Intraplate Basalts Research Articles

Hf isotope compositions of basaltic rocks from Jeju Island, South Korea: Implications for the origin of the dichotomous isotopic distribution of late Cenozoic magmatism in East Asia

The mantle sources of late Cenozoic intraplate basalts in East Asia can be divided into two distinct domains on the basis of Sr–Nd–Pb isotope systematics: DMM–EM1 in the north and DMM–EM2 in the south. However, the origin of this dichotomous isotopic distribution remains unclear. Here, we present new Hf isotope data for basaltic rocks from Jeju Island, South Korea, and compare the data with published data for coeval basalts from East Asia to understand the nature of the enriched components. The Jeju basalts have 176Hf/177Hf ratios of 0.282861–0.282965 and εHf values of +3.2 to +6.8. In an HfNd isotope plot, the basalts plot along the terrestrial array, without marked decoupling between Hf and Nd isotopes. Late Cenozoic basalts in East Asia fall mainly in the field of global ocean-island basalts. However, basalts in NE Asia tend to be much less radiogenic than those in SE Asia. In addition, some samples deviate from the terrestrial array, reflecting the involvement of various lithologies in the source. Major- and trace-element contents of the Asian basalts indicate the presence of subducted oceanic crust (eclogite/pyroxenite) in the mantle source. However, such crust is not a plausible source for the isotopically enriched components, given the less radiogenic Pb isotope compositions and higher 207Pb/206Pb ratios (0.816–0.917) of the late Cenozoic basalts compared with the HIMU end-member. Instead, subducted sediments could have been the enriched source. The representative EM1-type Wudalianchi basalts in NE Asia have high (Zr/Zr*)N ratios that show a negative correlation with εHf values, indicating the presence of ancient zircon-bearing sediments in the source. In contrast, SE Asian basalts have high Th/La ratios, suggesting the involvement of pelagic clay-rich sediments. A lack of meaningful correlations between Th/La ratios and εHf values indicates that the sediments are relatively young and were recycled with only a short residence time in the mantle source. PbNd isotope modeling shows that ancient (~2.2 Ga) sediments were involved in the mantle source of the NE Asian basalts, compared with relatively young sediments in the mantle source of the SE Asian basalts.

Late Neoproterozoic meta-gabbro in northeastern margin of the Yangtze Block: Magmatism related to late breakup of the Rodinia

Late Neoproterozoic tectonic evolution of the Yangtze Block is crucial for better understanding the breakup of Rodinia. The reported mafic magmatism related to the breakup event mainly distributed in the northwestern margin of the Yangtze Block. For the first time this study found a Late Neoproterozoic meta-gabbro pluton in the northeastern margin of the Yangtze Block. By means of petrography, zircon U–Pb geochronology, geochemistry and zircon Hf isotope, the formation age, petrogenesis and tectonic setting of the meta-gabbro pluton were determined and discussed. Dating results show that the pluton was formed at 642–636 Ma, reflecting an important magmatic event during the late Neoproterozoic. Geochemical characteristics indicate a tholeiite series for the meta-gabbro rocks, with positive anomalies of Th, La, Pb, Nd and negative anomalies of Nb, Ta, Zr, which are similar to intraplate basalts contaminated by the crust. The zircon 176Hf/177Hf ratios range from 0.282356 to 0.282563, with εHf(t) values ranging from + 0.74 to + 1.44. The two-stage Hf model ages range from 1289 Ma to 1748 Ma. Magma source of the meta-gabbro was mainly derived from an enriched mantle, which was evolved from the lithospheric mantle metasomatized by slab-derived melts and contaminated by a small amount of lower crust materials during magma ascent. Together with regional tectonics, we inferred that the meta-gabbro pluton was formed under the setting of intracontinental rift. Our result not only enriches the late Neoproterozoic magmatic activity events in the northern margin of the Yangtze Block, but also provides new evidence for the block involved in the late breakup of the Rodinia.

Bifurcation of mantle plumes by interaction with stagnant slabs in the mantle transition zone: Evidence from late Cenozoic basalts within Southeast Asia

Mantle plumes can encounter stagnant slabs in the mantle transition zone (MTZ) during their ascent from the lower mantle to Earth’s surface. However, the interactions between stagnant slabs and mantle plumes remain poorly understood. This study presents new data on the spatial thermal−lithological−chemical heterogeneity in the mantle over a vast area of Southeast Asia that reflects the variable influence of a late Cenozoic mantle plume. Site U1433 of International Ocean Discovery Program (IODP) Expedition 349 in the southwest sub-basin of the South China Sea (SCS) is located near the center of this seismically imaged mantle upwelling. This area also contains mid-ocean ridge basalt (MORB) derived from a peridotite-dominated and normal temperature mantle source with a depleted Nd-Hf and enriched 207Pb/206Pb isotopic composition. In contrast, MORB from Site U1431 of IODP Expedition 349 within the eastern sub-basin of the SCS and other intraplate basalts from the Hainan-Leizhou area, the eastern Indochina block, and elsewhere in the SCS are dominantly sourced from higher temperature pyroxenite-enriched mantle material that records mantle plume activity. These observations are in contrast with previous models involving a single plume rising from the lower mantle beneath the SCS that then tilted toward Hainan Island, China. Instead, it is likely that the observations from this region are indicative of plume-stagnant slab interaction. The shallow upper mantle beneath Site U1433 has not been influenced by a mantle plume as the presence of stagnant slabs within the MTZ prevented further upwelling of the plume. In contrast, a mantle plume passed through the weaker MTZ within the eastern sub-basin and circum-SCS areas, allowing plume-related magmatism to occur. The bifurcation of a deep-rooted mantle plume by stagnant slabs in the MTZ produced a cluster of small mantle upwellings that ascended to shallow depths, consistent with new multiscale global tomographic data for Southeast Asia.

Subduction‐Induced Asthenospheric Flow Around the Songliao Basin in NE China Revealed by Shear Wave Splitting Measurements of Dense Seismic Arrays

AbstractThe traditional mantle plume model fails to explain late Cenozoic intraplate volcanism in Northeast China (NEC). We constrained the pattern of upper mantle deformation to study the origin of intraplate volcanism by quantifying the shear wave splitting parameters captured by three NW‐SE linear seismic arrays in NEC. The dense station spacing (10 km) allowed us to image the small‐scale variations in anisotropic structures in unprecedented detail. The WNW‐ESE oriented subduction‐parallel anisotropy is likely induced by the asthenospheric return flow in the big mantle wedge convection associated with Pacific subduction. Therein the most notable feature is a toroidal pattern of anisotropy beneath the Songliao Basin, which is distinguished by a larger splitting time (approximately 1.0 s) relative to weak anisotropy in the center. The toroidal anisotropy pattern coincides with a high‐velocity anomaly extending from uppermost mantle down to ∼300 km depth, presumably indicating early stage foundering lithosphere. The asthenospheric return flow then moved around the foundering lithosphere and induced the corresponding toroidal anisotropy. The decompression partial melting of upwelling asthenospheric materials erupted along weak zones generating late Cenozoic intraplate basalts on both flanks of the Songliao Basin. The high‐density seismic array reveals that the Pacific subduction and early stage lithospheric foundering processes controlled the genesis of late Cenozoic volcanism in NEC.

Kinetic zinc isotope fractionation in olivine phenocrysts records magma evolution history of intra-plate basalts

In the past few years, zinc isotope systematics of basaltic magmas have been widely used as novel proxies for terrestrial mantle heterogeneity induced by recycled crustal materials and for planet formation and evolution. The influence of crystal-melt isotope disequilibrium during magma differentiation on the zinc isotopic composition of basaltic melts, nonetheless, has received little attention. In addition, no quantitative constraint has yet been given for the diffusion-driven Zn isotope fractionation between olivine crystals and melts. Here we present high-precision zinc isotope data (δ66ZnJMC-Lyon) for a series of olivine phenocrysts separated from intra-plate alkali basalts from the Jiaodong Peninsula in Eastern China, together with in-situ chemical analysis. Olivine phenocrysts have Zn isotopic compositions which are too light in comparison with the host basaltic melts (Δ66Znol-melt = −0.41‰ to −0.10‰; n = 16) to be explained by equilibrium isotope fractionation at magmatic temperatures. Instead, the decrease of δ66Zn values with decreasing Mg# (i.e., Mg/[Mg + Fe2+] × 100) and increasing Zn + Fe2+ contents in olivine phenocrysts suggests diffusion-driven kinetic fractionation during olivine crystallization. The data is well-fitted with a diffusion model in which Zn together with Fe2+ diffuse from surrounding melt into olivine crystals and approximately equal flux of Mg diffuses into melt due to the large chemical gradient, yielding a kinetic Zn isotope fractionation factor βZn of 0.07. By utilizing the fractional crystallization model under equilibrium or disequilibrium conditions, it is suggested that diffusion-induced isotope disequilibrium during olivine crystallization may drive Zn isotopic composition of the residual melt toward heavier values. When using zinc isotope systematics of any basaltic magma to probe the source heterogeneity, this impact must be taken into account if the Zn isotope disequilibrium widely exists in olivine phenocrysts. Based on the time-related quantitative diffusion model, our results show that the magma residence time for strongly alkali basaltic lavas (∼20 days) is 10–60 times shorter than that for weakly alkali basaltic lavas (6 months to 4 years). This supports the generation of weakly alkali basalts via interaction between silica-undersaturated melts and the surrounding lithospheric mantle, implying a probable common mechanism in intra-plate basalts that could be comparable globally. Thus, Zn isotope disequilibrium between olivine crystals and melts could provide valuable information on the evolution history of intra-plate basaltic magmas.

Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite

Intraplate basalts generally show a geochemical continuum from alkali to tholeiitic basalts. However, the genetic link between these two types of rocks has remained controversial. The Early Jurassic Karamay basalts in the West Junggar terrane, southern Central Asian Orogenic Belt (CAOB), erupted to form a small-volume outcrop in the stable continental intraplate region. The basalts are characterized by aphyric textures without any visible phenocrysts. Thus, they are different from the ubiquitous porphyritic-textured intraplate basalts and have a composition close to that of the mantle-derived primary melt. In contrast to chemically and petrographically well-defined alkali and tholeiitic basalts, the Karamay basalts exhibit transitional compositions spanning from alkali (with normative olivine+nepheline and normative olivine+hypersthene) to tholeiitic (with normative quartz+hypersthene), providing an important case to address the geochemical continuum of intraplate basalts. Similar to the alkali basalts in eastern China, the Karamay basalts have isotopic imprints of sedimentary carbonates, i.e., significantly lighter Mg (δ26Mg = −0.54‰ to −0.34‰) and heavier Zn (δ66Zn = 0.36–0.46‰) isotopes than the normal mantle. However, they display initial ((87Sr/86Sr)t ratios of 0.7047–0.7051, positive εNd(t) values (3.3–4.2) and positive anomalies of Nb-Ta-Ti-Zr-Hf, which are not expected in the case of incorporation of recycled carbonates. This contradiction can be reconciled by considering a decarbonation reaction between carbonates (i.e., dolomite and magnesite) and co-existing eclogite in the subducted oceanic slab at pressure >5 GPa, leaving light Mg and heavy Zn isotope signatures in the stagnant eclogite residue in the deep mantle. Combining the geochemical compositions of our samples with the geologic evidence, and considering the previous results of melt-peridotite reaction experiments, we conclude that the Karamay basalts might have originated from the interaction of silica-rich tholeiitic melt derived from the recycled decarbonated eclogite with fertile peridotite during its ascent. Our study highlights that intraplate alkali basalts, especially silica-rich ones (e.g., with SiO2>45 wt.%), can be transformed from tholeiitic melts through reaction with peridotite mantle, and demonstrates that deeply recycled oceanic crust stagnated in mantle can serve as a main source for alkaline lavas.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow

Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slab-derived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow

Abstract Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slabderived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.

Geochemical evidence for the generation of intraplate basalts from a carbonated eclogite source: A case study from Cenozoic Siziwangqi basalts

Geochemical evidence for the generation of intraplate basalts from a carbonated eclogite source: A case study from Cenozoic Siziwangqi basalts

Mantle plume–stagnant slab interaction controls the generation of a mixed mantle source for continental intraplate basalts

Continental intraplate basalts from southeastern Eurasia show ocean island basalt-like geochemical signatures and heterogeneous isotopic compositions suggesting mixing between isotopically enriched and depleted end-member sources. Although the isotopically enriched end-member(s) in these basalts which are related to oceanic crustal recycling have been investigated, the nature of the isotopically depleted end-member(s) remains unclear. We present geochemical characteristics of the southeast Cambodia basalt and compare them with other Cenozoic intraplate basalts in the South China Sea region. All these basalts contain a component with low Ce/Pb and Nb/Th ratios, moderate 87Sr/86Sr (~0.7040) and εNd (~ + 5), high εHf (~ + 9.5), and slightly positive ΔεHf value. These chemical and isotopic signatures are not consistent with the depleted local asthenosphere. We suggest a FOZO-like component, which is most likely derived from lower mantle material, represents the low-Ce/Pb component. Combined with geophysical observations, we propose that the interaction between the upwelling mantle plume and the stagnant slabs controls the formation of these intraplate basalts from the South China Sea region. This study highlights the important role of stagnant slabs in the mantle transition zone in modifying the compositions of mantle plumes, ultimately resulting in upper mantle heterogeneity.

Densely Sampled Global Dynamic Topographic Observations and Their Significance

AbstractTopography and bathymetry are principally supported by some combination of crustal and sub‐crustal density variations. However, dynamic topography is generated by vertical deflection of the Earth's surface as a result of mantle convection. Isolating and quantifying observable dynamic topography yields valuable information about mantle processes. Here, we investigate global dynamic topography by calculating residual depth anomalies throughout the oceanic realm and residual topographic anomalies across the continents. We correct for sedimentary and crustal loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide‐angle/refraction surveys, and receiver functions. In this way, an extensively revised and augmented global compilation of 10,874 oceanic residual depth measurements and 3,777 continental residual topographic measurements is constructed. In the oceanic realm, the methodology has been revised to improve accuracy and resolution. First, quartz/clay content of the sedimentary column is adjusted to remove minor skewness of residual depth anomalies as a function of plate age. Secondly, variation of bulk density as a function of crustal thickness is taken into account. Our global compilation is used to generate spherical harmonic representations of observable dynamic topography out to degree 40 (i.e., ∼1,000 km). Resultant spectra demonstrate that dynamic topographic power varies linearly with inverse wavenumber. The spectral slope directly reflects the way by which dynamic topography is generated by Stokes' flow. Our global results are consistent with independent and diverse geologic markers of uplift and subsidence together with Neogene‐Quaternary intraplate basalt magmatism.

Petrogenesis, Magma Source, and Geodynamics of Paleogene Mafic Rocks, Huimin Sag, Jiyang Depression, Eastern China

A suite of Paleogene mafic rocks was collected from boreholes in the Huimin Sag of the Jiyang Depression with the aim of investigating the petrogenesis and nature of mantle source for these rocks and further providing insights into the characteristic of related mantle plume. Whole-rock geochemical data indicate that the mafic rocks have relatively lower SiO2 (42.93%–48.57%) contents and similar characteristics to alkaline basalt and belong to transitional calc-alkaline series. These samples were clearly enriched in LREEs and depleted in HREEs and were also characterized by the enrichment of LILEs, incompatible elements, and HFSEs, similar to those of the Ocean Island Basalt (OIB). In addition, they exhibited Pb enrichment; Y, Pr, and Yb depletion; absence of Nb-Ta anomalies; high Hf and low Zr; and Rb/Yb ratios exceeding 1.0, indicating characteristics of intraplate rift-type alkaline basalt. The samples exhibited (Th/Ta)PM and (La/Nb)PM ratios less than 1 and plotted within the OIB, EMI, and EMII fields, indicating that crustal components had no role in the generation of the rocks. With the exception of individual samples that have a distinctive range of ε Nd values, the majority of samples have complex ε Nd values of -1.15 to 5.56, indicating a mixture of different sources, which was also apparent in the δ18O-87Sr/86Sr diagram, in which the samples plot close to the downward nonlinear curve. Based on the isotopic and trace elemental analyses, these igneous rocks are intraplate rift-type alkaline basalt and are of mantle plume origin. The variations in 87Sr/86Sr, 143Nd/144Nd, ε Nd values, LREEs, and HFSEs were probably due to the different locations of the mantle plume for different samples. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle, continental, or oceanic crust.

Upper mantle control on the W isotope record of shallow level plume and intraplate volcanic settings

Several studies have revealed small heterogeneities in the relative abundance of 182W, the radiogenic nuclide of short-lived 182Hf (t1/2 = ∼9 Ma), in terrestrial rocks. Whereas the majority of Archean rocks display 182W excesses relative to bulk silicate Earth, many young ocean island basalts show small 182W deficits, in particular if they are sourced from deep-rooted mantle plumes. The origin of this anomaly is still ambiguous, proposed models focus on core-mantle interaction or the presence of reservoirs in the lower mantle that have been isolated since the Hadean. In order to evaluate the role of upper mantle reservoirs, we report the first 182W data for intraplate basalts where a deep plume origin is still debated (Ascension Island, Massif Central, Siebengebirge and Eifel) and intraplate volcanic rocks associated with either plume or subduction zone environments (Italian Magmatic Provinces) and compare them to new data for basalts that have a deep mantle plume origin (La Réunion and Baffin Island). The proto-Iceland plume basalts from Baffin Island have uniform and modern mantle-like μ182W of around 0 despite extremely high (3He/4He). In contrast, basalts from both volcanic edifices from La Réunion span a range from modern upper mantle values to deficits as low as μ182W = −8.8 ppm, indicating a heterogeneous source reservoir. The μ182W in all other intraplate volcanic provinces overlap the composition of modern upper mantle to within 3 ppm. The absence of resolvable 182W anomalies in these intraplate basalts, which partially tap the lithospheric mantle, suggests that primordial components are neither present in the central and southern European lithosphere nor in the European asthenospheric reservoir (EAR). The general absence of 182W anomalies in European plume-related basalts can either be explained by a shallow mantle source or by the absence of isotopically anomalous and isolated domains in the deep mantle beneath the northern hemisphere, as also suggested by geophysical evidence.

Discovery of Late Mesozoic volcanic seamounts at the ocean-continent transition zone in the Northeastern margin of South China Sea and its tectonic implication

The Northern South China Sea (SCS) margin experienced the switch from Andean-type active subduction to passive extension during late Mesozoic era. However, the mechanism of cessation of paleo-Pacific subduction in the SCS region is unclear, and the location of suture zone between paleo-Pacific plate and South China block is obscure. In this study, we report new 40Ar/39Ar ages and whole-rock Hf isotopes of basalts from two poorly studied volcanic seamounts (namely, Puyuan and Beipo) at the ocean-continent transition zone in the Northeastern SCS margin, with an emphasis on the existence of exotic, buoyant oceanic plateau and its geodynamic effect on subduction cessation. Results indicate that the OIB-type Puyuan and E-MORB-type Beipo volcanic seamounts were erupted at 154.1 Ma and 93.2 Ma, respectively. Whole-rock trace element, Sr-Nd-Pb-Hf isotopic compositions, and subduction polarity point out that these samples were oceanic intraplate basalts formed within the paleo-Pacific plate, independent of the coeval, Late Mesozoic paleo-Pacific subduction system in the region. The ages and geochemical data of the two volcanic seamounts, combined with geochemical data of Late Mesozoic Pacific oceanic plateaus and seismic data of the Northeastern SCS margin, show that there may exist a Late Mesozoic exotic, buoyant oceanic plateau in the region. The collision between the oceanic plateau and the South China block could lead to the cessation of paleo-Pacific plate subduction, and changes in tectonic regime during the Late Cretaceous era (ca. 100–65 Ma). The pre-existing, Late Mesozoic suture zone has been well constrained by the remnant oceanic plateau and continental arc, and may play a key role in the formation of Cenozoic rifting tectonic units in the region.

Decoupled Zn-Sr-Nd isotopic composition of continental intraplate basalts caused by two-stage melting process

Ocean island basalts (OIBs) with Zn isotopic ratios higher than the normal mantle (δ66Zn = 0.17 ± 0.08‰) or mid-ocean ridge basalts (MORBs; δ66Zn = 0.27 ± 0.06‰) generally also have an enriched Sr-Nd isotopic signature, suggesting carbonate-bearing eclogites, whose protolith is inferred to be subducting altered oceanic crust, in their mantle source. On the contrary, continental intraplate basalts with high δ66Zn usually show depleted Sr-Nd isotopic signatures (i.e., decoupled Zn-Sr-Nd isotopic composition). To elucidate the origin of the decoupled Zn-Sr-Nd isotopic composition in continental intraplate basalts, we report the discovery of both coupled and decoupled Zn-Sr-Nd isotopic data for a suite of Cenozoic continental intraplate basalts from the Zhejiang province, Southeast China. These basalts display clear spatial and temporal geochemical variations, with early-stage inland low-silica samples presenting moderately enriched Sr-Nd isotopic signatures and high δ66Zn (coupled Zn-Sr-Nd isotopic composition, similar to OIBs), and later-stage coastal high-silica samples that display a pronounced δ66Zn decrease with increasing SiO2 and 87Sr/86Sr and with decreasing alkali contents and 143Nd/144Nd (decoupled Zn-Sr-Nd isotopic composition). The early-stage basalts with coupled high Zn-Sr-Nd isotopic signatures are also more enriched in incompatible elements than any other basalts from eastern China reported so far. We explain the spatial and temporal geochemical variations of these basalts as the result of two main melting events: 1) the low-silica early-stage magmatism mostly occurs inland and results from high-pressure partial melting of a carbonated eclogite-bearing asthenospheric mantle. Because of the presence of a thick lithosphere limits the melting of the depleted mantle component, the signature of the Zn-Sr-Nd isotopically enriched, and more fusible carbonated eclogite is preserved. 2) At the later stage, magmatism mostly occurs on the coast where the subcontinental lithosphere is thinner. Hence, decompression melting progresses to shallower depth, resulting in an increase of the contribution from the depleted peridotite matrix and a dilution of the signal from the isotopically enriched fusible component. Further upwelling and in-situ melting at the base of the subduction-modified sub-continental lithospheric mantle (SCLM) explains both the decoupled Zn-Sr-Nd isotopic signature of the coastal basalts and their major and trace element variability. We further propose that decompression melting is driven by small-scale convection resulting from variations of lithospheric thickness. Our data highlight the importance of dynamic melting of carbonated eclogite-bearing asthenosphere and subsequent lithospheric melting in preservation and destruction of the coupled enriched Zn-Sr-Nd isotopic signature of carbonated eclogite component and generation of the apparent decoupled Zn-Sr-Nd isotopic signal commonly observed in continental intraplate basalts.

Zinc isotopic systematics of the Mt. Baekdu and Jeju Island intraplate basalts in Korea, and implications for mantle source lithologies

Zinc isotope data for late Cenozoic intraplate basalts from Mt. Baekdu (North Korea) and Jeju Island (South Korea) were determined to characterize their mantle source lithologies. The average δ66Zn values (relative to JMC-Lyon) are 0.34‰ ± 0.08‰ (2SD; N = 13) for the Mt. Baekdu basalts and 0.37‰ ± 0.08‰ (2SD; N = 12) for the Jeju basalts, which are higher than values for refractory peridotites (0.18‰ ± 0.06‰) from the lithospheric mantle beneath the North China Craton. Given that Zn isotopic fractionation is limited during lithospheric processes, such as crustal contamination, magmatic differentiation, degassing, or thermal/chemical diffusion, the elevated δ66Zn values indicate the incorporation of recycled carbonates into the sub-lithospheric mantle source. Plots of Zn/Fe versus δ66Zn or δ26Mg for the Korean basalts can be explained by mixing of at least three source lithologies: (1) carbonated peridotite; (2) recycled siliciclastic sediments; and (3) eclogite. Strontium–Nd–Pb isotopic systematics show that the mantle source follows the DMM-EM1 array for the Mt. Baekdu basalts and a DMM-EM2 array for the Jeju basalts. Both the enriched components have isotopically light Zn and heavy Mg, which are typical of recycled siliciclastic sediments. Combined with the Pb isotope and trace element data, the Zn isotopic compositions suggest that the EM1 component could be ancient K-hollandite-bearing siliciclastic sediments and the EM2 component could be recently recycled clay-rich pelagic sediments.

Petrology of Mt. Kıra continental alkali lavas with arc-like signature, Batman, SE Anatolia, Turkey: Evidence for mafic juvenile lower crust assimilated intraplate basalts in the collision- and mantle flow-driven geodynamic setting of Arabian Foreland

Mineral phase variations and compositions of Early Pliocene (40Ar39Ar ages ranges 5.092 ± 0.030–4.892 ± 0.024 Ma) continental alkali basalts cropped out on Mt. Kıra, NE of Batman Province, SE Anatolia, and located approximately 75 km east of well-known Karacadağ Volcanic Complex (KVC) in Diyarbakır Province, record indications of pre-eruptive assimilation of the lower crust by OIB-like basaltic magmas from Arabian Foreland, which are proved by available data of juvenile lower crustal xenoliths in northern Syria Cenozoic basalts. Diopsitic augites with low Mg#’s (0.65–0.73) in Mt. Kıra basalts depict a compositional prone to clinopyroxenes (Mg#, 0.60–0.76) in Syrian lower crustal xenoliths and coexist with Ca- and Mn-rich fayalitic olivines (Fo63–36), Ca-poor (An72–13) plagioclases, and FeTi oxides. Mt. Kıra lavas, classified as hawaiites, are characterized by the narrow ranges of low SiO2 (46.30–47.68 wt%), Al2O3 (13.74–14.41 wt%), MgO (3.48–4.16 wt%), and high total alkali (Na2O + K2O, 5.26–5.89 wt%), Fe2O3 (13.97–15.38 wt%), TiO2 (3.44–3.61 wt%), CaO (7.20–8.99 wt%), and P2O5 (1.54–1.71 wt%) contents, and with low Cr (< 14 ppm) and Ni (< 20 ppm) values. Except for unusual ZrHf depletion, they exhibit typical arc-like (or crust-like) sharp negative anomalies of NbTa and Ti and possess high concentrations of Y (45–51 ppm), Sc (28–33 ppm), and Heavy REE (e.g., Dy, 9.25–10.73 ppm; Er, 4.74–5.27 ppm; Yb, 3.79–4.44 ppm). Two-component mixing model, and radiogenic 87Sr/86Sr (0·7,044,010–0.705396) and 143Nd/144Nd (0.512676–0.512812) isotopic compositions, which overlap with bulk-silicate earth, for Mt. Kıra basalts, provide evidence for the mafic juvenile lower crustal assimilation by OIB-like basaltic magmas derived from a common enriched mantle source. High Ba/La, Ba/Ta, Ba/Nb, Zr/Nb, and low Nb/La, La/Sm, Nb/Th, La/Yb, and Dy/Yb ratios, which coincide with those of Syrian lower crustal xenoliths, appears as geochemical hallmarks for decoupling of juvenile lower crust assimilated Mt. Kıra, NW Syria and some of KVC basalts from least contaminated ones in Arabian Foreland. The enrichments in Y, Fe, and Ti and depletions in Al and Sr in Mt. Kıra lavas, however, are attributed to post-eruptive chemical weathering processes. Based on petrological and tectonic data, it is proposed that the northern Arabian Foreland Cenozoic volcanism was triggered by the combined effect of both collision- and mantle flow-driven forces with a kinematic linkage. Asthenospheric mantle flow and upwelling, which supply a thermal source, caused the deep mantle processes and interaction between asthenosphere and lithosphere, which was formed the amphibole-bearing garnet lherzolitic (asthenospherized) metasomes (mantle flow-induced metasomatism) at the base of the SCLM accreted from the ductile asthenosphere, prior to partial melting. Partial melting of such re-fertilized metasomes and surrounding wall rock was most likely produced hybrid parental magmas via mixing variable proportions of melt fractions from a garnet lherzolite source with melt fractions from an amphibole-bearing garnet lherzolite source. On the other hand, concurrently running of collision-induced temporarily changing regional stress -compressional or extensional- regimes caused i) the development of surface tectonic structures in a dynamic topography that reactivated the pre-Neogene deep tectonic structures, ii) multiphase rapid uplifts, and iii) prominent deep channeling for the upwelling enriched mantle-derived hybrid magmas to penetrate. The shallow level lower crustal assimilation of this OIB-like hybrid mantle melts, generated the Mt. Kıra, NW Syria, and some of KVC basalts in the Arabian Foreland, during magmas en route to the surface.

Magnesium isotope geochemistry of the carbonate-silicate system in subduction zones

The lighter magnesium (Mg) isotopic signatures observed in intraplate basalts are commonly thought to result from deep carbonate recycling, provided that the sharp difference in Mg isotopic composition between surface carbonates and the normal mantle is preserved during plate subduction. However, deep subduction of carbonates and silicates could potentially fractionate Mg isotopes and change their chemical compositions. Subducting silicate rocks that experience metamorphic dehydration lose a small amount of Mg, and preserve the original Mg isotopic signature of their protoliths. When the dehydrated fluids dissolve carbonate minerals, they may evolve into lighter Mg isotopic compositions. The solubility of carbonate minerals in fluids decreases in the order of calcite, aragonite, dolomite, magnesite and siderite, leading to selective and partial dissolution of carbonate minerals along the subduction path. At the island arc depth (70–120 km), the metamorphic fluid dissolves mainly Mg-poor calcites, and thus the fluid has difficulty modifying the Mg isotopic system of the mantle wedge and associated arc basalts. At the greater depth of the back arc system or continental margin (>150 km), the supercritical fluid can dissolve Mg-rich carbonate minerals, and its interaction with the mantle wedge could significantly imprint the light Mg isotopic signature onto the mantle rocks and derivatives. Meanwhile, the carbonate and silicate remaining within the subducting slab could experience elemental and isotopic exchange, during which the silicate can obtain a light Mg isotopic signature and high CaO/Al2O3, whereas the carbonates, particularly the Ca-rich limestone, shift Mg isotopes and MgO contents towards higher values. If this isotopic and elemental exchange event occurs widely during crustal subduction, subducted Ca-rich carbonates can partially transform into being Mg-rich, and a portion of recycled silicates (e.g. carbonated eclogites) can have light Mg isotopic composition alongside carbonates. Both serve as the low-δ26Mg endmember recycled back into the deep mantle, but the latter is not related to deep carbonate recycling. Therefore, it is important to determine whether the light Mg isotopic signatures observed in intraplate basalts are linked to deep carbonate recycling, or alternatively, recycling of carbonated eclogites.

Evolution of the Continental Margin of South to Central Vietnam and Its Relationship to Opening of the South China Sea (East Vietnam Sea)

AbstractThe continental margin of south to central Vietnam is notable for its high elevation plateaus many of which are covered by late Cenozoic basalt flows. It forms the westernmost margin of a wide continental rift of the South China Sea (East Vietnam Sea), and uplift has been considered a result of either rifting or younger intraplate basalt magmatism. To investigate margin development apatite thermochronometry was applied to a dense array of samples collected from across and along the margin of south to central Vietnam. Results, including thermal history models, identified a distinct regional episode of fast cooling between c. 37 and 30 Ma after which cooling rates remained low. The fast cooling coincides with a period of fast extension across the South China Sea (East Sea) region that preceded continental break‐up recorded by Oligocene grabens onshore. A thermal model is used test different processes that might influence the inferred cooling including a distinct pulse of exhumation; a decrease in exhumation followed by an associated transient decrease in geothermal gradients and, underplating coincident with rifting. Thermal relaxation following Mesozoic arc magmatism is ruled out as geotherms returned to background rates within 20 Myrs of emplacement, well before the onset of fast cooling. Models support fast cooling attributed to accelerated erosion during early stages of rifting. Some additional heating from either underplating and/or hot mantle upwellings is also possible. No evidence was found to support regional uplift associated with the intraplate magmatism, enhanced monsoon‐driven erosion, or seafloor spreading dynamics.

Slab tear as a cause of coeval arc-intraplate volcanism in the Alexandra Volcanic Group, New Zealand

Coeval arc and intraplate basaltic magmas are intimately linked to back-arc volcanism in the Alexandra Volcanic Group of North Island, New Zealand. We present a comprehensive suite of new elemental and SrNd isotope geochemical data in the context of detailed new stratigraphy to resolve the fine-scale systematics of arc-intraplate magmatism at large basaltic stratovolcanoes (Pirongia and Karioi) and their surrounding vents. Time-chemistry reconstructions for Pirongia reveal cyclical contamination of arc magmas by deeper set intraplate sources, with each cycle ending in eruptions of lavas with transitional compositions. Further from the trench at Karioi, the magma system is characterised by on-off alternations of arc and intraplate basalt that eventually transition to arc-dominant magmatism. A two-component model is proposed to explain all geochemical variation in the field through mixing between arc and intraplate sources. Spatio-chemical data along a 65 km transect of the field indicates that as distance from the trench increases, the arc signature wanes and intraplate becomes increasingly prevalent in terms of erupted composition and volume. We conclude that trough-normal slab tear has facilitated the vertical and lateral ascension of asthenosphere into the mantle wedge, causing decompression melting to form intraplate basalts, and triggering secondary melting of the wedge to generate arc basalt. The Alexandra Volcanic Group is one of only several fields on Earth where arc and intraplate basalt are documented from the same vent systems.