Basalts and serpentinite from the Puerto Rico Trench, 1. Petrology

Typical abyssal tholeiites occur on both north and south walls of the Puerto Rico Trench. Probably they were erupted in the Mid-Atlantic Ridge in Cretaceous time and afterwards were moved to the present position. In addition, doleritic basalts containing phenocrysts of pigeonite rimmed by augite were found from the north wall, and these rocks presumably represent a new type of abyssal tholeiites characterized by $\text{CaO}\text{Na}{}_2\text{O}$ ratios lower than those of ordinary abyssal tholeiites.A new analysis of serpentinite from the north wall is also recorded here.     

Magmatic Complexes of the Vetlovaya Marginal Sea Paleobasin (Kamchatka): Composition and Geodynamic Setting

This study presents new geochemical and isotope data on igneous rocks of the Vetlovaya marginal sea paleobasin (part of the Late Mesozoic–Cenozoic margin of the northwestern Pacific). The results show that the rock complexes of this marginal sea basin comprise igneous rocks with geochemical compositions similar to those of normal oceanic tholeiites, enriched transitional tholeiites, and ocean island and back-arc basin basalts. Island-arc tholeiitic basalts are present only rarely. The specific geochemical signatures of these rocks are interpreted as being related to mantle heterogeneity and the geodynamic conditions in the basin.     

Characteristics of platinum-group elements in basalts from spreading axis of Mariana Trough

Total platinum-group elements (PGEs) abundances in basalts from the spreading axis of Mariana Trough ranged from 0.418×10~(-9) to 1.022×10~(-9), and primitive mantle-normalized PGE patterns are of positive slope showing the relative enrichment of PPGE (platinum,palladium,rhodium) and gold relative to IPGE. Compared with other mantle-originated rocks, these basalts have lower PGE contents and wider ranges of primitive mantle-normalized ratios of palladium content to iridium one,palladium content to platinum one and palladium content to gold one exhibiting relative platinum and iridium depletion. Characteristics of PGE patterns indicated that the studied Mariana Trough basalts originated from low partial melting, and the MORB mantle beneath the spreading center had been contaminated by the arc-island mantle. In the aspect of trace elements, Mariana Trough basalts showed the enrichment of LILE, lead and LREE, indicating that they had been influenced by subduction compositions. All these demonstrated that Mariana Trough basalts are products of partial melting from a mixed mantle (the contamination of MORB mantle by arc-island mantle).     

南马里亚纳海槽岩石学特征

南马里亚纳海槽岩性比较复杂：有玄武岩，根据标准矿物组合，进一步划分为碱性玄武岩，橄揽拉斑玄武岩和石英拉斑玄武岩；火山碎屑岩，包括凝灰岩和角砾凝灰岩；以及方辉橄榄岩．大部分玄武岩样品是距今五百万年以后形成的，个别岩石样品的形成年龄为八百万年左右．不同类型的玄武岩是含Ｈ２Ｏ的地幔岩块在底辟上升过程中，局部熔融、分离的产物；火山碎屑岩是火山喷发的火山灰、火山砾被水化学分解的物质胶结而成；方辉橄榄岩是地幔岩浆底辟上升直接侵入结晶的产物．     

Peculiarities of the magmatism and tectonics of the Knipovich Ridge

Petrological-geochemical data were obtained for intrusive rocks (gabbroids) recovered on the eastern flank of the Knipovich Ridge by deep-sea site 344 (DSDP, Leg 238). It was found that these rocks are similar to basalts and basaltic glasses studied in the adjacent sections of the ridge rift zone [7, 8]. This indicates that the intrusive rocks and erupted lavas are comagmatic. The gabbroids, basalts, and their quenched glasses were derived by differentiation in different-depth chambers and feeder channels. The petrochemical features of the gabbros and basalts (low Fe content, oxidized magnetic minerals) caused their weak magnetic properties. Owing to the multidirectional movements of the oceanic blocks, the bodies of the intrusive and effusive rocks their lost strict linearity and produced the mosaic anomalous magnetic field of the Knipovich Ridge.     

Rare earth element geochemistry of hydrothermal deposits from Southwest Indian Ridge

The REE compositions of hydrothermal deposits and basalt samples from the Southwest Indian Ridge (SWIR) were determined with ICP-MS.The results show that there are significant differences between different types of samples although all samples show relative LREE enrichment.The contents of REE in hydrothermal sulfides and alterated rocks samples are lower (from 7.036 × 10 6 to 23.660 × 10 6),while those in the white chimney deposits are relatively higher (ranging from 84.496 × 10 6 to 103.511 × 10 6).Both of them are lower than basalts.Chondrite-normalized REE distribution patterns show that sulfides and alterated rocks samples are characterized by significant positive Eu anomalies.On the contrary,white chimney deposits have obvious negative Eu anomalies,which may be caused by abundant calcite existing in the white chimney samples.Both the content and distribution pattern of REE in sulfides suggest that REE most possibly is originally derived from hydrothermal fluids,but influenced by the submarine reducing ore-forming environment,seawater convection,mineral compositions as well as the constraint of mineral crystallizations.     

Peculiarities of the rare-earth element distribution in the modern bottom sediments of the White Sea and the lower reaches of the Severnaya Dvina River

The study of the rare-earth element (REE) systematics in the modern bottom sediments of the White Sea and the lower reaches of the Severnaya Dvina River showed that they were derived by the simple mixing of the detrital material from two geochemically contrasting provenances: the Kola-Karelian geoblock almost completely consisting of Archean rocks and the northwestern Mezen syneclise made up of the Upper Vendian, Paleozoic, and Mesozoic rocks. This is best manifested by the changes in ?_Nd(0). In terms of the Gd_N/Yb_N and Eu/Eu*, most of the studied samples are comparable with the Post-Archean craton complexes, some of which resemble the average composition of the Archean mudstone. Based on the ΣREE and La_N/Yb_N, the modern bottom sediments are subdivided into two groups: (1) those close to basalts and granites and (2) those approximating common sedimentary rocks. From the lower reaches of the Pinega River to the Tersky coast, the maximal average (La/Yb)_RPSC, (Gd/Yb)_RPSC, La_N/Yb_N, and Gd_N/Yb_N ratios were determined in the samples taken at the boundary of Dvina Bay with the Basin, i.e., in the sediments with the highest content of the pelitic component. In general, the geochemical composition of the modern bottom sediments of almost the entire White Sea area was defined by input of the eroded products of the mature continental crust with the Severnaya Dvina River, the main river of the region. Such a setting, when the formation of the sediments in a peri- or intracontinental marine basin is controlled by one large river system, presumably may be propagated to the sedimentation in the Laptev Sea, the eastern Kara Sea, the Bay of Bengal, and other basins.     

西南印度洋脊龙旂热液区蚀变岩岩石学特征及对热液流体循环的指示

慢速?超慢速扩张洋脊的海底热液活动区多出露类型多样的蚀变岩石,记录了地壳深部的流体与围岩的相互作用,为研究深部热液流体特征以及循环过程提供了样本。本研究选取了中国大洋第30、34和40航次在超慢速扩张西南印度洋脊龙旂热液区(A区、B区和C区)利用电视抓斗采集的蚀变玄武岩、蚀变辉长岩、蚀变辉石岩和蛇纹岩等蚀变岩样品,利用光学显微镜、电子探针开展了岩相学和矿物化学分析。岩相学结果表明,龙旂热液区蚀变岩石样品约95%发生了地壳浅部的脆性变形作用,靠近龙旂1号热液区(A区)约有5%的蚀变岩石混合发育了脆性变形及脆性?塑性变形特征。研究区岩石蚀变属于中?低温变质作用,变质相近似绿片岩相,变质矿物组合为绿泥石?绿帘石?钠长石?阳起石?榍石。其中,A区的蚀变岩中的绿泥石形成温度(201～341℃)以及蛇纹石、阳起石、绿泥石等蚀变矿物的Fe元素含量(17.5%～27.5%)都高于龙旂3号热液区(B区和C区)的绿泥石形成温度(239～303℃)和Fe元素含量(16.8%～26.5%),这也与在该区观测到高温的热液喷口相符合。本研究认为龙旂热液区所在洋脊段发育的拆离断层为热液流体的向上运移提供了通道,洋壳扩张后期轴部的岩浆熔体在轴侧区域的岩浆侵入或喷发活动可能为热液循环提供了热源。     

Geochemistry and zircon U-Pb ages of the Oligocene sediments in the Baiyun Sag,Zhujiang River Mouth Basin

In this study, element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag, Zhujiang River Mouth Basin. The experimental results are discussed with respect to weathering conditions, parent rock lithologies, and provenances. The chemical index of alteration and the chemical index of weathering values of mudstone samples from the lower Oligocene Enping Formation indicate that clastic particles in the study area underwent moderate weathering. Mudstone samples exhibit relatively enriched light rare earth elements and depleted heavy rare earth elements, "V"-shaped negative Eu anomalies, and negligible Ce anomalies. The rare earth element distribution curves are obviously right-inclined, with shapes and contents similar to those of post-Archean Australian shale and upper continental crust, indicating that the samples originated from acid rocks in the upper crust. The Hf-La/Th and La/Sc-Co/Th diagrams show this same origin for the sediments in the study area. For the samples from the upper Enping deltas, the overall age spectrum shows four major age peaks ca. 59–68 Ma, 98–136 Ma, 153–168 Ma and 239–260 Ma. For the Zhuhai Formation samples,the overall age spectrum shows three major age peaks ca. 149 Ma, 252 Ma and 380 Ma. The detrital zircon shapes and U-Pb ages reveal that during Oligocene sedimentation, the sediments on the northwestern margin of the Baiyun Sag were supplied jointly from two provenances: Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift, and the former supply became stronger through time. Thus, the provenance of the Oligocene deltas experienced a transition from an early proximal intrabasinal source to a late distal extrabasinal source.     

Rocks from the Gibbs fracture zone and the Minia seamount near 53°N in the Atlantic Ocean

Rocks dredged and drilled from both the rift mountains of the Mid-Atlantic Ridge (Minia seamount) and from the northern wall and the median ridge of the adjacent Gibbs fracture zone near 53°N include tholeiites, serpentinized and mylonitized peridoties, and gabbroic rock.The tholeiites include: (1) pyroxene-tholeiites, commonly without phenocrysts and containing less than 15% Al₂O₃; (2) plagioclase-tholeiites with small (1.1–2.2 mm length) plagioclase phenocrysts and an Al₂O₃ content varying between 14–17%; and (3) high-alumina plagioclase tholeiites with large (> 2.5 mm length) plagioclase phenocrysts and more than 17% Al₂O₃. The apparently transitional differences among the three groups support the possibility that differentiation by crystal fractionation of the high-alumina plagioclase-tholeiites gave rise to the plagioclase-tholeiites with less Al₂O₃ and smaller phenocrysts and to the pyroxene-tholeiites. A small portion of the basalts ampled show effects of low-grade metamorphism. The peridotites may represent evidence of intrusive emplacement of peridotitic material beneath the tholeiitic rocks.     

Petrology, Geochemistry and Nd-Sr-Pb Isotopic Properties of Volcanic Rocks in Daheishan Island, Penglai, Shandong Province

The major elements, trace elements, K-Ar age and Sr-Nd-Pb isotopic systems of the Cenozoic volcanic rocks in Daheishan Island and Cishan, Penglai, Shandong Province are measured. The volcanic rocks ( olivine-nephelinite and nepheline-basanite ) in DaheishanIsland erupted periodically in an interval of 0.32 Ma, from 8.72 Mm 8.39 Ma, 8.08 Ma to 7.73Ma. The volcanic rocks are all rich in light REEs. They are similar to the OIB-type alkali basalt in the trace elements normalized model by primordial mantle: rich in high field elements such as Nb and Ta, and imcompatible elements such as Cs, Rb, Ba, Th, U. The volcanic rocks show a depletion of K and Rb elements. It is suggested by the trace elements that the olivine-nephelinite in Daheishan Island is originated fi‘om deep resources under the continental mantle, ε Nd (0) values of the volcanic rocks in Daheishan Island and Cishan are 5.31 - 8.51 and 7.33 respectively, suggesting that the volcanic rocks are from the depleted mantle resources, which have higher Sm/Nd ratios than the CHUR. ^143Nd /^144Nd ratios of Daheishan Island olivine-nephelinite and Cishan alkali basalts are 0.512 910 - 0.513 074 and 0.513 014 resoeetivelv. The ^87Sr /^86Sr of Daheishan lsland volcanic rocks are lower than that of Cishan, 0.703 427 - 0.703 482 and 0.703 895 respectively. The Daheishan Island olivinenephelinite has the Pb isotopic values as follows: ^206Pb /^204pb= 18.028 9 ～17.972 8,^207Pb /^204Pb= 15,435 8 - 15.402 2 and ^208Pb /^204Pb=38.087 6 - 37.997 5, lower than those of Cishan basanite. The Cishan basanite has ^206Pb / ^204Pb = 18.240 1, ^208Pb /^204Pb = 15.564 5 and ^208Pb /^204Pb=38.535. The authors suggest that the olivine-nephelinite in Daheishan Island is similar to the E-type MORB or Hawaii OIB, and the alkali basalts in Cishan similar to the Kerguelen OIB. The dominant mantle components of DM PREMA and perhaps DM( Dupal type ) are the dominant mantle components for volcanic rocks in Daheishan Island and Cishan. The PREMA component plays an important role.     

马里亚纳海槽扩张轴(中心)玄武岩铂族元素特征

马里亚纳海槽扩张轴中心玄武岩铂族元素(PGE)总量变化范围为0.418×10-9~1.022×10-9,其原始地幔标准化配分模式呈正倾斜型,表现出PPGE(包括Pt,Pd,Rh)和金的相对富集.与其他幔源岩石相比,该区玄武岩PGE总量较低,(Pd/Ir)N,(Pd/Pt)N,(Pd/Ru)N变化较大,表现出铂、铱的相对亏损和明显的铂负异常(Pt/Pt*=0.01~0.15).PGE的分布特征一方面反映了该区地幔熔融度不高,另一方面,大离子亲石元素LILE(铷、锶、钾等)、铅和轻稀土LREE略富集,暗示扩张中心之下的软流圈地幔源受到了岛弧地幔的混染.上述特征反映了马里亚纳海槽玄武岩是MORB型与岛弧型地幔源不同程度混合后部分融熔的产物.     

Bicyclic alkanes in source rocks of the Triassic Yanchang Formation in the Ordos Basin and their inconsistency in oil-source correlation

The Triassic Yanchang Formation is the main source rocks for Mesozoic oil in Ordos Basin. The formation includes 10 oil-bearing beds (Ch 1–Ch 10), that each can be further divided into two to three intervals. Abundant C₁₂–C₁₄ and C₁₅–C₁₆ bicyclic alkanes have been detected in the formation in the Xifeng oilfield, Ordos Basin. The C₁₂–C₁₄ group is dominated by C₁₂ and C₁₃, and the C₁₅–C₁₆ group contains abundant C₁₅. The groups show three distribution patterns: A) the C₁₂–C₁₄ group is the major component in the non-source rocks of the Ch 7-1 and Ch 8-1 intervals; B) both groups are abundant and are common in source rocks of the Ch 7-3 interval; and C) the C₁₅–C₁₆ group is the major component in source rocks of the Ch 7-3 interval and also in sediments that contain type Ⅰ or partial sapropel type Ⅱ₁ organic matter (OM) in the Ch 7-2 and Ch 8-1 intervals. Although thermal maturities of the source rocks in the Ch 7 section are similar, they show significant differences with respect to the drimane isomerisation index, which indicates that the drimane rearrangement is controlled by thermal evolution of the sediments, but may also be closely related to the depositional environment. This study determined that reducing environments are more conducive to preservation of drimane than oxic environments. The drimane isomerisation index and the value of the hopane parameter Tm/Ts are positively correlated. The parameter Tm/Ts varies over a wide range within the sequence, and the large variations may be a result of terrigenous OM input by turbidity currents and/or gravity flows, mixed with the autochthonous sediments. Abundant homodrimane in both source rocks may reflect reducing environments in deep lakes and major input of higher plant OM. Organic-rich shale and oil shale in the Ch 7-3 interval of the Yanchang Formation are the primary sources of oil in reservoirs in the Xifeng area. The crude oil is rich in bicyclic alkanes that are dominated by C₁₅–C₁₆ as source rocks with pattern C for bicyclic alkanes, which indicates an origin mainly from the Ch 7-3 interval. The main peaks in all of the crude oils are associated with 8β(H)-drimane and lower abundance of rearranged drimane. However, most of the source rocks have a main peak associated with 8β(H)-homodrimane or rearranged drimane. Weak microbial action, selective degradation and water washing may be the cause of the significant difference in bicyclic sesquiterpane composition between the crude oil and the source rocks. The result suggests that oil-source correlations based on the bicyclic sesquiterpanes are questionable.     

中太平洋海山富钴结壳与基岩关系的研究

对我国首次取得的中太平洋海山基岩和富钴结壳样品的结构构造、矿物和化学组成特征的研究表明,该区主要基岩类型为碱性玄武岩、磷块岩、磷酸盐化碳酸盐岩和燧石等,碱性玄武岩分布最广,并均有不同程度的风化.观察表明,富钴结壳的载体可以是各种岩性的基岩.玄武岩、风化火山岩和磷块岩上的结壳厚度比燧石的大,有较高的经济价值,是选矿冶炼的主要对象.对于调查区内富钴结壳形态总体上可分为三种类型:板状、砾状和结核状结壳.从以下四个方面探讨结壳与基岩之间的关系:(1)海水、基岩和结壳中主要金属元素含量特点;(2)通过聚类分析方法探讨结壳与基岩之间的亲疏程度;(3)结壳的分布特征;(4)不同基岩类型对结壳生长的影响.分析结果表明,调查区内的火山岩及磷块岩对结壳形成的贡献大(比其他类型岩石).     

雪宝顶碱性花岗岩岩石地球化学与成矿控制

对雪宝顶碱性花岗岩具有高丰度的W、Sn、Be含量和W、Sn矿化花岗岩的微量元素组合特征进行了分析，结果显示：稀土元素具有∑REE含量与LREE／HREE比值偏低和负Eu异常的特点；元素比值K／Rb，U／Th，Nb／Ta的规律性变化以及矿脉产状特征等表明W-(Sn)-Be脉状矿床形成于岩浆演化晚期，与岩浆期后的热液活动有关。     

西南印度洋中脊热液产物稀土元素组成变化及其来源

对西南印度洋中脊热液区不同热液产物稀土元素(REE)进行了分析,探讨了热液产物形成过程中稀土元素组成变化及其来源。研究结果表明:不同热液产物稀土元素总量变化范围从3.47×10-7到4.80×10-5,轻重稀土比值(LREE/HREE)从2.06到6.16,表明轻重稀土有较大程度分异,δEu异常(δEu=0.86~3.88)和δCe异常(δCe=0.40~0.86)显示热液产物中REE呈Eu富集和Ce亏损特征。稀土元素球粒陨石标准化模式呈现两种类型:(1)呈轻微富集LREE的平坦模式,REE大于2×10-5;(2)呈显著富集LREE和正Eu异常模式,REE小于5×10-7。模式1类似于洋壳火山岩REE配分模式,而模式2与西南印度洋中脊黑烟囱REE模式相似,也与典型洋中脊热液喷口流体和硫化物LREE富集和正Eu异常模式类似。热液产物中稀土元素含量变化和模式特征以及Mg与LREE极强正相关关系可能反映了西南印度洋中脊硫化物形成在热液流体与海水混合沉淀的初始阶段,后期经历了广泛的热液流体再循环和海水蚀变过程。     

Litho-stratigraphic effect on Variscan fluid flow within the Prague synform,Barrandian: Evidence based on C,O, Sr isotopes and fluid inclusions

The Palaeozoic sedimentary sequence of the Prague synform (Ordovician–Devonian) in the centre of the Bohemian massif underwent Variscan deformation and thermal overprint events. Variscan veins widespread throughout the sedimentary strata have precipitated from syntectonic aqueous and hydrocarbon-rich fluids. Homogenization temperatures of aqueous inclusions increased from 70 up to 226 °C in the Cambrian rocks underlying the Prague synform. Seawater, modified due to intensive water–rock interaction, was the main fluid component. Fluid flow was limited and restricted to the lithostratigraphic compartments forming a rock-buffered system. Stable isotopic modelling (C, O) and final interpretations of the confined hydrostratigraphic fluid migration was supported by the ⁸⁷Sr/⁸⁶Sr ratio in veins and wall rocks. Siliciclastic Cambrian and Ordovician rocks and the associated intersecting veins yielded similar isotopic signatures, and consequently the fluid migration is restricted to layer- and formation-scales. Gradually downwards increasing fluid temperature and compositional changes reflect burial at oil-window conditions. An open fluid system could be expected in proximity of major faults of the Prague syncline and at the top of the sedimentary sequence.     

慢速扩张洋中脊热液成矿的典型实例——青藏高原北部德尔尼铜矿地质对比研究

青海德尔尼铜矿是发育在青藏高原北部的块状硫化物矿床,其赋矿围岩为阿尼玛卿蛇绿岩套中的超基性岩,代表了古特提斯洋的残骸。通过对德尔尼铜矿的详细地质解剖,认为其保留了大量海底热液喷流的地质记录,包括:矿体上部普遍发育的薄层喷流岩;多孔状硫化物中保留下来的胶状结构、草莓结构和角砾结构;矿石的主要矿物组成;与黄铁矿碎屑同期形成的方解石、长英质胶结物;与大西洋Rainbow、TAG热液硫化物矿床相似的矿体分带性。根据洋脊玄武岩中TiO2全岩含量估算德尔尼蛇绿岩所代表的这一段古特提斯洋洋中脊为慢速-超慢速扩张洋中脊,半扩张速率为1.1~2.5cm/a。类比现今洋中脊热液硫化物成矿过程,认为德尔尼铜矿经历了海底喷流、冷却保存和俯冲侵位等3个阶段,其中海底喷流阶段可能与洋底核杂岩具有成因联系,俯冲侵位过程中的矿体和超基性岩、玄武岩受逆冲断层控制。与世界上其他陆地上保存的类似矿床相比,德尔尼铜矿时代年轻(石炭纪),矿床结构和构造及其围岩蚀变特征保存完整,是慢速-超慢速洋脊超基性围岩热液成矿作用的典型实例,可以称之为德尔尼型成矿作用,研究其成矿过程,对研究和理解现今洋中脊超基性岩系统热液成矿作用(尤其是深部成矿作用)具有重要的意义。     

Marine magnetic anomalies off Ratnagiri,western continental shelf of India

Total magnetic intensity and bathymetric surveys were carried out in the offshore area of Ratnagiri on the western continental shelf of India and an isomagnetic anomalies map at a contour interval of 50 nT was prepared which reveals N-S trends of magnetic anomalies. Two-dimensional model and spectral studies of these anomalies were carried out, and subsurface models of the geology in the area have been derived from anomalies at a number of places. The results suggest that the anomalies occur over a magnetic crystalline basement at a depth of 1–1.2 km which is similar (in magnetization) to onshore basalts of northwest India. These anomalies are believed to be an expression of a considerable thickness (around 1.7 km) of basalt, underlain by sediments. Identification of these basalts in offshore areas along the northwestern continental shelf of India would support (1) the idea that the onshore Deccan basalts of western India and the rhyolitic tuffs at the Laccadive ridge system (DSDP Site 210) are related to the same volcanic events, and (2) subsequent downfaulting of onshore Deccan basalts into the Arabian Sea and submergence below the Tertiary sediments.     

Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous,and lower in High Field Strength Elements (HFSE).These rocks are higher in Large Ion Lithophile Elements (LILE),thorium and uranium contents,positive lead anomalies,negative Nb-Ta anomalies,and enrichment in Light Rare Earth Elements (LREE).Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB),and rhyolites from the northern Okinawa Trough have the highest 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios.The NECS shelf margin basalts have lower 87 Sr/ 86 Sr ratios,ε N d and σ 18 O than the northern Okinawa Trough silicic rocks.According to 40 K– 40 Ar isotopic ages of basalts from the NECS shelf margin,rifting of the Okinawa Trough may have been active since at least 3.65–3.86 Ma.The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere.The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough,and the formation of basaltic magmas closely relates to the thinning of continental crust.The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough,which could have been generated by the interaction of basaltic melt with an enriched crustal component.From the Ryukyu island arc to East China,the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE),suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate,and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.