Cenozoic Basalts In Eastern China Research Articles

Li and O isotopes of mantle xenoliths from deep fault-related Cenozoic basalts in eastern China: The role of subducted components in the generation of the heterogeneous lithospheric mantle

It has been known that the lithospheric mantle underneath eastern China is geochemically heterogeneous, with different isotopic compositions across the South China and North China blocks. The distribution of Cenozoic basalts (<23 Ma) with abundant mantle xenoliths in eastern China is mainly controlled by lithospheric-scale faults. In this paper, we report new Li and O isotopic data for these mantle xenoliths and demonstrate that the mantle was variably modified via the Pacific subduction. Most olivine and orthopyroxene grains from the peridotite xenoliths in the South China block display lower δ18O values (4.43 to 5.77‰ and 4.77 to 5.55‰; respectively) than those of normal mantle, possibly reflecting impregnation of the slab melt of altered gabbroic precursors in the mantle transition zone (MTZ). The positive correlation between Li (1.40–6.54 ppm) and δ7Li (−28.41 to 20.83‰) of olivine grains suggests that the SCLM was metasomatized by hydrated slab-derived, isotopically heavier Li melts penetrating the frontal mantle wedge. The decoupling of Li and O isotopic systems implies that both hydrated mantle wedge and MTZ control the mass transportation and melt/fluid flow circulation beneath South China via the Pacific subduction. Comparatively, the higher δ18O values (4.90–6.41‰) and negative correlations between Li (1.39–6.02 ppm) and δ7Li (−23.55 to 22.54‰) in olivine grains from the North China block suggest a major contribution from the eclogitic slab that was metasomatized by the carbonatitic agent. Our findings corroborate the wide storage of carbonated eclogites in the MTZ beneath North China. We highlight that the stagnant slab in the MTZ might have controlled the refertilization of the lithospheric mantle beneath North China.

Late Cenozoic basaltic lavas from the Changbaishan-Baoqing Volcanic Belt, NE China: Products of lithosphere-asthenosphere interaction induced by subduction of the Pacific plate

Cenozoic intraplate basalts are low in volume but widespread in eastern China. They are predominantly alkaline and have oceanic island basalt (OIB)-like trace element compositions. Despite numerous studies, the origin of Cenozoic basalts in eastern China remains elusive. Possible roles of lithosphere thickness, subduction of Pacific plate and lithosphere-asthenosphere interaction in triggering spatial geochemical variations have not yet been clarified. Here, we have carried out mineral chemistry, major and trace element and SrNdHf isotope analyses of the late Cenozoic (<20 Ma) basaltic rocks from the Changbaishan-Baoqing Volcanic Belt (CVB), NE China, which revealed clear spatial compositional variations. The North CVB is dominated by basanites and alkali basalts with OIB-like trace element patterns and depleted SrNdHf isotopic compositions (87Sr/86Sr = 0.7039–0.7047, εNd = 3–5.6; εHf = 6.7–12), which may have been derived from low degree partial melting of a depleted source from asthenospheric mantle beneath a thick lithosphere. On the other hand, the South CVB consists of both alkali and sub-alkali lavas (alkali basalts, tholeiites and basaltic andesites) that display generally higher SiO2, Sm/Nd, Ba/Nb, Th/U, and lower Nb/Th, La/Sm and more enriched SrNdHf isotopic compositions (87Sr/86Sr = 0.7038–0.7056, εNd = −2.4 – +3.2; εHf = 3–8.2). These rocks may have been produced by larger degrees of partial melting of asthenospheric mantle beneath a relatively thin lithosphere. Ancient metasomatized lithospheric mantle might also have contributed to their genesis. In addition, the similar ranges of Mn, Ni and Fe/Mn for olivine phenocrysts from both the North and the South CVB suggest that they may have been derived from hybrid mantle sources containing similar proportions of peridotite and pyroxenite/eclogite components. We propose that decompression melting of upwelling asthenosphere and mechanical-chemical erosion of basal lithosphere related to lithosphere-asthenosphere interaction responsible for the genesis of the CVB magmas were likely associated with upper mantle convection and back-arc extension induced by deep subduction of the Pacific plate and its stagnancy in the transition zone.

Magma–magma interaction in the mantle recorded by megacrysts from Cenozoic basalts in eastern China

ABSTRACTRecently, besides magma–rock and rock–rock reaction, magma–magma interaction at mantle depth has been proposed as an alternative mechanism to produce diverse compositions of mantle. Clinopyroxene and garnet megacrysts can be formed at this condition since this process is suggested to trigger the high-pressure crystallization of these minerals. Studying on this type of megacrysts provides us important information on the genesis of intraplate basalts and the chemical heterogeneity of mantle, which has not been reported before. Here we present major, trace elements and Sr isotopes of clinopyroxene and garnet megacrysts hosted by Cenozoic basalts from Penglai, Shandong province of eastern China. The megacrysts are suggested to be formed by crystallization from magma because of their moderate Mg# (74.0–79.9 for clinopyroxene and 58.8–65.0 for garnet) and good correlations between Mg# and other elements (e.g. CaO, TiO2, Nd and Lu). The potential crystallized temperature and pressure are estimated to be ~1156°C at 2.6–3.2 GPa, which should occur at the top of asthenosphere or lithosphere–asthenosphere boundary based on the lithospheric thickness in this area (~60–70 km). Since the megacrysts show variable Sr isotopes, and their primary magmas show negative correlation between 87Sr/86Sr and Hf/Sm ratios, as well as positive correlation between Ba/Th and Nb/U for clinopyroxenes, it indicates a mixing origin. Cenozoic basalts from Shandong show a mixing trend, and high-pressure fractionation of clinopyroxene and garnet is suggested to occur during the mixing process because some basalts show significantly higher Sm/Yb and lower Ca/Al ratios than others, which again supports our interpretations. When compared to megacrysts and host basalts from other locations of eastern China, similar geochemical variations and a deviation trend relative to the mixing trend are also observed. It indicates that magma–magma interaction can be a common process for formation of intraplate basalts and basalt-borne megacrysts.

Origin of Late Cenozoic Abaga–Dalinuoer basalts, eastern China: Implications for a mixed pyroxenite–peridotite source related with deep subduction of the Pacific slab

Continental intraplate basalts (15.42–0.16Ma) from Abaga–Dalinuoer volcanic field (ADVF) in central Inner Mongolia of eastern China, as a part of Cenozoic volcanic province along eastern margin of the Eurasian continent, provide a good opportunity to explore potential links between deep subduction of the Pacific slab and continental intraplate volcanism. In this study, we report an integrated dataset of whole-rock K–Ar ages, major and trace elements and Sr–Nd–Pb isotopes, and olivine major and minor elements for the Abaga–Dalinuoer basalts (ADBs), and propose that mantle source lithology of the ADB magmas may consist of both pyroxenite and peridotite. The ADBs display low SiO2 (42.3–50.2wt.%), high MgO (7.3–11.4wt.%) and moderate K2O+Na2O (3.8–6.4wt.%), and can be subdivided into basanites, alkali basalts and tholeiitic basalts that are all characterized by ocean island basalt (OIB)-like rare earth elements (REE) and enrichment in both large ion lithosphile elements (LILE) and high field strength elements (HFSE). Olivine phenocrysts have higher Ni and Fe/Mn and lower Mn, Ca and Ca/Fe relative to those from peridotite melts, but exhibit clearly lower Ni contents (<2500ppm) compared with expected Ni range (>3000ppm) for olivines crystallized from olivine-free pyroxenite melts. Estimated compositions of the ADB primary magmas, together with olivine compositions, suggest an iron-rich mantle source related with silica-deficient pyroxenite that is most likely derived from deeply subducted Pacific oceanic crust. Additionally, peridotite and recent subducted sediments are also required to account for high Ni and MgO in primary magmas together with their trace elements and Sr–Nd–Pb isotope systematics. We suggest that a mixed pyroxenite–peridotite source lithology can better match observed whole-rock and olivine signatures in the ADB, compared with either peridotite only or olivine-free pyroxenite only source lithology. In our model, pyroxenite melts would either react with or mechanically mix with peridotite, and the proportion of pyroxenite melts may range from 30% to 45% for mechanical mixing scenario. A continuous spectrum from tholeiitic to alkali melts revealed by melt-peridotite reaction experiment can explain formation of primary magmas of basanites, alkali basalts and tholeiitic basalts by increasing melting degree of a similar mantle source. Relatively higher 206Pb/204Pb of the ADB may suggest more significant role of recent (<0.5Ga) subducted Pacific oceanic materials, in contrast to other Cenozoic basalts in eastern China (e.g., Changbai basalts) that exhibit varying contributions from ancient (>1.5Ga) subducted continental sediments. We emphasize that coupled geochemical and geodynamic links (i.e., subduction polarity) between deeply subducted Pacific slab and continental intraplate volcanism in eastern China may exist, which directly support the involvement of deeply subducted Pacific materials in petrogenesis of the ADB. From the perspective of plate motion kinetics, decompression partial melting of upwelling fragmented Pacific slab (silica-deficient pyroxenite+recent subducted sediments) may be triggered by rollback of deeply subducted Pacific slab during Late Cenozoic times. Continental intraplate volcanism in the ADVF generally started with termination of opening of the Japan Sea, suggesting that deep subduction of the Pacific slab may have been an important geodynamic mechanism responsible for tectono-magmatic evolution of northeastern Asia. We suggest that the ADBs have the potential to shed light on genetic links between continental intraplate volcanism and deep subduction of the Pacific slab in geochemical and geodynamic processes.

Mantle metasomatism did not modify the initial H2O content in peridotite xenoliths from the Tianchang basalts of eastern China

Metasomatism induced by melts/fluids is ubiquitous in the lithospheric mantle and can potentially modify the initial water content of the mantle. However, the preservation of correlations between H2O content and partial melting indices (e.g., Yb content in clinopyroxene, Cr/(Cr+Al) in spinel) and the lack of correlations between H2O content and metasomatic indices (e.g., La/Yb in clinopyroxene) in peridotite xenoliths from several localities suggest that variations in the initial H2O content were controlled by partial melting processes rather than by subsequent metasomatic event(s) (Hao et al., 2014; Denis et al., 2015). However, the applied partition coefficients of H2O between peridotite and melt (Dperidotite/melt=0.1–0.3) in the partial melting models cast doubts on the reasonability of such explanations. Whether metasomatism always modifies the initial H2O content of the lithospheric mantle remained a topic of debate. In this paper, we measure major and trace element concentrations and H2O contents of minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts in eastern China by using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The H2O contents (weight in ppm) of clinopyroxene, orthopyroxene and olivine are 70–280ppm, 35–140ppm and below detection limit (<2ppm H2O), respectively. Although water diffusion during xenolith ascent cannot be excluded for olivine, pyroxenes largely retain the initial H2O content of the mantle source, as supported by (1) the correlation between H2O content and major element content of pyroxene, and (2) the equilibrium H2O partitioning between clinopyroxene and orthopyroxene. The calculated whole-rock H2O contents range from 14 to 93ppm (average 52±25ppm) assuming 0.1 for the H2O partition coefficient between olivine and clinopyroxene. Although no hydrous minerals are found, the enrichment in light rare earth elements and large ion lithophile elements of clinopyroxene indicates cryptic mantle metasomatism. However, variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) are not correlated, suggesting that mantle metasomatism did not modify the initial H2O contents after the melting event. Instead, the H2O content correlates with the melting indices demonstrating that partial melting is the primary factor controlling the variations in the initial H2O contents. Notably, variations in whole rock H2O contents of the Tianchang peridotites can be modeled as a simple melting process of a MORB mantle source using the reported partition coefficients of H2O (Dperidotite/melt=0.005–0.03), providing a robust example suggesting that metasomatism does not always change the initial H2O content in the lithospheric mantle.

The water content and hydrogen isotope composition of continental lithospheric mantle and mantle-derived mafic igneous rocks in eastern China

The water contents of minerals and whole-rock in mantle-derived xenoliths from eastern China exhibit large variations and are generally lower than those from other on- and off-craton lithotectonic units. Nevertheless, the water contents of mineral and whole-rock in Junan peridotite xenoliths, which sourced from the juvenile lithospheric mantle, are generally higher than those elsewhere in eastern China. This suggests that the initial water content of juvenile lithospheric mantle is not low. There is no obvious correlation between the water contents and Mg# values of minerals in the mantle xenoliths and no occurrence of diffusion profile in pyroxene, suggesting no relationship between the low water content of mantle xenolith and the diffusion loss of water during xenolith ascent with host basaltic magmas. If the subcontinental lithospheric mantle (SCLM) base is heated by the asthenospheric mantle, the diffusion loss of water is expected to occur. On the other hand, extraction of basaltic melts from the SCLM is a more efficient mechanism to reduce the water content of xenoliths. The primary melts of Mesozoic and Cenozoic basalts in eastern China have water contents, as calculated from the water contents of phenocrysts, higher than those of normal mid-ocean ridge basalts (MORB). The Mesozoic basalts exhibit similar water contents to those of island arc basalts, whereas the Cenozoic basalts exhibit comparable water contents to oceanic island basalts and backarc basin basalts with some of them resembling island arc basalts. These observations suggest the water enrichment in the mantle source of continental basalts due to metasomatism by aqueous fluids and hydrous melts derived from dehydration and melting of deeply subducted crust. Mantle-derived megacrysts, minerals in xenoliths and phenocrysts in basalts from eastern China also exhibit largely variable hydrogen isotope compositions, indicating a large isotopic heterogeneity for the Cenozoic SCLM in eastern China. The water content that is higher than that of depleted MORB mantle and the hydrogen isotope composition that is deviated from that of depleted MORB mantle suggest that the Cenozoic continental lithospheric mantle suffered the metasomatism by hydrous melts derived from partial melting of the subducted Pacific slab below eastern China continent. The metasomatism would lead to the increase of water content in the SCLM base and then to the decrease of its viscosity. As a consequence, the SCLM base would be weakened and thus susceptible to tectonic erosion and delamination. As such, the crust-mantle interaction in oceanic subduction channel is the major cause for thinning of the craton lithosphere in North China.

Geochemical and Sr–Nd isotopic constraints on the petrogenesis of late Cenozoic basalts from the Abaga area, Inner Mongolia, eastern China

Over the past 30years, the Cenozoic basalts in eastern China have been the subject of many investigations, but their origin remains highly controversial. The Abaga–Dalinuoer volcanic field in Inner Mongolia consists of an approximately 10,000km2 lava plateau and more than 300 volcanic cones and extends northwestward to the adjacent Dariganga lava plateau of Mongolia, forming one of the largest Cenozoic volcanic fields in eastern Asia. In this paper, we concentrate on the Abaga volcanic field that comprises more than 200 monogenetic cones, including scoria cones, lava flows, and maars. Phreatomagmatism has contributed to the generation of several volcanic edifices, and the diameter of the maars craters can reach 3 to 6km, which are among the largest in eastern China. The volcanic rocks in the Abaga area can be divided into alkaline basalts and tholeiites. According to trace element and Sr–Nd isotopic characteristics, the Abaga magmas were primarily derived from a relatively homogeneous garnet peridotite source within the asthenosphere. Variable degrees and depths of partial melting played a key role in the genesis of magma from alkaline basalts to tholeiites. In contrast to the surrounding Cenozoic volcanic fields, such as Datong, Jining, and Hanuoba, in the western block of the North China Craton, the Abaga volcanic region in the southern part of the Central Asian Orogenic Belt is distinguished by the deepest melting and the thickest lithosphere. Negative Ba-, Rb-, K-, and Ti anomalies and relatively high 87Sr/86Sr ratios suggest the presence of residual phlogopite/amphibole during mantle melting. The origin of the Abaga magma most likely required involvement of fluids released from the stagnant Pacific slab in the mantle transition zone.

Thermal state and structure of the lithosphere beneath eastern China: A synthesis on basalt-borne xenoliths

Application of reliable thermobarometer on garnet-bearing mantle xenoliths and granulite xenoliths entrained by Cenozoic basalts in eastern China reveals two main types of geotherm. The first type, as exampled by Hannuoba (汉诺坝), Mingxi (明溪) and probably Northeast China, is characterized by constant slope of data in the P-T space. The second type, as exampled by the high geotherms of Nushan (女山) and probably Xinchang (新昌), is characterized by variable slopes, with the samples with pressure 2 MPa define a virtually vertical slope. The different slopes in the second type of geotherm may correspond to different heat transfer mechanisms, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle. This observed transition in thermal transfer mechanism is consistent with theoretical modeling. The two types of geotherm are not mutually exclusive, because the second type may characterize the thermal state of whole lithospheric section including both mechanical boundary layer (MBL) and thermal boundary layer (TBL), while the first type may only depict the MBL. The variable geotherms for different regions are indicative of a heterogeneous lithospheric structure in eastern China. (a) Eastern North China craton (NCC) is characterized by a second-type geotherm, corresponding to a thin lithosphere (∼70 km). Comparison of the equilibrium temperatures of spinel peridotites with this geotherm constrains the depth to Moho in eastern North China craton to be 30 km. In contrast, western NCC (Hannuoba: the first-type geotherm) possesses a relatively low thermal gradient, indicative of a thick lithosphere (>90–100 km) and a thick crust-mantle transition zone. The dramatic change in crustal and mantle structure across the DTGL (Daxing’anling (大兴安岭)-Taihangshan (太行山) gravity lineament) is consistent with recent seismic studies. (b) There is a decrease in thermal gradient and in lithospheric thickness from the coast (Xinchang: the second-type geotherm) to the inland (Mingxi: the first-type geotherm) in South China (from ∼80 km to >90 km), which is collaborated with westward variation in basalt geochemistry. (c) The weak convex-upward pattern of the geotherm in Qilin (麒麟) and Leizhou (雷州) Peninsula is peculiar, probably reflecting a transitional feature between conductive and advective heat transfer. It may result from impregnation of mantle plume on the base of the lithosphere. These new results not only provide a basic framework for the ongoing 4-D lithosphere mapping project in eastern China, but also yield important implications for deep processes that operated over the past.

Chemical and stable isotopic constraints on the nature and origin of volatiles in the sub-continental lithospheric mantle beneath eastern China

We report concentrations and stable isotopic data of CO 2 , CO, CH 4 , H 2 O and H 2 volatile species from mantle xenoliths hosted in Cenozoic basalts in eastern China. The volatiles are extracted by stepwise heating and mostly released in two temperature intervals, ∼ 400–600 °C and ∼ 800–1200 °C, respectively. We identified two types of volatiles: (1) “initial" volatiles, which were originally dissolved in crystal structures and trapped in fluid inclusions in the interiors of mineral crystals, are released at the 800–1200 °C interval, and composed of H 2 (66.6 mm 3.STP/g on average), CO 2 (50.7), CO (35.8) and CH 4 (0.6). The δD and δ 13C of CH 4 exhibit “normal" mantle signatures. However, variable δ 18O of CO 2 (0.6‰ to 16.6‰) and lighter δ 13C values of CO 2 and CO point to the involvement of a recycled crustal component. We interpret the latter as subducted oceanic crust with terrigenous sediments. (2) metasomatic volatiles, which were incorporated into minerals as fluid inclusions at edges and healed fractures of the recrystallizing minerals probably during mantle metasomatism, are released at the 400–600 °C interval, and composed mainly of CO 2 (∼ 9.82 mm 3.STP/g on average) with minor CO, H 2 and CH 4 , and are characterized by δ 13C of biogenic CH 4 , lighter and variable δD of H 2 O (− 110.7‰ to − 280.9‰), lighter δ 13C of CO 2 and CO and variable δ 18O of CO 2 (1.0‰ to 11.5‰), suggesting that the metasomatic volatiles may have originated form devolatilization of altered recycled oceanic crust with terrigenous sediments. We speculate that the metasomatic volatiles could have come from dehydration of the subducted paleo-Pacific lithosphere that lies horizontally in the transition zone beneath eastern China. Such recycled crustal fluids may have been important in metasomatizing the sub-continental lithosphere beneath eastern China and in causing thinning of the lithosphere and associated magmatism in the Mesozoic.

Hf isotopes of zircon megacrysts from the Cenozoic basalts in eastern China

Cenozoic basalts are widely distributed in eastern China, and some of them contain zircon megacrysts which are considered to be constituent mineral of the subcontinental lithospheric mantle (SCLM) and petrogenetically related to mantle metasomatism induced by addition of crustal materials. Using the Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS), zircon megacrysts from the Cenozoic basalts at Changle in Shandong, Mingxi in Fujian, and Penglai in Hainan provinces have been used for Hf isotopic analyses. The data indicate that there is no significant deviation for the different zircon grains in each locale, except those from Penglai. The obtained176Hf /177Hf ratios are 0.28302–0.28308 for Changle, 0.28297–0.28300 for Mingxi, and 0.28288–0.28293 for Penglai, with correspondingeHf values of 8.7–10.8, 7.0–7.9, and 3.9–5.7, respectively. These data display that there existed some regional heterogeneity, but the Hf model ages clustere in the Phanerozoic. Therefore, it is inferred that metasomatism of the lithospheric mantle beneath eastern China took place in the Phanerozoic, most probably in the Mesozoic-Cenozoic. However, the formation time of the lithospheric mantle is not clearly constrained based on the present Hf isotopic data.

Hf isotopes of zircon megacrysts from the Cenozoic basalts in eastern China

Hf isotopes of zircon megacrysts from the Cenozoic basalts in eastern China

Geochemical Records of Mantle Processes in Mantle Xenoliths from Three Cenozoic Basaltic Volcanoes in Eastern China

Abstract Rare earth element (REE) contents, and Sr and Nd isotopic compositions were measured for three suites of mantle xenoliths from the Kuandian, Hannuoba and Huinan volcanoes in the north of the Sino‐Korean Platform. From the correlations of Yb contents with Al/Si and Ca/Si ratios, the peridotites are considered to be the residues of partial melting of the primitive mantle. The chondrite‐normalized REE compositions are diverse, varying from strongly LREE‐depleted to LREE‐enriched, with various types of REE patterns. Metasomatic alteration by small‐volume silicate melts, of mantle peridotites previously variably depleted due to fractional melting in the spinel peridotite field, can account for the diversity of REE patterns. The Sr/Ba versus La/Ba correlation indicates that the metasomatic agent was enriched in Ba over Sr and La, suggestive of its volatile‐rich signature and an origin by fluid‐triggered melting in an ancient subduction zone. The Sr and Nd isotopic compositions of these xenoliths, even from a single locality, vary widely, covering those of Cenozoic basalts in eastern China. The depleted end of the Sr‐Nd isotope correlation is characterized by clearly higher 143Nd/144Nd and a broader range of 87Sr/86Sr compared to MORB. The low‐143Nd/144Nd and high‐87Sr/86Sr data distribution at the other end of data array suggests the existence of two enriched mantle components, like EM1 and EM2. The correlations between 143Nd/144Nd and 147Sm/N4Nd ratios in these xenoliths suggest at least two mantle metasomatic events, i.e. events at 0.6–1.0 Ga and 280–400 Ma ago.

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Continental intraplate basalts (15.42–0.16 Ma) from Abaga–Dalinuoer volcanic field (ADVF) in central Inner Mongolia of eastern China, as a part of Cenozoic volcanic province along eastern margin of the Eurasian continent, provide a good opportunity to explore potential links between deep subduction of the Pacific slab and continental intraplate volcanism. In this study, we report an integrated dataset of whole-rock K–Ar ages, major and trace elements and Sr–Nd–Pb isotopes, and olivine major and minor elements for the Abaga–Dalinuoer basalts (ADBs), and propose that mantle source lithology of the ADB magmas may consist of both pyroxenite and peridotite. The ADBs display low SiO2 (42.3–50.2 wt.%), high MgO (7.3–11.4 wt.%) and moderate K2O + Na2O (3.8–6.4 wt.%), and can be subdivided into basanites, alkali basalts and tholeiitic basalts that are all characterized by ocean island basalt (OIB)-like rare earth elements (REE) and enrichment in both large ion lithosphile elements (LILE) and high field strength elements (HFSE). Olivine phenocrysts have higher Ni and Fe/Mn and lower Mn, Ca and Ca/Fe relative to those from peridotite melts, but exhibit clearly lower Ni contents (< 2500 ppm) compared with expected Ni range (> 3000 ppm) for olivines crystallized from olivine-free pyroxenite melts. Estimated compositions of the ADB primary magmas, together with olivine compositions, suggest an iron-rich mantle source related with silica-deficient pyroxenite that is most likely derived from deeply subducted Pacific oceanic crust. Additionally, peridotite and recent subducted sediments are also required to account for high Ni and MgO in primary magmas together with their trace elements and Sr–Nd–Pb isotope systematics. We suggest that a mixed pyroxenite–peridotite source lithology can better match observed whole-rock and olivine signatures in the ADB, compared with either peridotite only or olivine-free pyroxenite only source lithology. In our model, pyroxenite melts would either react with or mechanically mix with peridotite, and the proportion of pyroxenite melts may range from 30% to 45% for mechanical mixing scenario. A continuous spectrum from tholeiitic to alkali melts revealed by melt-peridotite reaction experiment can explain formation of primary magmas of basanites, alkali basalts and tholeiitic basalts by increasing melting degree of a similar mantle source. Relatively higher 206Pb/204Pb of the ADB may suggest more significant role of recent (< 0.5 Ga) subducted Pacific oceanic materials, in contrast to other Cenozoic basalts in eastern China (e.g., Changbai basalts) that exhibit varying contributions from ancient (> 1.5 Ga) subducted continental sediments. We emphasize that coupled geochemical and geodynamic links (i.e., subduction polarity) between deeply subducted Pacific slab and continental intraplate volcanism in eastern China may exist, which directly support the involvement of deeply subducted Pacific materials in petrogenesis of the ADB. From the perspective of plate motion kinetics, decompression partial melting of upwelling fragmented Pacific slab (silica-deficient pyroxenite + recent subducted sediments) may be triggered by rollback of deeply subducted Pacific slab during Late Cenozoic times. Continental intraplate volcanism in the ADVF generally started with termination of opening of the Japan Sea, suggesting that deep subduction of the Pacific slab may have been an important geodynamic mechanism responsible for tectono-magmatic evolution of northeastern Asia. We suggest that the ADBs have the potential to shed light on genetic links between continental intraplate volcanism and deep subduction of the Pacific slab in geochemical and geodynamic processes.

Mantle Redox State Evolution in Eastern China and Its Implications

Abstract Using the secondary spinel standard, the authors have precisely measured the Fe3+/Σ Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, and estimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine‐orthopyroxene‐spinel oxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle in eastern China are higher in the south than in the north. Among them, the oxygen fugacity of the North China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, while that of Northeast and South China are the same as that of the global continental mantle. The variations of mantle redox state in eastern China are mainly controlled by the C‐O‐H fluids derived from the asthenospheric mantle. According to the mantle oxidation state, it can be concluded that the C‐O‐H fluids in the lithospheric mantle of eastern China consist mainly of CO2 and minor H2O, but CH4‐rich fluids should come from the asthenosphere where the oxidation state is lower.

The Characteristics of Spinels in Mantle‐derived Xenoliths in Cenozoic Basalts from Eastern China and Their Thermal States

Abstract: The colours and chemical composition variations of 160 spinels in peridotite and pyroxenite xenoliths from Cenozoic basalts in eastern China and their petrogeneses have been studied in detail. The relationships between major elements of spinels are discussed. The equilibrium temperatures, pressures and oxygen fugacities of spinels and their coexisting olivines, orthopyroxenes and clinopyroxenes have been determined using the Brey—kohler's T—P calculation methods (1990) and Ballhaus' fo2 calculation method (1991). The relationships between the composition and the equilibrium temperatures, pressures or oxygen fugacities of spinels in peridotite xenoliths from the basalts and the stable field of the spinels in the upper mantle have been shown.

Major- and trace-element compositions of Cenozoic basalts in eastern China: Petrogenesis and mantle source

The studied Cenozoic basalts from eastern China have SiO 2 contents ranging from 38.63 to 55.24 wt%, and Na 2O + K 2O from 3.1 to 9.4 wt%. They may be divided into three main types — tholeiitic, alkalic and potassic magmas. Variable trace-element abundances, together with the diverse major-element compositions, suggest that their origins involve both partial melting of mantle rocks at different depths, and subsequent fractional crystallization of clinopyroxene and olivine. Variations in the ratios of highly incompatible elements of the basalts in eastern China demonstrate two mantle components. The enriched mantle component, primarily represented by the basalts in NE China, show high LILE/Nb, Ba/Th, Ba/U, Th/U and low La/Ba ratios, typical of the Walvis Bay Ridge, Gough and Tristan da Cunha oceanic island basalts (OIB). These trace-element features support the identification of EM1 in their mantle source previously made by G.H. Xie and coworkers using Sr, Nd and Pb isotopes. This Dupal mantle source is inferred to be lithospheric and related to metasomatism during subduction, rather than due to the direct incorporation of continental crustal material into the mantle. The second component is represented by basalts from SE China which have the trace-element signatures of normal OIB, i.e. they do not have a Dupal anomaly. It appears most likely that upwelling of the asthenosphere and magma ascent induced the melting of overlying heterogeneous mantle lithosphere to generate the Cenozoic basalts of eastern China.

Major element compositions and rare-earth element abundances of Cenozoic basalts in eastern China: Implications for a pressure control over LREE/HREE fractionation in continental basalt

Major element compositions and rare-earth element (REE) and transition element (Ni, Cr and V) abundances have been determined on 44 basalt samples from eastern China. These basalts have SiO2 contents ranging from 38.63 to 55.24 (wt. %), and Na2O+K2O from 3.1 to 9.4 (wt. %). Ni and Cr abundances are largely variable, respectively falling in ranges 60–605 and 78–1150 ppm. REE abundances, especially light rare-earth elements (LREE), are highly variable. La/Sm and La/Yb ratios vary from 2.8 to 7.6 and 1.8 to 8.1.