Ophiolite Generation Research Articles

Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori ophiolite

Abstract Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

Heterogeneous sub-ridge mantle of the Neo-Tethys: Constraints from Re-Os isotope and HSE compositions of the Xigaze ophiolites

Luqu harzburgites from the Xigaze ophiolites represent ultra-refractory mantle domains in the Yarlung-Tsangbo Suture Zone, which is accepted as relics of the Neo-Tethys. These ultra-depleted harzburgites provide an excellent opportunity to constrain the ophiolite generation from the perspectives of Re-Os isotopes and highly siderophile elements. The depleted HSE patterns of several samples suggest that these harzburgites had experienced higher extents of partial melting than the normal abyssal peridotites. These conclusions were also evidenced by the petrological observations, such as the low modal contents of clinopyroxenes and the rare occurrence of residual sulfides. Re-depleted model ages (TRD) of two Luqu harzburgites with ultra-low absolute Re abundances yield an estimate of ca. 2.0 Ga for an ancient melting event, which implies that the Luqu peridotites have ancient domains. Even though the compositions of IPGE (Os, Ir, and Ru) in all Luqu samples are quite homogeneous and comparable to those of the primitive upper mantle (PUM), the variations of PPGE (Pt and Pd) and Re indicate that the Luqu harzburgites had experienced late-stage melt-rock interaction. These infiltrating melts are constrained to be silica-rich and sulfur-saturated and might be different from the melt accounted for the metasomatic clinopyroxene. The sulfide-rich melt agents are speculated to be derived from recycling ancient components, which is supported by several samples with elevated 187Os/188Os compositions and enriched Re abundances. Considering the dynamic environments and evolution history of the Neo-Tethys, these ultra-depleted mantle domains represented by Luqu harzburgites are readily formed by multi-stage processes of partial melting and melt-rock interactions in the mid-ocean ridges. Also, fragments of ancient subducted oceanic slabs within the mantle probably provide the sources of the recycling melts pervasive in the Xigaze mantle.

Single subduction zone for the generation of Devonian ophiolites and high‐P metamorphic belts of the Variscan Orogen (NW Iberia)

AbstractWithin the Variscan Orogen, Early Devonian and Late Devonian high‐P belts separated by mid‐Devonian ophiolites can be interpreted as having formed in a single subduction zone. Early Devonian convergence nucleated a Laurussia‐dipping subduction zone from an inherited lithospheric neck (peri‐Gondwanan Cambrian back‐arc). Slab‐retreat induced upper plate extension, mantle incursion and lower plate thermal softening, favouring slab‐detachment within the lower plate and diapiric exhumation of deep‐seated rocks through the overlying mantle up to relaminate the upper plate. Upper plate extension produced mid‐Devonian suprasubduction ocean floor spreading (Devonian ophiolites), while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication. Accretion of the remaining Cambrian ocean heralded Late Devonian subduction of inner sections of Gondwana across the same subduction zone and the underthrusting of mainland Gondwana (culmination of NW Iberian allochthonous pile). Oblique convergence favoured lateral plate sliding, and explained the different lateral positions along Gondwana of terranes separated by Palaeozoic ophiolites.

Reconciling Orogenic Drivers for the Evolution of the Bangong‐Nujiang Tethys During Middle‐Late Jurassic

AbstractIdentifying arc‐trench systems along with spatial and temporal variations in their record of tectono‐magmatic events is crucial for determining the orogenic divers and evolution of orogenic systems. New geochronological and geochemical data of Jurassic igneous rocks, as well as detrital zircon data from contemporaneous sedimentary units, within the eastern Bangong‐Nujiang suture in central Tibet indicate the existence of an approximately 1,200‐km Middle‐Late Jurassic magmatic arc system. This arc system can be divided into two distinct along‐strike segments, which are characterized by magmatic activity extending from 166 to 160 Ma in the east and 170–148 Ma in the west, followed by magmatic gaps at 160–120 and 148–125 Ma, respectively. An accretionary prism, magmatic arc, and retro‐arc sedimentary units are identified from south to north in the eastern segment. The 166–160 Ma arc includes high‐K calc‐alkaline granitoids, and high‐Mg andesites, dacites, and rhyolites, which collectively can be interpreted to originate from partial melting of ancient lower crust and mélange diapirs above a north dipping subduction zone. Our analysis reveals the existence of an overall compressional arc‐trench system along strike, which overlaps with a phase of 170–160 Ma ophiolite generation and a rock association of 160–148 Ma slab‐derived adakites and oceanic island basalt‐type rocks, and is followed by an overall magmatic gap during 148–125 Ma with subsequent 125–105 Ma extensive magmatism. We infer that these records may reflect sequential tectonic events, including subparallel ridge‐trench collision (170–160 Ma), slab window formation (160–148 Ma), subsequent subduction termination (148–125 Ma), and final Lhasa‐Qiangtang amalgamation (125–105 Ma).

The effects of partial melting, melt–mantle interaction and fractionation on ophiolite generation: Constraints from the late Cretaceous Pozantı-Karsantı ophiolite, southern Turkey

The Pozantı-Karsantı ophiolite consists of a mantle unit and overlying crustal rocks. The mantle peridotite is composed of harzburgitic to dunitic rocks that are depleted in Al2O3 and CaO compared to primitive mantle. The low whole-rock Al and Ca values are consistent with the high Cr# [= 100 × Cr/(Cr + Al)] values of spinel that range from 44 to 70. These spinels generally have low TiO2 (< 0.06 wt.%) concentrations, although spinels with high-Cr compositions in certain samples exhibit enrichments of up to 0.16 wt.% TiO2. The concentrations of chondrite-normalized rare earth elements (REEs) indicate depletion toward heavy to middle REEs. However, the entire mantle peridotite samples exhibit marked enrichment in light REEs and large-ion lithophile elements (LILEs) compared to middle REEs. The heavy REE patterns of some peridotite samples are comparable to the calculated melting curves representing various degrees of melting and are modeled by ~ 24% to 30% melting in the spinel stability field. However, some samples are more depleted in the middle REEs than heavy REEs. The patterns of heavy to middle REEs do not follow the melting lines produced by various degrees of melting in the spinel stability field. The heavy REE composition of these peridotite samples suggests that partial melting began in the garnet stability field and continued into the spinel stability field; they represent ~ 22% to 26% melting under various pressure conditions. High anorthite content of plagioclase as well as Nb-depleted composition of mafic cumulates and isotropic gabbros indicate that the crustal rocks of the Pozantı-Karsantı ophiolite formed from a melt that has been produced by melting of depleted source. This melt is thought to have enriched in light REE and fluid mobile elements that were released from the subducting slab during the subduction of Neotethyan ocean. The whole-rock light REE and LILE enrichments and the higher TiO2 concentrations of the high-Cr# spinels in some mantle peridotite samples cannot be explained by simple melt extraction in a mid-ocean ridge (MOR) environment and require melting and enrichment processes in a supra-subduction zone tectonic setting. However, these features of the peridotites can be explained by the interaction of light REEs and TiO2-rich melts and fluids with peridotites that were depleted during melting events in an MOR environment. This interaction, took place in a supra-subduction zone environment, may have increased the light REE concentrations in the peridotites and caused the re-equilibration of the Ti-depleted spinel to produce Ti-richer spinel and therefore explains the geochemical signatures of the Pozantı-Karsantı peridotites.

Rapid forearc spreading between 130 and 120 Ma: Evidence from geochronology and geochemistry of the Xigaze ophiolite, southern Tibet

The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone (YZSZ, Tibet) which also includes the Gangdese arc and the Xigaze forearc basin. This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped Rare Earth Element (REE) patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of Light Rare Earth Element (LREE)-enriched boninitic melts. The Xigaze crustal rocks belong to two groups: Mid-Ocean Ridge Basalt (MORB)-like rocks and boninitic rocks showing a uniform LREE depletion and flat to LREE enrichment on chondrite-normalized patterns, respectively. Geochemically, both groups show the influence of subducting oceanic slab-derived fluids. LA-ICPMS zircon U–Pb and Lu–Hf analyses from dolerite and quartz diorite dikes, which intruded into the mantle peridotite, and dolerite sheeted sills show that they were generated between 127 and 124Ma. The zircons possess positive εHf(t) values ranging from +7.5 to +17.3. Taking into account the geological and geochronological characteristics of the central-western YZSZ, we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab rollback during subduction initiation between 130 and 120Ma. Following this stage of spreading, the forearc was stabilized and the zone of melting migrated beneath the Gangdese arc producing the voluminous Late Cretaceous granitoids displaying depleted mantle-type Hf isotopic compositions. Our model provides a new explanation for the generation and evolution of forearc-type ophiolites.

Geochemistry and tectonic significance of ophiolites along the İzmir–Ankara–Erzincan Suture Zone in northeastern Anatolia

Abstract The Ankara–Erzincan suture zone includes large bodies of ophiolite and ophiolitic melange in northeastern Anatolia. The ophiolitic bodies are (1) Refahiye (Erzincan), (2) Şahvelet (Erzurum), (3) Karadağ (Erzurum) and (4) Kırdağ (Erzurum). The ophiolite-related units include well-preserved sections of oceanic lithospheric and accretionary melanges with local blueschist assemblages. The ophiolite-related units in NE Anatolia are unconformably overlain by Campanian–Maastrichtian-aged sediments that were later imbricated with the ophiolitic rocks. Geochemical data for the individual ophiolite sections indicate a tholeiitic composition, depletion in Nb, enrichment in large ion lithophile elements, parallel to slightly depleted high field strength element patterns (compared with normal-mid ocean ridge basalt), and slightly light rare earth element-depleted to parallel rare earth element trends. These features suggest progressive source depletion towards island arc tholeiites and finally boninites. A fore-arc setting is proposed for the generation of the ophiolites. In contrast, the volcanic rocks from the mélange units exhibit tholeiitic to alkaline compositions and either depletion or enrichment of rare earth element and high field strength elements. Seamount-type alkaline and subduction-related tholeiitic basaltic rocks were apparently juxtaposed during subduction/accretion. Models involving either a single north-dipping subduction zone or two north-dipping subduction zones may be applicable. Both models involve the generation of supra-subduction zone-type ophiolites in a forearc setting, an accretionary prism (with blueschists) and a volcanic arc during the Late Cretaceous. The ophiolites, ophiolitic melange and related blueschists were emplaced either northwards onto the Pontide margin or southwards over the passive margin of the Tauride platform.

The neoarchean ophiolite in the North China craton: Early precambrian plate tectonics and scientific debate

Archean greenstone belts and their possible inclusion of fragments of ophiolites is an important research subject, since it is correlated with the nature of early oceanic crust, and can yield information on the nature of early planetary lithospheres, the origin of TTG (tonalite-trondhjemite-granodiorite) continental crust, the formation of early cratons and continents, and is related to when plate tectonics started in the Earth’s evolutionary history. This article briefly reviews the North China craton’s Archean ophiolite argument and proposes further studies aimed at understanding the generation of greenstone belts and Archean ophiolites, and suggests some key scientific questions that remain to be answered.

Isotope dating of Lherzolite-type ophiolites in the Tamvatnei massif, Koryak-Kamchatka fold region: Zircon-based SHRIMP U-Pb data on quartz dolerite

The Tamvatnei ophiolite massif is located in the external part of the Koryak-Kamchatka accretionary orogen, within the limits of the Anadyr-Koryak Fold System (Late Cretaceous accretion), composed largely of complexes of enzymatic island arcs and backarc basins referred to the Jurassic-Early Neocomian. Lherzolite type ophiolites of the Tamvatnei massif, located in the harzburgite province of the Mainitsa and Algan-Velikorechenskii terranes and being a regional anomaly in terms of geological structure and rock composition, were not dated by paleontological methods. The geological data indicate only their pre-Senonian age of formation, and K-Ar datings for mafites by the rock bulk compositions cover nearly the whole Early Cretaceous. The isotope dating of ten accessory zircon grains from quartz diorite, which finished the formation of magnesian series of gabbro-diabases belonging to the Tamvatnei hypoabyssal ophiolite complex and that of magnesian andesites from the lower stratum of the volcanogenetic complex, yielded a concordant age of 138.9 ± 1.9 Ma. These results, as well as the previously acquired date for amphibole from amphibole microgabbro belonging to the hypoabyssal complex (120.0 ± 2.4 Ma; after P. U. Layer), indicate that Tamvatnei lherzolite-type ophiolites formed in the Valanginian-Aptian, i.e., later than the Late Mesozoic suprasubduction ophiolites from the adjacent Mainitsa and Algan terranes, whose volcanogenic-siliceous strata are dated at the Jurassic-Valanginian. The obtained results prove the model for Tamvatnei ophiolite generation in an introarc basin founded during breakup of the crust of the Mainitsa-Algan island-arc system.

Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology

Ophiolites are on-land remnants of oceanic lithosphere, and most of the more extensive ophiolites apparently formed above a subduction zone, a tectonic setting known as a suprasubduction-zone setting. Thin sheets of high-grade metamorphic rocks, known as metamorphic soles, crop out structurally beneath many suprasubduction-zone ophiolites. Such rocks may have formed during the inception of subduction beneath young and hot oceanic lithosphere. Disagreement exists as to whether suprasubduction-zone ophiolites are emplaced over the same subduction zone over which they once formed or over a later one. High-grade metamorphic rocks (blocks-in-melange and coherent sheets) from the Franciscan Complex may represent a metamorphic sole beneath the suprasubduction-zone Coast Range ophiolite. Trace-element and isotopic data indicate that the Franciscan high-grade metamorphic rocks formed in a suprasubduction-zone envi ronment, requiring the existence of a pre-Franciscan sub duction zone, whereas later-subducted, lower-grade oceanic rocks are of mid-ocean-ridge or oceanic-island basalt affi ni ties. The Coast Range ophiolite and Franciscan high-grade rock protoliths formed over a pre-Franciscan subduction zone that may have dipped westward. The high-grade Franciscan rocks were metamorphosed at the inception of east-dipping subduction beneath the Coast Range ophiolite, and the ophiolite was subsequently emplaced over this later subduction zone. Suprasubduction-zone protolith signatures have been obtained for other metamorphic soles beneath suprasubduction-zone ophiolites, suggesting that our proposed model of suprasubduction-zone ophiolite generation over one subduction zone and emplacement over a second one may be globally applicable. Regional geology suggests that this dual subduction-zone model may also apply to suprasubduction-zone ophiolites with midocean-ridge and/or oceanic-island basalt soles.

Geochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoic volcanic rocks from the Central Eastern Desert of Egypt: New insights into the ∼750 Ma crust-forming event

Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt. These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191ppm) and Ni (69ppm). Their REE patterns are slightly fractionated [(La/Yb)N=1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial ɛNd (+5.1 to +8.9) and Nd model ages (0.64–0.79Ga) indicate that the crust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavolcanic rocks are dated at ∼750Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust.

Radiolarian biostratigraphic constraints on the generation of the Nidar ophiolite and the onset of Dras arc volcanism: Tracing the evolution of the closing Tethys along the Indus – Yarlung-Tsangpo suture

The Nidar ophiolite in Ladakh Himalaya is a substantial fragment of oceanic lithosphere preserved along the western continuance of the Indus - Yarlung-Tsangpo suture zone. It was generated in a supra-subduction zone setting and is linked in development to the Dras intra-Tethyan volcanic arc. Earlier reported Hauterivian - Aptian radiolarians from the volcano-sedimentary succession atop the ophiolite were used to constrain the age of the ophiolite and duration of arc volcanism. Re-assessment of radiolarian taxonomy and biostratigraphy reveals an upper Barremian to upper Aptian range for the Nidar volcano-sedimentary section, the same as for the marine sedimentary cover atop the Dazhuqu ophiolite in the southern Tibet. The lower limit of its stratigraphic range places a minimum temporal constraint on the Nidar ophiolite generation in the late Barremian (ca 126 Ma). This compares well with radiometric age for the ophiolite (124± 1 Ma). The sedimentary successions of both ophiolites bear resemblance in the record of associated arc volcanism. Dras arc volcanism in the Nidar section began in the early Aptian. The Nidar and Dazhuqu ophiolites are interpreted as distal chronological equivalents developed in association with the same intra-Tethyan subduction system. Parallels between the evolution of this Cretaceous intra-Tethyan subduction system and other systems in the region remain uncertain.

Linking accretionary orogenesis with supercontinent assembly

Age relations for assembly of Gondwana and Pangea indicate that the timing of collisional orogenesis between amalgamating continental bodies was synchronous with subduction initiation and contractional orogenesis within accretionary orogens located along the margins of these supercontinents. Final assembly of Gondwana occurred between c.570 and 510 Ma, amalgamating the various components of East and West Gondwana. This was coeval with a switch from passive margin sedimentation to convergent margin activity along the Pacific margin of the supercontinent. Timing of subduction initiation along the Pacific margin ranges from 580 to 550 Ma as evidenced by the first appearance of arc derived detrital zircons in the upper Byrd Group sediments and the oldest supra-subduction zone plutons along the Antarctic segment of the margin. A phase of extension marked by supra-subduction zone ophiolite generation at 535–520 Ma is preserved in greenstone successions in eastern Australia and overlaps the onset of Ross–Delamerian contractional orogenesis between 520 and 490 Ma, inboard of the plate margin that coincides with the cessation of collisional orogenesis between the amalgamating blocks of Gondwana. Supra-subduction zone igneous activity was continuous throughout this period indicating that subduction was ongoing. The final stages of assembly of the Pangean supercontinent occurred between c.320 and 250 Ma. Major plate boundary reorganization during this time was accompanied by regional orogenesis along the Pacific margin. The East Gondwana margin segment experienced transpressional and transtensional activity from c.305 Ma until c.270 Ma, after which convergence along the plate margin was re-established. In eastern Australia this involved a migration of arc magmatism eastward into the old subduction complex indicating a stepping out of the plate margin. Synchronous with this phase of plate re-adjustment was the Gondwanide Orogeny (305–230 Ma) affecting the entire Pacific margin of Pangea. Temporal relations across supercontinents between interior collisional and marginal accretionary orogenies suggest a linked history between interior and exterior processes perhaps related to global plate kinematic adjustments. Orogenesis in accretionary orogens occurs in the absence of colliding bodies during ongoing subduction and plate convergence and must therefore be driven by a transitory coupling across the plate boundary. Correspondence of coupling with, or immediately following, subduction initiation and plate boundary reorganization, suggests it may reflect plate re-adjustments involving a temporary phase of increased relative convergence across the plate boundary.

Multiple ophiolite generation preserved in the northern Philippines and the growth of an island arc complex

Although all oceanic arcs grow through the addition of subduction-generated magmas, the geology of the northern Philippines demonstrates that a major contribution to arc crustal growth can come from repeated, episodic, intra-arc, back-arc, and/or fore-arc oceanic crust generation with subsequent preservation of the basic–ultrabasic units in the arc complex. At least five episodes of oceanic crust generation are represented in the northern Philippines by preserved ophiolitic sequences and recent intra-arc seafloor spreading. Each episode is distinct in age as confirmed by modern dating techniques, with the ages ranging from pre(?)-Jurassic to ∼Quaternary. Although the Philippines is widely regarded as an amalgamation of allochthonous terranes, a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the Philippine arc complex. Rather, the evidence suggests that most—and possibly all—of the ophiolites were generated as back-arc, fore-arc, or intra-arc crust within the Philippine arc complex. Hence, there is a close spatial association of several ophiolitic terranes of diverse ages spanning ∼150 Myr that formed as part of the arc complex. Such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting, which has experienced changing and possibly overlapping subduction from the east and west sides (in the current reference frame). Disruption of the ophiolitic basement terranes has been, and continues to be, effected primarily by wrench faulting. This style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite-arc terranes in continents and the petrologic evolution of island arcs.

Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung–Tsangpo suture zone, Tibet

Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung–Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (&lt;10 Ma) interval from late Barremian to late Aptian. Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins associated with various spreading centres. An increasing flux of arc-derived volcaniclastic sediment up-section indicates nearby volcanic arc activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.

Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung Tsangpo suture zone, Tibet

Abstract Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and 40Ar/39Ar geochronology

Cordilleran ophiolites typically occur as basement for accreted terranes. In the Klamath Mountains, ophiolitic terranes were progressively accreted by underthrusting beneath North America. The Josephine ophiolite is the youngest of the Klamath ophiolites and forms the basement for a thick Late Jurassic flysch sequence (Galice Formation). This ophiolite‐flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the Klamath Mountains above and a coeval plutonic‐volcanic arc complex below. The outcrop pattern of the roof (Orleans) thrust indicates a minimum displacement of 40 km, and geophysical studies suggest &gt;110 km of displacement. The basal (Madstone Cabin) thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km. A rapid sequence of events, from ophiolite generation to thrust emplacement, has been determined using 40Ar/39Ar and Pb/U geochronology. Ophiolite generation occurred at 162–164 Ma, a thin hemipelagic sequence was deposited from 162 to 157 Ma, and flysch deposition took place between 157 and 150 Ma. Tight age constraints on thrusting and low‐grade metamorphism associated with ophiolite emplacement (Nevadan orogeny) are provided by abundant calc‐alkaline dikes and plutons ranging in age from 151 to 139 Ma. Deformation and metamorphism related to the Nevadan orogeny appears to have extended from ∼155 to 135 Ma. Most of the crustal shortening took place by thrusting, constrained to have occurred from ∼155 to 150 Ma on both the roof and basal thrusts. Minimum rates of displacement are 2.4 and 3.6 mm/year for the basal and roof thrusts, respectively, but correlations with coeval thrusts yield rates of 8.4 and 22 mm/year (within the range of plate velocities). The high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western North America and an active arc from ∼155 to 150 Ma. A steep thermal gradient was present in the Josephine‐Galice thrust sheet from ∼155 to 150 Ma, with amphibolite facies conditions developed along the basal thrust. After accretion of the ophiolite by underthrusting, the ophiolite and overlying flysch underwent low‐grade dynamothermal regional metamorphism from 150 to 135 Ma. The upper age limit is tightly constrained by a 135 Ma K‐feldspar cooling age, syntectonic plutons as young as 139 Ma, and a Lower Cretaceous angular unconformity. Very rapid exhumation is indicated by the late Valanginian to Hauterivian age (∼130 Ma) of the unconformably overlying strata, suggesting unroofing by extensional tectonics.

A Cretaceous to Paleocene‐Eocene South China sea basin origin for the Zambales Ophiolite Complex, Luzon, Philippines?

Abstract The Eocene Zambales Ophiolite Complex that exhibits transitional mid‐ocean ridge basalt‐island arc tholeiite (MORB‐IAT) characteristics was formed in a subduction‐related marginal basin. The different surrounding marginal basins of the Philippines, namely, the South China Sea, Sulu Sea Basin, Celebes Basin and the West Philippine Basin have all been modeled to be of probable provenance of this ophiolite complex. Certain information (e.g. age, rock geochemistry, paleomagnetic rotations) and limitations, nevertheless, are inconsistent with the ophiolite complex being generated in these regions. Recent geophysical evidence suggests that the southwest sub‐basin of the South China Sea Basin is probably Cretaceous to Paleocene‐Eocene in age. This makes it possible to speculate that the Zambales Ophiolite Complex could have come from this sub‐basin. The present day rifting of the southern Izu‐Mariana arc can be taken as a modern day analog of this type of ophiolite generation.

Dating of late Proterozoic ophiolites in Egypt and the Sudan using the single grain zircon evaporation technique

Zircons from gabbro and plagiogranite in late Proterozoic ophiolites of the Arabian-Nubian Shield (ANS) in Egypt and the Sudan, as well as post-ophiolite granites have been dated using the stepwise evaporation method. Zircons from a plagiogranite in the Wadi Ghadir ophiolite, Eastern Desert of Egypt, yielded a mean 207Pb/ 206Pb age of 746±19 Ma, while a gabbro and diorite associated with the Abu Swayel ophiolite nappe ∼ 250 km southwest of Wadi Ghadir provided zircon 207Pb/ 206Pb ages of 729±17 and 736±11 Ma, respectively. Zircons from layered gabbro along the western margin of the Jabal Gerf ophiolite just north of 22°N were dated at 741±21 Ma, indistinguishable from the Wadi Ghadir and Abu Swayel results. A plagiogranite sample from the Onib ophiolite in the northern Red Sea Hills, Sudan, contains zircons with a 207Pb/ 206Pb age of 808±14 Ma, while granites which crosscut layered gabbro in this complex have zircon ages of 713±12 and 714±5 Ma respectively. The youngest granite at Onib was dated at 646±10 Ma and may reflect a phase of postaccretionary intraplate magmatism. Our zircon data document ocean crust generation between ∼ 810 and ∼ 730 Ma ago in the Nubian segment of the ANS, and the age data compare well with ophiolite generation in Saudi Arabia. However, some of our data cast doubt on large-scale correlations of suture belts between the Arabian and Nubian segments of the shield.

Generation and obduction of ophiolites: Constraints from the Bay of Islands Complex, western Newfoundland

Constraints provided by the Bay of Islands ophiolite complex (BOIC), western Newfoundland, suggest that its generation, as well as its obduction, is related to collision of the Dunnage Zone island arc system with the irregular continental margin of eastern North America. The constraints include the supra‐subduction zone setting of the BOIC; its extensional environment of generation; the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the North American continental margin; and the restriction of the BOIC, as well as other Appalachian ophiolites and associated Taconian allochthons, to reentrants in the North American margin. During the early phases of collision of the Dunnage arc with the North American margin, subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation. The first develops through trench rollback, leading to ophiolite generation in the overriding arc plate outboard of the reentrant, whereas the second develops in a transtensional environment with spreading centers offset by strike‐slip faults which propagate through the overriding plate away from the indenter promontory. The configuration of the BOIC spreading ridge normal to the continental margin, and the direction of initial obduction parallel to the margin, favor generation of the ophiolite in an overall strike‐slip regime. With complete subduction of the oceanic remnant, extension in the overriding plate ceases, and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin. Diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the Dunnage Zone, and for termination of arc volcanism along the North American margin.