Ophiolitic Remnants Research Articles

Geochemistry and petrogenesis of ophiolitic rocks from the Indus Suture Zone (ISZ), Ladakh Himalaya: insights for depleted mantle beneath an intra-oceanic island arc complex

ABSTRACT The Indus Suture Zone preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Parts of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include ultramafic-mafic cumulates, gabbros, and volcanics emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, these rocks show sub-alkaline tholeiitic characteristics with basalt to basaltic-andesite compositional variation. Based on modal mineralogy, the ultramafic cumulates classify as olivine-websterite, the mafic cumulates as olivine-norite, and the gabbros as norite to hornblende-gabbro. The ultramafic cumulates depict depleted chondrite normalized REE patterns [(La/Yb)N = 0.6–1.1] while the mafic cumulates display depleted to enriched REE patterns [(La/Yb)N = 0.6–3.2]. Fractionated patterns are observed in gabbros [(La/Yb)N = 1.6–4.1], while flat and enriched chondrite normalized REE-patterns in volcanics [(La/Yb)N = 1.0–12.3] similar to NMORB, and EMORB. The multi-element patterns depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to the primitive mantle. Presence of Mg-rich olivines, orthopyroxenes, and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic-mafic cumulates reflect their derivation from previously depleted mantle sources at high-pressure and temperature comparable to the base of modern intra-oceanic island arc tholeiitic sequences. The studied rocks exhibit close similarity to an intra-oceanic subduction system contemporaneous to the Dras-Shergol-Suru-Thasgam ophiolitic slices of western Ladakh.

“Rootless” Ophiolites above the Exhuming Pelagonian Core Complex, Northern Greece

The Mesohellenic ophiolites (MHO) in the Western Hellenides are part of an oceanic slab emplaced onto Pelagonian (Pangaean) continental rocks in the mid-Jurassic with a documented NE ophiolite emplacement. Ophiolitic outliers to the east of the MHO are oceanic lithospheric fragments, not complete ophiolite bodies, preserved above exhumed Pelagonia continental rocks. As these fragments lack connection to original root zone provenance, we refer to these as the “rootless” ophiolites.Pelagonian exhumation, possibly triggered by transcurent shear along its continental margin with the Pindos basin, began by the Late Jurassic and continued into the mid-Cretaceous. Exhumation affected the emplaced oceanic slab in the following ways: i) The metamorphic facies of the basal mélange separating the ophiolite from the Pelagonian basement grades from phyllitic to schist and amphibolite-schist over the exhumed Pelagonia. ii) Ophiolitic remnants are metasomatized where in contact with the exhumed Pelagonian rocks. iii) Remnant ophiolitic fragments are rotated and largely disassociated from their original relative pseudostratigraphic positions in their parent slab. iv) No amphibolite emplacement soles are preserved beneath ophiolitic remnants found directly above Pelagonia.East of Vourinos, remnants of the slab were tectonically entrapped between the exhuming Pelagonian core and its sedimentary overburden, and demonstrate extensional, largely gravitational displacements as well as rotation from original emplacement vectors. Primary constrictive slab emplacement features are obscured, but a general westerly sense of kinematics via listric and extensional faults have been imprinted. In the exhumation model, this "SW topping" direction cannot be interpreted as indicative of an eastern origin of the Pindos Basin ophiolites from the Vardar Zone, but rather as a local response to the uplift of Pelagonia and active deformation of the sedimentary overburden.

Paleoproterozoic ophiolite remnants in the northern margin of the North China Craton: Evidence from the Chicheng peridotite massif

Precambrian ophiolites provide important clues to the onset of modern-style plate tectonics. Peridotite massifs exposed in Chicheng area in the northern margin of the North China Craton occur as discrete tectonic lenses or boudins within Archean-Paleoproterozoic meta-volcanic and meta-sedimentary rocks of the Hongqiyingzi Group, along with retrograde eclogites. These serpentinized peridotites are composed dominantly of harzburgites and dunites, with minor ultramafic cumulates. Here we report the petrography, whole-rock chemical compositions, and mineral chemistry of the Chicheng peridotites with a view to evaluate their genesis and tectonic significance. The high Mg-number (91.2–93.5), low Al2O3 and CaO contents of harzburgites and dunites, as well as the high Fo values of olivine and Cr-number of spinel from these rocks suggest that the rocks represent highly refractory residues of partial melting. The ultramafic cumulates on the top of the peridotite massif might represent the early ultramafic cumulates from partial melt, as indicated by their relatively low Mg-number (87.0 and 91.8), and slightly higher Al2O3 and CaO contents. We infer that the Chicheng peridotites represent fragments of ancient sub-oceanic lithospheric mantle. The peridotites and the coexisting retrograded eclogites together with the gneisses in Chicheng might form part of a tectonically dismembered Paleoproterozoic ophiolite. The ophiolite was emplaced along the continental margin and subducted to the depth of eclogites facies during the formation of the Trans-North China Orogen. The metasomatized orthopyroxene in the Chicheng peridotites and flat REE pattern of whole rocks indicate melt/fuild metasomatism on residual peridotites during the post-melting stage. Amphiboles mostly show tremolite composition and were formed by retrograde metamorphism as displayed by their low Na, K, Ti and high Si contents. The ubiquitous existence of metamorphic amphiboles in Chicheng peridotites suggests that the ophiolite was subjected to amphibole-facies metamorphism before their exhumation in the Phanerozoic. The recognition of the ophiolite and its evolution history suggest that modern-style plate tectonics had begun at least in the Paleoproterozoic.

Weathering of Ophiolite Remnant and Formation of Ni Laterite in a Strong Uplifted Tectonic Region (Yuanjiang, Southwest China)

The Yuanjiang Ni deposit in southwestern margin of the Yunnan Plateau is the only economically important lateritic Ni deposit in China. It contains 21.2 Mt ore with an average grade of 1.05 wt % Ni and has been recognized as the second largest Ni producer in China following the Jinchuan super-large magmatic Ni–Cu deposit. This Ni deposit is hosted within the lateritic regolith derived from serpentinite within the regional Paleo-Tethyan Ophiolite remnants. Local landscape controls the distribution of the Ni mineralized regolith, and spatially it is characterized by developing on several stepped planation surfaces. Three types of lateritic Ni ores are identified based on Ni-hosting minerals, namely oxide ore, oxide-silicate mixed ore and silicate ore. In the dominant silicate ore, two phyllosilicate minerals (serpentine and talc) are the Ni-host minerals. Their Ni compositions, however, are remarkably different. Serpentine (0.34–1.2 wt % Ni) has a higher Ni concentration than talc (0.18–0.26 wt % Ni), indicating that the serpentine is more significantly enriched in Ni during weathering process compared to talc. This explains why talc veining reduces Ni grade. The geochemical index (S/SAF value = 0.33–0.81, UMIA values = 17–60) indicates that the serpentinite-derived regolith has experienced, at least, weak to moderate lateritization. Based on several lines of paleoclimate evidence, the history of lateritization at Yuanjiang area probably dates to the Oligocene-Miocene boundary and has extended to the present. With a hydrology-controlled lateritization process ongoing, continuous operation of Ni migration from the serpentinite-forming minerals to weathered minerals (goethite and serpentine) gave rise to the development of three types of Ni ore in the regolith. Notably, the formation and preservation of the Yuanjiang lateritic Ni deposit has been strongly impacted by regional multi-staged tectonic uplift during the development of Yunnan Plateau. This active tectonic setting has promoted weathering of serpentinite and supergene Ni enrichment, but is also responsible for its partial erosion.

An Upper Crustal Ophiolite Remnant within the Feather River Ultramafic Belt, California: Tectonomagmatic Origins and Implications for Its Evolution

The 150-km-long Feather River ultramafic belt (FRB) of the northern Sierra Nevada, California, consists of serpentinized ultramafic rocks, gabbroic rocks, with lesser amounts of amphibolitic rocks. The gabbroic rocks contain metagabbro and dikes within it. Based on the electron microprobe analyses, the metagabbro consists of typical igneous composition for clinopyroxene and early hornblende, indicating that they bear low grade metamorphism. The gabbroic rocks show Nb and Ta depletion in primitive-mantle normalized plots that is similar to typical island arc rocks. We present that the metagabbro within the ultramafic rocks is forming in the suprasubduction zone setting. Tectonic models are presented for the metagabbro and amphibolitic rocks in FRB. The low P burial metamorphism of the upper plate metagabbro and the high grade amphibolitic rocks are typical of relationships of upper plate ophiolites to metamorphic soles. The amphibolitic rocks record as metamorphic sole forming during subduction initiation beneath the ultramafic rocks and metagabbro. The unusual and protracted tectonic history of the FRB is similar to other ophiolite belts of the Cordillera of California but differs in the lack of crustal rocks.

Multiple convergences along an Archean craton margin: Clues from Proterozoic ophiolite remnants, granites and granulite domains along the SE margin of India

A collage of disparate tectonostratigraphic units intermittently stitched together in the Paleoproterozoic to Neoproterozoic constitutes the southeastern margin of the Archean Eastern Dharwar Craton (EDC). Two small ophiolitic slivers namely the 1.9 Ga Kandra Ophiolite Complex and the 1.33 Ga Kanigiri Ophiolitic Mélange (KOM) indicate two separate subduction-related intra-oceanic arcs outboard of the EDC margin in the Paleoproterozoic and the Mesoproterozoic, respectively. Post-tectonic granites intrusive into the KOM, adjoining domains of the Nellore Schist Belt and the Nallamalai Fold Belt add to the bulk of the collage which also includes the Ongole domain of the southern Eastern Ghats Belt. Generation of UHT granulite and migmatitic granite during 1.68-1.54 Ga probably indicating collision related tectonothermal activity, relate the Ongole domain to the Rayner complex of East Antarctica, which have similar isotopic signatures. The tectonic collage was welded with the EDC margin only in the Neoproterozoic as the intracratonic to cratonic marginal basins namely the Cuddapah Basin and the Kurnool Basin had open sea connections from the Paleoproterozoic to the Neoproterozoic. We re-examine the geological, structural and geochronological data in order to understand the applicability of a plate tectonic paradigm toward the growth of the EDC margin in the Proterozoic, and possible global connections.

Two-stage terrane assembly in Western Gondwana: Insights from structural geology and geophysical data of central Borborema Province, NE Brazil

Combined geophysical and structural data from the Transversal Subprovince of the Borborema Province (NE Brazil) highlight the internal structure of, and interrelationships between, the constituent terranes. Radiometric and magnetic maps show distinctive signatures for the Archean-Paleoproterozoic Alto Moxotó and Meso-to Neoproterozoic Alto Pajeú and Pernambuco-Alagoas terranes. Mapped radiometric and first and second order magnetic lineaments, associated with Euler deconvolution, enable correlation between geophysical data and major structures. In addition to early related deformation of the Alto Moxotó Terrane, combined analysis of late transposition foliations, lineations and kinematic criteria reveal a complex structural evolution marked by two distinct assembly stages. The first phase is characterized by thrust tectonics with top-to-the-south vergence, resulting in the juxtaposition of the allochtonous Alto Pajeú Terrane with the structurally underlying Alto Moxotó Terrane. The terrane boundary is delineated by the Serra de Jabitacá Shear Zone, which is associated with low dipping collisional granitic sheets and ca. 1.0 Ga mafic-ultramafic rocks interpreted as obtudcted ophiolite remnants. Later strike-slip movements strongly folded and obliterated thrust-related markers, and the continental scale E-W Pernambuco Lineament is interpreted as the result of lateral assembly between the composite Alto Pajeú-Alto Moxotó terranes and the Pernambuco-Alagoas Terrane during the metamorphic peak of the Brasiliano orogeny. Evolution of the Borborema Province reflects accretionary processes during the assembly of Western Gondwana.

The 2.58 Ga São José do Jacuipe gabbro-anorthosite stratiform complex, Itabuna-Salvador-Curaçá Orogen, São Francisco Craton, Brazil: Root of the Neoarchaean Caraiba continental arc?

Mafic-ultramafic complexes are common in ancient orogenic belts and can represent arc roots, obducted ophiolite remnants, continental intrusions, subducted oceanic plateaux or lithospheric mantle fragments. The Archaean-Palaeoproterozoic Itabuna-Salvador-Curaçá̧ Orogen hosts several mafic-ultramafic complexes, whose petrogenetic understanding is difficult owing to the high-grade metamorphism and deformation. One of these complexes is the 2583.7 ± 7.7 Ma São José do Jacuípe Gabbro-Anorthosite Complex, composed of massive to stratiform, deformed and metamorphosed anorthosites, leucogabbros, and gabbros with minor pyroxenites. Field relationships of the mafic-ultramafic rocks combined with calcic-pyroxene chemistry, negative Nb anomalies in mantle-normalised diagrams, and dominantly positive εNd(t) values suggest similarities with arc mafic magmatism. The gabbro-anorthosite complex matches the ages of arc orthogneisses (2574 ± 6 Ma) and norite-hypersthenite complex (2580 ± 10 Ma) of the Caraiba Complex and is thus interpreted as a fragment of the root of the Neoarchaean Caraíba magmatic arc within the Itabuna-Salvador-Curaçá Orogen.

Ophiolitic Remnants from the Upper and Intermediate Structural Unit of the Attic-Cycladic Crystalline Belt (Aegean, Greece): Fingerprinting Geochemical Affinities of Magmatic Precursors

The ophiolitic rocks of the Attic-Cycladic crystalline belt are considered of key importance for understanding the Mesozoic tectonic evolution of the Aegean region. Unresolved questions concern their tectono-stratigraphic relationships across the region. The mode of occurrence of the Cycladic ophiolites varies, as they appear as: (a) dismembered blocks (olistoliths) within the supra-detachment units of Paros and Naxos; (b) mélange formations in the upper structural unit of western Samos and Skyros; and (c) meta-ophiolitic mélange in the Cycladic Blueschist Unit (CBU) from central Samos. The trace element geochemistry and Pb-Sr-Nd isotopes of the mafic ophiolitic rocks indicate four geochemical groups: (a) the upper unit metabasites from Paros and western Samos (Kallithea) display an evolved basaltic composition (Mg# 40.2–59.6), with low Zr/Nb values (5–16) and high Ce/Y values (1.3 to 2.6) compared to MORB, indicating island-arc tholeiite affinities; (b) Naxos upper unit metabasalts show spider diagrams patterns indicating ocean island basalt (OIB-type) affinities; (c) Central Samos metagabbros (CBU) are primitive rocks with Back-Arc Basin basalt affinities; (d) the Skyros metadolerites and Tinos (Mt Tsiknias) and S. Evia (CBU) metagabbros, cluster as a separate geochemical group; they exhibit high MgO values (>10 wt %), very low TiO2 values (0.1–0.2 wt %), Y and Yb, and depleted trace element N-MORB normalized patterns, similar to volcanic rocks formed in modern oceanic fore-arc settings, such as boninites. A combination of the Pb- and Sr-isotopic compositions of Cycladic metabasites indicate that the Pb and Sr incorporated in the Cycladic ophiolites correspond to mixtures of magmatic fluids with seawater (206Pb/204Pb = 18.51–18.80; 207Pb/204Pb = 15.59–15.7; 208Pb/204Pb = 39.03–39.80 and initial 87Sr/86Sr80 = 0.705–0.707). Furthermore, peridotite relicts from Samos, Paros, and Naxos—irrespective of the structural unit—display chemical affinities of ocean floor peridotites formed in a supra-subduction zone. The characteristics of harzburgite relicts in Cycladic serpentinites and Skyros indicate a highly residual nature of the mantle source. Geochemical data from this study shed further light on the complex structure of the oceanic lithosphere from which the Cycladic ophiolites originated.

Petrogenetic significance of the eclogites from the Arquía Complex on southwestern Pijao, Central Cordillera (Colombia Andes)

Las eclogitas descubiertas en el suroeste de Pijao, Cordillera Central (Colombia Andes), son ricas en almandino (Xalm = 0.50-0.58; XFe = 0.69-0.83) y grosularia (Xgrs = 0.25-0.30), con cantidades menores de piropo (Xprp = 0.12-0.23) y relativamente baja espesartina (Xsps = 0.01-0.05), y la onfacita presenta contenidos de jadeíta, acmita y augita en el clinopiroxeno analizado de 7,00, 3,19 y 89,80% en moles; XMg= 0.82. Las eclogitas sufrieron una trayectoria PT en sentido horario con un ligero aumento de temperatura posterior al Ppeak (a 10.5 kbar y 535°C) a un máximo (Tpeak) a 8.5 kbar and 575°C y un posterior enfriamiento durante la descompresión a 4.5 kbar and 530°C. Los datos geoquímicos indican protolitos que corresponden a basaltos de tipo dorsales oceánicas normales (NMORB). Todas estas características sugieren que los remanentes ofiolíticos están involucrados en la subducción y arquitectura final del Complejo Arquía. La progresiva convergencia condujo a la formación de las asociaciones de la facies eclogites a la máxima profundidad en la corteza continental. Las eclogitas reportadas en este estudio representan un excelente ejemplo de rocas metamórficas de alta presión en cinturones orogénicos, muy útil para la comprensión de subducción de placas y los procesos de colisión de los continentes.

Metamorphic sole formation, emplacement and blueschist facies overprint: early subduction dynamics witnessed by western Turkey ophiolites

AbstractThe largest ophiolite on Earth, in western Turkey, is a key place to study obduction and early subduction dynamics. Ophiolite remnants derived from the same Neotethyan branch (preserved as a result of long‐lived Late Cretaceous continental subduction and later obduction) are underlain by hundred‐metre‐thick extensive metamorphic soles. These soles formed synchronously, at c. 93 Ma, and were welded to the base of the ophiolite, thereby dating the start of intra‐oceanic subduction. This contribution focuses on the structure, petrology and pressure–temperature evolution of the soles and other subduction‐derived units. Peak pressure–temperature conditions were estimated at 10.5 ± 2 kbar and 800 ± 50 °C for the sole by means of pseudosection calculations using Theriak/Domino and at 12 kbar and 425 °C for the unique, enigmatic blueschist facies overprint of the sole. This study provides clues to the mechanisms of sole underplating during early subduction, later cooling, and the nature of the western Turkey ophiolite.

Carboniferous and Permian evolutionary records for the Paleo‐Tethys Ocean constrained by newly discovered Xiangtaohu ophiolites from central Qiangtang, central Tibet

AbstractReconstructing the evolutionary history of the Paleo‐Tethys Ocean remains at the center of debates over the linkage between Gondwana dispersion and Asian accretion. Identifying the remnants of oceanic lithosphere (ophiolites) has very important implications for identifying suture zones, unveiling the evolutionary history of fossil oceans, and reconstructing the amalgamation history between different blocks. Here we report newly documented ophiolite suites from the Longmu Co‐Shuanghu Suture zone (LSSZ) in the Xiangtaohu area, central Qiangtang block, Tibet. Detailed geological investigations and zircon U‐Pb dating reveal that the Xiangtaohu ophiolites are composed of a suite of Permian (281–275 Ma) ophiolites with a nearly complete Penrose sequence and a suite of Early Carboniferous (circa 350 Ma) ophiolite remnants containing only part of the lower oceanic crust. Geochemical and Sr‐Nd‐O isotopic data show that the Permian and Carboniferous ophiolites in this study were derived from an N‐mid‐ocean ridge basalts‐like mantle source with varied suprasubduction‐zone (SSZ) signatures and were characterized by crystallization sequences from wet magmas, suggesting typical SSZ‐affinity ophiolites. Permian and Carboniferous SSZ ophiolites in the central Qiangtang provide robust evidence for the existence and evolution of an ancient ocean basin. Combining with previous studies on high‐pressure metamorphic rocks and pelagic radiolarian cherts, and with tectonostratigraphic and paleontological data, we support the LSSZ as representing the main suture of the Paleo‐Tethys Ocean which probably existed and evolved from Devonian to Triassic. The opening and demise of the Paleo‐Tethys Ocean dominated the formation of the major framework for the East and/or Southeast Asia.

Detrital chrome spinel evidence for a Neotethyan intra-oceanic island arc collision with India in the Paleocene

Models that support a single collision scenario for India and Eurasia are incompatible with the evidence that an intra-oceanic island arc (IOIA) existed within the Neotethyan Ocean. Understanding the spatial and temporal extent of any IOIA is crucial for India-Eurasia collision studies as the entire ocean, including any intra-oceanic features, must have been consumed or emplaced prior to continental collision. Here, we review what is known about the Neotethyan IOIA and report evidence from sedimentary successions in NW India and southern Tibet to constrain when and where it was emplaced. We use detrital mineral geochemistry and supporting provenance and age data to identify the source of the sediments and compare the timing of erosion of IOIA-derived material in both regions.Detrital chrome spinels, extracted from distinct sedimentary horizons in southern Tibet (Sangdanlin) and NW India (Ladakh), exhibit similar average geochemical values (TiO2=0.09 and 0.24%, Cr#=0.66 and 0.68 and Mg#=0.45 and 0.53, respectively) and supra-subduction zone (SSZ), forearc peridotite signatures. Furthermore, they overlap with in-situ chrome spinels reported from the Spongtang Ophiolite in NW India and the Sangsang Ophiolite in southern Tibet. As with many of the ophiolitic remnants that crop out in and adjacent to the Yarlung-Tsangpo and Indus suture zones (YTSZ and ISZ respectively), the Spongtang and Sangsang ophiolites formed in an IOIA setting. Linking the source of the detrital chrome spinels to those analysed from remnant IOIA massifs in the YTSZ and ISZ is strong evidence for the emplacement of the IOIA onto the Indian margin.The timing of the IOIA collision with India is constrained by the depositional ages of the chrome spinel-bearing sediments to the end of the Paleocene (Thanetian) in southern Tibet and the Early Eocene in NW India. This indirectly provides a maximum age constraint of Late Paleocene-Early Eocene for intercontinental collision between India and Eurasia. Additionally, this study highlights the importance of targeting distinct sedimentary horizons in collision zones to find evidence for discrete tectonic events that may be obfuscated by later collisions.

Structure and peridotite of Gibraltar arc southern bloc: gravimetric and aeromagnetic evidences

The southern Gibraltar Arc’s bloc (Rif’s belt) has a complex geological setting including peridotites outcropping in the Rif and Betic. They are among the largest infracontinental mantle rock outcrops worldwide. In spite of several geological studies, the deep structure of this area remains poorly defined. The aim of this work is giving a reliable geometry of the major geological structures of the Rif, especially the peridotite bodies and faults or geological contacts. The results obtained from a multiscale analysis of geological contacts applied to gravity and aeromagnetic data were used to generate a structural map of the studied area. This method in association with the Euler method allowed estimating the strike direction, shape, dip, and depth of contacts. The map gives a model of the region and confirms the existence of several faults, localized or inferred, from former geological studies. This study suggests a large extension of the Beni Bousera peridotites on the offshore from Jebha to Ceuta, and highlights an extended anomaly related to deeper crops probably ophiolite remnant of the suture zone.

Structures, microfabrics and textures of the Cordilleran-type Rechnitz metamorphic core complex, Eastern Alps.

Rechnitz window group represents a Cordilleran-style metamorphic core complex, which is almost entirely located within nearly contemporaneous Neogene sediments at the transition zone between the Eastern Alps and the Neogene Pannonian basin. Two tectonic units are distinguished within the Rechnitz metamorphic core complex (RMCC): (1) a lower unit mainly composed of Mesozoic metasediments, and (2) an upper unit mainly composed of ophiolite remnants. Both units are metamorphosed within greenschist facies conditions during earliest Miocene followed by exhumation and cooling. The internal structure of the RMCC is characterized by the following succession of structure-forming events: (1) blueschist relics of Paleocene/Eocene age formed as a result of subduction (D1), (2) ductile nappe stacking (D2) of an ophiolite nappe over a distant passive margin succession (ca. E–W to WNW–ESE oriented stretching lineation), (3) greenschist facies-grade metamorphism annealing dominant in the lower unit, and (4) ductile low-angle normal faulting (D3) (with mainly NE–SW oriented stretching lineation), and (5) ca. E to NE-vergent folding (D4). The microfabrics are related to mostly ductile nappe stacking to ductile low-angle normal faulting. Paleopiezometry in conjunction with P–T estimates yield high strain rates of 10− 11 to 10− 13 s− 1, depending on the temperature (400–350 °C) and choice of piezometer and flow law calibration. Progressive microstructures and texture analysis indicate an overprint of the high-temperature fabrics (D2) by the low-temperature deformation (D3). Phengitic mica from the Paleocene/Eocene high-pressure metamorphism remained stable during D2 ductile deformation as well as preserved within late stages of final sub-greenschist facies shearing. Chlorite geothermometry yields two temperature groups, 376–328 °C, and 306–132 °C. Chlorite is seemingly accessible to late-stage resetting. The RMCC underwent an earlier large-scale coaxial deformation accommodated by a late non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime. The RMCC was rapidly exhumed during ca. 23–18 Ma.

Bimodal stable isotope signatures of Zildat Ophiolitic Mélange, Indus Suture Zone, Himalaya: implications for emplacement of an ophiolitic mélange in a convergent setup

Zildat Ophiolitic Melange (ZOM) of the Indus Suture Zone, Himalaya, represents tectonic blocks of the fragmented oceanic metasediments and ophiolite remnants. The ZOM is sandwiched between the Zildat fault adjacent to a gneissic dome known as Tso Morari Crystalline (TMC) and thin sliver of an ophiolite called as the Nidar Ophiolitic Complex. The ZOM contain chaotic low-density lithologies of metamorphosed oceanic sediments and hydrated mantle rocks, in which carbonates are present as mega-clasts ranging from 100 meters to few centimeters in size. In this work, calcite microstructures, fluid inclusion petrography and stable isotope analyses of carbonates were carried out to envisage the emplacement history of the ZOM. Calcite microstructure varies with decreasing temperature and increasing intensity of deformation. Intense shearing is seen at the marginal part of the melange near Zildat fault. These observations are consistent with the melange as a tectonically dismembered block, formed at a plate boundary in convergent setup. The δ18O and δ13C isotope values of carbonates show bimodal nature from deeper (interior) to the shallower (marginal, near the Zildat fault) part of the melange. Carbonate blocks from deeper part of the melange reflect marine isotopic signature with limited fluid–rock interaction, which later on provide a mixing zone of oceanic metasediments and/or hydrated ultramafic rocks. Carbonates at shallower depths of the melange show dominance of syn-deformation hydrous fluids, and this has later been modified by metamorphism of the adjacent TMC gneisses. Above observations reveal that the melange was emplaced over the subducting Indian plate and later on synchronously deformed with the TMC gneissic dome.

Petrological vestiges of the Late Jurassic-Early Cretaceous transition from rift to back-arc basin in southernmost Chile: New age and geochemical data from the Capitán Aracena, Carlos III, and Tortuga ophiolitic complexes

The ophiolitic remnants of the Upper Mesozoic Rocas Verdes basin in southernmost South America were studied from the perspectives of petrography, chemistry of minerals, bulk-rock geochemistry, and U-Pb geochronology. The study aimed to unravel the tectonic, magmatic, and metamorphic evolution of a suprasubduction rift zone that underwent a transition to a back-arc basin. The rifting phase and bimodal magmatism within the Rocas Verdes basin started prior to or during the Late Jurassic, as indicated by a gabbro in contact with pillow basalts that dated at 154 Ma. In the Late Jurassic Capitán Aracena and Carlos III complexes, tholeiitic basalts are geochemically comparable to enriched mid-oceanic ridge basalts. Back-arc basin development continued for 35 myr until the Early Cretaceous, as suggested by the ages of detrital zircons in cherty layers within pillow basalts and metamorphic titanite that crystallized during seafloor metamorphism near the spreading/magmatic axis. In the Early Cretaceous Tortuga Complex, tholeiitic basalts are comparable to normal mid-oceanic ridge basalts. Non-deformative metamorphism converted the primary mineralogy of the ophiolites to low- to intermediate-grade metamorphic assemblages formed during ocean-floor type alteration in a suprasubduction setting. Fossilized bacteria, preserved as rounded aggregates of titanite microcrystals, were identified in the pillow basalts up to the Early Cretaceous. The Rocas Verdes basin closed during the Andean orogeny, which started during the Late Cretaceous, and ophiolites were tectonically juxtaposed and thrust over the sedimentary infill of the quasi-oceanic basin in which they developed. The tectonic emplacement of the ophiolitic complexes was complete before the latest Cretaceous, as indicated by crystallization ages of granites intruded into the ophiolitic complexes.

The record of the Late Palaeozoic active margin of the Palaeotethys in NE Iran: Constraints on the Cimmerian orogeny

The Cimmerian orogen resulted from the collision and accretion of several Perigondwanan blocks to the southern margin of Eurasia between the Late Triassic and Early Jurassic, following the closure of the Palaeotethys ocean. Remnants of this orogen discontinuously crop out in N (Alborz range) and NE Iran (Mashhad–Fariman area) below the syn- to post-collisional clastic successions of the Shemshak Group (Upper Triassic–Middle Jurassic) and the Kashaf Rud Formation (Bajocian). In NE Iran rock associations exposed in the Binalood Mountains, Fariman and Darreh Anjir areas include mafic–ultramafic intrusive rocks, basalts, silicoclastic turbidites and minor limestones, which have been interpreted in the past as ophiolitic remnants of the Palaeotethys ocean. Original stratigraphic, structural, geochemical and geochronological data, described in this paper, suggest a different interpretation. The volcano-sedimentary units of Fariman and Darreh Anjir complexes where deposited during Permian in a subsiding basin were siliciclastic turbidites, derived from the erosion of a magmatic arc and its basement, interfinger with carbonates and basaltic lava flows with both transitional and calc-alkaline affinity. The coexistence of magmatic rocks with different geochemical signature and the sedimentary evolution of the basin can be related to a supra-subduction setting, possibly represented by a fault-controlled intra-arc basin. The Fariman and the Darreh Anjir complexes are thus interpreted as remnants of a magmatic arc and related basins developed at the southern Eurasia margin, on top of the north-directed Palaeotethys subduction zone long before the collision of Iran with Eurasia. They were later involved in the Cimmerian collision during the Triassic. New radiometric ages obtained on I-type post-collisional granitoids postdating the collision-related deformational structures suggest that the suture zone closed before mid-Norian times. Deformation propagated later northward into the Turan domain involving the Triassic successions of the Aghdarband region.

The significance of Late Devonian ophiolites in the Variscan orogen: a record from the Vosges Klippen Belt

The present work examines the lithological, structural, geochemical and geochronological records from the Klippen Belt located in the southern Vosges Mountains (NE France). The Klippen Belt is represented by discontinuous exposures of serpentinized harzburgite, ophicalcite, gabbro, gneiss and polymictic conglomerate overlain by deep marine pelitic sediments. Structural data and Bouguer anomalies reveal that the Klippen Belt coincides with a significant discontinuity now occupied by a granitic ridge. Gabbro geochemistry indicates a MOR-type affinity similar to recent slow-spreading ridges, but positive Ba, Sr, Th or U anomalies do not exclude the influence of fluids expelled from a subduction zone. A Sm–Nd isochron age of 372 ± 18 Ma is thought to reflect gabbro emplacement from a highly depleted mantle source (eNd = +11.3), and U–Pb zircon ages from a gneiss sample indicate that the basement found in the Klippen has a Neoproterozoic origin. Combined data indicate the formation of a deep basin during Late Devonian rifting. The Klippen lithologies could testify for the presence of an ocean–continent transition environment subsequently inverted during the Early Carboniferous. Basin inversion during the Middle Visean was probably controlled by rift-related structures, and resulted in folding of the sedimentary successions as well as exhumation along thrust zones of deep parts of the basin represented by the Klippen Belt. Based on correlations with the neighbouring Variscan massifs, it is proposed that the southern Vosges sequences represent a back-arc basin related to the North-directed subduction of the southern Palaeotethys Ocean. This geodynamic reconstruction is tentatively correlated with similar ophiolitic remnants in the northern part of the French Massif Central (Brevenne) and with the evolution of the southern Black Forest. The Late Devonian ophiolites are interpreted as relicts of small back-arc marginal basins developed during general closure of the Palaeozoic subduction systems.

Reply to comment by C. Pin and J. Rodríguez on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites”

Reply to comment by C. Pin and J. Rodríguez on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites”