Ophiolitic Lavas Research Articles

The petrogenesis of modern and ophiolitic lavas reconsidered: Ti-V and Nb-Th

Tectonic discrimination diagrams are a key tool for understanding ancient volcanic rock origins. In this contribution we compile over 15,000 whole rock compositions to re-evaluate the Ti-V discrimination diagram and compare it to another commonly used tool, the Nb/Yb-Th/Yb diagram. We have reformulated the Ti-V diagram into a log–log plot to distinguish samples more clearly at lower concentrations. The compilation shows that MORB are dominated by Ti/V = 20–43, whereas juvenile arc tholeiites and boninites are characterized by Ti/V < 20 generally, although there is minor overlap at the boundary (Ti/V = 20–22). Plume-related volcanic rocks (ocean island basalts, oceanic plateaux, and continental flood basalts) generally have Ti/V > 43, although there may be significant overlap with MORB-like ratios for ridge-centered OIB and for some oceanic plateaux. About 56% of alkaline OIB have Ti/V > 70. Back-arc basins are dominantly MORB-like. Melt models show that MORB and juvenile arc volcanics most likely formed under different ƒO2 conditions, but are permissive of similar ƒO2 if the arc rocks form by much higher melt fractions. The Nb/Yb vs. Th/Yb plot clearly distinguishes most oceanic basalts (MORB, plateaux, OIB) from subduction-related volcanic rocks (boninite, juvenile arc tholeiite, calc-alkaline) and from flood basalts. We propose here a new two-proxy diagram of Ti/V vs. Th/Nb, which incorporates the advantages of both.

Regionally variable timing and duration of celadonite formation in the Troodos lavas (Cyprus) from Rb-Sr age distributions

Celadonite (K[Mg2+,Fe2+][Fe3+,Al3+](Si4O10)(OH2)) is a common void-filling mineral in ocean floor and ophiolite lavas that forms during low-temperature, off-axis, hydrothermal alteration. Its occurrence provides important information about the sink of K from seawater into the upper oceanic crust. New age determinations for celadonitic samples from a ~20 km section of the lavas in the Troodos ophiolite, along with published ages, show that most celadonite formed within the first ~20 Myrs after crust accretion. However, significant regional differences in the duration of celadonite formation are observed. In comparison to a region of flat paleo-seafloor, in a paleo-seafloor topographic low celadonite started forming later and continued forming for ~20 Myrs longer. The paleo-seafloor low is associated with abundant hydrothermal sediments which are interpreted to have played a key role in controlling the chemical conditions required for celadonite precipitation. It is hypothesised that celadonite formed for longer largely because these hydrothermal sediments acted as a source for labile Fe for a longer time than in the region where the Fe required for celadonite formation was mainly leached from silicate phases. In the region of flat paleo-seafloor, sediment did not begin accumulating for ~20 Myrs after crustal accretion allowing prolonged oxidative alteration of the upper crust. This led to Fe released by silicate phase breakdown being incorporated in stable Fe3+ containing phases, such as Fe-oxyhydroxides, rather than being transported as aqueous Fe2+ to sites where celadonite could form. If this model is correct, the uptake of K from seawater into the oceanic crust may be highly spatially heterogeneous and intimately linked to the subsurface redox conditions and the distribution of hydrothermal sediments.

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.

Immobile Element Fingerprinting of Ophiolites

Much of our understanding of ocean ridges has come from the collection and analysis of glasses recovered from ridge axes. However, applying the resulting methodologies to ophiolite complexes is not straightforward because ophiolites typically experience intense alteration during their passage from ridge to subduction zone to mountain belt. Instead, immobile element proxies for fractionation indices, alkalinity, mantle temperature, mantle flow and subduction addition may be used to classify ophiolite lavas and fingerprint the precise setting of the ridge at which an ophiolite formed. The results can help us recognise and interpret past spreading centres and so make plate tectonic reconstructions.

Field relations, geochemistry and origin of the Upper Cretaceous volcaniclastic Kannaviou Formation in western Cyprus: evidence of a southerly Neotethyan volcanic arc

Abstract The Kannaviou Formation (up to 750 m thick) accumulated in a deep-sea setting in west Cyprus during Campanian–Early(?) Maastrichtian time. The formation depositionally overlies Upper Cretaceous ophiolitic lavas, including those associated with serpentinite-hosted arcuate lineaments. The Kannaviou Formation locally overlies ophiolitic serpentinite, indicating that mantle rocks were exposed on the seafloor prior to sediment deposition. Geochemical analyses of basalts that depositionally underlie the Kannaviou Formation, within the arcuate lineaments, indicate close similarities with the boninitic lavas of the South Troodos Transform Fault Zone in south Cyprus. Abundant volcanogenic and minor terrigenous and pelagic sedimentary rock material is present within the Kannaviou Formation, while kaolinite is common within interbedded red clays. Suitable terrigenous source lithologies are present in the deformed continental margin/deep-sea sedimentary rocks of the Mamonia Complex in west Cyprus. Whole-rock chemical analysis of sandstones of predominantly volcaniclastic origin indicates an intermediate arc-like composition. Electron microprobe analysis shows that glass is silicic, with a tholeiitic fractionation trend. Similar arc-like volcanic rocks of Late Cretaceous age are exposed in the western Kyrenia (Girne) Range, north Cyprus. The provenance of the Kannaviou Formation provides evidence of Late Cretaceous northwards subduction of the South Neotethys beneath a continental margin to the north. Supplementary material: The sample locations, complete chemical analyses, electron probe data and x-ray diffraction results are available at www.geolsoc.org.uk/SUP18579

Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex, Central Eastern Desert, Egypt

Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)] decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for the dunite bodies (TiO2 = ~<0.07–0.22 wt%; Al2O3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09–0.28 wt%; Al2O3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38–0.51 wt%; Al2O3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc system above a southeast-dipping subduction zone.

The Dehshir ophiolite (central Iran): Geochemical constraints on the origin and evolution of the Inner Zagros ophiolite belt

The Late Cretaceous Dehshir ophiolite is an important element within the Inner Zagros (Nain-Baft) ophiolite belt and contains all components of a complete “Penrose ophiolite,” including tectonized harzburgites, gabbros, sheeted dike complexes, pillowed basalts, and rare ultramafic-mafic cumulates. The cumulate rocks of this ophiolite are composed of plagioclase lherzolite, clinopyroxenite, leucogabbro, and pegmatite gabbro. All the massifs in the Inner Zagros ophiolite belt are overlain by Turonian-Maastrichtian pelagic limestones (93.5–65.5 Ma). Clinopyroxene compositions of Dehshir mafic rocks are similar to those of both boninites and island-arc tholeiites. Nearly all spinels from the inner ophiolite belt are similar to those of highly depleted harzburgites from intra-oceanic forearcs, although some Dehshir harzburgite spinels plot within the field for abyssal (mid-ocean-ridge basalt) peridotites. All components of the Dehshir and other ophiolites of this belt show strong suprasubduction-zone affinities, from harzburgitic mantle to ophiolitic lavas. Volcanic rocks have a mixture of dominantly arc-like (island-arc tholeiite, boninite, and calc-alkaline) and subordinate mid-ocean-ridge basalt–like compositional features, usually with mid-ocean-ridge basalt–like rocks at the base and arc-like rocks at the top. Our data for the Dehshir ophiolite and the similarity of these results to those for Iranian inner and outer belt ophiolites compel the conclusion that a geographically long, broad, and continuous tract of forearc lithosphere was created at about the same time during the earliest stages of subduction along the southern margin of Eurasia in Late Cretaceous time.

New and revised radiolarian biochronology for the sedimentary cover of ophiolites in the Lesser Caucasus (Armenia)

Abstract In order to improve our understanding of the palaeogeographic and geodynamic evolution of the Tethyan realms preserved in the Lesser Caucasus we here review the existing data for the sedimentary cover of ophiolites preserved in Armenia. Particular attention is given to those dated sedimentary rocks that are in direct genetic contact with ophiolitic lavas, as they provide constraints for submarine oceanic activity. The oldest available ages come from the Sevan–Akera suture zone that point to a Late Triassic oceanization. Data from both the Sevan and Vedi ophiolites provide evidence for Middle Jurassic (Bajocian) submarine activity, that continued until at least the Late Jurassic (Mid/Late Oxfordian to Late Kimmeridgian/Early Tithonian), as dated recently in Stepanavan and in this study for the Vedi ophiolite.

Geochemical variations in a depleted fore-arc mantle: The Ordovician Thetford Mines Ophiolite

The ∼ 5 km thick mantle of the Ordovician Thetford Mines Ophiolite is dominated by harzburgite (≤ 5–6% clinopyroxene), cut by dunitic to orthopyroxenitic dykes. The plutonic crust is dominated by cumulates of boninitic affinity. The first lava cycle is dominated by boninites (and their differentiation products) with subordinate arc tholeiites; while the second cycle is exclusively of boninitic affinity. Boninites typically occur in fore-arc environments and are interpreted to be derived from previously depleted mantle as a result of decompression and influx of subduction-related fluids and melts. Thetford Mines Ophiolite mantle harzburgites are MgO-rich, and poor in CaO, Al 2O 3, TiO 2, Na 2O, and incompatible trace elements, with U-shaped primitive-mantle-normalized extended trace element profiles. Heavy rare-earth element abundances suggest that the Thetford Mines Ophiolite harzburgites are residues from ∼ 25 to 30% of fractional melting, or 30 to 35% critical melting (with 20% of each melt increment remaining in the source); as calculated from a fully-parameterized partition coefficient dataset. Tholeiites would represent 15–20% melting, while the boninites could represent the products of an additional 10–20% melting of the residues of tholeiite extraction. However, these models cannot explain the enrichment in light rare-earths (LREE) and large ion lithophile elements (LILE) seen in these otherwise refractory harzburgites or in the complementary lavas. A better match to the geochemical signatures is obtained if melting occurs synchronously with a continuous influx of slab-derived fluid and melt. About 0.005% of this arc component produces a much better fit to the Thetford Mines Ophiolite data. This amount is consistent with previously published Nd isotopic data on closely related rocks. However, bulk sediment contamination yields model residues that are too low in LILE Th, U, Nb, Ta, and LREE compared to Thetford Mines Ophiolite harzburgite, but produces model liquids that fit almost perfectly with Thetford Mines Ophiolite lavas. On the other hand, models involving contamination with 5% equilibrium melt of subducted continental margin sediment closely resemble Thetford Mines Ophiolite mantle, whereas the model liquids are too enriched in LILE, Th, U, Nb, Ta, and LREE compared to the lavas. One possible explanation for this conundrum is that melts generated at depth reacted with mantle rocks on their way to the surface, enriching the mantle and depleting the lavas in these elements. Despite an overall excellent fit to most data, some anomalies remain unexplained, for example, model residues show positive Ti peaks, whereas Thetford Mines Ophiolite mantle rocks do not.

Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process

The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian–Santonian (88–83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: (La/Yb) N = 0.6–0.9; (Nb/Th) N = 0.17–0.57; ( 143Nd/ 144Nd) i = 0.51273–0.51291; ( 87Sr/ 86Sr) i = 0.70370–0.70565; ( 207Pb/ 204Pb) i = 15.4587–15.5411; ( 208Pb/ 204Pb) i = 37.4053–38.2336; ( 206Pb/ 204Pb) i = 17.9195–18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. 87Sr/ 86Sr ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites. The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb) N = 0.2–2.3; (Nb/Th) N = 0.1–2.8; ( 143Nd/ 144Nd) i = 0.51264–0.51276; ( 87Sr/ 86Sr) i = 0.70386–0.70557; ( 207Pb/ 204Pb) i = 15.4888–15.5391; ( 208Pb/ 204Pb) i = 37.2729–37.8713; ( 206Pb/ 204Pb) i = 17.6296–17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb) N = 1.10–7.92; (Nb/Th) N = 0.10–0.94; but display a less radiogenic Nd isotopic composition than basalts [( 143Nd/ 144Nd) i = 0.51263] and more radiogenic ( 87Sr/ 86Sr) i ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. 40Ar/ 39Ar dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3 ± 0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/Yb) N = 6–14; (Nb/Th) N = 0.23–0.76; ( 143Nd/ 144Nd) i = 0.51262–0.51271; ( 87Sr/ 86Sr) i = 0.70338–0.70551; ( 207Pb/ 204Pb) i = 15.5439–15.6158; ( 208Pb/ 204Pb) i = 38.3724–39.3623; ( 206Pb/ 204Pb) i = 18.4024–19.6744]. They have significantly more radiogenic lead isotopic compositions than ophiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In addition, this oceanic + plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb) N = 2.07–2.31; (Nb/Th) N = 0.08–0.15; ( 144Nd/ 143Nd) i = 0.51271–0.51282; ( 87Sr/ 86Sr) i = 0.70452–0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to significant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block.

Discovery of Middle Jurassic (Bajocian) Radiolaria from the sedimentary cover of the Vedi ophiolite (Lesser Caucasus, Armenia)

The Vedi ophiolite, situated southeast of Yerevan (Armenia), represents part of the Neotethyan oceanic lithosphere preserved in the Lesser Caucasus. This ophiolite unit constitutes a large tectonic klippe, a result of obduction during the Upper Cretaceous (Coniacian–Santonian). Relatively well-preserved Radiolaria extracted from radiolarites overlying ophiolitic lavas along the Vedi River consist of Middle Jurassic (Bajocian, U.A.Z. 3–4) species, typical of the Tethyan tropical bioprovince. Assemblages are dominated by Nassellaria and characterised by the presence of species Cyrtocapsa mastoidea, Hexasaturnalis hexagonus, Laxtorum (?) hichioense, Stichocapsa japonica and Striatojaponocapsa plicarum s.l. This microfauna provides evidence for the oldest age available so far for the sedimentary cover of the Vedi ophiolite.

Petrological and geochemical study of the Late Cretaceous ophiolite of Khoy (NW Iran), and related geological formations

This paper, based on 113 new whole rock analyses and about 3500 electron microprobe analyses of the mineral phases, is dedicated to the petrography and geochemistry of the Khoy ophiolites and related formations, NW Iran. It is complementary to a previous paper published in this Journal, where we gave a detailed description of the geology of the Khoy area, including various geological field sections, two geological maps in colour, new micropaleontological data and 27 new 40K– 40Ar datings (Khalatbari-Jafari, M., Juteau, T., Bellon, H., Whitechurch, H., Cotten, J., Emami, H., 2004. New geological, geochronological and geochemical investigations on the Khoy ophiolites and related formations, NW Iran. J. Asian Earth Sci. 23, 507–535). Our conclusions are: (a) The petrographic study confirms the field data showing the existence of two ophiolite complexes in the region of Khoy. (b) The Late Cretaceous ophiolitic lavas of the Khoy region exhibit very homogeneous T-MORB-type multi-element plots, suggesting that they were formed at oceanic spreading centers, by partial melting of a depleted mantle source, probably contaminated by one or several regional mantle plumes, responsible for their moderate enrichment in LREE. They do not show any negative anomaly for Nb, Zr or Ti, which allows us to exclude a genesis in a ‘supra-subduction’ environment. (c) The Late Cretaceous ophiolite of Khoy was created at a slow-spreading oceanic ridge. (d) Cryptic variations along extrusive and layered gabbros sections suggest frequent replenishment and magma mixing events in the magma chambers. (e) The ‘supra-ophiolitic turbiditic series’ overlying the Late Cretaceous ophiolite was accumulated in a subduction trench running along the northwestern margin of the Iran Block. This trench was fed with detrital volcanic fragments from both sides: T-MORB basalt fragments from the ocean-side, and arc-type basalts from the continent-side. (f) The meta-ophiolites of Khoy probably also represent slow-spreading conditions, and the porphyroclastic to mylonitic tectonites preserved in these metamorphic slices attest to extreme conditions of ductile shearing, characteristic of oceanic fracture zones.

Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions

The multielemental composition of 31 lavas sampled in the extrusive section of the Oman ophiolite was determined by inductively coupled plasma‐source mass spectrometry (ICP‐MS). This study allowed us to define clear geochemical criteria to characterize the different lavas types in Oman. Most of the Oman ophiolite extrusive sequence is composed of lavas of composition similar to present‐day MORB with the exception of slight Nb‐Ta negative anomalies (V1 magmatism). V1 lavas display REE patterns and Zr/Hf ratios in the range of MORB (Zr/Hf = 33.15–38.7). In contrast, the overlaying V2 lavas, which outcrop only in the northern part of the ophiolite, are REE depleted relative to V1 and display low Zr/Hf ratios (23.6–30.5). V2 lavas may be further classified into two sub‐groups. V2 type I lavas display a continuous decrease from HREE to LREE. The upper V2 type II lavas are more depleted in HREE and MREE but they are enriched in LREE. They are also distinguished by their low Nb/Ta ratios (10.53–11.65) relative to other Oman basalts (V1: 12.5–14; V2 Type I: 12.35–14.4). As for N‐MORB, melting of an upwelling mantle beneath an oceanic spreading centre formed V1 lavas. V2 resulted from fluid‐enhanced melting of previously depleted mantle residual after V1 extraction. This process probably occurred at different melt/rock ratios thus resulting in the two V2 sub‐groups, and was enhanced in the northern part of the ophiolite. V3 lavas overlie the V1–V2 sequence in the Salahi area. They display incompatible element rich patterns (LREE &gt; MREE &gt; HREE) and a high Nb/Ta ratio typical of within‐plate basalts. In the different studied areas, the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data.

Ophiolites of the Kunlun Mountains, China and their tectonic implications

Three ophiolite belts, ranging in age from Cambrian to Triassic, provide valuable data on the tectonic evolution of the Kunlun Mountains which lie along the northern margin of the Tibetan Plateau. All of the ophiolites contain dismembered but nearly complete assemblages of peridotite, mafic and ultramafic cumulates, pillow and massive lavas and radiolarian cherts, (1) The oldest belt contains ophiolites of Cambrian age, but the initial spreading of the ocean basin most likely began in the Late Proterozoic. These ophiolites outcrop in the central part of the eastern Kunlun, extend for at least a few hundred kilometers, and are believed to reflect a small ocean basin. Closure of this ocean led the accretion of south Kunlun block to the north Kunlun block and Tarim craton. Ophiolitic lavas in this belt have island are affinities. (2) A second belt of early Carboniferous ophiolites extends nearly 600 km along the northern margin of the western Kunlun. Lavas in this belt are basalt, basaltic andesite and andesite, all of which have compositions characteristic of volcanic arcs. This ocean basin possibly developed on the basis of the early Paleozoic oceanic basin. Closure of this ocean basin by southward subduction in the early Permian produced a suite of calc-alkaline volcanic rocks in what is now the central part of the western Kunlun. (3) The third belt extends nearly 1200 km along the southern margin of the Eastern Kunlun and contains numerous ophiolites of Early Permian to Middle Triassic age. These ophiolites are highly tectonized, containing volcanic rocks with the geochemical characteristics of mid-ocean ridges, oceanic islands and volcanic arcs. This belt is tentatively interpreted as the suture zone between Gondwana and Eurasia.

Back-arc basin origin for the East Sulawesi ophiolite (eastern Indonesia)

The East Sulawesi ophiolite is one of the three largest ophiolites in the world. It displays all the components of a typical sequence, from residual mantle peridotites to cumulate gabbros, sheeted dolerites, and lavas of normal mid-oceanic-ridge basalt (MORB) composition. Trace element data on the lavas and dolerites, and particularly their depletion in Nb compared to neighboring incompatible elements, suggest a subduction-zone environment for their origin. The chemical similarity between the East Sulawesi ophiolite lavas and those from the Eocene Celebes Sea back-arc basin crust together with their identical age strongly suggest a back-arc tectonic environment for this ophiolite, which represents a fragment of the Eurasian plate obducted onto the East Sulawesi basement of Australian origin.

Kerguelen islands isotope geochemistry: Mantle origin and evidence for recycled material

The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian–Santonian (88–83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: (La/Yb)N = 0.6–0.9; (Nb/Th)N = 0.17–0.57; (143Nd/144Nd)i = 0.51273–0.51291; (87Sr/86Sr)i = 0.70370–0.70565; (207Pb/204Pb)i = 15.4587–15.5411; (208Pb/204Pb)i = 37.4053–38.2336; (206Pb/204Pb)i = 17.9195–18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. 87Sr/86Sr ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites. The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb)N = 0.2–2.3; (Nb/Th)N = 0.1–2.8; (143Nd/144Nd)i = 0.51264–0.51276; (87Sr/86Sr)i = 0.70386–0.70557; (207Pb/204Pb)i = 15.4888–15.5391; (208Pb/204Pb)i = 37.2729–37.8713; (206Pb/204Pb)i = 17.6296–17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb)N = 1.10–7.92; (Nb/Th)N = 0.10–0.94; but display a less radiogenic Nd isotopic composition than basalts [(143Nd/144Nd)i = 0.51263] and more radiogenic (87Sr/86Sr)i ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. 40Ar/39Ar dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3 ± 0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/Yb)N = 6–14; (Nb/Th)N = 0.23–0.76; (143Nd/144Nd)i = 0.51262–0.51271; (87Sr/86Sr)i = 0.70338–0.70551; (207Pb/204Pb)i = 15.5439–15.6158; (208Pb/204Pb)i = 38.3724–39.3623; (206Pb/204Pb)i = 18.4024–19.6744]. They have significantly more radiogenic lead isotopic compositions than ophiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In addition, this oceanic + plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb)N = 2.07–2.31; (Nb/Th)N = 0.08–0.15; (144Nd/143Nd)i = 0.51271–0.51282; (87Sr/86Sr)i = 0.70452–0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to significant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block.

Ophiolitic sulfide mineralization at Tilt Cove, Newfoundland; controls by upper mantle and crustal processes

Massive Cu-Zn sulfide (+ or - Au + or - Ag) orebodies are found at a number of localities in the Betts Cove ophiolite, the main ones being mined at Betts Cove and Tilt Cove. The mineralization is mainly pyrite and chalcopyrite, with lesser amounts of sphalerite, as well as traces of galena. Magnetite is locally abundant along with coarse specular hematite in the East mine. About 42,000 oz of gold was produced from both the massive ore and associated with sphalerite in banded and stringer sulfides within the ophiolitic pillow lavas. Mineralization is accompanied by intense black chloritization as well as more restricted stockwork pyrite and silica. The sulfide minerals exhibit the same tectonic fabrics as the host rocks, indicating their predeformation origin.Chemical data for 150 samples from the Tilt Cove area show primary chemical trends related to both olivine fractionation and advanced partial melting of the upper mantle. Silica, Fe, Mg, Ca, Na, and K were highly mobile during alteration with major variations related to gain or loss of silica--the lowest silica rocks approaching compositions of pure chlorite. Several trace elements were relatively immobile during alteration, including Ti, Zr, Y, P, Cr, and Ni. The lavas can be clearly separated into a high Zr group which overlaps samples from the ophiolite in the Betts Cove area and the compositional fields of mid-oceanic ridge basalts and island-arc tholeiites, and a low Zr group extremely depleted in both Ti and Zr. The low Zr samples, which are all from the Tilt Cove area where the largest massive sulfide orebodies are found, have a number of geochemical characteristics analogous to boninites. Rare earth element concentrations are lower in the lower Zr lavas, which have concave-upward patterns, than in the higher Zr lavas, which have convex-upward patterns.We endorse the suggestions of earlier workers that these lavas were formed by wet melting of depleted upper mantle caused by fluids produced through dehydration during subduction of previously metasomatized oceanic crust. We would go further, however, and suggest that these fluids were also enriched in base metals and sulfur, which were also incorporated into the lavas, either in solution or as immiscible sulfide droplets. The Tilt Cove orebodies were subsequently derived from these lavas by the generally accepted mechanism, i.e., leaching of metals by circulating seawater. It is estimated that the separate 3- to 5-million-ton sulfide orebodies at Tilt Cove were formed by leaching of approximately 2 km 3 of pillow lava containing approximately 300 ppm S, with no more than a approximately 200-m depth of circulation over a very broad area of approximately 12 km 2 . A smaller leaching volume would be adequate if higher concentrations of sulfur were available in the source rocks.

Ti-V plots and the petrogenesis of modern and ophiolitic lavas

Plots of Ti vs. V for many modern volcanic rock associations are diagnostic of tectonic setting and can be used to determine possible tectonic settings of ophiolites as well. The basis of this plot is the variation in the crystal/liquid partition coefficients for vanadium, which range with increasing oxygen fugacity from > 1 to ≪ 1. Since the partition coefficients for Ti are almost always ≪ 1, the depletion of V relative to Ti is a function of the fO2 of the magma and its source, the degree of partial melting, and subsequent fractional crystallization. Volcanic rocksfrom modern island arcs have Ti/V ratios of ⩽20, except for calc-alkaline volcanics which show the effects of magnetite fractionation. MORB and continental flood basalts have Ti/V ratios of about 20–50 and alkaline rocks have Ti/V generally >50. Back-arc basinbasalts may have either arc-like or MORB-like Ti/V ratios, and sample suites from single back-arc basins may have Ti/V ratios ranging from 10 to 50. This range in Ti/V ratios in samples from a restricted geographical area may be diagnostic of the back-arc setting. The Ti/V plot is applied here to published data on ophiolites from a variety of postulated settings and in general supports the conclusions of previous investigators. Ophiolites from the western Mediterranean (Corsica, northern Apennines) and the “lower” Karmoy volcanics have Ti/V trends similar to MORB; the “upper” Karmo'y volcanics have alkaline Ti/V ratios. Lavas and tonalites in the Papuan ultramafic belt, the high-Mg andesites of Cape Vogel, and the upper pillow lavas at Troodos all have Ti/V ratios < 20, consistent with formation in an island arc setting. More specific evaluation of the tectonic setting of these and otherophiolites requires application of detailed geologic and petrologic data as well as geochemistry. The Ti/V discrimination diagram, however, is a potentially powerful adjunct to these techniques.

Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt

The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria). The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas. While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin.

Boninite at a continental margin

A lava interbedded with Cretaceous sediments deposited on the continental margin of New Zealand has phenocrystic chromite, olivine, low-Ca and high-Ca pyroxenes with quench morphology, normative quartz, and relatively high MgO, Cr and Ni, but very low Ti and Zr; it is shown here to conform to the term bononite as defined by Cameron et al.1. Such rocks are usually found in geologically oceanic situations, as in the Bonin, and Mariana arc–trench systems of the Western Pacific, or in ophiolites that represent uplifted, oceanic crust formed close to oceanic plate margins, particularly spreading centres, though not necessarily at mid-ocean ridges2–6. The New Zealand boninite closely resembles certain ophiolitic lavas, yet its association is not ophiolitic, and this part of New Zealand never was, nor ever became, an oceanic spreading centre7. Coeval basalts are alkalic, not tholeiitic as is usual and the occurrence warns against the use of basalt petrochemistry as an indicator of tectonic environment in the absence of supporting geological evidence.