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There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic (Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry (SIMS) zircon U-Pb dating yields a weighted mean 207Pb/206Pb age of 873±6 Ma for the granitoid. It displays mantle-like zircon eHf(eHf(873 Ma)=+9.3 to +10.9) and whole-rock Nd (eNd(873 Ma)=+3.5) values but supracrustal δ18O values, the latter mostly varying between 9‰ and 10‰. The calculated whole-rock δ18O value of 11.3‰±0.6‰ matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust (MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high δ18O values observed in ancient juvenile continental crust, such as Archean tonalite-trondhjemite-granodiorite suites.

The southern Altaids is of critical significance for a better understanding of the orogenic architecture and continental growth in Central Asia. The tectonic setting of the South Tianshan, a typical Paleozoic accretionary orogen within the southern Altaids, is controversial. This study reports new structural relations, geochemistry, and geochronology of previously assigned Late Silurian–Early Devonian strata in the Karaghol area of the South Tianshan. The constituent rocks are pillow basalts, ribbon cherts, limestones, siliceous mudstones, and turbiditic sandstones. Field relations show that these rocks are characterized by fault-bound tectonic slices and melanges with typical block-in-matrix structures. The pillow basalts show N-MORB, E-MORB, and OIB geochemical fingerprints, and our new LA-ICP-MS zircon U–Pb dates indicate that the maximum eruption age of the OIB-type basalts was ca. 386 Ma, whereas the Maximum Depositional Ages (MDAs) of the turbiditic sandstones were ca. 369 Ma and 355 Ma. The age spectra of the detrital zircons, which define prominent peaks at ca. 370 Ma, ca. 420 Ma, ca. 770 Ma, ca. 960 Ma with subordinate Proterozoic ages, are a good match with the igneous ages of intermediate to felsic magmatism in the Yili–Central Tianshan arc. We conclude that the previously assigned Late Silurian–Early Devonian strata in the Karaghol area are mostly dismembered structural components and melanges that belonged to an accretionary complex of Late Devonian to Early Carboniferous age, into which fragments of a Late Devonian seamount along with South Tianshan oceanic crustal rocks were likely emplaced during north-dipping subduction in the Early Carboniferous. This well-documented accretionary orogen in the Chinese South Tianshan is a key constraint on the accretionary orogenesis of the southern Altaids.

[Example of collonade and entablature cooling joints within a ponded lava of the Rosebank Field Upper Volcanics revealed by formation microimager log data alongside a field analogue example from the Isle of Staffa. , Abstract The Rosebank Field is located in the Faroe‐Shetland Basin and hosts hydrocarbons within siliciclastic sediments interlayered with volcanic packages of the Late Paleocene to Early Eocene aged Flett Formation. Within this study the volcanic sequences are investigated based on an integrated appraisal of available drill cuttings, sidewall cores, core and wireline logs including image log and geochemical logs from eight wells supported by 3D seismic data. The Rosebank lower (RLV), middle (RMV) and upper (RUV) volcanic sequences are inter‐layered with Colsay Member (C1–C4) fluvial to shallow marine siliciclastic intervals. A comprehensive cross‐field borehole based lithofacies interpretation is presented characterising simple, compound and ponded effusive lava flow facies along with pillow lavas, invasive lava flows, volcaniclastic sediments and complex lava–sediment interactions. Geochemical analyses of core, sidewall core, and hand‐picked cuttings spanning the field reveal separate high‐titanium (RHT) and relatively lower‐titanium (RLT) basaltic magma suites. These compositions can be identified and correlated across much of the field utilising geochemical logging data which, in combination with the geochemical analyses, reveals a two‐part stratigraphic sub‐division of each of the RLV, RMV and RUV. Geochemical logging data is also used to define a volcanic proxy (Fe/10+Ti) which utilises the elevated iron (Fe) and titanium (Ti) within all effusive and volcaniclastic basaltic lithologies to differentiate siliciclastic from volcaniclastic sediments where other logging parameters overlap. By comparing the borehole analyses with seismic data, a localised eruptive vent is interpreted within the north of the field. Finally, a cross‐field volcanic model is presented and compared with relevant global field analogues, providing a constrained spatial framework for sub‐surface modelling of inter‐volcanic sequences.]

Avroman Mountain has a northwest–southeast trend of Zagros Collisional Belt and elongates parallel to the Iranian border in Northeastern Iraq, Kurdistan Region. Previous studies found many volcanic bodies on the mountain and considered them either as Triassic and Cretaceous dismembered ophiolite fragments or as basaltic dykes. The present work focused on the restudy of these bodies and proved that they are neither dykes nor ophiolite rocks. On the contrary, the present study considers these bodies as successions of altered volcaniclastic sandstones (greywackes) and conglomerates that were transported from the Urumieh-Dokhtar Magmatic Arc (UDMA) as fresh or old reworked sediments, during Paleocene–Eocene, and deposited inside the Sanandij-Sirjan Zone (SSZ). Additionally, many clues are found that manifest their sedimentary origins such as presences of laminations, a southward decrease in grain sizes, the parallelism of all the bodies to the surrounding layer of Qulqula Radiolarian Formation (QRF) and Avroman Formation, folded successions, graded beddings, poorly sorted textures, irregular and broken grains outlines, a gradation from conglomerate to sandstone. Other evidences are absences of pillow basalts, amygdales, volcanic vents or cones, volcanic mineralizations, volcanic flow structures and textures, chilled borders, contact metamorphism, sedimentary xenoliths, volcanic bombs, or lapilli, and crosscutting relations between the sedimentary bodies and host rocks. These bodies are nearly located between the Avroman and Qulqula Radiolarian formations at the top and the base, respectively. Finally, these bodies have tectonic, depositional, and chronological close relations with the volcaniclastic successions of the Mawat, Bulfat, and Penjween areas. Stratigraphically, they belong to Walash Formation (or Group) and were deposited during the Paleocene–Eocene.

Gaussberg is a nunatak composed of lamproite pillow lava situated on the coast of East Antarctica. It is the most isolated Quaternary volcanic centre in Antarctica but it is important palaeoenvironmentally and petrologically out of all proportion to its small size. The edifice has a likely low, shield-like, morphology c. 1200 m high and possibly up to 10 km wide, which is unusually large for a lamproite construct. Gaussberg was erupted subglacially at 56 ± 5 ka, which places it late in the last glacial, close to the peak of marine isotope stage 3. The coeval ice sheet was c. 1300 m thick, and c. 420 m has been removed from the ice surface since Gaussberg erupted. Lamproite is a rare ultrapotassic mantle-derived magma, and Gaussberg is one of two type examples worldwide. Although traditionally considered as related in some way to the Kerguelen plume, it is more likely that the Gaussberg magma is a product of a separate magmatic event. It is ascribed to the storage and long-term (Gy) isolation of sediment emplaced by subduction in the Transition Zone of the deep mantle, followed by entrainment and subsequent melting in a plume.

Preserved rocks in the Jurassic Ferrar Large Igneous Province consist mainly of intrusions, and extrusive rocks, the topic of this chapter, comprise the remaining small component. They crop out in a limited number of areas in the Transantarctic Mountains and southeastern Australia. They consist of thick sequences of lavas and sporadic occurrences of volcaniclastic rocks. The latter occur mainly beneath the lavas and represent the initial eruptive activity, but also are present within the lava sequence. The majority are basaltic phreatomagmatic deposits and in at least two locations form immense phreatocauldrons filled with structureless tuff breccias and lapilli tuffs with thicknesses of as much as 400 m. Stratified sequences of tuff breccias, lapilli tuffs and tuffs are up to 200 m thick. Thin tuff beds are sparsely distributed in the lava sequences. Lava successions are mainly 400–500 m thick, and comprise individual lavas ranging from 1 to 230 m thick, although most are in the range of 10–100 m. Well-defined colonnade and entablature are seldom displayed. Lava sequences were confined topographically and locally ponded. Water played a prominent role in eruptive activity, as exhibited by phreatomagmatism, hyaloclastites, pillow lava and quenching of lavas. Vents for lavas have yet to be identified.

The newly discovered early Paleozoic Delenuoer ophiolite, in the western margin of the Central Qilian Shan, is composed of serpentinized peridotite, cumulate gabbro, diabase, massive basalt, and pillow basalt. This study presents geochronological and geochemical data for the cumulate gabbro and basalt. LA–ICP–MS U–Pb dating of zircons from the cumulate gabbro yielded a magmatic crystallization age of 472 ​± ​4 ​Ma. The basalts have normal mid-ocean ridge basalt (N-MORB) compositions and a narrow range of eNd(t) values (+4.5 to ​+5.3), which indicates they were derived from a depleted mantle source. On the basis of regional geological constraints, it is proposed that the Delenuoer ophiolite is a westward extension of the South Ophiolite Belt (Yushigou–Youhulugou–Donggou–Dongcaohe Ophiolite Belt) in the North Qilian Shan. The Delenuoer ophiolite, along with the Gulangxia–Delenuoer fault, defines the westernmost part of the tectonic boundary between the North and Central Qilian Shan. This ophiolite may have formed during southward subduction of the Qilian Ocean slab during the early Paleozoic.

How many volcanic bodies are being confused with plutonic ones worldwide? The purpose of this study is not to provide an answer to this question, but rather to illustrate this issue through an example from the French Armorican Variscides. It concerns a magmatic body cross-cutting highly strained terranes in the Ouessant Island, regarded for decades as a granitoid (monzogranite) on the basis of both its coarse-grained texture and its mineralogy. However, the volcanic origin of the metamorphosed series flanking this foliated body is here recognized by pillow lavas, deposit layers, and fiamme-bearing volcaniclastics, all emplaced onto the soft-substrate floor of a fault-bounded basin. Among other things, similarities between feldspar megacrysts/porphyrocrysts in both the volcaniclastics and the adjoining (formerly) monzogranitic massive body lead us to reinterpret the latter as a trachydacitic extrusion. In our model, the corresponding viscous lava progressively flowed in the basin, recovering the earlier volcanic formations and inducing load effects on the underlying soft sediments, along with compaction of the previously deposited pumices, to produce fiamme. The interpretation of the South-Ouessant area as a Visean transtensional volcano-sedimentary basin provides a new perspective on the distribution of the Variscan pull-apart basins in the Armorican Massif.

Volcaniclastic deposits, pillow lavas, dikes and sills of the intraplate Waiareka-Deborah Volcanic Field in North Otago were emplaced into and onto the Zealandia Eocene-Early Oligocene continental ...

The Western Kunlun orogenic belt (WKOB) located south of the Tarim Basin and the north-western margin of the Tibetan Plateau, was previously considered a complex orogenic belt that closed along the...

New and compiled geochemical, isotopic, and geochronological data allow us to propose a new explanation for Paleogene oceanic magmatic rocks along the Iran-Iraq border. These rocks are represented by a thick pile (>1000m) of pillow lavas and pelagic sediments and underlying plutonic rocks. These are sometimes argued to represent a Paleogene ophiolite but there are no associated mantle rocks. Integrated zircon U-Pb ages, bulk rock major and trace element and radiogenic isotope data indicate that these rocks are more likely related to forearc rifting due to extreme extension during Late Paleogene time which also triggered high-flux magmatism in the Urumieh-Dokhtar Magmatic Belt and exhumation of core complexes in Iran. These observations are most consistent with formation of the Paleogene oceanic igneous rocks in a >220 km long forearc rift zone. Supplementary material:https://doi.org/10.6084/m9.figshare.c.4972994

Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lat...

Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO2 activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO ( 60) and low TiO2 (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO2-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO2-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO2-H2O-CO2 system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction.

The accretionary orogeny of the Central Asian Orogenic Belt (CAOB) has been a popular topic over the past decades due to its complex tectonic process. The Carboniferous magmatism in the Sawur Mountains of the northern West Junggar plays an important role in understanding the accretionary process of West Junggar. Detailed geological mapping has revealed that the Carboniferous Heishantou Formation in the Sawur Mountains consists mainly of pillow lavas, basaltic andesites, dacites, tuffs and sedimentary rocks, which were intruded by the Lasite intrusion. The pillow lavas display low SiO2 and TiO2 as well as high MgO and FeOt contents and enrichment in light rare earth elements (LREEs) and Nb–Ta, suggesting derivation from an enriched lithospheric mantle in an oceanic island setting. With the oceanic crust subduction, they were scraped off and accreted in a fore-arc setting. Basaltic andesites exhibit high SiO2 and Al2O3 contents, low Mg#, Zr, and Nb contents as well as Nb/La ratios, and a clear depletion of Nb–Ta, indicating derivation from a mantle wedge affected by the inputs of slab-derived fluids in an island arc setting. Dacites possess calc-alkaline and adakitic geochemical features with high Sr and Ba contents, high Th/Yb ratios, enrichment in LREEs, low Nb/La ratios, a depletion of Nb–Ta and positive eHf(t) values (10.3–15.0). Accordingly, they are interpreted as being derived from the melting of subducted oceanic crust mixed with minor sediments in an island arc setting. Magmatic zircons from dacite yield a weighted 206Pb/238U mean age of 326.8 ± 1.6 Ma, constraining the crystallization age of the dacite as well as the timing of the oceanic crust subduction. The Lasite intrusion that intruded the Heishantou Formation is K-feldspar granite, exhibiting pronounced I-type affinity and a high potassic calc-alkaline series. This granite shows low Na2O and Mg# (36.9–40.3) values; low Zr and Y contents as well as 10,000 × Ga/Al ratios; high LREE, K and Pb contents; and a depletion of Nb and Ta. Altogether, the low (87Sr/86Sr)i (0.7037–0.7043) and positive eNd(t) (6.22–6.36) and eHf(t) (10.1–15.5) values suggest that the granites were derived from the partial melting of a juvenile lower crust affected by mantle materials in a subduction setting. The zircon weighted 206Pb/238U mean age of 316.6 ± 2.8 Ma represents the intrusive age of the granite and related subduction during the late Carboniferous. All of the above geochemistry and geochronology of the magmatic rocks constrain the subduction–accretion tectonics in the Sawur Mountains of the northern West Junggar during ~ 327–317 Ma and confirm the subduction of the Paleo-Asian Ocean (PAO) lasted until the late Carboniferous in this region.

Research subject. A meridional Tastau rift structure located in the Famenian sub-latitude rift system of Central Kazakhstan was investigated, including the specific features of its constituent sediments and volcanism, as well as the stages of its development.Materials and methods. The study was based on data collected during a long period of fieldwork, including a detailed study of the sequence of sedimentation processes, large-scale geocarting, sampling for various types of precision analyses.Results. The typomorphic features of the main rock types were determined. It was shown that basaltoid rocks occurred in the form of pillow structures with a hyaloclastite matrix between the pillows. Siliceous shales are saturated with thin ash silicic acid material. A geological scheme of the Tastau structure and its specific features was presented in a series of cross-sections. The developmental stages of this structure were established. The structure was referred to the rift type.Conclusion. The material and structural features of the investigated Tastau section are shown to be typical of the entire rift system of Central Kazakhstan. The pre-rift (Frasnian) stage is characterized by continental volcanism in the form of highpotassium and ultra-potassium rhyolite ignimbrites composing an encialic island-arc structure. These volcanites belong to the shoshonite and high-potassium lime-alkaline petrochemical series. The rift stage itself began in the Early Famenian age with the formation of a system of narrow deep troughs, into which the sea from the neighbouring Zhongaro-Balkhash sea region started to ingress. The conglomerates are characterized by thin flattened isometric pebbles, whose well-polished surface is similar to that of pebbles in modern sea beaches. The sandstones exhibit a high level of lamination and are characterized by poorly pelletized clastic material. The clastic material is represented by the underlying Frasnian rhyolitic ignimbrites carried in from the sides of the trough. The mature rift stage is characterized by the accumulation (under marine conditions) of pillow basalts and hyaloclastites and thin-layered silicite-clay-carbonate shales with abundant ash. The postrift stage is responsible for the formation of layered limestones of the Lower Tournaisian, developed also widely beyond the specified rift trough.

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. Keywords: Ophiolite, Volcanic arc, Early Cretaceous, Nain, Ashin, Iran

The Ess ophiolite, one of the most important ophiolitic massifs of the Arabian Shield, consists of a lower succession of serpentinized mantle rocks overlain by an ultramafic cumulate sequence, layered and isotropic gabbros, sheeted dykes, pillow lavas and pelagic sediments. The Ess mantle section is composed mainly of serpentinized peridotites with chromitite pods, dunite, wehrlite and pyroxenite. Extensive metasomatism and alteration has transformed the ultramafic rocks to talc-carbonates, magnesite deposits and listvenite, especially along shear zones and fault planes. Nevertheless, relics of primary chromian spinel, olivine and pyroxenes are observed. Both primary and metamorphic olivines can be recognized in dunite; the latter is marked by very high forsterite content (97–98), low NiO content ( 0.6) and low TiO2 content (

The structure of the oceanic upper volcanic crust is less understood at slow-spreading ridges than at faster ones. Its construction is dominated by pillow lavas, reflecting lower effusion rates than those at fast spreading ridges, where sheet and lobate flows are common and flow off-axis while thickening the extrusive volcanic layer. Based on optical and high-resolution bathymetry data from the Lucky Strike segment (Mid-Atlantic Ridge), we document a mode of volcanic emplacement that likely operates at some magmatically robust slow- and ultra-slow spreading ridge segments, in the presence of strong gradients in magma supply. In these settings, sheet flows may efficiently transport melt away from magmatically robust segment centers in the along-axis direction, steered by normal faults, and exploiting along-axis topographic gradients, with limited across-axis flow. Surface lineations of sheet lava flows tend to be subparallel to fault scarps and the overall segment orientation, and show whorls, well-developed lava channels with associated levees, and surface fold structures at flow fronts. This mode of lava emplacement transitions away from the segment center to pillow-dominated seafloor near the segment ends, associated often with hummocks and axial volcanic ridges. This results in local lava emplacement due to a melt supply that is lower than at the segment center. We propose that fault-steering of lava flows along-axis, limiting off-axis transport as observed at fast-spreading systems, may be common at both slow- and ultra-slow spreading ridges with significant along-axis changes in magma supply linked to topographic gradients. As a result, the nature and properties of the extrusive volcanic layer may vary significantly along axis owing to changes in the modes of volcanic emplacement, as transitions from sheet flows to pillow lavas may impact the porosity structure and hence the seismic properties of the extrusive layer in the oceanic upper crust.

We present geological, bulk-rock geochemical and Sr–Nd–Hf isotopic data for mafic rocks from the West Philippine Basin (WPB). These mafic rocks comprise pillow basalts characterized by a vesicular structure. The mid-ocean ridge basalt (MORB)-normalized trace element patterns of basalts from the study area display depletions in Nb. In addition, the chondrite-normalized lanthanide patterns of basalts from the WPB are characterized by significant depletions in the light lanthanides and nearly flat Eu to Lu segments. The investigated rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703339–0.703455 and high εNd(t) values (8.0 to 8.7). Furthermore, basalts from the WPB have 176Hf/177Hf ratios that range from 0.28318 to 0.28321 and high εHf(t) from 15.2 to 16.3. Semi-quantitative modeling demonstrates that the parental melts of basalts from the study area were derived by ~20% adiabatic decompression melting of a rising spinel-bearing peridotite source. The Sr–Nd–Hf isotopic compositions of basalts from the WPB indicate that their parental magmas were derived from an upper mantle reservoir possessing the so-called Indian-type isotopic anomaly. Interpretation of the isotopic data suggests that the inferred mantle source was most likely influenced by minor inputs of a sediment melt derived from a downgoing lithospheric slab. Collectively, the petrographic and geochemical characteristics of basalts from the study area are analogous to those of mafic rocks with a back-arc basin (BAB)-like affinity. As such, the petrogenesis of basalts from the WPB can be linked to upwelling of an Indian-type mantle source due to lithospheric slab subduction that was followed by back-arc spreading.

International audience; Melt supply at the ultraslow-spreading Southwest Indian Ridge (SWIR) has been shown to vary from nearly amagmatic, leading to ultramafic seafloor, to magmatically robust, producing fully volcanic seafloor. The center of the SWIR 50°28′E segment represents a magmatically robust endmember. High-resolution bathymetry and backscatter, near-bottom magnetic data, and visual observations were acquired to infer spatiotemporal variations in upper-crustal construction over the past 780 kyr. Tectonic strain inferred from observed faults represents

The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, and moss agates. The silica matrix of black agates is only composed of length-fast and zebraic chalcedony, micro- and macro-crystalline quartz, and quartzine. In addition to silica minerals, calcite, chlorite, feldspar, sulphides, and carbonaceous matter were also recognised. The black colour of agates is related to the presence of disseminated carbonaceous matter (CM) with a bulk content of less than 1 wt.%. Raman spectroscopy revealed that CM from black agates might be attributed to poorly ordered CM. The metamorphic temperature for CM from moss and spherulitic agates was determined to be close to 330 °C, whereas CM from concentrically zoning agates is characterised by a lower temperature, 264 °C. The potential source of CM in moss and spherulitic agates is associated with the hydrothermal fluids enriched in CM incorporated from underlaying carbon-bearing shungite rocks. The concentrically zoning agates contained heterogeneous CM originated both from the inter-pillow matrix and/or hydrothermal fluids.

The rocks of Raialo Group in the Alwar sub-basin of North Delhi Fold Belt were investigated with a focus on meta-basic lava flows of the Tehla Formation. The basaltic rocks, now occurring as amphibolites, are medium- to fine-grained and show evidence of greenschist to lower amphibolite facies metamorphism, characterized by dominant plagioclase, hornblende, actinolite, chlorite, and minor, epidote, opaque, and Fe–Ti oxide mineral assemblage. The presence of amygdules and vesicles, well preserved primary igneous texture, and relict pyroxenes, at places, confirm their basaltic lineage. In a single outcrop, located to the northeast of Tehla village, well-preserved pillow structures have been discovered. The pillow structures vary in size (few cm to >1 m) and shape (spheroid, elongate and kidney-shaped), and inter-pillow spaces are filled up with chert, at places. This is the first report of pillow basalt occurrence from the North Delhi Fold Belt. In addition to offering evidence of submarine basaltic volcanism, the mega-pillows, gabbroic lenses, and presence of ultramafic rocks in the region suggest a possible ophiolite setting that needs a detailed petrographic and geochemical evaluation.

DOI: 10.17014/ijog.8.2.233-253 The study of tholeiitic basalt is a general-classic study from geotectonic MORB, ocean island (OIB), continental rift, volcanic-arcs {IAB or Active Continental Margin (ACM)}. However, the geotectonic study of the tholeiitic volcanic-arcs is still unclear at the moment. In general, the arc tholeiitic is directly pointed to an island-arc volcanic, and the result of google search engine defines no existence of tholeiitic geochemistry which is formed from continental-arc volcanic (ACM). The problem lies in the model of discrimination diagram which is not able to discriminate ACM from the island arc volcanic. The spider diagram shows relatively similar of patterns as well as in the use of the isotope 143Nd/144Nd versus 87Sr/86Sr. Tholeiitic Kebasen pillow lava exhibits a slightly hydrothermal alteration (propyilitic alteration) which consists of plagioclase (labradorite-bytownite), olivine, pyroxene (diopside), hornblende, volcanic glass and other secondary minerals (such as iddingsite, zeolite, carbonate, sericite and opaque minerals). The results of the interpretation using the overlay diagram of Mg# and FeO(t)/MgO, diagram Nb/La vs. La/Yb, the overlaid diagram between the diagram of Zr/Y vs. Zr, newly developed diagram for sedimentary recycling (Th/Ce vs. SiO2) reveal the Kebasen lava is a differentiated tholeiitic rock with relatively low of Mg# (Mg# 0.1); furthermore, the trace element constituent is interpreted based upon the melting of oceanic slab (Zr/Y ~ 3). The magmatism of Kebasen lava is potentially formed at temperature of ~ 1240 o C and a pressure of ~ 1.7 GPa at the depth of ~ 56 Km.

We report in situ observations that reveal the presence of the thorid genus Eualus at hydrothermally active sites. The shrimp Eualus amandae Nye, Copley & Linse, 2013 was first collected in non-venting sites but near areas of hydrothermal activity, on the East Scotia Ridge segment E9 and in the Kemp Caldera, South Sandwich Arc. During a recent expedition of RV Polarstern, specimens of Eualus amandae were observed via a remotely operated vehicle (ROV) at the East Scotia Ridge segment E2 and Kemp Caldera. The animals were seen in shimmering water sites on pillow basalts and at active hydrothermal orifices, both unambiguously hydrothermally active. These sites were also characterised by other vent marginal fauna, such as deep-water comatulid crinoids or stoloniferean cnidarians. The shrimp family Thoridae is more diverse in Antarctic waters than other shrimp families and these records suggest two independent origins of hydrothermal-related habitats in Lebbeus, and now Eualus. These records expand the understanding of the contribution of geothermal activity to larger patterns of Antarctic deep-sea biodiversity.

The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. In this manuscript, we present a newly discovered ophiolite in the Yakou-Banpo area of the southern Lancangjiang belt in the Sanjiang area, SW China. The ophiolite displays a complete Penrose-type ophiolite sequence, composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity. Furthermore, positive Ɛ Nd (t) (+4.5 to +6.7) and zircon Ɛ Hf (t) values (+12.4 to +14.3), as well as mantle-like δ 18 O values (~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. SHRIMP zircon U Pb dating yield weighted mean ages of 305 ± 3 Ma, 310 ± 2 Ma, and 313 ± 6 Ma. Hence, the Yakou-Banpo ophiolite belt formed during the Late Carboniferous (313–305 Ma) and records the existence of a mature ocean basin. We propose that an arc – back-arc system developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

The West Junggar, located in the southernmost part of the Central Asian Orogenic Belt (CAOB), is a key region for understanding the Paleozoic evolution of the CAOB. Issues of the timing of initial subduction and tectonic unit connections in northern West Junggar still remain controversial. In this study, we report a new ophiolitic melange named the E'min ophiolitic melange in northern West Junggar. The tectonic blocks in the E'min ophiolitic melange are mainly composed of serpentinized peridotite, serpentinite, gabbros, pillow basalts, and cherts, with a matrix consisting of highly deformed serpentinites. A gabbro exhibits a zircon SHRIMP U-Pb age of 476 ± 2 Ma, and the zircon grains have δ18O values similar to those of mantle zircons. Those basalt samples display depletions of light rare earth element (REE) relative to heavy REEs. They exhibit weak enrichment of Ba and Th, and moderate depletion of Nb and Ta. The basalts display similar geochemical characteristics to that of fore–arc basalts in the present-day fore–arc setting. The gabbros exhibit high MgO and compatible element contents, but low TiO2, total REE and high field strength element (HFSE) contents. They exhibit light REE depletion, enrichment in large-ion lithophile elements, and depletion of HFSEs. The boninite-like geochemical patterns of the gabbros indicate that they were formed in a subduction-related environment, and were derived from an extremely depleted mantle source infiltrated by subduction-derived fluids and/or melts. The E'min ophiolitic melange has a geochemical make-up similar to those of suprasubduction-zone (SSZ)-type ophiolites formed in a forearc setting. Hence, we propose that the E'min ophiolitic melange formed in a forearc setting and may represent the initial subduction in northern West Junggar. Based on geochronological data, we propose that the E'min ophiolite, together with the Kujibai, Hoboksar and Hongguleleng ophiolites, formed during a similar period and comprise a huge E–W trending ophiolitic belt.

The Central Asian Orogenic Belt (CAOB) is one of the largest and long-lived accretionary collages worldwide, and provides an ideal opportunity to address the fundamental geological processes and continental growth of accretionary orogens. However, the tectonic history of the CAOB is still controversial. Previous studies mainly focused on the process and model of accretion and subduction, but did not pay much attention to oceanic intraplate volcanoes. In this paper, we provide an overview of the salient features of twelve OIB-hosting accretionary complexes with ages from Late Neoproterozoic to Early Carboniferous in western the CAOB, and distinguish rock association of typical seamounts from accretionary complexes and shear zones, including pillow basalt, volcanic breccia, limestone, olistostromes and terrestrial clastic rocks. Combined with unique structural features of seamount, the stratigraphic framework of seamount was preliminarily reconstructed in Paleo-Asian Ocean, but the distribution and scale of seamounts is still not clear. Geochemically, volcanic rocks from seamounts are mainly alkaline basalts with litte tholeiitic basalts. The alkaline basalts are characterized by LREE enrichment and HREE depletion, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The Nd, Sr and Pb isotopic features indicate that magma sources have enriched components and show fluid metasomatism. In short, this is consistent with geochemical features of Louisville, Ontong Java and Hawaiian seamounts which are related to mantle plumes, but do not contain high-Mg picrite and komatitic basalt. Based on our observations, and in combination with previous work, we present an innovative model in which the CAOB was involved multiple intra-oceanic arc with seamounts subduction and accretion, which is similar to the present-day Pacific and can better explain the tectonic evolution of the CAOB. However, further studies are needed to document the distribution, scale and mantle dynamics of seamounts in the CAOB.

We have studied geochemical characteristics, mineralogy and origin of the manganese deposits in Bela ophiolitic complex. Geochemical investigation was conducted in order to discuss the elemental correlations and to infer the probable origin of manganese deposits in Bela ophiolites. Ore microscopy was conducted to identify different manganese minerals and their paragenesis. Psilomelane and braunite were found to be the major ore minerals. Mineral paragenesis sequence as observed was braunite forming first. Psilomelane formed in later stages because it has been found that psilomelane is altering the braunite. Magnetite was observed as a secondary mineral as vein filling and cutting through both the braunite and psilomelane, therefore, younger than the psilomelane and braunite. The gangue minerals observed in the studied samples were quartz, cryptocrystalline silica and calcite. The correlations among different major and trace elements showed diversity of relations. MnO showed negative correlation with Fe2O3 (− 0.73), Si2O (− 0.27), positive correlation with Al2O3 (0.54), TiO2 (0.36), MgO (0.22), Pb (0.23), Ni (0.07), Cr (0.12), and no correlation was established with Zn, Cu, Co. Binary diagram of Si versus Al and ternary discrimination diagrams of Fe–(Ni + Co + Cu) × 10–Mn and Ni–Zn–Co showed hydrothermal-diagenetic-type deposits. Furthermore, the present study suggests that the enriched manganiferous fluid during its upward movement within the Tethys oceanic crust near spreading center started precipitating Mn along with Fe on the sea floor. Later on, these manganese minerals were obducted on land between Indian plate and Helmond block of Eurasian plate in existing position accompanying pillow basalt and pelagic sediments.

The Paleoproterozoic Usagaran Orogenic Belt in East Africa is an exemplary site of preserved relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic crust. This paper describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist-facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid-immobile elements. The (La/Sm)N ratios (0.62–1.09) and REE patterns point to normal mid-ocean ridge basalt (N-MORB) and transitional MORB (T-MORB) mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts are an analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920–1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Thus, the Konse pillow basalts might have been displaced from their suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Dunites in the mantle section of the Oman ophiolite contain olivines which show both a wide range of compositions (Fo86.2 to Fo94) and very high magnesium numbers. These data are combined with experimental liquidus olivine-melt data to show that the range of olivine compositions requires that the dunites formed from a range of melt compositions and that some of these melts were very magnesian, with MgO concentrations up to MgO = 18–20 wt%. These observations are consistent with the finding of MgO-rich melt inclusions from chromitites in the mantle section of the Oman ophiolite (Rollinson et al., Lithos, 2018). It is proposed that the high Mg-olivines formed in equilibrium with high-Ca boninites, found in the upper section of the pillow lavas sequence of the ophiolite. A model is developed whereby high MgO boninites fractionated olivine through a process of melt-rock reaction/fractionation with the enclosing harzburgite to create a range of dunitic compositions and evolved boninitic lavas. Field evidence shows that the emplacement of boninites was late in the evolution of the Oman ophiolite indicating that the mantle dunites of boninitic origin formed late in the history of the ophiolite. High-Ca boninites form through the shallow, hydrous melting of the mantle wedge in a subduction setting and the presence of boninitic dunites and lavas further supports the view that the Oman ophiolite formed in a fore-arc setting through subduction-induced spreading.

Geotourism is a relatively new branch of tourism dealing with geosites, which are special places that exhibit aspects that characterize the complex history of the Earth. This contribution aims to report an inventory of features of geological interest occurring in the Cayirbagi-Calkoy area in the Eastern Black Sea Region in Turkey. Among these is Cal Cave, a site that attracts local and foreign visitors and can potentially contribute to the development of tourism in the study region. Further, there is a well-developed karstic system in the area. A number of caves and dolines in Cal-Camili Natural Park are a part of this system. There are also volcanic and volcano-sedimentary rocks outcrops in the region. Numerous forms such as dikes, pillow lava, and exfoliation structures show the formation dynamics of rocks and characterize the region. The Sahinkaya Member is among the unique places in the Eastern Pontides region, where the K–Pg boundary can be observed. Along with “pure” geological features, the region also offers visitors many other historical, cultural, and natural resources. Cal-Camili Natural Park was raised to the status of natural park for its biological (plant and wildlife) diversity and the integrity of its landscape. Rising from green forests, the white rocky structure of the Sahinkaya Member stands out against the surrounding dark landscape. The combination of valuable elements of different natures (i.e. geologic, naturalistic, historical) could allow sustainable local development.

The East Taiwan Ophiolite (ETO) occurs as blocks and thrust sheets associated with the Lichi Melange in the Coastal Range of eastern Taiwan. The blocks consist of serpentinized harzburgite, serpentinite breccia, gabbro, dikes of dolerite and plagiogranite, pillow basalts, and red clay within a mud-and serpentinite-rich melange matrix. New U-Pb zircon dating of a pegmatite gabbro yields a weighted mean age of 16.65 +/- 0.20 Ma. This age is earlier than the North Luzon Arc but overlaps with the late-stage spreading of the South China Sea. ETO glassy basalt has low K2O, MgO and high CaO contents, similar to MORB. REE and trace element patterns show both N-MORB patterns with LREE depletion and E-MORB patterns with slight LREE enrichment. A few samples show slight depletion in Nb-Ta and Ti and enrichment in Rb, Ba, U and Sr, indicating a hint of subduction influence. Most ETO basalt plots within the overlapping fields of N-MORB and BABB on Ti-V, Cr-Y, Nb/Yb-Th/Yb, and Hf/3-Th-Ta discrimination diagrams. These geochemical compositions are emblematic of mid-ocean ridge or back-arc lava, like South China Sea basalt. We interpret ETO basalt and gabbro as fragments of the subducted South China Sea basement that were scrapped off and accreted to the Luzon forearc during the process of subduction initiation along the Manila Trench. Blocks of mantle material in the melange may originate from the upper plate of the arc-continent collision and were mixed with lower plate crustal material in a subduction channel now represented by the Lichi Melange.

As an indicator of extensional tectonic events, MORB-like magmas could provide great insights into tectonic evolution of subduction zones. In this study, we carry out an integrated study of geochronology, geochemistry and Sr Nd isotopic compositions for the Early Paleozoic MORB–like Lashuixia gabbro in Hualong, southern Central Qilian block. U Pb zircon dating yields magma crystallization age of 506 ± 6 Ma for the gabbro. Geochemically, the gabbros display weakly fractionated REE patterns with (La/Yb)N = 1.17–1.61 and flat trace element patterns except for enrichment of large ion lithophile elements (LILEs) and depletion of Nb. Isotopically, the gabbros display pronouncedly positve eNd(t) values of +5.8 − +7.0, but their initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7046–0.7070) are slightly high because of post-magmatic alteration and metamorphism. Their geochemical and isotopic features indicate that several endmembers (i.e. MORB-sourced asthenospheric mantle, subcontinental lithospheric mantle and oceanic slab-derived fluid) were involved into their source region. Our modal batch partial melting calculation indicates that 7–17% partial melting of 90% asthenospheric mantle plus 10% subcontinental lithospheric mantle at spinel-stable depth accords well with most incompatible trace element compositions of the gabbros. In combination with regional data, we propose that rock association of arc-type pillow lava (ca. 494 Ma), Nb-enriched basalt (NEB)-like diabase (ca. 491 Ma), boninite (ca. 483 Ma) and later OIB-like basalt (ca. 468 Ma) in the Lajishan belt witnessed evolution of this belt from continental arc splitting to back-arc basin spreading due to rollback of the subducted North Qaidam-West Qinling Oceanic slab. We suggest that the Lashuixia gabbro, ca. 10 Ma earlier than the arc-type pillow lava, recorded the initial back-arc lithospheric extension. As with those in many modern arc-basin systems, MORB-like gabbro with overprinting of other endmembers in paleo-subduction zones could not only be an important indicator for back-arc extension, but also make a constraint on the time when the back-arc extension initiated.

Chemical fluxes associated with low-temperature, off-axis, alteration of the upper oceanic crust (seafloor weathering) may play an important role in controlling the composition of the ocean and the long-term carbon cycle. However, it is challenging to quantify these fluxes and how they change with changing bottom water temperature over geological timescales. Here we study the exchange of major elements associated with seafloor weathering in the exceptionally preserved Cretaceous Troodos ophiolite and compare them to less well constrained data from drill cores from the modern ocean basins. Calcite O-isotope thermometry from four traverses through the lavas reveals a well-ventilated region at the top of the lava pile where alteration occurred at near-constant temperatures similar to that of bottom water. The lithological makeup of the crust appears to control the thickness of this region, with increased abundance of sheet flows marking the base of this zone. Maintaining low-temperatures in the well-ventilated region requires large fluid fluxes facilitated by the high permeability of the pillow lavas. Large-scale addition of CO2 to the crust only occurs in this well-ventilated region indicating that the alkalinity producing reactions required for calcite precipitation occurred at temperatures sensitive to bottom water temperature. Comparison of whole rock geochemical data for samples from the well-ventilated zone with volcanic glass compositions shows that the changes in rock compositions due to seafloor weathering are large (roughly −4 wt% SiO2, +0.5 to 1 wt% MgO, −5 wt% CaO, −0.5 wt% Na2O, +3.5 wt% K2O and +2.5 wt% CO2). Comparison to data from drill cores from modern oceanic crust that was also altered under well-ventilated conditions suggests that bottom water temperature plays a major role in controlling the chemical exchange between the ocean and lavas; parameterizations for the associated fluxes are provided. Because elemental exchange between the ocean and oceanic crust globally depends strongly on bottom water temperature, fixed values for these fluxes cannot be used in models of the evolution of seawater chemistry or subduction fluxes.

The study of volcanogenic and volcanogenic–sedimentary rocks (Early Proterozoic pillow lava of Karelia and South Africa), where diverse fossilized bacteria (prokaryotes) and probably even eukaryotes shows that, during this early period, conditions of cooling lava flows and igneous rocks were favorable for bacterial development and colonization.

Geological characteristics and geochemical analyses are reported for the early Carboniferous pillow basalt in the Huilvshan region (west Junggar, Northwest China), with the aim to indicate its petrogenesis, magma source characteristics, and tectonic implication. This pillow basalt consists of clinopyroxene and plagioclase with trace amounts of magnetite, apatite, and chromian spinel. It is tholeiitic in composition with low concentrations of Na2O + K2O (1.52–4.74 wt.%). Similar to the N-MORB, the samples of this pillow basalt have nearly flat chondrite-normalized REE patterns ((La/Yb)N = 0.87–1.47) with insignificant Eu anomalies (Eu/Eu* = 0.84–1.18), and show no obvious enrichments of LILEs and insignificant depletions in HFSEs. Petrology and geochemical characteristics suggest that this pillow basalt is the product of MORB-like magma derived from a depleted mantle corresponding to ≤4% partial melting of spinel lherzolite. SIMS analysis of the zircons separated from tuff interlayered with basalt gives a weighted average U–Pb age of 328 ± 3 Ma (MSWD = 1.4), which represents the magma eruption time in the Huilvshan region. From these observations, in combination with the previous work, we conclude that an extensional tectonic regime dominated the tectonic activity of west Junggar during early Carboniferous.

We present an integrated study of ophiolite complexes and island arc rocks from the Tianshui-Wushan Accretionary Belt, West Qinling Orogen. The West Qinling Orogen is important because it links the Qinling orogen to the east and the Qilian and Kunlun orogens to the west. The link between these orogens is commonly assumed, but has little study in detail. Zircon U-Pb analyses from ophiolitic rocks indicate the oceanic lithosphere formed in the Cambrian (530–500 Ma). Pillow lavas in the ophiolite complex show geochemical signatures of enriched MORB, suggesting they represent remnants of an oceanic plateau or seamounts. The island arc rocks include a volcanic complex with basalt-andesite and boninite of Late Ordovician age (460–440 Ma), and a serpentinized peridotite massif. The serpentinized peridotite most likely represent a highly refractory mantle residue with subsequent melt-rock interaction at ~450 Ma, suggesting that it formed in a forearc setting. The co-occurrence of ophiolite complexes and island arc rocks indicates that Tianshui-Wushan Accretionary Belt is an oceanic suture zone caused by oceanic subduction in the Early Paleozoic. The island arc rocks most likely represent the early product of an Izu-Bonin-Mariana (IBM)-type intra-oceanic arc, developed in response to a collision between an oceanic plateau and a continental margin. Our study permits a tectonic correlation between the Tianshui-Wushan Accretionary Belt in the West Qinling Orogen and the South Qilian Accretionary Belt in the Qilian Orogen, thereby establishing the continuity between the Early Paleozoic orogenic belts along the southern margin of the North China Craton.

Exhumation of basement rocks on the seafloor is a worldwide feature along passive continental margins and (ultra-) slow-spreading environments, documented by dredging, drilling or direct observations by diving expeditions. Complementary observations from exhumed ophiolites in the Alps allow for a better understanding of the underlying processes. The Aiguilles Rouges ophiolitic units (Val d’Hérens, Switzerland) are composed of kilometre-scale remnants of laterally segmented oceanic lithosphere only weakly affected by Alpine metamorphism (greenschist facies, Raman thermometry on graphite: 370–380 °C) and deformation. Geometries and basement-cover sequences comparable to the ones recognized in actual (ultra-) slow-spreading environments were observed, involving exhumed serpentinized and carbonatized peridotites, gabbros, pillow basalts and tectono-sedimentary cover rocks. One remarkable feature is the presence of a kilometric gabbroic complex displaying preserved magmatic minerals, textures and crosscutting relationships between the host gabbro and intruding diabase, hornblende-bearing dikelets or plagiogranite. The bulk major and trace element chemistry of mafic rocks is typical of N-MORB magmatism (CeN/YbN: 0.42–1.15). This is supported by in-situ isotopic signatures of magmatic zircons (εHf = + 13 ± 0.6) and apatites (εNd = + 8.5 ± 0.8), determined for gabbros and plagiogranites. In-situ U–Pb dating was performed on zircons by laser ablation-ICP-MS, providing ages of 154.9 ± 2.6 Ma and 155.5 ± 2.8 Ma, which are among the youngest for oceanic gabbros in the Alps. Our study suggests that the former Aiguilles Rouges domain was characterized by tectonism and magmatism resembling present-day (ultra-) slow-spreading seafloor. It also suggests that the Tethyan lithosphere is laterally segmented, with punctuated magmatism such as the Aiguilles Rouges gabbros and carbonated ultramafic seafloor covered by basalts and Jurassic tectono-sedimentary deposits.

Ophiolites, found in orogenic belts, are slices of ancient oceanic lithosphere obducted on land during continental collision and ocean closure. They provide valuable insights into submarine volcanological and petrological processes. Palaeotectonic interpretations of ophiolites have heavily depended on geochemical data, despite the considerable submarine alteration and even metamorphism commonly observed in ophiolites. No independent checks on the geochemistry-based inferences are usually provided or sought. Here, we present a hitherto unavailable volcanic facies perspective on the ~ 140 Ma Nidar ophiolite, exposed > 4100 m above sea level in the Ladakh region of the Indian Trans-Himalaya. We describe features of pillow lavas and hyaloclastite forming the oceanic crust and of peperite and silicic volcanic ash layers in the overlying sedimentary cover (mainly radiolarian cherts, dated at 132–112 Ma). The whole volcanosedimentary sequence is inconsistent with a mid-ocean ridge setting. We interpret it as having formed in a shallow (~ 2.5 km) submarine environment, with ongoing explosive silicic eruptions, in an Early Cretaceous, compositionally bimodal, intra-oceanic island arc in the Neo-Tethys Ocean. Geochemical-isotopic data on the Nidar ophiolite have previously been used to argue for an intra-oceanic arc origin. We suggest that a volcanic facies approach to the study of ophiolites can be a valuable guide to their palaeodepths and palaeotectonic settings.

Neoproterozoic ophiolites are widely distributed in the Central and Southern Eastern Desert of Egypt. The most important and well-known ophiolites of the Central Eastern Desert (CED) are exposed in Ghadir area. They occur as intact ophiolitic sequence of oceanic crust, blocks and fragments in an ophiolitic melange and as dismembered ophiolite components. The whole rock association is foliated and metamorphosed to the greenschist facies up to lower amphibolite facies. The intact ophiolite sequence is composed mainly of metagabbro, sheeted dykes, and pillow lavas. Exotic fragments of oceanic lithosphere tectonically incorporated in a foliated volcaniclastic matrix forming tectonic ophiolitic melange. The Ghadir pillowed metabasalts are classified as high-Ti MORB-type, which are comparable to MORB or BABB ophiolites and appear to be derived from an evolved magma in an intra-oceanic back-arc basin. The intact ophiolite sequence is remnants of oceanic crust formed the floor of the basin and became tectonically obducted during the closure of the back-arc basin in which they formed. The formation of the Ghadir ophiolitic melange by tectonic processes in a back-arc basin is followed by tectonic emplacement of some dismembered ophiolitic rocks along structural contacts. The whole MORB ophiolites of the Eastern Desert (ED) of Egypt were suggested to be formed in a back-arc basin due to collision of an island arc system with the African continent. They occur in different geological settings, and the compositional variation within these rocks was assigned to geochemical variations in a single tectonic setting of back-arc basin rather than to different tectonic settings.

The Durkan Complex is a key tectonic element of the Makran accretionary prism (SE Iran) and it has been interpreted as representing a continental margin succession. We present here a multidisciplinary study of the western Durkan Complex, which is based on new geological, stratigraphic, biostratigraphic data, as well as geochemical data of the volcanic and meta-volcanic rocks forming this complex. Our data show that this complex consists of distinct tectonic slices showing both non-metamorphic and very low-grade metamorphic deformed successions. Stratigraphic and biostratigraphic data allow us to recognize three types of successions. Type-I is composed by a Coniacian – early Campanian pelagic succession with intercalation of pillow lavas and minor volcaniclastic rocks. Type-II succession includes a volcanic sequence passing to a volcano-sedimentary sequence with Cenomanian pelagic limestones, followed by a hemipelagic sequence. This succession is characterized by abundant mass-transport deposits. Type-III succession includes volcanic and volcano-sedimentary sequences, which are stratigraphically covered by a Cenomanian platform succession. The latter is locally followed by a hemipelagic sequence. The volcanic rocks in the different successions show alkaline geochemical affinity, suggesting an origin from an oceanic within-plate setting. Our new results indicate that the western Durkan Complex represents fragments of seamounts tectonically incorporated in the Makran accretionary wedge during the latest Late Cretaceous–Paleocene. We propose that incorporation of seamounts in the frontal prism caused a shortening of the whole convergent margin and possibly contributed to controlling the deformation style in the Makran Accretionary Wedge during Late Cretaceous–Paleocene times.

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n&rsquo, Tinzla, Assif n&rsquo, Tasriwine, and Assif n&rsquo, Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 &plusmn, 5 Ma, which is consistent with the 761.1 + 1.9/&minus, 1.6 and 762 + 1/&minus, 2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica&ndash, low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 &plusmn, 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

Abundant ophiolites are distinctive components of the Arabian-Nubian Shield (ANS), providing important clues about its origin and hosting many of its valuable mineral resources. The ANS ophiolites were thrust onto pre-Pan-African continental shelves, causing deformation, mylonitization, and cataclasis in the footwall rocks. Most of the ANS ophiolites are fragmentary or incompletely preserved but nevertheless each contains at least some elements of the classic ophiolite sequence. The ANS contains a few complete Penrose-type sequences, composed of metamorphosed peridotites followed upward by pyroxenite/dunite, cumulate gabbro, massive gabbro, pods of plagiogranite, sheeted diabase dikes, pillow basalts, and deep marine sediments. The ANS ophiolites show a wide range of ages (870–680 Ma); they predate or overlap with the island arc stage (~770 to ~670 Ma) of the assembly of the shield. All the ANS ophiolites are strongly deformed, metamorphosed, and affected by several types of alteration. Alteration resulted in the development of magnesite, talc-carbonates, rodingite, and listvenite, particularly, along shear zones. Extensive carbonate alteration is common in the ultramafic sections, concentrated in talc-carbonates, listvenites, and in veins and dikes of magnesite, calcite, and dolomite; the sources of CO2-bearing fluids and the age of pervasive carbonate alteration are quite uncertain despite their importance. Lower greenschist facies regional metamorphism predominates in the ANS ophiolites, but there are excursions to amphibolite grade. The ultramafic upper mantle rocks of the ANS ophiolites are, without exception, highly serpentinized harzburgites and dunites. The mineralogy of the protolith is apparent petrographically from the abundance of mesh and bastite textures—characteristic of serpentine developed after olivine and orthopyroxene, respectively—and from rare preserved relics of fresh olivine, orthopyroxene, and Cr-spinel. Fresh cores of Cr-spinel are always surrounded by ferritchromite and Cr-magnetite rims. The ultramafic members of the ANS ophiolites are consistently high in Mg# (mostly > 0.89); enriched in Ni, Cr, and Co; and depleted in Al2O3 and CaO. These features are most consistent with the general consensus that the ANS ophiolites all formed in supra-subduction zone settings. However, authors continue to disagree on the detailed setting, i.e., fore-arc or back-arc spreading centers. We contend that the evidence strongly favors the fore-arc setting for the majority of the ANS ophiolites. Specifically, in serpentinites, the relics of Cr-spinel have Cr# mostly >60, and the relics of olivines and orthopyroxene are Mg-rich, both suggesting that the serpentinites are residual to high degrees of melt extraction, as found most commonly (though not exclusively) in modern fore-arc peridotites. The ophiolitic sequences of the ANS are worthy targets for mineral exploration. They host a variety of important mineral deposits, including chromite, magnesite, talc, platinum-group elements, base metals (Cu–Ni–Co), and gold. There is a spatial and genetic relationship between carbonatized ultramafics, subsequent granite intrusions, and gold mineralization.

The fossil biota of the upper Valanginian–lower Hauterivian sediment hosted vent (SHV) environment of Zengővarkony comprises 39 species, 55 genera and higher taxa representing seven phyla. Thirteen of these taxa are reported from Zengőrvarkony as new species or new subgenera/genera, and this high endemicity is linked to a unique palaeoenvironment. The palaeoenvironment is developed in an outer shelf epicontinental setting on an elevated block or slope, and is linked to a small sedimentary iron-ore body interbedded with pillow lavas being a result of ferriferous exhalations and biogenic processes. Biogeographically, the brachiopods indicate the Sub-Tethyan domain. Serial sectioning of brachiopods revealed a taxon-dependent rich and diverse microfauna (sponges, foraminifers, gastropods) preserved inside the shells. Internally preserved microfauna in brachiopods reveals differentiation among preventive strategies. It implies different preventive efficiencies of alternative defence strategies of higher brachiopod taxa. The unique Zengővarkony SHV environment attracted both nektonic and benthic organisms by creating different ecotopes around the hydrothermally driven bottom environment. This environment belongs to the group of rare, shelfal, hydrothermally influenced palaeoenvironments formed on continental crust. Water depth was most probably between 100 and 150 m. Recent observations from a similar modern volcanic built-up, and its effects on the marine environment located on continental crust around the Aeolian Islands (Tyrrhenian Sea) support the recognition of this fossil SHV environment and helps to understand better its bathymetry, ecological conditions and ore-formation.

71 Gulch Volcano, located in the western Snake River Plain, southwestern Idaho (USA), was formed by a basaltic fissure eruption into Pliocene Lake Idaho. Deposits at and below the eruptive surface record the nature of explosive and non-explosive sediment-magma and water-magma interactions. The paleoenvironment and the volcanic plumbing system of 71 Gulch eruption were reconstructed from a detailed study of the now-exposed eruptive and subsurface deposits. Estimated water levels during the eruption range from 1 to 53 m across the entire volcano based on the distribution of pillow lavas. Billowed dikes and fluidal peperites are two types of intrusive features found ≤ 12 m below the syn-eruptive surface that have similar continuous margins representing ductile behavior and emplacement into soft, deformable sediment. In comparison, tabular dikes and blocky peperites occur throughout 41 m thick of exposed deposits below the syn-eruptive surface. A conical, diatreme-like vent structure dominates the subsurface exposure, indicating excavation by phreatomagmatic explosions. Localized spatter within the main subsurface vent suggests temporary exclusion of water and thus a period of brief vent growth above the lake level. Evidence of sediment-magma mingling through blocky and fluidal peperites shows how differing subsurface depths impact the resulting mingling textures.

The Neoproterozoic metavolcanic rocks constitute major rock sequences in the Egyptian Nubian Shield. A recent detailed overview of the volcanisms and volcanic sequences particularly the metavolcanic rock unit is treated in this chapter. The geologic overview is stressed on a brief description of the metavolcanics at 41 better studied occurrences, a brief description of some common petrographic characteristics of these rocks, a discussion of their metamorphism, geochemistry, and age-dating. Two essential types: ophiolitic metavolcanics (Oph Mv) and arc metavolcanics (Arc Mv) have been discussed in detail. They were overprinted by greenschist and lower amphibolite facies metamorphism. The ophiolitic (Old) metavolcanic rocks (Oph Mv) are concentrated in the Central Eastern Desert (CED) and Southern Eastern Desert (SED) of Egypt, comprise pillow lavas and sheeted dykes. The pillowed lavas display pillow structure with abundant amygdales and scarcely massive. The sheeted dykes’ are repeated and continuous vertical styles. The rock types of the pillow lavas include spilite, aphyric and variolitic metabasalt, metabasaltic andesite and/or meta-andesite. The sheeted dykes exhibit subophitic, ophitic, and blastodiabasic metabasalt and rare metadolerite and meta-andesite. The arc (Young) metavolcanic rocks (Arc Mv) are generally widespread in the southern Sinai, NED, CED, and northern parts of the SED of Egypt. They belong to the island-arc mafic to felsic metavolcanic rocks. The Arc Mv includes massive and schistose flow and/or pillow lavas, commonly associated with pyroclastic and tuffs. They comprise two associations of mafic and felsic: blastodiabasic and porphyritic metabasalt and meta-andesite association, and porphyritic meta-andesite, metarhyodacite and metadacite association. Geochemically, the Arc Mv has higher ranges and means of SiO2, Al2O3, CaO, Na2O, Sr, Nb, Zr, and V and lower MgO, MnO, TiO2, Ba, Rb, Cr, and Ni compared with that of Oph Mv, suggest their fractionation sources. On discrimination diagrams, transitional tectonic environment among MORB, within-plate, and island-arc basalt types of the Oph Mv, revealing simulating their back-arc marginal basin. However, N-MORB affinity for the lavas and fore-arc character for the associated ophiolitic metaultramafics have been also reported. These variations reflect source region heterogeneities, and differences in degree of partial melting as well as corroborating the variability of oceanic domain of the ED Mv. The complied REEs data can be grouped into: Oph Mv dominating in the CED and SED and Arc Mv of Southern Sinai, NED and CED. The Oph Mv consequently comprises two subgroups: (i) LEEs-depleted pattern (similar to the N-MORB-type) and (ii) LREEs-enriched pattern (analogous to transitional MORB-IA). On the other hand, the Arc Mv has REEs patterns similar to that of IA (e.g., LREEs-enriched). These variations also reflect source region heterogeneities, differences in degree of partial melting, and variability of oceanic-continental terrains domain of the Egyptian shield Mv. Bimodal arc associations of both mafic and felsic compositions (Shadli-type) are concentrated in CED and northern parts of the SED. The mafic end-members are tholeiitic basalt and basaltic andesite whereas the felsic ones comprise calc-alkaline dacite and rhyolite. The mafics are comparable to N-MORB and arc-related non-cumulative mafic rocks. Felsic end-members are subduction-related source of arc-related magmas of thickened low-K mafic lower crust. The Oph Mv is relatively yielded age ranges from 750–700 Ma, whereas, the Arc Mv is relatively range in ages from 750–610 Ma, suggesting an overlap and no discernible difference in age are found between both sequences. These ages may represent the time of eruption of these metavolcanics in the crustal growth of the Egyptian Nubian Shield.

The Eastern Desert (ED) Neoproterozoic ophiolites are tectonically important elements of the Arabian–Nubian Shield. Although affected by various degrees of dismemberment, metamorphism, and alteration, almost all of the diagnostic Penrose-type ophiolite components can be found, namely, lower units of serpentinized peridotite tectonite and cumulate ultramafics and upper units of layered and isotropic gabbros, plagiogranites, sheeted dykes and pillow lavas. The contacts between the lower unit (mantle section) and the upper unit (crustal section) were originally magmatic, but in all cases are now disrupted by tectonism. The mantle sections of the ED ophiolites are exposed as folded thrust sheets bearing important and distinctive lithologies of serpentinized peridotites of harzburgite and dunite protoliths with occasional podiform chromitites. The ED ophiolites show a spatial and temporal association with suture zones that indicate fossil subduction zone locations. Multiple episodes of regional metamorphism mostly reached greenschist facies with less common amphibolite facies localities. CO2-metasomatism resulted in the development of talc–carbonate, listvenite, magnesite, and other carbonate-bearing meta-ultramafic rocks. Geochemical data from the ED serpentinites, despite some confounding effects of hydration and alteration, resemble modern oceanic peridotites. The ED serpentinites show high LOI (≤20 wt%); Mg# mostly higher than 0.89; enrichment of Ni, Cr, and Co; depletion of Al2O3 and CaO; and nearly flat, depleted, and unfractionated chondrite-normalized REE patterns. The modal abundance of clinopyroxene is very low if it is present at all. Chromian spinel survived metamorphism and is widely used as the most reliable petrogenetic and geotectonic indicator in the ED ophiolite mantle sections. The high-Cr# (mostly ~0.7) and low-TiO2 (mostly ≤ 0.1 wt%) characters of chromian spinel indicate a high degree of partial melt extraction (≥30%), which is commonly associated with fore-arc settings and equilibration with boninite-like or high-Mg tholeiite melts. Based on the general petrological characteristics, the ED ophiolitic chromitites are largely similar to Phanerozoic examples that have been attributed to melt–peridotite interaction and subsequent melt mixing in fore-arc settings. The comparison between the ED Neoproterozoic mantle peridotites and Phanerozoic equivalents indicates considerable similarity in tectonomagmatic processes and does not support any major changes in the geothermal regime of subduction zones on Earth since the Neoproterozoic era. The mantle sections of ED ophiolites are worthy targets for mining and exploration, hosting a variety of ores (chromite, gold, and iron/nickel laterites) and industrial minerals (talc, asbestos, and serpentine).