首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Volcanic rocks on the island of Lipari show the entire range of Sr, Nd, Pb isotopic compositions displayed by other islands in the Aeolian archipelago. The rapid isotopic evolution of subaerial volcanic rocks on Lipari towards crustal values together with the appropriate isotopic composition of the neighbouring Calabrian crust (Serre) indicate that many geochemical characteristics observed in the lavas can be attributed to contamination and mixing with crustal materials and melts. Interpretation of the data is complicated by the fact that underplating onto the crust-mantle boundary and the specific lithologies present in the crustal section differ underneath each individual sector of the island. In the central and northern parts of the island, metapelitic rocks were incorporated to provide the more radiogenic Sr isotopic compositions of some lavas. The products from M. Guardia in the southern part of Lipari, where activity is restricted to the last 30–40 ka, bear geochemical similarities to the island of Vulcano, where it is proposed that considerable remobilization of the crust took place in the presence of mafic mantle-derived melts. On Lipari the petrogenetic processes of magma mixing and assimilation dominate over fractional crystallization, and the observed increase of K2O over Na2O can be correlated with contributions from metapelitic crustal lithologies. It is suggested that the variability in isotopic composition and the budget of alkalis (Na2O versus K2O) in the lavas can be explained by invoking a heat source from an intruding asthenospheric MORB-type mantle into a cooler lithospheric crust/mantle during the opening of the Tyrrhenian basin.  相似文献   

2.
长白山区二道白河流域早更新世玄武质熔岩的成因   总被引：2，自引：1，他引：1  
马晗瑞  杨清福  盘晓东  武成智  陈聪 《岩石学报》2015,31(11):3484-3494
采用岩石化学和同位素分析方法,研究了二道白河流域早更新世玄武质熔岩的成因。玄武质熔岩由钠质拉斑玄武岩和钾质粗面玄武岩、玄武质粗面安山岩组成。它们的REE分配形式比较相近,表明它们来自共同的源区。Sr、Nd、Pb同位素示踪表明,二道白河流域早更新世玄武质熔岩岩浆源区接近于似原始地幔。它们的Mg#=100Mg O/(Mg O+Fe O)低于中国东部新生代玄武岩原始岩浆的Mg#(60~68),Ni(27.76×10-6~200.6×10-6)低于原始地幔,Rb/Sr(0.05~0.09)、Ba/Rb(15.64~264)高于原始地幔,说明这些岩石不是源自原始地幔。玄武质熔岩的DI变化于42~67,具有高Ca、高Sr、Eu正异常,微量元素图解显示玄武岩保留部分熔融趋势,粗面玄武岩、玄武质粗安岩具有结晶分异趋势,岩浆上升过程中发生了不同程度的地壳混染作用。玄武质熔岩的Nb/Ta之比为14.8~15.8,与勘察加半岛深俯冲带火山类似。Nb/Ta-(Na2O-K2O)关系图解显示研究区玄武质岩浆的形成与俯冲板片的部分熔融有关。  相似文献   

3.
The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr87/Sr86, 0.7042 to 0.7090; Pb206/Pb204, 16.31 to 17.30; Pb208/Pb204, 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.Publication authorized by the Director, U.S. Geological Survey.  相似文献   

4.
The strontium isotopic compositions have been determined for twelve tholeiitic basalts dredged from the Gordo and Juan de Fuca Rises. Sr87/Sr86 ratios range from 0.7012 to 0.7031 and average 0.7026. These data, combined with other data from the East Pacific Rise indicate that tholeiite basalts being erupted along the active rises, in the Pacific Ocean, contain less radiogenic Sr87 than basalts erupted on the islands. These isotopic differences between the ocean-ridge tholeiite and the more alkali island basalts indicate that variations in Rb/Sr have persisted in the mantle for billions of years. The possible origins and distribution of these heterogeneties are discussed.  相似文献   

5.
The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

6.
Twenty representative rocks ranging from lamprophyric to granitic composition, from the Spanish Peaks igneous Complex, south-central Colorado, were analyzed for Sr isotopic compositions and their concentrations of K, Rb, Sr and Ba. The various igneous rocks from this Cenozoic complex do not have a comagmatic relationship from the evidence of their Sr isotopic compositions. Due to the generally low Sr87/Sr86 isotopic ratios, the possibility of the highly radiogenic underlying Precambrian basement as the source of magma generation can be ruled out. The sources for the magmas of this igneous complex must be in the upper mantle or the lower crust. Model calculations using elemental distribution coefficients and assumed mantle materials suggest that the abundant lamprophyric magmas in this region could be derived from a phlogopite-bearing hornblende peridotite by a small degree of partial melting (<5%) at lower pressure environment (<50 km). Other possibilities for lamprophyric magma generation were also examined. The slightly higher Sr87/Sr86 ratios observed in the granitic rocks are interpreted as reflecting the nature of this source-the lower crust. Alternatively, they may suggest a limited contamination of the original liquid by upper crustal material. For the entire igneous complex, mixing of two independent magmas, lymprophyric and granitic, is suggested to be the mechanism responsible for the complicated and diverse chemical characteristics.  相似文献   

7.
Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr87/Sr86 ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr87/Sr86 ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr87/Sr86 ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr87/Sr86 ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr87/Sr86 ratio or (ii) selective diffusion of material of high Sr87/Sr86 ratio into kimberlitic fluids.  相似文献   

8.
G.G. Pe  A. Gledhill 《Lithos》1975,8(3):209-214
Isotopic ratios of strontium in 9 volcanic rocks from the south-eastern part of the Hellenic arc range from 0.7037 to 0.7075. Within individual series of differentiation, there seems to be a correlation between Sr87/Sr86 and K2O/SiO2.All strontium isotope data for the Hellenic arc are reviewed. Comparable (but slightly smaller) ranges of Sr isotope ratios are found in other island arcs with continental basement. To explain the high values of Sr87/Sr86 ratio for the Hellenic arc, a selective addition of Sr87 from the wall rock, and a process of assimilation involving water, perhaps from subducted sediments, are suggested. Since closely-spaced individual volcanic centres of similar ages have very different Sr isotope ratios, and since the range of Sr isotopic composition in individual centres is quite large, the variation is unlikely to be due to primary variation in mantle composition.  相似文献   

9.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

10.
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ 18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.  相似文献   

11.
The mid‐Cretaceous Spences Bridge Group (SBG) comprises a series of basaltic to rhyolitic lavas and related volcaniclastic rocks (Pimainus Formation) overlain by a succession of mainly amygdaloidal andesites (Spius Formation) related to the closure of the Methow–Tyaughton basin and accretion of the Insular terrane in the North American Cordillera. Geochemical variation in the SBG is related primarily to metasomatic processes in the mantle wedge. Pimainus lavas include low‐ to high‐K, tholeiitic and calc‐alkaline types, and have isotopic compositions (εNd(100Ma) = + 5.2 to + 7.0, εSr(100Ma) = − 10 to − 20, 206Pb/204Pb = 18.82 to 18.91, 207Pb/204Pb = 15.55 to 15.60, 208Pb/204Pb = 38.24 to 38.43) between the ranges for primitive arcs and accreted terrane crust. Crustal sources are identified only for some low–medium K dacite and rhyolite compositions. The occurrence of intermediate compositions with high MgO contents (up to 6 wt%) and the presence of adakitic trace element features in medium–high K felsic lavas attests to metasomatism of the mantle wedge by slab melts during Pimainus volcanism. Spius lavas have comparable K2O and Pb isotopic compositions to the Pimainus, even higher MgO (up to 9.2 wt%), and display a mild intraplate character in having up to 0.6 wt% P2O5, 15 ppm Nb, and 240 ppm Zr. Spius Nd−Sr isotopic compositions (εNd(100Ma) = + 5.3 to + 6.9, εSr(100Ma) = − 14 to − 25) define an array extending from Pimainus to alkaline seamount compositions. The low εSr values, elevated high field strength element contents, and moderate silica contents suggest Spius volcanism was related to the introduction of small melt fractions from the asthenosphere into the mantle wedge which had previously generated Pimainus melts. The range of compositional types in the Pimainus Formation constrains tectonic scenarios to include an elevated slab thermal regime, likely from approach of an ocean ridge system toward the continental margin. Spius volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab‐hinge roll‐back on closure of the Methow–Tyaughton basin. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

12.
The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high 207Pb/204Pb ratio, as has been found in some other volcanic arcs.87Sr/86Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.  相似文献   

13.
Si87/Sr86 ratios have been determined for lavas and py lastic rocks from three basalt-andesite centers along the Lesser Antilles arc—Mt. Misery on the island of St. Kitts, Soufriere on the island of St. Vincent, and Carriacou, an island of The Grenadines. The average Si87/Sr86 content of these rocks is 0.7038 for Mt. Misery, 0.7041 for Soufriere, and 0.7053 for Carriacou. All the Sr87/Sr86 values from each center are the same within analytical uncertainty (±0.0002). The constancy of strontium isotopic data within each center supports the hypothesis that basalts and andesites for each specific center investigated are generated from the same source — in agreement with petrographic and major- and minor-element data. Strontium isotopic compositions and elemental concentrations, particularly of strontium and nickel, indicate that this source was mantle peridotite and that the relationship between the respective basalts and andesites is probably fractional crystallization.Publication authorized by the Director, U.S. Geological Survey.  相似文献   

14.
Volcanism along the northwest boundary of the Arabian Plate found in the Gaziantep Basin, southeast Turkey, is of Miocene age and is of alkaline and calc-alkaline basic composition. The rare earth element data for both compositional series indicates spinel–peridotite source areas. The rare earth and trace elements of the alkaline lavas originate from a highly primitive and slightly contaminated asthenospheric mantle; those of the calc-alkaline lavas originate from a highly heterogeneous, asthenospheric, and lithospheric mantle source. Partial melting and magmatic differentiation processes played a role in the formation of the petrological features of these volcanics. These rocks form two groups on the basis of their ~(87) Sr/~(86) Sr and ~(143) Nd/~(144) Nd isotopic compositions in addition to their classifications based on their chemical compositions(alkaline and calc-alkaline). These isotopic differences indicate a dissimilar parental magma. Therefore, high Nd isotope samples imply a previously formed and highly primitive mantle whereas low Nd isotope samples may indicate comparable partial melting of an enriched heterogeneous shallow mantle. Other isotopic changes that do not conform to the chemical features of these lavas are partly related to the various tectonic events of the region, such as the Dead Sea Fault System and the Bitlis Suture Zone.  相似文献   

15.
Average 87Sr/86Sr ratios for lavas from Quaternary and Pleistocene volcanoes of the Kurile island arc, NW Pacific, decrease from 0.7035 in the south to 0.7032 in the north. The northern Kuriles are characterised by K2Oricher volcanics and by an older crust. Varying ratios show no simple relation to crustal thickness or geochemical indicators of crustal contamination. This is thought to reflect the immature character of the crust — its simatic composition, low Rb/Sr ratios and youthfulness. Older lavas from the Kuriles (Lower Tertiary, Miocene) have similar or slightly higher 87Sr/86Sr ratios; some have suffered slight alteration and possibly crustal contamination. Quaternary volcanics from the Kurile and Aleutian arcs have the lowest 87Sr/86Sr ratios of all circum-Pacific arcs and this may be ascribed to (a) the isotopic individuality of the landward North American plate and/or (b) the high degree of mechanical coupling between the Pacific and North American plates reducing the amount of subducted 87Sr-rich sediments and seawater. An isotopic boundary between island arcs is located in central Hokkaido. The primary basaltic magmas of the Kuriles were derived from mantle recently contaminated by radiogenic Sr. Subsequent fractionation to andesites and dacites occurred by closed-system fractional crystallization.  相似文献   

16.
Triassic granodiorites in South China (SC) provide an opportunity to examine crust–mantle interactions that may have been caused by a mantle plume. Here we present a combined study of chronological, geochemical, and Sr–Nd–Hf isotopic compositions for Dashenshan granodiorites. These are high-K, calc-alkaline, I-type granodiorites that yield a U–Pb zircon age of 211 ± 3 Ma. They are metaluminous to weakly peraluminous (A/CNK < 1.1), with 3.04–3.89 wt.% Na2O and 3.24–3.86 wt.% K2O, and Na2O/K2O ratio ranging from 0.79 to 1.11. These granodiorites contain 67.7–72.6 wt.% SiO2 but show moderate Mg# values (44.2–57.8) and variable contents of Ni (3.6–29.9 ppm) and Cr (7.6–53.5 ppm). They exhibit light rare earth element (REE) enrichment and flat, heavy REE patterns with negative Eu anomalies (Eu/Eu* = 0.52–0.87). They also display strongly negative Ba, Sr, Nb, Ta, P, and Ti anomalies and positive Rb, Th, K, and Pb anomalies. Dashenshan granodiorites have high whole-rock initial 87Sr/86Sr ratios (0.7121–0.7172), negative εNd (t) values (–8.8 to –6.8), and negative zircon εHf (t) values (–6.6 to –3.3). These results suggest that the Dashenshan granodiorites were generated by a mixing between crustal melt and mantle-derived magma in an extensional setting. We conclude that generation of the Dashenshan pluton may reflect an interaction between a mantle plume and the overlying SC crust.  相似文献   

17.
A model for Sr evolution in the West Greenland-Labrador region is proposed based on the available data for granitic gneisses ranging in age from 3.6 AE to 1.9 AE and for recent continental tholeiites from West Greenland. The evolution of initial Sr isotopic composition is consistent with a two-stage model for the mantle in this area with a constant Rb/Sr ratio of 0.014 ± 0.002 from ～ 4.45 AE through the present. The model suggests the mantle may have been depleted in Rb ～ 4.45 AE ago if the Earth was originally chondritic with respect to Rb and Sr. This age of differentiation is consistent with recently proposed terrestrial differentiation based on Pb Pb analyses of ancient Amîtsoq gneiss feldspars (～4.47 AE, Gancarz and Wasserburg, 1977). A linear regression of the gneisses, if extrapolated to the present, predicts the present value of Sr87/Sr86 of the mantle in this area to be 0.7034 ± 0.0003 which is the measured Sr isotopic composition of the recent Svartenhuk tholeiites. Various implications of the model with respect to early Earth history, mantle evolution and pre-emplacement histories of gneissic suites are discussed.  相似文献   

18.
This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (87Sr/86Sr=0.70597–0.70653; 206Pb/ 204Pb=18.862–19.018; 207Pb/204Pb=15.640–15.686; 208Pb/ 204Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (87Sr/86=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high 87Sr/86Sr (71141–0.71240) and low 206Pb/204Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H2O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.  相似文献   

19.
The picritic lavas of Baffin Island represent one of the most primitive Phanerozoic volcanic suites in the world with MgO contents ranging from 22 wt% (29 Mg, cation unit = Mg/100 cations) for olivine-rich lavas to 11 wt% (16 Mg) for olivine-poor lavas. Two magma types can be recognized on the basis of trace element and isotopic geochemistry. N-type magma, which dominates the high-MgO lavas, has depleted LREE patterns [(La/Sm)N0.6–0.7] typical of N-MORB, K/Ti<0.05, and 87Sr/86Sr <0.7032. E-type magma, which dominates the lower MgO lavas, has flat to slightly enriched LREE patterns [(La/Sm)N1.1–1.2] typical of E-MORB, K/Ti>0.5 and 87Sr/86Sr ranging between 0.7032–0.7039. These two magma types are, however, virtually indistinguishable in terms of major clements and many other trace elements. The E and N-type samples are intermixed throughout the volcanic succession, indicating that both types of magma erupted contemporaneously. Although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation, crystal fractionation cannot account for the difference in the LREE between E-type and N-type lavas. Crustal contamination involving a lower crust composition cannot reproduce the more magnesian E-type lavas.and can only repoduce the lowest (La/Sm)N ratios of the E-type lavas if high degrees of assimilation (50%) have occurred. Partial melting models can reasonably account for the distinct (La/Sm)N ratios and the similar Zr/Y values of the two magma types, but fail to reproduce the observed abundances or REE, Sr, Y and Zr. Compositionally different mantle sources are required to explain the two distinct magma types observed in Baffin Island. A model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of N and E-type magmas in the Baffin Bay picritic suite. The Baffin Island E-type lavas are less enriched in Sr, Y and Zr contents and have lower Zr/Y, for similar (La/Sm)N than the other E-type lavas of the northern North Atlantic region.  相似文献   

20.
Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO2, 18O of +6.9 to 7.0, and 87Sr/86Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, 18O values up to +7.6, and 87Sr/86Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K2O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%, 18O of +5.6 to 9.9 and 87Sr/86Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%, 18O of +7.7 to 8.6, and 87Sr/86Sr between 0.7047 and 0.7056.These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes.The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号