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U-Pb zircon geochronology and isotopic records have played an influential role in our understanding of convergent margin dynamics. Orogenic cyclicity models link tectonic regimes with magmatic isotopic signatures in advancing orogens, relating compressional regimes with evolved signatures and extension with juvenile signatures; however, such frameworks may not apply for retreating orogens, which commonly produce substantial crustal heterogeneities during backarc rifting and ocean spreading. We explore the Mesozoic to Cenozoic Patagonian Andes tectonic evolution, combining U-Pb zircon ages, bulk rock eNd, and new detrital zircon eHf from the retroarc basin to understand the associated magmatic arc evolution during retreat and advance of the margin. Our results reveal a protracted phase of isotopically juvenile magmatism between 150 and 80 Ma, which began during backarc extension and persisted long after the margin switched to a contractional regime. We propose that the prolonged juvenile isotopic trend started mainly due to trenchward migration of the arc during backarc extension (150-120 Ma) and persisted due to partial melting of underthrusted juvenile attenuated and oceanic crust during backarc basin closure (120-80 Ma). This interpretation implies that tectonic stress alone does not predict isotopic trends, and factors like assimilation or the composition of underthrusted crust are important controls on magmatic isotopic composition, especially in retreating and transitional orogens. [ABSTRACT FROM AUTHOR]

The increased velocity associated with decaying seismicity in the subducting oceanic crust usually has been attributed to its eclogitization. However, the degree of the eclogitization of subducting oceanic crust at depth remains unclear due to the lack of velocity data for eclogite at pressures of >3 GPa. In this study, the P- and S-wave velocities of eclogite aggregates composed of omphacite and garnet (Omp100, Omp80Grt20, and Omp- 50Grt50) were measured simultaneously at pressures of up to 8 GPa using ultrasonic interferometry. The velocities of the eclogite aggregates increased with rising pressure and garnet content. By determining the velocities at different pressures using the finite strain equation, the elastic moduli of the eclogites and their pressure derivatives were determined to be KS0 = 119-134 GPa, KS0‰ = 5.2-5.3, G0 = 73-80 GPa, and G0‰ = 1.5-1.6. The high-wave velocity and low VP/VS ratio of the eclogites, combined with the seismic tomography and seismicity distribution, jointly constrain the depth and composition of eclogitization in the subducted oceanic crust of Northeast Japan, and a new 1-D velocity structure is proposed. We also compiled and calculated the depth of the eclogitization and the garnet content of the subducting oceanic crusts in 27 typical locations around the Pacific Ocean. The depth of the eclogitization was positively correlated with the age of the subduction zone, and the garnet content was estimated to be 12%-32% in our model. [ABSTRACT FROM AUTHOR]

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE-striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via fieldbased, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (~500-550 °C and ~5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep-dominated deformation. A strain rate of 10-12 s-1 and a differential stress of 25-39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47-0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence. [ABSTRACT FROM AUTHOR]

The western margin of SW Gondwana was a place of active convergence between the Gondwana shield and the oceanic lithosphere of the Panthalassa Ocean during most of the Paleozoic. However, several studies have indicated that especially to the north of latitude 22°S, the Devonian-early Carboniferous was a time of relative quiescence without magmatic activity, metamorphism, or deformation. This interval has been termed the "Devonian Problem." As the Devonian-Carboniferous is extensively well represented by over 4000-m-thick sedimentary sequences in the Tarija Basin in the Southern Bolivian Subandean Zone, provenance analyses--U-Pb isotope analysis on detrital zircon, Sr-Nd whole-rock isotope analysis, and X-ray diffraction (XRD)--were conducted on stratigraphically controlled sedimentary units to attempt to constrain the tectonic setting of the basin during that time. U-Pb on zircon provenance analysis indicates that only the Carboniferous units show input from comparatively young sources (<14%), with ages between 420 Ma and 320 Ma, which can be correlated with a Devonian magmatic arc. The dominant source areas for Devonian-Carboniferous sediment were the Sierras Pampeanas to the southwest and the Arequipa-Antofalla Massif and Famatinian Arc to the west, besides a few zircon grains (<18% of dates) that were possibly derived from pre-Andean inliers with ages typically >1800 Ma. The combined Sr-Nd isotope and XRD results for the pelites and the multidimensional scaling (MDS) analysis indicate that the Carboniferous units most likely represent reworked material from older units, with a minor contribution from the western part of the basin. All currently available data lead us to propose that the Tarija Basin developed during the Devonian-Carboniferous in a foreland basin setting related to the convergent margin, with important glacial sedimentary input from the continent during the Carboniferous. The absence of a contribution from a concomitant magmatic arc for the Devonian units of the Tarija Basin and the scarce magmatic zircon input (<14%) into the Carboniferous units indicate a shift from flat-slab subduction during the Devonian to normal subduction during the Carboniferous following complete delamination of the flat slab. Our comparison of the detrital record for the Tarija Basin with that of the Paraná, Chaco-Paraná, Sauce Grande, Paganzo, Navidad Arizaro, Madre de Dios, and Karoo basins supports an active margin setting for the SW margin of Gondwana during this time interval. [ABSTRACT FROM AUTHOR]

Genesis of the large-volume alkaline crust at active continental margin is still enigmatic for geologists worldwide. The point at issue is whether or not subducted oceanic crusts get involved and how they interact with the mantle source of the alkaline crust. Late Mesozoic juvenile alkaline crusts with high εNd(t)-εHf(t) values are widely distributed in the Great Xing'an Range of NE China, as parts of an arc magmatic belt related to the Mongol-Okhotsk Ocean closure. We carried out multiisotope analyses and 2-D high-resolution numerical modeling to trace the mantle source nature of the alkaline crust. The alkaline rocks show similar trace elements with the I-type enriched mantle and are originated from an upwelling oceanic-island basalt-like mantle. Their high field strength element depleted arc features indicate the crustal material addition in the source region. Low δ18O, mantle-like Sr-Nd-Hf and light Mg isotope compositions, limited δ7Li variations, no Nd-Hf decoupling, and our mixing calculation preclude continental crustal assimilation, marine-sediment melt and/or altered oceanic crust (AOC)-fluid metasomatism, and bulk marine sediment involvement, and provide evidence of the bulk AOC addition in the mantle source. Lower δ18O values than the mantle and relatively low δ7Li values further confirmed the involved AOC to be a high-temperature (high-T) AOC. Our multiisotope tracing successfully fingerprints the recycled high-T AOC into the source region of the alkaline juvenile crust. Then, our 2-D high-resolution numerical modeling reconstructs the high-T AOC recycling processes driven by mélange melting. [ABSTRACT FROM AUTHOR]

The nature and tectonic evolution of the Bangong-Nujiang Tethyan Ocean remain unresolved, which seriously restricts our understanding of the role of this ocean in the Tethys tectonic domain and hinders in-depth study of the geodynamics of the Tethyan tectonic evolution. The study of intraplate oceanic-island fragments in the Bangong-Nujiang Suture Zone of Northern Tibet, which are remnants of the oceanic crust, is important for determining the nature and tectonic evolution of the Bangong-Nujiang Tethyan Ocean. Here, we present a combined analysis of petrological, geochronological, geochemical, and Sr-Nd isotope data for the five intraplate oceanic-island fragments preserved in the Bangong-Nujiang Suture Zone. The oldest intraplate oceanic-island fragment in the Bangong-Nujiang Suture Zone is of Late Permian-Triassic age, and the underlying oceanic lithosphere is Late Permian or older, which indicates the development of the Bangong-Nujiang Tethyan Oceanic basin at this time. The opening time of the Bangong-Nujiang Tethyan Ocean must be even earlier than the Late Permian. At least four Triassic intraplate oceanic-island fragments have been recognized in the Bangong-Nujiang Suture Zone, and their assemblages and geochemistry are mostly similar to present-day oceanicisland basalts that erupted onto the thick lithosphere of mature oceanic basins. All of these observations indicate that the Bangong-Nujiang Tethyan Ocean evolved into a mature oceanic basin during the Triassic. The youngest intraplate oceanic-island fragments are of Late Jurassic-Early Cretaceous ages, which effectively argues for the continued presence of the Bangong-Nujiang Tethyan Ocean at this time. The abundant data of the intraplate oceanic-island fragments provide strong evidence that the Bangong-Nujiang Tethyan Ocean was a broad oceanic basin that separated Cimmeria from Gondwana during the Permian-Early Cretaceous and experienced complex evolution over at least 160 m.y. [ABSTRACT FROM AUTHOR]

Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8-7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ∼0.02‰. Approximately 6-9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab-an application that can be applied to arc magmas worldwide. [ABSTRACT FROM AUTHOR]

The newly identified Neoproterozoic Yuanmou high-Nb mafic and adakitic rocks at the western margin of the Yangtze Block in South China are investigated for petrogenesis and geodynamic implications. Zircon U-Pb ages show the adakitic granodiorite host and mafic microgranular enclaves (MMEs) were synchronously emplaced at 811-806 Ma, and were cut by an 802 ± 11.5 Ma mafic dike. The medium-grained granodiorites consist mainly of plagioclase, quartz, K-felspar, biotite, and amphibole. High SiO2 (62.8-67.5 wt%), Al2O3 (13.1-16.8 wt%) contents, high Sr/Y (17.4-49.0), and (La/Yb)N (16.3-52.6) but low Y (6.83-16.1 ppm) and Yb (0.74-1.82 ppm) concentrations of the granodiorites point toward their calc-alkaline and adakitic nature. Further, their high Dy/Yb (1.52-2.31) and Gd/Y (2.35-4.43) and low K2O/Na2O (0.22-0.48) and Th/La (0.14-0.20) ratios, constant bulk rock εNd(t) (-0.5 to -1.5) and zircon εHf(t) (0.0 to +2.3) values, suggest that the granodiorites were derived from partial melting of a mafic lower crustal source. The MMEs in the granodiorites and mafic dike intrusions are composed of amphibole, plagioclase, clinopyroxene, and olivine, and have similar chemical compositions, such as high Nb (15.7-41.9 ppm) and TiO2 (2.13-3.39 wt%) contents and high Nb/U (18.7-30.9) values, akin to alkaline high-Nb basalts (HNB). Their variable Ba/Nb (7.68-79.2), Nb/Zr (0.09-0.16), high Nb/Th (5.47-6.68), and low Th/Zr (0.01-0.03) and La/Nb (1.03-1.37) ratios indicate a mantle source modified by melts derived from subducted oceanic crust and sediments. The Yuanmou HNB with high εNd(t) (+4.8 to +6.9) were probably generated during back-arc extension by decompression melting of a mantle source metasomatized by adakitic slab melts formed during earlier stages of the subduction. This implies that the location of the back-arc was far from the subducted oceanic slab, possibly due to a slab rollback process. In this scenario, HNB intrusions provided the heat for partial melting and adakitic melt formation in the mafic lower crust. The model suggests that the Yuanmou and other similar associations of HNBs and adakitic rocks emplaced along the western margin of the Yangtze Block are products of a widespread Neoproterozoic back-arc extension. [ABSTRACT FROM AUTHOR]

U-Pb dating of single detrital zircon grains by laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP-MS) paired with Hf and O isotopic and trace-element analyses provide first-order indicators of the Late Cretaceous--Cenozoic evolution of the southern Ecuadorian magmatic arc. Detrital zircon U-Pb ages define significant clusters that are tentatively interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43 Ma. A major accretionary event in the Late Cretaceous (75-65 Ma) is marked by a broad range of zircon isotopic values (εHf[t] > 20 and δ18O > 8‰) that suggest melting of both the lower and upper crust (most likely of continental affinity) as well as enriched mantle components. Highly fractionated signatures in trace-element patterns and Eu/Eu* combined with mantle-like δ18O and juvenile εHf values characterize zircons from 60 to 45 Ma, suggesting that the Late Cretaceous--middle Eocene arc originated from an enriched mantle and likely reflects the persistence of overthickened crust previously attributed to the main Late Cretaceous accretionary period. Subsequently, negative shifts in εHf(t) isotopic composition from 45 to 30 Ma are paired with mantle-like δ18O values as well as decreases in U/Yb and Eu/ Eu*. These signatures could be attributed to magma emplacement in a thinner crust and the existence of a broad extensional magmatic arc extending from the current forearc toward areas near the craton; however, other scenarios cannot be excluded. This event was characterized by enriched mantle melt sources with residence times pointing to known crustal events (Sunsás) in the Amazonian craton. From 30 to 10 Ma, the isotopic record slightly evolved toward a depleted mantle signature with a substantial increase in fractionation. Our results combined with previously published isotopic records from detrital zircon grains found in modern rivers suggest that, for at least the last 30 m.y., the southernmost Northern Andes magmatic arc has been segmented, with the emplacement of juvenile magmas to the north and more enriched magmas related to the recycling of ancient continental crust and/or subducted sediments to the south--aspects found in other Northern Andes settings in which the continental arc was constructed in both oceanic and continental crust. [ABSTRACT FROM AUTHOR]

Recycled oceanic crust is believed to have played an important role in the formation of continental flood basalts, whereas the involvement of large amounts of CO2 derived from recycled marine carbonate in their mantle sources is highly debated. Zn isotopes have great potential to trace recycled carbonate due to the distinctly different δ66Zn values between marine carbonates and the mantle. Representative continental flood basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in Tarim, NW China, were collected to investigate their mantle sources, and their Zn isotopes were studied systematically for the first time. The Zn isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰) are higher than values in the primitive mantle (δ66Zn = 0.16‰ ± 0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ66Zn = 0.24‰-0.31‰). Considering their high fractionation (e.g., Mg# = 0.28-0.37) and potential involvement of crustal contamination (87Sr/86Sri between 0.70720 and 0.70779; εNd between -3.2 and -1.9), their Zn isotopes may not conclusively point to a carbonated mantle source. In contrast, Yingmai and Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and 0.39‰ ± 0.03‰), nonradiogenic 87Sr/86Sri (0.70459-0.70518) and εNd(t) (between -1.3 and 0.1) signatures, and less fractionation (Mg# = 0.46-0.71), implying the involvement of recycled carbonate components in their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti anomalies, low CaO/ Al2O3 ratios, and high SiO2 contents preclude direct melting of carbonate-bearing mantle. Alternatively, these features may suggest the melting of decarbonated subducted eclogite and variable interaction with subsolidus peridotite. This assumption is consistent with the positive correlations of δ66Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of mixing between eclogite-derived high-δ66Zn melt and peridotite-derived mantle-like δ66Zn melt. Our study provides a new model to reconcile the geochemical features of the Tarim continental flood basalts and highlights the potential role of decarbonated eclogite in the formation of continental flood basalts. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two continental collisional orogenic events and magmatism in early Paleozoic and early Mesozoic. However, possible magmatic additions to the continental crust growth of the EKO in different tectonic stages of early Paleozoic collisional orogeny have been overlooked. Three phases of early Paleozoic plutons from the Xiangride-Kuhai area in the east of the EKO have been chosen for detailed investigation and the results are reported here. The oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex. The monzodiorite has a sanukitoid-like composition with high TiO2 and Y contents and is interpreted as being derived from partial melting of metasomatized mantle wedge lherzolite. The granodiorite is typified by its high SiO2 content, high Sr/Y ratio, and depleted Hf isotope, and is interpreted as an adakite-like melt derived from the melting of a subducted Proto-Tethys oceanic crust. The magmatism can be linked to northward subduction of the Proto-Tethys Ocean between 520 and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced during ca. 450-431 Ma with an I-type granitic composition. Of these, the ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source in the orogen. The ca. 450-434 Ma monzogranite and granodiorite in the Walega and Zhiyu intrusive complexes exhibit variable element and isotope compositions. They would have been generated by magma mixing of felsic melts from the old crust and mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431 Ma quartz diorite in the Walega Intrusive Complex is formed through crustal assimilation and fractional crystallization of mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion >60%. Stage 2 suite was emplaced during the closure of Proto-Tethys oceanic branches and subsequent continental collision during 450-426 Ma. Magmatism diminished between ca. 426 and 410 Ma during exhumation of the continental lithosphere as indicated by the presence of retrograde eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are adakite-like or have an Atype granitic composition and are enriched in Nd-Hf isotopes. They might have been derived from the remelting of old and juvenile continental crust in a post-collisional extensional setting during 410-390 Ma. Identification of partial melts, derived from the subducted Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1 and 2 plutons, show that the subcrustal materials have been significantly transferred to the overlying continental crust. Hence the magmatism in oceanic subduction (Stage 1) and continental collision (Stage 2) settings contributes to the early Paleozoic juvenile continental crust growth of the EKO. The post-collisional extensional setting (Stage 3) is dominated by the reworking of a pre-existing continental crust. The early Paleozoic continental crust growth processes in the EKO are different from the previous view in which the continental collision orogens have no crust growth, and inconsistent with the proposal that crust growth is significant only in a continental collision setting. [ABSTRACT FROM AUTHOR]

Lawsonite eclogites are major hosts of H2O and trace elements and thus key for long-term deep element cycling in subduction zones. Existing cycling models suggest that the subducting oceanic crust transforms to lawsonite-eclogite assemblages; yet the scarcity of lawsonite eclogites in the rock record questions to what extent the oceanic crust transforms to law-sonite-eclogite assemblages during subduction. Here, we use petrological modeling coupled with geodynamic calculations for a typical subduction zone to show that the occurrence of lawsonite eclogites is controlled by the maturation of the subduction zone. We find that law-sonite eclogite does not form in infant subduction zones; with time, the oceanic crust forms lawsonite assemblages, but prograde heating obliterates lawsonite except in rocks exhumed prior to such heating. Lawsonite-eclogite assemblages in the oceanic crust form and survive prograde metamorphism only in mature and cold subduction zones but still necessitate specific characteristics during exhumation to preserve lawsonite. We show that the stability of lawsonite in mature subduction zones is hindered by hybridization between the mafic crust and the overlaying mantle wedge material; by contrast, lawsonite proportion increases with intense seafloor alteration and Ca-Al metasomatism. These latter processes are thus key for enhancing recycling. We argue that lawsonite-driven mass cycling to the deep mantle is important in mature subduction zones, but the role of lawsonite eclogite in carrying mass deep into Earth in the infant stage of a subduction zone is minor. [ABSTRACT FROM AUTHOR]

Subduction initiation is a pivotal process in plate tectonics. Models of subduction initiation include the collapse of passive margins, oceanic transform faults, inversion of oceanic core complexes, and ridge failure but have ignored the potential effects of continental crust relicts within the oceanic crust. In this paper, we explore the role of microcontinents on subduction initiation through two-dimensional thermo-mechanical numerical modeling. We consider three scenarios with variable ages of oceanic crust surrounding the microcon-tinent and parametrically examine the microcontinent characteristics (size, crustal thickness, thermal gradient, and rheology), oceanic plate age, and convergence rates. Results suggest that moderate-size (≥300 km) microcontinents can nucleate subduction initiation at the junction between continental and oceanic plates. A large part of the microcontinent would be dragged into the subduction zone, and the subsequent asthenosphere upwellings would incorporate part of the microcontinent. Our numerical models add a new hypothetical scenario for subduction initiation, especially for those places where a young and buoyant plate subducts beneath an older and denser oceanic plate. Moreover, they can explain the origin of exotic crust materials and ultrahigh-pressure minerals in supra-subduction zone ophiolites. [ABSTRACT FROM AUTHOR]

Diamonds can form within eclogite (remnants of ancient subducted oceanic crust) in the subcontinental lithospheric mantle (SCLM), pieces of which may then be transported to the surface as xenoliths in kimberlites. These diamond-bearing xenoliths offer a rare opportunity to study diamonds in their host lithology. The morphology and surface features of diamonds record evidence of attack by diamond-aggressive melts/fluids, which results in resorption, rounding, and destruction of octahedral growth forms. The location and timing of such diamond-destruction processes remain controversial, and the kimberlite magma itself is often considered to be the primary culprit. We used X-ray computed tomography scanning to present a view of diamond morphology and distribution within 24 diamondiferous eclogites in unprecedented detail. These scans clearly capture diamond growth and destruction that occurred within the SCLM, prior to kimberlite entrainment. We show that euhedral diamonds in these eclogites are predominantly step-faced octahedra. This morphology is preserved even when the diamonds are exposed at the surface of the eclogites, indicating that kimberliteinduced resorption was not significant. Six eclogites contain only rounded diamonds with no distinct crystal faces, and their exposed surfaces on the exterior of the xenoliths are highly irregular, indicating diamond-destruction in the SCLM. In three cases, single xenoliths host both resorbed diamonds and step-faced octahedra, indicating multiple metasomatic events, some of which were diamond-aggressive and others diamond-friendly. These diamondiferous xenoliths provide snapshots of diamond growth and destruction in the SCLM, caught in the act. [ABSTRACT FROM AUTHOR]

Highly siderophile elements (HSEs), including Re and Os, are used extensively as geochemical tracers and geochronometers to investigate the formation and evolution of Earth's crust and mantle. Mantle rocks are commonly serpentinized, but the effect of serpentinization on the distribution of HSEs is controversial because HSEs are commonly hosted by rare, micrometerto sub-micrometer-scale grains of platinum group minerals (PGMs) of ambiguous origin that are challenging to identify, characterize, and interpret. In this study, atom probe tomography (APT) is used to characterize two spatially close PGM grains hosted by a partially serpentinized harzburgite from Macquarie Island, Australia. The APT data reveal an extraordinary level of detail that provides insights into the origin of a complex Cu--Pt alloy grain (average composition Cu4Pt). The grain hosts Fe-, Ni-, and Pt-rich sub-grains associated with Rh, variably overlapping networks of Pd- and Cd-enrichment, and OH-rich volumes identified as fluid inclusions. Osmium and Ru are hosted by an idioblastic laurite (RuS2) grain. Compositional, textural, and phase-diagram constraints are consistent with a modified pre-serpentinization origin for the PGMs, and a comparison between observed and calculated grain distributions indicate that while Os isotope ratios were probably unaffected by serpentinization, whole-rock and grain-scale HSE and isotopic ratios may have been decoupled during serpentinization. [ABSTRACT FROM AUTHOR]

Although long-lived detachment faulting plays an important role in fluid circulation and in accommodating tectonic extension at slow-spreading oceanic ridges, it is still unclear how the fluid-enriched faults contribute to the observed seismic anisotropy in the lower crust. We investigated sheared and altered gabbros along the detachment fault zones from the Xigaze ophiolite in the southern Tibetan Plateau. Results demonstrate that the positive feedback between fluid circulation and shearing, linked by dissolution-precipitation creep of amphibole, resulted in fluid enrichment during strain localization along the fault zones. Based on this shearing-enhanced fluid circulation model, our calculations of the seismic properties show that amphiboles (de)formed by dissolution-precipitation creep along the fault zones largely contribute to the seismic anisotropy (P and S waves) and S-wave delay time in the lower crust at slow-spreading ridges, with the polarization directions of fast shear waves being subparallel to the ridges. The strength of resulting seismic anisotropy is largely a function of crustal thickness, fault zone attitude, and metasomatism intensity. This study provides a novel explanation for the origin of seismic anisotropy in the lower oceanic crust at slow-spreading ridges. The conclusion may also have implications for the origin of seismic anisotropy at fast-spreading ridges where there are high melt supplies. [ABSTRACT FROM AUTHOR]

The Northwestern Hawaiian Ridge is an age- progressive volcanic chain sourced from the Hawaiian mantle plume. Proximal to the Northwestern Hawaiian Ridge are several clusters of smaller seamounts and ridges with limited age constraints and unknown geodynamic origins. This study presents new bathymetric data and 40Ar/39Ar age determinations from lava flow samples recovered by remotely operated vehicle (ROV) from two east–west-trending chains of seamounts that lie north of the Pūhāhonu and Mokumanamana volcanoes. The previously unexplored Naifeh Chain (28°48′N,167°48′W) and Plumeria Chain (25°36′N, 164°35′W) contain five volcanic structures each, including three guyots in the Naifeh Chain. New 40Ar/39Ar age determinations indicate that the Naifeh Chain formed ca. 88 Ma and the Plumeria Chain ca. 85 Ma. The Cretaceous ages, coupled with a perpendicular orientation of the seamounts relative to absolute Pacific plate motion at that time, eliminate either a Miocene Hawaiian volcanic arch or Cretaceous mantle- plume origin. The seamounts lie on oceanic crust that is modeled to be 10–15 Ma older than the corresponding seamounts. Here, two models are put forth to explain the origin of these enigmatic seamount chains as well as the similar nearby Mendelssohn Seamounts. (1) Diffuse lithospheric extension results in the formation of these seamounts until the initiation of the Kula- Pacific spreading center in the north at 84– 79 Ma, which alleviates the tension. (2) Shear-driven upwelling of enriched mantle material beneath young oceanic lithosphere results in an age- progressive seamount track that is approximately perpendicular to the spreading ridge. Here we show that all sampled seamounts proximal to the Northwestern Hawaiian Ridge are intraplate in nature, but their formations can be attributed to both plume and plate processes. [ABSTRACT FROM AUTHOR]

The formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive oceancontinent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT. [ABSTRACT FROM AUTHOR]

The geological history of the Arctic is constrained within the framework of the assembly and breakup of three supercontinents. The first of these was preceded by the crystallization of the oldest dated rocks on Earth and consolidation of the Arctic region's Archean cratons between 2.82 and 2.54 Ga. Following the emplacement of regional mafic dike swarms between 2.51 and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia) supercontinent between 2.0 and 1.6 Ga, and the distribution of low-thermalgradient eclogite (indicative of continental subduction) and ophiolite (indicative of obduction of oceanic crust onto a continental margin) suggests that diagnostic plate-tectonic processes were well in place by the early Paleoproterozoic. Basin formation, flood basalts, and dike swarms are features of the partial(?) breakup of Nuna (Columbia) by 1.5-1.27 Ga. The extent to which specific dike swarms led to continental breakup and a rift-to-drift transition remains unclear. Assembly of the second supercontinent (Rodinia, 1.4-0.9 Ga) is recorded by a network of Grenvillian and Sveconorwegian collisional orogenic belts. Prominent features of Rodinia breakup (780-615 Ma) in the Arctic are extensive dike swarms and regional-scale glacial-periglacial deposits associated with the Sturtian (717-661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball Earth glaciations. Assembly of the third supercontinent, Pangea, between 600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr). Pangea breakup (rifting since 250 Ma and oceanic spreading since the Cretaceous) led to the emplacement of Cretaceous and Paleogene flood basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and Arctic Ocean, and orogens characterized by relatively small but far-traveled accreted terranes with provenance in Laurentia, Baltica, and Siberia. Paleogeographic similarities and geological correlations among Laurentia, Baltica, Siberia, and the North China craton suggest that Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by introversion, whereas unique paleogeographic traits for Pangea within the Arctic region point to supercontinent formation by extroversion. [ABSTRACT FROM AUTHOR]

Whether or not oceanic crust basalts are affected by plate subduction is a hot topic of debate. The South China Sea is one of the largest marginal basins in the western Pacific Ocean and has been surrounded by subduction of the Pacific plate and Indian plate, yet, to date, no study has clearly shown evidence of subduction in the geochemistry of volcanism in the basin due to a lack of sampling of igneous crust basalts on the seafloor. The International Ocean Discovery Program Expedition 349 cored seafloor basalts near the fossil spreading ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) subbasins in the South China Sea. The recovered basalt samples indicated a pyroxenite-bearing peridotite mantle source. Here, we report Mg isotopic data from 14 of these oceanic crust basalt samples. The δ26Mg values of most basalts from the three drill holes were higher (up to -0.10‰) than that of the average mantle (-0.25‰). The lack of correlations of δ26Mg with geochemical indices of magmatic processes (e.g., MgO, CaO/Al2O3, La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had insignificant effects on the Mg isotopic compositions of the South China Sea basalts. Thus, the variations in Mg isotopes were inherited from their mantle sources. Considering the highly varied Ce/Pb ratios and elevated 87Sr/86Sr values but mantle-like 143Nd/144Nd values, we propose that the varied δ26Mg values were likely caused by metasomatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ26Mg values of the South China Sea basalts resulted from mixing among pyroxenite-bearing peridotite mantle, the nearby Hainan plume materials, and subducting serpentinite-released fluids. Therefore, these Mg isotopes suggest that the mantle source of the South China Sea basalts was influenced by subducted materials, providing further evidence of the initial expansion, formation, and evolution of the South China Sea during plate subduction. [ABSTRACT FROM AUTHOR]

Newly formed oceanic crust is altered by seawater and carbonated at low temperatures over poorly defined periods of time. We applied in situ U-Pb dating to investigate 28 carbonate veins from Ocean Drilling Program Hole 801C, which is situated in the oldest Jurassic-age oceanic crust preserved in the western Pacific Ocean. Our results indicate that Pacific Ocean crust began accreting at 192 ± 6 Ma, which is ~25 m.y. earlier than previously recognized. Carbonation peaked at 171 ± 5 Ma and continued at a low rate for more than ~65 m.y. after accretion. Jurassic carbonation rates varied over ~10 m.y. timescales but encompassed a range similar to that observed today. These data suggest that carbonation rates are relatively insensitive to changes in atmospheric CO2, but confirm the longevity of seafloor alteration as a critical control in global volatile cycling. [ABSTRACT FROM AUTHOR]

Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460-400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450-440 Ma and addition of juvenile mantle-derived melts at 420-410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking. [ABSTRACT FROM AUTHOR]

We report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western European Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements, including Cr, Mn, and rare earth elements. Thermodynamic modeling revealed that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite, and chlorite between 505 °C and 525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geologic and geochronologic constraints indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 m.y. These observations can only be explained by the presence of extremely low permeability values (<10-22 m²) to keep the fluid confined at the meter scale within vugs on such time scales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into Earth's mantle, with implications for volatile recycling budgets. [ABSTRACT FROM AUTHOR]

The architecture of a mush-dominated plumbing system in active volcanic areas conditions the magma pathways feeding eruptions. Open-system processes along these pathways and the associated time scales are directly related to monitoring data and eruptive behavior. Despite crystal mush--dominated systems being common in active volcanoes, previous studies have not focused on the integration of data from the mush sectors feeding different eruptions, supplying a partial view of the pre-eruptive magmatic processes and hindering the interpretation of the monitoring signals during unrest periods. We focus on the Marsili seamount (Tyrrhenian Sea), where the mineral data document processes within a magmatic system vertically extended throughout the local oceanic crust and made of a mush framework spotted with eruptible meltand crystal-rich pockets. We undertook a study of Marsili olivine crystals that constrains the time scales of three pre-eruptive scenarios, dominated by open-system processes: (1) disaggregation of the deep Marsili volcano mush zone that occurred over a time scale of years prior to the eruption; (2) rapid ascent (days) of mantle-derived basaltic magma that, in some cases, intercepts shallow plagioclase-rich pockets; and (3) multiple mixing events between melt- and crystal-rich mush zones occurring approximately 1-2 mo and 0.5-3 yr before the eruption. Our results highlight the importance of contemporaneously studying eruptions in different locations on a volcano edifice for a better comprehension on how mush-dominated plumbing systems work as a whole and how this must be considered during the interpretation of monitoring data. [ABSTRACT FROM AUTHOR]

An ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)--like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621-0.12008 and very low 187Re/188Os values of 0.031-0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2-4.1) and relatively high (87Sr/86Sr)i ratios (0.7046-0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09-0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6-1.65 wt%), Cr (0.22-6.26 ppm), and Ni (0.77-4.74 ppm) compositions and low Mg# (22.4-31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰-5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy--type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge and ocean-island basalts provide vital but incomplete insights into mantle chemistry. For example, high-anorthite plagioclase is generally too refractory and incompatibleelement depleted to have crystallized from the melts that carry it to the surface. Moreover, erupted basalts rarely preserve the extreme isotopic and incompatible-element depletions found in some primitive melt inclusions and residual abyssal peridotites. By integrating experimental observations with analyses of natural crystals and glasses, we infer that high-anorthite plagioclase grows from high-degree melts of refractory but otherwise unexotic mantle sources with depleted incompatible-element compositions. The widespread occurrence of high-anorthite plagioclase in oceanic basalts and the oceanic crust hence indicates that refractory melts pervade the uppermost mantle and lower crust. We thus suggest that refractory melts play much a greater role in crustal accretion than typically recognized and that refractory and feasibly depleted peridotite is more prevalent in the upper mantle than previously thought. [ABSTRACT FROM AUTHOR]

Flat-slab subduction has affected parts of North America, South America, and Asia over the past 250 m.y. In these areas, reconstructions show that the subducting plate became subhorizontal below the continent for ~5 to >30 m.y., followed by foundering of the slab and resumption of steep-angle subduction. Using two-dimensional numerical models, we examine the factors that control the development and removal of a flat slab. Models are based on the Late Cretaceous to Oligocene Farallon flat slab below the southwestern United States. We find that the primary control on subduction geometry is the oceanic plate density structure. Subduction of a buoyant oceanic plateau creates a flat-slab segment that moves inboard at approximately the rate of continental trenchward motion (4-5 cm/yr). Steepening is initiated with eclogitization of the oceanic plateau crust. Once the plateau density exceeds that of the mantle, the slab undergoes rollback through progressive trenchward-directed detachment from the continent at a rate of 2-10 cm/yr. Rollback is enhanced by: (1) weakening of the overlying continental mantle lithosphere, inferred to result from slab-derived hydrous fluids, and (2) a slowdown in plate velocities; the rate and amount of oceanic eclogitization are second-order effects. Conversely, rollback is hindered by a strong oceanic plate and interactions between the slab and high-viscosity lower mantle. For the ~2000-km-long Farallon slab, the Conjugate Shatskey Rise plateau must have remained buoyant for 20-30 m.y. after subduction. This was followed by rapid rollback caused by both plateau eclogitization and continental weakening, leaving an area of thinned and hydrated continental lithosphere. [ABSTRACT FROM AUTHOR]

Granitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantlederived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive eNd(t) values indicate juvenile crust whereas negative eNd(t) values indicate ancient crust. Positive eNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82-64.41 wt%, and it was emplaced at 50-48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93-68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted wholerock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044-0.7048, positive eNd(t) values of 0.6-3.9, eHf(t) values of 6.5-10.5, zircon eHf(t) values of 1.6-12.1, and zircon d18O values of 5.28-6.26?. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched wholerock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070-0.7085, negative eNd(t) values of -5.2 to -3.9 and neutral eHf(t) values of 0.9-2.3, and relatively lower zircon eHf(t) values of -2.8-8.0 and slightly higher zircon d18O values of 6.25-6.68?. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone. [ABSTRACT FROM AUTHOR]

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sedimentderived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sedimentderived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their wholerock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian-Early Triassic (ca. 254-248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show d11B and d18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed backarc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas. [ABSTRACT FROM AUTHOR]

The deep upper mantle is the main source of high-temperature magmatism, but the only known naturally occurring samples of high-pressure mantle constituents are mineral inclusions in diamonds. Trace elements in olivine crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal that these crystals formed in the deep upper mantle as high-pressure phenocrysts, and some perhaps even formed in the mantle transition zone (410-600 km) where they began as wadsleyite. The crystals were entrained within ascending komatiite magma and conveyed to the surface. The olivine crystals have the highest contents of Al2O3 (0.3 wt%) recorded in any terrestrial olivine, which is indicative of formation at high pressure. The deep mantle gave rise to Archean komatiites, extraordinarily hot magmas (up to 1700 °C), which provide insight into Earth's early mantle evolution and the formation of most ancient continental and oceanic crust. In spite of extensive research since their discovery over 50 years ago, the origins of komatiites have remained contentious. Plumes--thermochemical instabilities originating at the core-mantle boundary--are the most likely source, but no direct evidence of a deep mantle origin of komatiite has yet been recognized. [ABSTRACT FROM AUTHOR]

We present pore-fluid geochemistry and heat-flow data along the SWIM1 fault in the Horseshoe Abyssal Plain (northeastern Atlantic Ocean). The SWIM1 fault is part of the transcurrent plate boundary between Africa and Eurasia and cuts through as much as 5-kmthick sediments overlying >140 Ma oceanic lithosphere. In a number of places, restraining segments (as long as 15 km) of the SWIM1 fault generate anticlines (positive flower structures) that protrude as ∼100-m-high hills above the abyssal plain. Heat flow and gradients of dissolved constituents in pore water are enhanced at these seafloor highs. Transport-reaction modeling confirms that slow advection of deep-seated fluids, depleted in Mg and enriched in Sr and CH4, can explain the observations. The geochemical signature is similar to the one observed at deep-sea mud volcanoes located eastward on the SWIM1 fault. The upward-migrating fluids have interacted with carbonate rocks at maximum 5 km depth, which represent the oldest sedimentary unit on top of the basement. We argue that deep-rooted fluids can generally be mobilized and transported upward along flower structures that formed in restraining-bend segments of long strike-slip faults. Such tectonic settings represent largely unrecognized corridors for mass exchange between lithosphere and ocean. [ABSTRACT FROM AUTHOR]

The nature and source of magmatism associated with large igneous provinces (LIPs) remain disputed. Here we investigate the role of recycled materials that contributed to mantle heterogeneity in the Tarim Large Igneous Province (TLIP) in China through integrated Zn-Mg-Sr-Nd isotopes of a rare tephritic rock suite. The Sr--Nd isotopes [(87Sr/86Sr)i = 0.70368-0.70629; εNd(t) = -0.25-4.64] and δ26Mg values (-0.23‰ to -0.34‰) of the tephritic porphyries are consistent with a normal mantle origin. In contrast, the whole rock and pyroxene phenocrysts yield δ66Zn values of +0.28‰ to +0.46‰ and +0.30‰ to +0.39‰, which are slightly heavier than those of the terrestrial mantle (+0.16 ± 0.06‰) and mid-oceanic-ridge basalts (MORBs) (+0.27 ± 0.05‰). We exclude the possibility that the heavy Zn isotopes of the Wajilitag tephritic porphyries are caused by magmatic processes such as fractional crystallization and partial melting and correlate the isotopic features to the role of altered oceanic crust along with magnesite in the mantle source. The Wajilitag tephritic porphyry displays trace-element patterns similar to those of the melts from natural hornblendite, especially those of hornblendite + peridotite. Additionally, the geochemical characteristics such as enrichment in Nb and Ta, depletion in K, high TiO2, and constant Na2O/K2O ratios also suggest that the tephritic porphyries were derived from an amphibole-bearing source contributed by altered oceanic crust along with sedimentary carbonates. Our study provides insight into the contribution of subducted materials to the mantle heterogeneity beneath the TLIP and highlights the role of subduction in the deep carbon cycle and subducted slab-lithosphere-plume interaction in the generation of LIPs. [ABSTRACT FROM AUTHOR]

The mechanisms and processes by which subducted slab interacted with mantle plume remain controversial, as direct observation of such interaction is difficult to impossible. Compositional heterogeneity of large igneous provinces (LIPs) additionally makes plume-slab interaction hard to detect. Oxygen isotopes are sensitive enough to trace the source of magmas. Here we provide evidence for plume-slab interaction mainly based on in situ zircon Hf-O isotope analyses, as well as whole-rock elemental and Sr-Nd-Hf isotope analyses, on the Late Permian and Early Triassic A-type granites on the margin of the Emeishan LIP in SW China. These granites show typical A-type geochemical characters, such as high total alkali (7.93-9.68 wt%) and field strength element (HFSE, e.g., Zr and Nb) contents, and high FeOT/(FeOT+MgO) (0.87-0.98) and Ga/Al (3.67-5.06) values. The Late Permian (ca. 259 Ma) and Early Triassic (ca. 248 Ma) granites show high Nb/Th (>3.0) and low Y/Nb (<1.2) and Yb/Ta (<2.0) ratios similar to the oceanic island basalts and have near-zero εNd(t) (-0.83 to -0.13 and -0.15 to +0.16, respectively) and depleted εHf(t) (+2.71 to +3.39 and +2.62 to +3.55, respectively). In situ zircon O-Hf analyses yielded anomalously low δ18O (0.2-2.0‰ and 3.2-4.8‰, respectively) and positive εHf(t) (1.6-7.0 and 3.9-8.8, respectively), suggesting varying proportions of hydrothermally altered oceanic crust in their source region. Our results imply that significant amounts of altered Paleotethyan oceanic crust have been subducted in the upper mantle beneath the western South China Block. The nearby rising Emeishan mantle plume may have rapidly entrained and incorporated these oceanic crustal materials to the shallow mantle so that their low-δ18O isotope feature was preserved. Subsequent decompression-related partial melting of this hybrid source formed parental rocks of the low-δ18O A-type granites. Our findings also suggest that LIPs could obtain their compositional (especially oxygen isotope) diversity through the interaction between the subducting slab and rising mantle plume. [ABSTRACT FROM AUTHOR]

Whether orogenic gold deposits formed from crustal or subcrustal sources is debated, and their link to orogenic processes is ambiguous. Gold mineralization in the Triassic East Kunlun-West Qinling Orogen, China, displays a spatial zonation in terms of its ages and stable isotope compositions. In the West Qinling segment, most gold deposits formed in a back-arc setting at 220~210 Ma during a collisional episode within late slab rollback. These deposits have dominant δ34S of 5-15‰ and δ18Ofluid of 10-14‰, whereas those formed in the suture zone at 210-170 Ma, during a post-collisional episode after slab break-off, have lower d34S of -5-+5‰ and d18Ofluid of 6-10‰. In the East Kunlun segment, those deposits that formed in a continental-arc setting and its related suture zone at 240-200 Ma, in collisional to post-collisional episodes associated with slab break-off, have δ34S and δ18Ofluid values that are essentially similar to those in the West Qinling suture. d34S values of ore sulfide separates and rims of zoned pyrites that have mantle-like signatures, in contrast with crustal signatures of host rocks, are indicative of subcrustal ore-fluid sources. The combined chronological and stable isotope shifts are consistent with a model in which ore fluids for gold mineralization in a back-arc setting were sourced from mantle lithosphere that was metasomatized by subducted oceanic sediment; whereas those in a continental-arc setting--including its suture zone--were sourced from fluid derived from altered oceanic crust. This study thus provides new insights into the complexity of orogenic gold systems in evolving orogens. [ABSTRACT FROM AUTHOR]

A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive ?Hf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative Hf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism. [ABSTRACT FROM AUTHOR]

Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95MoNIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (-0.18‰ to +0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (-0.65‰ to -0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis. [ABSTRACT FROM AUTHOR]

The crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ~6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes. [ABSTRACT FROM AUTHOR]

Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46-45, 40-36, and 19-18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46-45 and 40-36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo-transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body. [ABSTRACT FROM AUTHOR]

Oceanic arc subduction systems are the loci of substantial recycling of oceanic crust and production of juvenile arc crust that differentiates to more evolved felsic crust. Inevitably, some juvenile sediments are subducted with the oceanic crust. However, distinguishing the incorporation of juvenile sediments in oceanic arcs is not always straightforward, because they may not measurably shift many geochemical signatures, such as Sr and Nd isotopes, of oceanic arcs. Nevertheless, combined zircon U-Pb, Hf, and O isotope data can provide a powerful tool to decipher sedimentary flux into oceanic arc magmas, and here we report a case study for the late Paleozoic A-type granites from the West Junggar oceanic arc in the southern Central Asian Orogenic Belt. These plutons contain hastingsite and iron biotite diagnostic minerals and have high alkali, FeOT/MgO, Zr, and Ga/Al, but possess low CaO contents, and strongly negative Eu, Sr, and Ba anomalies, demonstrating their close affinity with A-type granites. Zircon U-Pb analyses indicate that these A-type granites emplaced in the Late Carboniferous to Early Permian (ca. 307-298 Ma). Their high zircon εHf(t) values (+12.4 to +15.5), suggest that the magmas were derived from a mantle or juvenile crustal source. However, their δ18Ozrn (+7.2‰ to +11.9‰) values are significantly higher than that of the mantle, and modeling using Hf-O isotope and rare earth element data indicate the assimilation of sedimentary materials at a proportion of ~50%. Our data suggest that juvenile sediments (e.g., greywacke) played an important role in the formation of the studied A-type granites. The re-melting of sedimentary material induced by the late Carboniferous ridge subduction can promote the transition from an intra-oceanic arc to continental crust. Our results show that the subduction and re-melting of juvenile sediments in oceanic arc systems could be an important mechanism for the maturation of oceanic arc crust. [ABSTRACT FROM AUTHOR]

The Archean ocean supported a diverse microbial ecosystem, yet studies suggest that seawater was largely depleted in many essential nutrients, including fixed nitrogen. This depletion was in part a consequence of inefficient nutrient recycling under anoxic conditions. Here, we show how hydrothermal fluids acted as a recycling mechanism for ammonium (NH4 +) in the Archean ocean. We present elemental and stable isotope data for carbon, nitrogen, and sulfur from shales and hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in Western Australia. This suite documents the transfer of NH4 + from organic-rich sedimentary rocks into underlying sericitized dacite, similar to what is seen in hydrothermal systems today. On modern Earth, hydrothermal fluids that circulate through sediment packages are enriched in NH4 + to millimolar concentrations because they efficiently recycle organic-bound N. Our data show that a similar hydrothermal recycling process dates back to at least 3.24 Ga, and it may have resulted in localized centers of enhanced biological productivity around hydrothermal vents. Last, our data provide evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen, and, when subducted, it satisfies the elemental and isotopic source requirements for a low-N, but 15N-enriched, deep mantle nitrogen reservoir as sampled by mantle plumes. [ABSTRACT FROM AUTHOR]

The prolonged convergence along the Southeast Asian margin from the Mesozoic to Cenozoic shaped the region into a complex tectonic collage of microblocks of diverse origin and evolutionary history. Among these microblocks, the formation and paleogeographic evolution of the Palawan continental terrane remain issues of uncertainty and controversy, especially regarding the petrogenesis of the oldest rocks and the conjugate relationship with the South China margin. This study examined these issues from the perspective of detrital fingerprints (including geochemistry, heavy mineral, and zircon U-Pb geochronology) of Mesozoic to Cenozoic strata from Palawan Island and basins of the northern South China Sea. The across-margin comparison of provenance signatures, favored by a comprehensive data compilation and a revision of the stratigraphic framework using the youngest zircon ages, provides insights into regional paleogeographic reconstructions from the Jurassic to the Miocene. The results reveal provenance shifts that correspond to the paleogeography of the Palawan continental terrane evolving from an accretionary complex in the Jurassic to a rifted margin in the early Cenozoic and finally to a microcontinent in the middle Cenozoic. Based on comparable provenance signatures, the terrane is interpreted to have been conjugated to the northeastern South China Sea margin from the Mesozoic until the Oligocene spreading of the South China Sea. The terrane likely existed as a northern passive margin of the proto--South China Sea from the latest Cretaceous to early Cenozoic before the oceanic crust of the latter was emplaced over the former during the Oligocene--Miocene. [ABSTRACT FROM AUTHOR]

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977-1981 to understand the region's tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ~3-5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments. [ABSTRACT FROM AUTHOR]

Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4-5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5-6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ~7 km thick, and the lower crust supports seismic velocities of 7.2-7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge-type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust. [ABSTRACT FROM AUTHOR]

Continental lithospheric mantle (CLM) may have been built from subducted slabs, but the apparent lack of concurrent oceanic crust in CLM, known as the mass imbalance problem, remains unresolved. Here, we present a simple dynamic model to evaluate the likelihood of losing dense eclogitized oceanic crust from CLM by gravitational instability. Our model allowed us to assess the long-term evolution of such crust removal, based on how thermal and viscosity profiles change over time across the continental lithosphere. We found that the oceanic crust incorporated early into CLM can quickly escape to the asthenosphere, whereas that incorporated after a certain age would be preserved in CLM. This study provides a plausible explanation for the mass imbalance problem posed by the oceanic ridge origin hypothesis of CLM and points to the significance of preservation bias inherent to the studies of cratonic diamonds. [ABSTRACT FROM AUTHOR]

Photochemical processes generate mass-independent fractionation (MIF) of mercury (Hg) isotopes in the atmosphere-ocean system, and the subduction of marine sediments or hydrated oceanic crust may recycle the resultant Hg isotope signature into the volcanic-arc environment. This environment typically hosts epithermal gold deposits, which are characterized by a specific Hg-Sb-As metal association. We investigated the Hg isotopic composition of seven volcanic-arc-related epithermal gold deposits in northeast China and revisited the isotopic composition of Hg in hydrothermal ore deposits in circum-Pacific and Mediterranean volcanic arcs. The gold ore samples in northeast China mostly display positive Δ199Hg values (0.11‰ ± 0.07‰, 1σ, n = 48) similar to those observed in the Pacific Rim (0.07‰ ± 0.09‰, 1σ, n = 182) and the Mediterranean Cenozoic volcanic belt (0.09‰ ± 0.08‰, 1σ, n = 9). Because Hg in marine sediments and seawater has positive Δ199Hg, we infer that Hg-bearing epithermal deposits in active continental margin settings receive most Hg from recycled seawater in marine sediments, through the release of Hg by dehydration from the subducting oceanic slab. However, negative to near-zero Δ199Hg values were observed in Hg-bearing deposits in the South China craton (-0.09‰ ± 0.05‰, 1σ, n = 105) and in the intraplate magmatic-hydrothermal Almadén Hg deposit in Spain (-0.02‰ ± 0.06‰, 1σ, n = 26), which are considered to relate to basement and mantle sources, respectively. Hg isotopes have the potential to trace lithospheric Hg cycling. [ABSTRACT FROM AUTHOR]

In Southeast Asia, the Jinshajiang-Ailaoshan suture in Southwest China and the Song Ma zone in Vietnam were generally accepted as the amalgamation boundary of the South China and Simao/Indochina blocks. However, the tectonic location of the Paleotethyan suture in North Laos and its tectonic affinity remain ambiguous. To address this issue, we present the geochemical and geochronological data of a set of mafic and plagiogranitic rocks along the Song Ma suture zone in North Laos. Three groups of rocks with distinctive petrological, geochronological, and geochemical signatures were identified. The ca. 370 Ma Group 1 plagioclase amphibolite was preserved as xenoliths in the ca. 260 Ma granitoids. It exhibits a normal mid-oceanic-ridge basalt (N-MORB)-like, multielemental pattern with highly positive εNd(t) (+3.3 to +10.7) and εHf(t) (+6.68 to +7.41) values and low δ18O values (5.3‰-5.5‰), that are interpreted as products of the Late Devonian - earliest Carboniferous continental rift setting. The ca. 270 Ma Group 2 gabbro/diabase-diorite exhibits arc-like geochemical affinity with the εNd(t), εHf(t), and δ18O values of -5.85 to -3.76, -3.8 to +3.2, and 6.6‰-7.6‰, respectively. It is derived from the metasomatized wedge. The ca. 260 Ma Group 3 plagiogranite shows Nb-Ta and Ti negative anomalies and positive εNd(t) (+3.21 to +4.21), εHf(t) (+8.4 to +13.6), and δ18O (5.9%-6.8%) values, which are indicative of its derivation from mafic oceanic crust in a subduction setting. In combination with the published data, these results support the development of the Carboniferous-Permian Paleotethyan branch in North Laos, which represents the tectonic location of the Song Ma suture zone that spatially connects the Jinshajiang-Ailaoshan suture zone in Southwest China and the Chenxing-Bangxi suture zone in Hainan Island, South China. [ABSTRACT FROM AUTHOR]

A three-dimensional (3-D) S-velocity model for the crust and upper mantle beneath the South China Sea and Indonesia is presented, determined by means of Rayleigh wave analysis, in the depth range from 0 km to 400 km. The crustal and lithospheric mantle structure of this study area was previously investigated using several methods and databases. Due to their low resolution, a 3-D structure for this area has not been previously determined. The determination of such a 3-D S-velocity model is the goal of the present study. The most conspicuous features of the crust and upper mantle structure include the S-velocity difference between the Java Sea and the Banda Sea regions and a transitional boundary between these two regions. This model confirms the principal structural features revealed in previous studies: an oceanic crust structure in the center of the South China Sea, crustal thinning from the northern continental margin of the South China Sea to this oceanic crust, and the existence of a high-velocity layer in the lower crust of the northern continental margin. This study concludes that the north of the South China Sea is a nonvolcanic-type continental margin, solving the open question of whether the continental margin of the northern South China Sea is volcanic or nonvolcanic. A new map of the asthenosphere's base is also presented. [ABSTRACT FROM AUTHOR]

Phase equilibria modeling of sodic-calcic amphibole-epidote assemblages in greenstones in the northern Appalachians, USA, is compatible with relatively shallow subduction of the early Paleozoic Laurentian margin along the Laurentia-Gondwana suture zone during closure of a portion of the Iapetus Ocean basin. Pseudosection and isopleth calculations demonstrate that peak metamorphic conditions ranged between 0.65 GPa, 480 °C and 0.85 GPa, 495 °C down-dip along the subducted Laurentian continental margin between ~20 km and ~30 km depth. Quantitative petrological data are explained in the context of an Early Ordovician geodynamic model involving shallow subduction of relatively young, warm, and buoyant Laurentian margin continental-oceanic lithosphere and Iapetus Ocean crust beneath a relatively warm and wet peri-Gondwanan continental arc. A relatively warm subduction zone setting may have contributed to the formation of a thin, ductile metasedimentary rock-rich channel between the down-going Laurentian slab and the overriding continental arc. This accretionary channel accommodated metamorphism and tectonization of continental margin sediments and mafic volcanic rocks (greenstones) of the Laurentian margin and provided a pathway for exhumation of serpentinite slivers and rare eclogite blocks. Restricted asthenospheric flow in the forearc mantle wedge provides one explanation for the lack of ophiolites and absence of a wellpreserved ultra-high-pressure terrane in central and northern Vermont. Exhumation of the subducted portion of the Laurentian margin may have been temperature triggered due to increased asthenospheric flow following a slab tear at relatively shallow depths. [ABSTRACT FROM AUTHOR]

Abundant zoned olivine xenocrysts from Early Cretaceous basalts of the Yixian Formation in western Liaoning Province, China, contain critical information about the nature and evolution of the lithospheric mantle of the northern North China Craton. These olivine xenocrysts are large (600-1600 µm), usually rounded and embayed, with well-developed cracks. Their cores have high and uniform forsterite (Fo) contents (88-91), similar to the peridotitic olivine entrained by regional Cenozoic basalts. Their rims have much lower Fo contents (74-82), comparable to phenocrysts (72-81) in the host basalts. These characteristics reveal that the zoned olivine has been disaggregated from mantle xenoliths and thus can be used to trace the underlying lithospheric mantle at the time of basaltic magmatism. The olivine cores have high oxygen isotope compositions (d18OSMOW = 5.9-7.0?) relative to the normal mantle value, suggesting that the Early Cretaceous lithospheric mantle was enriched and metasomatized mainly by melts/fluids released from subducted oceanic crust that had experienced low-temperature hydrothermal alteration. Preservation of zoned olivine xenocrysts in the Early Cretaceous basalts indicates that olivine-melt/fluid reaction could have been prevalent in the lithospheric mantle as an important mechanism for the transformation from old refractory (high-Mg) peridotitic mantle to young, fertile (low-Mg), and enriched lithospheric mantle during the early Mesozoic. [ABSTRACT FROM AUTHOR]

In the past few years, the so-called mélange model has been offered as an alternative to the long-standing model of enrichment of the subarc mantle by separate additions of fluid and sediment components from the subducting plate. In the mélange model, components from the subducting plate become physically mixed at the slab-mantle interface. Partial melting of the peridotite subsequently occurs after being hybridized by the mélange material that diapirically rises into hotter portions of the wedge. Here, we present the first Ba isotope study of lavas from the Tonga-Kermadec arc (southwest Pacific Ocean) and show that Ba isotopes distinguish between fluid and melt derived from different subducted components. This provides fresh constraints on the debate. Remarkable along-strike Ba isotope variations were observed and are best explained by contributions from variable proportions of sediment and altered oceanic crust (AOC) fluid from the subducting plate. Combined Ba-Sr-Pb isotope relationships indicate that sediment melt and AOC fluid were added to the source of the arc lavas separately at different times. This is inconsistent with the mélange model, at least in this arc [ABSTRACT FROM AUTHOR]