滇西地区上地幔铅同位素组成的确定及其应用

滇西地区基性－超基性岩、碱性岩中的长石、石榴子石、橄榄石等单矿物及羊拉铜矿区二叠世海相玄武岩的铜铅同位素组成可以代表原始地幔的铅同位素组成，由此确定的该区上地幔铅同位素组成变化范围为^206Pb/^204Pb=17.877-18.506,平均18.108,^207Pb/^204Pb=15.470-15.587,平均15.479,^208Pb/^204Pb=37.797-38.567,平均38．177。研究发现，由于受地壳物质的混染和成岩后U、Th衰变产生的放射成因铅的影响，这些幔源岩石全岩铅同位素组成已不能代表原始上地幔的铅同位素组成；金顶铅锌矿床的铅并非上地幔来源，而是相当于下地壳来源的铅与兰坪盆地沉积岩铅二者的混合铅。     

滇西上地幔铅同位素演化模式及滇西富碱斑岩铅同位素组成的诠释

大量的Pb同位素数据资料表明,滇西上地幔铅具两阶段演化历史,但又不是很好地吻合Stacey-Kramers的两阶段模式。本文依据Stacey-Kramers建造两阶段模式的基本原理,结合滇西地区上地幔铅同位素组成资料,拟合出了能更好地反映滇西上地幔铅同位素演化历史的二阶段模式并应用此模式对滇西富碱斑岩及其相关岩石的铅同位素组成做出诠释。     

下地壳麻粒岩包体研究进展及其意义

产于火山岩中的下地壳麻粒岩包体是人们窥视深部地壳的一个窗口 ,现已成为人们探索大陆下地壳物质组成和演化最直接的研究对象。下地壳岩石包体一般具有麻粒岩相矿物组合 ,主体由镁铁质麻粒岩组成 ,以出现紫苏辉石矿物为标志 ,下地壳包体的平衡温度为 70 0～10 4 0℃ ,平衡压力在 0 .8～ 1.4GPa之间。在绝大多数地区 ,基性下地壳包体占优势 ,其共同特征是低SiO2 、高CaO和Mg# ,过渡族元素和不相容元素含量变化很大 ,不同构造背景和不同岩石类型的包体同位素成分也有差异 ,全球各大陆下地壳包体Nd、Pb同位素组成分布具有“块体效应”特征。利用出露于地表的下地壳麻粒岩包体可以探讨下地壳物质组成和成分 ,建立下地壳物性剖面和岩石圈模型 ,并可作为幔源岩浆底侵作用的重要判据之一。世界范围内下地壳麻粒岩包体研究表明 ,在探讨下地壳的组成特点上 ,由火山岩中麻粒岩包体和出露的麻粒岩地体两类岩石样品所获得的下地壳组成特点存在比较大的差异 ,前者较后者反映的下地壳组成更偏基性。下地壳包体的类型与成因对揭示大陆地壳的增生、演化方式以及壳—幔交换作用具有重要的指示意义     

下地壳麻粒岩包体研究进展及其意义

产于火山岩中的下地壳麻粒岩包体是人们窥视深部地壳的一个窗口，现已成为人们探索大陆下地壳物质组成和演化最直接的研究对象。下地壳岩石包体一般具有麻粒岩相矿物组合，主体由镁铁质麻粒岩组成，以出现紫苏辉石矿物为标志下地壳包体的平衡温度为700－1040℃，平衡压力在0．8－1．4GPa之间。在绝大多数地区，基性下地壳包体占优势，其共同特征是低SiO2、高CaO和Mg^ ，过渡族元素和不相容元素含量变化很大，不机构造背景和不同岩石类型的包体同位素成分也有差异，全球各大陆下地壳包体Nd、Pb同位素组成分布具有“块体效应”特征。利用出露于地表的下地壳麻粒岩包体可以探讨下地壳物质组成和成分，建立下地壳物性剖面和岩石圈模型，并可作为幔源岩浆底侵作用的重要判据之一。世界范围内下地壳麻粒岩包体研究表明，在探讨下地壳的组成特点上，由火山岩中麻粒岩包体和出露的麻粒岩地体两类岩石样品所获得的下地壳组成特点存在比较大的差异，前者较后者反映的下地壳组成更偏基性。下地壳包体的类型与成因对揭示大陆地壳的增生、演化方式以及壳－幔交换作用具有重要的指示意义。     

金顶超大型铅锌矿床的成矿金属来源——来自铅同位素组成的制约

金顶矿床是世界著名的超大型铅锌矿床,其巨量的金属堆积引起许多学者对成矿金属来源的关注。前人通过铅同位素示踪研究,提出了成矿金属来自地幔、上地壳、下地壳及不同端员混合等不同认识。理论分析表明,这些观点认识的差异可能源于不同作者分析铅同位素数据存在测试误差。基于此,笔者在金顶矿床选择了7个代表性硫化物样品,再次进行了铅同位素分析。结果显示,矿床铅同位素组成为~(206)Pb/~(204)Pb=18.3945~18.4429、~(207)Pb/~(204)Pb=15.6412~15.6583、~(208)Pb/~(204)Pb=38.6266~38.6772,在铅同位素演化模式图解(Zartman et al.,1979)中数据点分布集中,处于"造山带"和"上地壳"演化曲线之间,未显示出明显的线性分布特点,表明金顶矿床成矿金属来源主要为壳源;区域对比表明,金顶矿床明显比白秧坪矿带铅锌矿床贫放射性成因铅,而与区域VMS型矿床铅同位素组成更为接近,这表明金顶矿床与白秧坪矿带矿床有着不同的金属物源区,其金属可能来自盆地底部晚三叠世火山岩或其内早期的VMS矿化。     

兰坪盆地大型矿集区多金属矿床的铅同位素组成及铅的来源

滇西兰坪中、新生代陆相分地中蕴藏有金顶超大型铅锌矿床、白秧坪大型银矿床及羊拉、金满等一系列大、中型铜矿床，矿点、矿化点星罗棋布，为一成矿时代新、矿床规模大、矿床种类丰富的大型矿集区。分布于盆地边缘的矿床与火山活动有一定关系，盆地内部的矿床产于中、新生代沉积岩中。铜矿床沿澜沧江深大断裂带东侧分布，向东至盆地中部，为铅锌银分布区，再向东出现汞锑砷矿化。该矿集区不同矿床的铅同位素组成及铅的来源，可分为三种类型，（1）金顶超大型铅锌矿床以具有低同痊素比值的铅为主，铅的主体来源相当于下地壳铅，整个矿床的铅为下地壳铅与盆地沉积岩铅的混合铅；（2）金满铜矿床及盆增内其它铜铅锌矿床的铅来自盆地沉积岩；（3）位于盆地边缘的着拉铜矿床，早期热水沉积成矿阶段铅的来源与二迭系玄武岩铅一致，为幔源铅，晚期热液成矿阶段铅来自地壳重熔岩浆作用。     

山东香夼斑岩型铅梓矿床的地球化学特征及成因探讨

香夼矿床是一个较为典型的斑岩型铅锌矿床,成矿母岩为燕山期花岗闪长斑岩,稀土元素呈现明显的Eu亏损,为地壳重熔型浅成、超浅成岩浆岩.矿床主要有用元素Pb、Zn、Cu、Ag、Au、S、Cd、Se、Te等均较高.矿床硫同位素δ~(34)S值靠近零,铅同位素组成表明矿石铅来源于岩浆,矿石铅与岩浆岩岩石铅组成一致,主要为上地幔来源的铅,有少量地壳铅参与成矿.H、O同位素表明成矿流体为岩浆水,成矿温度在230-350℃之间,方铅矿、闪锌矿单矿物微量元素显示出与岩浆热液型铅锌矿床相同的元素组合.     

云南澜沧铅锌银铜矿床稳定同位素地球化学研究

澜沧铅锌银铜多金属硫化物矿床产于石炭系火山岩和碳酸盐岩中。矿床硫同位素研究表明硫具胡来自下地壳或上地幔的特征。据铅构造模式认为矿石铅为上地壳铅和地幔铅的混合物，具有多来源的特点。矿物及其包裹体的氢、氧同位素组成反映出成矿溶液早期以岩浆水为主，稍晚有大气降水混入。脉石矿物主方解石的碳同位素组成表明成矿过程中热液同围岩进行了广泛的物质成份交换。为与燕山期次火山活动有关的热液矿床。     

中天山尾亚杂岩体的铅同位素组成特征

对尾亚杂岩体钾长石铅同位素组成的详细研究发现,它们符合幔-壳二元混合模式。即地幔来源的铅和不同数量的地壳来源的铅的混合。经计算,其中约有25％—37％的铅来自上地幔,而63％—75％的铅来自地壳,该区陆壳平均年龄约2450Ma。本区岩石铅同位素组成与中国东部不同,显示贫钍富铀的特征。铅同位素的研究进一步证明尾亚杂岩体系幔-壳混合源的同熔系列花岗岩。     

扬子地块东段大陆岩石圈地幔的物质成分

根据苏皖地区新生代碱性玄武岩中的尖晶石二辉橄榄岩包体的化学成分，讨论了扬子地块东段大陆岩石圈地幔的物质成分。以包体的平均成分代表该区岩石圈地幔的平均成分，依照地幔地球化学演化趋势，估计了该区岩石圈地幔的原始组成。采用最近提出的二辉石地质温度计，计算了包体的平衡温度，讨论了该区岩石圈地幔物质成分在垂向上的分布和变化，上地幔橄榄岩中的玄武质组分在垂向上由上而下亏损程度逐渐减小。包体中稀土元素丰度模式反映了该区岩石圈地幔演化的多阶段性。除了程度不同的部分熔融作用外，局部地段曾有隐性地幔交代作用发生，造成了地幔橄榄岩中主量成分和稀土元素的解耦现象。     

Isotopic evidence for genetic relations between acid and alkaline rocks in Italy

Pb isotopic composition and concentrations were determined for whole-rock samples from the Pliocene-Pleistocene, acid, Tuscan Igneous Province, Italy, in order to clarify their relation to the Cenozoic alkaline volcanism of Central Italy. The Pb of the Tuscan rocks is fairly uniform in its isotopic composition, despite variable and high ₈₇Sr/₈₆Sr ratios (0.711–0.725; Barberi et al, 1971).A comparison of the Pb isotopic composition in the Tuscan and the potassic alkaline rocks strongly suggests a continuous mixing relation between both rock series, with partial melts of mantle material and argillaceous sedimentary rocks as likely end-members. Such an interrelation could also provide a simple explanation for the range in the Sr isotopic ratio within the Tuscan rocks. The favoured model envisages a similar history for both rock-series; it is suggested that their main petrographic differences result from a lower portion of mantle-derived material in the parental material for the Tuscan rocks, and from the mineral phases present in the residue during partial melting of the source region.     

桐柏地区变质杂岩和侵入岩类Pb同位素组成特征及其地质意义

桐柏地区存在两个铅同位素组成明显不同的基底块体,在桐柏－商城断裂北侧,基底岩石以高放射性成因铅同位素组成为特征,而桐柏－商城断裂南侧,基底岩石以低放射性成因铅同位素组成为特征,它们分别可与南阳盆地以西的北秦岭和南秦岭构造带中基底岩石的铅同位素组成相对比,从而证明桐柏地区是秦岭造山带的构造东延部分．同时,通过桐柏地区侵入岩铅同位素组成特征的分析表明,早古生代侵入岩浆主要来源于北秦岭高放射性成因铅同位素组成的块体,而中生代侵入岩浆则来源于南秦岭低放射性成因铅同位素组成的块体,指示在南阳盆地以东,相当于北秦岭构造块体的地壳深部存在南秦岭的陆壳物质,反映南秦岭的陆壳物质早期向北俯冲叠置于北秦岭块体之下,并作为北秦岭中生代花岗质岩浆的源区     

Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits,East Fennoscandia,Kola Peninsula,Russia

The Lovozero alkaline massif—an agpaitic nepheline syenite layered intrusion—is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376?±?28 Ma, based on a ²⁰⁶Pb/²⁰⁴Pb versus ²³⁸U/²⁰⁴Pb isochron and 373?±?9 Ma, from a ²⁰⁸Pb/²⁰⁴Pb versus ²³²Th/²⁰⁴Pb isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The ²⁰⁶Pb/²⁰⁴Pb values fall within the depleted MORB field, with some rocks having lower ²⁰⁷Pb/²⁰⁴Pb but higher ²⁰⁸Pb/²⁰⁴Pb values. Together with other related carbonatites having both lower and higher ²⁰⁶Pb/²⁰⁴Pb values, the combined KACP rocks form an extended linear array defining either a?~2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4–2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5–10% contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.     

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths,North China Craton: O and Pb isotopic evidence

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ¹⁸O values (6.0–7.7‰) higher than normal mantle δ¹⁸O values (～5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.     

安徽沿江地区岩浆岩的深部构造信息

本区分布着十分广泛的中生代岩浆岩,根据岩浆岩的物质组成、岩石组合、地壳结构、成矿作用及Ｎｄ、Ｓｒ、Ｐｂ同位素组成与比值变化等特征,可划分出３个岩浆岩带。在内带发育幔源型碱性玄武岩浆演化系列的基性—中酸性侵入岩,伴生有矽卡岩型Ｃｕ、Ｆｅ、Ｓ、Ａｕ矿床。北外带以中酸性侵入岩为主,属于下地壳重熔产物。南外带以Ｓ型花岗岩为主,显示出由上地壳重熔的特点。南、北两带均为钙碱性演化系列,并伴生斑岩型铜矿床。     

A Pb isotope investigation of the Lovozero Agpaitic Nepheline Syenite,Kola Peninsula,Russia

For the first time Pb isotope composition was established in Lovozero rocks and raremetal ores, which is important for identifying their sources. The world’s largest layered intrusion of agpaitic nepheline syenite-the Lovozero alkaline massif—is located near the center of the Kola Peninsula in Russia. This superlarge complex plutonic body hosts the economically important loparite and eudiallyte deposits [1]. These deposits contain immense resources of REE, Nb, Ta, Zr, and constitute a world class mineral district. The Lovozero massif belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Previous bulk rock studies have shown that the initial Sr and Nd isotope ratios of Lovozero rocks plot in the depleted mantle quadrant of Sr-Nd diagrams [2]. More recently, Hf isotope data obtained by Kogarko et al. (3) confirm that the Lovozero and Khibina massifs with ?_Hf between 6 and 8 are derived predominantly from a depleted mantle source. It was shown that Sr, Nd, and Hf abundances are significantly elevated in the Kola alkaline rocks, and thus their isotopic compositions are relatively insensitive to minor contamination by the overlying crustal rocks. By contrast, Pb in the KACP rocks is a much more sensitive indicator of a crustal component. In this paper we investigate the lead isotopic signature of all resentative types of Lovozero rocks (Table 1) in order to further characterize their mantle sources. The Lovozero massif consists of four intrusive phases. Rocks of phase I (mostly nepheline syenites) comprise about 5% of the total volume, phase II (urtites, foyaite, lujavrites) forms the main portion of the massif comprising 77% in volume, and phase III (eudialyte lujavrites) contributes about 18%. Country rocks are represented by Devonian effusive rocks and Archean gneisses.     

宁芜地区中生代岩浆岩的成因——岩石学与Nd,Sr,Pb同位素证据

宁芜中生代火山盆地产出火山岩与侵入岩，火山岩以玄武粗安岩，粗安岩和粗面岩为主，安山岩和响岩少量，火山岩以高钾富碱为特征，已确定为橄榄安粗岩系。侵入岩以辉长闪长玢岩－一辉长闪长岩为主，以高钠低硅为特征，并有辉长岩和花岗岩产生，据地质学和Ｎｄ，Ｓｒ，Ｐｂ同位素资料，侵入岩与火山岩属同一个岩浆系列，是碱性玄武岩浆在下地壳经过轻度ＡＦＣ混合后，侵入上地壳，在轻度混染的情况下，经过以结晶分离为主的岩浆分异形     

Granulite xenoliths from western Saudi Arabia: the lower crust of the late Precambrian Arbian-Nubian Shield

Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial⁸⁷Sr/⁸⁶Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield T_DM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.     

Enriched Subcontinental Lithospheric Mantle in the Northern Part of the South Indian Granulite Terrain: Evidence from Yelagiri and Sevattur Syenite Plutons, Tamil Nadu, South India

In the southern part of the Indian Peninsula, there are a number of alkaline plutons of Proterozoic age. In the northern part of the South Indian granulite terrain, the Yelagiri (syenite, pyroxenite) and Sevattur (syenite, pyroxenite, carbonatite) plutons intrude Archaean epidote-hornblende gneisses. Geochemical and isotopic characteristics of the Yelagiri and Sevattur plutons indicate that the syenitic magmas formed from highly differentiated mantle-derived alkali basalts. The Yelagiri and Sevattur syenites are characterized by evolved Sr and Nd isotopic compositions, pronounced enrichment in LILE and large negative Nb anomalies. Trace element and Sr-Nd isotope characteristics of the Yelagiri and Sevattur syenites are similar to those of the subduction-related alkaline rocks. The scarcity of geological evidence for subduction activity at the time of syenite intrusion during Neoproterozoic does not support a link between the alkaline magmatism and subduction. However, our data are consistent with the model of derivation of the Yelagiri and Sevattur syenites from the subcontinental lithospheric mantle, which was previously enriched by slab derived component. The geochemical and isotopic signatures of other mantle-derived intrusive rocks in the northern part of the South Indian granulite terrain with ages ranging from ca. 2.5 Ga to 0.75 Ga also support the above idea.     

冀西北水泉沟杂岩体成因的Nd同位素证据

水泉沟碱性杂岩体是冀西北金矿集中区最重要的赋矿岩体。长期以来，关于水泉沟杂岩体的成因一直争议很大，严重制约了本区金矿床理论研究水平的提高。本文对水泉沟杂岩体及其围岩太古宙变质岩进行了系统的Nd同位素研究。研究结果表明，水泉沟杂岩体不可能是太古宙变质岩混合岩化或重熔的产物，而是由来自地幔的岩浆与下地壳局部熔融的物质不均匀混合形成的。