Search

Despite their importance, no detailed mineral analyses have been conducted for volcanic rocks of Ocean Drilling Project Site 801C in the Pigafetta Basin, Western Pacific Ocean. From the eight volcanic sequences that comprise Site 801C, we performed comprehensive studies on the alkali basalts from sequence 1 (Upper Volcanic Group; UVG) and the tholeiitic basalts from sequences 3, 4, 6, and 8 (Lower Volcanic Group; LVG). To understand the magmatic evolution occurring at fast-spreading ridges, we studied the petrography, major and trace element compositions of minerals, mineral texture, and major and trace elements and Nd isotopic compositions for whole rocks. Alkali basalts from the UVG are characterized by enriched light rare earth elements (LREE) and large ion lithophile elements (LILE), relatively depleted heavy REE, and slightly low Nd isotopic ratios (ɛNd = 5.8–6.4), similar to the composition of ocean island basalts. Tholeiites from the LVG display significant depletions of LREE and LILE resembling the geochemistry of normal mid-ocean ridge basalts, and show much lower degrees of REE fractionation ((La/Yb) N = 0.50–0.74) and relatively high Nd isotopic ratios (ɛNd = 9.8–11.2). Plagioclase, alkali feldspar, clinopyroxene, and olivine in the UVG basalts display obvious normal zoning texture, indicating the slow ascent and magma evolution. The UVG alkali basalts were likely produced by the simple binary mixing of melts from a chemically and isotopically heterogeneous mantle. The comprehensive analyses suggest that the basaltic magma from Site 801C is the result of N-MORB fast-spreading ridge tholeiitic volcanism, which was followed after a certain interval (approximately 7 Ma) by within-plate alkaline volcanism from a plume influenced fertile mantle. The fast-spreading rate of the Pigafetta Basin basement was approximately 38–58 mm/y, and the within-plate alkaline volcanism was located ~ 400 km away from the active axis. Image 3 • Tholeiites from Site 801C are formed at fast spreading ridge. • Off-axis alkalic volcanism in Site 801C is located ~ 400 km away from the active axis. • The fast-spreading rate of the Pigafetta Basin basement was 38–58 mm/y. [ABSTRACT FROM AUTHOR]

The multi-stage magmatic plumbing system beneath the Ulleung, Dokdo, and Oki-Dogo Island has attracted considerable attention but has seldom been investigated in detail. Herein, we present a comprehensive geochemical analysis of volcanic rocks sampled at the Ulleung Island, including whole–rock major and trace elements as well as Sr and Nd isotopic compositions. Late Cenozoic alkaline volcanic rocks from the Ulleung Island are commonly divided into basaltic rocks (tephrite basanite and basalt) and acidic rocks (trachyte and rhyolite) based on the large petrologic and geochemical variations. All rocks are significantly enriched in light rare-earth elements (REEs) and large-ion lithophile elements and relatively depleted in heavy REEs. The acidic rocks have slightly enriched 87 Sr/ 86 Sr and ε Nd ratios (0.704895–0.705173; −2.48 to −1.91) and distinctly negative Ba, Sr, P, Eu, and Ti anomalies relative to the basaltic rocks (0.704733–0.705006, -2.61– -1.17). A multi-stage magma evolution model was finally developed beneath the Ulleung, Dokdo, and Oki-Dogo Island, characterized by at least two magma reservoirs at different depths approximately ranging between 20 and 100 km. These ascending magmas were retained in a shallower reservoir for ~1.0, ~0.5, and ~4.5 Ma, respectively, prior to eruptions with significant crystal fractionation. Mantle-derived basaltic melts either directly travelled to the surface to erupt as tephrite-basanite and basalt, or were stagnated in shallower reservoirs where they differentiated into acidic magma to erupt as trachyte and rhyolite. The Late Cenozoic Korean Peninsula basalts are mainly divided into two groups, i.e., the depleted mantle (DM) and enriched mantle 1 (EM1) mixing trend beneath the Ulleung and Dokdo Island, and the DM-EM2 mixing connection beneath the Jeju Island, which likely indicates the traversed boundary beneath Korean Peninsula that represents a simple lateral continuation of the boundary between the southeast and northeast China Blocks. [ABSTRACT FROM AUTHOR]

A genetic link between the Georgia-Islas Orcadas and Meteor guyots on opposite sides of the South Atlantic was proposed based on seafloor morphology and tectonic reconstructions. Here we test this model with geochemistry. We present new major and trace element and Sr, Nd, Pb, Hf radiogenic isotope data from volcanic and sedimentary rocks from Meteor Rise (ODP Site 703) and Georgia-Islas Orcadas Rise (ODP Sites 698, 699, and 701). Site 701 samples have slightly depleted isotopic compositions similar to southern Mid-Atlantic Ridge (SMAR) basalts, representing the upper mantle. Site 698, 699 and 703 volcanic rocks have initial Sr-Nd-Pb-Hf isotope ratios ((87Sr/86Sr) i = 0.70278–0.70660; (143Nd/144Nd) i = 0.51217–0.51283; (206Pb/204Pb) i = 18.09–18.75; (176Hf/177Hf) i = 0.28246–0.28299), extending from the Shona Hotspot track field (consisting of Richardson guyot, Agulhas Ridge and Meteor guyot samples) towards Proterozoic continental crust. These data support derivation from a common mantle plume, having formed contemporaneously as a single continuous hotspot track, which was subsequently divided by a ridge jump. Modelling of the isotope data show that binary mixing between an average Shona anomaly composition and an upper continental crustal component with a composition similar to the Site 698 sandstones can explain the data by 10–30% upper crustal assimilation to generate the composition of the Site 698, 699 and 703 volcanic rocks. Therefore, our geochemical data confirm the dismemberment of the Shona Hotspot track by a ridge jump and subsequent seafloor spreading. • Site 698, 699, 701, and 703 samples are derived from a common mantle plume. • Tectonic dismemberment of Shona Hotspot volcano is confirmed by geochemistry. • Site 698, 699 and 703 samples have suffered from 10 to 30% upper crustal assimilation. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E -MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies. [ABSTRACT FROM AUTHOR]

We take two typical Northeast Asia bimodal volcanoes as examples to explain the general features of Cenozoic bimodal ocean island basalt (OIB)-type volcanism in Northeast Asia. We present mineralogical, petrological, geochemical, isotopic, and full-waveform seismic tomographic evidence for the existence of two-layer magma chambers of Late Cenozoic volcanic activity beneath Ulleung Island and Mt. Changbai (Paektu). Ulleung Island volcanic rocks, which are composed of alkaline basalt, phonotephrite, trachyte, and phonolite, belong to the alkaline magma series and display enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slight depletion of heavy rare earth elements (HREEs), enriched 87Sr/86Sr (0.70475–0.70507) and 143Nd/144Nd (0.51250–0.51255) isotopic values, and enriched 207Pb/204Pb (15.544–15.626) and 208Pb/204Pb (38.750–38.954) values, similar to the geochemistry of OIB. Ulleung Island felsic volcanic rocks are characterized by significant negative Ba, Sr, P, Eu, and Ti anomalies (δEu = 0.14–0.35) and positive Pb anomalies, slightly higher 87Sr/86Sr isotopic ratios relative to those of mafic volcanic rocks, although the mafic and felsic samples have similar Sr, Nd, and Pb isotope compositions without significant differences. Ulleung Island and Mt. Changbai volcanic activities are likely related to the involvement of subduction-related compositions, but parts of the Mt. Changbai samples have been contaminated by crustal components to a certain extent. Mafic volcanic rocks of Ulleung Island are segregated from a deeper mantle source within a pressure range of 10.1–21.2 kbar compared with felsic volcanic rocks, which exhibit fractional crystallization of clinopyxene, spinel, plagioclase, and olivine. To explain this, a two-layer magma chamber beneath Ulleung Island with depths of 30–40 and 60–80 km is proposed, which is supported by mineral crystallization pressure and a three-dimensional full-waveform seismic tomography model. We also suggest that similar magma eruption processes and a two-layer magma chamber with depths of ~10 and 40–60 km also exist beneath Mt. Changbai. Taking the typical Cenozoic bimodal samples from Ulleung Island and Mt. Changbai as examples, we argue that two-layer magma chambers exist beneath Cenozoic bimodal OIB-type volcanic activities in Northeast Asia. [Display omitted] • A two-layer magma chamber beneath the Ulleung Island is proposed. • Fractional crystallization might play a key role for Ulleung Island volcanic rocks. • Northeast Asia volcanism is related to the involvement of subduction-related compositions. • The general features of Cenozoic bimodal OIB-type volcanism in Northeast Asia are proposed. [ABSTRACT FROM AUTHOR]

Ontong Java Plateau (OJP) is the largest known igneous province preserved on Earth. The hypotheses for the origin of the OJP volcanic activities remain controversial. Here we report new major, trace-element, and Sr–Nd–Pb isotopic compositions from volcanic rocks from Ocean Drilling Program Sites 1183, 1185, 1186, and 1187 of the OJP. Samples from the OJP are tholeiitic in composition and can be divided into low- and high-Ti groups, display flat rare-earth elements (REEs) distributions that are generally similar to normal mid-ocean-ridge basalt (N-MORB) patterns with slightly positive Nb, Ta, Th, U, and Ti anomalies and negative K, Pb, and Sr anomalies. However, OJP samples have significantly enriched 87Sr/86Sr (0.703641–0.704629), 143Nd/144Nd (0.512941–0.512972), and 206Pb/204Pb (18.493–18.940) radiogenic isotopic ratios, indicating that at least three distinct FOZO-type, EM1-type, and HIMU-type enriched components are involved. Combining with the method of principal component analysis, the special geochemical compositions with depleted N-MORB-incompatible elements and enriched isotopes of OJP volcanic samples could be explained by the off-axis ridge–plume interaction model. Due to the influence of strong ridge tensile forces, the off-axis plume magmatic mantle would flow to the ridge axis and undergo decompression melting, resulting in the depletion of the highly incompatible elements but the invariability of the isotopic compositions due to the relatively long half-life of the radiogenic elements. We speculate that these isotopically enriched N-MORB-type basalts may be an exclusive product of an off-axis ridge–plume interaction. Unlabelled Image [ABSTRACT FROM AUTHOR]