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The structure and lithological sequences of ophiolites provide critical information for reconstructing their tectonic setting and thus, the history of the paleo-oceanic basins in which they formed. Our systematic mapping of three of the best-preserved massifs in the Xigaze ophiolite from the middle segment of the Yarlung Zangbo Suture Zone (YZSZ) in South Tibet, revealed a nearly intact structure of the ophiolitic sequence that has largely escaped emplacement and post-emplacement dismemberment. One of the most important results is the identification of a nearly E-W striking, serpentinized shear zone (SPSZ) preserved within the sequence. The SPSZ is rooted in the serpentinized front of the upper mantle and is mainly composed of meta-gabbroic and rodingite lenses floating in a serpentinite matrix. Extensive talc-serpentinite metasomatism, shearing and cataclastic deformation in the SPSZ reveal a complex interplay of fluid metasomatism and progressive ductile-brittle deformation in hundreds of meters of shear zone between the mantle and the crust. The structural and petrological features of the SPSZ are very similar to those of detachment faults within modern in situ oceanic core complexes (OCCs) in the Mid-Atlantic Ridge and the Southwest Indian Ridge. Zircon U Pb ages from the footwall of the fault, and the age of radiolarian chert overlying it suggest that the detachment faulting took place between ca. 125 to 123 Ma. Combined with the stratigraphic and geochronological relationships between the ophiolite and the overlying sediments, we suggest that the Xigaze ophiolite developed in the supra-subduction zone on the southern margin of the Lhasa terrane during rollback of the Neo-Tethyan oceanic slab. Recognition of detachment faulting in the Xigaze ophiolites, along with other features, suggest that the Xigaze ophiolite formed at a spreading ridge, whereas the other large ultramafic massifs in YZSZ, may have formed in an off-axis location on the slow-spreading ridge.

In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158–161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cretaceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive eHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of eHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

The discovery of ultrahigh pressure (UHP) minerals in orogenic belts has revolutionized our understanding of subduction and the return flow of previously deeply subducted material to Earth's surface as part of the cycling and interaction of crustal and mantle systems. One class of UHP minerals is found as inclusions in orogenic peridotite-hosted podiform-chromite systems, within Phanerozoic ophiolites and ophiolitic melanges. Such inclusions have opened a window into processes of recycling of crustal materials to the deep mantle and back through subduction and mantle convection in Phanerozoic orogens. Here, we document the first occurrence of an UHP mineral hosted in an ophiolitic podiform chromitite melange from the Neoarchean paired metamorphic belt of the Central (Taihang) Orogenic Belt, Northern China. Numerous inclusions of rutile, apatite, dolomite, and amphibole are interpreted to be crustal-derived; they occur in podiform chromite grains hosted in a 2.6–2.5 Ga ophiolitic melange now part of the North China Craton and formed by subduction of oceanic and continental material. Microstructures and phase relationships in a multi-phase inclusion of TiO2(II), rutile, apatite, and tremolite yield minimum P-T conditions of 7.5 GPa at 1000 °C, indicating that the crustal host, including carbonates, were subducted to depths > 270 km, transferred to the mantle of the overriding plate, and returned to the surface by 2.5 Ga. We suggest that slab rollback forced upward mantle flow, incorporating entities from the lower plate, perhaps in serpentinite diapirs, resulting in adiabatic melting that allowed crustal material to be trapped in chromite grains crystallizing in high-Mg melts. Contrasting bulk moduli and thermal contraction of the inclusions and host chromite protected the inclusions from P-induced back-reaction during exhumation. Together, these features show that the 1600 km long Central (Taihang) Orogenic Belt is emerging as the world’s first well-documented Phanerozoic style orogen, with classic tectonic zonation, ophiolitic melanges, paired metamorphism, local evidence for UHP conditions, foreland basins, and late to post orogenic magmatism. Applying the null hypothesis, we explain this high degree of similarity by invoking the operation of Phanerozoic style plate tectonics, at least throughout the 1600 km length of the COB, and by geological comparison, in other similar aged geological terrains globally. From this we infer modern-style plate tectonics was operating in the Neoarchean.

Chromian-spinel (chromite) is a common mineral in ophiolitic rocks and the study of chromite from the mantle sections of ophiolites can help shed light on their petrogenetic origin and tectonic setting. The Proterozoic Miaowan Ophiolite Complex (MOC) in the Yangtze Craton contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with serpentinised dunites. Most chromite grains display compositional zoning due to alteration but the fresh cores preserve primary igneous compositions. Podiform chromitites in the MOC dunites and harzburgites are compositionally similar to typical ophiolitic chromitites elsewhere. The chromite grains contain numerous inclusions of clinopyroxene and amphibole with minor amounts of olivine, chlorite, base-metal sulphides (BMS) and platinum-group minerals (PGM). The abundant hydrous mineral inclusions within the chromite grains suggest a hydrous mantle source. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts in a forearc setting. However, chromite grains in the harzburgites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the MOC peridotites suggest that the chromitites in the MOC formed in a forearc tectonic setting during reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.

This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation.Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment.There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor.Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex.The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 ∗ Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.

The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1–4 from oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6 wt% and positive correlations of LOI with Al2O3, but negative correlations with SiO2, CaO and Na2O. Fresh dacites (LOI Plagioclase in the fresh dacite has relatively high Sr contents and 87Sr/86Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039–0.7138), indistinguishable from those of the fresh rocks. Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustally-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower–middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks.

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with

The NWW-striking Qinling Orogen formed in the Triassic by collision between the North China and Yangtze Cratons. Triassic granitoid intrusions, mostly middle- to high-K, calc-alkaline, are widespread in this orogen, but contemporaneous intrusions are rare in the southern margin of the North China Craton, an area commonly considered as the hinterland belt of the orogen. In this paper, we report zircon U–Pb ages, elemental geochemistry, and Sr–Nd–Hf isotope data for the Laoniushan granitoid complex that was emplaced in the southern margin of the North China Craton. Zircon U–Pb dating shows that the complex was emplaced in the late Triassic (228 ± 1 to 215 ± 4 Ma), indicating that it is part of the post-collisional magmatism in the Qinling Orogen. The complex consists of, from early to late, biotite monzogranite, quartz diorite, quartz monzonite, and hornblende monzonite, which span a wide compositional range, e.g., SiO2 = 55.9–70.6 wt.%, K2O + Na2O = 6.6–10.2 wt.%, and Mg# of 24 to 54. The biotite monzogranite has high Al2O3 (15.5–17.4 wt.%), Sr (396–1398 ppm) and Ba (1284–3993 ppm) contents and relatively high La/Yb (mostly 14–30) and Sr/Y (mostly 40–97) ratios, but low Yb (mostly 1.3–1.6 ppm) and Y (mostly14–19 ppm) contents, features typical of adakitic rocks. The quartz monzonite, hornblende monzonite and quartz diorite have a shoshonitic affinity, with K2O up to 5.58 wt.% and K2O/Na2O ratios averaging 1.4. The rocks are characterized by strong LREE/HREE fractionation in chondrite-normalized REE pattern, without obvious Eu anomalies, and show enrichment in large ion lithophile elements but depletion in high field strength elements (Nb, Ta, Ti). The biotite monzogranite (228 Ma) has initial 87Sr/86Sr ratios of 0.7061 to 0.7067, eNd(t) values of − 9.2 to − 12.6, and eHf(t) values of − 9.0 to − 15.1; whereas the shoshonitic granitoids (mainly 217–215 Ma) have similar initial 87Sr/86Sr ratios (0.7065 to 0.7075) but more radiogenic eNd(t) (− 12.4 to − 17.0) and eHf(t) (− 14.1 to − 17.0). The Sr–Nd–Hf isotope data indicate that the rocks were likely generated by partial melting of an ancient lower continental crust with heterogeneous compositions, as partly confirmed by the widespread presence of early Paleoproterozoic inherited zircons. Mafic microgranular enclaves (MMEs), characterized by fine-grained igneous textures and an abundance of acicular apatites, are common in the Laoniushan complex. Compared with the host rocks, they have lower SiO2 (48.6–53.7 wt.%) and higher Mg# (51–56), Cr (122–393 ppm), and Ni (24–79 ppm), but equivalent Sr–Nd isotope compositions, indicating that the MMEs likely originated from an ancient enriched lithospheric mantle. The abundance of MMEs in the granitoid intrusions suggest that magma mixing plays an important role in the generation of the Laoniushan complex. Collectively, it is suggested that the Laoniushan complex was a product of post-collisional magmatism related to lithospheric extension following slab break-off. Formation of the adakitic and shoshonitic intrusions in the Laoniushan complex indicates that the Qinling Orogen had evolved into a post-collisional setting by about 230–210 Ma.

Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosphere formed in distinctly different tectonic settings during their igneous evolution. Recent studies in Archean and Proterozoic greenstone belts have shown that the Precambrian rock record may also include exposures of a diverse suite of ophiolite complexes as part of craton development in the early history of the Earth. We review the salient features of the Precambrian ophiolite record to highlight what has been learned about Precambrian oceanic spreading systems since the original Penrose definition of ophiolites in 1972. Some of the diagnostic, characteristic, typical, and rare aspects of ophiolites of all ages are presented in a table in order to help determine if tectonically deformed and metamophosed sequences in Precambrian shield areas may be considered as ophiolites. The results of this comparative study are important in that they enable researchers to more realistically characterize allochthonous mafic/ultramafic rock sequences as ophiolitic or non-ophiolitic. This approach is more deterministic in contrast to some other arbitrary classification schemes requiring three or four of the Penrose-style ophiolitic units to be present in the Precambrian record for a specific rock sequence to be considered ophiolitic. Once these tectonic fragments are recognized as remnants of ancient oceanic lithosphere, great progress shall be made in understanding early Earth history. We discuss the significance and implications of the Precambrian ophiolite record to constrain the mode and nature of the plate tectonics that operated in deep time.

Miyashiro (1973) famously initiated a debate on the tectonic setting of ophiolite complexes by proposing that ‘the Troodos ophiolitic complex was probably formed in an island arc’. This paper evaluates and updates Miyashiro's work by: (a) using the Mehegan–Robinson set of 137 fresh volcanic glass analyses to sidestep the controversy over the effect of alteration on major element classification diagrams; (b) using the volcanic glass database for Troodos analogues such as back-arc basins, slab edges, forearcs and subduction initiation terranes; (c) revising Miyashiro's classifications by including the boninitic series and subdividing the tholeiitic series into high- and medium-Fe series; and (d) extending Miyashiro's methodologies to new developments in the interpretation of major element data. We conclude that the Troodos Massif is made up of oceanic crust built from a high-Si8, moderate-Fe tholeiitic magma, overlain by boninites. Its low K8/H8 is consistent with near-trench crustal accretion and its Na8–Fe8 systematics indicate that TP = c.1400 °C for the Lower Lavas, consistent with rapid slab roll-back and/or sideways influx of hot mantle. Overall, the geochemical characteristics and geological setting support models in which the Troodos Massif formed by slab roll-back following subduction initiation, probably near a slab edge.

A newly discovered eclogite belt in the eastern part of the Lhasa Block, Tibet, is about 500–1000 m wide and at least 60 km long in an E–W direction. The eclogites occur as tectonic slices in garnet-bearing, mica–quartz schist. They are generally fresh and form thick, massive layers that consist chiefly of garnet (Grt) + omphacite (Omp) + phengite (Phe) + rutile (Rut) + quartz (Qtz). P–T calculations based on the Grt–Omp–Phe mineral assemblage yielded peak metamorphic conditions of 2.7 GPa and 730 °C, close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high-pressure metamorphic belt. Petrochemical data suggest that the eclogite protoliths were typical MORB basalts, derived from depleted mantle. SHRIMP U–Pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 ± 15 to 292 ± 13 Ma, with an average value of 262 ± 5 Ma. The MORB eclogites are interpreted to be remnants of Paleo-Tethyan oceanic lithosphere. The eclogites, along with Permian island arc volcanic rocks to the north, are believed to mark a Carboniferous–Permian suture zone dividing the Lhasa Block into a northern and southern segment. This newly identified suture zone suggests that the border of the Paleo-Tethyan Ocean jumped southward from north of the Bangong-Nujiang suture to within what is now the Lhasa Block.

Ophiolites are key features for unravelling the geologic history of orogenic belts, especially in China, which was formed by amalgamation of numerous tectonic blocks from Precambrian time to the present. Although much has been learned about ophiolites over the last 30 years, questions remain as to their definition, composition, classification, origin and tectonic significance. Most ophiolites, particularly those in China are incomplete bodies composed of serpentinized peridotite, minor cumulate rocks and sparse pillow lavas. Sheeted dikes are rare in most ophiolites and the structural relationships between the different lithologies are commonly unclear. Such bodies should be considered ‘possible’ ophiolites unless the lithologies can be clearly linked structurally or compositionally. The former classifications of ophiolites into Cordilleran- or Tethyan-types and Harzburgite- or Lherzolite-types have been superseded by a more process-oriented classification proposed by Dilek (2003) that relates them to different magmatic and tectonic processes. This classification makes it easier to reconstruct paleoenvironments and processes in the geologic record. Although ophiolites were originally interpreted as fragments of normal ocean lithosphere, most such bodies have been shown to contain clear suprasubduction zone geochemical fingerprints. Some ophiolites consist entirely of suprasubduction zone components whereas others are compound bodies formed originally at mid-ocean spreading centers and then modified by later suprasubduction zone melts. Percolation of suprasubduction zone melts through the overlying mantle wedge can produce dunite pods and dikes, commonly associated with podiform chromitites, and may be responsible for the formation of transitional zone dunites. Ophiolite emplacement is thought to require subduction of lower density material beneath the body, a process that is easy to explain in subduction zone environments. Emplacement can occur either during subduction rollback or by final closure of the ocean basin in which the ophiolite formed. Much new work on Chinese ophiolites is currently underway and new interpretations of these bodies are being developed. This Special Issue presents the results of recent studies of ophiolites from most of the major orogenic belts in China.

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (

Three suites of granitic rocks are present in the early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt, which has eclogite ages of 466-495 Ma. The first suite consists of quartz monzodiorite + granodiorite + monzogranite, the second of monzogranite + two-mica granite + muscovite granite + syenogranite, and the third of granodiorite + monzogranite + biotite granite. The country rocks into which these bodies were intruded consist chiefly of medium- and high-grade Proterozoic gneisses and schists and Paleozoic sedimentary and volcanic rocks. Monzogranite of the first suite has a zircon SHRIMP age of 473 Ma, whereas syenogranite in the second suite has an age of 446 Ma, and granodiorite in the third suite has an age of 397 Ma. Rocks of the first and third suites have similar mineralogies consisting of plagioclase, hornblende, quartz, alkali feldspar, and biotite, whereas those of the second suite consist of potassium feldspar, quartz, muscovite, biotite, and plagioclase. Rocks of the first a...

Whole-rock major, trace, and platinum-group elemental (PGE) data, and major and trace element compositions of diopsides are reported for peridotite xenoliths from (1) early Mesozoic volcanic breccias in Xinyang, located at the southern margin of the North China Craton, and (2) Cenozoic basalts in Hebi and Shanwang, both of which are situated within the craton and lie on the North-South Gravity Line and the Tanlu fault zone, respectively. The early Mesozoic Xinyang xenoliths are harzburgites containing

Stratigraphic relationships and bulk-rock geochemical data indicate that Upper Permian metabasalts in the Songpan–Ganzi and Yidun terranes, on the eastern margin of the Tibetan Plateau, are part of the Emeishan large igneous province, which is believed to have formed from the Emeishan mantle plume. Eruption of the Emeishan basalts at 260 Ma was coincident with rifting of the western margin of the South China Craton to form the Songpan–Ganzi ocean basin. The spatial and temporal coincidence between basalt eruption and continental rifting, as well as regional doming prior to eruption, suggest that continental break-up was a response to the Late Permian Emeishan plume. The Songpan–Ganzi ocean basin was rapidly filled with Triassic flysch deposits, then deformed and uplifted during Mesozoic collision between the North China and South China Blocks and the Tertiary collision of India and Eurasia.

Sheeted dike complexes, in which dike intrudes dike without intermediate screens of gabbro or pillow lava, have long been considered key features of oceanic lithosphere and ophiolites formed in extensional environments. The presence of a sheeted complex implies an approximate balance between spreading rate and magma supply, such that there is just enough melt to fill newly formed fractures produced by spreading. Such a balance appears to exist at mid-ocean ridges, where both the spreading rate and magma supply are probably linked to mantle convection, and thus sheeted dikes appear to be a major part of the ocean crust. In contrast, ophiolites, which are formed or modified in suprasubduction zone environments, rarely have large, well-developed sheeted dike complexes, because magma supply and spreading rate are not linked in the same way. In suprasubduction zone environments, the spreading rate is controlled largely by the rate of slab rollback, whereas the magma supply is controlled by the local temperature profile, the lithology of the subducting crust and mantle wedge, the history and degree of melting of the mantle source, and the abundance and nature of fluids. Because spreading rate and magma supply are rarely balanced in these environments, we suggest that sheeted dikes, rather than being key elements of ophiolites, may instead be unusual features in such bodies. Thus, care must be exercised in using ophiolites to investigate spreading processes at mid-ocean ridges.

The Aoyougou mafic-ultramafic complex lies in the Paleozoic orogenic belt of the western part of the North Qilian Mountains near Aoyougou valley in Gansu Province, northwestern China. It consists of serpentinite, a cumulate sequence of gabbro and diorite, pillowed and massive lavas, diabase dikes, and chert, an assemblage tentatively interpreted as an ophiolite. SHRIMP dates on zircons from the diabase dikes indicate a crystallization age of 1777 ± 28 Ma. The basalts show light rareearth element enrichment and have relatively high TiO2 and low Al2O3 contents, characteristic of present-day E-MORB or ocean-island lavas. All of the lavas have relatively low MgO contents and Mg numbers [100Mg/(Mg+Fe)], indicating a somewhat evolved character. The diabase dikes have flat chondrite-normalized REE patterns and are significantly more primitive than the lavas. Based on the lava geochemistry, the nature of the serpentinite, and the regional geology, the Aoyougou complex is interpreted as a fragment of oceanic litho...

Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

Metamorphic rocks found at the base of the Jurassic Donqiao ophiolite of northern Tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif. The rocks form a sequence some 8 m thick, varying from high-grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down. The mineral paragenesis is similar to other such aureoles, and indicates that temperatures in excess of 750 °C may have been reached during metamorphism. The lack of high-pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone. Ar–Ar geochronology on amphibole separates provides dates of 175–180 Ma for the displacement of the ophiolite, significantly older than the age of emplacement estimated from stratigraphie relationships. The ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment.

Three ophiolite belts, ranging in age from Cambrian to Triassic, provide valuable data on the tectonic evolution of the Kunlun Mountains which lie along the northern margin of the Tibetan Plateau. All of the ophiolites contain dismembered but nearly complete assemblages of peridotite, mafic and ultramafic cumulates, pillow and massive lavas and radiolarian cherts, (1) The oldest belt contains ophiolites of Cambrian age, but the initial spreading of the ocean basin most likely began in the Late Proterozoic. These ophiolites outcrop in the central part of the eastern Kunlun, extend for at least a few hundred kilometers, and are believed to reflect a small ocean basin. Closure of this ocean led the accretion of south Kunlun block to the north Kunlun block and Tarim craton. Ophiolitic lavas in this belt have island are affinities. (2) A second belt of early Carboniferous ophiolites extends nearly 600 km along the northern margin of the western Kunlun. Lavas in this belt are basalt, basaltic andesite and andesite, all of which have compositions characteristic of volcanic arcs. This ocean basin possibly developed on the basis of the early Paleozoic oceanic basin. Closure of this ocean basin by southward subduction in the early Permian produced a suite of calc-alkaline volcanic rocks in what is now the central part of the western Kunlun. (3) The third belt extends nearly 1200 km along the southern margin of the Eastern Kunlun and contains numerous ophiolites of Early Permian to Middle Triassic age. These ophiolites are highly tectonized, containing volcanic rocks with the geochemical characteristics of mid-ocean ridges, oceanic islands and volcanic arcs. This belt is tentatively interpreted as the suture zone between Gondwana and Eurasia.

Mineral chemical data have been collected from the 6 km thick mantle section of the Table Mountain massif (Bay of Islands Ophiolite Complex, Newfoundland). The samples chosen represent the geochemical background, i.e. exclude samples from the vicinity of bands, dykes and pods of dunites and pyroxenites within the host harzburgites and (rare) lherzolites. Three large mineral-chemical domains can be distinguished as a function of depth below the crustal gabbros. The intermediate mantle section (2–5 km beneath the gabbros) has a very depleted geochemical pattern with Cr# [ 100 Cr ( Cr + Al ) ] in spinel > 44, low Ti. The basal kilometer of the mantle section shows a steep geochemical gradient towards less depleted mineral chemistries (Cr# as low as 10). Both the intermediate and basal mantle rocks fit into a progressive partial melting trend. The top of the mantle section (0–2 km below the gabbros) shows Cr# between 30 and 44, but also higher Ti and ferric iron in spinel, and Na in clinopyroxene. Microstructural evidence for melt-related clinopyroxene is locally preserved and, in one area, plagioclase lherzolites occur. The topmost mantle section is considered to have been affected by melt infiltration. The geochemical variation is larger than elsewhere, suggesting that melt-infiltration, or at least the process of trapping of melt, was heterogeneous. In most melt-infiltrated harzburgites, plagioclase is absent. Other peridotites show good microstructural evidence for melt-infiltration but little chemical evidence to substantiate it. Both features are discussed and can be explained by a variety of processes demonstrating the complexity of melt-infiltration.

Publisher Summary High-pressure‑ultrahigh-pressure (HP‑UHP) metamorphic belts record a range of tectonic processes from deep subduction of oceanic lithosphere to collisional orogeny. It is now clear that low-density continental crust with high rheological strength can be subducted to mantle depths of 100‑150 km and then quickly exhumed. Currently, more than 20 UHP metamorphic belts have been found in continental collision belts along plate convergence boundaries. The study of HP‑UHP metamorphism has been a challenging frontier issue in earth sciences. The HP‑UHP terranes in the eastern Tethyan Orogenic belt in China and neighboring regions record the former presence of numerous small ocean basins and small continental blocks. The presence of these small continental blocks led to local subduction of the oceanic lithosphere that floored the many Proto-Tethyan and Paleo-Tethyan ocean basins. This is particularly evident in the Qinghai-Tibet Plateau, where eclogite-bearing HP‑UHP belts mark subduction of oceanic lithosphere beneath many microcontinental blocks. Subduction of continental lithosphere was mainly at the margins of large continental blocks. Deep subduction of continental lithosphere generally involved either scissor-like collision between large blocks or shear-type collision of tongue-shaped lithospheric slabs.

The Clarno Formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows, and cut locally by irregular hypabyssal stocks, dikes and sills. Lateral lithofacies variations are pronounced, and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation. A range of sedimentary environments is represented in the volcaniclastic sequences, including near-vent flow and breccia accumulations, bouldery high-gradient braided streams, and relatively low-gradient sandy-tuff braidplains associated with paludal deposits. We infer that the coarse-grained volcaniclastic rocks of the Clarno Formation accumulated largely in volcanic flank and apron settings. The formation's stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands, and only a few, scattered remnants of large central vent volcanoes are known. The absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop. We infer that the arc was composed of small- to medium-size volcanoes arranged haphazardly across a broad area. The sedimentary succession most probably accumulated in a series of intra-arc depressions formed by diffuse block rotation driven by oblique subduction during the Eocene.

The Troodos Massif has played a unique role in the development of the ophiolite concept. The importance of Troodos derives from its coherent nature, its excellent exposures and easy access, and the fact that it was one ofthe first such bodies to be studied in detail. Early studies demonstrated the genetic relationship between ophiolitic units and elucidated the nature and significance of sheeted dikes, leading to the recognition that ophiolites are fragments of ocean lithosphere formed in spreading environments. Information derived from Troodos played a key role in formulating a new definition of ophiolites in 1972, one that has remained essentially unchanged for 30 years. The suprasubduction zone nature of ophiolitic lavas was first recognized in Troodos, providing a new understanding of where ophiolites originate in the ocean basins and raising questions as to the validity of applying the ophiolite model to the greater part of the oceanic lithosphere. Detailed mapping and drilling of Troodos has shown that the ophiolite was constructed from multiple subrift magma chambers, and has elucidated the relationships between the plutonic complex and the sheeted dikes. The base metal orebodies of Troodos were shown to be hydrothermal deposits before black smokers were discovered on the seafloor, and they now provide opportunities for detailed, three-dimensional studies of fossil hydrothermal systems. Finally, study of Troodos provided convincing evidence for uplift of oceanic lithosphere by partial underthrusting of continental lithosphere, a model now widely favored for many cases of ophiolite emplacement. Although our understanding of ophiolites has been greatly enhanced by studies of Troodos, many questions remain and we expect that the massif will continue to play a significant role in ophiolite research.

International audience; Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle–ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally ≤5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins.

The Reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99.7%. Subaerial lava flows comprise 54% of the core, dikes 41%, and clastic rocks about 5%. In addition, a 1.3-km-thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled. The lava flows are chiefly low-magnesia basalts and ferrobasalts with lesser amounts of icelandite, basaltic andesite, and olivine tholeiite. In the drill core, complete flows range from 1.2 to 19.7 m in vertical thickness and average 7.1 m; in the exposed section, flow thicknesses range from 1 to 30 m and average 5.2 m. Most of the flows are characterized by an upper scoriaceous zone 1–3 meters thick, grading downward through a zone of fractured basalt into a massive flow interior. Some flows also have a thin basal breccia. Olivine tholeiites and some low-magnesia basalts are commonly mottled, whereas ferrobasalts and icelandites have well developed flow banding. Most of the flows are fine- to medium-grained, aphyric to sparsely phyric, and moderately vesicular. A few are moderately plagioclase phyric; others have scattered phenocrysts of clinopyroxene and olivine. Over 100 intrusive units are recognized in the core, and they span approximately the same compositional range as the flows. They are characterized by having steeply inclined chilled contacts, a uniform massive aspect, and an absence of vesicles or brecciation. Most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures. Many of the dikes are also aphyric, but some contain notable plagioclase or olivine phenocrysts. Groundmass textures range from fine-grained, intersertal to coarse-grained, ophitic. Clastic units occur typically as thin, varicolored layers between lava flows, rarely exceeding 0.5 m in thickness. These are chiefly fine- to coarse-grained, poorly- to well-bedded lithic tuffs showing varying degrees of reworking. Most are silicic to intermediate in composition, but basaltic units are also present. Two rhyolitic ash flow tuffs occur in the section, one of which is 30 m thick. We interpret the lavas to be the flanking portions of broad low-angled central volcanoes centered on silicic complexes. Some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms. Most of the dikes were probably intruded laterally into the lava pile; thus they do not represent direct vertical feeders from deep magma bodies.

Major element data are reported for lavas and dikes from a 3.2-km section through the eastern Iceland lava pile. The section includes a 1.2-km subaerial exposure and a 1.9-km sequence drilled by the Iceland Research Drilling Project (IRDP). Alteration increases down-section, although primary magma chemistry may be interpreted from FeO* Al2O3, TiO2, P2O5 and (to a lesser extent) SiO2 in addition to immobile trace elements. Lava compositions range from MgO-rich tholeiite to SiO2-rich icelandite, while dikes range from tholeiite to tholeiitic andesite. Three stratigraphic divisions based on volcanologic and trace element criteria (e.g., Ce/Yb and Zr/Y) are confirmed by major elements. Upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas. Variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase. During build-up of the pile, eruptive phases commenced with silica-rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates. Dike chemistry resembles that of upper group lavas and also the nearby Breiddalur volcano. Middle group lavas probably derive from a buried silicic complex, while the lower group may represent the flank of a more deeply buried complex. Chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic-silicic complexes. Magma supply, fractionation, and mixing are considered in the framework of such kinematic models, although it is premature to draw direct analogies to the active Neogene zone of Iceland.

Petrologic, geochemical, and geophysical studies of cores from DSDP sites 417 and 418 show that the Cretaceous eruptive crust was formed at a stable slow-spreading axis similar to the present Mid-Atlantic Ridge, probably not close to major transform fractures. Interpretations of whole-rock and glass chemical variation suggest that magma fractionation occurred in temporally and spatially transient storage reservoirs at various depths between the zone of magma segregation and the ridge crest. We suggest that melt generation, uprise, and eruption are part of a related episode of activity involving rapid upward propagation of fractures. Eruptive episodes are recorded by stratigraphic sequences of contiguous, mostly cogenetic, lavas, separated by breccias, sediment, and/or extreme alteration zones suggestive of lengthly quiescent intervals. The section drilled at 417D comprises three such episodes (in 365 m of basement), whereas seven are recorded in 418A (in 540 m). Patterns of downhole changes in magnetic inclinations and polarities strongly support the suggestion that iterative burial of successive lava episodes occurs with crustal spreading from the axis. This model predicts that the eruptive layer is imbricated and that it shows increasing complexity of lithology, chemical compositions, and magnetic character with depth. Studies of crack distribution and laboratory measurements of core physical properties indicate preferential zones of water circulation which decrease with depth. Permeability of the lava pile is largely controlled by the imbricate eruptive structure.

The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of the Tibetan Plateau. The volcanic rocks are grouped into four units (1–4) from the oldest to youngest. Units 1, 3 and 4 are composed of olivine trachybasalt, basaltic trachyandesite and trachyandesite, and Unit 2 consists of hornblende dacite. The rocks of Units 1, 3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol% and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to typical arc volcanics. Despite minor crustal contamination, 87Sr/86Sr ratios (0.706–0.709), eNd values (−3.2 to −8.7), and eHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (206Pb/204Pb = 18.02–18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting.

Deep sea drilling in the eastern Indian Ocean shows that the oceanic crust off Western Australia is approximately 140 million years old and becomes younger to the west; this dates the initial opening of the Indian Ocean.

Any comprehensive model for the origin of the Troodos complex and other Mideast ophiolite complexes must explain the geochemical evidence for subduction-zone involvement, the thin inferred oceanic crust, the extensional environment indicated by sheeted dikes, the existence of fault zones perpendicular to the inferred spreading axes, and the discontinuous nature of ophiolite exposures around the Arabian block. The Andaman Sea region of the Indian Ocean may provide an actualistic model for the origin of these ophiolites. There, spreading takes place in short segments above a subduction zone in the region of active spreading. The Andaman Sea model substantially accounts for the known geologic and geophysical data from the circum-Arabian ophiolite belt, and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt. The model also predicts that Andaman Sea spreading-center magmas may be similar in composition to those found in the Mideast ophiolites.

The Salton Sea geothermal field lies in the Salton Trough, the landward extension of the Gulf of California, an area of active crustal spreading. Surface volcanic rocks of the field consist of five small rhyolite domes extruded onto Quaternary sediments of the Colorado River delta. Two domes are linked by subaqueous pyroclastic deposits; the others are single extrusions with or without marginal lava flows. The domes are low-calcium, alkali rhyolite with 1 to 2 percent crystals. Similar silicic rocks found in wells have been extensively altered by geothermal brines. Basaltic rocks occur as xenoliths in the domes and as subsurface dikes, sills, or flows. The xenoliths consist of low-potassium tholeiitic basalt similar to that of the East Pacific Rise. Subsurface basaltic rocks are mineralogically similar to the xenoliths but have undergone extensive hydrothermal alteration. Numerous partly melted granitic xenoliths in the domes show various degrees of either cotectic melting along quartz-feldspar grain boundaries or disequilibrium incongruent melting of hydrous ferromagnesian minerals. These rocks are sodic granite containing notably higher SiO/sub 2/, CaO, and Na/sub 2/O and lower total iron than the enclosing rhyolite. Compositions and textures suggest that the granite xenoliths are fragments of basement rather than the crystallized equivalentsmore » of the rhyolite magma. The bimodal basalt-rhyolite assemblage of the Salton Sea geothermal field is believed to have formed by partial fusion in two stages of mantle peridotite, forming successive rhyolitic and basaltic melts. After formation, the rhyolitic magma was partly contaminated by continental crust material.« less

Aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics. Five zones are recognized in the Troodos ophiolite and in situ oceanic crust: (1) a sea-floor weathering zone (SWZ), (2) a low-temperature zone (LTZ), (3) a transition zone (TZ), (4) an upper dike zone (UDZ), and (5) a mineralized zone (MZ). The MZ may be overlain by the SWZ, LTZ, or TZ, but in the known examples it is always underlain by the UDZ. The development of the SWZ is dependent upon the duration of cold-seawater circulation, which is dependent upon seafloor topography and the rate and nature of sedimentation. The boundary between the low- and high-temperature zones is largely controlled by permeability contrasts. Lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust. This, along with the evidence for disequilibrium assemblages, implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects.