A Nd and Sr isotopic study of the Ivrea zone,Southern Alps,N-Italy

The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ₆₀₇ ^Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by phlogopite metasomatism. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ₂₉₃ ^Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic crustal residence ages of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.     

Transition from calc-alkaline to potassium-rich magmatism in subduction environments: geochemical and Sr, Nd, Pb isotopic constraints from the island of Vulcano (Aeolian arc)

The problem of mantle metasomatism vs. crustal contamination in the genesis of arc magmas with different potassium contents has been investigated using new trace element and Sr–Nd–Pb isotopic data on the island of Vulcano, Aeolian arc. The analysed rocks range in age from 120 ka to the present day, and cover a compositional range from basalt to rhyolite of the high-K calc-alkaline (HKCA) to shoshonitic (SHO) and potassic (KS) series. Older Vulcano products (>30 ka) consist of HKCA–SHO rocks with SiO₂=48–56%. They show lower contents of K₂O, Rb and of several other incompatible trace element abundances and ratios than younger rocks with comparable degree of evolution. ⁸⁷Sr/⁸⁶Sr ranges from 0.70417 to 0.70504 and increases with decreasing MgO and compatible element contents. ²⁰⁶Pb/²⁰⁴Pb ratios display significant variations (19.31 to 19.76) and are positively correlated with MgO, ¹⁴³Nd/¹⁴⁴Nd (0.512532–0.512768), ²⁰⁷Pb/²⁰⁴Pb (15.66–15.71) and ²⁰⁸Pb/²⁰⁴Pb (39.21–39.49). Overall, geochemical and isotopic data suggest that the evolution of the older series was dominated by assimilation–fractional crystallisation (AFC) with an important role for continuous mixing with mafic liquids. Magmas erupted within the last 30 ka consist mostly of SHO and KS intermediate and acid rocks, with minor mafic products. Except for a few acid rocks, they display moderate isotopic variations (e.g. ⁸⁷Sr/⁸⁶Sr=0.70457–0.70484; ²⁰⁶Pb/²⁰⁴Pb=19.28–19.55, but ²⁰⁷Pb/²⁰⁴Pb=15.66–15.82), which suggest an evolution by fractional crystallisation, or in some cases by mixing, with little interaction with crustal material. The higher Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr=0.70494–0.70587) of a few, low-volume, intermediate to acid rocks support differentiation by AFC at shallow depths for some magma batches. New radiogenic isotope data on the Aeolian islands of Alicudi and Stromboli, as well as new data for lamproites from central Italy, are also reported in order to discuss along-arc compositional variations and to evaluate the role of mantle metasomatism. Geochemical and petrological data demonstrate that the younger K-rich mafic magmas from Vulcano cannot be related to the older HKCA and SHO ones by intra-crustal evolutionary processes and point to a derivation from different mantle sources. The data from Alicudi and Stromboli suggest that, even though interaction between magma and wall rocks of the Calabrian basement during shallow level magma evolution was an important process locally, a similar interpretation can be extended to the entire Aeolian arc. Received: 27 September 1999 / Accepted: 24 May 2000     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Origin of Triassic magmatism of the Southern Alps (Italy): Constraints from geochemistry and Sr-Nd-Pb isotopic ratios

During Middle-early Late Triassic (∼243–235?Ma) a diffuse igneous activity developed in the Southern Alps (Italy). Sparse lava flow and pyroclastic succession remnants of such Southern Alps Triassic Igneous Rocks (SATIR) crop out in the Brescian Prealps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Julian Alps. Plutonic rocks are found in two main plutonic complexes (Monzoni and Predazzo) and a small stock (Cima di Pape). Coeval igneous products can be traced eastward to Austria and Dinarides, for a total length of ∼450?km.The coeval formation of major late Anisian-late Ladinian carbonate platforms in the subsiding eastern Dolomites, and significant uplift and subaerial erosion in the western Dolomites suggest, for these areas, the occurrence of large-scale strike-slip and extensional tectonics with the development of horst-and-graben structures.This study reports the first complete review of the SATIR activity, including new mineral chemical data on 14 samples, 61 major and trace element whole rock analyses and 7 Sr-Nd-Pb isotopic ratios for volcanic and plutonic samples from Dolomites (lavas plus Monzoni and Predazzo plutonic rocks) and Vicentinian Alps lavas. Despite the variable post-magmatic modifications, the large areal distribution of the products and their wide spectrum of chemical compositions, these samples show rather common geochemical and mineralogical characteristics and define major and trace element trends that can be associated with nearly close-system magmatic evolution. Minor upper crustal contaminations can be observed in specific cases, mostly in the most differentiated products (SiO₂ >70?wt%).A specific characteristic of SATIR is their calcalkaline to shoshonitic affinity, resembling the derivation from subduction-modified mantle sources, a feature at odds with the coeval strike-slip and extensional tectonics. Geochemical modelling, petrological considerations and geological constraints allow us to propose a model in which the SATIR mantle sources reflect previous subduction metasomatism (likely occurred during Variscan times). These metasomatised mantle sources were reactivated ∼90–100 Myr after the end of the subduction, when continental rifting caused a raise of the geotherms and passive upwelling of asthenospheric mantle.     

Origin of Sr,Nd and Pb isotopic systematics in high-Sr basalts from central Arizona

Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (⁸⁷Sr/⁸⁶Sr = 0.7052 to 0.7055, _Nd= –9.2 to –10.7); alkali olivine basalts (⁸⁷Sr/ ⁸⁶Sr = 0.7049 to 0.7054, _Nd= –2 to 0.2); basanite and hawaiites (⁸⁷Sr/⁸⁶Sr = 0.7049 to 0.7053, _Nd= –3.5 to –7.8); and quartz tholeiites (⁸⁷Sr/⁸⁶Sr = 0.7047, _Nd= –1.4 to –2.6). Post-faulting lavas have lower ⁸⁷Sr/⁸⁶Sr (<0.7045) and _Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.     

Baseline determination of the atmospheric Pb,Sr and Nd isotopic compositions in the Rhine valley,Vosges mountains (France) and the Central Swiss Alps

The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with ²⁰⁶Pb/²⁰⁷Pb and ⁸⁷Sr/⁸⁶Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (²⁰⁶Pb/²⁰⁷Pb: 1.158–1.167; ⁸⁷Sr/⁸⁶Sr: 0.719–0.725; ε_Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (⁸⁷Sr/⁸⁶Sr: 0.714 − 0.716; ε_Nd: −3.6 to −8.1). The ¹⁴³Nd/¹⁴⁴Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the ¹⁴³Nd/¹⁴⁴Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.     

Fluid inclusion evidence for progressive folding during decompression in metasediments of the Voltri Group (Western Alps, Italy)

Quartz veins syntectonic to distinct folding events in metasediments from the Voltri Group (Ligurian Alps) were studied in order to compare fluid and structural evolution. Studied veins (VS1, VS2, VS3) pertain to three distinct generations of folds (F₁, F₂, F₃) that formed during the retrograde metamorphic evolution. Two types of fluids characterize the different generations of veins and are represented essentially by aqueo-carbonic mixtures of moderate salinity with decreasing densities (1.01–0.41 g/cm³). The chemical evolution is characterised by a progressive decrease of H₂O, from early fluids associated with opening of VS1 and VS2 (XCO₂≈0.08) to fluids related to VS3 formation (XCO₂≈0.3). The close match between the fluids in VS1 and VS2 suggests that the development of two superimposed systems of folds (F₁ and F₂ folds) occurs under very similar P–T conditions, during a progressive and continuous deformational event at glaucophanic and/or barroisitic metamorphic grade. A different evolution is outlined for the formation of VS3 during low greenschist grade. Successive isochores allow us to define a retrograde decompression path for the Voltri Group. Present results indicate that fluid inclusions are powerful markers to constrain the P–T–t conditions of different folding events.     

郯庐断裂带晚中生代富碱侵入岩Nd、Sr、Pb同位素特征及源区性质探讨

郯庐断裂带是中国东部一条巨大的深大断裂带,它纵切华北克拉通、扬子克拉通、兴蒙-吉黑造山带和大别-苏鲁造山带四个大地构造单元,特别是在该断裂带出露许多晚中生代富碱侵入岩体,构成一条长达近2000km的富碱侵入岩带.本文是在前人研究基础上首次从大尺度对整个郯庐断裂带富碱侵入岩的Nd、Sr、Pb同位素做了比较系统深入的研究,发现四个大地构造单元的富碱侵入岩具有明显不同的Nd、Sr、Pb同位素特征,暗示四大构造单元富碱侵入岩的源区性质差别较大:兴蒙-吉黑造山带晚中生代富碱侵入岩源于和亏损地幔有成因联系的年青地壳(εNd(t)为ft.,平均值 2.25);华北克拉通、扬子克拉通和大别-苏鲁造山带三大构造单元晚中生代富碱侵入岩都主要源于EMI型富集地幔(εNd(t)均为负值).但是这三大构造单元岩石圈地幔富集程度差别较大.大别-苏鲁造山带岩石圈地幔相对富集程度最高(εNd(t)平均值为-17.06);华北克拉通次之(εNd(t)平均值为-10.03);扬子克拉通岩石圈地幔富集程度最低(εNd(t)平均值为-6.41).本文认为大别.苏鲁造山带岩石圈地幔相对最富集的原因可能是在早中生代扬子克拉通陆壳向华北克拉通深俯冲之后的折返过程中,因压力释放有利于熔/流体的形成、析出和向上交代并改造华北岩石圈地幔使之成为富集地幔.     

Interaction between ultrapotassic magmas and carbonate rocks: Evidence from geochemical and isotopic (Sr, Nd, O) compositions of granular lithic clasts from the Alban Hills Volcano, Central Italy

Magma-carbonate rock interaction is investigated through a geochemical and Sr-Nd-O isotope study of granular lithic clasts (ejecta) from the Alban Hills ultrapotassic volcano, Central Italy. Some samples (Group-1) basically represent intrusive equivalents of Alban Hills magmas. A few samples (Group-2) are ultramafic, have high MgO (∼30 to 40 wt%) and δ¹⁸O‰, and originated by accumulation of mafic phases that crystallised from ultrapotassic melts during assimilation of dolomitic rocks. Group-3 ejecta consist of dominant K-feldspar, and show major element compositions similar to phonolites, which, however, are absent among the Alban Hills volcanics. Finally, another group (Group-4) contains corroded K-feldspars, surrounded by a microgranular to porphyritic matrix, made of igneous minerals (K-feldspar, foids, clinopyroxene, phlogopite) plus wollastonite, garnet, and some cuspidine. Group-4 ejecta are depleted in SiO₂ and enriched in CaO with respect to Group-3.The analysed ejecta have similar ¹⁴³Nd/¹⁴⁴Nd (0.51204-0.51217) as the Alban Hills lavas, whereas ⁸⁷Sr/⁸⁶Sr (0.70900-0.71067) is similar to lower. Whole rocks δ¹⁸O‰ ranges from +7.0 to +13.2, reaching maximum values in ultramafic samples. A positive correlation with CaO is observed in single rock groups. Large Ion Lithophile Element (LILE) abundances and REE fractionation are generally high, and extreme values of Th, U and LREE are found in some Group-3 and Group-4 rocks.Mineralogical, petrological and geochemical data reveal extensive interaction between magma and carbonate wall rocks, involving both dolostones and limestones. These processes had dramatic effects on magma compositions, especially on phonolites, which were transformed to foidites. Evidence of such a process is found in Group-4 samples, in which K-feldspar is observed to react with a matrix that represents strongly undersaturated melts formed by interaction between silicate magma and carbonates. Trace element data also testify to a very important role for F-CO₂-H₂O-S fluids during magma-wall rock interaction. Fluid transfer was responsible for extreme enrichments in Th, U, and LREE especially observed in Group-3 and Group-4 rocks. Implications of these processes for potassic magma evolution in Central Italy are discussed.     

内蒙古敖包吐萤石矿床的Sr、Nd、Pb同位素地球化学特征

敖包吐萤石矿床是内蒙古北部苏莫查干地区单一萤石矿集区中的一个代表性矿床,产于早二叠世大石寨组火山-沉积岩与早白垩世敖包吐花岗岩的接触带上。文章通过分析该矿床岩、矿石的微量元素和稀土元素,揭示出萤石的成矿作用可分为2个阶段,即交代作用和充填作用。交代作用过程中大石寨组的结晶灰岩可能为萤石的形成提供了部分Ca来源,萤石矿石的稀土元素配分模式与海水基本类似,具有Ce负异常;成矿作用后期主要表现为充填作用,形成颗粒粗大的萤石,表现为重稀土元素富集的特征,并随着萤石的沉淀析出,稀土元素总量逐渐下降,反映出成矿流体经历了较长期的演化过程。各地层单元、花岗岩体和萤石矿石的Sr、Nd、Pb同位素研究表明,萤石的放射性同位素组成具有壳、幔源混合的特点,成矿物质来源具有多源性。早白垩世敖包吐花岗岩可能是萤石中F的主要来源,而大石寨组的结晶灰岩则可能提供了Ca。另外,Pb、Nd同位素的极大不均一性,有可能是成矿流体在运移过程中对艾力格庙群放射性组分的选择性吸收的结果。萤石成矿作用与钾玄岩的时空关系暗示了萤石的成矿过程可能是中国东部岩石圈减薄和下地壳的置换地质事件的结果。在构造转型的过程中,燕山中期富碱的酸性花岗岩浆的活动分异出富含F的成矿流体,与幔源流体混合,沿区域重新活化的深大断裂和大石寨组的层间破碎带上升,交代其间的灰岩透镜体,从而形成敖包吐中型萤石矿床。     

Relationships between the chemical and isotopic (Sr,Nd, Hf,and Pb) heterogeneity of the mantle

The reasons for the isotopic heterogeneity of the mantle are analyzed in this paper on the basis of published isotopic data. It was shown that the observed variations in the Sr, Nd, Hf, and Pb isotopic compositions of oceanic basalts cannot be explained by mixing of a finite number of homogeneous reservoirs (components). The considerable variations in the contents of Rb, Sr, Sm, Nd, Lu, Hf, U, Th, and Pb and ratios of these and other trace elements in tholeiitic basalts indicate that the chemical heterogeneity of mantle-derived rocks is inherited in part from their sources. Oceanic tholeiitic basalts show a tight correlation between the variances of Nd, Hf, Sr, and Pb isotopic ratios and the variances of respective radiogenic additions that could be accumulated in these rocks over a time period of 〈t〉 = 1.8 Gyr. This paradox clearly indicates that variations in all the mentioned isotopic systems in the mantle cannot be understood without the analysis of the geochemical heterogeneity of rocks.The close to lognormal distributions of lithophile trace elements in oceanic tholeiitic basalts and the character of correlations between them suggest that magmatic differentiation was the major mechanism of the formation of chemical heterogeneity in the mantle. The role of metasomatism in the global transport of trace elements and formation of the geochemically heterogeneous mantle is probably rather limited. Intrusive processes within the mantle could result in the development of chemical and, after a period of time, isotopic anomalies in the mantle. Simple calculations show that long-lived geochemical anomalies related to alkaline magmatism could be responsible for EM-I type isotopic anomalies, and geochemical anomalies produced in the mantle by enriched tholeiitic melts could be sources of EM-II type isotopic anomalies. The analysis of the distribution of the isotopic compositions of mantle-derived igneous rocks in various “isochron” coordinates suggested that the formation of geochemical anomalies in the mantle is a long-term process lasting for hundreds of millions of years. Nonetheless, trends approaching 4.5 Ga were never observed in such diagrams, i.e., the mantle is in general rejuvenated in all isotopic systems. Both on global and local scales, there are no mantle domains that have remained geochemically closed and isolated since the Earth’s formation. The entire mantle is involved in material exchange processes.The development of isotopic systems in the mantle was explored by means of statistical modeling accounting for the tendency of a continuous increase in the chemical heterogeneity of the mantle source and the tendency of obliteration of the isotopic heterogeneity owing to the convective mixing in the mantle. The modeling demonstrated that the character of the isotopic heterogeneity of the mantle is statistically consistent with the character of its chemical heterogeneity. The mantle isotopic anomalies HIMU, EM-I, and EM-II were generated by two simultaneous processes: the magmatic differentiation of mantle material and its not very efficient mixing.     

内蒙古白乃庙地区铜(金)和金矿床钕、锶和铅同位素研究

内自古白乃庙地区产出有白乃庙铜（金）矿床（南、北矿带）、白乃庙金矿床和徐尼乌苏金矿化带，是华北陆台北缘金属成矿带的重要组成部分。笔者测定和解释了上述三个矿床的钕、锶和铅同位素比值和变化特征。研究认为：白乃庙铜（金）矿床南、北矿带在成矿时代、成矿物质来源和形皮过程方面存在明显差异，南矿带条带状、块状硫化物矿石的形成作用始于１１３０×１０￣６ａ前的基性火山喷发活动。相比之下，北矿带细脉浸染状和脉状铜－钼－金矿石与加里东期（４４０×１０￣６ａ）花岗闪长斑岩的侵位和结晶分异密切相关。白乃庙金矿床和徐尼乌苏金矿化带是岩浆热液、大气降水混合，并且对基底岩层或绿片岩围岩进行淋滤、萃取的结果。白乃庙地区铜（金）矿床和金矿床（点）成矿物质来源和形成机制的厘定，对于了解华北地台北缘内蒙古中南段金属成矿带的形成与演化具有重要意义。     

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.     

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。     

Origin and evolution of 2 Ga old carbonatite complex (Ihouhaouene,Ahaggar, Algeria): Nd and Sr isotopic evidence

Associated syenitic rocks and carbonatites from Ihouhaouene, Algeria, have been investigated for their Sr and Nd isotope and trace element geochemistry. A zircon U-Pb emplacement age (1994 +22 -17 Ma) has been obtained from the carbonatites. The REE characteristics, among which the significant and approximately constant negative Eu anomaly and the evolution of (La/Ce)N and (Yb/Lu)N ratios which increase sympathetically with total REE abundances, are considered to be of purely magmatic origin. They are used to constrain the genetic links between syenites and carbonatites. Sr and Nd isotopes suggest a similar source for carbonatites and syenites, which is enriched compared to a chondritic reference: Nd(T)=-6.4 to -8.6 and ISr(T)=0.7097. These features are interpreted as evidence of contamination of a mantle-derived magmatic precursor by continental crust, occurring in a magmatic chamber.     

Granulite-amphibolite facies metasediments from the Serre (Calabria,Southern Italy): their protoliths and the processes controlling their chemistry

In the northwestern Serre, a near continuous section through the Palaeozoic lower continental crust crops out. This paper deals with granulite facies metasedimentary rocks of this section. Fifty-five chemical analyses (major elements, Sc, V, Cr, Co, Ni, Rb, Sr, Y, Nb and Ba) are given. Chemical data indicate that the original sedimentary sequence was made up of arenites in the lower part and mainly Fe and Mg rich pelites in the upper part. The metapelites experienced anatexis and removal of some melt, whereas the meta-arenites suffered only incipient anatexis and became dehydration granulites. The various rock types display an overall relatively low K/Rb ratio due mainly to the stability of K- and Rb-retaining phases. A weighted mean composition of the fragment of the lower continental crust outcropping in the Serre has been established by integrating the chemical data for metasediments (55 analyses), metabasites (92 analyses) and dioritic and tonalitic gneisses (10 analyses) with the field data. This composition compared to the compositions of the bulk continental crust and of other sections considered as belonging to the lower crust, confirms the hypothesis that (1) the lower crust is heterogeneous, and (2) it is more basic than the continental crust.     

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.     

Pb,Sr, Nd,and Hf isotopic constraints on the origin of Hawaiian basalts and evidence for a unique mantle source

Pb, Sr, Nd, and Hf isotopic relationships among basalts from the Hawaiian Islands suggest that these basalts were derived from three sources; the oceanic lithosphere (Kea end member), the depleted asthenosphere (posterosional end member) and a deep-mantle plume (Koolau end member).Hawaiian tholeiites are derived within the lithosphere and the isotopic trends collectively defined by the tholeiite data are interpreted as a plume-lithosphere mixing trend. The isotopic characteristics of late-stage basalts are derived from the tholeiite source (lithosphere + plume) with additional input from the lithosphere, asthenosphere, or both. These basalts probably originate from near the asthenosphere-lithosphere boundary. Posterosional basalts are derived from the depleted asthenosphere, but their isotopic characteristics have been slightly modified by either the plume or the source of previously erupted volcanics. The isotopic data require little or no mixing of asthenospheric material into the plume during tholeiite production and thus are consistent with the concept of a rapidly ascending, fluid-rich plume. In addition to providing a source of heat, the plume may supply volatiles to both the sources of tholeiites and posterosional basalts.The isotopic characteristics of the Koolau (plume) component are unique among OIB sources. If undifferentiated or “primitive” mantle material still exists, then the radiogenic-isotope data for Koolau in combination with rare gas data for Hawaiian basalts in general suggest that the Hawaiian plume may be derived from such material. In any case, the Hawaiian Islands data, when compared to those of other OIB, serve to illustrate the isotopically diverse nature of mantle sources.     

Mantle source heterogeneity in subduction zones: constraints from elemental and isotope (Sr,Nd, and Pb) data on Vulcano Island,Aeolian Archipelago,Italy

Mineralogy and Petrology - Vulcano is part of the Aeolian volcanic arc in the southern Tyrrhenian Sea. Its products were emplaced through multiple episodes of edifice building and collapse since...     

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.