by the Ankara-Erzincan ophiolitic suture zone, which is the remnant of the northern branch of the Neo-Tethys, namely the Ankara-Erzincan Ocean. This ocean branch opened during the Lias between these two continental fragments and started to close at the beginning of the Late Cretaceous, by the consumption of its floor, along two north-dipping subduction zones. The northern one was along the southern margin of the Sakarya Continent. As a result of this subduction zone, an ensialic magmatic arc, some fore-arc basins and a mélange belt developed from north to south on the Sakarya Continent. The second subduction zone, located to the south, gave rise to a mélange belt and an island arc developed with and on it. Hot-spot magmatics (seamounts), which were scraped from the subducting oceanic crust, also accreted into this ophiolitic-volcanic belt. At the end of the late Cretaceous, the central part of the Ankara-Erzincan Ocean closed due to the collision of the Kırşehir and the Sakarya continents. As a result of this collision, mélange belts and ensimatic arc volcanics formed the Ankara-Yozgat suture between these continental fragments.

Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project

SUMMARY Densely spaced wide-angle data from stations recording airgun shots along BABEL profile 2 in the northern Gulf of Bothnia were analysed, modelled, and compared with the normal-incidence recordings. Profile 2 extends from the metasedimentary and metavolcanic Proterozoic Central Svecofennian Province into the volcanic Northern Svecofennian Province of magmatic arc affinity and crosses the Proterozoic-Archaean boundary of the northern Baltic Shield as defined from isotope studies. We used a 2-D traveltime inversion to derive P- and S-wave velocity-depth models and a distribution of Poisson’s ratios along the profile. A series of north-dipping wide-angle reflectors characterize the middle and lower crust in the southern part of the model, coinciding with similar events in previous multichannel wide-angle data and the CMP recordings. Several subhorizontal upper to mid-crustal wide-angle reflectors and a well-resolved Moho boundary at 42 to 45 km depth coincide with reflective zones or changes in reflectivity patterns of the CMP data. P-wave velocities increase from 5.7 km s-’ at the near-surface to 6.4 and 6.6 km s-l in the mid-crust and up to 7.6 km s-l in the lowermost crust. P, velocities reach 8.0 to 8.2 km s-’ below the Moho in the northern part of the model but are less than 8.0 km s-’ in the southern part. Poisson’s ratios, calculated from the P- and S-wave models, range from 0.21 to 0.25 in the uppermost crust to about 0.30 in the lower crust and upper mantle. Our velocity and Poisson’s ratio models indicate that the upper crust contains predominantly felsic, quartz-rich rocks. The increase of P-wave velocity and Poisson’s ratio in the mid- to lower crust is explained by increasingly mafic composition. We propose a geological model across the northern Gulf of Bothnia that combines our velocity-depth model with results from the seismic normal-incidence experiment, previous geophysical information, and geological surface observations. The model consists of a possibly multiple Proterozoic north-dipping subduction zone with remnants of oceanic crust and zones of imbricated metasediments and mafics on top. There is evidence for a metasedimentary fore-arc basin in the middle crust and a layer of mafic to ultramafic cumulates at the base or below parts of the lower crust. South-dipping mid- and lower crustal zones of high reflectivity in the northern part of profile 2 indicate that Archaean crust underlies the Northern Svecofennian Province. The major features of this Proterozoic collision zone, such as the suggested subduction slab, zones of crustal reflectors dipping parallel to the slab, a metasedimentary basin, and mafic to ultramafic cumulates between the subduction slab and overlaying Moho, are well-preserved analogues of recent oceaniccontinental collision and subduction zones.

Skarn CuAu orebodies of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia

The major CuAu skarn deposits of the Gunung Bijih (Ertsberg) district in central Irian Jaya are products of hydrothermal systems that developed in association with Pliocene magma emplacement in an active continental margin. The CuAu skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Australian continental margin entered a north-dipping subduction zone at ∼ 12 Ma. The intermediate-composition intrusions consist of fine-grained porphyritic stocks, dikes, and sills that have K-Ar ages ranging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration. The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Group carbonate strata along the periphery of the Pliocene Ertsberg intrusion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg East (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores. The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. Diopsidic clinopyroxene is common, particularly in GBT. Anhydrite and phlogopite are abundant in the GBT complex, and the anhydrite:calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least three types of garnets have been identified at the Dom and show a progressive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is texturally associated with early retrograde alteration. Differences among the skarn orebodies are related in part to variable protolith composition that affected skarn development within different stratigraphic positions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodies probably are hosted by the Eocene Faumai Formation. The DOZ and lower IOZ orebodies, dominated by magnesian skarn alteration, appear to be developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.

Atmur-Delgo suture: A Neoproterozoic oceanic basin extending into the interior of northeast Africa

Field and remote-sensing data have enabled identification of a previously unrecognized discontinuous belt of ophiolitic nappe remnants in northeastern Sudan. The belt crops out for 200 km in a west southwest to southwest direction from the western margin of the Arabian-Nubian shield into the gneissic terrane previously accepted as part of the Archean to Paleoproterozoic Nile craton. Although highly dismembered and metamorphosed, the belt contains all the components of a Phanerozoic-type ophiolite. The ophiolite belt is interpreted as a suture zone (Atmur-Delgo suture), manifesting collision (∼700 Ma) between a passive-margin terrane in the south with an island-arc terrane in the north, following the consumption of an oceanic basin along a north-dipping subduction zone. Our results indicate that previously recognized crustal provinces in northeastern Africa must be reassessed critically.

Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden

Abstract Palaeoproterozoic, ∼ 1.9 Ga old, subaerial felsic to mafic volcanics of the Kiruna-Arvidsjaur Porphyry Group (KAPG) are abundant in northernmost Sweden. The felsic-dominated KAPG is situated north of a proposed north-dipping subduction zone along the boundary between the Archaean craton in the northeastern part of the Baltic Shield and juvenile Proterozoic crust in the south. The geochemistry of the volcanics indicates that plate-tectonic and volcanic-arc processes similar to recent ones occurred in northern Sweden ∼ 1.9 Ga ago. The volcanics were formed in extensional as well compressional environments. Calc-alkaline volcanics mark compression in the south and in the east. Mildly alkaline volcanics occurring in the west were formed in an extensional setting. The same rift zone gave rise to a basin filled with a thick sequence of younger sediments. On the basis of regional differences in geochemistry and petrology, the KAPG has been divided into the Kiruna, Arjeplog. Arvidsjaur and Lulea subprovinces. The volcanics in the Skellefte ore district south of the KAPG occurrence area have a volcanic-arc character and can be related to the volcanics of the Arvidsjaur subprovince immediately to the north; however, the latter are subaerial whereas the Skellefte volcanics represent a marine facies. The northerly Kiruna subprovince represents a different tectonic environment with a mafic to intermediate, trachytic volcanism that indicates extensional environments. The pattern of the different geotectonic settings indicates that the development of the ∼ 1.9 Ga old volcanics cannot be assigned solely to the action of a subduction zone dipping northwards beneath the Skellefte district.

Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: Timing and processes

Various types of eclogite, including coesite-bearing varieties formed under high- P-high- T conditions (up to 27 kbar, 700–800°C), and glaucophane schist occur in the Dabie Mountains and the SuLu terrane, eastern China. Dating of these high-pressure rocks by the SmNd mineral isochron and 40 Ar 39 Ar method suggests that the occurrence of the high-pressure metamorphism was during the early Triassic. LREE-enriched chondrite-normalized patterns for type-II eclogites and low initial ϵ Nd of − 14 to − 3 and the 87 Sr 86 Sr-values (0.706–0.710) for various types of eclogites suggest that their protoliths were mainly derived from Precambrian island arc or intraplate basalts in the basement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mountains suggests that the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabie Mountains in the late Jurassic and Cretaceous (at 134 Ma). It is proposed that the collision between the North China Block and Yangtse Block began in the late Permian or early Triassic with a north-dipping subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Triassic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histories for a coexisting gneiss and eclogite pair from Shima area suggest that the eclogite and their country rocks are not always coherent to each other. Some of them may have been juxtaposed through tectonic processes from different levels.

The Ayia Varvara Formation of SW Cyprus: A product of complex collisional tectonics

The Ayia Varvara Formation is a wedge of amphibole schists and intercalated metasediments within the intensely deformed pre-upper Maastrichtian volcanosedimentary Mamonia Complex. All contacts are steep faults against Upper Triassic transitional to alkaline lavas with reefoidal limestones, subalkaline depleted Troodos-type lavas, and serpentinised harzburgite. Ophiolite-related metamorphics are believed to have been formed by dynamothermal processes, beneath young oceanic crust during emplacement. Characteristically, such rocks might be expected to show a decrease of metamorphic grade away from the contact. Mineral analyses from the Ayia Varvara metamorphics, however, show no systematic variations that would indicate a metamorphic gradient. Hornblende remains generally calcic and of tschermakitic hornblende composition with an almost complete substitution of Mg and Fe 2+, suggesting disequilibrium. Plagioclase is essentially albitic and remains so for a distance of some 800 m from the harzburgite contact. Trace element analyses of the amphibolites and surrounding extrusives define four lava types. Types 1–3 are transitional to alkaline intraplate lavas believed to be remnants of intraoceanic islands and atolls. Type 4-lavas are extremely depleted in the HFS elements and are very similar to the Arakapas transform-fault lavas. The amphibolites have, in part, a trace-element geochemistry that is close to mid-ocean ridge basalts (MORB), a type that has not previously been recorded from Cyprus. The small area around Ayia Varvara records a long and complicated tectonic history that provides an insight into the evolution and emplacement of the Troodos ophiolite. The Mamonia Complex is interpreted as a passive margin sequence with intra-oceanic islands that was accreted onto a Late Cretaceous north-dipping subduction zone. Suprasubduction spreading produced the Troodos ophiolite and associated fracture zones. Spreading and subduction ceased in the late Maastrichtian when the newly-formed Troodos crust underwent a 90° anticlockwise rotation which juxtaposed Mamonia and Troodos (Arakapas) fracture zone rocks along arcuate, steep, strike-slip faults. At least some of the Ayia Varvara metamorphic rocks most likely represent Triassic oceanic crust and sediments metamorphosed during subduction by the overriding young Troodos crust.

Long-lived continent-ocean interaction in the Early Proterozoic Ungava orogen, northern Quebec, Canada

The Early Proterozoic Ungava orogen contains evidence for the development of a longlived (>80 m.y.) and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc-continent collision. A magmatic arc was built on 2.00 Ga oceanic crust above a north-dipping subduction zone between about 1.90 and 1.86 Ga. Development of a south-verging continental thrust belt containing units associated with prior rifting of the Archean Superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca. 1.87 Ga. Subduction of the Superior province continental crust is inferred to have led to a reversal in subduction polarity. A younger, south-dipping subduction zone is suggested by a suite of 1.85-1.83 Ga quartz diorite to granite plutons that intrude the upper- plate oceanic crust and the older arc. The arc-continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement.

Geochemistry and significance of metavolcanic rocks from the Bou Azzer-El Graara ophiolite (Morocco)

The Bou Azzer-El Graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc-alkalic arc sequence or as a composite forearc and accretionary melange. Seventy-nine new major and trace element whole-rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation. The results of this study indicate that the Bou Azzer metavolcanics comprise three petrographically and geochemically distinct groups. 1. (A) A siliceous metavolcanic series, ranging from basaltic andesites to rhyolites with calc-alkalic characteristics, is the principal component of the northern section of the complex. It includes distinct quartz-bearing, rhyolite flows. Isolated massive pyroclastic layers are typically associated with this series. These rocks are interpreted as forearc basin volcanics and pyroclastics produced during late Proterozoic, north-dipping subduction. 2. (B) Low-Ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite. These rocks may represent fragments of the forearc basement below the siliceous series. 3. (C) High-Ti metabasalts which range from enriched tholeiitic (Zr/Y<4) to mildly alkalic (Zr/Y>4) compositions are the principal volcanic block in the southern, melange-like terrane of the complex. These have high Ba and Sr contents (100–200 ppm average Ba, 260 ppm average Sr). They are identical to basaltic compositions from Tachdamt and Bleida on the southern margins of the ophiolite, which have previously been interpreted as a rifted continental margin sequence. These rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north-dipping subduction zone or rift margin basalts, which have been assembled in a tectonic melange as a consequence of a forearc-continent collision. Both the low-Ti and high-Ti metabasalt groups include samples previously identified as ocean-ridge or back-arc volcanics. However, there is no compelling evidence for the occurrence of these lava compositions in this region. The distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the Bou Azzer-El Graara complex.

Aptian-Albian palaeogeography of Neo-Tethyan domain

The Aptian-Albian interval (124.5-97.0 Ma) was a critical time both globally and for the Tethyan domain. In the Tethyan domain it was the time when a united Neo-Tethyan subduction zone became established between the future site of the Alps and Southeast Asia and greatly accelerated the rate of north-south convergence throughout the Tethyan region. A consequent widespread transgression on both Laurasian and Gondwanian margins deposited Orbitolina limestones and associated clastics on wide continental shelf areas. On the sea-floor of the Neo-Tethys ocean, organic-rich siliceous black shales, radiolarites, and turbidites with minor amounts of carbonates were deposited. Siliceous black shales accumulated under euxinic conditions of restricted small ocean basins of the Neo-Tethyan realm and along their margins. These areas received large amounts of organic debris from deltaic areas and coastal plains of the neighbouring continental shelves. Following the establishment of the north-dipping subduction zone, the Laurasian passive margin was largely converted to an active one, fringed by a large number of magmatic arcs that greatly influenced the sedimentation on the margin. Land areas and continental basins were widespread on this margin in northern Tibet, China and Indo-China. This was a consequence of the Late Triassic to Early Jurassic continental collissions and accompanied intracontinental convergence that lasted well into the Cretaceous. A detailed Aptian-Albian facies map and a crude palinspastic palaeogeographic map of the entire Tethyan realm are prepared in order to express the contrasts in sedimentation on Laurasian and gondwanian margins and to show the compatibility of the presented data with the most recent and reliable reconstructions of the major tectonic elements in Tethysides.