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 15 毫秒
1.
《Lithos》1987,20(3):207-223
The late Quaternary phonolitic pumice deposits of Tenerife, Canary Islands, are the product of a periodically-tapped, periodically-replenished, zoned alkaline magma system. Nepheline syenite blocks occur as lithics in the deposits, and provide solidified representatives of the phonolite magmas. The blocks are considered to have crystallised in the roof zone of the active magma system at a depth of roughly 4 km beneath the Las Cañadas caldera. Conversion of highly-differentiated phonolitic magma to solid nepheline syenite was achieved between 770° and 680°C. Quenched glass of extreme composition in one erupted block provides a sample of the last interstitial liquid; comparison of this sample with phonolitic pumice, and with fully crystallised syenites, allows reconstruction of the magmatic, interstitial and subsolidus crystallisation histories of nepheline syenite. Phases represented by the phenocryst assemblage of the most differentiated phonolitic pumices (sanidine, sodalite, nepheline, Na-poor pyroxene, biotite, sphene and magnetite) formed an initial crystal “mush”, containing abundant trapped liquid, on the walls of the magma chamber. Interstitial crystallisation, involving the conversion of pyroxene to Na-rich compositions, continued growth of felsic phases, and partial to complete consumption of biotite, sphene, magnetite and apatite, was accompanied by extreme sodium enrichment in the residual liquid. Stellate aegirine, lavenite, loparite and Mn-rich ilmenite are among the final products of magmatic crystallisation. Modification of feldspar primocrysts persisted into subsolidus conditions.It is suggested that complete solidification of an open-system alkaline magma chamber results in the formation of an alkaline alkaline ring complex.  相似文献   

2.
Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of >11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4–6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H2O-rich (~6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H2O-saturated liquidus at ~4 km. H2O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently ‘hot’ and ‘cold’ granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons.  相似文献   

3.
浙江普陀花岗杂岩体包含若干石英闪长质包体,该类包体中存在三种不同类型的斜长石:正常环带的斜长石、筛孔构造的斜长石和酸性斜长石为核的“反环带”斜长石。根据斜长石的环带构造特征和成分分析,认为本区的岩浆演化过程大致如下:下部基性岩浆注入到上覆酸性岩浆中并进行混合作用,酸性岩浆中已结晶的富钠质斜长石晶体进入偏基性的混合岩浆中,部分熔融形成筛孔构造;随着端员岩浆的进一步混合,富钠质斜长石晶体与中性混合熔体仅形成粗糙的边界,而保留原先构造特征;同时混合岩浆可以直接结晶出正常环带斜长石,呈单颗粒或以膜的形式包围其它环带构造的长石。本文还通过与平潭甬闪辉长岩杂岩体内筛孔斜长石的对比,认为斜长石的环带构造和成分可以反映岩浆源区特征和岩浆演化历史。  相似文献   

4.
An analytical and experimental study of zoning in plagioclase   总被引：1，自引：0，他引：1  
A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals.  相似文献   

5.
Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U–Pb age. Recrystallisation provides a mechanism for resetting zircon U–Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U–Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.Dedicated to Borwin Grauert on the occasion of his sixtieth birthday  相似文献   

6.
The 1995–1999 eruption of the Soufriere Hills volcano,Montserrat, has produced a crystal-rich andesite containingquench-textured mafic inclusions, which show evidence of havingbeen molten when incorporated into the host magma. Individualcrystals in the andesite record diverse histories. Amphibolephenocrysts vary from pristine and unaltered to strongly oxidizedand pseudomorphed by anhydrous reaction products. Plagioclasephenocrysts are commonly reverse zoned, often with dusty sievetextures. Reverse zoned rims are also common on orthopyroxenephenocrysts. Pyroxene geothermometry gives an average temperatureof 858 ± 20°C for orthopyroxene phenocryst cores,whereas reverse zoned rims record temperatures from about 880to 1050°C. The heterogeneity in mineral rim compositions,zoning patterns and textures is interpreted as reflecting non-uniformreheating and remobilization of the resident magma body by intrusionof hotter mafic magma. Convective remobilization results inmixing together of phenocrysts that have experienced differentthermal histories, depending on proximity to the intruding maficmagma. The low temperature and high crystallinity are interpretedas reflecting the presence of a cool, highly crystalline magmabody beneath the Soufriere Hills volcano. The petrological observations,in combination with data on seismicity, extrusion rate and SO2fluxes, indicate that the current eruption was triggered byrecent influx of hot mafic magma. KEY WORDS: Montserrat; eruption; magma mixing; mafic inclusion; sieve texture  相似文献   

7.
颜丽丽  贺振宇  刘磊  赵志丹 《地质通报》2015,34(203):466-473
浙江雁荡山是中国东南部燕山晚期巨型火山-侵入杂岩带的重要组成部分。对其中央侵入相石英正长斑岩的暗色微粒包体中的斑晶和基质斜长石进行了详细的内部结构和成分分析,揭示了斜长石复杂环带的成因和相关的岩浆作用过程。斑晶斜长石由熔蚀的核部和表面干净的幔部组成,边部包裹有钾长石膜。核部斜长石呈浑圆状或港湾状,内部发育筛状结构,An成分显著低于幔部斜长石,代表来自酸性岩浆房中早期结晶的斜长石捕掳晶。同时,幔部斜长石与自形、表面干净的基质斜长石具有类似的An含量,且两者均含有针状磷灰石的包裹体,应结晶自与暗色微粒包体相应的基性岩浆。长石的复杂结构记录了雁荡山火山-侵入杂岩形成过程中的岩浆混合作用和岩浆演化过程。岩浆混合之后的火山喷发活动,造成岩浆房的压力突然减小,温压条件达到钾长石结晶的区域,在石英正长斑岩的斑晶斜长石和暗色包体中的斑晶与基质斜长石外均形成钾长石膜,构成反环斑结构。  相似文献   

8.
This study documents the chemical and textural responses of zircon in the Elba igneous complex, with particular reference to the 7- to 7.8-Ma-old Monte Capanne pluton in relation to its coeval volcanic counterpart (Capraia), using BSE imaging and quantitative electron microprobe analyses. The Monte Capanne pluton displays multiple field and geochemical evidence for magma mixing. The samples we have investigated (including monzogranitic, mafic enclave and dyke samples) display similar zircon textures and are associated with an extremely large range of trace and minor element (Hf, Y, HREE, Th, U) compositions, which contrast with relatively simple textures and zoning patterns in zircons from a Capraia dacite. We have used a relatively simple textural classification (patchy zoning, homogenous cores, oscillatory zoning and unzoned zircon) as the basis for discussing the chemical composition and chemical variation within zircons from the Monte Capanne pluton. Based on these data and other works (Dini et al. 2004 in Lithos 78:101–118, 2004) , it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the Monte Capanne magma chamber. In particular, mixing was responsible for the development of the patchy-zoning texture in the zircon cores, which was associated with reactions between other accessory phases (including monazite, apatite, allanite), which we infer to have significantly affected the Th distribution in zircon. Zircons from the MC pluton displaying “homogeneous cores” have chemical affinities with zircons in the coeval Capraia volcanic system, consistent with the participation of a Capraia-like mantle end-member during mixing. Further zircon growth in the MC pluton produced the oscillatory zoning texture, which records both long-term (crystal fractionation) and transient (recharge with both silicic and mafic magmas) events in a hybrid magma chamber. It is inferred that Hf and the Th/U ratio cannot be used alone to infer magmatic processes due to their dependency on temperature, nor are they a diagnostic feature of xenocrystic grains. This study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system.  相似文献   

9.
The emplacement of the Mesoproterozoic G?temar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The G?temar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8?km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx.  相似文献   

10.
Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (Ⲓ% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An-trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present - at increasing recharge rates with time - in the plumbing system throughout the volcano's history.  相似文献   

11.
U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with 70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between 230Th and 238U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (226Ra)/(230Th)i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.  相似文献   

12.
Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO~T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.  相似文献   

13.
Compositionally zoned plutons are an important feature of theSierra Nevada batholith, California. Two such plutons have beenexamined to determine the mechanism by which crystals separatefrom a magma. The Tinemaha pluton shows continuous compositionalvariation from 58 to 67% SiO2, whereas the McMurry Meadows plutonis bimodal, with an outer margin of mafic granodiorite (59–60%SiO2) and an inner core of granite (66–69% SiO2). Extremedifferentiates also occur as small isolated masses within thesuite and may contain up to 76% SiO2. Both plutons are uniformin strontium isotopic composition but are different from eachother, with initial 87Sr/86Sr values of 0?70719 and 0?70651respectively. The Tinemaha pluton is both horizontally and vertically( 1000 m) zoned, with fractionation occurring both inward fromthe contacts and upward. The vertical trends in relative mineralproportions are not consistent with crystal settling. Both thevertical and horizontal variations in the chemical compositionof 50 elements, in mineralogy, and in accessory mineral lightrare-earth element zoning, are all directly relatable to side-wallcrystallization which created a less-dense melt that buoyantlymoved upward along the wall towards the top of the magma chamber.The different rates for diffusive heat exchange and compositionaldiffusion within the magma initiated the double-diffusive gradientin the magma chamber. Compositional variations in the side-wallcrystal accumulation zone occur as boundary layer melts evolve,reflecting changes in the bulk convecting magma. The compositionalgap in the McMurry Meadows pluton is the result of a similarbut more efficient side-wall fractionation process, relatedto a higher proportion of melt to crystals in the initial magmaand a slower rate of side-wall solidification as a result ofthe thermal blanket created by the enclosing Tinemaha pluton.  相似文献   

14.
The compositional zoning of a garnet population contained within a garnet-grade metapelitic schist from the Lesser Himalayan Sequence of Sikkim (India) provides insight into the rates and kinetic controls of metamorphism, and the extent of chemical equilibration during porphyroblast crystallisation in the sample. Compositional profiles across centrally sectioned garnet crystals representative of the observed crystal size distribution indicate a strong correlation between garnet crystal size and core composition with respect to major end-member components. Systematic steepening of compositional gradients observed from large to small grains is interpreted to reflect a progressive decrease in the growth rate of relatively late-nucleated garnet as a result of an increase in interfacial energies during progressive crystallisation. Numerical simulation of garnet nucleation and growth using an equilibrium approach accounting for chemical fractionation associated with garnet crystallisation reproduces both the observed crystal size distribution and the chemical zoning of the entire garnet population. Simulation of multicomponent intracrystalline diffusion within the population indicates rapid heating along the pressure–temperature path, in excess of 100 \(^{\circ }\)C Myr\(^{-1}\). Radial garnet growth is correspondingly rapid, with minimum rates of 1.4 mm Myr\(^{-1}\). As a consequence of such rapid crystallisation, the sample analysed in this study provides a close to primary record of the integrated history of garnet nucleation and growth. Our model suggests that nucleation of garnet occurred continuously between incipient garnet crystallisation at \(\sim\)520 \(^{\circ }\)C, 4.5 kbar and peak metamorphic conditions at \(\sim\)565 \(^{\circ }\)C, 5.6 kbar. The good fit between the observed and predicted garnet growth zoning suggests that the departure from equilibrium associated with garnet nucleation and growth was negligible, despite the particularly fast rates of metamorphic heating. Consequently, rates of major element diffusion in the intergranular medium during garnet crystallisation are interpreted to have been correspondingly rapid. It is, therefore, possible to simulate the prograde metamorphic history of our sample as a succession of equilibrium states of a chemical system modified by chemical fractionation associated with garnet crystallisation.  相似文献   

15.
Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal.It is proposed that the plagioclase cores formed during injection of primitive magma into a previously existing magma chamber, that the mantle formed during mixing of a partially mixed magma and the remaining magma already in the chamber, and that the inner rim formed when the mixed magma was in a sheeted dike system. The large plateau at the outside of the mantle may have formed during the injection of the next batch of primitive magma into the main chamber, which may trigger an eruption. This model is consistent with fluid dynamic calculations and geochemically based magma mixing models, and is suggested to be the major mechanism for generating the disequilibrium conditions in the magma.  相似文献   

16.
The spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr.B.P. tephra deposit of Laacher See Volcano. The melt showed vertical chemical zonation immediately prior to eruption, and throughout most of the magma volume only a small fraction of crystals were present. The eruption also ejected crystal-rich nodules, ranging from mafic to felsic in composition, which are samples of the materials crystallising at the boundaries of the chamber. New data on nodule petrography and chemical compositions of whole-rocks, minerals and interstitial glasses are presented. Volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol%, i.e. the crystals interlock. One exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol%. Bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt. Felsic nodules show much less pronounced differences with their interstitial melt. Felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls. Mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor. Modal mineralogy, glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber, a broader range of mineral compositions are often found. Mineral assemblages were especially diverse in the upper part of the chamber. It is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption, that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor, that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls, and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and/or time.  相似文献   

17.
Carbonatites of the Jacupiranga alkaline–carbonatite complex in São Paulo State, Brazil, were used to investigate mineral–fluid interaction in a carbonatite magma chamber because apatite showed a marked discontinuity between primary fluid inclusion-rich cores and fluid inclusion-poor rims. Sylvite and burbankite, apatite, pyrite, chalcopyrite and ilmenite are the common phases occurring as trapped solids within primary fluid inclusions and reflect the general assemblage of the carbonatite. The apatite cores had higher Sr and REE concentrations than apatite rims, due to the presence of fluid inclusions into which these elements partitioned. A positive cerium anomaly was observed in both the core and rim of apatite crystals because oxidised Ce4+ partitioned into the magma. The combined evidence from apatite chemistry, fluid inclusion distribution and fluid composition was used to test the hypotheses that the limit of fluid inclusion occurrence within apatite crystals arises from: (1) generation of a separate fluid phase; (2) utilization of all available fluid during the first stage of crystallization; (3) removal of crystals from fluid-rich magma to fluid-poor magma; (4) an increase in the growth rate of apatite; or (5) escape of the fluids from the rim of the apatite after crystallization. The findings are consistent with fractionation and crystal settling of a carbonatite assemblage in a fluid-stratified magma chamber. Secondary fluid inclusions were trapped during a hydrothermal event that precipitated an assemblage of anhedral crystals: strontianite, carbocernaite, barytocalcite, barite and norsethite, pyrophanite, magnesian siderite and baddeleyite, ancylite-(Ce), monazite-(Ce) and allanite. The Sr- and REE-rich nature of the secondary assemblage, and lack of a positive cerium anomaly indicate that hydrothermal fluids have a similar source to the primary magma and are related to a later carbonatite intrusion.  相似文献   

18.
Internal structures in zircons from granitoids from the late Archaean Darling Range Batholith show secondary features revealed by HF etching, which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid. Zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation. The most striking feature is the development of high trace element concentration areas found in zircons from a number of granites. These structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band, about half way between the outer edge and the centre of the zircon. In any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim. This suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones. This explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger SHRIMP ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres. Also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon. This structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands. All structures can be transgressed by generally rounded lobes and patches of low U, weakly nebulously zoned zircon. This is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid, resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon. Received: 6 January 1998 / Accepted: 8 May 1998  相似文献   

19.
The Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)m. Melt inclusion H2O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with DClfl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)m with temperature. We suggest that the zoning can be caused by accumulation of CO2-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H2O fugacity and/or increasing Clm result in increasing (Cl/OH)m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO2-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO2 emissions at the surface.  相似文献   

20.
Twenty-five mafic microgranular enclaves of the Lavras Granite Complex in southern Brazil were studied petrographically and geochemically to establish their origin and to investigate the processes involved in their differentiation. Mesoscopic and microscopic textures indicate that they are products of magma mingling between a basic end member of probable mildly alkaline affinity and host shoshonitic and alkaline granitic rocks. The hybridisation process involved at least the following mechanisms: (i) chemical diffusion of volatiles and very mobile elements such as K to the less polymerised liquids, leading to the crystallisation of hydrated mafic minerals; (ii) chemical diffusion of Ti and P to the less polymerised liquids, leading to titanite and apatite crystallisation; (iii) mechanical accretion in the basic magma of early crystallised host granite phases that promoted enrichment of their major constituents and of trace elements with high partition coefficients in these phases; (iv) chemical diffusion of elements such as Rb, Nb, Y, and Yb with high Kd in the major enclave phases, from host magma into the basic enclaves. These processes occurred simultaneously, probably before the dispersion of basic batch magma forming the mafic microgranular enclaves, and caused hybridisation and complex geochemical patterns. The patterns are very different from the linear trends predicted for near-equilibrium systems such as those of magma mixing or fractional crystallisation.  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