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1.
Twenty six whole rocks, seven matrix and fifty three mineral separates from the compositionally zoned late Quaternary Laacher See tephra sequence (East Eifel, W Germany) were analyzed by instrumental neutron activation. These data document the chemical variation within the Laacher See magma chamber prior to eruption with a highly fractionated phonolite at the top and a more mafic phonolite at its base as derived from other data. Incompatible elements such as Zn, Zr, Nb, Hf, U, light and heavy rare earths are extremely enriched towards the top whereas compatible elements (e.g. Sr, Sc, Co, Eu) are strongly depleted. Semicompatible elements (Ta and some middle REE) are depleted at intermediate levels. This chemical variation is shown by whole rock and matrix data indicating the phonolite liquid was compositionally zoned regardless of phenocryst content. Hybrid rocks (phonolite-basanite) show the largest concentrations for compatible elements. All elements (except Rb) display continuous compositional variations with regard to the stratigraphic position of pumice samples. From these data we are able to distinguish three main units: An early erupted highly fractionated magma, the main volume of evolved phonolite and a mafic phonolite as the final products. The extreme variation of trace element distribution coef ficients (K) for 9 mineral phases with respect to stratigraphic position (resp. matrix composition) cannot be explained by conventional mechanisms. We postulate a significant modification of the trace element content of the phonolite melt by liquid-liquid controlled differentiation processes subsequent to and/or contemporaneous with (fractional) crystallization which caused disequilibrium between phenocrysts and host matrix. Therefore, our “distribution coefficients” deviate from equilibrium partition coefficients equivalent to the amount of this post crystallization modification of the matrix composition. The relationship between varying K and matrix composition is demonstrated by a ΔK-ΔM-diagram (variation of K versus variation of matrix, M). Different parts of this diagram relate to different parameters (T, P, polymerization, complex-building, equilibrium crystallization in a zoned magma column and post crystallization disequilibrium effects) which are responsible for the variation of distribution coefficients in general. The ΔK-ΔM-diagram may allow to distinguish between different processes affecting the distribution coefficients measured in natural volcanic rocks from a differentiating magma system.  相似文献   

2.
Field relations in dissected volcanic terrains and the internalevidence of persistent low-pressure cotectic character in eruptedbasalts point to the frequent and substantial modification ofliquid compositions by some form of partial crystallizationwithin the crust In contrast, the highly compatible trace elementsdo not display the marked variations and extreme depletionswhich are predicted to result from perfect fractional crystallization(PFC). Imperfect fractional crystallization, refilling of magmachambers during fractionation and in situ crystallization areimportant factors which can help to explain this apparent paradox.This paper explores another effect, the integration of residualliquids from differing extents of partial crystallization, whichcan help to resolve this paradox, even while still permittingperfect fractional crystallization at all points in the magmachamber. Integration of such residual liquids through the thicknessof the crystallization zone is explicit, although not implemented,in the model of in situ crystallization proposed by Langmuir('Nature 340, 199–205, 1989). It may be separated as aprocess for purposes of mathematical modelling from the basicconcept of partial crystallization of small packets of magmawith remixing of the residual liquids into the main body ofmagma. Integration of melts from differing extents of partialcrystallization might in principle also be applied to the caseof lateral variations in the mass fraction crystallized withposition in the magma chamber. Integrated PFC itself can developresidual liquids which differ little from products of equilibrium(batch) crystallization (FTC) at the same average mass fractionof liquid remaining in both incompatible and compatible traceelement concentrations. For one specific combination of parametersthese integrated liquids are identical in composition at allvalues of the distribution coefficient to the EPC liquid. Atother values of the parameters the integrated liquids may even—anew paradox— have higher relative concentrations of highlycompatible elements than the EPC products. Any integration ofresidual liquids from different mass fractions of PFC rapidlyeliminates what have in the past been taken to be the diagnosticdifferences between PFC and EPC Integration of EPC liquids (towardswhich the products of imperfect fractional crystallization processeswill tend) produces even more pronounced effects, with highlycompatible elements less depleted even than in EPC and far lessdepleted than would be predicted by simple models. When interpretedaccording to oversimplified models, sequences of residual liquidsproduced in such processes might appear to be inconsistent withproducts of a partial crystallization process and to requirea process of progressively smaller mass fractions of meltingof inhomogeneous and progressively more refractory (higher mg-number)source regions. KEY WORDS: highly compatible elements; in situ crystallization; boundary layer; integrated crystallization *e-mail: oharamj{at}cardiff.ac.uk  相似文献   

3.
天目山火山岩的反序现象及其成因   总被引：2，自引：0，他引：2  
浙江天目山地区的中生代火山岩由流纹岩、英安岩、安山岩组成,其喷发存在反序现象。笔者用带状岩浆房的模式解释了该区火山岩的特点,应用各种物理化学方法建立了岩浆房中存在的温度、密度和粘度梯度。主要元素及微量元素的行为及其定量模拟表明,分离结晶是导致岩浆成分变化的主要机理。然而,各种矿物在岩浆房中的沉降速度计算表明,尤其是酸性岩浆中,晶体的重力沉降是不可能的。因此,岩浆房中存在的分离结晶现象主要是通过对分离作用进行的。  相似文献   

4.
The process of coupled assimilation and fractional crystallization (AFC) is one of the most popular petrogenetic concepts that explains magmatic differentiation. Conventional geochemical models for this process assume that crystals are removed instantaneously from the magma body as they are produced; however, recent advances in isotopic microanalysis have clarified that the crystals are suspended within the magma body for a certain period, affecting the whole-rock composition in response to the intra-grain isotopic zoning. This study develops a mass balance model for simultaneous assimilation and imperfect fractional crystallization (AIFC) to describe the effects of suspended crystals on the path of magma evolution. The mass balance differential equations for the liquid and suspended crystals are solved simultaneously. The analytical solution of the AIFC equations gives a quantitative account of the evolution paths of trace elements and isotopes within bulk crystals, liquid, and magma (crystals plus liquid). The chemical path of the magma differs markedly from that predicted by the conventional AFC model.  相似文献   

5.
Cumulate and Cumulative Granites and Associated Rocks   总被引：1，自引：0，他引：1  
Abstract. Processes that move crystals relative to melt, that is crystal fractionation, are of major importance in producing variations that are observed within cogenetic suites of granites. In low‐temperature granite suites, crystal fractionation initially involves the progressive separation of crystals residual from partial melting from that partial melt. Once separation of those crystals, or restite, has been completed, further fractionation may occur through the separation of crystals that had precipitated from the melt, the process known as fractional crystallization. High‐temperature granite magmas are largely or completely molten and elements such as Ca, Mg and Fe, and their associated minor elements, are in that case dissolved in the melt. Such magmas, particularly those that are more potassic and hence contain a higher fraction of low temperature melt, may evolve compositionally through fractional crystallization. Cumulate rocks result, comprising a framework of cumulus minerals with interstitial melt. In this process some of the melt is also displaced to form more felsic rocks. Such cumulate rocks may have distinctive chemical compositions, but that is often not the case. Distinctive features include SiC>2 contents near or below 50 % in rocks that are transitional in the field to more felsic granites, very high Cr and Ni, very low K, P, Ba, Rb and Zr, and anomalous abundances of the anorthite components Ca and Al. These rocks may also have positive Eu anomalies. Cumulate rocks do not necessarily have distinctive textures, at least as such features are understood at this time. Fractional crystallization can also involve the movement of precipitated crystals relative to melt. We refer to rocks as cumulative when formed from the fractions in which the abundance of crystals has increased. The production of cumulative granites typically occurs at more felsic melt compositions than is the case for cumulate granites, and this process may have its greatest significance in the fractional crystallization of the felsic haplogranites. Relative to felsic granites of broadly similar compositions lying on a liquid line of descent, cumulative granites contain more Ca, reflecting the addition from elsewhere of plagioclase crystals with solidus compositions. The abundances of Sr and Ba may be high to very high, and sometimes there are positive Eu anomalies. Cumulative I‐type granites may have low abundances of Y and the heavy REE, while the S‐type granites can be very distinctive with anomalously high abundances of Th and the heavy REE resulting from the concentrating of monazite. Generally, but not always, those who propose fractional crystallization as a mechanism for producing compositional variation within a suite of granites do not state whether the rocks in that particular case are thought to lie on a liquid line of descent or are cumulates/cumulative, although it is generally presumed that they were melts. Our experiences in eastern Australia have shown that the mechanism of fractional crystallization was quantitatively not as important during granite evolution as many workers would expect. However, there are some excellent examples of that process, most notably the Boggy Plain Supersuite. Overall in eastern Australia, varying degrees of separation of restite is a much more common mode of crystal fractionation, and that may also be seen to be the case for some other granite provinces if they are examined with that possibility in mind.  相似文献   

6.
Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial 87Sr/86Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.LDGO Contribution no. 2964  相似文献   

7.
The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization.  相似文献   

8.
Major and trace element compositions of rocks and coexisting phenocrysts of the Thingmúli volcano suggest a revision of the existing models for the formation of intermediate and silicic melts in Iceland. The new data define two compositional tholeiitic trends with a significant gap between them. A high-iron trend (HFe) contains 6–14 wt% total FeO in silicic rocks with c. 1 wt% MgO, as well as sodic plagioclase and hedenbergite phenocrysts. A low-iron trend (LFe) contains 3–5 wt% FeO at c. 1 wt% MgO, which is typical of Iceland but higher than MORB compositions. The most evolved phenocrysts of the LFe trend do not reach iron-rich end members. The HFe trend is interpreted as a result of fractional crystallization; numerical modelling using the MELTS algorithm suggests that crystallization took place under redox conditions constrained to one-log unit below the fayalite-magnetite-quartz oxygen buffer (FMQ-1). The LFe trend is explained by a combination of mixing between rhyolite and ferrobasalt, assimilation of hydrated crust and fractional crystallization under higher redox conditions (FMQ). The two trends and the gap are best defined in a plot of Mg# versus SiO2 that is useful to unravel petrogenetic processes. For example, intermediate and silicic rocks of the Holocene volcanic systems of spreading rifts (e.g. Krafla), propagating rifts (e.g. Hekla) and off-rifts (Öræfajökull) also fall into high- and low-iron fields and outline a gap similar to Thingmúli. The identification of two compositional trends in erupted intermediate and silicic volcanic products shows that processes in the deep roots of single volcanic systems are highly diverse and likely controlled by local variations in the thermal gradients and the extend of hydrothermal alteration. Generalizations about the relationship between the compositions of intermediate and silicic rocks and plate tectonic setting, therefore, should be avoided.  相似文献   

9.
Melt inclusions in quartz phenocrysts from a single clast of pumice near the base of the plinian pumice fall of the Bishop Tuff were studied to test ideas concerning separation of melt and crystals in silicic magmas. Ten analyzed inclusions from the pumice clast are of high silica rhyolite composition with very low contents of the highly compatible elements Ba, Sr, and Eu, consistent with extensive fractionation. The concentrations of U, La, Ce, Mg, and Ca of these ten melt inclusions vary considerably as determined by ion microprobe. Petrologic considerations indicate that uranium is an incompatible element with a maximum bulk partition coefficient D of about 0.2 and that the evolution of the uranium content of the melt was controlled by crystallization of the magma. A minimum of 33 wt% perfect fractional crystallization is required to explain the observed range in uranium. However, only 17 wt% crystals occurred in the pumice clast. The greater calculated fraction of crystals requires significant separation of crystals and melt before the eruption of the plinian pumice fall in spite of the fact that crystal mixing (settling, etc.) did not occur in the Bishop magma.  相似文献   

10.
冷水银铅锌矿的火山—侵入杂岩体出露于信江中生代火山盆地南缘。岩石化学、微量元素、稀土元素和铷锶同位素分析表明冷水火山岩及次火山岩是同源产物,是同熔岩浆分异的结果。岩浆中成矿物质Cu、Fe、Pb、Zn、S、H_2O的数量与下地壳、上地慢或洋壳物质的部分熔融程度有关。冷水火山—侵入杂岩是钙碱性岩浆于不同演化阶段形成的岩石组合并具有化学成分上的相似性和继承性。岩浆房中具有良好的分带现象,上部为富钾的流纹质岩浆,向下逐渐变为基性岩浆。岩浆由钙碱性向偏碱性演化,促使成矿元素大量富集,在一定条件下形成矿床。  相似文献   

11.
Geological and experimental evidence indicate that the three-phasefield, plagioclase-alkalifeldspar-liquid, may terminate in severaldifferent ways. The possible terminations have been developedfrom Schreinemakers' rules governing the disappearance of three-phasefields. In igncous rocks, these different terminations may arisefrom variations in the relative amounts of additional componentsin magmas, or from changes of total pressure, or from structuralchanges that effect the extent of solid solution in feldspar. The three-phase region originates from the intersection of thesolidus and the feldspar solvus. The available evidence regardingthis intersection is reviewed, as is the evidence for the existenceand form of the boundary curve on the feldspar liquidus. Thedata are used to project a series of isobaric polythermal andisobaric isothermal diagrams for each possible termination.Subsequent discussions relate the theoretical arguments to thenatural evidence, suggest the more probable geological environmentsof some of the terminations, and indicate characteristic featuresof each termination. It may be possible to identify the type of termination involvedin the crystallization of some rocks. The necessary data arethe amounts and compositions of both kinds of coexisting feldsparsand of the feldspar components of the coexisting liquid at oneor more stages of the crystallization process. Volcanic rocksare most suitable for such studies. The terminations explain many of the compositional relationshipspossible between feldspar crystals and silicate melt under geologicaland experimental conditions. These compositional relationshipsare fundamental to understanding the crystallization of igneousrocks and the formation of melts by partial fusion. Other geologicalapplications include a simple explanation for some resorbedfeldspars, the separation, correlation, and comparison of porphyriticrock units, interpretation of compositional changes of successivezones of zoned feldspars, and mantling of one feldspar by another.With all types of termination, extensive fractionation yieldsa liquid rich in Or and Ab that may crystallize to alkali feldsparor feldspars.  相似文献   

12.
The Los Humeros volcanic center, located 180 km east of MexicoCity, is one of several silicic centers in the ‘back-arc’portion of the Mexican Neovolcanic Belt. Eruptive products spanthe compositional range from high-silica rhyolite to basalt.During the last 0?46 Ma, three major explosive eruptions andan extended period of lava flow emplacement periodically sampledan integrated magma reservoir that was initially zoned fromrhyolitic uppermost levels to andesitic and perhaps basalticlower levels, with compositional gaps in the ranges 63–67and 72–75 per cent SiO2. The compositional zonation canlargely be explained by fractional crystallization, but mustbe accompanied by assimilation to explain the range of Sr andNd isotopic ratios. Higher than predicted concentrations ofNi, Cr, and strongly incompatible elements such as Rb and Bain andesites suggest continuous replenishment of a fractionatingchamber by mantle-derived basalts. The volumetric predominanceof rhyolite in the early history of the center points to a longperiod of accumulation of differentiates without eruptive withdrawal. Once volcanic activity started tapping the chamber, eruptiverates seem to have exceeded the rate of regeneration of differentiatedmagma. Although there is overlap in the compositional zonationof the products of successive eruptive events, the dominantvolume of each is always more mafic than that of the precedingeruption, indicating only limited regeneration of differentiatedmagmas during repose periods. This seems to have been a consequenceof the chamber remaining in approximate thermal balance duringthe last 0?46 Ma, as shown by similar Fe-Ti oxide temperaturesfor given magma compositions, regardless of age. Calculationssuggest that the chamber received thermal input through theinjection of basalt at an average rate of 0?1 km3 per thousandyears. Apparently this thermal input was too small to generatedifferentiated magma by partial melting of the wall rocks ofthe chamber, but large enough to offset conductive or hydrothermalcooling that would promote differentiation by extensive crystallization.  相似文献   

13.
马芳  薛怀民 《地质学报》2017,91(2):334-361
处于浙-赣火山岩带东北缘的湖(州)-安(吉)盆地内的火山岩/潜火山岩从中性到酸性,中间没有明显的成分间断,以中酸性—酸性组分占绝对优势,中性组分相对较少,缺少基性组分,代表一套连续的中性—酸性岩浆系列。岩石化学总体表现为富碱和高钾的特征,中性岩属橄榄玄粗岩系列,中酸性—酸性岩类属高钾钙碱性系列。盆地内的火山岩在地球化学上均表现为富集大离子亲石元素和轻稀土元素,而高场强元素Nb、Ta、Ti等则有一定程度的亏损。火山岩中主量和微量元素的变异规律揭示分离结晶作用是盆地内岩浆演化的主要机理,但岩浆演化的不同阶段分离的矿物相有所差异,斜长石自始至终都是分离结晶的重要矿物相,在中性—中酸性岩浆演化阶段,角闪石可能也是重要的分离矿物,而中酸性—酸性岩中钾长石和黑云母的分离结晶也起着重要影响。本文所作的精确定年结果表明,盆地内三期火山活动产物的LA-ICPMS锆石U-Pb年龄在误差范围内几乎一致,介于128~130 Ma之间,指示盆地内主要火山活动持续的时间很短。另外测得粗安岩集块的SHRIMP锆石U-Pb年龄为136±1 Ma,可能意味着该盆地初期有少量偏基性的火山活动。推测湖安火山岩盆地发育于活动大陆边缘的后造山环境,岩浆的形成可能受控于岩石圈的拆沉或俯冲板片的断落,母岩浆主要是由镁铁质下地壳物质部分熔融形成的,几乎没有地幔物质的参与,所形成的安山质-英安质岩浆在浅部岩浆房中发生过强烈的分异演化。  相似文献   

14.
Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle.  相似文献   

15.
The Precambrian Dullstroom Formation of South Africa, which is predominantly composed of basaltic andesites interbedded with subordinate sedimentary and felsic volcanic strata, represents the first phase of an extended period of magmatism that was responsible for the Bushveld Magmatic Province, including the extrusive Rooiberg Group and the intrusive Bushveld Complex. New geochemical and isotopic data for the Dullstroom Formation are presented in an effort to elucidate the petrogenetic processes operative during the initiation of this magmatic episode. The volcanic units of the central portion of the Dullstroom Formation have been subdivided into at least three interbedded compositional groups: low Ti mafic to intermediate units, high-Ti mafic to intermediate units, and high Mg felsic units. High Ti and low Ti volcanic units are similar in some compositional characteristics to basalts of the nearby northern and southern provinces, respectively, of the Mesozoic Karoo continental flood basalts. Isotopic and compositional data for low Ti Dullstroom strata are consistent with bulk assimilation into a melt similar in composition to a southern Karoo basalt of 20% upper continental crust accompanied by 20% fractional crystallization of pyroxene and plagioclase. Isotopic and compositional data for high Ti Dullstroom strata are consistent with magma mixing of 30% northern Karoo K-rich basalt and 70% southern Karoo basalt followed by 20% assimilation of upper continental crust and 20% fractional crystallization of pyroxene and plagioclase. Compositions of high Mg felsic volcanic strata are consistent with 25% assimilation of a mixture of silica-rich sedimentary rock and upper continental crust into a melt similar in composition to low Ti volcanic units with 25% fractional crystallization of pyroxene and plagioclase. However, it has been suggested that compositions of these high Mg felsic strata may also be consistent with interaction of a crustal melt. Assimilation, fractional crystallization, and magma mixing that apparently affected these Dullstroom Formation volcanic strata may have occurred in a series of shallow magma chambers. These data are consistent with the suggestion that Dullstroom Formation volcanic rocks are the result of a mantle plume. Mantle plume origin also is suggested by the large volume of intrusive and extrusive strata associated with this magmatic episode. These data do not support the hypothesis that the Bushveld Complex and the Rooiberg Group formed by impacts of a cluster of comets or asteroids. Received: 14 October 1998 / Accepted: 17 May 1999  相似文献   

16.
An indirect method was used to study Na, K, Rb, Cs, Sr and Ba partition coefficients between crystals and silicate melt. Equilibria between a hydrothermal solution and the melt at 800°C and 2 kb and between a hydrothermal solution and crystals at 750°C and 2 kb were separately achieved.For major element partitioning (Na and K), the results obtained here are in good agreement with those of Tuttle and Bowen (1958) which allow us to follow crystal evolution during a fractional crystallization process where the growth of zoned crystals takes place.For minor elements Rb, Cs, Sr, Ba, melt/aqueous solution partition coefficients depend on Na/K as well as the silica content of the melt. These effects are rather small for Rb and Cs, but are much more important for the alkaline earths. The feldspar/aqueous solution partition coefficients also depend on Na/K.The variations of the partition coefficients feldspar/melt are complex in the part of the Qz-Ab-Or diagram located below the cotectic line.During fractional crystallization following the Rayleigh law (assuming that there are no kinetic phenomena) Sr (D > 10) is almost totally removed from the melt in the early stages whereas Cs (D < 0.1) remains in the melt during the whole process. Rb and Ba have partition coefficients closer to unity. The variation of these coefficients, due to changes in bulk composition of liquid and crystals during fractional crystallization, can lead to complex zoning with possible concentration maxima at some stages. Similar phenomena can be expected in non-ideal natural solid solutions, even if no discontinuities can be detected in the physicochemical evolution of the parent magma.  相似文献   

17.
东天山石炭纪企鹅山群火山岩岩石成因   总被引：13，自引：1，他引：12  
土屋矿区南北大沟企鹅山群火山岩的岩石地球化学研究表明:东天山企鹅山群火山岩主要为拉斑系列,少量为钙碱系列;岩石类型为玄武岩、玄武安山岩、英安岩和流纹岩。稀土、微量元素和Sr、Nd同位素特点揭示:该火山岩系形成于大陆裂谷环境;其源区主要为软流圈地幔,同时有岩石圈地幔源组分卷入,酸性岩浆是玄武质岩浆结晶分异的产物。  相似文献   

18.
卢成忠  顾明光 《中国地质》2007,34(4):565-571
杭州南部新元古界青白口系上墅组火山岩,由基性岩和酸性岩组成,缺乏SiO2在57×10-2～68×10-2的中性及中酸性成分,构成一套双峰式火山岩组合。基性火山岩与酸性火山岩的稀土元素和微量元素特征存在明显差异,表明酸性火山岩并不是由基性岩浆分离结晶产生,而是分别由不同的源区熔融形成,基性火山岩起源于亏损程度较低的地幔或来源于亏损地幔的岩浆受到陆壳物质的混染,酸性火山岩主要由上地壳经不同程度的部分熔融而形成,其形成的构造环境为陆内至陆缘张裂环境。上墅组双峰式火山岩的形成,是华南Rodinia超大陆裂解事件的岩石学记录,揭示了华南Rodinia超大陆裂解始于青白口纪。  相似文献   

19.
The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.  相似文献   

20.
Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved granites and aplites which are encountered mostly at the plutons roof and margins. Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization. The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the differentiates. The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation of mineral/melt partition coefficients (K d s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models constructed using the K d s thus obtained along with modal abundances from petrographic estimates. Lamont-Doherty Geological Observatory Contribution Number 3701  相似文献   

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