Ophiolitic Units Research Articles

Short-lived intra-oceanic arc-trench system in the North Qaidam belt (NW China) reveals complex evolution of the Proto-Tethyan Ocean

Abstract Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527–506 Ma gabbro, 515–506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495–486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.

The Central-Sudetic ophiolites – Remnants of the SSZ-type Devonian oceanic lithosphere in the European part of the Variscan Orogen

The Variscan Central-Sudetic Ophiolites (CSO) are located in the Sudetes,the NE segment of the Bohemian Massif. The Devonian CSO display highly depleted, harzburgite mantle sections containing gabbroic dykes and local occurrences of mostly isotropic, large gabbroic bodies as well as volcanic rocks. The ultramafic rocks locally show melt percolation-derived clinopyroxene-olivine aggregates and chromitites. The low REE composition and depletion in LREE relative to HREE of the clinopyroxene as well as the chromite Cr# and Mg# values are typical for phases formed from refractory melts occurring in the supra-subduction zone environment. The gabbroic bodies consist of differently evolved, mostly cumulate rocks, while the volcanic rocks form a relatively monotonous basalt sequence. The trace element compositions of both the plutonic and volcanic rocks display depleted N-MORB affinities, their derivation from a refractory mantle source is further reflected by previously published depleted mantle-like Sr-Nd isotopic compositions. The ultramafic and mafic members of the CSO show greenschist- to lower amphibolite-facies metamorphic overprints.The occurrence ofdepleted harzburgites and isotropic gabbros forming the mantle and the crustal sections of the CSO, respectively, indicate intermediate- to slow-spreading conditions and aheterogeneous structureof the oceanic lithosphere, untypical for layered ophiolites. The geochemical characteristics of the melt-percolation phases of the ultramafic rocks, the bulk-rock compositions of the mafic rocks and the homogeneous volcanic sequences suggest a formation of the CSO in a mature back-arc basin setting, lacking subduction-derived contamination. Spreading rate conditions and geochemical affinities of the CSO are similar to those of other Devonian ophiolites localized along the Variscan Belt, for instance the Upper Ophiolitic Units in the NW Iberian Massif (Spain), thereby confirming the general absence of N-MORB type lithosphere in the European Variscan Orogen.

Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley)

A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone (SSZ)) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso- and micro-structural observations suggests that T1 was near-coeval with respect to the D2, while T2 developed during D4.A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at ~46–41 Ma, while S2 foliation developed at ~40–36 Ma and was nearly coeval with the SM mylonitic foliation (~39–36 Ma).Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.

Metamorphic gabbro and basalt in ophiolitic and continental nappes of the Zermatt region (Western Alps)

The composition of meta-gabbro and meta-basalt occurring abundant and widespread in all nappes of the nappe stack exposed in the Zermatt region of the Western Alp shows distinct patterns related to the geodynamic origin of metamorphic basic rocks. Eclogitic meta-basalts of the ophiolitic Zermatt-Saas Unit (ZSU) show enriched MORB signatures. The meta-basalts (eclogites) of the continental fragment of the Theodul Glacier Unit (TGU) derive from pre-Alpine metamorphic continental intraplate basalts. Meta-basalts (eclogites) from the continental basement of the Siviez-Mischabel nappe (SMN) derive from MORB thus a genetic relation to the TGU eclogites can be excluded. All basic igneous rocks experienced post-magmatic alteration by fluid-rock interaction ranging from processes at the seafloor, in the shallow crust, during subduction zone hydration, in the exhumation channel and late Alpine regional metamorphisms. The consequences of these alteration processes can be identified at various levels in the rock composition data. It was found that the REE data are little affected by fluid-rock alteration. Some trace elements, notably Cs, Rb, and Ba are typically massively altered relative to igneous compositions in all three groups of meta-basalts. Generally, meta-basalts from the TGU and the SMN preserved the features of the original composition whilst the ZSU meta-volcanic rocks experienced massive alteration. For the ZSU meta-volcanic rocks it is evident that Zr was gained and Y lost during high-pressure fluid-rock interaction indicating a mobile behavior of the two elements during HP-metamorphism in contrast to their behavior in hydrothermal near-surface fluid-rock interaction.

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

AbstractDevastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear‐heating event. The well‐preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti‐poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10−6 [SI] volume, with P′ ranging from 1.05 to 2.5) to reconstruct the co‐seismic deformation parameters, that is, fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included mostly normal mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite‐hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution.

Fossil oceanic core complexes in the Alps. New field, geochemical and isotopic constraints from the Tethyan Aiguilles Rouges Ophiolite (Val d\u2019H\xe9rens, Western Alps, Switzerland)

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.

Protolith and tectonic implications of the No. Ⅳ pluton in Xiarihamu area, East Kunlun, Qinghai Province

东昆仑造山带西段夏日哈木地区IV号岩体是镁铁-超镁铁质侵入体还是蛇绿岩单元仍存争议。本文对IV号岩体开展了野外地质特征、岩相学、地球化学及锆石U-Pb年代学的研究。IV号岩体主体为斜长角闪岩，呈透镜状产在片麻岩中。斜长角闪岩主要由角闪石、斜长石组成，含少量石英、黑云母、榍石和铁钛氧化物。具有基性岩的化学成分，属于拉斑玄武岩系列岩石，稀土配分模式中Eu显示正异常（δEu=1.3~1.6），ε_Nd（t）值为+3.5~+5.3，推测其原岩可能为堆晶辉长岩，明显不同于含铜镍矿的Ⅰ号镁铁-超镁铁岩体，与该地区出露的榴辉岩（退变榴辉岩）可以对比。斜长角闪岩中的锆石具有弱环带，采用LA-MC-ICP-MS方法得出的²⁰⁶Pb/²³⁸U加权平均年龄为421±4Ma （MSWD=2.8），Th/U比值为0.01~3.26，指示锆石为变质-深熔成因，代表了斜长角闪岩的形成时代。夏日哈木地区的斜长角闪岩和该地区出露的榴辉岩及榴闪岩的原岩可能都是早期形成于陆壳环境的基性岩（大陆边缘的基性侵入岩或喷出岩），在430Ma左右发生陆壳深俯冲作用时形成榴辉岩，420~410Ma榴辉岩发生不同程度退变形成榴闪岩和斜长角闪岩。由此推测，IV号岩体的构造属性与该地区的榴辉岩属性相同，形成于陆壳环境，其原岩不属于蛇绿岩单元。斜长角闪岩的出现指示了该地区原特提斯洋壳已俯冲消亡，深俯冲的陆壳开始折返，这对认识东昆仑早古生代的构造演化和铜镍矿形成的地质背景具有重要意义。;Whether the protolith of the No. IV pluton in Xiarihamu area in the western part of the East Kunlun Orogenic Belt belongs to a mafic-ultramafic intrusive body or a component (gabbro) of the ophiolitic unit remains debated. Here, we reported the occurrence and the petrographic, geochemical and zircon U-Pb geochronological analysis results of this pluton to clarify the above disagreement. Petrographically, this pluton is an amphibolite block occurring as lens within a set of gneisses. The amphibolite rocks mainly consist of hornblende and plagioclase, with minor quartz, biotite, titanite and Fe-Ti oxide. The samples collected from amphibolite pluton have SiO₂ contents varying from 43% to 50% with Mg^# values from 42 to 73, and chondrite-normalized REE patterns show they have a positive Eu anomaly (δEu=1.3~1.6), implying their protoliths were cumulative mafic rocks and belong to tholeiitic series. Furthermore, the amphibolite rocks have positive whole-rock ε_Nd(t) values of +3.5~+5.3, similar to those of the eclogite or retrograded-eclogite and different from those of the No. I Ni- and-Cu-bearing mafic-ultramafic rocks in Xiarihamu area. Zircon grains from the amphibolite pluton show weakly zonation on their CL images with Th/U ratios varying from 0.01 to 3.26, suggesting they have a metamorphic-anatexis origin rather than a magmatic one. The zircon grains yield a ²⁰⁶Pb/²³⁸U weight mean age of 421±4Ma (MSWD=2.8), which represent the formation time of the pluton. In other words, the formation of this pluton can be divided into two stages: the previous cumulate gabbro underwent a high-pressure metamorphism and formed eclogite in the subduction stage, then followed by a retrograde stage which led to the transformation from eclogite to amphibolite. To be more clearly, the protoliths of the amphibolite, eclogite and retrograded-eclogite in the Xiarihamu area are mafic rocks (mafic pluton or eruption) formed in a continental margin, they turned into eclogites during a deep continental subduction approximately at 430Ma, and finally became retro-eclogite and amphibolite with various retrograde metamorphism at 420~410Ma. Therefore, the protolith of the No. IV pluton is not a gabbro component of the ophiolitic unit. Our studying results indicate that the evolution of this region experienced a closing of Paleozoic Proto-Tethys oceanic basin and then an exhumation of subducted continental crust, which is important for understanding the Paleozoic tectonic evolution of East Kunlun Orogen and the geological setting of the Cu-Ni mineralization.

Multistage Magmatism in Ophiolites and Associated Metavolcanites of the Ulan-Sar’dag Mélange (East Sayan, Russia)

We present new whole-rock major and trace element, mineral chemistry, and U-Pb isotope data for the Ulan-Sar’dag mélange, including different lithostratigraphic units: Ophiolitic, mafic rocks and metavolcanites. The Ulan-Sar’dag mélange comprises of a seafloor and island-arc system of remnants of the Paleo-Asian Ocean. Detailed studies on the magmatic rocks led to the discovery of a rock association that possesses differing geochemical signatures within the studied area. The Ulan-Sar’dag mélange includes blocks of mantle peridotite, podiform chromitite, cumulate rocks, deep-water siliceous chert, and metavolcanic rocks of the Ilchir suite. The ophiolitic unit shows overturned pseudostratigraphy. The nappe of mantle tectonites is thrusted over the volcanic-sedimentary sequence of the Ilchir suite. The metavolcanic series consist of basic, intermediate, and alkaline rocks. The mantle peridotite and cumulate rocks formed in a supra-subduction zone environment. The mafic and metavolcanic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc calc-alkaline andesitic basalts, andesites, and dacites; (3) tholeiitic basalts of mid-ocean ridges; and (4) oceanic island basalts. U–Pb dating of zircons from the trachyandesite, belonging to the second geochemical type, yielded a date of 833 ± 4 Ma which is interpreted as the crystallization age during mature island-arc and intra-arc rifting stages. The possible influence of later plume magmatic-hydrothermal activities led to the appearance of moderately alkaline igneous rocks (monzogabbro, trachybasalt, trachyandesite, subalkaline gabbro, and metasomatized peridotites) with a significant subduction geochemical fingerprint.

The Peltetec ophiolitic belt (Ecuador): a window to the tectonic evolution of the Triassic margin of western Gondwana

ABSTRACT New field and petrographic observations, whole-rock major- and trace-element geochemical data, Nd and Sr isotope systematics and U-Pb SHRIMP zircon ages of the Peltetec metaophiolite and related rocks (Ecuador) allow the characterization of the igneous and sedimentary protoliths and the evaluation of the geodynamic evolution of the active western margin of Gondwana during the Mesozoic. The Peltetec ophiolitic belt, exposed along the western margin of the Cordillera Real (Ecuador) and previously considered of Cretaceous age, comprises slices of metamorphosed (greenschist facies) and variably deformed peridotites, gabbros (locally cumulate), basaltic lavas and dykes, hydrothermal breccias and sedimentary rocks. The geochemical characteristics of the tholeiitic basaltic rocks indicate a Sr-Nd isotope depleted mantle source with variable imprint of subduction zone-derived or continental crust-assimilated components, as indicated by enrichment in Th, negative anomaly of Nb and relative depletion in high field strength elements (HFSE). Geochemical diversity allows defining different groups of noncumulate igneous rocks with oceanic island arc and NMORB-like back-arc affinities. U-Pb zircon ages from a metabasite yielded 228.1 ± 1.7 Ma, interpreted as the crystallization age of the magma. These data allow proposing the generation of the Peltetec ophiolite in a back-arc setting as a consequence of late Triassic extension of the western continental margin of Gondwana. Granites located at the western margin of the Peltetec Unit with S-type affinity and equal Triassic age (U-Pb age of 228.2 ± 0.8 Ma) document coeval anatexis of the continental crust as a consequence of heating associated with the same extensional event that originated the ophiolite. Geologic correlation allows enlarging such tectonic scenario to include ophiolitic units of similar age in Colombia.

Tectonic Evolution of Neotethys Ocean: Evidence of Ophiolites and Ocean Plate Stratigraphy from the Northern and Southern belts in the Western Yarlung Zangbo Suture Zone, Tibet

AbstractThe Yarlung Zangbo suture zone (YZSZ) separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith to the north (Xu et al., 2015; Yang et al., 2015). The western YZSZ in southern Tibet is divided by the Zhongba terrane into the northern belt (NB) and southern belt (SB). Ophiolites in the NB are dismembered as ophiolitic mélanges. Peridotite, cumulated gabbro, ocean plate stratigraphy (OPS) of seamount remnants and pelagic‐hemipelagic sequence as blocks in serpentinite matrix are mainly observed, from west to east, in Dajiweng, Baer, Kazhan, Cuobuzha, Zhalai, Gongzhu. Ophiolites in the SB are absent ophiolitic units of sheeted dikes and MORB‐like pillow lavas, occur as much larger peridotite massifs (i.e., Dongbo, 400 km2; Purang, 650 km2; Xiugugabu, 700 km2; Dangqiong, 300 km2) which are intruded by mafic dike swarms and overlain by volcanic sedimentary OPS (Liu et al., 2018). We propose that the SB mafic–ultramafic rocks and volcanic sedimentary OPS represent fragments of an early Cretaceous continental margin ophiolite whose magmatic evolution was influenced by 140–137 Ma plume magmatism (Liu et al., 2015; Zheng et al., 2019). Relics of Late Paleocene to very Early Eocene deep‐marine basin were developed in Saga and Gyirong (Ding, 2003; Li et al., 2018). In contract, the NB ophiolitic mélanges report a travel log of an oceanic plate ranging from Middle Triassic to Early Cretaceous.

The Capo Castello Shear Zone (Eastern Elba Island): Deformation at the Contact between Oceanic and Continent Tectonic Units

Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.

Sulphur and lead isotope geochemistry of the Dursunbey (Balıkesir) lead–zinc deposit

The Biga Peninsula (northwest Anatolia) hosts several Pb–Zn occurrences, and the Dursunbey Pb–Zn deposit is one such important Pb–Zn province of Turkey.The basement rocks in the study area comprise metamorphic rocks of the Menderes Massif (Simav Metamorphites, Sarıcasu Formation, and Budağan Limestone) and ophiolitic units of the İzmir-Ankara Suture Zone, known as the Dağardı Mélange. These units were intruded by the Miocene Alaçam granite. Cover units start with the Early Miocene Yeniköy Formation (comprising alternating sandstone, claystone, marl, and clayey limestone, and tuff additives) and are terminated by Civanadağ tuffs, Akdağ volcanics, and the Toklargölü Formation, consisting of unattached grains and alluvium deposits.Mineralisation that is trending along N50°W 30–35°NE occurs between the Alaçam Granite and the carbonate rocks of the Menderes Massif Metamorphites. Microscopic analysis revealed that the mineralisation contains chalcopyrite, pyrite, sphalerite, galena, fahlerz (tetrahedrite, tennantite), pyrrhotite, pyrargyrite-proustite, magnetite, and haematite as major ore minerals.The δ34S values range from 2.4 to 4.1‰ in pyrite (n = 2, mean = 3.25‰), from 1.7 to 3.4‰ in chalcopyrite (n = 4, mean = 2.8‰), 2.2–3.5‰ in sphalerite (n = 6, mean = 3.05‰), but from only 0.2–1.8‰ (n = 8, mean = 1.12‰) in galena. The low δ34S values of the sulphide minerals (varying over a narrow range) may indicate that sulphur in the ore-depositing solutions had a magmatic origin.The Pb isotope ratios of galena ranged from 18.729 to 18.742 for 206Pb/204Pb, 15.589 to 15.606 for 207Pb/204Pb, and 38.589 to 38.647 for 208Pb/204Pb. In one sphalerite sample, these ratios were measured as 18.738, 15.614, and 38.648, respectively.The Pb isotope model ages are between −90 and −45, which are consistent with those of J-type lead. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb diagrams, such ages plot below the average Pb-isotope crustal growth curve, indicating that lead in the studied mineralisation had an orogenic source.

The Ganj Complex reinterpreted as a Late Cretaceous volcanic arc: Implications for the geodynamic evolution of the North Makran domain (southeast Iran)

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and oceanic tectonic units. Among these, the Ganj Complex was up to now regarded as an ophiolite unit. This Complex is represented by a >2000 m thick sequence consisting of a dyke-swarm showing transition to a volcano-sedimentary sequence with local interlayerings of turbidite beds. This sequence grades-up to a turbidite sequence whose calcareous nannofossil biostratigraphy indicates a Turonian-Coniacian age. Volcanic rocks and dykes are represented mainly by intermediate and acidic rocks and minor basalts. They show volcanic arc affinity, with calc-alkaline and island arc tholeiitic compositions, which locally show adakite signature. In the turbiditic arenites, volcanic-derived debris is predominant and it is associated with moderate amounts of coheval calcareous rock fragments and bioclasts; thus suggesting sediment provenance from intrabasinal volcanic arc and carbonatic platform source rocks.In contrast to previous interpretations, our new data indicate that the Ganj Complex represents remnants of a Turonian-Coniacian volcanic arc that was most likely forming in a relatively shallow marine environment in an extensional intra-arc setting. We therefore propose a new tectonic model in which the Ganj volcanic arc formed in the southern margin of the Lut block as a consequence of the north-dipping subduction of the North Makran oceanic basin. We suggest that this subduction was coeval with the subduction of the Southern Neo-Tethys, which was active to the south of the North Makran Ocean.

Seismically-induced serpentine dehydration as a possible mechanism of water release in subduction zones. Insights from the Alpine Corsica pseudotachylyte-bearing Monte Maggiore ophiolitic unit

The Monte Maggiore ophiolitic unit of Alpine Corsica consists of an intact to variably hydrated spinel lherzolite intruded by Jurassic gabbro dykes. A widespread serpentinization, marked by low-pressure serpentine polymorphs, results from sea water interaction with mantle rocks during Jurassic sea-floor spreading of the Piemonte-Liguria ocean. A second serpentinization event, marked by the presence of antigorite, is related to the Cretaceous to Paleogene subduction of the Piemonte-Liguria oceanic lithosphere. Tectonic pseudotachylyte veins locally crosscutting the unit, display several original characteristics. (1) They are hosted by serpentinite, showing that seismic ruptures can propagate through hydrated mantle rocks. (2) The host serpentinite in contact with pseudotachylyte veins shows a thin (500 μm or less) rim of secondary olivine newly crystallized at the expense of serpentine. The heat necessary for serpentine dehydration is likely provided by the frictional melt. Antigorite-bearing clasts (themselves partly dehydrated along their boundaries) reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability. This indicates that frictional meting occurred at pressures between 0.7 and 0.85 GPa, that is, at ~20 to ~30 km depths in the subducting Piemonte-Liguria oceanic slab. Since the dehydration of serpentine into olivine is only observed at the host-rock selvages of pseudotachylyte veins, it cannot be related to the crossing, by the subducting slab, of the regional 610 ± 100 °C isotherm of the subduction zone, temperature at which antigorite starts to dehydrate into olivine. An estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake, the average amount is about 1.36 L/m2 or 1.36 × 105 m3 for the entire rupture surface (assumed to be 100 km2). If the released water is not incorporated in the nearby frictional melt, it can flow away from the slip zone and contribute to the various fluid-rock interactions active in subduction zones. Alternatively, if the water cannot escape from the slip zone and if the pore pressure is locally high enough, it could trigger aftershocks following the initial seismic rupture.

Fe-Ti-Zr metasomatism in the oceanic mantle due to extreme differentiation of tholeiitic melts (Moa-Baracoa ophiolite, Cuba)

Ti-rich amphibole, Mg-rich ilmenite, baddeleyite, zirconolite, srilankite, and zircon are important high-field-strength elements (HFSE) bearing phases in the Potosí chromitite bodies located in the Moho Transition Zone of the Cretaceous Moa-Baracoa suprasubduction zone ophiolite (eastern Cuba). Such HFSE-bearing phases were found in the interaction zone between gabbroic intrusions and chromitite pods. In addition to HFSE-bearing minerals, the studied samples are composed of Fe3+ and Ti-rich chromite, olivine (Fo86-90), clinopyroxene (En44-49), plagioclase (An51-56), orthopyroxene (En84-94), F-rich apatite, and Fe-Cu-Ni sulfides. The studied ilmenite hosting Zr oxides (baddeleyite, zirconolite, and srilankite) contains up to 13 wt.% MgO. The Potosí zirconolite is the first record of this mineral in ophiolitic chromitites and non-metamorphic ophiolite units, and it has relatively high REE contents (up to 10 wt.% of REE2O3) and the highest concentrations in Y2O3 (up to 11 wt.%) reported so far in zirconolite from terrestrial occurrences. Zircon is observed forming coronas surrounding ilmenite grains in contact with silicate minerals, and is characterized by very low U and Pb contents. The zircons formed after high temperature Zr diffusion in ilmenite (exsolution) and a subsequent reaction along grain boundaries following crystallization. Finally, U-Pb dating of baddeleyite exsolutions within ilmenite yielded an average age of 134.4 ± 14 Ma, which provides the first ever dating for a metasomatic event in Potosí that matches well (within uncertainty) the formation age of the oceanic crust of the eastern Cuba ophiolite. We propose that the occurrence of HFSE- and REE-bearing minerals in the Potosí chromite deposit is the result of a two stage process: first, water-rich and HFSE-rich residual melts are produced by intercumulus crystal fractionation after an evolved MORB (BABB)-like melt; and secondly, these residual melt fractions escaped the solidifying mush and extensively reacted and metasomatized the surrounding chromitites, crystallizing HFSE- and REE-bearing minerals and Fe-Cu-Ni sulfides.

Melt-rock interaction between granitic pegmatites and hosting amphibolites from the Chiavenna Ophiolitic Unit (Tanno Pegmatitic Field, Central Alps, North Italy)

The Tanno pegmatitic field, placed southward of Chiavenna (Central Alps, Sondrio, Italy), develops a large number of subplanar dykes that crosscut the Chiavenna Unit, an ophiolitic complex mainly composed, in the study area, of amphibolite rocks. This study focuses on the contact between a pegmatitic dyke and the amphibolitic country rock. We distinguished four zones across the contact: I) inner amphibolite, II) contact amphibolite, III) contact pegmatite, IV) inner pegmatite. The inner amphibolite, not affected by melt-rock interaction, is composed of amphibole, phlogopite, ilmenite, titanite and rutile. Two amphibole generations occur, both of them showing a patchy compositional zoning. Amphibole I are Mg-hornblende, whereas Amphibole II have a pargasitic composition. The contact amphibolite shows an enrichment of mica belonging to the phlogopite-biotite series, titanite and the presence of fluorapatite and plagioclase (Ab45-60), that is absent in the inner amphibolite. Close to the contact, amphiboles display no zoning and gain an Mg-horneblenditic composition. The contact pegmatite has quartz, albitic plagioclase, garnet (almandine-spessartine series), muscovite, K-feldspar and fluorapatite. It shows a comb texture, with elongation of plagioclase crystals normal to the contact itself. Far from the contact, the inner pegmatite has an increasing grain-size and a less organized texture. In this zone several accessory phases occur, including gahnite, columbite-(Fe), monazite-(Ce), xenotime-(Y), uraninite and betafite. Whole rock analyses suggest that a chemical exchange, concerning both major elements and trace elements, occurred between the pegmatitic melt and the hosting amphibolite. A considerable increase of SiO2, Na2O and, to a lesser extent, of Al2O3 is observed from the amphibolite towards the pegmatite; K2O and CaO show a decrease at the same extent. The REE pattern in the pegmatite highlights an enrichment in HREE at the contact. Mineral chemistry confirms this trend with variations observable in plagioclase, gradually more albitic from the amphibolite to the pegmatite. Mineralogical characters and geochemical features allow to classify the Tanno pegmatite in the LCT (Lithium, Cesium, Tantalium) family. Based on the metamorphic peak conditions reported from the Lepontine Dome the ambient conditions during pegmatite intrusion were ca. 550°C and 5 kbar. The reduced thermal difference between pegmatite and wall rock explains the diffuse contact observed by X-ray micro-computed tomography. The collected data suggest a chemical interaction between melt and wall rock, according to the following reaction taking place in the amphibolite Amphibole I + Amphibole II + Ilmenite + Pegmatitic melt → Amphibole III + Plagioclase + Phlogopite + Titanite + Fluorapatite

The basement geology of Japan from A to Z

AbstractThe Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.

Origin and geochemical evolution of groundwaters at the northeastern extend of the active Fethiye-Burdur fault zone within the ophiolitic Teke nappes, SW Turkey

Thermal and nonthermal, mineral and non-mineral spring waters in different physical and chemical characters occur along the active Fethiye-Burdur fault zone (FBFZ) in the southwestern Turkey and discharge mainly within the supra-subduction-type ophiolitic units of the Teke nappes. This research aims to determine the physical and hydrogeochemical characteristics, origin, and geochemical evolution of the groundwaters discharging from the same lithology and tectonic zone within the northeastern extend of the active Fethiye-Burdur fault zone. A total of representative nine groundwater samples were collected from the study area. Of which, one of the samples is thermal, and others are nonthermal groundwaters. One of the nonthermal groundwaters is mineral water type, and the other one is hyperalkaline (pH > 10). Based on the analytical results, the waters in the study area are divided into six different groundwater facies: (i) Mg-Ca-HCO3-SO4, (ii) Mg-HCO3-CO3, (iii) Ca-HCO3, (iv) Ca-Mg-HCO3, (v) Mg-HCO3, and (vi) Ca-Na-Cl-CO3 facies. The chemical characteristics of the groundwaters are mainly related to mineralogic and hence chemical composition of sub-surface rocks, residence time of the groundwater, and water-rock intereactions, whether or not the conditions closed to CO2. Neutral to moderate alkaline (pH 10) Ca-Na-Cl-CO3-type waters within deeper ultramafic rock aquifers under the conditions closed to CO2. Mainly altered mafic-ultramafic silicates and carbonate rocks are the major sources of mineralization in the study area. On the other hand, H2S (most probably magmatic origin) reacted with carbonate rocks underlying the ophiolitic melange units produced CO2(g). Dissolution of CO2 within groundwaters is thought to cause arising of acidic conditions, higher degree dissolution of mafic-ultramafic silicates, and mineral water formation.

The Ribeirão da Folha ophiolite-bearing accretionary wedge (Araçuaí orogen, SE Brazil): New data for Cryogenian plagiogranite and metasedimentary rocks

The Araçuaí orogen and West Congo belt make up a singular confined orogenic system, embraced by the San Francisco – Congo craton: the AWCO. It includes a southern sector with ophiolite bodies and magmatic arc, and a northern sector free of them, suggesting the precursor basin was an embayment partially floored by oceanic crust. The northernmost ophiolitic rock-assemblage found in the Araçuaí orogen comprises metamafic and meta-ultramafic rocks with signatures of ocean-floor magmas, and associated pelagic to oceanic metasedimentary rocks of the Ribeirão da Folha Formation. Although tectonically dismembered and metamorphosed, those rocks resemble the classical ophiolite pseudostratigraphy. The Ribeirão da Folha Formation comprises rocks expected to be found in the upper units of an ophiolite edifice, like Al-rich micaschist (pelagic pelite), graphite-rich schist (black shale), sulfide-bearing metachert, diopsidite with massive sulfide, and banded iron formations (chemical-exhalative sediments), and sulfide-bearing fine-grained ortho-amphibolite with thin metachert intercalations (mafic volcanic unit). That formation hosts tectonic slices of banded ortho-amphibolite (dolerite) with plagiogranite veins, and coarse-grained massive ortho-amphibolite (gabbro), representing dismembered slivers from deeper mafic units, and slices of meta-ultramafic rocks from the deepest ophiolite units. Zircon crystals from a plagiogranite vein yielded the U-Pb SHRIMP age of 645 ± 10 Ma, providing a new time constraint for ocean-floor emplacement. Micaschist samples show chemical attributes typical of distal passive margin pelites. Among three progressive deformation phases, the main ductile phase (Dn) shows kinematic indicators related to top to SW mass transport, associated with intermediate P-T (St, Ky, Sil) metamorphic zoning. Although the few youngest grains of detrital zircon from three siliciclastic samples have distinct ages (around 599 Ma, 741 Ma, and 816 Ma), their wide-range age spectra and Hf signatures suggest similar sediment sources. The wide lithological variety and stratigraphic complexity along with the intricate tectonic framework of the Ribeirão da Folha region, comprising thrust slices of ophiolitic rocks tectonically interleaved with older rift-related rocks, characterize an accretionary wedge that was scrapped off the subducted slab and involved in collisional tectonics, marking the AWCO suture zone for some 250 km between the Guanhães basement block (lower plate) and Rio Doce magmatic arc (upper plate).

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

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