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1.
Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg#=0.39-0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb=17-51; Zr=93-274; La=17-55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb=4.2-5.5; Nb/Ta=17.5ǂ.4; (La/Sm)n=7.3ǃ.1); isotope ratios are restricted in range (87Sr/86Sr=0.70393-0.70436; 143Nd/144Nd=0.51272-0.51280; 206Pb/204Pb=19.87-19.92; 207Pb/204Pb=15.68-15.70; 208Pb/204Pb=39.56-39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# >60) and hawaiite to benmoreite (Lenderut; Mg#=0.48-0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper (La/Sm)n=17.4ǃ.2). 87Sr/86Sr (0.70358, 0.70391), 143Nd/144Nd (0.51280, 0.51267), 206Pb/204Pb (19.96,20.17), 207Pb/204Pb (15.66,15.76) and 208Pb/204Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low143Nd/144Nd (0.512388-0.512453) ratios for their 87Sr/86Sr (0.70370-0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions (206Pb/204Pb=17.93-19.01; 207Pb/204Pb=15.43-15.58; 208Pb/204Pb=37.91-39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P-T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.  相似文献   

2.
The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of 40 Ar-39Ar ages: 147–106 Ma, 97–81 Ma and 68–49 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=7–11.1; La/Nb = 0.77–1; 87Sr/86Sr; =0.7042–0.7065143Nd/144Nd;i = 0.5125–0.5127; 206Pb/204Pbi = 17.59–18.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.6–6.8;La/Nb=0.53–0.66; 87Sr/86 Sri=0.7029–0.7037; 143Nd/144Ndi=0.5126–0.5129;206Pb/204Pbi = 18.54–20.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume *;Corresponding author.  相似文献   

3.
牙克石地区出露一套早白垩世玄武质火山岩,其SiO2含量为52．81%-53．39％,K2O含量为1．86％-2．87％,岩性为玄武质粗面安山岩。富集大离子亲石元素Rb和Ba,高场强元素Nb和Ta亏损明显,Zr和H阮明显异常,8Eu为0．77-0．82。从同住素的特点看,（^87Sr／^86Sr）,变化于0．704762-0．704941之间,εNd（t）为2．00～2．54;在εNd（t）-（^87Sr／^86Sr）闺解上,样品投影点落入洋岛玄武岩（OIB）和美国盆岭省范围内。^206Pb／^204pb为18．3288-18．4225,^207b／^204Pb为15、4566-15．4893,^208Pb／^204Pb为37．9401-38．0523：在。^207Pb/^24Pb-^206Pb.^204pb和,^208b／^304pb-^306Pb／^204pb图解上．样品投影点都落在亏损洋中脊玄武岩地幔附近。综合考虑本区火山岩的地质、地球化学特点,认为其来源于被俯冲洋壳交代的岩石圈地幔。  相似文献   

4.
Mt. Jefferson is an andesite-dacite composite volcano in the Cascade Range, the locus of andesite and dacite-dominated volcanism for at least 1 million years. A large trace element data set for Mt. Jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model (FC or AFC) for the generation of Mt. Jefferson andesites. Several incompatible element (Zr, Nb, Y) concentrations decrease in the range from basalt to andesite, and then increase in the range from andesite to rhyodacite. Others (Ba, Rb, La, Th) remain constant or show a slight increase in the basalt to andesite range, with modest increases from andesite to rhyodacite. Systematic variations in highly incompatible element ratios such as Ba/La and Rb/Th suggest magma mixing dominates the trace element signatures. Rhyodacites are isotopically uniform (87Sr/86Sr=0.70325-0.70343; 206Pb/204Pb=18.75-18.85; ‘18O=6.3ǂ.3), whereas andesite and dacite are more variable (87Sr/86Sr=0.70291-0.70353; 206Pb/204Pb=18.59-18.86; ‘18O=6.0ǂ.6). Typical basaltic andesite has 87Sr/86Sr=0.70326-0.70358, 206Pb/204Pb=18.78-18.85, and ‘18O=5.9ǂ.4. Sr-rich (>1,000 ppm) basaltic andesite is more variable (87Sr/86Sr=0.70300-0.70360; 206Pb/204Pb=18.70-18.89; ‘18O=5.9ǂ.4). The data define mixing arrays with one end member at 87Sr/86Sr=0.7029; 206Pb/204Pb=18.59, another at rhyodacite, and a third at 87Sr/86Sr=0.7036; 206Pb/204Pb=18.89. The first end member is defined by Sr-rich (800-1,200 ppm) andesite with high Al2O3, and low K2O, Ba, and Rb/Th; the third one by K2O- and very Sr-rich (>2,000 ppm) shoshonite. Isotopic data for basalts in northern Oregon preclude any fractionation relationship between basalt and either rhyodacite or Sr-rich andesite (e.g., the minimum 206Pb/204Pb ratio in basalt is 18.83). Considered in light of geophysical models for the Cascades, these data suggest two types of crustal melting beneath the arc. Rhyodacite may be generated at 25-30 km depth by partial melting of arc basalt-like amphibolite at 850-900 °C. Sr-rich andesite may be formed by partial melting of depleted MORB-like mafic granulite at 35-45 km depth at 1,000-1,100 °C. Experimental and REE evidence supports these interpretations as does the restriction of Sr-rich andesite in the Cascades to the area south of the 100 mW/m2 heat flow contour between Mt. Jefferson and Mt. Hood. Thick crust and high heat flow are necessary to produce such andesite.  相似文献   

5.
. We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#̾.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6-5.1) and negative Nb anomalies (Nb/La=0.37-0.62), but low Zr/Nb ratios (7-14). Initial )Nd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39-18.45, 207Pb/204Pb=15.49-15.61, 208Pb/204Pb=37.06-38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11-130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8-15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6-2.4] and higher )Nd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81-1.30), but Zr/Nb is high (21-48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low )Nd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high )Nd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.  相似文献   

6.
青藏高原东北缘多福屯第三纪钠质基性火山岩及构造启示   总被引：4，自引：2，他引：2  
位于青藏高原东北缘的多福屯第三纪火山岩,为钠质基性火山岩系。该火山岩(La/Yb)N值在6~11,ΣREE平均为117,样品普遍具有不相容元素富集、Pb亏损以及弱的Nb、Ta正异常,Nb/U和Ce/Pb平均值分别为30和17;样品的87Sr/86Sr值为0.7041~0.7069,Nd初始值为0.5129(εNd(t)=6),Δ207Pb/204Pb和Δ208Pb/204Pb分别在11~19和73~84之间,结合高的Sr初始值,表现了特征的Dupal(高放射成因铅)异常。地球化学特征表明,该火山岩属于似OIB性质的陆内火山岩,源区具有DM与EMⅡ混合特点。个别岩石表现出明显陆壳混染的迹象。火山岩源区特征反映了OIB类岩石的复杂性,可能由区域原、古特提斯地幔继承而来,因而是原地的和固有的,并非与青藏高原物质向东挤出有关。通过与邻区同时代火山岩的对比推断,整个高原东北缘新生代火山岩幔源区具有小尺度的不均一性。高原东北缘新生代火山岩的发育可能与较大规模的贺兰-川滇南北复合构造有关。  相似文献   

7.
The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.  相似文献   

8.
浙江东南部晚中生代上、下火山岩系（以下简称上、下岩系）中均有玄武岩产出,本文对这些玄武岩分别进行了元 素地球化学和Sr-Nd-Pb同位素研究。不同岩系玄武岩的主量元素均表现出富碱、富Al等特征。但微量元素存在差异,下岩 系天台和青田样品具有轻稀土富集以及Ba, Pb和Sr富集,Eu负异常,Nb, Ta, Zr和Hf亏损的特征。上岩系玄武岩的元素特征 也有差别,永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山等地样品的元素特征表现出的性质与下岩系样品相似,武义玄 武岩样品没有Ta, Nb亏损特征,金衢盆地玄武岩的元素特征则介于两者之间。对应的,这些玄武岩样品的同位素组成也有 明显差异,下岩系玄武岩的初始同位素组成范围为 I Sr=0.70850~0.70897,εN（d t） = -5.6~-4.1,（206Pb/204Pb） i =18.21~18.38,（207Pb/204Pb） i =15.55~15.58,（208Pb/204Pb） i =38.26~38.49,接近下岩系中酸性岩浆岩的范围,反映了下地壳物质对其岩浆源区的显著影响。上岩系玄武岩有明显差异,表现出与元素特征对应的分组现象。其中永嘉花坦、宁波玄坛地、新昌镜岭和永嘉镜架山样品 I Sr = 0.70734~0.70936, εN（d t）= -7.1~-2.1,（ 206Pb/204Pb） i =18.01~18.40,（ 207Pb/204Pb） i = 15.54~15.62,（ 208Pb/204Pb） i=37.99~38.62, 具有富集特征, 可能来自活动大陆边缘; 而武义和金衢盆地样品的 I Sr=0.70533~0.70589, εNd（ t） =0.4~3.3,（206Pb/204Pb） i =17.23~18.11,（ 207Pb/204Pb） i =15.46~15.53,（ 208Pb/204Pb） i =36.91~38.43,具有类似OIB特征,趋向亏损地幔端元。上下岩系玄武岩的元素和同位素组成的研究表明,玄武岩的物质来源有较明显的差别,且表现出随时间变化的特征。其中下岩系玄武岩源区中可能有古老岩石圈地幔、下地壳物质和俯冲蚀变洋壳物质的贡献,而上岩系中玄武岩源区有可能是类似下岩系玄武岩性质的岩石圈、软流圈地幔和下地壳物质等的贡献。浙东南晚中生代岩石圈演化的动力学过程可能与太平洋板块俯冲有关,但不能排除岩石圈地幔拆沉的影响,具体的讨论还需要更多的岩石学和/或地幔包体资料的补充。  相似文献   

9.
SIMONETTI  A.; BELL  K. 《Journal of Petrology》1994,35(6):1597-1621
Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial 143Nd/144Nd(0.51265–0.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial 87Sr/86Sr(0.70319–0.70361) ratios are variable. The 18Osmow (9.53–14.15%0)and 13CPDB (–3.27 to –1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between 18O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial 143Nd/144Nd (0.51246 0.51269) and initial 87Sr/86Sr(0.70344–0.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(207Pb/204Pb 15.63–15.71; 206Pb/204Pb 19.13–19.78)and silicate rocks (207Pb/204Pb 15.61–15.72; 206Pb/204Pb18.18–20.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination.  相似文献   

10.
本文对中国东南沿海不含幔源包体的中生代玄武岩和含幔源包体的新生代玄武岩进行了微量元素和Nd-Sr-Pb同位素对比研究。中生代玄武岩呈Ta、Nb和Hf负异常,低Ce/Pb、Nb/U比值和高La/Nb比值,与岛弧火山岩和陆壳岩石的微量元素特征相类似,说明在岩浆生成和上升过程中,幔源组分受到了陆壳组分的混染。新生代玄武岩呈Ta、Nb正异常和Pb负异常,高Ce/Pb、Nb/U比值和低La/Nb比值,与海岛玄武岩(OIB)相类似,Nd-Sr同位素成分与夏威夷玄武岩类似,因而它们未受明显的陆壳混染。143Nd/144Nd与206Pb/204Pb之间的负相关关系和87Sr/86Sr与206Pb/204Pb之间的正相关关系说明本区新生代玄武岩起源于中等亏损程度的软流圈地幔,并与EMII富集地幔组分发生了混合。  相似文献   

11.
Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/YbCN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (Nd from −9.3–−20.1) isotopic compositions. TDM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (206Pb/204Pb=16–17.3, 207Pb/204Pb=15.1–15.6, 208Pb/204Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.  相似文献   

12.
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.  相似文献   

13.
Lavas from Heard Island, located on the Kerguelen Plateau inthe southern Indian Ocean, exhibit the largest range (e.g.,87Sr/86Sr=0.7047–0.7079) of isotopic compositions yetobserved on a single oceanic island. Isotopic compositions arewell correlated and are accompanied by systematic changes inincompatible trace element ratios, particularly those involvingNb. These variations are interpreted as resulting from mixingbetween two components. One is characterized by high 87Sr/86Sr,low 206Pb/204Pb and 143Nd/144Nd ratios, and negative Nb andEu anomalies, and is derived ultimately from the upper continentalcrust. The other has lower 87Sr/86Sr, and higher 206Pb/204Pband 143Nd/144Nd ratios, and lacks the depletions in Nb and Eu.Two possible compositions are considered for the low-87Sr/86Srcomponent of the source. The first is at the low-87Sr/86Sr endof the Heard Island data array, represented most closely bylavas from the Laurens Peninsula. However, trace element variationssuggest that these lavas might not be representive of the Heardplume. The second is close to the low-87Sr/86Sr end of the isotopicarray for lavas from the main volcano. In this case a lithosphericmantle origin is suggested for the Laurens Peninsula lavas.The relationships between isotopic data, major element compositions,and incompatible trace element ratios indicate that the continent-derivedmaterial is probably present in the mantle source, where itmakes a maximum contribution of <4 wt.% for all but one HeardIsland sample. However, if the Kerguelen Plateau is a submergedcontinental block, shallow-level contamination cannot be ruledout. The binary mixing model developed to explain the Heard Islandgeochemical variations is extended to include other Indian Oceanoceanic island and mid-ocean ridge basalts (OIB and MORB). Weshow that isotopic compositions of Indian Ocean OIB are consistentwith sampling of a regional reservoir in which the same twocomponents exist in variable proportions (generally 1–5wt.% of the continent-derived component). The distinctive isotopiccompositions of Indian Ocean MORB are consistent with mixingof a similar component into an Atlantic-or Pacific-like MORBmantle source. The relatively unradiogenic 206Pb/204Pb isotopiccompositions of these ‘enriched’ Indian Ocean mantlecomponents are unlike any present-day marine sediments and indicatethat their source has had 238U/204Pb ratios (µ) much lowerthan typical upper continental crust for > 1 Ga. These agespre-date the formation of Gondwana (600-130 Ma) and thereforedo not support sediment subduction beneath Gondwana as the causeof enrichment in the sub-Indian Ocean mantle. We propose thatthe enrichment of Indian Ocean OIB sources was due to subductionof upper-crustal material beneath a Proterozoic precursor ofGondwana at 1–2 Ga. The enrichment of the Indian OceanMORB sources could have had a similar origin, or could havebeen derived from sub-continental lithospheric mantle returnedto the asthenospheric mantle, perhaps during the break-up ofGondwana (200–130 Ma).  相似文献   

14.
西天山喇嘛苏岩体年代学、地球化学及成矿意义   总被引：1，自引：0，他引：1  
文中主要对西天山喇嘛苏岩体进行SHRIMP锆石U-Pb年龄、主微量及Sr-Nd-Pb同位素测定,阐明岩体成因及形成构造背景。喇嘛苏岩体主要由石英二长闪长岩、花岗闪长斑岩和英云闪长斑岩组成。其中,石英二长闪长岩形成于(394.8±4.9)Ma,花岗闪长斑岩和英云闪长斑岩形成于(380.9±3.9)Ma,略晚于石英二长闪长岩。岩体具有埃达克质岩的特征,且显示从钙碱性向高钾钙碱性演化的趋势,稀土元素配分曲线显示相对富集轻稀土((La/Yb)N:3.55～15.52)及中等的负或正Eu异常(δEu:0.53～1.12)。岩体具有较高的Sr含量((322～808)×10-6)和较低的Y含量((12.90～18.86)×10-6)。微量元素特征显示岩体富集LILE亏损HFSE,并具有Nb、Ta和Ti负异常。岩体初始Sr-Nd同位素组成为εNd(t)=-4.29～+0.75和ISr=0.706 052～0.708 263,Nd模式年龄为1.03～1.46Ga。花岗闪长斑岩和英云闪长斑岩的铅同位素特征为206Pb/204Pb=18.500～19.044,207Pb/204Pb=15.575～15.626,208Pb/204Pb=38.443～38.864;石英二长闪长岩为206Pb/204Pb=18.694～18.711,207Pb/204Pb=15.622～15.630,208Pb/204Pb=38.648～38.660。所有地球化学特征显示喇嘛苏岩体是俯冲洋壳部分熔融形成的熔体,上升过程中与受俯冲带沉积物交代的地幔楔相互作用,且有少量古老地壳的混染而形成。岩体形成于晚泥盆世准噶尔残余洋盆向伊犁—中天山地块俯冲的大陆弧背景,与该区Cu(Au)矿化有较密切的联系。  相似文献   

15.
Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; )Nd=0.2-1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '18O=8.0 and '13C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.  相似文献   

16.
Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.  相似文献   

17.
研究中天山地块卡瓦布拉克杂岩带的成因,对重建中天山南缘古生代洋陆构造格局及其演变过程具有重要意义。在查明杂岩带中-基性岩主体岩类矿物学及岩石学特征的基础上,分析了该岩类的主、微量元素和Sr Nd Pb同位素组成。结果显示:其属于偏基性的富钠、低钾岩石系列,富集大离子亲石元素(Rb、U、Sr)和轻稀土元素(LREE),亏损高场强元素(Nb、Ta、Zr)和重稀土元素(HREE)。全岩Sr Nd同位素组成具有较低的锶初始比值(ISr=0706~0709)和较高的εNd(t)值(684~976),并具有较低的放射性成因铅同位素组成,初始铅同位素比值(206Pb/204Pb)i=18015~18239、(207Pb/204Pb)i=15573~15593、(208Pb/204Pb)i=37871~38207。结合前人研究成果和区域地质资料,认为其母岩浆应该来自被交代的年轻富集岩石圈地幔,代表岛弧岩浆,说明卡瓦布拉克杂岩带中-基性岩可能代表该区晚古生代岩浆弧的一部分。  相似文献   

18.
In the Mediterranean area, lamproitic provinces in Spain, Italy, Serbia and Macedonia have uniform geological, geochemical and petrographic characteristics. Mediterranean lamproites are SiO2-rich lamproites, characterized by relatively low CaO, Al2O3 and Na2O, and high K2O/Al2O3 and Mg-number. They are enriched in LILE relative to HFSE and in Pb, and show depletion in Ti, Nb and Ta. Mediterranean lamproites show huge regional variation of Sr, Nd and 207Pb/204Pb isotopic values, with 87Sr/86Sr range of 0.707-0.722, εNd range from −13 to −3, and 207Pb/204Pb range of 15.62-15.79.Lamproitic rocks are derived from melts with three components involved in their origin, characterized by contrasting geochemical features which appear in 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd space: (i) a mantle source contaminated by crustal material, giving rise to crust-like trace element patterns and radiogenic isotope systematics, (ii) an extremely depleted mantle characterized by very low whole-rock CaO and Al2O3, high-Fo olivine and Cr-rich spinel, which isotopically resembles European peridotitic massifs and lithospheric mantle; (iii) a component originating from the convecting mantle, characterized by unradiogenic 87Sr/86Sr and radiogenic 143Nd/144Nd and 206Pb/204Pb. These components demand multistage preconditioning of the lamproite-mantle source, involving an episode of extreme depletion, followed by involvement of terrigenous sediments, and finally interaction with melts originating from the convecting mantle, some of which are probably carbonatitic.We use our data on Mediterranean lamproites to characterize the mantle composition under the whole Alpine-Himalaya belt. Lamproites are an integral part of postcollisional volcanism, and are the most extreme melting products from a mantle which is ubiquitously crustally metasomatized. Enriched isotope signatures in Himalayan volcanics can also be explained by the involvement of subducted sediments instead of by proterozoic mantle lithosphere.  相似文献   

19.
Neogene plateau lavas in Patagonia, southern Argentina, eastof the volcanic gap between the Southern and Austral VolcanicZones at 46·5° and 49·5°S are linked withasthenospheric slab window processes associated with the collisionof a Chile Ridge segment with the Chile Trench at 12 Ma. Thestrong ocean-island basalt (OIB)-like geochemical signatures(La/Ta <20; Ba/La <20; 87Sr/86Sr = 0·7035–0·7046;143Nd/144Nd = 0·51290–0·51261; 206Pb/204Pb= 18·3–18·8; 207Pb/204Pb = 15·57–15·65;208Pb/204Pb = 38·4–38·7) of these Patagonianslab window lavas contrast with the mid-ocean ridge basalt (MORB)-like,depleted mantle signatures of slab window lavas elsewhere inthe Cordillera (e.g. Antarctic Peninsula; Baja California).The Patagonian lavas can be divided into a voluminous  相似文献   

20.
207Pb/204Pb versus 206Pb/204Pb model ages using Shonkin Sag data and published analyses for magmas of the Cenozoic Wyoming-Montana alkaline province (WYMAP) provide evidence of an Archean age for the subcontinental lithospheric mantle (SLM) associated with the Wyoming craton. The SLM imprint on magmas is expressed as Ba, Ta, Nb and Ti "anomalies" which correlate with radiogenic isotopic data, and it resembles a subduction imprint on Cenozoic south-western USA basalts (SWUSAB). However the latter give Proterozoic Pb isotope model ages. Although the Archean and Proterozic model ages may represent mixing lines, the fact that they resemble the ages for continental crust cut by WYMAP and SWUSAB respectively indicates that the age of the underlying SLM helped control the "isochron" slopes and inferred "ages". Lower 143Nd/144Nd and 206Pb/204Pb but comparable 87Sr/86Sr for WYMAP suggest that SLM associated with Archean cratons has lower Sm/Nd, U/Pb and Rb/Sr ratios than SLM associated with SWUSAB Proterozic terranes, regardless of when the subduction imprint or imprints developed. WYMAP magmas have high Pb/Zr ratios indicating that Archean SLM, like Archean continental crust, is enriched in Pb compared to Proterozoic SLM. If the enrichment was Archean, it implies that higher Archean heat flow enhanced Pb transfer from the subducting slab to overlying lithospheric mantle and crust. A subducted sediment imprint on the SLM is also consistent with high i18O values for the Shonkin Sag. Low TiO2 in WYMAP may reflect a residual mantle TiO2 phase. If so, the Nb "missing" from crustal and oceanic mantle reservoirs may reside in rutile of Archean SLM. Isotopic similarities between WYMAP and EM1 oceanic island basalts may reflect the presence of delaminated, Archean SLM in the oceanic mantle, although low Pb/Zr ratios and a lack of Ti, Nb and Ta anomalies in oceanic island basalts deserve further investigation.  相似文献   

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