Late Paleozoic subduction system in the northern margin of the Alxa block,Altaids: Geochronological and geochemical evidences from ophiolites

The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO₂ and low K₂O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al₂O₃/TiO₂ ratios, but low TiO₂ and SiO₂ contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.     

Geochemistry and petrogenesis of the Eocene back arc mafic rocks in the Zagros suture zone,northern Noorabad,western Iran

The northern Noorabad area in western Iran contains several gabbro and basalt bodies which were emplaced along the Zagros suture zone. The basalts show pillow and flow structures with amygdaloidal textures, and the gabbroic rocks show massive and foliated structures with coarse to fine-grained textures. The SiO₂ contents of the gabbros and basalts are similar and range from 46.1–51.0 wt.%, and the Al₂O₃ contents vary from 12.3–18.8 wt.%, with TiO₂ contents of 0.4–3.0 wt.%. The Nb concentrations of some gabbros and basalts are high and can be classified as Nb-enriched arc basalts. The positive εNd(t) values (+3.7 to +9.8) and low ⁸⁷Sr/⁸⁶Sr_(initial) ratios (0.7031–0.7071) of both bodies strongly indicate a depleted mantle source and indicate that the rocks were formed by partial melting of a depleted lithospheric mantle and interaction with slab fluids/melts. The chemical composition of trace elements, REE pattern and initial ⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd ratios show that the rocks have affinities to tholeiitic magmatic series and suggest an extensional tectonic regime over the subduction zone for the evolution of these rocks. We propose an extensional tectonic regime due to the upwelling of metasomatized mantle after the late Cretaceous collision in the Harsin-Noorabad area. These rocks can be also considered as Eocene back arc magmatic activity along the Zagros suture zone in this area.     

Late Devonian OIB alkaline gabbro in the Yarlung Zangbo Suture Zone: Remnants of the Paleo-Tethys?

The Yarlung Zangbo Suture Zone (YZSZ) is believed to be composed of material largely derived from the destruction of the Neo-Tethys that occurred from early Mesozoic to early Cenozoic. We report here geochronological and petrological data obtained for newly discovered alkaline gabbro blocks embedded in a mélange zone of the western YZSZ. Single zircon U–Pb analyses from one representative gabbro sample by SIMS (Secondary Ion Mass Spectrometry) yielded a combined crystallization age of about 363.7 ± 1.7 Ma (1σ). In situ Hf isotopic analyses yielded ε_Hf(t) values of + 2.6 to + 5.5, suggesting an enriched mantle source. All of the gabbro samples show typical Ocean Island Basalt (OIB) affinity with little or no continental crust contamination. They also display strong geochemical similarities with the Hawaii basalts and the Xigaze seamount basalts suggestive of their intra-oceanic setting. These observations, in combination with the Early Carboniferous layered gabbros reported at Luobusa, indicate that these rocks could represent remnants of the Paleo-Tethys. We propose that a branch ocean separating the Western Qiangtang terrane and the Lhasa terrane from the Gondwana continent might have been present during the Late Devonian and the Early Carboniferous, providing new constrains on the configuration of Paleo-Tethys in Tibetan Plateau during early Late Paleozoic.     

Juxtaposition of adakite,boninite, high-TiO2 and low-TiO2 basalts in the Devonian southern Altay,Xinjiang, NW China

We present petrographic and geochemical data on representative samples of the Devonian adakite, boninite, low-TiO₂ and high-TiO₂ basalts and associated rocks in the southern Altay areas, Xinjiang, NW China. These volcanic rocks mostly occur as tectonic blocks within suture zones between the Siberian and Junggar plates. Adakite occurs in the Suoerkuduke area ca. 40 km south of Fuyun, and actually represents a poorly-sorted massive volcaniclastic deposit, mostly consisting of a suite of hornblende andesite to pyroxene andesite. The geochemical features of the adakite suggest its generation by melting of subducted oceanic crust. Boninite occurs in the Saerbulake area ca. 20 km southwest of Fuyun, as pillowed lava or pillowed breccia. It is associated with high-TiO₂ basalt/gabbro and low-TiO₂ basalt. The boninites are metamorphosed, but contain relict clinopyroxene with Mg# (=100*Mg/(Mg+Fe)) of 90–92, and Cr₂O₃ contents of 0.5–0.7 wt% and chromian spinel with Cr/(Cr+Al) ratio of 0.84. The bulk rock compositions of the boninites are characterized by low and U-shaped REE with variable La/Yb ratios. They are classified as high-Ca boninite. The Cr-rich cpx phenocryst and Chromian spinel suggests that the boninites were formed by melting of mildly refractory mantle peridotite fluxed by a slab-derived fluid component under normal mantle potential temperature conditions. Basaltic rocks occur as massive flows, pillowed lavas, tuff breccia, lapilli tuff and blocks in tectonic mélanges. Together with gabbros, the basaltic rocks are classified into high-TiO₂ (>1.7 wt%) and low-TiO₂ (<1.5 wt%) types. They show variable trace element compositions, from MORB-type through transitional back-arc basin basalt to arc tholeiite, or within plate alkalic basalt. A notable feature of the Devonian formations in the southern Altay is the juxtaposition of volcanic rocks of various origins even within a limited area; i.e. the adakite and the boninites are associated with high-TiO₂ and low-TiO₂ basalts and/or gabbros, respectively. This is most likely produced by complex accretion and tectonic processes during the convergence in the Devonian–Carboniferous paleo-Asian Ocean between the Siberian and Junggar plates.     

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite,central Eastern Desert,Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P₂O₅ anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Geochemistry of Neoproterozoic mafic intrusions in the Panzhihua district (Sichuan Province,SW China): Implications for subduction-related metasomatism in the upper mantle

Metasomatism above subduction zones is an important process that produces heterogeneous mantle and thus a diversity of igneous rocks. In the Panzhihua district, on the western margin of the Yangtze Block (SW China), two Neoproterozoic mafic intrusions, one olivine gabbro and one hornblende gabbro, have identical ages of 746 ± 10 and 738 ± 23 Ma. Both of the gabbros are tholeiitic in composition and have arc-like geochemical compositions. The hornblende gabbros have K₂O concentrations ranging from 0.70 to 1.69 wt.% and show enrichment of Rb, Ba, U, Th and Pb and depletion of Nb,Ta and Ti. They have variable ⁸⁷Sr/⁸⁶Sr ratios (0.7045–0.7070) with constant ɛNd(t) values (−0.12 to −0.93). The olivine gabbros have relatively low K₂O (0.19–0.43 wt.%), are depleted in Rb and Th relative to Ba and U, and have obvious negative Nb–Ta and Zr–Hf anomalies on primitive mantle-normalized trace element diagrams. Their ɛNd(t) values range from −0.64 to −1.73 and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7070 to 0.7075. Both types of gabbro experienced fractional crystallization of clinopyroxene, plagioclase, amphibole and minor Fe–Ti oxide. The parental magmas of the olivine and hornblende gabbros were formed by about 20% partial melting of garnet–spinel lherzolite and spinel lherzolite, respectively. According to trace elemental ratios, the hornblende gabbros were probably derived from a source strongly modified by subducted slab fluids, whereas the olivine gabbros came from a mantle source modified by subducted slab melts. The close association of the olivine gabbros and hornblende gabbros suggests that a steep subduction zone existed along the western margin of the Yangtze Block during Neoproterozoic time. Thus, the giant Neoproterozoic magmatic event in South China was subduction-related.     

The Zhaheba ophiolite complex in Eastern Junggar (NW China): Long lived supra-subduction zone ocean crust formation and its implications for the tectonic evolution in southern Altaids

The composite Zhaheba ophiolite complex, exposed in Eastern Junggar in the Southern Altaids, records an unusually long record of oceanic crust and magmatic arc evolution. The Zhaheba ophiolite complex consists of ultramafic rocks, gabbro, diorite, basalt and chert intruded by diabase dikes and diorite porphyry. These rocks are overlain by a several-km-thick section of tuffaceous rocks, volcaniclastic sedimentary rocks, and intermediate volcanic rocks. The igneous rocks of the ophiolite complex show negative Nb and Ta anomalies and LREE enrichment relative to HREE, suggesting the influence of fluids derived from a subducting oceanic slab. The LA-ICPMS U–Pb age of zircons from gabbro is 495.1 ± 3.5 Ma. Zircon ages from diorite and basalt are 458.3 ± 7.2 Ma and 446.6 ± 6.0 Ma, respectively. The basalt is locally overlain by bedded chert. Diabase dikes and diorite porphyry yield the U–Pb ages of 421.5 ± 4.1 Ma and 423.7 ± 6.5 Ma, respectively. The age of stratigraphically lower part of the overlying volcanic–volcaniclastic section is constrained to be about 410 Ma, the maximum depositional age of the tuffaceous sandstone from U–Pb detrital zircon ages. Late rhyolite at the top of the stratigraphic section yielded a U–Pb zircon age of 280.3 ± 3.7 Ma. The age and stratigraphic relationships for the Zhaheba ophiolite complex and related rocks suggest that the period of ~ 70 Ma of initial supra-subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 Ma. Many other ophiolites in the southern Altaids appear to record similar relationships, and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra-subduction zone. The occurrence of the Zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic Tethyan SSZ ophiolites but similar to some ophiolites in North America. Although the Zhaheba ophiolite belt is flanked by the Dulate arc in the north and Yemaquan arc in the south, it cannot stand a suture between two arcs. It is suggested that Devonian–Carboniferous Dulate arc was built on the late Cambrian–middle Ordovician Zhaheba supra-subduction oceanic crust. The late Carboniferous rocks and early Permian rocks in Dulate arc are interpreted to form in the extensional process within Zhaheba–Dulate arc composite system.     

Field and geochemical characteristics of Mesoarchean to Neoarchean volcanic rocks in the Storø greenstone belt,SW Greenland: Evidence for accretion of intra-oceanic volcanic arcs

The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Sm_cn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Yb_cn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Sm_cn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.     

Zircon ages and geochemical compositions of the Manlay ophiolite and coeval island arc: Implications for the tectonic evolution of South Mongolia

Numerous small dismembered ophiolite fragments occur in South Mongolia, but they are very poorly studied. The lack of age data and geochemical analysis hampers our understanding of the Paleozoic tectonic evolution of the region. We conducted detailed studies on the Manlay ophiolitic complex and Huree volcanic rocks south of the Main Mongolian Lineament (MML) to provide some constraints on these rocks. The Manlay ophiolite consists of dunite, harzburgite, pyroxenite, gabbro, plagiogranite, basalt and chert, locally with chromite mineralization in dunite. The gabbro and plagiogranite yielded SHRIMP zircon weighted mean ²⁰⁶Pb/²³⁸U ages of 509 ± 5 Ma and 482 ± 4 Ma, respectively. The basalt and dolerite samples of this complex show enrichment in LREE and LILE and negative Nb, Ta and Ti anomalies, and the chrome spinel from the chromitite lens in the dunite is characterized by high Cr^# and low TiO₂ contents. These features suggest a supra-subduction zone (SSZ) origin for the ophiolitic complex. The Huree volcanic rocks, ranging from basalt to dacite, display enrichment in LREE and LILE, weak Eu anomalies and distinctly negative Nb, Ta and Ti anomalies, consistent with those of typical magmas in a subduction environment. An andesite sample from this arc yielded a SHRIMP ²⁰⁶Pb/²³⁸U zircon age of 487 ± 5 Ma, which is the oldest reliable age for an island arc in South Mongolia. Recognition of an Early Paleozoic ophiolitic complex and a coeval island arc indicates that South Mongolia underwent a period of active volcanism during Late Cambrian to Ordovician. Additionally, the tuff overlying the ophiolitic complex and a granite intruding the ophiolite have SHRIMP zircon U–Pb ages of 391 ± 5 Ma and 304 ± 4 Ma, respectively. Combining the available data, we propose that the Early Paleozoic subduction–accretionary complexes likely constitute the basement of the Late-Paleozoic arc formations and correlate with the Lake Zone in western Mongolia.     

Recognizing OIB and MORB in accretionary complexes: A new approach based on ocean plate stratigraphy,petrology and geochemistry

We present a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy (OPS), and on the petrology and geochemistry of basalts from mid-oceanic ridges (MORB) and oceanic islands (OIB) using examples from four accretionary complexes (AC) in SW Japan: Akiyoshi, Mino–Tamba, Chichibu and Shimanto. The key to the problem is the model of OPS, which includes an association of igneous and sedimentary rocks that form on an oceanic plate during its travel from a mid-oceanic ridge to a subduction zone. We propose the reconstruction of the tectonic settings of basalts according to their relationships with associated OPS sediments, their petrogenesis and their geochemical features. Five types of OPS are recognized in the accretionary complexes of SW Japan: (1) sandstone/shale; (2) sandstone/shale and chert; (3) sandstone/shale, chert and MORB; (4) sandstone/shale, chert, MORB and gabbro (± peridotite); (5) seamount OPS including OIB, cap carbonates, slope clastics and basal shale/chert. The alkaline, tholeiitic or calc-alkaline composition of basaltic melts, which are typical of oceanic islands, mid-oceanic ridges and island-arcs, respectively, can be identified by the sequence in crystallization of their major phenocrysts, i.e. olivine (ol), clinopyroxene (cpx) and plagioclase (pl), and by their compositions. Alkaline and calc-alkaline mafic lavas are characterized by an ol → cpx → pl succession, whereas tholeiitic melts by their ol → pl ± cpx succession. Titanium-rich minerals, e.g., Ti–augite, kaersutite, Ti–biotite, are typical of alkaline lavas. The application of geochemistry-based tectonic discrimination diagrams is also a powerful tool, if not supported by geological and petrological data, may result in confusion due to magma contamination, post-magmatic alteration, and secular change of mantle thermal conditions. We propose that a direct comparison of normalized multi-element patterns and key binary plots from older volcanic rocks with their modern analogues provides a more viable and reliable method of basalt discrimination. Our OPS–petrology–geochemistry method allows us to confirm the above conclusions that the lavas of the Akiyoshi, Mino–Tamba and Southern Chichibu AC formed in oceanic islands, because they are associated with seamount OPS sediments, crystallized from ol to cpx and pl, contain Ti–augite and kaersutite and are enriched in TiO₂, LREE and Nb. In this paper we present geochemical data from the Inuyama basalts of the Mino–Tamba AC and from the Toba complex in the huge Mikabu greenstone belt of the Chichibu AC. The Inuyama basalts are in contact with Jurassic pelagic cherts, but their geochemical features are confusing; they contain phenocrysts of ol, Ti–augite and kaersutite and therefore probably formed in seamounts. The Toba volcanic rocks are a part of the huge ophiolite belt; they have flat to slightly LREE-enriched REE patterns, are characterized by an ol → cpx succession of phenocrysts and they plot in the OIB field in binary plots suggesting they formed in an oceanic plateau.     

Structure,age, and tectonic development of the Huoshishan–Niujuanzi ophiolitic mélange,Beishan, southernmost Altaids

The Huoshishan–Niujuanzi ophiolitic mélange (HNO) is located near the central part of the Beishan Orogenic Belt in the southernmost Altaids. The HNO consists of ultramafic rocks, cumulate gabbros, gabbros, plagiogranites, diorites, diabases, basalts, andesites, rhyolitic volcaniclastic rocks and siliceous sedimentary rocks, many of which are in a schist matrix (Gongpoquan Group). Geochemical data of the mafic rocks indicate a calc-alkaline or a mixture of calc-alkaline and tholeiitic rocks with negative Nb, Ta and positive Pb, Ba and La anomalies, suggesting formation in an island arc or supra-subduction zone setting. A gabbro from a block in the mélange in the Niujuanzi area has a zircon age of 435.0 ± 1.9 Ma and a plagiogranite with an age of 444.3 ± 1.9 Ma, and another gabbro from the Huoshishan area has an age of 410.5 ± 3.7 Ma. The schist matrix has a zircon age of 512 ± 5.3 Ma and contains Silurian, Devonian and Carboniferous fossils, thus the mélange formed in the late Carboniferous or later. Our structural analysis of fault planes in the HNO, the crenulation cleavages (S₂) of the schist, and fold axial planes of early Permian sandy limestone/quartz veins and late Permian sandstones indicates that the mélange underwent a north-to-south compression, and the orientation of stretching lineations, slickensides and fold hinge lines implies that the HNO experienced top-to-the north (or -northwest) movement. The entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late Paleozoic. The emplacement age of the HNO was probably near the end-Permian based on the age of the youngest rocks in the ophiolitic mélange, and by the presence of a late Permian unconformity. From our work, integrated with published regional data, we outline a comprehensive geodynamic model for the central BOC.     

Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan,Kyrgyzstan

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi–Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic–carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid-oceanic ridge (MORB-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO₂, LREE (La/Sm_n = 1.9), and Nb (Zr/Nb_av. = 10), have differentiated HREE (Gd/Yb_n = 2.0), medium to low εNd (~ 5.7) and are characterized by clear Nb positive anomalies in normalized multi-element plots (Nb/Th_pm = 1.3, Nb/La_pm = 1.1). The OIBs formed by relatively low degrees of melting (< 5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORB-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher εNd values (~ 9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several seamount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian–Bashkirian.     

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean ²⁰⁶Pb/²³⁸U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO₂ gap between 55% and 65%. The mafic rocks display negative ɛ_Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ_Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La_N/Yb_N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.     

Crustal contamination of Late Neogene basalts in the Dien Bien Phu Basin,NW Vietnam: Some insights from petrological and geochronological studies

Early Pliocene (Zanclean) basalts in the Dien Bien Phu pull-apart basin in NW Vietnam, associated with the presently sinistral Dien Bien Phu Fault Zone, have been dated by the K–Ar method at 4.4–4.9 and 5.4–5.2 Ma. Rapid migration of basaltic magma to the surface in the Dien Bien Phu Fault Zone may be due to Pliocene transtension of the crust in this region, resulting from asthenospheric upwelling induced by lateral displacement of the mantle. The basalts are moderately phyric ( < 10%) and consist of olivine (hyalosiderite), plagioclase (bytownite–labradorite) and orthopyroxene (bytownite–labradorite) phenocrysts, and a fine-grained crystalline matrix (olivine–hortonolite, plagioclase–labradorite, clinopyroxene–pigeonite and augite, K-feldspar). The presence of Fe-rich olivine and orthopyroxene phenocrysts indicates that the basalts are SiO₂-saturated/oversaturated olivine tholeiites which formed under water-undersaturated conditions. The Dien Bien Phu basalts contain both mantle-derived (pyroxenites, dunites, gabbros) and crustal (sillimanite/mullite + Mg–Fe spinel), wallrock xenoliths, indicative of crustal contamination during the ascent of the basaltic magma. The basalts show selective enrichment in some mobile elements (K, Rb, Sr and Th), a feature considered to be a result of metasomatism. These rocks, classified on the basis of their normative composition as quartz tholeiites, could represent primary olivine tholeiites/basalts, in which the geochemical signatures were modified by the processes of contamination.     

The Rio Capim volcanic–plutonic–sedimentary belt,São Francisco Craton,Brazil: Geological,geochemical and isotopic evidence for oceanic arc accretion during Palaeoproterozoic continental collision

The collision of oceanic arcs with continents is a common plate tectonic process in the Phanerozoic, but its recognition in the Precambrian is hampered by deformation and metamorphism. The Rio Capim volcanic–plutonic–sedimentary belt lies in sharp tectonic contact with Archaean rocks of the Uauá block in the northern part of the São Francisco craton. Field relationships and high-precision geochronology indicated that the Rio Capim basalts, gabbros, diorites, and dacites were emplaced approximately at 2148–2143 Ma, and later intruded by 2128 Ma-old diorite to tonalite plutons. All rocks were metamorphosed under amphibolite to granulite facies conditions mainly between 2080 Ma and 2070 Ma, but deformation may have lasted until about 2040 Ma as estimated from syn-deformation zircon and titanite grains. The association of basalt, andesite, dacite, and their plutonic counterparts, combined with their positive ε_Nd(t) values and incompatible trace element geochemical signatures similar to island arc magmas, support the proposition that the Rio Capim belt was a Palaeoproteorozoic intra-oceanic arc sequence that collided with a continent, of which the Mesoarchaean Uauá block is a remnant. The implications for the regional evolution and metallogenesis are also discussed.     

Mineral chemistry and genesis of the Permian Cihai and Cinan magnetite deposits,Beishan, NW China

Cihai and Cinan are Permian magnetite deposits related to mafic-ultramafic intrusions in the Beishan region, Xinjiang, NW China. The Cihai mafic intrusion is dominantly composed of dolerite, gabbro and fine-grained massive magnetite ore, while gabbro, pyrrhotite + pyrite-bearing clinopyroxenite and magnetite ore comprise the major units in Cinan. Clinopyroxene occurs in both deposits as 0.1–2 mm in diameter subhedral to anhedral grains in dolerite, gabbro and clinopyroxenite. High FeO contents (11.7–28.9 wt%), low SiO₂ (43.6–54.3 wt%) and Al₂O₃ contents (0.15–6.08 wt%), and low total REE and trace element contents of clinopyroxene in the Cinan clinopyroxenite imply crystallization early, at high pressure. This clinopyroxene is FeO-rich and Si and Ti-poor, consistent with the clinopyroxene component of large-scale Cu-Ni sulfide deposits in the Eastern Tianshan and Panxi ares, as well as Tarim mafic intrusion and basalt, implying the Cinan mafic intrusion and sulfide is related to tectonic activity in the Tarim LIP. The similar mineral chemistry of clinopyroxene, apatite and magnetite in the Cihai and Cinan gabbros (e.g., depleted LREE, negative Zr, Hf, Nb and Ta anomalies in clinopyroxene, lack of Eu anomaly in apatite and similarity of oxygen fugacity as indicated by V in magnetite), indicate similar parental magmatic characteristics. Mineral compositions suggest a crystallization sequence of clinopyroxenite/with a small amount of sulfide – gabbro – magnetite ore in the Cinan deposit, and magnetite ore – gabbro – dolerite in Cihai. The basaltic magma was emplaced at depth, with magnetite segregation (and formation of the Cinan magnetite ores) occurring in relatively low fO₂ conditions, after clinopyroxenite and gabbro fractional crystallization. The evolved Fe-rich basaltic magma rapidly rose to intermediate or shallow depths, forming an immiscible Fe-Ti oxide magma as fO₂ increased and leaving a Fe-poor residual magma in the chamber. The residual magmas was emplaced at different levels in the crust, forming the Cihai gabbro and dolerite, respectively. Finally, the immiscible Fe-Ti oxide magma was emplaced into the earlier formed dolerite because of late magma pulse uplift, resulting in a distinct boundary between the magnetite ores and dolerite.     

Discovery of Neoarchean suprasubduction zone ophiolite suite from Yishui Complex in the North China Craton

The Archean tectonic realm of the North China Craton (NCC) is considered in recent models as a collage of several microblocks which were amalgamated along zones of ocean closure during late Neoarchean. Here we report the finding of a dismembered ophiolite suite from the southern margin of the Jiaoliao microblock in the interior of the unified Eastern Block of the NCC. The suite is composed of lherzolite, pyroxenite, noritic and hornblende gabbro, and hornblendite intruded by veins and sheets of leuco granite. Together with transposed layers and bands of metavolcanics and amphibolites, banded iron formation (BIF), and diabase dykes in the adjacent locations, the Yishui complex corresponds well with a dismembered suprasubduction zone ophiolite suite. Clinopyroxene in the pyroxenite and gabbroic rocks is Mg rich and range in composition from augite to diopside. Among orthopyroxenes, those in lherzolite show the highest X_Mg of 0.84–0.85. Plagioclase in hornblende gabbro shows high anorthite content (An_50–64). Calcic amphiboles in the gabbroic rocks range in composition from ferropargasite to ferro-edenite, edenite and pargasite. Spinel inclusions in lherzolite are Cr-rich magnesiospinel. Geochemically, the mafic rocks from Yishui complex show subalkaline basaltic source, whereas the granitoids show volcanic arc affinity. The hornblende gabbro and gabbro, lherzolite and hornblendite show compositional similarity to E-MORB and N-MORB respectively. The lherzolite and hornblendite possess arc-related ultramafic cumulate nature, with overall features straddling the fields of IAT and IAT-MORB. The geochemical features are consistent with evolution in a suprasubduction regime with no significant crustal contamination. The majority of zircon grains in the Yishui suite exhibit magmatic texture and high Th/U ratios. Zircon grains from hornblendite define ²⁰⁷Pb/²⁰⁶Pb upper intercept age of 2538 ± 30 Ma. Zircons from the granite show ages of 2538 ± 16 Ma and 2503 ± 21 Ma, and those from the gabbros yield ages of 2503 ± 16 Ma and 2495 ± 10 Ma. The well defined major age peak at 2500 Ma is broadly coeval with Neoarchean ages reported from the microblocks in the North China Craton. The zircon Lu–Hf data from the Yishui suite display εHf(t) values between − 2.5 and 5.0, with corresponding model ages suggesting magma derivation from Neoarchean juvenile sources together with limited reworked Paleo-Mesoarchean crustal components.Our study is the first report of Neoarchean suprasubduction-type ophiolites from a locality far from the margins of the major crustal blocks and suture zones in the NCC and strengthens the concept that the craton is a mosaic of several microblocks with intervening oceans that closed along multiple subduction zones. We envisage that the amalgamation between the Xuhuai and the Jiaoliao microblocks resulted in the accretion of the Yishui suprasubduction zone ophiolitic assemblages onto the southern margin of the Jiaoliao microblock. The Neoarchean microblock amalgamation in the North China Craton provides new insights into continental growth in the early Earth and confirms that modern style plate tectonics might have been initiated early in the history of our planet.     

Evidence of Mid-ocean ridge and shallow subduction forearc magmatism in the Nagaland-Manipur ophiolites,northeast India: constraints from mineralogy and geochemistry of gabbros and associated mafic dykes

We discuss here the mineralogical and geochemical characteristics of mafic intrusive rocks from the Nagaland-Manipur Ophiolites (NMO) of Indo-Myanmar Orogenic Belt, northeast India to define their mantle source and tectonic environment. Mafic intrusive sequence in the NMO is characterized by hornblende-free (type-I) and hornblende-bearing (type-II) rocks. The type-I is further categorized as mafic dykes (type-Ia) of tholeiitic N-MORB composition, having TiO₂ (0.72–1.93 wt.%) and flat REE patterns (La_N/Yb_N = 0.76–1.51) and as massive gabbros (type-Ib) that show alkaline E-MORB affinity, having moderate to high Ti content (TiO₂ = 1.18 to 1.45 wt.%) with strong LREE-HREE fractionations (La_N/Yb_N = 4.54–7.47). Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and a fertile mantle/plume source at the spreading center. On the other hand, type-II mafic intrusives are hornblende bearing gabbros of SSZ-type tholeiitic composition with low Ti content (TiO₂ = 0.54 wt.%–0.86 wt.%) and depleted LREE pattern with respect to HREE (La_N/Yb_N = 0.37–0.49). They also have high Ba/Zr (1.13–2.82), Ba/Nb (45.56–151.66) and Ba/Th (84.58–744.19) and U/Th ratios (0.37–0.67) relative to the primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge contaminated by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Their subduction related origin is also supported by presence of calcium-rich plagioclase (An_16.6–32.3). Geothermometry calculation shows that the hornblende bearing (type-II) mafic rocks crystallized at temperature in range of 565°–625 °C ± 50 (at 10 kbar). Based on these available mineralogical and geochemical evidences, we conclude that mid ocean ridge (MOR) type mafic intrusive rocks from the NMO represent the section of older oceanic crust which was generated during the divergent process of the Indian plate from the Australian plate during Cretaceous period. Conversely, the hornblende-bearing gabbros (type-II) represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge slightly modified by the hydrous fluids released from the subducting oceanic slab during the initial stage of subduction of Indian plate beneath the Myanmar plate.     

Petrogenesis and tectonic significance of the early Paleozoic Delenuoer ophiolite in the Central Qilian Shan,northeastern Tibetan Plateau

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