Ophiolitic Basalts Research Articles

Petrogenesis of mafic rocks from northwest Iran (Piranshahr) and comparison with northeast Iraq ophiolites: Implications for slab window magmatism in an evolving Neotethys arc

AbstractThe Piranshahr ophiolite is located in northwest Iran. The Mawat ophiolite in northeast Iraq is the western continuation of the Piranshahr ophiolite. The whole rock geochemistry of mafic rocks from the Piranshahr ophiolite (in the Gerdikavalan regions) is studied and compared with mafic rocks from the Mawat ophiolite in northeast Iraq (in the Hasanbag, Walash and Naopurdan regions). Two groups can be identified among the ophiolitic basalts in northwest Iran and northeast Iraq. The first one is represented by Late Cretaceous calc‐alkaline basalts, displaying typical active continental margin signatures and thought to have been derived from the melting of enriched mantle sources modified by continental crust and subduction fluids. The second group is characterized by Eocene–Oligocene tholeiitic basalts, displaying signatures intermediate between mid‐ocean ridge basalt (MORB) and oceanic island basalt (OIB) and back‐arc basin (BAB) like magmas and suggested to have been derived from the melting of mixed asthenospheric‐lithospheric mantle sources in a slab window. We propose that the subduction signature is acquired by interaction of the mantle advected through the slab window in the upper part of the subducting plate.

Makran ophiolitic basalts (SE Iran) record Late Cretaceous Neotethys plume-ridge interaction

ABSTRACT The Makran complex in southeast Iran provides a spectacular subduction-related accretionary complex to understand the mechanism of oceanic accretion and the evolution of subduction zones. In this paper, we present new major and trace element data as well as isotopic compositions of mafic volcanic blocks from the Makran ophiolitic mélange complex (OMC). Our aim is to assess the genesis of these rocks and discuss their implications on the evolution of Neotethys Ocean. These volcanic blocks are composed mainly of basalts with minor trachytes. The Makran lavas are occasionally interlayered with tuff layers. Zircons from these tuffs give U-Pb ages of 95 Ma, which is well in accordance with the reposted microfossil data for the interlayered pelagic limestones with pillow lavas. Makran basalts can be geochemically subdivided into four groups; normal to transitional MORB, enriched-MORB, Plume-type MORB and alkaline (-OIB-like) basalts. The OIB-like pillow lavas are represented by high values of Th/Tb (6.3–7.4) which are higher than other basalts (group 1 = 0.3–0.8; groups 2 = 0.7–1.6; group 3 = 1.58–1.36).143Nd/144Nd(t) ratios for basalts ranges from 0.51247 to 0.51292, whereas 87Sr/86Sr(t) isotopic composition of the OMC lavas varies from 0.704433 to 0.709466. The Pb isotopic composition of the lavas are quite high, ranging from 15.49–15.66 for 207Pb/204Pb(t), 18.09–19.12 for 206Pb/204Pb(t) and 37.80–39.23 for 208Pb/204Pb(t). The chemistry of these rocks suggests that they were formed most likely in an oceanic setting with clear plume-ridge interaction. These rocks can form from partial melting of a highly heterogeneous mantle source, which is extensively metasomatized with deep mantle OIB-type components. We suggest these rocks have been generated in an oceanic ridge with plume-ridge interaction, similar to the Iceland-Reykjanes Ridge, before being fragmented and accreted into the Makran accretionary complex.

The crust-mantle evolution of the Anabar tectonic province in the Siberian Craton: Coupled or decoupled?

To clarify the tectonic-thermal evolution of the Anabar tectonic province in the central Siberian Craton, we performed an isotope-geochemical study of 20 xenoliths from the Udachnaya, Zarnitsa, and Komsomolskaya kimberlite pipes to represent different crustal levels. Most mafic granulites have Proterozoic Nd model ages and geochemical characteristics close to those of intraplate basalts, whereas some mafic and intermediate granulites with Archean model ages exhibit geochemical features of supra-subduction ophiolitic basalts. Analysis of U-Pb ages and hafnium isotopic composition of zircon indicates that the main tectonic-thermal events modified the crust at 2.7 and 1.9–1.8 Ga, which is consistent with ages of mantle depletion events from previous studies. All zircons have Archean Hf model ages (TDMC3.6-3.2 Ga). Overall, thermal events with ages of 2.9–2.8, 2.7, 2.4, 1.97 and 1.8 Ga have remarkable influence on the studied zircons. Tectono-thermal events at 2.4 1.97, 1.9 and 1.8 Ga with no addition of juvenile material are recorded by zircons from xenoliths of mafic and intermediate granulites and metadiorites. A compilation of isotope-geochemical data demonstrates that instead of age-stratified, the crust of the Anabar tectonic province consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence the crust and mantle of the Siberian Craton has been coupled since the Paleoarchean.

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran: Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.

Petrological and tectono-magmatic significance of ophiolitic basalts from the Elba Island within the Alpine Corsica-Northern Apennine system

Two distinct ophiolitic units, which represent remnants of the Jurassic Ligurian-Piedmont Ocean, crop out in the Elba Island. They are the Monte Strega unit in central-eastern Elba and the Punta Polveraia-Fetovaia unit in western Elba. Ophiolitic rocks from the Monte Strega unit are commonly affected by ocean floor metamorphism, whereas those from the Punta Polveraia-Fetovaia unit are affected to various extent by thermal metamorphism associated with the Late Miocene Monte Capanne monzogranitic intrusion. Both ophiolitic units include pillow lavas and dykes with compositions ranging from basalt to basaltic andesite, Fe-basalt, and Fe-basaltic andesite. Basaltic rocks from these distinct ophiolitic units show no chemical differences, apart those due to fractional crystallization processes. They display a clear tholeiitic nature with low Nb/Y ratios and relatively high TiO2, P2O5, Zr, and Y contents. They generally display flat N-MORB normalized high field strength element patterns, which are similar to those of N-MORB. Chondrite-normalized rare earth element patterns show light REE / middle REE (LREE/MREE) depletion and marked heavy (H-) REE fractionation with respect to MREE. This HREE/MREE depletion indicates a garnet signature of their mantle sources. Accordingly, they can be classified as garnet-influenced MORB (G-MORB), based on Th, Nb, Ce, Dy, and Yb systematics. We suggest that the Elba Island ophiolitic basalts were generated at a magma starved, slow-spreading mid-ocean ridge. REE, Th, and Nb partial melting modelling shows that the compositions of the relatively primitive Elba Island ophiolitic basalts are compatible with partial melting of a depleted MORB mantle (DMM) source bearing garnet-pyroxenite relics. Hygromagmatophile element ratios suggest that basalts from both ophiolitic units were originated from chemically very similar mantle sources. A comparison with basalts and metabasalts from Alpine Corsica and northern Apennine ophiolitic units shows that the composition of the inferred mantle source for the Elba Island basalts is similar to that of some Lower Schistes Lustres metabasalts of Alpine Corsica ophiolites, and some basalts from the Internal Ligurian units of northern Apennine. In contrast, it slightly differs from those of other ophiolitic units of Alpine Corsica and northern Apennine. The chemical differences observed between basalts and metabasalts from different Ligurian-Piedmont ophiolitic units were likely associated with different partial melting degrees of either DMM source or garnet-pyroxenite relics and/or different mixing proportions of melts derived from them, as well as to different compositions of garnet-pyroxenite relics.

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.

Late Devonian–early Permian accretionary orogenesis along the North Tianshan in the southern Central Asian Orogenic Belt

The Palaeozoic orogenic process in the North Tianshan of the southern Central Asian Orogenic Belt is controversial. Systematic field study indicates that the ophiolitic fragments of the North Tianshan are mainly thrust slices and blocks of a late Palaeozoic accretionary complex, which was intruded by granitoids. U-Pb zircon dating of plagiogranites from the North Tianshan ophiolite yielded a mean age of 343.1 ± 2.7 Ma. These are typical oceanic plagiogranites but with a supra-subduction zone (SSZ) signature. Ophiolitic basalts display N-MORB, E-MORB, and OIB compositions. One gabbro with an age of 301.9 ± 2.2 Ma shows E-MORB geochemistry mixed with N-MORB and OIB. Some andesites show clear island arc characters indicated by enrichment of LILEs relative to HFSEs. Mean ages of 344.9 ± 4.2 and 298.7 ± 2.4 Ma were obtained for a granite porphyry and a mylonitic granite, respectively. The two granitoids display an island arc geochemical signature evidenced by enrichment of LILEs and depletion of HFSEs. Combined with an eastward migration of Late Devonian to Carboniferous arc magmatism and related Cu-Au-Mo deposits, we propose that trench retreat and slab roll-back took place during subduction of the Junggar Ocean spreading ridge beneath the North Tianshan arc, and that the accretion may have lasted into early Permian time, an important late stage of the long-lived accretionary orogenesis in the southern Central Asian Orogenic Belt.

A new method of discriminating different types of post-Archean ophiolitic basalts and their tectonic significance using Th-Nb and Ce-Dy-Yb systematics

In this paper, a new discrimination diagram using absolute measures of Th and Nb is applied to post-Archean ophiolites to best discriminate a large number of different ophiolitic basalts. This diagram was obtained using >2000 known ophiolitic basalts and was tested using ∼560 modern rocks from known tectonic settings. Ten different basaltic varieties from worldwide ophiolitic complexes have been examined. They include two basaltic types that have never been considered before, which are: (1) medium-Ti basalts (MTB) generated at nascent forearc settings; (2) a type of mid-ocean ridge basalts showing garnet signature (G-MORB) that characterizes Alpine-type (i.e., non volcanic) rifted margins and ocean-continent transition zones (OCTZ). In the Th-Nb diagram, basalts generated in oceanic subduction-unrelated settings, rifted margins, and OCTZ can be distinguished from subduction-related basalts with a misclassification rate <1%. This diagram highlights the chemical variation of oceanic, rifted margin, and OCTZ basalts from depleted compositions to progressively more enriched compositions reflecting, in turn, the variance of source composition and degree of melting within the MORB-OIB array. It also highlights the chemical contributions of enriched (OIB-type) components to mantle sources. Enrichment of Th relative to Nb is particularly effective for highlighting crustal input via subduction or crustal contamination. Basalts formed at continental margin arcs and island arc with a complex polygenetic crust can be distinguished from those generated in intra-oceanic arcs in supra-subduction zones (SSZ) with a misclassification rate <1%. Within the SSZ group, two sub-settings can be recognized with a misclassification rate <0.5%. They are: (1) SSZ influenced by chemical contribution from subduction-derived components (forearc and intra-arc sub-settings) characterized by island arc tholeiitic (IAT) and boninitic basalts; (2) SSZ with no contribution from subduction-derived components (nascent forearc sub-settings) characterized by MTBs and depleted-MORBs. Two additional discrimination diagrams are proposed: (1) a Dy-Yb diagram is used for discriminating boninite and IAT basalts; (2) a Ce/Yb-Dy/Yb diagram is used for discriminating G-MORBs and normal MORBs. The proposed method may effectively assist in recovering the tectonic affinity of ancient ophiolites, which is fundamental for establishing the geodynamic evolution of ancient oceanic and continental domains, as well as orogenic belts.

Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean–continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66–71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (CeN/SmN=0.1–0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet+clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis, Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm–Nd and Lu–Hf isotope correlations. The Nd–Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (εNd=+4.7 to +7.6, εHf=+4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (δ18O=+4.9‰ to +5.2‰) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts.

Large P–T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite mélange

The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite mélange that contains blocks of mafic blueschist and high-pressure, granulite facies, metapyroxenite (Sm–Nd metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite facies assemblages corresponding to the high-pressure intermediate-type metamorphic facies series. Adjacent rocks of the serpentinite mélange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite facies), dolerite (actinolite–oligoclase sub-facies) and gabbro (amphibolite facies), all with assemblages that are diagnostic of the low-pressure metamorphic facies series. The difference in metamorphic facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite mélange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite mélange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.

Mid-Cretaceous Hawaiian tholeiites preserved in Kamchatka

We report geochemical data on a peculiar group of Albian-Cenomanian (120-93 Ma) basalts preserved in ophiolites on the Kamchatsky Mys peninsula (Kamchatka, Russia) that share trace element and isotopic compositions with enriched tholeiites from the Detroit and Meiji Seamounts in the Hawaiian-Emperor Seamount chain. Melt inclusions in chromium spinel from these rocks, representative of melt composition unaffected by post-magmatic alteration, exhibit Hawaiian-type (Th/Ba)n (0.25-0.77; i.e., distinctively low compared to the majority of oceanic island basalts and mid-oceanic ridge basalts). Low 208 Pb*/ 206 Pb* of ~0.93 in rocks and high (Nb/La)n = 1.1-4.6 in melt inclusions suggest the presence of a distinctive Kea-type component in their source. We propose that the ophiolitic basalts represent older (Early to middle Cretaceous) products of the Hawaiian hotspot (older than preserved on the northwest Pacifi c seafl oor) that were accreted to the forearc of Kamchatka. The presence of similar compositional components in modern and Cretaceous Hawaiian hotspot lavas suggests a persistent yet heterogeneous composition of the mantle plume, which may have sampled ≥15% of the core-mantle boundary layer over the past ~100 m.y.

Diachronism in the sedimentary cover around the Troodos ophiolitic massif (Cyprus)

Abstract The detailed micropaleontologic study of the sedimentary series overlying the youngest ophiolitic basalts of the Troodos massif (Cyprus), confirms the late Campanian age of the top of the oldest terrigenous cover (Kannaviou Formation) topping these ophiolites themselves assigned to Cenomanian-Turonian. It also demonstrates for the first time the strong diachronism of the Lefkara Formation chalky limestones, unconformably overlying either the Kannaviou Fm. sandstones and clays, or the oceanic basalts. This diachronism over short distances and the on-lap of the Lefkara Fm. are revealed by varying stratigraphic gaps within the Paleocene-Miocene interval and are indicated by new age datings of the transgressive basal part of the Lefkara Fm. along six principal sections: late Thanetian (Zone P5) at the Pano Lefkara typelocality; early Ypresian (Sub-zone P6a) at Agia Anna; earliest Bartonian (Zone P13) at Kannaviou and Analiontas; early-middle Lutetian (Zones P10 to P11) at Kofinou; Langhian (Zone N8) at Choulou. Maastrichtian deposits are missing and Maastrichtian Globotruncanidae/Heterohelicidae only observed as individual reworked specimens within well-dated Tertiary levels. Finally, we discuss the influence of the Tertiary extensional brittle tectonics on this diachronism, the on-lap processes and the differential subsidences occurring after the late Cretaceous/? early-middle Paleocene folding (Laramian compression).

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.

THE BARGONASCO - UPPER VAL GRAVEGLIA OPHIOLITIC SUCCESSION, NORTHERN APENNINES, ITALY

The Northern Apennines ophiolitic successions pertain to the Ligurian orogenic units, which were thrust onto the Adria continental margin (Tuscan and Umbrian Units) and show two different types of stratigraphic settings (Fig. 1) (Abbate et al., 1980): a- as “oceanic basement” of the Vara Supergroup, one of the higher units of the tectonic pile (“Internal Ligurides”); b- as olistoliths of variable sizes, accompanied by ophiolitic breccias, included in the Upper Cretaceous - Eocene terrigenous successions of the “External Ligurides” tectonic units. The Val Graveglia-Bargonasco area represented in the map, is located in eastern Liguria, on the northwesternmost side of the Northern Apennine, where most and more complete ophiolitic sequences of type a- crop out (the “Mt. Porcile Unit, Fig. 1). They consist of mantle peridotites (from clinopyroxene-poor lherzolites to harzburgites) and gabbros, which form the “oceanic basement”, metamorphosed and tectonically denudated before the volcano-sedimentary cover (ophiolitic breccias, basalts, cherts, carbonate and carbonate- shaly pelagites, and finally siliciclastic turbidites) was deposited. The tectonic scheme (Fig. 1, partially modified from the “schema tettonico” of the geologic map) shows that the Porcile Unit, object of our study, includes seven subunits separated by minor thrusts, i.e., from west to east, Mt. Domenico, M. Bianco, Rio Gromolo, Mt. Ciazze, M. Bocco, Rivo Orti, Mt. Tregin Subunits. The stratigraphic successions of all the Subunits pertain to the Vara Supergroup (see the stratigraphic Scheme on the geologic map). This short note summarises a more extensive one, about the geologic evolution of this same area, which will be published in the Atti Ticinensi di Scienze della Terra, Pavia, Italy, 2004. We decided to write this short summary to advertise the map of this key area for understanding the geology of the Northern Apennine ophiolites.

A comparative study of Late Cretaceous ophiolitic basalts from New Zealand and New Caledonia: implications for the tectonic evolution of the SW Pacific

Our understanding of the tectonic evolution of the SW Pacific between the Cretaceous and the Miocene is incomplete. During this period there were a number of ophiolitic complexes formed and emplaced, including the Tangihua Ophiolite Complex of New Zealand and the Poya terrane ophiolites of New Caledonia. Both ophiolitic complexes contain rocks from only the uppermost levels of the oceanic crust, have a similar age of formation, and were emplaced during the Eocene. Consequently, it is thought that the two systems are somehow related. However, the Poya terrane and the Tangihua Ophiolite Complex have significantly different geochemistry. The Poya terrane basalts are predominantly P- and N-MORB tholeiites generated during the opening of a small marginal basin north-east of New Caledonia, whereas the Tangihua basalts range from back arc tholeiites to calc-alkaline arc basalts suggesting formation in an arc–back arc setting. The similarities in age, emplacement and stratigraphy between the two systems must be a result of the tectonic regime operating within the SW Pacific as the new geochemical information clearly shows that the Poya terrane and the Tangihua Ophiolite Complex are not directly related.

Emplacement of the Resurrection Peninsula ophiolite in the southern Alaska forearc during a ridge‐trench encounter

The 57±1 Ma Resurrection Peninsula ophiolite of southern Alaska's Chugach terrane formed in a near‐trench environment as the Kula‐Farallon ridge was being subducted beneath North America. The magmatic sequence includes pillow lavas, sheeted dikes, gabbros, trondhjemites, and a poorly‐exposed ultramafic section. The lavas show mid‐ocean ridge basalt and arc‐like geochemical signatures, interpreted to reflect compositionally diverse melts derived from near‐fractional melting of a variably depleted mantle source, mixed with variable amounts of assimilated continentally‐derived flysch. A sedimentary sequence overlying the ophiolite, here named the Humpy Cove Formation of the Orca Group, preserves a continuous 3.6 plusmn;1.4 Ma record of turbidite sediments that were deposited on the ophiolite as it was transported to North America and emplaced in the Chugach accretionary prism. A transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent. Chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt, signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent. Hydrothermal metals are not resolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite, suggesting that they were diluted by turbidite sedimentation. The top of the section is truncated by the Fox Island shear zone, a 1 km thick, greenschist‐facies, west‐over‐east thrust related to the emplacement of the ophiolite into the accretionary wedge. The Fox Island shear zone is intruded by a 53.4±0.9 Ma granite, showing that the ophiolite formed, was transported to the North American continent, overthrust by a major accretionary prism‐related thrust, and intruded by granite all within 3.6±1.4 Ma. Using these estimates for the timing of formation and emplacement of the ophiolite, with palinspastic reconstructions and plate motion vector triangles, we suggest that the Resurrection Peninsula ophiolite was transported up to 500 km (461±79 km) relative to North America from the place of its formation at the Kula‐Farallon ridge prior to its emplacement into the North American margin. Since the best estimates of the amount of translation of the Chugach terrane come from paleomagnetism of the Resurrection Peninsula ophiolite, current estimates of margin‐parallel strike slip need to be reduced considerably.

Geochemical evidence used to test alternative plate tectonic models for pre-Upper Jurassic (Palaeotethyan) units in the Central Pontides, N Turkey

East–west trending, inferred Palaeotethyan subduction–accretion complexes form much of the pre-Upper Jurassic basement of the Central Pontides. A number of specific tectonic units are present that include variably metamorphosed basic igneous rocks, as dismembered ophiolites, coherent stratigraphical successions, ordered thrust sheets, and blocks in melange. About 200 samples from a c. 200 km long N–S traverse were analysed by a high-precision X-ray fluorescence technique, supplemented by electron microprobe analysis. The basic igneous rocks have not experienced significant chemical mobility after formation and are thus useful for discrimination of tectonic settings of formation using well known binary and ternary plots and mid-ocean ridge basalt (MORB)-normalized ‘spidergrams’. From N to S the main results are that basalts of the Kure Complex were erupted in a subduction-influenced tectonic setting, favouring interpretation as a dismembered ophiolite formed in a marginal basin along the S Eurasian margin. Thick, mainly volcanic, successions of the Cangaldag Complex show a subduction-influenced calc-alkaline composition, above a basement varying from MORB/island arc tholeiite type to high-Mg (boninitic) type, with an inferred origin as an oceanic arc above forearc basement. Meta-basites from the adjacent Bayam Melange indicate a supra-subduction zone origin, possibly as a dismembered forearc ophiolite (possibly related to the Cangaldag). A large overriding thrust sheet of mainly ultramafic rocks, the Elekdag, is interpreted as a supra-subduction zone ophiolite, based on depleted chrome spinel composition (analysed by microprobe). Eclogitic blocks in structurally underlying melange are of mid-ocean ridge (MOR) type. Further south, a northward-dipping, inferred subduction–accretion complex, the Domuzdag–Saraycikdag Complex, includes MOR-type meta-basites in the north, and more ‘enriched’ compositions in the south, of within-plate origin. Lastly, the geochemistry of meta-basites in the south, within the Kargi Complex, suggests a plume-MOR to within-plate type eruptive setting, without continental crustal influence. The geochemical data mainly support a northward rather than southward subduction model for the Palaeotethyan assembly of the Central Pontides, particularly since the Kure ophiolitic basalts in the north exhibit a subduction-influenced chemical signature, in contrast to all the units in the south which lack any identifiable subduction component. Comparisons with published geochemical data from the Karakaya Complex in the SW Pontides reveal similar units, interpreted as accreted MORB lithosphere and oceanic seamounts, but emphasize that the Cangaldag arc is presently the only known Palaeotethyan subduction arc-related unit in Turkey. Copyright © 1999 John Wiley & Sons, Ltd.

Supra-subduction zone ophiolites of Central Anatolia: geochemical evidence from the Sarikaraman Ophiolite, Aksaray, Turkey

AbstractThe Central Anatolian Crystalline Complex (CACC), situated between the northern and southern oceanic strands of Neotethys, contain a number of little-studied ophiolitic bodies of late Cretaceous age that have a bearing on the Mesozoic development of this region. The pillow lavas and sheeted dykes of the Sarikaraman Ophiolite were originally a comagmatic differentiated series of vesicular, aphyric and olivine-poor, plagioclase—clinopyroxene phyric tholeiites, but now exhibit greenschist facies assemblages. A set of late dolerite dykes cross-cutting the whole volcanic sequence are more chemically evolved and were probably derived from a different source. Relative to N-MORB the lavas and dykes are enriched in some LIL elements (K, Rb, Cs, U, Th and Sr) and depleted in HFS elements (Nb, Ta, Hf, Zr, Ti and Y) and light REE. In terms of immobile elements the ophiolitic basalts have the broad chemical characteristics of island are tholeiites that were formed in a supra-subduction zone setting, whereas the late dykes are more akin to N-MORB. In this respect the Sarikaraman Ophiolite is similar to other ophiolites found in the eastern Mediterranean region and emphasizes the preservation of this particular environment in the CACC. If all the Central Anatolian Ophiolites (of which the Sarikaraman Ophiolite is one example) were derived via southward thrusting from the Vardar-Izmir-Ankara-Erzincan Ocean branch to the north, age relationships suggest that this segment of ocean crust was relatively short-lived before obduction onto the CACC.

Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.

An Ordovician rift environment for the Memphremagog polymetallic massive sulphide deposit, Appalachian Ophiolite Belt, Quebec

The Memphremagog polymetallic massive sulphide deposit occurs at the contact between a pillowed volcanic sequence and a detrital sedimentary sequence consisting principally of shale and greywacke. The deposit, situated about 100 km east of Montréal in the Appalachians of southeastern Quebec, is composed of massive sulphide breccia. Major minerals include pyrrhotite, pyrite, sphalerite, galena, chalcopyrite, and calcite, with minor amounts of arsenopyrite, freibergite ((Cu,Ag,Fe)12Sb4S13), meneghinite (CuPb13Sb7S24), gudmundite (FeSbS), and kersteritic stannite (Cu2(Fe,Zn)SnS4). The mineral assemblage, metal content, and primary textures are comparable to modern sea-floor sulphide mounds enclosed by thick sedimentary sequences deposited in basins such as the Guaymas Basin in the Gulf of California.Although the footwall basalt unit is interstratified with the St-Daniel Formation of the Ophiolite Belt, it possesses a transitional geochemical signature of alkaline affinity that is uncharacteristic of ophiolitic basalts. We propose that this volcanic unit was emplaced within a transtensional basin of a rifted fore-arc system that formed during the Taconic orogeny as a result of diachronous oblique collision of an island- arc system (Ascot–Weedon?) with the North American continent. During magmatic activity, a hydrothermal system was imprinted on the volcanics and underlying sediments. Subsequent hydrothermal fluid emanations led to the formation of the Memphremagog sulphide deposit, which is quite distinct from sulphide deposits generally found within ophiolite belts.