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1.
Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.  相似文献   

2.
A group of oceanic islands and/or seamounts (hereafter, paleoseamounts) was produced by oceanic hot-spot magmatism in the Late Proterozoic-Early Cambrian in the southwestern margin of the Paleo-Asian Ocean. They were accreted to the Kuznetsk-Altai island arc in the Late Cambrian and were subsequently incorporated during the closing of the paleocean into the accretionary complexes of the western part of the Altai-Sayan area (southwestern Siberia, Russia). The major-and trace-element compositions and Sr and Nd isotopic systematics of pillow lavas and basalt flows from the Kurai (600 Ma) and Katun’ (550–530 Ma) paleoseamounts of Gorny Altai characterize the evolution of Hawaiian-type magmatism in the Paleo-Asian Ocean during that period. The obtained data show a significant change in lava composition between 600 and 550–530 Ma. The tholeiitic basalts of the Kurai Paleoseamount (600 Ma) from the southern part of Gorny Altai have lower incompatible element contents and higher 147Sm/144Nd values compared with the younger tholeiitic and alkali basalts of the Katun’ Paleoseamount (550–530 Ma), whose rocks are exposed in northern Gorny Altai. The trace-element compositions of the Katun’ lavas are similar to those of the Hawaiian tholeiites, and their 147Sm/144Nd ratios are lower than those of the Kurai basalts. It was suggested that the older Kurai Paleoseamount was formed above a thinner oceanic lithosphere, i.e., closer to a paleospreading axis compared with the younger Katun’ Paleoseamount. The observed temporal variations in the chemical and isotopic characteristics of lavas are probably related to differences in the degree of melting of the heterogeneous mantle owing to the different thickness of the oceanic lithosphere above which the Kurai and Katun’ paleoseamounts were formed. During the Ediacaran, a plume developed beneath the younger and, consequently, thinner lithosphere of the Paleo-Asian Ocean. The higher degree of melting in the mantle column resulted in a more considerable contribution from the refractory depleted material of the upper mantle. After 50–70 Ma, i.e., in the Early Cambrian, the plume affected a thicker lithosphere, its mantle column became shorter, and the degree of melting was lower. Owing to this, the basaltic melt was more contributed by incompatible element enriched less refractory material of the lower mantle.  相似文献   

3.
The 3500-m-thick sequence of volcanic rocks at Noril'sk, formed during a brief interval (～1 m.y.) at the Permian/Triassic time boundary (～251 Ma), represents the earliest part of the ～6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Noril'sk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to primary mantle melts; they varied from low-Mg basalts to olivine tholeiites or picrites, with normal tholeiites predominating.

The primitive lavas are subdivided into four groups (magma types) on the basis of trace-element ratios (principally, Gd/Yb, Th/U, La/Yb, Ta/La, Ti/Sc, and V/Yb) and isotopic data. Three of the groups include both basaltic and picritic primitive lavas (with low-Mg basalts present in one of them), whereas the fourth group is represented exclusively by tholeiites. Distinctions among the groups cannot be related to degree of melting, and isotopic data indicate that none of the magma types could have formed by mixing or contamination of other types. Apparently, only differences in source composition and/or depth of melting can explain the magmatic diversity.

This multitude of primitive magma types may be explained by melting in different layers of the upper mantle, which is complexly layered beneath Siberia to depths of 270 km. Moreover, no clear boundary between lithosphere and asthenosphere is evident in the deep seismic profile. A large-scale event is necessary to account for melting in different parts of the upper mantle and formation of the great volume of the Siberian flood basalts in ～1 m.y. Extension, caused by ascent of a mantle plume, would provide a reasonable explanation, but no plume-related uplift is documented in north-central Siberia prior to, or during, eruption of the volcanic sequence.  相似文献   

4.
The Ueno Basalts of central Japan comprise a monogenetic volcaniccone complex that was active between 2·76 and 1·34Ma. Basalts were erupted at more than 14 centers scattered overa region 40 km in diameter. Alkali basalt was erupted first,followed by sub-alkaline basalt. Quasi-concentric expansionof eruption centers coinciding with uplift and with decreasingalkalinity of the lavas suggests that Ueno magmatism originatedfrom a mantle diapir as it mushroomed at the base of the lithosphere.Depleted asthenospheric mantle (alkali basalt), enriched lithosphericmantle (sub-alkaline basalt), and crustal components are identifiedas chemical end-members in the petrogenesis of the Ueno Basalts.Incompatible trace element abundances indicate that the Uenoalkali basalts are typical within-plate basalts, whereas thesub-alkaline basalts show strong affinities with normal arclavas. Sr–Nd–Pb isotopic compositions indicate thatthe mantle source of the alkali basalts was more depleted thanthat of the sub-alkaline basalts. About 7% melting of asthenosphericmantle in the garnet-lherzolite stability field produced theprimitive alkali basalts and 12% melting of spinel lherzolitewithin the subcontinental lithosphere produced the primitivesub-alkaline basalts. Isotopic compositions and fluid mobile/immobileelement ratios broadly covary with SiO2 contents in the sub-alkalinesuite, and increasing silica content is associated with strongerEMII (Enriched Mantle II) isotope affinities and fluid mobileelement abundances. A progressive AFC (assimilation–fractionalcrystallization) model assuming assimilation of a low-K silicicmelt reproduces the chemical variations observed in the sub-alkalinesuite. Melting of a flattening mantle diapir at the base ofthe lithosphere is the dominant cause of Ueno magmatism, accompaniedby the assimilation of older arc crust. KEY WORDS: arc basalt; crustal assimilation; mantle heterogeneity; Ueno Basalts  相似文献   

5.
Hawaiian volcanoes, dominantly shields of tholeiitic basalt,form as the Pacific Plate migrates over a hotspot in the mantle.As these shields migrate away from the hotspot, highly alkaliclavas, forming the rejuvenated stage of volcanism, may eruptafter an interval of erosion lasting for 0·25–2·5Myr. Alkalic lavas with geochemical characteristics similarto rejuvenated- stage lavas erupted on the sea floor north ofOahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce,Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the NorthArch and the rejuvenated-stage Honolulu Volcanics were controlledduring partial melting by residual garnet, clinopyroxene, Fe–Tioxides and phlogopite. However, the distinctively high Ba/Thand Sr/Nd ratios of lava forming the North Arch and HonoluluVolcanics reflect source characteristics. These characteristicsare also associated with shield tholeiitic basalt; hence theyarise from the Hawaiian hotspot, which is interpreted to bea mantle plume. Inversion of the batch melting equation usingabundances of highly incompatible elements, such as Th and La,requires enriched sources with 10–55% clinopyroxene and5–25% garnet for North Arch lavas. The 87Sr/86Sr and 143Nd/144Ndratios in lavas forming the North Arch and Honolulu Volcanicsare consistent with mixing between the Hawaiian plume and adepleted component related to mid-ocean ridge basalts. Specifically,the enrichment of incompatible elements coupled with low 87Sr/86Srand high 143Nd/144Nd relative to bulk Earth ratios is best explainedby derivation from depleted lithosphere recently metasomatizedby incipient melt (<2% melting) from the Hawaiian plume.In this metasomatized source, the incompatible element abundances,as well as Sr and Nd isotopic ratios, are controlled by incipientmelts. In contrast, the large range of published 187Os/188Osdata (0·134–0·176) reflects heterogeneitycaused by various proportions of pyroxenite veins residing ina depleted peridotite matrix. KEY WORDS: Hawaiian plume; Honolulu Volcanics; North Arch; plume–lithosphere interaction; rejuvenated stage; trace element geochemistry; alkalic lavas  相似文献   

6.
韩江伟  熊小林  朱照宇 《岩石学报》2009,25(12):3208-3220
对雷琼地区21个晚新生代玄武岩样品的主量、微量元素和Sr、Nd、Pb同位素分别用湿化学法、ICP-MS和MC-ICPMS进行了测定.这些玄武岩主要为石英拉斑玄武岩,其次为橄榄拉斑玄武岩和碱性玄武岩.大多数样品的微量元素和同位素成分与洋岛玄武岩(OIBs)相似,而且随着SiO_2不饱和度增加,不相容元素含量也增加.除R4-1可能受到地壳混染外,其他样品相对均一的Nd同位素(ε_(Nd)=2.5-6.0)以及变化明显但范围有限的Sr同位素(0.703106～0.704481),可能继承了地幔源区的特征.~(87)Sr/~(86)Sr与~(206)Pb/~(204)Pb的正相关和~(143)Nd/~(144)Nd与~(206)Pb/~(204)Pb的负相关特征暗示DM(软流圈地幔)与EM2(岩石圈地幔)的混合.地幔捕虏体的同位素特征暗示EM2成分不可能存在于尖晶石橄榄岩地幔,而La/Yb和Sm/Yb系统表明岩浆由石榴石橄榄岩部分熔融产生,这意味着EM2成分可能存在于石榴石橄榄岩地幔.雷琼地区玄武岩的地球化学变化可以用软流圈地幔为主的熔体加入不同比例石榴石橄榄岩地幔不同程度熔融产生的熔体来解释:碱性玄武岩和橄榄拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较低程度(7%～9%)熔融体混合,而石英拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较高程度(10%～20%)熔融体的混合.  相似文献   

7.
The Hawaiian–Emperor Seamount Chain (ESC), in the northernPacific Ocean, was produced during the passage of the PacificPlate over the Hawaiian hotspot. Major and trace element concentrationsand Sr–Nd–Pb isotopic compositions of shield andpost-shield lavas from nine of the Emperor Seamounts providea 43 Myr record of the chemistry of the oldest preserved Hawaiianmagmatism during the Late Mesozoic and Early Cenozoic (from85 to 42 Ma). These data demonstrate that there were large variationsin the composition of Hawaiian magmatism over this period. Tholeiiticbasalts from Meiji Seamount (85 Ma), at the northernmost endof the ESC, have low concentrations of incompatible trace elements,and unradiogenic Sr isotopic compositions, compared with youngerlavas from the volcanoes of the Hawaiian Chain (<43 Ma).Lavas from Detroit Seamount (81 Ma) have highly depleted incompatibletrace element and Sr–Nd isotopic compositions, which aresimilar to those of Pacific mid-ocean ridge basalts. Lavas fromthe younger Emperor Seamounts (62–42 Ma) have trace elementcompositions similar to those of lavas from the Hawaiian Islands,but initial 87Sr/86Sr ratios extend to lower values. From 81to 42 Ma there was a systematic increase in 87Sr/86Sr of boththoleiitic and alkalic lavas. The age of the oceanic lithosphereat the time of seamount formation decreases northwards alongthe Emperor Seamount Chain, and the oldest Emperor Seamountswere built upon young, thin lithosphere close to a former spreadingcentre. However, the inferred distance of the Hawaiian plumefrom a former spreading centre, and the isotopic compositionsof the oldest Emperor lavas appear to rule out plume–ridgeinteraction as an explanation for their depleted compositions.We suggest that the observed temporal chemical and isotopicvariations may instead be due to variations in the degree ofmelting of a heterogeneous mantle, resulting from differencesin the thickness of the oceanic lithosphere upon which the EmperorSeamounts were constructed. During the Cretaceous, when theHawaiian plume was situated beneath young, thin lithosphere,the degree of melting within the plume was greater, and incompatibletrace element depleted, refractory mantle components contributedmore to melting. KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity  相似文献   

8.
Many objections have been raised as to the ability of subcontinental lithospheric mantle to produce voluminous amounts of basalt, because this upper part of the mantle is thought to be refractory, and the geotherm is rarely above the peridotite solidus at these depths under continents. However, in the Pacific Northwest of the USA during the Neogene, the subcontinental lithospehric mantle has been proposed as a key source for basalts erupted within the northern Basin and Range, and for the Columbia River flood basalts erupted on the Columbia Plateau. An alternative explanation to melting in the subcontinental lithospheric mantle, which equally well explains the chemical compositions thought to originate there, is that these magmas were contaminated by crust of varying ages. Calc-alkaline lavas, which occupy the Blue Mountains in the center of this region, hold clues to the latter process. Their elevated trace element ratios (e.g., Ba/Zr, K2O/P2O5), coupled with differentiation indicators such as Mg? [molar Mg/(Mg?+?Fe)], and Sr, Nd, and Pb isotopic compositions, can most reasonably be explained by crustal contamination. Appraisal of continental peridotite xenolith data indicates that high trace element ratios such as Ba/Zr in continental basalts cannot result from melting in the subcontinental lithospheric mantle. Instead, as with the calc-alkaline lavas, these high ratios in the tholeiites most likely indicate crustal contamination. Furthermore, the peridotite xenoliths do not have a relative depletion in Nb and Ta that is observed in most of the lavas within the region. Relatively minor volumes of tholeiites erupted in late Neogene times in the northern Basin and Range (Hi-Mg olivine tholeiites) and Columbia Plateau (Saddle Mountains basalts), are the only lavas which have trace element and isotopic compositions consistent with being derived from, or largely interacting with a subcontinental lithospheric mantle in the Pacific Northwest. In contrast to the prior studies, we suggest that the mantle sources for most of the basalts in this region were ultimately beneath the lithospheric mantle.  相似文献   

9.
The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283,87Sr/86Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar  相似文献   

10.
The Pb isotope compositions of amphiboles and clinopyroxenesin spinel peridotite and pyroxenite mantle xenoliths from theintra-plate Quaternary volcanic fields of the Eifel province(Germany) are strongly correlated with their Sr–Nd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and Sr–Nd–Pbisotope compositions similar to the depleted source of mid-oceanridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearingxenoliths from shallower depths (<45 km) provide evidencefor three temporally distinct episodes of mantle metasomatismin the subcontinental lithosphere: (1) aqueous fluids from anisotopically enriched (EM-like) mantle reservoir caused amphiboleformation during deformation in the shallow continental lithosphericmantle and may be subduction related, probably associated withthe last major tectonic event that influenced the area (Hercynianorogeny). (2) During a second phase of mantle metasomatism theEM-like lithospheric mantle was affected by melts from an ancient,HIMU-like (high time-integrated µ = 238U/204Pb) mantlesource. The HIMU-like component introduced by these fluids hada much more radiogenic Pb isotope composition than the asthenosphericsource of the widespread Cenozoic magmatism in Europe and maybe linked to reactivation of ancient subducted crustal domainsduring the Hercynian orogeny or to early Cretaceous deep-sourcedmantle plumes. (3) During a brief final stage the heterogeneouslyenriched EM–HIMU subcontinental lithosphere was locallymodified by basaltic melts migrating along fractures and veinsthrough the upper mantle as a consequence of the Cenozoic Eifelvolcanism. Although a DMM component is completely lacking inthe metasomatic fluids of the metasomatic episodes 1 and 2,the vein melts of episode 3 and the Cenozoic Eifel lavas requiremantle sources containing three end-member components (DMM–HIMU–EM).Thus, mobilization of the more depleted mantle material occurredat the earliest in the Tertiary, contemporaneously with thedevelopment of the extensive rift system and main melt generationin Europe. Alternatively, the variety of Sr–Nd–Pbisotope signatures of the metasomatic agents may have been producedby melting of isotopically distinct mantle domains in a heterogeneousuprising mantle plume. KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes  相似文献   

11.
Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher 206Pb/204Pband 87Sr/86Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting  相似文献   

12.
Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and 87Sr/86Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (206Pb/204Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.  相似文献   

13.
东天山自然铜矿化带玄武岩的起源、演化及成岩构造背景   总被引：9，自引：5，他引：4  
新疆东天山地区与玄武岩有关的自然铜矿化带位于东天山觉罗塔格构造带内,自西向东有十里坡、黑龙峰、长城山、东尖峰等主要矿(化)点,自然铜矿化主要发育在玄武岩、杏仁状玄武岩及凝灰岩夹层中。本文基于玄武岩的地球化学特征,研究东天山自然铜矿化带玄武岩是否与地幔柱有关、岩浆源区性质、岩浆演化、成岩构造背景等问题。研究显示,东天山自然铜矿化带玄武岩与地幔柱岩浆活动无直接关系,整个天山地区是否存在石炭-二叠纪地幔柱岩浆活动也需要进一步的研究; 玄武岩起源于亏损岩石圈地幔,是演化岩浆的产物; 演化的玄武质岩浆形成后,在岩浆房中或上升至地表的过程中没有发生明显的分离结晶作用,也没有受到明显的地壳物质混染; 与玄武岩对应的地幔橄榄岩平衡原生岩浆演化的高镁岩浆的产物,可能为东天山地区与铜镍硫化物成矿有关的基性-超基性岩,指示这些铜镍硫化物矿床可能与地幔柱岩浆活动也没有关系; 玄武岩形成于新疆北部后碰撞构造阶段的伸展期,是在拉张应力体制下,由于软流圈上涌导致岩石圈地幔部分熔融而形成。  相似文献   

14.
The Cameroon line comprises a 1600-km long Y-shaped chain of< 30 m.y. old volcanoes and <70 m.y. old plutons extendinginto mainland Africa from the Atlantic island of Pagalu. Thedistribution of basaltic volcanic centres is ideal for comparingsub-continental and sub-oceanic sources for basalts and forstudying the influence of the lithosphere on magma generation.We report Nd, Sr, Pb and O isotopic data for more than thirty(principally basaltic) samples from all the main volcanic centrestogether with data for two granulite facies xenoliths. Thosebasalts which display no obvious evidence of crustal contaminationyield initial 87Sr/86Sr ratios ranging from 0.7029 to 0.7035,Nd between +2 and +7 and 206Pb/204Pb between 19?0 and 20?6.The Nd and Sr isotopic compositions define a field on the lefthand side of the ‘mantle array’ (that is with relativelyunradiogenic Sr) and include some data which show overlap withcompositions observed for St. Helena. 208Pb/204Pb ratios extendto 40?4—amongst the more radiogenic observed for alkalibasalts. The Nd and Sr isotopic data are similar in oceanicand continental sectors indicating that the magmas are derivedfrom generally similar mantle sources. Despite this overallsimple picture, the source of the Cameroon line volcanics hasin fact been variable in both time and space. Pb is less radiogenicand Sr is more radiogenic in transitional to hypersthene-normativecompositions. There is a progression to more radiogenic leadisotopic compositions with time for the Cameroon line as a wholethat is most strikingly displayed in the 30 m.y. eruptive historyof Principe. These space-time data are difficult to reconcilewith conventional plume models or with some dispersed ‘plumpudding’ models. The heterogeneities require isolationtimes considerably longer than the age of the south Atlanticsea floor (120 Ma). The eruptive lavas with the most radiogenicPb observed (accompanied by unradiogenic Nd) precisely straddlethe continental edge (i.e. occur in both oceanic and continentalsectors) with no dependency on Nd and Pb concentrations. A modelis proposed which links these observations with the destructionof lithosphere, and the impregnation of the uppermost mantleby the St. Helena hot spot during the formation of the SouthAtlantic ocean. This mantle was subsequently melted to formthe Cameroon line which appears to be derived from a risinghot zone initiated by the early plume activity. The magmaticproducts reflect the mantle mixing that took place during continentalbreakup, the consequent cooling and thickening of the lithosphereand the continued interaction between rising plume componentsand this lithosphere. The depth from which magmas are currentlybeing tapped at the continent/ocean boundary is estimated atless that 150 km.  相似文献   

15.
The petrology and geochemistry of the Azores Islands   总被引：7，自引：0，他引：7  
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.  相似文献   

16.
峨眉山大火成岩省：地幔柱活动的证据及其熔融条件   总被引：138，自引：5，他引：138  
对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。  相似文献   

17.
保山卧牛寺组玄武岩为低钛拉斑玄武岩，具有大陆板内玄武岩的特征，总体与蛾眉山玄武岩中的低钛玄武岩相似。其分异程度较高，富集大离子亲石元素和高场强元素，有明显的Nb、Ta负异常，Zr和Hf正异常，Eu无异常或弱负异常。该玄武岩由3个大喷发旋回形成3个岩石单元，其中下部早期第1单元由致密块状玄武岩、斜斑玄武岩、杏仁玄武岩和凝灰岩组成，第2和第3单元中见少量橄榄玄武岩和粒玄岩以及辉绿岩脉。有较高的^86Sr／^87Sr值(0．705966—0．706657)、较低的^143Nd／^144Nd值(0．512212—0．512283)，εNd(t)多为负值，源区为介于EMI和EMⅡ端员之间的富集岩石圈地幔。上述地球化学和同位素特征均可与蛾眉山玄武岩下部低钛拉斑玄武岩对比，表明该期岩浆作用可能同为地幔柱活动的产物，并暗示蛾眉山大火成岩省向西有很大的延伸。  相似文献   

18.
Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.  相似文献   

19.
The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 105 km3) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 104km3, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/86Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle.Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within < 2 Ma during the Oligocene. Ignimbritic eruptions resumed in the Miocene during two episodes dated at 15.4 ± 0.2 Ma and 8.0 ± 0.2 Ma for the Debre Birhan area. The Rb-Sr isochron ages of ignimbrites (both Oligocene and Miocene rhyolites) are indistinguishable within uncertainties from the 40Ar/39Ar ages of the underlying flood basalts. The Oligocene ignimbrites and the underlying trap basalts are synchronous with a shift in the oxygen composition of foraminifera recorded in Indian and Atlantic Ocean cores. The temporal coincidence of Ethiopian Oligocene volcanism, which released immense volumes of S (> 1.4 × 1015 mol) and Cl (6.4 × 1015 mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.  相似文献   

20.
Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (87Sr/86Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.  相似文献   

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