Intra-oceanic Subduction Zone Research Articles

Bainang Terrane, Yarlung–Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction–accretion processes preserved on the roof of the world

The Bainang terrane, an intra-oceanic island arc subduction complex into which Tethyan oceanic rocks were accreted during the Cretaceous, is preserved within the Yarlung–Tsangpo suture zone of Tibet. The lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central Tethyan domain from Late Triassic to mid-Cretaceous time. These rocks are preserved within a south-verging imbricate thrust stack of thin (≪1 km thick) northward younging tectonic slices. Five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts. The northern tract, which accumulated on the north side of Neotethys, was probably separated from its southern counterpart by a mid-ocean ridge. Detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract. Oceanic plate stratigraphy of the northern tract records its northward travel and mid-Cretaceous (late Aptian) approach towards a south-facing intra-oceanic subduction zone. Rocks in the southern tract developed closer to the Indian subcontinent and experienced thermotectonic subsidence and Mid-Jurassic basic alkaline intraplate magmatism. They were probably accreted late in the Cretaceous. Variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion. The overall tectonostratigraphy of the terrane reflects its development in a remote intra-oceanic setting.

Development of concepts concerning the genesis and emplacement of Tethyan ophiolites in the Eastern Mediterranean and Oman regions

This article uses evidence mainly from the Eastern Mediterranean and Oman to help understand the origin and emplacement of ophiolites. Field studies in these regions emphasize the diversity of ophiolite-related sedimentary and tectonic processes. Topics to be discussed here include the sedimentary covers of ophiolites, their metamorphic soles, related melanges, and emplaced continental margin (slope/rise) units. Modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites. The writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones. Evidence from their intact sedimentary covers, however, indicates that such subduction was only incipient in most cases, as directly associated volcanic arcs or back-arc basins are rare. Metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open-ocean to continental slope/rise settings. Underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges. Continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the Eastern Mediterranean and Oman regions. Ophiolite emplacement was commonly achieved by trench–margin collision, although strike-slip (transpression) and large-scale accretion along subducting margins played a role in some cases. Taking account of the Mesozoic palaeotectonic setting of the Eastern Mediterranean region, it is widely believed that ophiolites of two main ages (Mid-Jurassic and Late Cretaceous) were formed in several small oceanic basins separated by rifted microcontinents. The history of Tethys in pre-Mesozoic time remains more open to question. Lastly, giant subduction-related ophiolites, as in Oman, Cyprus, or the Balkans, may not be forming anywhere in the world ocean today, but may instead relate to rare events in earth history when regional, to global plate, boundaries were drastically reorganised.

The Dzela complex, Polar Urals, Russia: a Neoproterozoic island arc

Abstract The Neoproterozoic Dzela complex in the southern part of the Polar Ural Mountains occurs within the Uralian fault zone. It represents residual oceanic lithospheric mantle, partial melt derived from it, and seafloor basalts. In low strain domains, a depleted mantle residue of ultramafic rocks (pyroxenite, lherzolite and dunite) and mafic rocks (gabbro, gabbro-norite and quartz gabbronorite) are preserved. Distinctive negative europium anomalies in the ultramafic rocks (and their metamorphosed equivalents) indicate their residual nature after basaltic melt extraction. Gabbroic rocks (and their metamorphosed equivalents) have elevated rare earth element (REE) concentrations, but similar overall REE patterns as the ultramafic rocks that host them, suggesting that the ultramafic rocks are the source of the gabbroic melts. Blueschist- and greenschist-facies metabasalts overlie the ultramafic and mafic rocks. Two groups of metabasalts are defined. One has NMORB-like REE patterns, but is uniformly more depleted than NMORB, and the other has slightly enriched (relative to NMORB) LREE/HREE patterns. The latter has high field-strength elements (HFSE) concentrations consistent with formation in an intraoceanic subduction zone setting. Dzela mafic rocks have been dated for the first time to 578 ± 8 Ma (2σ) (U-Pb zircon from quartz-bearing gabbro-norite), defining a Neoproterozoic crystallization age. The complex was probably accreted to the NE margin of Baltica ( sensu lato ) during the Timanian Orogeny. Locally reset U-Pb zircon ages of about 350 Ma probably record Uralian orogenesis and recrystallization associated with high-pressure/low-temperature metamorphism. The Dzela complex provides further evidence that ocean-continent collision was the driving mechanism for Timanian Orogeny.

Neotethyan ophiolitic rocks of the Anatolides of NW Turkey and comparison with Tauride ophiolites

The I2zmir–Ankara–Erzincan zone in Turkey is part of the Anatolide belt. In NW Anatolia, near Kütahya, the ophiolitic rocks within the I2zmir–Ankara–Erzincan suture zone have a tholeiitic character and oceanic back-arc chemistry. Amphibolites form a metamorphic sole to part of the ophiolite in the Kütahya region and yield an 40 Ar/ 39 Ar age of 93 ± 2 Ma, interpreted to record high-grade metamorphism and intra-oceanic thrusting of pre-existing ocean crust. Gabbros and dykes yield 40 Ar/ 39 Ar ages of 94 ± 13 Ma and 48 ± 12 Ma. The first is suggested to be the ophiolite formation age, and the second a regional metamorphic event. The ophiolitic rocks of the Kütahya region are composite and record several events in the evolution of a Neotethyan seaway situated in north–central Anatolia and farther east. They are interpreted to have formed in a small ocean basin marginal to the subducting I2zmir–Ankara–Erzincan ocean. The Kütahya ophiolites of the Anatolide belt are similar in age to the Tauride ophiolites, and the sub-ophiolite metamorphic rocks and dykes in the ultramafic parts of the ophiolites in the Anatolide–Tauride Block formed at the same intraoceanic subduction zone within the same Neotethyan ocean.

Petrological Characteristics of the Hantaishir Ophiolite Complex, Altai Region, Mongolia: Coexistence of Podiform Chromitite and Boninite

Petrological and geochemical examination shows that the Hantaishir ophiolite complex of the Altai region in Mongolia contains two large ultramafic massifs (the Taishir and Naran massifs), a sheeted dyke complex, and a serpentine mélange zone containing blocks of chromitite and peridotites. The ultramafic massifs consist mainly of harzburgite and dunite with a small amount of podiform chromitite. Cr/(Cr+Al) atomic ratios of chromian spinels are high in the peridotites and sheeted dykes (>0.7), and in the chromitites (>0.8). Fo contents of olivines in the peridotites span a relatively narrow range (91.8-93.0). Most of the sheeted dykes are boninite-type high-Mg andesites. The coexistence of podiform chromitite and boninite in the Hantaishr ophiolite complex suggests that the ophiolite formed in an intra-oceanic subduction zone, from a depleted mantle source.

Re–Os, Sm–Nd isotope- and REE systematics on ultramafic rocks and pillow basalts from the Earth's oldest oceanic crustal fragments (Isua Supracrustal Belt and Ujaragssuit nunât area, W Greenland)

Ultramafic rocks and associated pillow basalts from the western–northwestern sector of the Isua Supracrustal Belt (ISB, West Greenland) reveal a complex pattern of trace elements, mainly with respect to their rare earth elements (REE). The heterogeneity of LREE patterns, in combination with widely scattering εNd[ T=3.81 Ga] values between −3.7 and +12.4, indicate that the REE systematics of the samples were disturbed by one (or more) metamorphic events which took place some considerable time after the original crystallization of the Isua belt. Furthermore, LREE-rich metasomatic fluids associated with the emplacement of tonalites into the supracrustals may have significantly altered the primary trace element budget. Extreme variations in whole rock γOs[ T=3.81 Ga] values of these rocks ranging from −8500 to +9 are favorably interpreted to derive from Re-additions during fluid hydrothermal alterations accompanying the early and late Archean metamorphic events. In contrast, the Re–Os isotope system remained intact in ultramafic-hosted chromites from Isua and from the Ujaragssuit nunât area about 20 km SE of the ISB. An average γOs[ T=3.81 Ga] value of +1.3 ±2.5 (2 σ) for the chromite population covers the entire range defined by the chondritic average, the convecting primitive upper mantle (PUM) [Nature 383 (1996) 517; Geochim. Cosmochim. Acta 65 (2001) 1311] and models involving outer core entrainment at this time. Given the uncertainty in the origin of the ultramafic rocks and, thus, the chromites, and the very high degree of alteration of these rocks within the ISB, our data do not conclusively allow to further impinge on the petrogenesis of the ISB mantle-derived rocks. Our data are the first though to vaguely hint at an early Archean mantle with a possible slightly suprachondritic initial 187Os/ 188Os isotopic composition. Similarities of unaltered pillow basalts with Phanerozoic boninites [Chem. Geol. 184 (2002) 231] may indicate a formation in an intra-oceanic subduction zone like geodynamic process, in which extreme early (Hadean?) depletion of a mantle source by melt extraction was followed by a second stage of melting induced by enriched subduction components. Alternatively, core–mantle interaction as a plausible mechanism for possible weak 187Os enrichments in these rocks remains a valid hypothesis.

THE ARGOLIS PENINSULA IN THE PALAEOGEOGRAPHIC AND GEODYNAMIC FRAME OF THE HELLENIDES

Geological and biochronological studies on the Argolis Peninsula (Pelagonian - Subpelagonian Domain) -also based on petrological data on the ophiolitic rocks- allowed us to propose a new tectonic unit succession; from the bottom upwards: a- the Trapezona Unit (Early-Middle Triassic - Late Jurassic; Pelagonian continental margin); b- the Dhimaina Ophiolitic Unit (Middle Triassic-Late Jurassic/Early Cretaceous oceanic realm, covered by an Early Cretaceous- Eocene “Mesautochthon; c- the Iliokastron Melange Unit (Middle-Late Jurassic - Early Cretaceous) in the northern Argolis and the Adheres Melange Unit (Cretaceous-Paleocene) in the southern Argolis; d- the Faniskos Unit (Late Jurassic-Eocene). Based on a close examination of the paleogeography of the continental margin successions and of their possible relationships with the ocean floor successions, we propose a model for the evolution of the oceanic and continental domains of this section of the Hellenides during the Triassic-Eocene interval, and we try to frame it into the evolution of the whole Dinaric-Hellenic orogenic system. This model hypothesises the opening, during Middle-Late Triassic time, of a single ocean in the Dinaric-Hellenic realm (the Vardar Ocean), which continues its spreading phase until the Middle Jurassic when, an intra-oceanic subduction zone, testified by the presence of IAT volcanites and boninitic rocks developed contemporaneously with MOR basalts. During the Late Jurassic obduction of the ophiolitic units onto the continental margin began. During the Early Cretaceous, the complete thrusting of the Dhimaina Ophiolitic Unit onto the Trapezona Unit occurred. During Eocene, and particularly after the Ypresian, the different units reached their present tectonic setting.

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

The Beimarang mélange (southern Tibet) brings additional constraints in assessing the origin, metamorphic evolution and obduction processes of the Yarlung Zangbo ophiolite

The Beimarang massif is one of many ophiolitic remnants which crop out discontinuously along the Yarlung Zangbo suture zone in southern Tibet. The southern contact of these remnants is marked by a highly sheared serpentinite mélange made up of blocks of serpentinites, diabases, gabbros and composite blocks of serpentinites and mafic injections. The Beimarang mélange has been investigated in order to bring additional constraints on the origin, metamorphic evolution and obduction processes of the Yarlung Zangbo ophiolite. Petrography and geochemical data suggest that the ultramafic components are similar to moderately depleted upper mantle peridotites. They may represent the already cooled and serpentinized Tethyan upper mantle which was trapped in a mantle wedge at the onset of a north-dipping Early Cretaceous intra-oceanic subduction zone located south of the Gangdese arc. These peridotites were then intruded by back-arc-like mafic magmas whose moderately depleted mantle source was affected by a subduction component. Ultramafic and mafic secondary mineral assemblages suggest that early low-( P/ T) metamorphic intra-oceanic conditions reached the amphibolite facies (>550 °C) before being retrograded down to the pumpellyite–prehnite facies (<280 °C). The Beimarang mélange, interpreted as an obduction mélange formed near a spreading ridge, was subjected to metamorphic conditions in the pumpellyite–prehnite facies which favored re-serpentinization of the peridotites and partial rodingitization of the mafic rocks. Unlike subduction-related mélanges and their associated lithological units found in the Ladakh area, we found no evidence of high-( P/ T) conditions in lithologies from the Beimarang mélange.

Boninite-like volcanic rocks in the 3.7–3.8 Ga Isua greenstone belt, West Greenland: geochemical evidence for intra-oceanic subduction zone processes in the early Earth

The 3.7–3.8 Ga Isua greenstone belt of southwest Greenland is characterized by variably metamorphosed, metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks. The voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units. The sedimentary rocks consist mainly of banded iron formation, cherts, conglomerates and siliciclastic turbidites. The least altered metavolcanic amphibolites (the Garbenschiefer unit) from the Central Tectonic Domain of the Isua greenstone belt are characterized by high Mg-number (0.60–0.80), MgO (7–18 wt.%), Al 2O 3 (14–20 wt.%), Ni (60–645 ppm) and Cr (60–1920 ppm) contents, but low TiO 2 (0.20–0.40 wt.%), Zr (12–30 ppm), Y (6–14 ppm) and rare earth element (REE) concentrations. These compositional features collectively represent a coherent mafic to ultramafic suite. Chondrite-normalized REE patterns are concave upward. On the primitive mantle-normalized extended trace element diagrams, they are characterized by relative depletion of Nb, but with an enrichment of Zr, relative to neighboring REEs. Alteration, deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition. The average initial ε Nd value of these metavolcanic rocks is +2. Collectively, these geochemical characteristics are comparable to those of Phanerozoic boninites. Given the observation that in the Tertiary, boninites are exclusively associated with intra-oceanic subduction environments (e.g., Izu–Bonin–Mariana subduction system), this suggests that intra-oceanic subduction zone-like geodynamic processes were operating as early as 3.7–3.8 Ga.

PETROLOGY OF THE KURANCALI PHLOGOPITIC METAGABBRO: AN ISLAND ARC-TYPE OPHIOLITIC SLIVER IN THE CENTRAL ANATOLIAN CRYSTALLINE COMPLEX

In the Central Anatolian Crystalline Complex (CACC), besides the ophiolitic bodies with more or less recognisable sequence and preserved magmatic pseudostratigraphy, there are many massive and layered gabbroic masses occurring as isolated outcrops. Most of these mafic rocks were interpreted as dismembered parts of an allochthonous ophiolitic assemblage (Yaliniz et al, 2000), derived from the northerly located Izmir-Ankara branch of the Alpine Neotethys. An overall supra-subduction zone genesis has been envisaged for the Central Anatolian Ocean (CAO). However, remarkable differences in the geochemical characteristics of different isolated outcrops suggest differences in the source areas and tectonic settings within the intra-oceanic subduction zone. One of these gabbro units, the Kurancali Metagabbro, occurs as an isolated body in the central part of CACC. It is thrust along a steep south vergent thrust-plane onto the uppermost units of the Central Anatolian Metamorphics (CAM). The main body of the Kurancali Metagabbro is characterised by a distinct compositional layering. The layered gabbros are represented by pyroxene and hornblende gabbros. Phlogopite-rich plagioclase-hornblende gabbro occurs mainly as pegmatitic dikes intruding the layered gabbro sequence. The layered gabbros in general consist mainly of diopsidic augites, brown hornblendes, plagioclase. Secondary phases are phlogopitic mica, brownish green hornblende replacing clinopyroxenes, and fibrous greenish actinolitic hornblende partially or completely replacing brown hornblende. The primary dark micas display phlogopitic composition within the range of annite30.13-42.60 and plogopite 69.9-57.4 The analysed pyroxenes are diopsidic (En32.3Fs18.5Wo49.1 - En34.9Fs17.9Wo48.2). The whole-rock geochemistry of the gabbros indicates the presence of two distinct groups of rocks; a subordinate group of phlogopite gabbro with island arc calc-alkaline affinity and a dominating layered gabbro sequence with island arc tholeiite characteristics. They are extremely enriched in LIL elements, indicative of an alkaline metasomatism in the source region and they display geochemical features of transitional back arc basin basalts (BABB)/island arc basalts (IAB)- and IABtype oceanic crust. Based on their geochemical similarities with modern island arc basements, we suggest that the Kurankali Metagabbro may represent the basement of an initial island arc, generated in a supra-subduction zone setting within the Izmir-Ankara branch of Neotethys.

The role of fluids during formation and evolution of the southern Superior Province lithosphere: an overview

Models for fluid flow and hydrothermal alteration for the Abitibi greenstone belt are reviewed in the light of Lithoprobe results in the region. In the Abitibi greenstone belt, eruption of volcanic sequences over 2750-2700 Ma was accompanied by pervasive low-temperature hydrothermal alteration at high water/rock ratios, giving systematic 18O-enrichment. Archean ambient ocean water bottom temperatures were likely ca. 30°C, and δ18O ~0 ± 1‰. Chert-iron formations precipitated from low temperature hydrothermal discharge. Base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high-temperature (~300-400°C) fluids in convective cells sited above subvolcanic intrusions. The ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high-temperature, low water/rock exchange with the volcanic pile to NaCl (3-7 wt.%) or CaCl2-NaCl (up to 30 wt.%) brines of δ18O = 0-8‰. These volcanic associated hydrothermal deposits are associated with greenstone belt asemblages in the northern Abitibi subprovince that were emplaced as a series of thrust slices over the Opatica plutonic belt. In the southern Abitibi subprovince the hydrothermal deposits were associated with a series of rift basins (Noranda, Val d'Or, etc.), formed on top of accreted oceanic assemblages comprising primitive arcs and plateaus, or in protoarcs, and associated with oblique convergence. Contemporaneous erosion of emergent arcs and the older cratonic provenance terrane of the Pontiac subprovince by orographic rainfall, and submarine weathering, fed first-cycle vol cano genic sediments to depositional basins in the Abitibi, but siliciclastic sediments of mixed old 3 Ga continent and 2.7 Ga arc provenance in the Pontiac subprovince. Abitibi subprovince turbidites were more weathered and 18O-enriched than Pontiac subprovince equivalents. Subduction-accretion assembly of the Opatica-Abitibi and Pontiac terranes involved allochthonous thrusting of the Abitibi subprovince over the Pontiac subprovince. There were several pulses of granitoid magmatism during accretionary assembly over 2695 to 2674 Ma. Syn- to late-tectonic tonalites were generated by melting of hot young hydrous ocean crust in a shallow-dipping intraoceanic subduction zone. The intrusions exsolved small quantities of magmatic fluids that formed Cu-Zn showings. Late-tectonic shoshonites formed at [Formula: see text]80 km in subarc mantle wedge by slab dehydration-wedge melting. This late-stage of arc development involved transfer of significant quantities of gas-rich alkaline magmas 80+ km through the lithosphere along the accretionary terrane bounding structures, and produced small phosphorus and barite deposits. Synmagmatic metamorphism was of the high-temperature low-pressure type, and occurred in several pulses; water/rock ratios were generally low distal from structures. Tens of thousands of cubic kilometres of fluids generated by dehydration reactions at the base of the subduction-accretion complex, during thermal relaxation following collision and the main granitoid pulses, advected up terrane boundary structures and locally generated lode gold deposits. At the highest structural levels these fluids mixed with Archean meteoric water where δ18O &lt; 0. A second metamorphism (M2) occurred over 2645 to 2611 Ma leading to melting of Pontiac sediments and formation of S-type granites. Deposits of Mo, Th, and P were precipitated from magmatic fluids of δ18O 8-9‰. M2 variably reset radiogenic and stable isotope systems in nonrobust minerals of vol canogenic massive sulphide and lode gold deposits. Hypersaline CaCl2 formation brines evolved in Paleoproterozoic glaciogenic sediments; these penetrated into the Archean basement where they redistributed gold and are pervasively present as low-temperature secondary brine inclusions. The Matachewan (2.5 Ga) and Hearst dyke swarms drove higher temperature advection of the brines, and Ag-Co-Ni sulpharsenide deposits formed by thermal evolution of the brines driven by the Nipissing diabase dyke swarm at ~2219 Ma. Local resetting of 40Ar/39Ar spectra between 2550 and 2200 Ma was the product of tectonic pumping of fluids along reactivated Archean structures, possibly due to coupling of the 200 km thick mantle lithosphere to Archean crust. Seismic evidence for late overprinting of the lower crust and growth of 2450 Ma zircon rims in lower crustal assemblages were associated with this event. There was also fluid activity at 1950 to 1850 Ma due to the Hudsonian orogen induced Kapuskasing event. Hypersaline CaCl2-rich brines formed in the Paleozoic sedimentary cover (~500 Ma), penetrated deep (&gt;5 km) into the Archean basement, and comprise vast reservoirs of hypersaline brines deep in the Shield. The brines precipitated prehnite-laumontite veins that record hundreds of increments of dilation. Subglacial 18O-depleted fluids penetrated to shallow ([Formula: see text]1 km) depths in the Quaternary; they form mixing lines with the hypersaline end member. Extremely D-depleted (-400 to -100‰) CH4 and H discharge in mining districts of the Shield. The depleted H may form by radiolysis of H2O and (or) by a Fischer-Tropsch type process. The hypersaline brine end-member was shifted to the left of the meteoric water line by exchange with D-depleted H.

Melting of Refractory Mantle at 1middle dot5, 2 and 2middle dot5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Boninites are an important ‘end-member’ supra-subduction zone magmatic suite as they have the highest H2O contents and require the most refractory of mantle wedge sources. The pressure–temperature conditions of boninite origins in the mantle wedge are important to understanding subduction zone initiation and subsequent evolution. Reaction experiments at 1·5 GPa (1350–1530°C), 2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C) between a model primary high-Ca boninite magma composition and a refractory harzburgite under anhydrous and H2O-undersaturated conditions (2–3 wt % H2O in the melt) have been completed. The boninite composition was modelled on melt inclusions occurring in the most magnesian olivine phenocrysts in high-Ca boninites from the Northern Tongan forearc and the Upper Pillow Lavas of the Troodos ophiolite. Direct melting experiments on a model refractory lherzolite and a harzburgite composition at 1·5 GPa under anhydrous conditions (1400–1600°C) have also been completed. Experiments establish a P, T ‘melting grid’ for refractory harzburgite at 1·5, 2 and 2·5 GPa and in the presence of 2–3 wt % H2O. The effect of 2–3 wt % dissolved H2O produces a liquidus depression in primary boninite of ∼112 ± 19°C at a given temperature. The H2O-bearing melts, recalculated to 100 wt % anhydrous, are ∼2–6 wt % higher in MgO, ∼1–2 wt % higher in SiO2 and ∼1–1·5 wt % lower in FeO, compared with nominally anhydrous melts at the same P and T. These differences are consistent with a change in the melting reaction, resulting in a higher contribution of orthopyroxene to the melt phase, compared with anhydrous conditions. We conclude that high-Ca boninite petrogenesis requires temperatures as high as ∼1480°C at depths of ∼45 km in the mantle wedge; these are constraints for any proposed model of intra-oceanic subduction zones. A comparison of the results from the boninite–harzburgite reaction experiments with the direct melting experiments on refractory lherzolite and harzburgite indicates that the influence of subduction components (included in the composition of the added model boninite) is to cause high-pressure melting cotectics to move towards the olivine apex (i.e. to relatively higher pressures) of the molecular normative projection from diopside onto the base of the ‘basalt tetrahedron’ [Jd + CaTs + Lc–Qz–Ol] compared with anhydrous melting of normal mantle in the absence of a subduction component. The subduction component involved in high-Ca boninite petrogenesis in addition to H2O has relatively high Al2O3 and Na2O contents. The experimental data from this and other studies empirically quantify the absolute effect of dissolved H2O (0·2–21 wt %) on the liquidus depression of olivine-saturated basaltic melts with ∼<3 wt % total alkalis as follows: \[\ olivine\ liquidus\ depression\ ({^\circ}C)\ =\ 74.403\ {\times}\ (H_{2}O\ wt\ \%)^{0{\cdot}352}.\ \] The equation that describes this empirical relationship is non-linear with an error of ∼9 relative percent.

A Geotraverse Across Northwestern Turkey: Tectonic Units of the Central Sakarya Region and their Tectonic Evolution

Abstract In the Central Sakarya area of Turkey there are two main Alpine continental units, separated by a south verging ophiolitic complex which represents the root zone of the İzmir-Ankara Suture Belt. The Central Sakarya Terrane in the north includes two ‘Variscan’ tectonic units in its basement. The Söğüt Metamorphic rocks represent a Variscan ensimatic arc complex and the Tepeköy Metamorphic rocks are characteristically a forearc-trench complex. The unconformably overlying Triassic Soğukkuyu Metamorphic rocks correspond to a part of the Karakaya Formation and are interpreted as a Triassic rift basin assemblage. These units are unconformably overlain by a transgressive sequence of Liassic-Late Cretaceous age that represents the northeastward deepening carbonate platform of the Sakarya Composite Terrane. The middle tectonic unit (the Central Sakarya Ophiolitic Complex) comprises blocks and slices of dismembered ophiolites, blueschists and basic volcanic rocks with uppermost Jurassic-Lower Cretaceous radiolarite-limestone interlayers. Geochemical data from basalt blocks suggest mid-ocean ridge basalt (MORB)- and suprasubduction-type tectonic settings within the Neotethyan İzmir-Ankara Ocean. The southern tectonic unit includes basal polyphase metamorphosed clastic rocks (Sömdiken Metamorphics), intruded by felsic and basic dykes and overlain by thick-bedded marbles. This assemblage is unconformably overlain by continental clastic rocks gradually giving way to thick-bedded recrystallized limestones, cherty limestones and pelagic limestones intercalated with radiolarites, and finally by a thick high pressure-low temperature (HP-LT) metamorphic synorogenic flysch sequence. This succession is identical to the passive continental margin sequences of the Tauride Platform. It is suggested that this passive margin was subducted during the Late Cretaceous in an intraoceanic subduction zone and affected by HP-LT metamorphism. The emplacement of the allochthonous oceanic assemblages and the collision with the Central Sakarya Terrane was complete by the end of the Cretaceous.

Mesozoic–Tertiary tectonic evolution of Albania in its regional Eastern Mediterranean context

Research carried out since the opening of Albania to the ‘western’ geological community in the 1990s allows the geology and tectonics of this small Balkan country to be integrated into an overall interpretation of Neotethys in the Eastern Mediterranean region, specifically Greece and the former Yugoslavia region. Albanian geology is most famous for the occurrence of contrasting ophiolites of Jurassic age. The country is sub-divided into three main geotectonic units. Those in the west (i.e. Sazani, Ionian, Kruja, Krasta–Cukali and the Albanian Alps) relate to the western margin of Apulia, effectively part of North Africa from Early Mesozoic time onwards. In the east is the Korabi zone, interpreted as a microcontinent located further northeast within Neotethys. Between the two is the Mirdita zone, dominated by two contrasting Jurassic ophiolites: the ‘Western-type’ ophiolite and the ‘Eastern-type’ ophiolite. Taking account of evidence from Greece and former Yugoslavia, the following tectonic evolution is proposed. From Early Mesozoic time onwards, Neotethys in Albania existed as a northward continuation of the Pindos oceanic basin in Greece (Pindos–Mirdita ocean). A Korabi microcontinent was rifted off Apulia in the Early–Mid Triassic, followed by passive margin subsidence until late Middle Jurassic time. A rifted, slow-spreading, MOR-type ophiolite formed within the western Pindos–Mirdita oceanic basin in Late Triassic?–early Late Jurassic time (using present co-ordinates). The Shkoder–Peje (Scutari–Peć) lineament in the north of Albania is interpreted as an important transform fault zone that influenced the regional tectonic evolution during the entire Mesozoic–Early Tertiary rift/drift/emplacement history of the Pindos–Mirdita ocean. Taking account of evidence from the Greek and former Yugoslavia ophiolites, the easterly Albanian ophiolites formed within the Pindos–Mirdita ocean above a westward-dipping intra-oceanic subduction zone in the early Late Jurassic. Metamorphic soles were formed during initial intra-oceanic displacement (ca. 162–174 m.y.), and underlying melanges originated as subduction–accretion complexes. The MOR, Western-type ophiolite evolved into the IAT–boninitic Eastern-type ophiolite after subduction began. During Tithonian–Berriasian time, trench–margin collision emplaced the ophiolites eastwards over the Korabi margin, coupled with redeposition of ophiolitic and continental margin material as exotic blocks, debris flow deposits and turbidites. These were emplaced onto the upper surface of the ophiolitic thrust sheet during the latest stages of emplacement onto the Korabi microcontinent. Following collision in the east, oceanic crust remained within a small, remnant Pindos–Mirdita oceanic basin. Passive conditions were restored in the Mid–Late Cretaceous, with shallow-water carbonate deposition on microcontinental units and both pelagic and redeposited carbonates in basinal settings. From the Maastrichtian onwards, Africa–Eurasia convergence resulted in large-scale westward thrusting, deforming the Apulian foreland in Oligo-Miocene time. The suture zone was re-activated in Oligo-Miocene time as the Albanie–Thassalie piggy-back-type basin in central and southern Albania. Albania as a whole was rotated 45° clockwise as part of the western Aegean arc in post-Eocene time. Coastal and offshore Albania experienced strong subsidence after Early Miocene time, forming the Periadriatic basin (including the Tirana depression), interpreted as a foredeep related to continuing regional convergence. Southern Albania experienced extensional tectonics in Plio-Quaternary time related to southward ‘roll-back’ of the Aegean active margin.

Photang thrust sheet: an accretionary complex structurally below the Spontang ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India

The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey. The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange. These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence. Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients. Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit. Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates. The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB). These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids. Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth. This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere. The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway. These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean.

Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Eu anom and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ce anom (1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ce anom (0.9–1.21) and Eu anom (0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation. It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.

Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey)

The Kiziltepe ophiolitic thrust sheet in the Bolkar Mountains of Turkey occurs between two subparallel ophiolite belts bounding the Tauride carbonate platform and represents a remnant of the Cretaceous Neo-Tethyan oceanic lithosphere. It is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra-oceanic subduction zone in the Neo-Tethys ∼92-90 Ma. Blueschist-facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing P/T with cooling. Regional tectonic constraints suggest a Maastrichtian age for the timing of this blueschist-facies metamorphism. Sodic amphibole-rich veins and crossite/Mg-riebeckite rims on hornblende suggest that growth of blueschist-facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries. We infer a counterclockwise P-T-t trajectory during which metamorphism was accompanied/succeeded by rapid uplift along the northern edge of the Tauride belt in Late Cretaceous-early Tertiary time.

Isotopic and trace element constraints on the genesis of a boninitic sequence in the Thetford Mines ophiolitic complex, Quebec, Canada

The Mont Ham Massif (part of the Thetford Mines ophiolite, south Quebec) represents a magmatic sequence made up of tholeiitic and boninitic derived products. A geochemical study confirms the multicomponent mixing models that have been classically advanced for the source of boninites, with slab-derived components added to the main refractory harzburgitic peridotite. An isochron diagram of the boninitic rocks is interpreted as a mixing trend between two components: (i) a light rare earth element (LREE) enriched component (A), interpreted as slab-derived fluid–melts equilibrated with sedimentary materials (εNd = −3, 147Sm/144Nd = 0.140), and (ii) a LREE-depleted component (B) (0.21 &lt; 147Sm/144Nd &lt; 0.23), interpreted as slab-derived fluid–melt equilibrated with recycled Iapetus oceanic crust and equated to the Nd-isotope characteristics of the Iapetus mantle (εNd = 9). A multicomponent source is also necessary to explain the Nd-isotope and trace element composition of the tholeiites, which are explained by the melting of a more fertile, lherzolitic mantle and (or) mid-ocean ridge basalt source (component C), characterized by a large-ion lithophile element depleted pattern and an Iapetus mantle Nd isotopic composition (εNd = 9), mixed in adequate proportions with the two previously infered slab-derived components (A and B). The genesis of the boninites of Mont Ham is not significantly different from those of boninites located in the Pacific. An intraoceanic subduction zone appears to be an appropriate geodynamic environment for the Mont Ham ophiolitic sequence.