Mechanical and thermal structure of a cylindrical plume in the earth's mantle

A mantle plume is understood as a hot, narrow, upwelling flow in the earth's mantle and accompanied by an efficient transfer of mass and energy from deep to upper layer of the earth. The cylindrical plume in earth's mantle plays an important role in explaining the origin of the surface hot spots and linear island chains. From the basic hydrodynamical equations, the detailed mechanical and thermal structure of a cylindrical plume of Newtouian fluids with temperature and pressure-dependent viscosity are given in the present paper. For two sets of rheological parameters the radial profiles of upward velocity, temperature and viscosity in the plume and radiuses of the plume at various depths have been calculated.

The thermal and mechanical structure of a two-dimensional plume in the earths mantle

On the condition that the distribution of velocity and temperature at the mid-plane of a mantle plume has been obtained (pages 213–218, this issue), the problem of determining the lateral structure of the plume at a given depth is reduced to solving an eigenvalue problem of a set of ordinary differential equations with five unknown functions, with an eigenvalue being related to the thermal thickness of the plume at this depth. The lateral profiles of upward velocity, temperature and viscosity in the plume and the thickness of the plume at various depths are calculated for two sets of Newtonian rheological parameters. The calculations show that the precondition for the existence of the plume, δT/L 1 (L = the height of the plume, δT = lateral distance from the mid-plane), can be satisfied, except for the starting region of the plume or near the base of the lithosphere. At the lateral distance, δT, the upward velocity decreases to 0.1 – 50% of its maximum value at different depths. It is believed that this model may provide an approach for a quantitative description of the detailed structure of a mantle plume.

In search of a hidden long-term isolated sub-chondritic 142Nd/144Nd reservoir in the deep mantle : implications for the Nd isotope systematics of the Earth

Here we search for evidence of the existence of a sub-chondritic 142Nd/144Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai’i with coupled elevations in 186Os/188Os and 187Os/188Os, from Iceland that represent mixing of upper mantle and lower mantle components, and from Gough with sub-chondritic 143Nd/144Nd and high 207Pb/206Pb, are favorable samples that could reflect mantle sources that have interacted with an Early-Enriched Reservoir (EER) with sub-chondritic 142Nd/144Nd. High-precision Nd isotope analyses of basalts from Hawai’i, Iceland and Gough demonstrate no discernable 142Nd/144Nd deviation from terrestrial standards. These data are consistent with previous high-precision Nd isotope analysis of recent mantle-derived samples and demonstrate that no mantle-derived material to date provides evidence for the existence of an EER in the mantle. We then evaluate mass balance in the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd. The Nd isotope systematics of EERs are modeled for different sizes and timing of formation relative to ε143Nd estimates of the reservoirs in the μ142Nd = 0 Earth, where μ142Nd is ((measured 142Nd/144Nd/terrestrial standard 142Nd/144Nd)−1 * 10−6) and the μ142Nd = 0 Earth is the proportion of the silicate Earth with 142Nd/144Nd indistinguishable from the terrestrial standard. The models indicate that it is not possible to balance the Earth with respect to both 142Nd/144Nd and 143Nd/144Nd unless the μ142Nd = 0 Earth has a ε143Nd within error of the present-day Depleted Mid-ocean ridge basalt Mantle source (DMM). The 4567 Myr age 142Nd–143Nd isochron for the Earth intersects μ142Nd = 0 at ε143Nd of +8 ± 2 providing a minimum ε143Nd for the μ142Nd = 0 Earth. The high ε143Nd of the μ142Nd = 0 Earth is confirmed by the Nd isotope systematics of Archean mantle-derived rocks that consistently have positive ε143Nd. If the EER formed early after solar system formation (0–70 Ma) continental crust and DMM can be complementary reservoirs with respect to Nd isotopes, with no requirement for significant additional reservoirs. If the EER formed after 70 Ma then the μ142Nd = 0 Earth must have a bulk ε143Nd more radiogenic than DMM and additional high ε143Nd material is required to balance the Nd isotope systematics of the Earth.

Sr-Nd-Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133Ma) 87Sr/86Sr ratios of 0.70538-0.70642, 143Nd/144Nd of 0.51233-0.51218, 206Pb/204Pb of 17.74-18.25, 207Pb/204Pb of 15.51-15.57, and 208Pb/204Pb of 38.18-38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr-Nd-Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γOs values range from+1.0 to+2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred. © 2013 Elsevier Ltd.

Investigation of Mantle Dynamics from Platinum Group Elements and Rhenium-Osmium Isotope Systematics of Mantle Xenoliths from Oahu, Hawaii

Intraplate volcanism that has created the Hawaiian-Emperor seamount chain is generally thought to be formed by a deep-seated mantle plume. While the idea of a Hawaiian plume has not met with substantial opposition, whether or not the Hawaiian plume shows any geochemical signal of receiving materials from the Earth’s Outer Core and how the plume may or may not be reacting with the overriding lithosphere remain debatable issues. In an effort to understand how the Hawaiian plume works I report on the first in-situ sulfides and bulk rock Platinum Group Element (PGE) concentrations, together with Os isotope ratios on well-characterized garnet pyroxenite xenoliths from the island of Oahu in Hawaii. The sulfides are Fe-Ni Monosulfide Solid Solution and show fractionated PGE patterns. Based on the major elements, Platinum Group Elements and experimental data I interpret the Hawaiian sulfides as an immiscible melt that separated from a melt similar to the Honolulu Volcanics (HV) alkali lavas at a pressure-temperature condition of 1530 ± 100OC and 3.1±0.6 GPa., i.e. near the base or slightly below the Pacific lithosphere. The 187Os/188Os ratios of the bulk rock vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratio strongly correlates with major element, High Field Strength Element (HFSE), Rare Earth Element (REE) and PGE abundances. These correlations strongly suggest that PGE concentrations and Os isotope ratios reflect primary mantle processes. I interpret these correlations as the result of melt-mantle reaction at the base of the lithosphere: I suggest that the parental melt that crystallized the pyroxenites selectively picked up radiogenic Os from the grain boundary sulfides, while percolating through the Pacific lithosphere. Thus the sampled pyroxenites essentially represent crystallized melts from different stages of this melt-mantle reaction process at the base of the lithosphere. I further show that the relatively low Pt/Re ratios of the Hawaiian sulfides and the bulk rock pyroxenites suggest that, upon ageing, such pyroxenites plus their sulfides cannot generate the coupled 186Os- 187Os isotope enrichments observed in Hawaiian lavas. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt-Re-Os isotope systematics of plume-derived lavas.

Controls of mantle potential temperature and lithospheric thickness on magmatism in the North Atlantic Igneous Province.

FUNDING This study was funded by University of Aberdeen. SUPPLEMENTARY DATA Supplementary data for this paper are available at Journal of Petrology online. ACKNOWLEDGEMENTS The authors wish to thank Claude Herzberg, Estaban Gazel and an anonymous reviewer for thoughtful and constructive reviews.

Timing and source constraints on the relationship between mafic and felsic intrusions in the Emeiashan large igneous province

Several I- and A-type granite, syenite plutons and spatially associated, giant Fe–Ti–V deposit-bearing mafic ultramafic layered intrusions occur in the Pan–Xi(Panzhihua–Xichang) area within the inner zone of the Emeishan large igneous province (ELIP). These complexes are interpreted to be related to the Emeishan mantle plume. We present LA-ICP-MS and SIMS zircon U–Pb ages and Hf–Nd isotopic compositions for the gabbros, syenites and granites from these complexes. The dating shows that the age of the felsic intrusive magmatism (256.2 ± 3.0–259.8 ± 1.6 Ma) is indistinguishable from that of the mafic intrusive magmatism (255.4 ± 3.1–259.5 ± 2.7 Ma) and represents the final phase of a continuous magmatic episode that lasted no more than 10 Myr. The upper gabbros in the mafic–ultramafic intrusions are generally more isotopically enriched (lower eNd and eHf) than the middle and lower gabbros, suggesting that the upper gabbros have experienced a higher level of crustal contamination than the lower gabbros. The significantly positive eHf(t) values of the A-type granites and syenites (+4.9 to +10.8) are higher than those of the upper gabbros of the associated mafic intrusion, which shows that they cannot be derived by fractional crystallization of these bodies. They are however identical to those of the mafic enclaves (+7.0 to +11.4) and middle and lower gabbros, implying that they are cogenetic. We suggest that they were generated by fractionation of large-volume, plume-related basaltic magmas that ponded deep in the crust. The deep-seated magma chamber erupted in two stages: the first near a density minimum in the basaltic fractionation trend and the second during the final stage of fractionation when the magma was a low density Fe-poor, Si-rich felsic magma. The basaltic magmas emplaced in the shallowlevel magma chambers differentiated to form mafic–ultramafic layered intrusions accompanied by a small amount of crustal assimilation through roof melting. Evolved A-type granites (synenites and syenodiorites) were produced dominantly by crystallization in the deep crustal magma chamber. In contrast, the I-type granites have negative eNd(t) [-6.3 to -7.5] and eHf(t) [-1.3 to -6.7] values, with the Nd model ages (T Nd DM2) of 1.63-1.67 Ga and Hf model ages (T Hf DM2) of 1.56-1.58 Ga, suggesting that they were mainly derived from partial melting of Mesoproterozoic crust. In combination with previous studies, this study also shows that plume activity not only gave rise to reworking of ancient crust, but also significant growth of juvenile crust in the center of the ELIP.

Decompression melting driving intraplate volcanism in Australia: Evidence from magnetotelluric sounding

A long-period magnetotelluric (MT) survey, with 39 sites covering an area of 270 by 150 km, has identified melt within the thinned lithosphere of Pleistocene-Holocene Newer Volcanics Province (NVP) in southeast Australia, which has been variously attributed to mantle plume activity or edge-driven mantle convection. Two-dimensional inversions from the MT array imaged a low-resistivity anomaly (10-30Ωm) beneath the NVP at ∼40-80 km depth, which is consistent with the presence of ∼1.5-4% partial melt in the lithosphere, but inconsistent with elevated iron content, metasomatism products or a hot spot. The conductive zone is located within thin juvenile oceanic mantle lithosphere, which was accreted onto thicker Proterozoic continental mantle lithosphere. We propose that the NVP owes its origin to decompression melting within the asthenosphere, promoted by lithospheric thickness variations in conjunction with rapid shear, where asthenospheric material is drawn by shear flow at a "step" at the base of the lithosphere.

Neoarchean continental growth through arc magmatism in the Nilgiri Block, southern India

The formation and growth of continental crust in the Archean have been evaluated through models of subduction-accretion and mantle plume. The Nilgiri Block in southern India exposes exhumed Neoarchean lower crust, uplifted to heights of 2500 m above sea level along the north western margin of the Peninsula. Major lithologies in this block include charnockite with or without garnet, anorthosite-gabbro suite, pyroxenite, amphibolite and hornblende-biotite gneiss (TTG). All these rock types are closely associated as an arc magmatic suite, with diffuse boundaries and coeval nature. The charnockite and hornblende-biotite gneisses (TTG) show SiO2 content varying from 64 to 73 wt.%. The hornblende-biotite gneisses (TTG) are high-Al type with Al2O3 >15 wt.% whereas the charnockites show Al2O3 <15 wt.%. The composition of charnockite is mainly magnesian and calcic to calc-alkaline. The mafic-ultramafic rocks show composition close to that of tholeiitic series. The low values of K(2)o (<3 wt.%), (K/Rb)/K2O (<500), Zr/Ti, and trace element ratios like (La/Yb)n/(Sr/Y), (Y/Nb), (Y + Nb)/Rb, (Y+Ta)/Rb, Yb/Ta indicate a volcanic arc signature for these rocks. The geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction. The primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements (LILE) and comparable high field strength elements (HFSE) in charnockite and hornblende-biotite gneisses (TTG) consistent with subduction-related origin. Primitive mantle normalized REE pattern displays an enrichment in LREE in the chamockite and hornblende-biotite gneisses (TTG) as compared to a flat pattern for the mafic rocks. The chondrite normalized REE patterns of zircons of all the rock types reveal cores with high HREE formed at ca. 2700 Ma and rims with low HREE formed at 2500-2450 Ma. Log-transformed La/Th-Nb/Th-Sm/Th-Yb/Th discrimination diagram for the mafic and ultramafic rocks from Nilgiri displays a transition from mid-oceanic ridge basalt (MORB) to island arc basalt (IAB) suggesting a MORB source. The U-Pb zircon data from the charnockites, mafic granulites and hornblende-biotite gneisses (TTG) presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700-2500 Ma. Together with the recent report on Neoarchean supra-subduction zone ophiolite suite at its southern margin, the Nilgiri Block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the Neoarchean. (c) 2014 Elsevier B.V. All rights reserved.

Arquitetura crustal da bacia de Almada no contexto das bacias da margem lesste da América do Sul

A Bacia de Almada, localizada no estado da Bahia, compartilha características similares com as outras bacias da margem leste do Brasil, quando é analisada segundo aspectos como os processos sedimentares e o regime de esforço dominante durante a sua formação. Observa-se uma diferença marcante em relação as outras bacias quando é analisada sob a ótica da composição da crosta transicional, uma vez que não se registra atividade vulcânica durante a fase rifte. A aquisição de um extenso levantamento sísmico 3D, com cabos de 6 km de comprimento e 9.2 segundos de tempo de registro (tempo sísmico duplo), resultaram em imagens sísmicas de boa qualidade das estruturas profundas do rifte. Adicionalmente, estudos de modelagem gravimétrica foram integrados com a análise sísmica para corroborar o modelo geológico. A Bacia de Almada é parte dos sistemas de rifte continentais, desenvolvidos durante o Berriasiano até o Aptiano, que antecederam a quebra do continente do Gondwana, evoluindo posteriormente para uma margem passiva divergente. O processo do rifteamento desenvolveu cinco sub-bacias de orientação NNE-SSO, desde posições terrestres até marinhas profundas, produzindo um arcabouço estrutural complexo. Os perfis da sísmica profunda mostram o afinamento progressivo da crosta continental até espessuras da ordem de 5 km, abaixo da sub-bacia mais oriental, com fatores de estiramento crustal próximo a 7 antes do desenvolvimento de crosta oceânica propriamente dita. As imagens sísmicas de boa qualidade permitem também o reconhecimento de sistemas de falhas lístricas que se iniciam na crosta superior, evoluem atravessando a crosta e conectando as sub-bacias para finalizar em um descolamento horizontal na crosta inferior estratificada. Adicionalmente, a bacia apresenta um perfil assimétrico, compatível com mecanismos de cisalhamento simples. As margens vulcânicas (VM) e não vulcânicas (NVM), são os extremos da análise composicional das margens divergentes continentais. Na Bacia de Almada não se reconhecem os elementos arquiteturais típicos das VM, tais como são as grandes províncias ígneas, caracterizadas por cunhas de refletores que mergulham em direção ao mar e por intenso vulcanismo pré- e sin-rifte nas bacias. Embora a margem divergente do Atlântico Sul seja interpretada tradicionalmente como vulcânica, o segmento do rifte ao sul do Estado da Bahia apresenta características não-vulcânicas, devido à ausência destes elementos arquiteturais e aos resultados obtidos nas perfurações geológicas que eventualmente alcançam a seqüência rifte e embasamento. Regionalmente a margem divergente sul-americana é majoritariamente vulcânica, embora a abundância e a influência do magmatísmo contemporâneo ao rifte seja muito variável. Ao longo da margem continental, desde a Bacia Austral no sul da Argentina, até a Bacia de Pernambuco no nordeste do Brasil, podem ser reconhecidos segmentos de caráter vulcânico forte, médio e não vulcânico. Nos exemplos clássicos de margens não vulcânicas, como a margem da Ibéria, a crosta transicional é altamente afinada podendo apresentar evidências de exumação de manto. Na Bacia de Almada, a crosta transicional apresenta importante estiramento embora não haja evidências concretas de exumação de manto. Os mecanismos responsáveis pela geração e intrusão dos grandes volumes de magma registrados nas margens divergentes são ainda sujeitos a intenso debate. Ao longo da margem divergente sul-americana há evidências da presença dos mecanismos genéticos de estiramento litosférico e impacto de plumas. Alternativamente estes dois mecanismos parecem ter tido um papel importante na evolução tectônica da margem sudeste e sul, diferenciando-as da margem continental onde foi implantada a Bacia de Almada.

Estudo do magmatismo máfico de complexos alcalinos do sudeste do Brasil

Nesta dissertação foram estudas rochas máficas dos complexos alcalinos de Morro de São João, Rio Bonito, Tanguá, Gericinó-Mendanha, Morro Redondo, Itatiaia e Passa Quatro. Essas rochas ocorrem na forma de diques e/ou sills. As amostras coletadas foram classificadas como lamprófiros, fonolitos, gabros e diabásios alcalinos. A análise geoquímica permitiu identificar um trend fortemente insaturado e um trend moderadamente alcalino para os complexos estudados. O primeiro é caracterizado por foiditos e fonolitos como membros parentais e mais evoluídos, respectivamente, enquanto o segundo tem basaltos alcalinos como membros parentais e traquitos como os mais evoluídos. Todas as amostras plotam no campo da série alcalina, sendo majoritariamente miaskíticas, sódicas ou potássicas. Adicionalmente, o estudo geoquímico indicou que os complexos alcalinos representam câmaras magmáticas distintas, onde diferentes processos evolutivos tiveram lugar. As modelagens apontaram dois processos de diferenciação distintos nos complexos estudados. Os complexos alcalinos de Morro de São João, Morro Redondo, Gericinó-Mendanha e Itatiaia estariam relacionados a processos de cristalização fracionada. Por outro lado, o Complexo Alcalino de Passa Quatro teria sido diferenciado por processos de cristalização fracionada com esvaziamento e posterior reabastecimento da câmara magmática (RTF). De um modo geral, esses modelos indicaram a presença de mais do que uma série magmática nos complexos estudados e a não cogeneticidade entre as séries agpaíticas e miaskíticas. A discriminação de fontes foi feita com base na análise dos elementos terras raras das amostras parentais de cada um dos complexos (gabro em Morro de São João e lamprófiro nos demais). No entanto, este procedimento não foi aplicado para o Complexo Alcalino de Morro Redondo, uma vez que todas as suas amostras apresentaram valores de MgO muito abaixo do típico para líquidos parentais. O líquido parental do Complexo Alcalino do Gericinó-Mendanha apresentou razões de La/Yb e La/Nb, maior e menor que a unidade, respectivamente, típicas de derivação a partir fontes férteis. Os líquidos parentais dos outros complexos alcalinos tiveram suas razões La/Yb e La/Nb maiores que a unidade, típicas de derivação a partir de fontes enriquecidas. Os modelos desenvolvidos revelaram que os líquidos parentais de cada um dos complexos estudados estariam relacionados a fontes lherzolíticas com granada residual. Além disso, a fusão parcial destas fontes teria ocorrido num intervalo de 1 a 7% dentro da zona da granada. Finalmente, as modelagens petrogenéticas elaboradas permitiram a proposição de um cenário geodinâmico, envolvendo a descompressão adiabática do manto litosférico e sublitosférico anomalamente aquecidos. As características geoquímicas dos líquidos parentais parecem ter sido controladas essencialmente pela mistura desses dois tipos de fontes.

Petrografia, litogeoquímica e datação Ar-Ar dos montes submarinos e dos rochedos de Martim Vaz - Cadeia Vitória-Trindade

Este trabalho de mestrado estudou a ilha de Martin Vaz e cinco montes submarinos da Cadeia Vitória-Trindade Columbia, Dogaressa, Davis, Jaseur e Montague. Martin Vaz é um conjunto de ilhas formado pela ilha principal - Martin Vaz, duas ilhotas íngremes e inacessíveis - a Ilha do Norte e a Ilha do Sul além de vários rochedos menores, como o Rochedo Agulha, espalhados a 48 km a leste de Trindade, perfazendo uma área total de 0,3 km. Martin Vaz, assim como os montes submarinos, pertencem a chamada Cadeia Vitória-Trindade, estão inseridos em um contexto tectônico regional cujo trend W-E sugere representar o track da pluma mantélica de Trindade quando da passagem da Placa Sul Americana sobre ela desde o Terciário (CROUGH et al., 1980; OCONNOR & DUNCAN, 1990, GIBSON et at., 1997). A petrografia das amostras de Martin Vaz indica haver basanitos parcialmente alteradas, melanocráticas, textura afanítica, porosas, apresentando vesículas em torno de 1,0-5,0 milímetros. Apresenta fenocristais de piroxênio além de alguns fenocristais de olivina verde-oliva translúcido variando de 1,0-3,0 milímetros. A ilha principal apresenta também diques e necks fonolíticos apresentando matriz microlítica alterada, orientada, de cor verde apresentando minerais ripiformes de cor branca (feldspato alcalino) e outros de cor violácea (titanoaugita) além de pequenos opacos. Pequenos fenocristais de aegerina-augita fortemente pleocroica, alguns apresentando geminação simples, por vezes zonado, apresenta extinção variando de c ∧ α ou X = 23 a 33 (medida de 10 grãos). Biotita laranja amarronzada com textura poiquilítica (1,0 mm), minúsculos cristais euédricos de titanita (raros), além de cristais pseudohexagonais isotrópicos alterados de analcita e carbonatos. As amostras utilizadas neste trabalho de mestrado possuem valor mínimo de 33.91 % SiO2 (TRIM-01D) e máximo de 52,2 (MVA-01) variando de ultrabásicas a básicas. Através da análise dos óxidos SiO2 e MgO é possível distinguir dois grupos de rochas para Martin Vaz: um ultramáfico magnesiano (<42% SiO2 e >7% MgO) e um básico (>45% SiO2) e, para os montes submarinos, dois grupos: um ultramáfico magnesiano (>9% MgO <42% SiO2) e um básico (>45% SiO2 e com valores de MgO em torno de 4%). As análises de Ar-Ar para as quatro amostras de Martin Vaz apresentam idades para o derrame de basanito variando de 320366 Ka (MVA-10) à 623127 Ka (MVA-04). A única amostra datada representando do dique de fonólito é a MVA-05B e obteve idade de 64984 Ka, indicando ser contemporânea ao derrame basanítico.

Tectonics in the northwestern West Philippine basin

The West Philippine basin (WPB) is a currently inactive marginal basin belonging to Philippine Sea plate, which has a complex formation history and various crust structures. Based on gravity, magnetic and seismic data, the tectonics in West Philippine basin is characterized by amagma spreading stage and strike slip fractures. NNE trending Okinawa-Luzon fracture zone is a large fracture zone with apparent geomorphology and shows a right-handed movement. The results of joint gravity-magnetic-seismic inversion suggest that the Okinawa-Luzon fracture zone has intensive deformation and is a transform fault. Western existence of the NW trending fractures under Ryukyu Islands Arc is the main cause of the differences between south and north Okinawa Trough. The Urdaneta plateau is not a remained arc, but remnant of mantle plume although its lava chemistry is similar to oceanic island basalt (OIB).

Major element, trace element, and Sr, Nd and Pb isotope studies of Cenozoic basalts from the South China Sea

The whole rock K-Ar ages of basalts from the South China Sea basin vary from 3.8 to 7.9 Ma, which suggest that intra-plate volcanism after the cessation of spreading of the South China Sea (SCS) is comparable to that in adjacent regions around the SCS, i.e., Leiqiong Peninsula, northern margin of the SCS, Indochina block, and so on. Based on detailed petrographic studies, we selected many fresh basaltic rocks and measured their major element, trace element, and Sr-Nd-Pb isotope compositions. Geochemical characteristics of major element and trace element show that these basaltic rocks belong to alkali basalt magma series, and are similar to OIB-type basalt. The extent of partial melting of mantle rock in source region is very low, and magma may experience crystallization differentiation and cumulation during the ascent to or storing in the high-level magma chamber. Sr-Nd-Pb isotopic data of these basaltic rocks imply an inhomogeneous mantle below the South China Sea. The nature of magma origin has a two end-member mixing model, one is EM2 (Enriched Mantle 2) which may be originated from mantle plume, the other is DMM (Depleted MORB Mantle). Pb isotopic characteristics show the Dupal anomaly in the South China Sea, and combined with newly found Dupal anomaly at Gakkel ridge in Arctic Ocean, this implies that Dupal anomaly is not only limited to South Hemisphere. In variation diagrams among Sr, Nd and Pb, the origin nature of mantle below the SCS is similar to those below Leiqiong peninsula, northern margin of the SCS and Indochina peninsula, and is different from those below north and northeast China. This study provides geochemical constraints on Hainan mantle plume.