SSZ-type Ophiolites Research Articles

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

The ephemeral history of Earth’s youngest supra-subduction zone type ophiolite from Timor

Ophiolites occur widely in orogenic belts, yet their origins remain controversial. Here we present a modern example with a geodynamic model from Timor, eastern Indonesia, where Earth’s youngest supra-subduction zone (SSZ)-type ophiolitic fragments are exposed. Zircon U-Pb ages and geochemical data indicate a short timespan (~10 to 8 Ma) for the magmatic sequence with boninitic and tholeiitic arc compositions. We interpret the Timor ophiolite as part of the infant Banda arc-forearc complex, which formed with the opening of the North Banda Sea and subsequent arc-continent collision along the irregular Australian continental margin. Our study connects the occurrence of small, short-lived ocean basins in the western Pacific with orogens around the globe where ephemeral SSZ-type ophiolites occur. These orogenic ophiolites do not represent preexisting oceanic crust, but result from upper-plate processes in early orogenesis and thus mark the onset of collision zone magmatism.

Petrogenesis and Geochronology of the Shazuoquan Ophiolite, Beishan Orogenic Belt: Constraints on the Evolution of the Beishan Ocean

The ophiolites in the Beishan Orogenic Belt provide important information about the evolution of the Beishan Ocean in the Paleozoic Era. We studied ophiolite petrology, geochemistry and isotopic chronology. The Shazouquan ophiolites consist of dunites, wehrlites, gabbros and anorthosites. Ophiolitic mélange belts are composed of matrixes and blocks, and different rocks are fault-bounded. Dunites and wehrlites are high in Mg#, Cr# and MgO, low in TiO2, relatively depleted in large-ion lithophile elements (Ti and P) and enriched in high-strength elements (U, Zr and Hf). They have a total REE of 1.25 × 10–6−5.39 × 10−6 and δEu of 1.12–3.54, which are similar to those of SSZ-type ophiolites, indicating that their parent magma source region may be a weakly depleted mantle source region. The anorthosite and gabbro are high in Al2O3, MgO and Mg#, low in TiO2, enriched in large-ion lithophile elements (Rb and Sr), and depleted in high-strength elements (Nb, Ta and Ti), but enriched in Zr and Hf. They have similar geochemical signatures to those of arc magmatic rocks. They are derived from the mantle peridotite formed against the tectonic background of subduction and modified by the fluid materials in the subduction zone. We collected anorthosite and gabbro, which were produced as ophiolite for U-Pb dating. The anorthosite yields a zircon U-Pb, aged 394 ± 11 Ma (MSWD = 0.84), and a gabbro zircon U-Pb, aged 466 ± 12 Ma (MSWD = 3.2), indicating that the Shazouquan ophiolite was formed in the Middle Ordovician–Early Devonian eras. Combining the above evidence, we conclude that the Beishan Ocean was in a subduction tectonic background from the Middle Ordovician to Early Devonian periods.

Cambrian intra-oceanic subduction within the southern branch of the Proto-Tethyan Ocean: Constraints from rhyolites in the Lajishan suture, NE Tibetan Plateau

The South Qilian suture in the Qilian Orogen is an early Paleozoic belt that developed as a result of closure of the southern branch of the Proto-Tethyan Ocean. It is characterized by widespread Cambrian intermediate-basic volcanic rocks which record the early-stage evolution of the paleo-ocean basin. However, Cambrian evolution of the paleo-ocean basin remains controversial due to the lack of research on the age and tectonic affinity of these volcanic rocks. In this study, we identified rhyolites from the intermediate-basic volcanic rocks in the South Qilian suture. They occur as pebbles in the breccias, or as veins and interlayers within the intermediate-basic volcanic rocks. Zircon U-Pb datings yield ages of 514.0 ± 2.8 Ma, 521 ± 4 Ma, 523 ± 5 Ma, and 513 ± 9 Ma for four rhyolites, respectively. The Cambrian rhyolites show variable La/Sm ratios and Rb contents and positive zircon εHf(t) values (+6.8 to +14.6) and whole-rock εNd(t) values (+2.1 to +4.6), suggesting that they were derived from partial melting of juvenile mafic crust. They also have high Al2O3 and CaO contents, as well as low K2O and trace element concentrations, which are similar to those of oceanic arc rhyolite. These early Cambrian oceanic arc rhyolites, along with the SSZ-type ophiolite (ca. 530–520 Ma) to the north and late-stage granitoid intrusions (ca. 474–450 Ma), indicate that the southern branch of the Proto-Tethyan Ocean experienced southward intra-oceanic subduction during the early Cambrian-early Ordovician period.

The Middle-Late Cretaceous Zagros ophiolites, Iran: Linking of a 3000 km swath of subduction initiation fore-arc lithosphere from Troodos, Cyprus to Oman

Abstract New trace-element, radiogenic Sr-Nd-Pb isotopic and geochronological data from Middle-Late Cretaceous Zagros ophiolites of Iran give new insights into the tectono-magmatic history of these supra-subduction zone (SSZ)-type ophiolites. The distribution of Middle-Late Cretaceous SSZ-type ophiolites in Iran comprises two parallel belts: (1) the outer Zagros ophiolitic belt and (2) the inner Zagros ophiolitic belt. These Middle-Late Cretaceous ophiolites were generated by seafloor spreading in what became the fore-arc and back-arc during the subduction initiation event and now define a ~3000-km-long belt from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. The Zagros ophiolites contain complete (if disrupted) mantle and crustal sequences. Mantle sequences from both outer-belt and inner-belt ophiolites are dominated by dunites, harzburgites, and lherzolites with minor chromitite lenses. Peridotites are also intruded by gabbros and a variety of mafic to minor felsic (plagiogranite and dacite) dikes. Crustal rocks comprise ultramafic-mafic cumulates as well as isotropic gabbros, sheeted dike complexes, pillowed and massive lavas, and felsic rocks. Our new zircon U-Pb ages indicate that the outer-belt and inner-belt ophiolites formed near coevally during the Middle-Late Cretaceous; 100–96 Ma for the outer belt and 105–94 Ma for the inner belt. Both incompatible-element ratios and isotopic data confirm that depleted mantle and variable contributions of subduction components were involved in the genesis of outer-belt and inner-belt rocks. Our data for the outer belt and inner belt along with those from better-studied ophiolites in Cyprus, Turkey, the UAE, and Oman lead to the conclusion that a broad, ~3000-km-long swath of fore-arc lithosphere was created during Middle-Late Cretaceous time.

Neoproterozoic Trench‐arc System in the Western Segment of Jiangnan Orogenic Belt, South China

AbstractThe Jiangnan orogenic belt is a key to understanding of the Neoproterozoic tectonic evolution of the South China Block. We investigate the mafic‐ultramafic suites of lherzolite, pyroxenite, gabbro, pillow basalt and gabbroic diorite as well as red jasper interbedded with marine marbles that are mainly exposed as fault‐trapped blocks in the Yuanbaoshan and Longsheng domains of the western Jiangnan belt. The post‐collisional granite plutons that intruded the ultramafic‐mafic rocks are developed well. Zircons in the gabbro yield crystallization ages of 867±10 Ma, 863±8 Ma, 869±9 Ma and 855±5 Ma whereas those from the granites show ages of 823±5 Ma, 831±5 Ma, 824±5 Ma and 833±6 Ma. The Neoproterozoic serpentinited ultramafic samples display minor REE enriched pattern with depletion of Rb, Ba, Nb, Ta and Ti, similar to those of SSZ type ophiolite. The coeval gabbro shows tholeiitic features and is characterized by negative Ba, Nb, Ta, Zr, Hf and Sr anomalies and LREE enriched patterns, with a minor negative Eu anomaly. Some zircon grains from the Longshen gabbro yield Neoarchean‐Paleoproterozoic ages (2859–2262 Ma), suggesting its continental arc setting. Geochemical signature of the mafic‐ultramafic rocks is consistent with subduction related setting. The pyroxene‐bearing diorite exhibits a distinctive arc affinity. The zircons from the gabbro show positive ∊Hf(t) values ranging from 3.9 to 13.8. The granitoids are typical S–type granites with high ACNK values (1.15–1.40) and negative ∊Hf(t) values (−15.1 to −3.2), and are classified as collision–related granites. Combined with the occurrences of mafic‐ultramafic rocks, siliceous marble and red jasper mixed with basalt, our new results suggest the presence of a Tonian (863–869 Ma) SSZ ophiolite system and continental arc‐type magmatism in the western Jiangnan orogen.

Amalgamation between the Yangtze and Cathaysia blocks in South China: Evidence from the ophiolite geochemistry

Neoproterozoic ophiolites in the southeastern margin of the Yangtze Block record tectonic evolution of South China. The ∼1.0 Ga Xiwan ophiolite and the ca. 830 Ma Fuchuan ophiolite indicate a long-term assembly of the Yangtze and Cathaysia blocks during the Neoproterozoic time. Peridotites from the Xiwan ophiolite show high Al2O3 (1.24–2.67 wt%) and low MgO contents (39.61–44.26 wt%), similar to the abyssal and back-arc peridotites that experienced low degrees of partial melting. Their U-shaped REE patterns and HFSE enrichment indicate weak melt metasomatism. Whole rock 187Re/188Os (0.09–0.29) and 187Os/188Os ratios (0.11526–0.12193) yield a melt-extraction age of 1.8–2.1 Ga. However, peridotites from the Fuchuan ophiolite are more refractory and have low Al2O3 (0.51–0.95 wt%) and high MgO contents (43.99–48.24 wt%), similar to the forearc peridotites that underwent high degrees of melt extraction. Their 187Re/188Os (0.01–1.08) and 187Os/188Os (0.11852–0.12867) variations may have resulted from fluxed melting induced by the subduction-related fluids. The peridotites, in combination with the overlying mafic oceanic crustal rocks, suggest that the Xiwan and Fuchuan ophiolites are SSZ-type ophiolites which were formed in back-arc and forearc settings, respectively, followed by the collision between the Yangtze and Cathaysia blocks at ca. 830 Ma.

Zircon U-Pb age, geochemistry and Sr-Nd isotope characteristics of the Duolong SSZ-type ophiolites in Geize County, Tibet: Evidence for intra-oceanic subduction of the Bangonghu-Nujiang Ocean during the Late Permian

西藏改则县多龙矿集区分布着大量的SSZ型蛇绿岩残片，是班公湖-怒江蛇绿岩带的重要组成部分，本文对代表性辉绿（玢）岩进行了锆石U-Pb定年、岩石地球化学及Sr-Nd同位素分析，获得辉绿玢岩加权平均年龄为252.1±1.5Ma（MSWD=0.09，n=33），表明多龙SSZ型蛇绿岩形成于晚二叠世。岩石烧失量普遍较高（1.57%~5.23%）的特征，表明样品普遍遭受蚀变；而且具有富硅、高铝、低MgO及低Mg#值（36.8~63.1）的特征，说明其经历过一定程度的分离结晶作用。多龙辉绿岩和辉绿玢岩富集大离子亲石元素（LILE：Rb、Cs、K、Sr、P、Pb），亏损高场强元素（HFSE：Ta、Nb、Ti、Zr、Hf）；稀土元素总量为23.27×10-6~58.95×10-6，δEu为0.83~1.35，（La/Yb）N为0.66~1.30，具有与岛弧拉斑玄武岩类似的稀土元素和总体一致的微量元素特征。岩石（87Sr/86Sr）i值为0.707345~0.708748，（143Nd/144Nd）i值为0.512669~0.512705，tDM2值为305~456Ma，eNd（t）值为+6.9~+8.8，表明其岩浆源区为亏损地幔源区，其地球化学成分具有洋内弧的特征，暗示其可能是洋内俯冲作用的产物。综合分析认为，班-怒洋壳于晚二叠世时（ca.252Ma）在板块汇聚边缘发生了类似西太平洋Izu-Bonin-Mariana的洋内俯冲作用，形成兼具MOR蛇绿岩亲缘性和俯冲带SSZ蛇绿岩地球化学印记的双重属性的多龙辉绿岩及辉绿玢岩的母岩浆；母岩浆经过结晶分异、上升侵位，最终于洋内俯冲带之上的弧前盆地环境下形成多龙辉绿岩和辉绿玢岩。

Geochronological and geochemical constraints on the origin of the Southeast Anatolian ophiolites, Turkey

The Southeast Anatolian ophiolites outcropping in the Southeast Anatolian Orogenic Belt (Southeast of Turkey) mark the closure of the southern branch of the Neotethys Ocean associated with the collision between the Arabian Plate and Anatolian microplate. We present new geochemical, zircon U–Pb age, zircon Lu–Hf, and Sr-Nd isotopic data on the Southeast Anatolian Ophiolites to understand their formation ages, magma genesis, and geotectonic implications. The ophiolites, which are related to island arc igneous rocks, consist of mantle peridotites and crustal rocks (less dunite, gabbros, sheeted dykes, massive, and pillow basalts). The flat rare-earth element (REE) patterns, depletion in Nb and Ta, and enrichment in LILEs (Ba, Rb, Th, Sr, Pb) of gabbros suggest close similarities with very low Ti (boninitic) lavas found in the forearc regions. Using laser ablation inductively coupled plasma–mass spectrometry, zircon separated from leucogabbros, diabase dykes, and plagiogranites yield U-Pb ages of 92 and 83 Ma, which are interpreted to represent the formation ages of the ophiolites. The zircons in the gabbros and plagiogranites are dominated by positive eHf(t) values (between +3.1 and + 17.45) with a few negative eHf(t) values. High eHf(t) features are consistent with derivation from Mid-oceanic Ridge Basalt (MORB)-source mantle. The negative eHf(t) values of the zircons suggest the involvement of subducted sedimentary rocks. The southeast Anatolian ophiolites represent an SSZ-type ophiolite and are part of the Late Cretaceous oceanic lithosphere of the southern branch of the Neotethys Ocean that opened during the Late Triassic and closed during the Late Cretaceous.

Early Permian Sunidyouqi suprasubduction-zone ophiolites in the central Solonker suture zone (Inner Mongolia, China)

Different final closing ages have been proposed for the evolution of the Paleo-Asian Ocean (PAO), including Late Silurian, pre-Late Devonian, Early Permian, Late-Permian and Late Permian–Early Triassic. Ophiolites represent fragments of ancient oceanic crust and play an important role in identifying the suture zone and unveiling the evolutionary history of fossil oceans. Our detailed geological, geochemical and geochronological investigations argue for the existence of Early Permian (297 Ma) SSZ type ophiolites in the Sunidyouqi area of central Inner Mongolia, China. The gabbros and basalts show LREE depleted REE patterns and left-leaning primitive mantle-normalized spider diagrams with variable negative Nb-Ta anomalies (Nb* = 0.24–1.28 and 0.29–0.55, respectively). The Sunidyouqi ophiolites were generated in a mature back-arc basin. The Sunidyouqi ophiolites share the same petrological, geochemical and geochronological characteristics with the other ophiolites along the Solonker suture zone, delineating a Late Paleozoic ocean and arc-trench system. This Late Paleozoic ocean and arc-trench system coincides with a Permian paleobiogeographical boundary, i.e. the boundary between the northern cold climate (Boreal faunal–Angaraland floral realm), and a southern warm climate (Tethys faunal–Cathaysian floral realm). A tectonic scenario was proposed at last for the closure of the SE PAO involving (1) Late Ordovician to Middle Permian continuous southward subduction beneath the northern margin of North China; (2) Carboniferous to Middle Permian continuous northward subduction the forming the Northern Accretionary Orogen; (3) Late Permian final closure of the SE PAO.

Classification of the Neoproterozoic ophiolites of the Central Eastern Desert, Egypt based on field geological characteristics and mode of occurrence

The present work presents new field observations on the geological characteristics of the Neoproterozoic ophiolites of the Central Eastern Desert (CED) of Egypt. Based on mode of occurrences, the investigated ophiolites appear to show characteristics of both MORB-type and SSZ-type ophiolites and classified from oldest to youngest into: (1) MORB intact ophiolites (MIO), (2) dismembered ophiolites (DO), and (3) arc-associated ophiolites (AAO). Ophiolite components include serpentinized peridotite and other metamorphosed ultramafics, metagabbros, sheeted dykes, pillowed and massive metabasalts, minor boninites, and pelagic sediments. The rocks of the MIO and DO were subjected to two main phases of deformation (D1 and D2) and metamorphism, whereas the AAO have been affected only by later phase of deformation (D2). Compositional variation of chrome spinel in blocks of metamorphosed ultramafic and serpentinite of dismembered ophiolites shows three distinct groups (G1, G2, and G3) based on their Cr#. Whereas G1 is similar to abyssal peridotites, G2 and G3 display mainly supra-subduction zone (SSZ) tectonic setting. The ophiolitic metagabbros display both N-MORB and supra-subduction zone tectonic setting (SSZ). The basaltic rocks of the MORB-intact ophiolites (MIO) and blocks of metabasalts in the melanges display N-MORB affinity, whereas the basaltic rocks associated with arc volcanics display SSZ tectonic setting. Collectively, the investigated ophiolites of the Central Eastern Desert (CED) fall into two groups, MORB or BABB and SSZ ophiolites. They are spatially and temporally unrelated, and thus, it seems likely that the two types are not petrogenetically related. Ophiolites occur in different geological settings, and they represent change of the tectonic setting of the ophiolites from MORB to SSZ with time.

Geochronological and geochemical constraints on the coexistent N-MORB- and SSZ-type ophiolites in Babu area (SW China) and tectonic implications

Our detailed geological, geochemical and geochronological investigations indicate the coexistence of late Paleozoic (270–265 Ma) normal middle ocean ridge basalt (N-MORB)-type and suprasubduction-zone (SSZ)-type ophiolites in the Babu area (SW China). Metaperidotite samples from the Babu ophiolites show high Os concentrations and low Re/Os, 187 Os/ 188 Os and 187 Re/ 188 Os ratios, and represent residual upper mantle. Metagabbro and metabasalt samples show light rare earth element (LREE)-depleted REE patterns and N-MORB-like primitive mantle-normalized spider diagrams without Nb–Ta and Zr–Hf anomalies, and depleted Sr–Nd isotopic compositions (ε Nd ( t ) = +6.9 – +8.3). These geochemical and Re–Os–Sr–Nd isotopic data show that the Babu ophiolites are derived from a depleted mantle source and represent ancient N-MORB-like oceanic crust. In addition, peridotites, basalts and boninites with SSZ signatures (e.g. Nb–Ta negative anomalies) were also reported in the Babu ophiolites consistent with their generation in a forearc or arc setting. In combination with the arc-related high-Mg # limburgite, basalt and andesite, these late Paleozoic ultramafic–mafic rocks are genetically related to Paleotethyan subduction, rather than being Emeishan mantle plume related. The coexisting N-MORB- and SSZ-type ophiolites in the Babu area show similar geochemical and geochronological characteristics to the Jinshajiang–Ailaoshan–Song Ma ophiolites. Supplementary material: A description of the geology of the Babu ultramafic–mafic complex, sample descriptions, detailed analytical methods and results, and a summary of previously published age data for late Paleozoic magmatic rocks along the China–Vietnam border are available at https://doi.org/10.6084/m9.figshare.c.4007569

The Shimian ophiolite in the western Yangtze Block, SW China: Zircon SHRIMP U-Pb ages, geochemical and Hf-O isotopic characteristics, and tectonic implications

The Shimian mafic-ultramafic rocks in the western Yangtze Block is a structurally dismembered complex which is intruded by the Neoproterozoic granite and faulted against the Neoproterozoic Suxiong Group. In this study, we divide the Shimian mafic-ultramafic rocks into two suites including the Late Mesoproterozoic Shimian ophiolite and Early Neoproterozoic gabbros, based on differences in the field occurrence, age, and geochemical characteristics. The Late Mesoproterozoic Shimian ophiolite mainly consist of serpentinites, gabbros, basalts, and rodingites. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on a gabbro sample yielded an age of 1066±11Ma, representing the formation age of the Shimian ophiolite. All basalt and gabbro samples are tholeiitic and have normal mid-ocean ridge basalt (N-MORB)-like immobile element patterns with negative Nb and Ti anomalies. The low zircon δ18O (4.6–5.6 ‰) and high positive zircon εHf(t) (+10.8 to +18.3) values indicate that these rocks were derived from a depleted mantle source. These features are comparable with those of the basaltic rocks in SSZ-type ophiolite. The Early Neoproterozoic gabbros intruded into the Shimian ophiolite at ca. 937Ma. The geochemical and isotopic characteristics of these gabbros are similar to those of the island arc basalts. Integrating previous studies with the data presented in this contribution, we suggest that the northwestern Yangtze Block has probably witnessed the Late Mesoproterozoic–Early Neoproterozoic diachronous subduction.

An Andean-type arc system in Rodinia constrained by the Neoproterozoic Shimian ophiolite in South China

The configuration of the supercontinent Rodinia has long been a matter of debate; the key controversy is the position of South China in Rodinia. We report an incomplete Neoproterozoic ophiolite sequence, including gabbros and serpentinized peridotites intruded by mafic dikes, near Shimian (Sichuan Province), along the western margin of the Yangtze Block in South China. Serpentinized peridotites have very low REE (0.14–1.16ppm) and trace element concentrations, and are interpreted as parts of a depleted mantle sequence. Chromites from the serpentinized peridotites have low TiO2 (<0.3wt%) and Fe2O3 (<7.6wt%), and moderate Cr/(Cr+Al) (0.48–0.67) and Mg/(Mg+Fe2+) (0.42–0.67) ratios and are clearly indicative of strong depletion of the host peridotites, similar to those of supra-subduction zone (SSZ). Mafic dikes and gabbros contain zircon grains with similar U-Pb ages of ∼800Ma, and are chemically akin to MORB-like tholeiitic basalts and boninite-series volcanic rocks reported in other SSZ ophiolites and in the Izu-Bonin-Mariana arc system. Therefore, the serpentinized peridotite, mafic dikes and gabbros together appear to form a SSZ-type ophiolite assemblage preserving the accretion of oceanic lithosphere to the western Yangtze Block. The age and location of the Shimian ophiolite rule out the possibility that South China was located in the centre of Rodinia. Instead, the Neoproterozoic arc-affinity igneous rocks at the western margin of the Yangtze Block are well correlated with those in Greater India and Madagascar. These three blocks thus together formed a giant Andean-type arc system along the NW margin of Rodinia that existed for more than 100Mys.

Collision of the Tacheng block with the Mayile-Barleik-Tangbale accretionary complex in Western Junggar, NW China: Implication for Early-Middle Paleozoic architecture of the western Altaids

Western Junggar in NW China, located to the southeast of the Boshchekul-Chingiz (BC) Range and to the north of the Chu-Balkhash-Yili microcontinent (CBY), played a key role in the architectural development of the western Altaids. However, the mutual tectonic relationships have been poorly constrained. In this paper, we present detailed mapping, field structural geology, and geochemical data from the Barleik-Mayile-Tangbale Complex (BMTC) in Western Junggar. The Complex is divisible into Zones I, II and III, which are mainly composed of Cambrian-Silurian rocks. Zone I contains pillow lava, siliceous shale, chert, coral-bearing limestone, sandstone and purple mudstone. Zone II consists of basaltic lava, siliceous shale, chert, sandstone and mudstone. Zone III is characterized by basalt, chert, sandstone and mudstone. These rocks represent imbricated ocean plate stratigraphy, which have been either tectonically juxtaposed by thrusting or form a mélange with a block-in-matrix structure. All these relationships suggest that the BMTC is an Early-Middle Paleozoic accretionary complex in the eastern extension of the BC Range. These Early Paleozoic oceanic rocks were thrust onto Silurian sediments forming imbricate thrust stacks that are unconformably overlain by Devonian limestone, conglomerate and sandstone containing fossils of brachiopoda, crinoidea, bryozoa, and plant stems and leaves. The tectonic vergence of overturned folds in cherts, drag-related curved cleavages and σ-type structures on the main thrust surface suggests top-to-the-NW transport. Moreover, the positive εNd(t) values of volcanic rocks from the Tacan-1 drill-core, and the positive εHf(t) values and post-Cambrian ages of detrital zircons from Silurian and Devonian strata to the south of the Tacheng block indicate that its basement is a depleted and juvenile lithosphere. And there was a radial outward transition from coral-bearing shallow marine (shelf) to deep ocean (pelagic) environments, and from OIB/E-MORB to N-MORB lava geochemistry away from the Tacheng block. Comparisons with published data suggest that these positive isotopic values, stratigraphic, structural and geochemical relationships can be best understood as an analogue of the relationships between the Ontong Java oceanic plateau and the Pacific oceanic crust. Therefore we propose that the basement of the Tacheng block was an Early Paleozoic oceanic plateau. The southern part of the Tacheng block was an accretionary complex and the northern part was an oceanic basin in the Early Paleozoic, the configuration of which is similar to that of the present Ontong Java oceanic plateau situated on the Pacific oceanic crust, and its accretion into the Solomon accretionary complex. The presence of Ordovician SSZ-type ophiolites, early Paleozoic blueschist and Silurian SSZ-type intrusions in the BMTC, and Early-Middle Paleozoic continental arc-related intrusive rocks in the northern margin of the CBY provide further corroboration of a former subduction zone between the southern West Junggar and the northern margin of the CBY. Furthermore, consideration of the fact that the Kokchetav-North Tianshan range was collaged to the southern margin of the CBY in the Ordovician-Devonian indicates that both ranges were amalgamated synchronously with the CBY constructing the Early-Middle Paleozoic architecture of western Altaids.

Cambrian Kherlen ophiolite in northeastern Mongolia and its tectonic implications: SHRIMP zircon dating and geochemical constraints

The Kherlen terrane, which contains the Kherlen ophiolitic complex, is located between two Precambrian continental blocks in the northeastern Mongolia. We present new geochemical and SHRIMP zircon U–Pb data for the Kherlen ophiolitic complex and for granitic plutons intruding the complex, providing constraints on the regional evolution in Early Paleozoic time. The Kherlen ophiolite, which is geochemically similar to SSZ-type ophiolites, was originated from two distinct mantle sources, a N-MORB-like source and an E-MORB-like source. A gabbro and a plagiogranite dike intruding the gabbro from the Kherlen ophiolite yielded similar SHRIMP zircon U–Pb ages of ca. 500Ma, suggesting that the ophiolite formed in Late Cambrian time. Post- or syn-collisional granites intruding the ophiolitic complex yielded crystallization ages of ca. 440Ma, which is interpreted to record the minimum age of the tectonic emplacement of the ophiolite. These new data demonstrate that the Kherlen ophiolite belt is an Early Paleozoic suture between the Ereendavaa and the Idermeg continental terranes, which is generally coeval with the Bayankhongor belt in central Mongolia, indicating that they are regionally correlated, and thus they define a major Early Paleozoic suture between two Precambrian continental blocks in the central-northeastern Mongolia.

Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr–Nd–Hf isotopic characterization

The Dongqiao ophiolite occurs in the central segment of the Bangong–Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30years’ studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U–Pb dating yielded ages of 188±1Ma for the layered gabbro and 181±1Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong–Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong–Nujiang suture zone. If so, the Tethys Ocean of the Bangong–Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

Timing and nature of the Xinlin–Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing’an Range, eastern Central Asian Orogenic Belt

Jifeng ophiolitic melange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing’an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin–Xiguitu Ocean associated with the collision between the Erguna block and Xing’an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic melange, the gabbros from newly discovered the Jifeng ophiolitic melange are studied with zircon U–Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U–Pb dating from the ophiolitic gabbros yields U–Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic melange. The gabbros display low SiO2, TiO2, K2O contents, high Na2O, LREE contents and indistinctive REE fractionation [(La/Yb)N = 1.97–2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the e Hf (t) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages (T DM1) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic melange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin–Xiguitu Ocean) existed between the Erguna block and Xing’an block. The Ocean’s formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing’an block.

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

Geochemical (including Re-Os isotopic) studies of the mantle rocks of ophiolites in the Bangong-Nujiang suture zone in central Tibet have provided a coherent picture of the evolution of the Bangong-Nujiang Tethyan oceanic mantle from mid-ocean ridge (MOR) to subduction-zone (SSZ) settings.Clinopyroxene (cpx)-harzburgites and lherzolites in the Bangong Lake ophiolite were formed in a MOR setting, as demonstrated by the Cr# of spinels (<0.60) and whole-rock LREE-depleted patterns. Suprachondritic 187Re/188Os ratios (up to 1.833) of cpx-harzburgites and their spinels can be explained by interaction with melts derived from high Re/Os sources. Re-depletion (TRD) model ages (0.48-0.55Ga) suggest these rocks may represent a Pan-African domain beneath the Gondwana continent. High TiO2 contents of spinels and whole-rock samples imply that the lherzolites were formed through a refertilization process. Similarly, Re-Os isotopic systematics of sulfides in the lherzolites (187Re/188Os: 0.173-1.717, 187Os/188Os: 0.12646-0.17340) demonstrate that they are mixtures of primary and secondary sulfides. 187Os/188Os ratios (0.1211-0.1226) of whole-rock lherzolites give TRD ages of 0.73-0.97Ga, indicating the presence of Neoproterozoic lithospheric mantle under the spreading ridges.Mantle rocks in the SSZ-type ophiolites from Bangong Lake, Dongqiao and Nagqu reflect the complex evolution of the Bangong-Nujiang oceanic mantle during the SSZ stage. Most harzburgites from the Bangong Lake ophiolite give TRD ages of 1.0-1.5Ga, possibly representing relics of a Mesoproterozoic lithospheric mantle. However, three samples have both high Os contents (1.32-4.45ppb) and near-chondritic 187Os/188Os (0.1260-0.1297), and may represent Mesozoic oceanic lithospheric mantle. 187Os/188Os ratios of dunites and harzburgites from the Dongqiao and Nagqu ophiolites vary from 0.1174 to 0.1316 and give TRD ages up to 1.43Ga, also suggesting the existence of a Mesoproterozoic lithospheric mantle which has experienced mantle refertilization and later metasomatism caused by percolation of several generations of melts and/or fluids.We interpret the old mantle domains in the Bangong-Nujiang ophiolite belt as relics of ancient sub-continental lithospheric mantle (SCLM), which may have survived during the opening of the Bangong-Nujiang Tethyan ocean basin, and subsequently were incorporated into Mesozoic oceanic lithospheric mantle.

Zircon U–Pb, O, and Hf isotopic constraints on Mesozoic magmatism in the Cyclades, Aegean Sea, Greece

Compared to the well-documented Cenozoic magmatic and metamorphic rocks of the Cyclades, Aegean Sea, Greece, the geodynamic context of older meta-igneous rocks occurring in the marble–schist sequences and melanges of the Cycladic Blueschist Unit is as yet not fully understood. Here, we report O–Hf isotopic compositions of zircons ranging in age from ca. 320 Ma to ca. 80 Ma from metamorphic rocks exposed on the islands of Andros, Ios, Sifnos, and Syros with special emphasis on Triassic source rocks. Ion microprobe (SHRIMP II) single spot oxygen isotope analysis of pre-Cretaceous zircons from various felsic gneisses and meta-gabbros representing both the marble–schist sequences and the melanges of the study area yielded a large range in δ18O values, varying from 2.7 ‰ to 10.1 ‰ VSMOW, with one outlier at −0.4 %. Initial eHf values (−12.5 to +15.7) suggest diverse sources for melts formed between Late Carboniferous to Late Cretaceous time that record derivation from mantle and reworked older continental crust. In particular, variable δ18O and eHf(t) values for Triassic igneous zircons suggest that magmatism of this age is more likely rift- than subduction-related. The significant crustal component in 160 Ma meta-gabbros from Andros implies that some Jurassic gabbroic rocks of the Hellenides are not part of SSZ-type (supra-subduction zone) ophiolites that are common elsewhere along the margin of the Pelagonian zone.