Ocean Island Basalts Research Articles

Geochemistry and magmatic petrology of meta-ophiolites from the Bajgan Complex (Makran Accretionary Prism, SE Iran): new insights on the nature of the Early Cretaceous Middle East Neotethys

The Bajgan Complex in the North Makran Domain (Makran Accretionary Prism) comprises disrupted meta-ophiolitic sequences originating from oceanic crust protoliths. They include ultramafic and mafic cumulates, isotropic gabbros, plagiogranites and basalts. Ultramafic–mafic cumulates and plagiogranites exhibit compositions akin to rocks formed in mid-ocean ridge settings. Isotropic gabbro and basalt protoliths can be subdivided into three distinct geochemical types. Type-1 rocks are sub-alkaline (Nb/Y < 0.1) with low Th, Nb and Ta contents and La N /Yb N ratios <1, resembling those of normal mid-ocean ridge basalts (N-MORB). Type-2 rocks display slight enrichment in Th, Ta, Nb (Nb/Y = 0.36–0.45) and La N /Yb N = 2.12–3.20, resembling the chemistry of enriched (E-) MORB. Type-3 basalts show an alkaline nature (Nb/Y = 0.88–1.82), significant Th, Ta and Nb enrichment, and high La N /Yb N ratios (7.01–20.08), resembling the chemistry of alkaline basalts (ocean island basalts, OIB). Petrogenetic modelling indicates that N-MORB protoliths originated from a depleted MORB mantle source, whereas E-MORB and OIB protoliths were generated from partial melting of sub-oceanic depleted sources that underwent varying degrees of OIB-type enrichment. The Bajgan meta-ophiolitic protoliths were formed within a Late Jurassic to Cretaceous oceanic basin influenced by mantle plume activity and plume–ridge interaction. Supplementary material : Field photographs, micrographs, rocks sampled, analytical methods and results, and supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7193937 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Geochemical constraints on ridge-plume interaction in the West Philippine Basin

The West Philippine Basin is not only a back-arc basin resulting from the subduction of the paleo-plates, but it has also undergone modification due to intraplate processes. However, the interaction between intraplate processes and back-arc spreading processes remains unclear. In this study, we present geochemical compositions of basaltic lavas from various tectonic sites in the West Philippine Basin. Lavas from the Benham Rise and its adjacent seamount, as well as basement crust samples from the West Philippine Basin, are similar to oceanic island basalts and mid-oceanic ridge basalts, respectively. The lavas from the Central Basin Fault lie between the two aforementioned types. Based on the mixing modeling of geochemical data, morphological structure of the West Philippine Basin, and age data for lavas, we developed a conceptual model for the three-stage tectonic evolution, with the simultaneous influence of back-arc spreading and mantle plume processes.

Iron valence systematics in clinopyroxene crystals from ocean island basalts

The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity (fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document}). However, our knowledge of how and why fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland and the Azores to explore whether they record information about magma Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} contents and magmatic fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document}, we find that up to half of the total Fe present can occur as Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}, with crystals from alkali systems typically containing more Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} than those from tholeiitic systems. Thus, Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} in these crystals is hosted within esseneite component (CaFe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}AlSiO6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{6}$$\\end{document}), though some may be hosted in aegirine component (NaFe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}Si2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document}O6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{6}$$\\end{document}) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} is related to the abundance of tetrahedrally coordinated Al (IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al), implying some steric constraints over Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} partitioning between clinopyroxene and liquid (i.e., DFe2O3cpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$D\\mathrm {^{{cpx-liq}}_{{Fe_{2}O_{3}}}}$$\\end{document} values), though this may not be an equilibrium relationship. For example, IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-rich {hk0}\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{hk0\\}$$\\end{document} prism sectors of sector-zoned crystals contain more Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} than IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-poor {1¯11}\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{\\bar{1}11\\}$$\\end{document} hourglass sectors. Moreover, IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}. Apparent clinopyroxene-liquid Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document}–Mg exchange equilibria (i.e., KD,Fe2+-Mgcpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K\\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$\\end{document} values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that KD,Fe2+-Mgcpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K\\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$\\end{document} values lies closer to experimentally reported values of 0.24-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document}0.26 than values of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}0.28 returned from a general model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (≥\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ge$$\\end{document}FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} degassing; our other glassy sample from Iceland was also affected by reductive SO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions on clinopyroxene-liquid equilibria involving Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document} and Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}.

Origin of the Site U1504 alkaline basalts in the South China Sea continental margin: Insights on deep mantle diversity and subduction dynamics under continental arcs

Alkaline basalts produced in continental arcs should contain information different from the arc tholeiite-calc-alkaline-series magmas, and their origin could provide unique constraints on deep mantle composition and material cycling. However, due to their sparse occurrence, alkaline basalts in continental arcs have not been studied thoroughly, which hinders our understanding of the mantle diversity and subduction dynamics under continental arcs. In this study, we present new 40Ar/39Ar ages, major and trace elements, and Sr-Nd-Hf isotopic data for the International Ocean Discovery Program Site U1504 alkaline basalts in the continental arc developed on the South China Block (SCB). These alkaline basalts were generated at ca. 121 Ma and display typical oceanic-island basalt geochemical characteristics. Their relatively high εNd(t) (3.5−3.7) and low (87Sr/86Sr)i (0.7034−0.7040) and La/Nb (0.5−1.0) values suggest that they were mainly derived from an asthenosphere mantle source. Compared to alkaline basalts in the SCB inland, U1504 alkaline basalts exhibit lower K2O/Na2O, Zr/Sm, Zr/Y, εNd(t), and εHf(t) values, indicating the addition of minor sub-continental lithospheric mantle. The enrichment of Nb, Ta, light rare earth elements, and slight depletion of Zr, Hf, and Ti, as well as elevated Fe/Mn and Sm/Yb and low CaO, indicate that their mantle lithology was mainly garnet pyroxenite. Based upon these findings and previous studies, the garnet pyroxenite was probably formed by the interaction of upwelling asthenosphere with slab edges in the scenario of break-off of the Paleo-Pacific Plate, and such interaction diversified the mantle chemistry beneath continental arcs. In conjunction with other reported alkaline basalt data, it is proposed that the enriched asthenosphere mantle beneath the SCB had formed sequentially from inland to coastal since the late Mesozoic, and this may be related to lateral and vertical flow in the deep asthenosphere controlled by the break-off of subducted plates.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Geologic fingerprints of the Rodinia breakup in the western North China Craton: Constraints from the Early Neoproterozoic (ca. 825–810 Ma) mafic volcanic and intrusive rocks in the Langshan tectonic belt

As an important component of the East Asian continental block, the North China Craton (NCC) is crucial for reconstructing Rodinia in the Neoproterozoic. However, whether the NCC is a component of the Rodinia supercontinent and records its breakup remains enigmatic. Here, we identified Langshan gabbro and mafic dikes with ages of ca. 829–826 Ma and mafic volcanic rocks with ages of 810 ± 3 Ma in the northeastern Alxa Block of the western NCC. The Langshan gabbro shows flat rare earth element (REE) patterns [(La/Yb)N = 1.3–1.5] and slight enrichment in most incompatible trace elements, similar to enriched mid-ocean ridge basalt. The mafic volcanic rocks show the geochemical features of oceanic island basalt, including high total alkaline content (7.86–8.47 wt%), enriched in light REEs (La/YbN = 8.0–9.1), and insignificant Eu anomalies. Together with contemporaneous felsic volcanic rocks identified in previous studies, the mafic volcanic rocks (ca. 810 Ma) define a bimodal association. The mafic intrusive and associated dike swarms from ca. 825–810 Ma along with the coeval Jinchuan ultramafic–mafic intrusions, followed by rifting sedimentary successions, were formed in an extensional rift setting, supporting the geologic fingerprints of the Rodinia breakup in the western NCC. The well-matched mafic magmatic barcodes, similar provenance, and sedimentary successions preserved in the Adelaide superbasin of southeastern Australia and the Langshan rift basin of the western NCC support a possible link between the NCC and Australia during the Early Neoproterozoic.

Multiple sulfur isotopes evidence deep intra-slab transport of sulfate-rich fluids

Sub-arc mantle oxidation is often attributed to sulfates from seawater transported in aqueous fluids derived from slab dehydration, but sulfur speciation and origin(s) in high-pressure fluids remain weakly constrained. Here, we use the four sulfur stable isotopes (32S, 33S, 34S, 36S) to decipher processes, e.g. mixing of two end-member reservoirs and open-system reaction occurring during sulfur devolatilization, and deduce the nature and origin of sulfur species in aqueous slab-derived fluids. We analyzed high pressure metamorphic rocks from the Monviso massif Lower Shear Zone (LSZ; Western Alps), recording evidence of an interface-parallel, channelized fluid flow near peak burial metamorphic conditions (∼550 °C and 2.7 GPa). We sampled variably metasomatized Fe-Ti metagabbros, hybrid (talc) schists and serpentinites containing sulfides supposed to preserve the aqueous fluid composition, as well as other alpine metagabbro, metabasalt and serpentinites from greenschist to eclogite-facies devoid of the studied metasomatic overprint.The millimeter to centimeter-sized sulfide crystals found in abundance within LSZ lithologies are mainly pyrites containing inclusions of the peak burial mineral assemblage (e.g. garnet, omphacite, rutile) indicating their crystallization at peak conditions or during incipient exhumation. The sulfides in the non-metasomatic metagabbros and metabasalts cover a narrow range in δ’34S of 0.89 ± 0.63 ‰ and Δ’33S of 0.017 ± 0.01 (n = 6) that could be inherited from hydrothermal sulfides formed on the seafloor. In contrast, the LSZ sulfides from the Fe-Ti metagabbros, serpentinites and hybrid schists (n = 24) are enriched in 34S and 33S (bulk δ’34S from 4.91 to 21.32 ‰ and Δ’33S from 0.02 to 0.06 ‰) compared to the sulfides from the non-metasomatic rocks. Mixing between reduced sulfur species cannot explain all the data without requiring an end-member composition as high as δ34S ∼ 22 ‰ and Δ33S ∼ 0.06 ‰, i.e. values rarely recorded by sulfides in the literature. Instead, we propose that this 33S-34S-enrichment results from open-system sulfate reduction to sulfides in aqueous fluids at 550°C, with an initial δ34S ∼ 12 ± 3 ‰ and Δ33S ∼ 0.025 ± 0.010 ‰. Because this signature is different from seawater sulfate, we suggest that sulfate was produced by hydrothermal sulfide oxidation during antigorite breakdown at 650°C, in agreement with Mg, Ni and Cr enrichments in the metasomatized HP mafic lithologies. This model is consistent with relatively oxidized 34S-enriched arc lavas and can potentially explain recycled sulfur with positive δ34S signature recorded by ocean island basalts.

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:Pmelt>Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

A new look at the geodynamic development of the Ediacaran–early Cambrian forearc basalts of the Tannuola-Khamsara Island Arc (Central Asia, Russia): Conclusions from geological, geochemical, and Nd-isotope data

Abstract Oceanic igneous rocks throughout the Altai-Sayan Fold Belt (ASFB) in central-southern Siberia are often considered to be late Precambrian–early Paleozoic accreted elements of oceanic crust – often of uncertain paleogeographic or geodynamic origin. We explore the role of suprasubduction zone settings in the formation of different ASFB terranes. Our study offers a non-accretionary perspective on the tectonomagmatic development of basalt-bearing units in the ASFB on the example of the forearc terrane of the Ediacaran–early Cambrian Tannuola-Khamsara island arc (herein termed Sayan-Tuvan forearc zone). We describe the geochemistry, structural geology, and stratigraphic relations of basalts of the Aldynbulak, Uttug-Khaia, and Chingin formations, which are integral parts of the Sayan-Tuvan forearc zone. The Aldynbulak basalt samples mainly fall in the compositional fields of ocean island basalts and enriched mid-ocean ridge basalts (E-MORB) and likely derived from a deep mantle source. The Uttug-Khaia and Chingin basalts are N- and E + T-MORB-like basalts, carrying forearc geochemical signatures. Specifically, the Chingin Formation contains boninite dikes and is associated with a boninite-bearing ophiolite. Boninites are commonly associated with forearc magmatism and thus a forearc formation setting is likely. Tectonic and stratigraphic considerations imply that the Aldynbulak basalts formed first, followed by the Uttug-Khaia and later the Chingin basalts and boninites. A schematic model, involving decompression melting of the mantle, is proposed for the development of the studied forearc basalt suites that are linked with the growth of the Tannuola-Khamsara island arc system 580–540 million years ago.

Geochemical Evidence of Plume Sources for High-MgO Lavas in the Western Kunlun Orogenic Belt

Abstract Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian–Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6–33.8 wt %) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2 ± 0.9 wt %. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite-dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg–Fe equilibria, Al-in-olivine and olivine–spinel thermometry and oxybarometer yield a mantle potential temperature of 1522–1556 °C and high oxygen fugacity of FMQ (fayalite-magnetite-quartz) + 0.93. The H2O contents in the picrite flows are estimated as 3.67 ± 1.0 wt %, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB (oceanic island basalt)-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd–O–Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.

Seismic Observation of a New ULVZ Beneath the Southern Pacific

AbstractWe present new observations of core‐diffracted shear waves which contain anomalous waveforms sampling the lowermost mantle beneath the southern Pacific region. Data in two distinct geometries, one from New Zealand to North America and the other from the Fiji and Solomon Islands to South America, show evidence of postcursor phases. The postcursor delays and move‐outs imply that they are caused by an ultra‐low velocity zone (ULVZ). Beamforming analyses of the observed diffracted postcursors show a strong backazimuthal deviation, suggesting this new ULVZ is likely to have a cylindrical shape similar to broad ULVZs sampled by shear diffracted waves elsewhere. Full‐waveform modeling suggests that the postcursors seen in North America might be due to the previously modeled ULVZ located to the west of the Galápagos, while those seen in South America are due to a previously unknown ULVZ beneath the Southern Pacific. We cannot fit observations in both geometries by a single ULVZ. For the new location, we propose one cylindrical ULVZ model with a radius of 400 km and a shear wave velocity decrease of 20% centered at geographical coordinates (−33.6, 130) close to the Pitcairn hotspot. Despite some uncertainty in the west‐east direction, this new ULVZ observation likely provides another example to support the hypothesis that ULVZs exist at the base of mantle plumes where primordial signatures are observed in the ocean island basalts.

Tectonic dismemberment and plume-ridge interaction in the Sub-Antarctic South Atlantic Ocean Basin: Confirmation from in situ geochemical and Sr isotopic compositions of minerals

The Sub-Antarctic South Atlantic Ocean has been inferred to have undergone a complex tectonic history involving dismemberment and plume-ridge interaction. Here we report new in situ major (electron probe microanalysis, EPMA), trace element (laser ablation−inductively coupled plasma−mass spectrometry, LA-ICP-MS), and Sr isotopic (laser ablation−multicollector−inductively coupled plasma−mass spectrometry, LA-MC-ICP-MS) compositions for minerals (olivine, clinopyroxene, and plagioclase) from the Northeast Georgia Rise (NGR; Sites 698 and 699), Islas Orcadas Rise (IOR; Site 701), and Meteor Rise (MR; Site 703) volcanic samples in the Sub-Antarctic South Atlantic Ocean. Plagioclases and clinopyroxenes from Sites 698, 699, and 703 are characterized by similar incompatible element patterns and Sr isotopic characteristics, suggesting that they are likely to have originated from a cogenetic mantle source. The plagioclases and clinopyroxenes exhibit an oscillatory, reverse, and normal zoning texture, and display clear evidence of Sr isotopic disequilibrium, suggesting dynamic and open fractional-crystallization processes as well as extensive mixing of compositionally distinct magmas. Rims and groundmass of plagioclase exhibit much more highly radiogenic Sr isotopes than their cores, likely indicating the involvement of both continental lithospheric and recycled oceanic crust. The in situ geochemical and isotopic compositions of these minerals exhibit the features of both the oceanic-island basalt−type Tristan-Gough mantle plume track and the normal mid-oceanic-ridge basalt−type Mid-Atlantic spreading ridge (MAR) and Agulhas spreading ridge (AR) track. We speculate that the NGR, IOR, and MR were formed from the same mantle source with volcanic flow of the Tristan-Gough mantle plume. Subsequently, tectonic movement along the MAR and AR separated the originally combined MR-IOR-NGR, resulting in the incursion of depleted asthenospheric mantle and the contamination contributions of continental and recycled oceanic crust components.

Calcium isotope variability among ocean islands reveals the physical and lithological controls on mantle partial melting

Major geodynamic processes, including mantle convection and plate tectonics, govern the exchange of mass between Earth's interior and its surface. These processes drive mass-transfer mechanisms such as partial melting and crustal recycling, which have generated a lithologically and compositionally heterogeneous mantle over time. Ocean island basalts (OIB) directly sample these heterogeneities, making them excellent tools to constrain the mantle’s dynamic geochemical evolution. In this study, we probe the connection between crustal recycling, mantle mineralogy, and partial melting using the stable Ca isotope compositions of OIB, ultimately contextualizing their Ca isotope variability using conventional mantle “endmember” components including DMM, HIMU, EM-1, and EM-2. Using the state-of-the-art Nu Sapphire collision cell MC-ICP-MS, we measured the Ca isotope compositions of 27 well-characterized OIB samples from 11 different ocean island groups. By supplementing our new data with additional high-precision Ca isotope measurements from the literature, we create an expanded dataset consisting of 15 ocean islands, facilitating a global-scale investigation of Ca isotope variability among hotspots. Our findings reveal that island-averaged Ca isotope compositions form two distinct groups on the basis of their radiogenic isotope compositions. The first group, consisting of OIB with un-enriched radiogenic isotope compositions (non-EM-type), exhibits robust negative correlations with both lithospheric thickness and primitive TiO2 contents, providing compelling evidence that Ca isotopes are sensitive to the degree of partial melting (F) as well as to the effects of crustal recycling. This joint signal appears to trace the process of decompression melting beneath oceanic lithosphere of variable thickness, as recycled lithologies rich in garnet and pyroxene (i.e., pyroxenites)—which generate large solid-melt Ca isotope fractionations—disproportionally control the Ca isotope composition of the final aggregate melt when F is limited by thick overlying lithosphere. In contrast, the island-averaged Ca isotope compositions of OIB with enriched radiogenic isotope compositions (EM-type) form a distinctly light negative correlation with primitive TiO2 contents without exhibiting a relationship with lithospheric thickness. In addition to lithology-dependent partial melting effects, the Ca isotope compositions of EM-type OIB appear to be sensitive to the compositional effects of crustal recycling, which may explain their decoupling from the physical controls of partial melting. Calcium isotopes show exceptional potential for elucidating the origin of both enriched and ultra-depleted mantle sources, emphasizing the need to prioritize localities with these signatures in future studies.

Paleozoic evolution of the Yukon-Tanana terrane of the North American Cordillera, NW British Columbia

Abstract The origins and primary relationships between tectono-stratigraphic units are fundamental to the terrane concept in accretionary orogens, but they are challenging to assess in metamorphic terranes. In NW British Columbia, three tectonically bounded metamorphic suites of the Yukon-Tanana terrane formed in distinct tectonic settings, based on high-spatial-resolution geochronology and immobile trace-element geochemistry. The Florence Range suite comprises late Neoproterozoic or younger to pre–latest Devonian metasedimentary rocks derived from continental crust, 360 ± 4 Ma calc-alkaline intermediate orthogneiss, and 357 ± 4 Ma amphibolite with oceanic-island basalt composition, consistent with rifting of a continental margin. The detrital signature is dominated by late Mesoproterozoic zircon, which indicates different sources than other parts of the Yukon-Tanana terrane. The Boundary Ranges suite comprises pre–Late Devonian metasedimentary rocks derived in part from a mafic source, amphibolite derived from subduction-zone metasomatized mantle, and 369 ± 4 Ma to 367 ± 7 Ma calc-alkaline felsic to intermediate orthogneiss. The Whitewater suite comprises meta-chert, graphite-rich metapelite, and amphibolite with back-arc basin basalt composition consistent with an anoxic basin near a volcanic source. Our data indicate that the Florence Range and Boundary Ranges suites were separate until at least the Early Mississippian and may have formed a composite terrane since the Permian, whereas the relationship with the Whitewater suite is uncertain. We compare the Paleozoic evolution of the Yukon-Tanana terrane in NW British Columbia with several modern analogues in the west and southwest Pacific Ocean.

Iron isotopic compositions of HIMU Ocean island basalts: Implications for the mantle source lithology

Basalts from the islands of Mangaia, Tubuai, Rurutu (old stage), and Raivavae (Rairua stage) in the South Pacific, and from St. Helena Island in the South Atlantic, are characterized by extremely radiogenic Pb isotopic compositions (e.g., 206Pb/204Pb > 20.5). These ocean island basalts (OIBs) define the famous HIMU (high μ, where μ = 238U/204Pb) component in the mantle ‘zoo’, which has traditionally been thought to represent ancient recycled oceanic crust with highly fractionated U/Pb and Th/Pb ratios. However, recent high-precision analyses of minor and trace elements in olivine phenocrysts from HIMU OIBs suggest a peridotitic mantle source rather than pyroxenitic/eclogitic remnants of subducted oceanic crust. To better constrain the lithology of the HIMU source is crucial for further understanding the nature of the HIMU component. Here we utilize stable iron (Fe) isotopes, a novel tool for identifying the mantle source lithology of basalts, to further investigate the lithology of the HIMU source by examining classic HIMU OIBs from the Cook-Austral volcanic chain in the South Pacific. The results show that these OIBs have δ57Fe values varying from 0.09‰ to 0.18‰, which are similar to those of normal mid-ocean ridge basalts (N-MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). Quantitative calculations indicate that the fractional-crystallization-corrected δ57Fecorr values (0.06–0.15‰) of these basalts can well be explained by partial melting of garnet peridotite with δ57Fe values ranging from 0.05‰ to 0.09‰. Such low δ57Fecorr values, however, are difficult to be reproduced by partial melting of eclogite with a MORB-like δ57Fe value (δ57Fe = 0.15‰). Our new Fe isotope data, combined with previously reported heavy whole-rock zinc isotopes (δ66Zn = 0.38 ± 0.03‰) as well as high Mn/Fe (100Mn/Fe = 1.5–1.7) and Ca/Al (mostly >15) ratios of olivine phenocrysts in the HIMU OIBs, further confirm that the lithology of the HIMU mantle source is carbonated peridotite.

Petrogenesis and Geodynamic Implications of Cretaceous Nb-Enriched Mafic Dykes in the East Kunlun Orogen, Northern Tibet Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd Isotopes

There is a magmatic lull period in the East Kunlun orogen (EKO) during the Jurassic to the Cretaceous. However, due to the lack of records of magmatic activity restricts our understanding of the late Mesozoic magmatic-tectonic evolution of the EKO. Herein, an integrated study of geochronology, whole-rock geochemistry and Sr-Nd isotopes were conducted for the Cretaceous mafic dykes in the EKO, Northern Tibet Plateau, to reveal their petrogenesis and geodynamic implications. LA-ICP-MS Zircon U-Pb dating reveals that the studied mafic dykes comprising diabase and diabase porphyry emplaced at ca. 80.9 ± 0.8 Ma. The Cretaceous mafic dykes have low contents of SiO2 (46.36 wt.%~47.40 wt.%) but high contents of MgO (6.79 wt.%~7.38 wt.%), TiO2 (1.91 wt.%~2.13 wt.%), Nb (12.4~18.3 ppm) and Nb/U ratio (31~39), resembling Nb-enriched mafic dykes. They exhibit chondrite-normalized rare earth element (REE) and primitive mantle-normalized trace element patterns, remarkably similar but not identical to the oceanic island basalts (OIB). The moderate REE fractionation ((La/Yb)N = 3.55~5.37), weak negative Eu anomalies (δEu = 0.87~0.97) and relative enrichment of Rb, Ba, K, as well as high contents of Cr and Ni and slightly depleted Sr-Nd isotopes (εNd(t) = −0.18~1.33), suggest that the studied dykes originate from a partial melting of spinel lherzolite and a little of garnet which was previously modified by subducted sediments. Combined with other evidence, we propose that the studied Cretaceous Nb-enriched mafic dykes in the Northern Tibet Plateau were formed in the intraplate setting, which may be a partial melting of the enriched mantle in the lower lithosphere caused by the activity of the East Kunlun strike-slip fault.

OIB-type mafic–ultramafic rocks in South Guangxi, South China: A record of interaction between Paleo-Tethys subduction and Emeishan large Igneous Province

Late Permian ultramafic–mafic igneous rocks in the Pingxiang area, Guangxi, South China include wehrlite, gabbro, diabase, and amygdaloidal basalt. Geochemical characteristics document interaction between the Emeishan Large Igneous Province (ELIP) and the mantle wedge related to subduction of the Paleo–Tethys. Gabbro and basalt show chondrite–normalized rare earth elements (REEs) and primitive mantle–normalized trace element patterns that parallel ocean island basalts (OIB) and mafic intrusions from the ELIP. Melts in equilibrium with clinopyroxenes, in both wehrlite and gabbro, have REEs and trace element compositions similar to OIB and high–Ti mafic rock of the ELIP. The clinopyroxenes also show a rift–related compositional trend and cover those from the ELIP rocks. These compositional features, along with their moderate initial 87Sr/86Sr and εNd(t) values (−0.8 to +3), are consistent with derivation from an enriched mantle plume source. This is the first time that ultramafic–mafic rocks are reported beyond the established extent of the ELIP and could represent an offshoot from the main mantle plume or indicate that the ELIP plume was considerably bigger than current proposals but has been significantly denuded by subsequent erosion. In addition, the Pingxiang rock units display slightly enriched εNd(t) values and slight negative Nb, Ta, and Zr relative to OIB rocks, indicating an enriched component in the mantle source. Formation of the ELIP spatially and temporally overlapped with subduction of the Paleo–Tethys ocean and interaction between the plume and mantle wedge accounts for the compositional diversity of the ELIP. Such interaction is probably common along the southwestern–southern margin of the Yangtze Block.

Late Paleozoic magmatism from northern Sibumasu terrane: Constraints on separation of Cimmeria from Gondwana

The Cimmerian is one of several continental terranes that separated from northern Gondwana during the Carboniferous-Permian period. Magmatic evidence for the rifting of the fragments from Gondwana is preserved in the Baoshan area of eastern Cimmeria. These magmatic rocks outcrop over an area of more than 10,000 km2 and are mainly composed of basalts and dolerites, with limited occurrences of picrites. Our new results show that the basalts are tholeiitic and exhibit enriched trends in light rare earth elements (LREE), negative anomalies in Nb-Ta-Ti, and enriched isotopic signatures in Sr-Nd-Pb. They underwent fractional crystallization of clinopyroxene and olivine, with insignificant crustal contamination. Suppositional primary melt compositions and REE modeling results suggest that they were mainly formed by 2–5 % degrees of melting of spinel lherzolites with minor garnet (<1%) in a shallow enriched EMII lithospheric mantle, with minor components from the depleted mantle. Lack of plume-related ocean island basalts, a narrow south-north distribution, and a relatively low melting temperature may contradict the plume model. The Woniusi basalts could have originated in a back-arc environment, but further research is required to gather a more complete spectrum of volcanic rocks associated with the arc. In contrast, we propose that they were formed on the lower-plate passive Gondwana margin after the northward subduction of the Paleo-Tethys Ocean since the Late Devonian. This subduction may result in a longer-lasting extension of the passive Gondwana margin. The Woniusi basalts are roughly coeval with the subduction of the mid-ocean ridge during the Latest Carboniferous. After the active ridge has subducted, the slab pull is strong enough to separate a terrane, because the ridge spreading no longer absorbs the strain caused by subduction. If this is the case, subduction of the Paleo-Tethys ocean-ridge would enhance slab-pull forces and finally trigger Woniusi magmatism.

Oxygen Fugacity of Global Ocean Island Basalts

AbstractMantle plumes contain heterogenous chemical components and sample variable depths of the mantle, enabling glimpses into the compositional structure of Earth's interior. In this study, we evaluated ocean island basalts (OIB) from nine plume locations to provide a global and systematic assessment of the relationship between fO2 and He‐Sr‐Nd‐Pb‐W‐Os isotopic compositions. Ocean island basalts from the Pacific (Austral Islands, Hawaii, Mangaia, Samoa, Pitcairn), Atlantic (Azores, Canary Islands, St. Helena), and Indian Oceans (La Réunion) reveal that fO2 in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenous fO2 (average = +0.5 ∆FMQ; 2SD = 1.5) relative to prior estimates of global mid‐ocean ridge basalts (MORB; average = −0.1 ∆FMQ; 2SD = 0.6), though many individual OIB overlap MORB. Specific mantle components, such as HIMU and enriched mantle 2 (EM2), defined by radiogenic Pb and Sr isotopic compositions compared to other OIB, respectively, have distinctly high fO2 based on statistical analysis. Elevated fO2 in OIB samples of these components is associated with higher whole‐rock CaO/Al2O3 and olivine CaO content, which may be linked to recycled carbonated oceanic crust. EM1‐type and geochemically depleted OIB are generally not as oxidized, possibly due to limited oxidizing potential of the recycled material in the enriched mantle 1 (EM1) component (e.g., sediment) or lack of recycled materials in geochemically depleted OIB. Despite systematic offset of the fO2 among EM1‐, EM2‐, and HIMU‐type OIB, geochemical indices of lithospheric recycling, such as Sr‐Nd‐Pb‐Os isotopic systems, generally do not correlate with fO2.

Geochronology, geochemistry, and tectonic setting of the Neoproterozoic magmatic rocks in Pan-African basement, West Ethiopia

The West Ethiopian plateau lies in the transition zone between the Arabian-Nubian shield and the Mozambique belt of the Pan-African orogen, underlain by Precambrian to Tertiary rocks in the Gimbi-Nejo area. The Precambrian basement consists mainly of intrusive-meta-intrusive and meta-sedimentary rocks with ice-rafting deposits. Based on the field survey, unique material records, deformation features, magmatism, and metamorphism indicate that the Gimbi-Nejo area likely underwent four tectonic stages during the Neoproterozoic era.The sedimentary formation of 980 ± 3.9 Ma rift valleys is represented by meta-clastic rocks, calc-silicate rocks, meta-basic rocks bearing marble in Chochi, and the Kata domain. Geochemically, meta-peridotite and meta-gabbros show tholeiitic features, while meta-granitoids are medium-K calc-alkaline peraluminous to metaluminous A-type (Ce/Nb 2.1–11.9; Y/Nb 1.2–8; Yb/Ta 2.6–14). Meta-gabbro samples with immobile HFSE (Th, Ta, Hf) and REE (La, Sm, Yb) fall in the within-plate alkaline basalt, oceanic island basalt (OIB), and continental rift basalt field. Meta-granitoid samples also fall in the within-plate granite field. The lithospheric subduction stage magmatic arc, composed of gabbros and granitoids, was emplaced around 827 ± 3.2 Ma. The granitoids include calcic tonalite and trondhjemite showing an adakitic signature with high Sr/Y (14.2–48.2) ratios. Gabbro samples fall in the island arc basalt and continental arc basalt field. Meta-peridotites and meta-gabbros to gabbros originated from the partial melting of garnet-spinel lherzolite to spinel lherzolite (<10 %) and garnet lherzolite to garnet-spinel lherzolite (<30 %) mantle source, respectively. The continental collisional area contains thrust folds and faults trending in a north–south direction. The ca. 797 ± 3.7 Ma calc-alkalic granitoid rocks along the suture zone are high-K peraluminous I-type granitoids (ASI < 1.1). The strike-slippage area is represented by sinistral ductile shearing deformation. The alkali-calcic granitoid rocks associated with shearing and decollements are low to high-K calc-alkaline peraluminous-metaluminous I-S type granitoids (ASI = 0.95–1.17). The U-Pb age of alkali-calcic granitoid is 564 ± 3.4 Ma. Syn-collisional calc-alkalic and late-post orogenic alkali-calcic granitoid samples both fall within the category of within the plate granite.The εHf (t) values of meta-granitoid (+9.4 to + 15.5), calcic granitoid (+7.6 to + 13.2), calc-alkalic granitoid (+7.4 to + 15.5), and alkali-calcic granitoid (+7.11 to + 17.12) suggest that these Neoproterozoic granitoids have been derived from a Meso-Neoproterozoic juvenile crust that formed through depleted mantle source. Depleted mantle magma likely ascended to the juvenile crust and remained there for a certain period (mean time = 122 Ma, 257 Ma, 248 Ma, and 256 Ma, respectively).