Ophiolite Complex Research Articles

Age and Paleogeodynamic Nature of the Kalinovka Ophiolite Complex, Sikhote-Alin Orogenic Belt

Age and Paleogeodynamic Nature of the Kalinovka Ophiolite Complex, Sikhote-Alin Orogenic Belt

Early Jurassic intra-oceanic subduction initiation along the Bangong Meso-Tethys: Geochemical and geochronological evidence from the Shiquanhe ophiolitic complex, western Tibet

Due to the unclear tectonic setting of the Jurassic ophiolites within the Bangong-Nujiang suture, how and when intra-oceanic subduction initiated within the Bangong Meso-Tethys remain controversial. Based on the new petrological, geochemical, and geochronological data reported in this paper, ∼190–180 Ma MORB-like ophiolites and OIB-like mafic rocks were identified from the Shiquanhe ophiolitic complex in the western Bangong-Nujiang suture. The MORB-like gabbros exhibit strong light rare earth elements (LREEs) and heavy rare earth elements (HFSEs) depletion and were derived from partial melting of a highly refractory depleted mantle source with limited slab-derived fluids. The OIB-like mafic rocks are enriched in LREEs and HFSEs and were originated from the melting of upwelling asthenospheric mantle without subduction additions. The data for the MORB-like ophiolites reported in this paper and previously published data demonstrate the formation of Early Jurassic fore-arc ophiolites in response to the Early Jurassic intra-oceanic subduction initiation within the Bangong-Nujiang suture. The Early Jurassic OIB-like rocks formed in the extensional tectonic setting, together with regional geological evidence, suggest that the Early Jurassic initial subduction event within the Bangong Meso-Tethys resulted in broad upper plate extension. Our study not only reveals the intra-oceanic subduction initiation age of the Bangong Meso-Tethys but also implies that subduction initiation can cause strong extension in a wider region of the upper plate in the Tethys oceanic system, not just in the fore-arc region.

Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India

Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India

Rodingites from the Veria-Naousa ophiolite (Greece): Mineralogical evolution, metasomatism and petrogenetic processes

In the Veria-Naousa ophiolitic complex (north Greece), rodingite appears mainly in the form of cross cutting dykes within serpentinised peridotites. It is distinguished into three types, based upon the provenance of its protoliths, textural characteristics, mineralogical assemblages and geochemical affinities. Type I rodigites were derived from boninitic diabasic protoliths and their mineralogical assemblage include garnet + clinopyroxene + chlorite. Type II rodingites were formed at the expense of gabbroic precursors, comprising clinopyroxene + garnet + vesuvianite ± quartz, whereas Type III rodingites replaced diabasic tholeiitic protoliths comprising of garnets + vesuvianite + clinopyroxene + chlorite. Rodingitisation resulted in desilification, decrease of alkalies, Al, Fe, Mg and increase in Ca contents. In Type I rodingites the MREE (middle rare earth elements) and HREE (heavy rare earth elements) were slightly reduced. Type II rodingites experienced LREE (light rare earth elements) depletions, whereas MREE and HREE remained fairly stable. Restricted mobility of REE in Type III rodingites is assigned to shallow-level rodingitisation under decreasing pH.Rodingitisation occured in two distinct stages at fore-arc settings. The first stage occured under mildly oxidising conditions and enhanced CO2/H2O ratios. This stage affected the protoliths of all rodingite types. The second rodingitisation stage occured under more oxidising conditions and lower CO2/H2O ratios, which corresponds to the exhumation stage of the serpentinite-rodingite formations. Types II and III rodingites were subjected to further rodingitisation under the increasing influence of slab-derived hydrous phases at shallower depths, leading to the formation of late-stage andradite and vesuvianite. All stages of rodingitisation are estimated to have occurred under relatively moderate temperatures and pressure (~300 to 450 °C; ~2–6 kbar respectively).

Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

Melting of mafic slab and mantle peridotite during ridge subduction of the Proto-Tethys Ocean (Qilian Orogen, NW China)

Ocean-ridge subduction and the induced slab windows are important to understand the generation of arc magmatism and growth of juvenile crust both in present and ancient subduction zones. The physical conditions of oceanic ridges control the compositions of magmas during ridge subduction, and therefore magmatic composition can act as a marker of ridge subduction in ancient orogenic belts. Here we present the petrological, geochemical and geochronological studies for sodic rhyodacites and a concurrent ophiolite complex (basalt/dolerite-gabbro and harzburgite) within the North Qilian orogen, NW China. The sodic rhyodacites are low Sr/Y tonalite-trondhjemite with depleted SrNd isotopic compositions, suggesting derivation from melting of normal mid-ocean-ridge basalts (N-MORB) at 449 ± 2.9 Ma. Phase equilibrium and geochemical modeling confirm that the sodic rhyodacites formed via the partial melting of N-MORB sources at 3–5 kbar/780–840 °C. The basalts/dolerites exhibit similar geochemical characteristics to typical N-MORB rocks, and the gabbro gives zircon UPb age of 450 ± 2.5 Ma. The petrogenesis of the basalts/dolerites is similar to the “blow-torch” effect often seen during ocean-ridge subduction. The harzburgites are similar to abyssal peridotites in mineral compositions and have equilibrium temperatures of 1230–1460 °C, suggesting the obduction of a hot asthenospheric mantle on the continental margin. Our results show the integrated melting processes of the upwelling mantle peridotite and the subducted oceanic crust within the slab window during ridge subduction of the Proto-Tethys Ocean. These results are also significant in understanding the petrogenesis of low Sr/Y tonalite-trondhjemites, which are widely preserved in Archean to modern terranes.

Geochemical characteristics of an ophiolitic complex from Mt. Tenzan area, Saga Prefecture, northern Kyushu

Geochemical characteristics of an ophiolitic complex from Mt. Tenzan area, Saga Prefecture, northern Kyushu

Petrogenesis of Heterogeneous Mantle Peridotites from the Banovina Ophiolite Complex, Croatia

Petrogenesis of Heterogeneous Mantle Peridotites from the Banovina Ophiolite Complex, Croatia

Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea‐like back‐arc basin

AbstractThe Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back‐arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back‐arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left‐sloping rare earth element (REE) patterns and relatively low εHf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (εHf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt‐modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid‐modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc‐alkaline, the Neoproterozoic back‐arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back‐arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back‐arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.

Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia

Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.

Platinum-Group Minerals in the Placer of the Kitoy River, East Sayan, Russia

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. The article shows for the first time the results of a study of placer platinum mineralization found in the upper reaches of the Kitoy River (the southeastern part of the Eastern Sayan (SEPES)). Using modern methods of analysis (scanning electron microscopy), the authors studied the microtextural features of platinum-group minerals (PGM), their composition, texture, morphology and composition of microinclusions, rims, and other types of changes. The PGM are Os-Ir-Ru alloys with a pronounced ruthenium trend. Many of the Os-Ir-Ru grains have porous, fractured, or altered rims that contain secondary PGE sulfides, arsenides, sulfarsenides, Ir-Ni-Fe alloys, and rarer selenides, arsenoselenides, and tellurides of the PGE. The data obtained made it possible to identify the root sources of PGM in the placer and to make assumptions about the stages of transformation of primary igneous Os-Ir-Ru alloys from bedrock to placer. We assume that there are several stages of alteration of high-temperature Os-Ir-Ru alloys. The late magmatic stage is associated with the effect of fluid-saturated residual melt enriched with S, As. The post-magmatic hydrothermal stage (under conditions of changing reducing conditions to oxidative ones) is associated with the formation of telluro-selenides and oxide phases of PGE. The preservation of poorly rounded and unrounded PGM grains in the placer suggests a short transport from their primary source. The source of the platinum-group minerals from the Kitoy River placer is the rocks of the Southern ophiolite branch of SEPES and, in particular, the southern plate of the Ospa-Kitoy ophiolite complex, and primarily chromitites.

Mapping topographic, geophysical and gravimetry data of Pakistana contribution to geological understanding of Sulaiman Fold Belt and Muslim Bagh Ophiolite Complex

Mapping topographic, geophysical and gravimetry data of Pakistana contribution to geological understanding of Sulaiman Fold Belt and Muslim Bagh Ophiolite Complex

Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

Constraining the timing of Arabia-Eurasia collision in the Zagros orogen by sandstone provenance (Neyriz, Iran)

AbstractThe Zagros orogen, formed by the collision of the Arabian and Eurasian continental margins, represents one of the largest and richest oil and gas provinces in the world. The Zagros fold-thrust belt records collision and convergence along the Neotethys suture zone. By coupling field observations, sandstone modal analysis, U-Pb zircon dating, and Hf isotopic data from the Upper Cretaceous to Pliocene sedimentary succession of the Neyriz region, this paper documents several major provenance changes that allow us to propose a refined scenario for the Zagros orogeny. An ophiolitic complex dated by detrital-zircon U-Pb geochronology as ca. 95 Ma provided detritus to Upper Cretaceous-Paleocene strata deposited along the northeastern margin of the Arabian lower plate (ophiolite provenance). Yet, on the southwestern margin of the Eurasian upper plate, upper Paleocene-lower Eocene strata indicate provenance from Mesozoic magmatic rocks yielding zircons dated as ca. 240 Ma and 170 Ma as well as the recycling of clastic rocks. Since the early Miocene, the sedimentary basin located on the Arabian plate received both ophiolitic detritus and magmatic-arc, recycled clastic, and axial-belt metamorphic detritus from Eurasia. U-Pb ages of detrital zircons reflect polyphase magmatism at 170 Ma, 95 Ma, and 40 Ma on the Eurasian active margin. Our results indicate that progressive accretion, uplift, and exhumation of the Zagros orogen was well under way by the beginning of the Miocene in the Neyriz region. Literature data from adjacent regions suggest that the Arabia/Eurasia collision may have occurred diachronously and later in the Kermanshah and Lurestan areas to the north.

Petrogenesis of mantle peridotite and cumulate peridotite rocks from the Nagaland Ophiolite Complex, NE India

Here, we report new geochemical data from mantle peridotite (spinel lherzolite, dunite, and harzburgite) and cumulate peridotite (websterite) rocks from the northernmost portion of the Nagaland Ophiolite Complex. Based on detailed textural, mineral (major element) chemistry, and whole‐rock (major and trace element) chemistry, we reveal the origin of these rocks in different tectonic settings and their modification via the melt–mantle interaction process. The spinel lherzolite rocks with low Cr# (0.16–0.25) chrome spinel are mantle residues formed after low degrees (~10–15%) of partial melting in a mid‐oceanic ridge setting, which has been subsequently modified in a subduction zone setting. However, the dunites and harzburgite samples are interpreted to be depleted mantle residue formed in a supra‐subduction zone (SSZ) setting. Additionally, the dunites are inferred to be of replacive origin representing former harzburgites. Based on Cr# of chrome spinel, the dunites are classified into low Cr# (0.53–0.57), medium Cr# (0.73–0.79), and high Cr# (0.89–0.90) dunites. High Cr# dunite with low‐TiO2 (0.01–0.06 wt%) possibly formed by interaction with a boninitic melt, whereas low‐medium Cr# dunite with high TiO2 (0.14–0.34 wt%) formed by interaction with a high‐Ti melt in an SSZ setting. Harzburgite samples with their highly resorbed orthopyroxene grain boundaries contain chrome spinels (Cr# = 0.84–0.88; TiO2 = 0.00–0.06 wt%) similar to the high Cr# dunite samples and are also interpreted to be formed by interaction with a boninitic melt. The websterite samples with their low Mg# (0.79) olivine and low Ni (345–413 ppm) content are interpreted to be of cumulate origin formed in an SSZ setting. However, the interstitial chrome spinel blebs with low Mg# (0.18–0.19), high Cr# (0.60–0.61), Fe3+ (0.20–0.21), and very high TiO2 (2.03–2.04 wt%) are interpreted to be a result of post‐cumulus interaction of interstitial chrome spinel with high‐Ti intercumulus melt. These mantle peridotites and cumulates of different tectonic origins were obducted and juxtaposed during the final stages of the India‐Asia collision.

Geology of Zagros metamorphosed volcaniclastic sandstones: a key for changing the Mawat Ophiolite Complex to a metamorphic core complex, Kurdistan Region, NE-Iraq

Mawat Ophiolite Complex is located about 36 km to the northeast of Sulaimani city and directly to the east-northeast of Mawat town near the border of Iran in the northeastern Iraq. The complex has about 600-km2 surface area and consists of high mountain terrains that subjected to intense geological investigations from the fiftieth of previous century till now. According to previous studies, the complex contains tens of igneous rocks such as basalt, metabasalt, tuff, diabase, metadiabase, diorite dykes, periodotite, serpentinite, serpentinite-matrix mélange, gabbro, metagabbro, harzbergite, pyroxenite, plagiogranite, pegmatite, granitiod rocks and dunite. They added occurrences of the volcanic and subvolcanic rocks in the form of dykes or basaltic flows. The present study tries to change the petrology and tectonics of whole complex from Ophiolite Complex to Metamorphic Core Complex. The revision includes refusal of all the above igneous rocks, instead they considered as medium grade regional metamorphism of different types of volcaniclastic sandstones (volcanic wackes), arenites and greywackes (impure sandstones) which sourced predominantly from remote volcanic source area inside Iran. The revision depended on several conjugate field and laboratory evidences inside the complex. These evidences such as absence of pillow basalt, volcanic flows, glass shards, volcanic cones, dykes, sills, contact metamorphism, dilatational structures and flow structures. Other evidences are presence of cross beddings, erosional surfaces, lensoidal channel fills, metamorphosed conglomerate, exposures of thousands of laminated planar beds and transition from fresh volcaniclastic sandstones to its medium grade metamorphosed counterparts, which previously considered as igneous rocks of ophiolite types. Another, evidence, in contrast to ophiolite section, the basalt location is at the base of the claimed ophiolite section while plutonic (dunite and peridotite) rocks located at its top. These locations of the two rocks contradict the definition of ophiolites. Accordingly, the present study changed the geological map of the whole Mawat area from igneous outcrops to metamorphosed volcaniclastic sandstones, arenites and greywackes that belong to Walash-Naoperdan Series. The parent rocks of the series transformed to different types of regionally metamorphosed rocks by deep burial during Eocene. During the burial, diageneses and metamorphisms enhanced by complex mixture of materials from different source areas and seawaters environments. Later, they uplifted, unroofed and exhumed during Pliocene as a core complex.

The structural positions of the ophiolite complex in the Earth crust of the western Tien-Shan

The westernmost parts of the Tien Shan region are located between two areas of crustal suturing, formed by the closure of the Turkestan Ocean, and probably the closure of a second ocean, the Gissar Ocean. Regional correlation of these sutures, however, has been problematic due to the lack of geological and geophysical data, as well as conflicting interpretations within the literature of various geological bodies. We summarize the information about Paleozoic ophiolites of westernmost parts of the Tien Shan for the international geoscientific audience from the literature and our own unpublished data. We focus on the best-known examples of Southern Tien Shan ophiolites which are remnants of Paleo-Asian Ocean, aligned in two main belts in Uzbekistan. Ophiolites reveal a wide age spectrum ranging from the Ordovician to the Devonian on the northern slope of Southern Tien Shan, and the Early Carboniferous on the southern slope. Considering all data on these ophiolites as well as regional considerations lets us conclude that a single ocean located subduction of the Turkestan Ocean basin under the northern Karakum-Tadjik terrane caused back-arc continentalo ruisft.i ngo iunththwearGdi ssar region in Early Carboniferous resulted in the formation of a small basin with oceanic crust. By late Carboniferous/early Permian times, both oceanic basins were subducted.

Cr - spinel and clinopyroxene compositions from metagabbro of Nui Ngoc complex in Tam Ky area: evidence of island - arc magma

The hornblende and biotite - bearing metagabbro of the Nui Ngoc complex were exposed into a few small blocks in the southwest Tam Ky city and were viewed as parts of the Tam Ky - Phuoc Son ophiolitic complex (TPO). These rocks were undergone mylonitic deformation and metamorphism. The rocks consist mainly of orthopyroxene, clinopyroxene, plagioclase, olivine, hornblende, biotite and very few microscopic Cr - spinel. The clinopyroxene and Cr - spinel minerals were analyzed for their composition by EPMA, the results showed that: (1) The clinopyroxenes have low Al2O3 (3,2÷3,5 wt %), TiO2 (0,70÷0,82 wt %) contents and (2) the Cr - spinels have low TiO2 (0,23÷0,58 wt %) content and Mg#100* (Mg/ (Mg+Fe2+)) (32÷42) but has medium Cr# (Cr/ (Cr+Al)) (45÷52).These features are similar to those of rocks formed in anoceanic - oceanicsubduction zone that was reported for the plagiogranite of Dieng Bong complex nearby. This study results combined with previous research results in the Tam Ky - Phuoc Son suture zone show the existence of island arc, continental arc, and MOR - magmatic types.

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.

Metamorphic evolution of a Tonian eclogite associated with an island arc of the southern Brasiliano Orogen

The early evolution of the southern Brasiliano Orogen is recorded in the São Gabriel Terrane, which consists of two Neoproterozoic (Tonian and Cryogenian) juvenile arcs, ophiolite complexes and supracrustal rocks. This study aims at providing new insights into the geodynamics processes and subduction process of this orogen by investigating the first Tonian eclogite (named Três Vendas eclogite) discovered in the southern Brasiliano Orogen. To determine the tectono-metamorphic evolution, we integrated field work with petrography, scanning electron microscopy, electron microprobe analyses of minerals and in situ LA-ICP-MS U-Pb geochronology with Lu-Hf and rare earth elements in zircon. The preserved high-grade metamorphic assemblage in these rocks has garnet, rutile, phengite, and omphacite, indicating that these rocks are (retro) eclogites. The presence of omphacite with up to 50% jadeite indicates minimum burial pressures of ≥15 kbar that are consistent with forward modeling of the garnet compositions. Omphacite was altered to fine-grained symplectites of hornblende, oligoclase, and diopside or augite with minor quartz during rapid exhumation. The second stage of retrogression involved reaction of garnet and diopside to plagioclase and hornblende symplectites (kelyphites) at amphibolite facies conditions. Kelyphitic hornblende-garnet pairs indicate peak temperatures of 710–760 °C. Whole rock composition and Zr–Y characteristics are consistent with tholeiitic island arc basalts, while their garnet compositions point to subduction zone eclogites. U-Pb ages of zircon cores yielded 909.5 ± 5.3 Ma and metamorphic rims 891.9 ± 8.2 Ma. Zircon age-corrected εHf isotope compositions range between +4 and +10, reflecting the moderately juvenile nature of these rocks. Zircon cores are distinguished by higher rare earth element (REE) contents and steeper middle REE to heavy REE (MREE-HREE) patterns, higher Th/U ratios and higher Sr and Ca contents than the metamorphic rims, and the rims are likely coeval with the kelyphites. The combined data indicate that the Três Vendas eclogites belong to a Tonian arc and were buried to >45 km, most likely in a subduction channel. The data from the first eclogite in southern Brazil have significant consequences for the interpretation of the Tonian evolution of the Brasiliano Orogen and, by extension, the Rodinia framework.