Chemical and Isotopic Characteristics of Gabbroic Xenoliths from Hannuoba,China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution.     

Study on the Sr,Nd,Pb and O Isotopes of Basic Dyke Swarms in the Wudang Block and Basic Volcanics of the Yaolinghe Group

The Sr,Nd and Pb isotopic characteristics of the Wudang basic dyke swarms and basic volcanics of the Yaolinghe Group show that they were derived from the same multi-component mixing source in the mantle.The Wudang basic dyke swarms have(^87Sr/^86Sr)i=0.6905-0.7061,εNd(t)=-1.9-5.0,△^208Pb/^204Pb=35.49-190.26,△^207Pb/^204Pb=Th/Ta and a wide range of La/Yb ratios;and the basic volcanics of the Yaolinghe Group have(^87Sr/^86Sr)i=0.6487-0.7075,εNd(t)=0.11-3.94，△^208Pb/^204Pb=-81.58-219.95,△^207Pb/^204Pb=4.44-16.68and higher Th/Ta and La/Yb ratios,indicating that their source is a mixture of DM and EMⅡ,and the basic volcanics of the Yaolinghe Group were contaminated by crust materials en rout to the surface.Based on the geochemical features of continental tholeiitic basalts and being products of differen tacies derived from the same source,it can be concluded that an important rifting event in the South Qinling basement block occurred during Neoproterozoic,followed by a setting of oceanic basic in the Early Paleozoic.     

On the Relationship between Mantle Metasomatism and Partial Melting-Evidence from Mantlederived Lherzolite Xenoliths from Nueshan,China

Studies of the mantle-derived iherzolites from Nushan show that in addition to CO2,there were present H2O and small amounts of CO, CH4, SO2,Cl and F in the initial mantlc fluids derived fron the asthenospheric mantlc plumc .The imitial fluids accumulated in some regions of the mantle, resulting in lowering of the mantle solidus (and liquidus) and partial melting of the upper mantle. Melts formed from low-degree of fluid-involved partial melting of the upper mantle would be highly enriched in incompatible elements.Fluies and melts are allthe metasomatic agents for mantle metasomatism, and the interaction between them and the depleted mantle could result in the substan-tial local enrichment of LREE and incompatible elements in the latter.In case that the concentration of H2O in the fluids (and melts) is lower ,only cryptic metasomatism would occur, in case that the concentration of H2O is higher,the degree of partial melting would be higher and hydrous metasomatic phases(e.g. amphiboles )would nucleate. Under such circumstances, there would occur model metasomatism.     

Geochronology of and Nd—Sr—Pb Isotopic Evidence for Mantle Source in the Ancient Subduction Zone beneath Sanshui Basin,Guangdong Province,China

The available petrochemical data indicate that volcanic rocks in the Sanshui basin of Guangdong Prov-ince are characteristic of the island-arc tholeiite-calc-alkaline series.Their K-Ar ages range from 43 to 64 Ma,corresponding to Early Tertiary.The Nd-Sr-Pb isotopic correlations give two binary mixing trends,indicating a mixed source of Leiqiong depleted mantle,seawater and altered sediments pertaining to subduction.Our studies have confirmed the existence of an ancient subduction zone beneath the South Chi-na continent,which is considered to have resulted from the suturing of various terrains in southern China.     

Sr,Nd,Pb Isotope Geochemistry and Magma Evolution of the Potassic Volcanic Rocks,Wudalianchi,Northeast China

Wudalianchi volcanic rocks are the most typical Cenozoic potassic volcanic rocks in easten China.Compositional comparisons between whole rocks and glasses of various occurrences indicate that the magma tends to become rich in silica and alkalis as a result of crystal differentiation in the course of evolu-tion.They are unique in isotopic composition with more radiogenic Sr but less radiogenic Pb.^87Sr/^86Sr is higher and ^143Nd/^144Nd is lower than the undifferentiated global values.In comparison to continental pot-ash volcanic rocks,Pb isotopes are apparently lower.These various threads of evidence indicate that the rocks were derived from a primary enriched mantle which had not been subjected to reworking and shows no sign of incorporation of crustal material.The correlation between Pb and Sr suggests the regional heterogeneity in the upper mantle in terms of chemical composition.     

相山火山—侵入杂岩Nd—Sr—Pb同位素地球化学特征

对相山火山－侵入杂岩Nd,Sr,Pb同位素组成及其底变质岩Pb同位素组成的研究表明，相山两旋回火山岩及火山期后的次火山岩具有较低的εNd(t)值（－7．46－9．40），较高的Isr值（0．70801－0．71201）和较古老的Nd模式年龄（1．54－1．70Ga)，且相对富集放射成因铅（^206Pb/^204Pb,^207Pb/^204Pb,^208Pb/^204Pb分别为17．686-18.323,15.523-15.730,38.143-38.936)。相山火山－侵入杂岩与该区出露的基底变质沉积岩在Nd,Pb同位素组成上既有明显的相似性，又有一定差别，因此，相山火山－侵入杂岩的源区主要为地壳岩石，但并不排除有部分幔源组分介入。ε     

大别山麻粒岩和TTG片麻岩的Sr,Nd,Pb同位素地球化学

北大别4个麻粒岩和4个TTG片麻岩样品的Sr,Nd同位素分析结果表明，样品普遍具有较高的锶同位素比值（^87Sr/^86Sr=0.7066-0.7461)，较低的Nd同位素比值（^143Nd/^144Nd=0.5108-0.5124)，表现出明显的壳源特征，结合铅同位素的组成来看，北大别麻粒岩及TTG片麻岩的同位素成分相当于－下地壳。岩石的特源为古老的地壳，大约在2－3Ga之间，少数样品的同位素特征可能暗示源区有年轻地壳物质的加入。     

张家口水泉沟正长岩杂岩体成因的REE和Sr、Nd、Pb同位素证据

水泉沟杂岩体位于尚义 赤城断裂的南侧,侵位于太古宙桑干群变质岩中。岩体形成于晚加里东至早海两期。其稀土元素含量为8.543×10~(-6)～211.6×10~(-6),随着岩石的CaO、MgO、FeO、Fe_2O_3含量降低,SiO_2、K_2O、Na_2O含量增大,稀土含量减小,稀土分布模式由右倾直线型变为近平直的“～”型。在铅同位素构造模式图上杂岩体落在地幔铅同位素演化线附近。在ε_(Nd)-ε-(Sr)图解上处于地幔演化线的下方。杂岩体的钕模式年龄低于围岩桑干群的形成年龄。岩体可能来源干上地幔与下地壳太古宙变质杂岩混熔作用所形成的正长岩岩浆。     

陕西金堆城钼矿区花岗岩Sr、Nd、Pb同位素特征及其地质意义

通过对陕西金堆城钼矿区花岗斑岩体和八里坡斑岩体进行地球化学测试,测得金堆城斑岩体的SiO2含量为72.89%～74.06%,MgO为0.07%～0.3%,稀土总量为43.29×10-6～93.94×10-6,稀土元素配分曲线呈右倾型,具有明显的铕负异常和弱的负铈异常(δEu为0.43～0.78,δCe为0.74～0.86),富集大离子亲石元素K、Rb、U、Th和Sr等元素,亏损Ba、P和Ti等元素。八里坡斑岩体的SiO2含量为69.87%～70.80%,Al2O3 14.93%～15.46%,MgO 0.28%～0.48%,Sr/Y比值大于60,稀土总量为125.23×10-6～139.63×10-6,铕为无异常或微弱的正异常(δEu为0.98～1.04),铈为微弱的负异常(δCe为0.92～0.96),富集Ba、U、K等大离子亲石元素,而亏损P、Ta和Ti等元素。金堆城斑岩体和八里坡斑岩体的岩石类型为I型花岗岩,Pb同位素显示金堆城斑岩体的Pb主要来自下地壳,但有地幔物质的加入,八里坡斑岩体的Pb主要来自下地壳。金堆城斑岩体的ε(Nd,t)值为较低负值(-13.8～-15.2),但ε(Sr,t)变化较大,为-46.4～13.6,八里坡斑岩体具有负低ε(Nd,t)值(-20.4)和正高ε(Sr,t)值(64.5～65.2)。金堆城花岗斑岩和八里坡花岗斑岩的Sr、Nd、Pb同位素与华北地块相似,这两个岩体的源区为华北地块组成部分。     

青海南部二叠纪火山岩Sr、Nd、Pb同位素特征及地质意义

对青海南部治多-杂多一带二叠纪火山岩进行了全岩Sr、Nd、Pb同位素分析, 样品的I_Sr=0.70333~0.70417, ε_Nd(t) =4.1~5.5, 206Pb/204Pb=17.547~17.986, 207Pb/204Pb=15.476~15.584, 208Pb/204Pb=37.778~37.976.结合岩石学、地球化学研究表明, 该套火山岩为OIB型, 形成于板内伸展拉张的裂谷环境, 属于青藏高原及邻区晚古生代裂谷系统, 原始岩浆可能来自亏损的古软流圈, 裂解机制与古地幔柱活动有关.样品铅同位素的V₁=31.39~46.78, V₂=22.38~42.45, 落入扬子铅同位素省区域, 表明早二叠世青海南部地区具亲扬子性, 揭示出昌都地块在晚古生代属于特提斯区, 为泛华夏陆块群或扬子板块的一个组成部分.      

汉诺坝玄武岩中地幔岩捕掳体REE和Sr,Nd同位素地球化学

本文报道汉诺坝玄武岩中地幔岩捕掳体的ＲＥＥ丰度和Ｓｒ、Ｎｄ同位素组成。不同岩石类型的ＲＥＥ配分模式和同位素组成反映地幔部分熔融程度和交代作用过程。二辉橄榄岩亏损轻稀土,是原始地幔经不同程度部分熔融的残留体。方辉橄榄岩具Ｕ型ＲＥＥ配分模式,是强烈亏损的地幔岩被熔体非化学平衡交代的结果。二辉岩脉状体富轻、中稀土,它同与脉状体接触的二辉橄榄岩可达化学平衡或近于化学平衡,而二辉岩脉状体的形成与玄武岩岩浆无成因关系。据对二辉岩脉状体和不含脉状体橄榄岩的Ｓｍ－Ｎｄ同位素定年,这种脉状体形成于３００Ｍａ左右。     

Metallogenic Province Derived from Mantle Sources: Nd, Sr, S and Pb Isotope Evidence from the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the most important regions for Cu, Au and polymetallic and rare metallic (Li, Be, Nb, Ta) mineralization over the world. Most of the ore deposits in the CAOB are closely associated with granitoids. Available Sr, Nd, S and Pb isotopic data indicate that the metallogenic epoch and sources of the mineral deposits in the CAOB are consistent with that of the regional granites. Available data suggest that mantle sources could have played an important role in the Paleozoic to Mesozoic mineralization in the CAOB.     

峨眉山玄武岩Sr、Nd、Pb同位素特征及其物源探讨

选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究.结果表明, 少数样品的Pb同位素与Hanan和Graham定义的C组分相似, 而大多数样品则不在C组分范围之内, 说明除地幔柱物质外, 有岩石圈物质的加入.在多元同位素图解上, 峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间, 表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释.结合已有的微量元素资料分析, 其中的地壳组分主要为下地壳, 而早期玄武质岩浆在上升过程中由于通道不畅通, 有较多的上地壳组分的混染.岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关.粗面岩的同位素组成和玄武岩接近, 说明粗面岩是玄武质岩浆分离结晶作用形成的.      

Characteristics of rare-earth elements （REE）, strontium and neodymium isotopes in hydrothermal fluorites from the Bailashui tin deposit in the Furong ore field,southern Hunan Province,China

In this paper the authors present the REE concentrations and Sr and Nd isotopic compositions of fluorites from the Bailashui tin deposit of the Furong ore field, southern Hunan Province. The results showed that the total amount of REE in fluorites is usually low, ranging from 0.705 to 8.785 μg/g with the chondrite-normalized REE distribution patterns similar to those of the Qitianling granites in the study area, characterized by LREE-enrichment patterns with pronounced negative Eu anomalies. The fluorites vary in Sr isotopic composition within the range of 0.7083-0.7091, the values are lower than those of the granites and higher than those of the host carbonate rocks in this area. The εNd（t） values of fluorites vary between -9.4 and ＋10.3, revealing that both the crust- and mantle-source materials were involved in the ore-forming hydrothermal fluids. Combined with previous studies on this ore deposit, the Bailashui tin deposit is temporally and spatially closely related with granitic magmatism in this area. The hydrothermal fluorites are the product of fluid/rock interactions between granitic magmatic hydrothermal fluid and marine carbonate rocks. The REE and F in the ore-forming fluid were derived from the granites, whereas Sr in the ore-forming fluid came mainly from the granitic magmatic hydrothermal fluid and marine carbonate rocks, although variations in Sr isotopic composition cannot be explained by a simple mixture of these two end-members. Evidence demonstrated that the ore-forming fluids are of crustal-mantle mixing origin, but that the fluids were probably incompletely homogenized and this may be caused by inhomogeneous mixing of the fluids of different sources.     

Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic–Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP.

In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO₂ (47–50%), high TiO₂ (2.5–3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb)_n=1.5–5.1). Their isotopic signature and ratios of very incompatible elements (εNd_i=+5.8 to +4.2, (⁸⁷Sr/⁸⁶Sr)_i=0.703–0.705, (²⁰⁷Pb/²⁰⁴Pb)_i=15.46–15.64) match a depleted PREMA (prevalent mantle)-like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5–10%) or a slight upper crustal assimilation–fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO₂ (51–58%), low TiO₂ (0.7–1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb–Ta anomalies. Isotopic signatures (εNd_i=+0.4 to −5.3, (⁸⁷Sr/⁸⁶Sr)_i=0.705–0.710, (²⁰⁷Pb/²⁰⁴Pb)_i=15.57–15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma–upper crust interaction through an AFC process (r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample.

This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust.     

南极乔治王岛中——新生代岩浆岩Sr—Nd—Pb同位素组成及源区特征

对乔治王岛１１个火山岩和２个侵入岩样品做了Ｓｒ－Ｎｄ－Ｐｂ同位素分析，其中（８７Ｓｒ／８６Ｓｒ）ｉ＝０．７０３２６～０．７０３９２，εＮｄ＝３．０２～６．７２，２０６Ｐｂ／２０４Ｐｂ＝１７．７７６～１８．５１５，２０７Ｐｂ／２０４Ｐｂ＝１５．．５０６～１５．５７１，２０８Ｐｂ／２０４Ｐｂ＝３７．８６８～３８．３０８。根据以上同位素组成及其相互关系并结合岩石学和微量元素特征及火山岩中熔融包裹体成分，得出以下结论：（１）研究的乔治王岛岩浆岩起源于亏损地幔（ＤＭＭ）和地幔流体交代成因的富集地幔ＥＭＩ混合形成的源区；（２）该源区具有广义Ｄｕｐａｌ异常，它正是由于ＥＭＩ组份加入即通过流体交代作用而产生的；（３）流体主要是俯冲深海沉积物脱碳酸盐作用而释放出的富ＣＯ２流体。