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1.
Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO2) at 0.8 GPa, 850°C, orthoclase (Or93) and sanidine (Or30) partially dissolved and were mantled by sodic plagioclase (An25–30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward —mostly in the form of parallel blades — toaard the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static boundary zone of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultancously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as the Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of dacitic composition. Mantled feldspars in some intrusive systems (i.e., rapakivi granites) may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes.  相似文献   

2.
Mantling of alkali feldspar megacrysts by oligoclase (‘rapakivi texture’) generally can be interpreted as the result of magma mixing, although decompression is a viable interpretation, especially for high-level intrusions. Coexistence of mantled and unmantled crystals can be explained by transfer of mantled crystals (‘antecrysts’) from a mixed (hybrid) rock to a host granitoid devoid of mantled crystals, for example, by disintegration of microgranitoid enclaves. Processes capable of explaining multiple oligoclase shells include repeated increase and release of volatiles, and repeated replenishment by more mafic magma. The shells could be formed by transfer of megacrysts into and out of a magma-mixing zone during flow in dyke-like conduits or in the fronts of mafic flows moving across cumulate layers in plutons. Ovoid megacrysts, which occur especially in Proterozoic rapakivi granitoids, are difficult to interpret but are better explained by growth processes than by magmatic corrosion. The common presence of simple twinning, partial crystal faces, euhedral plagioclase inclusions and granophyre-like intergrowths with quartz favours normal magmatic growth. The common ovoid shapes with local facets could reflect incomplete development of crystal faces, owing to relatively rapid growth. Granophyre-like intergrowths in the ovoids, local granophyre occurring as megacryst rims and in the groundmass, and the common presence of miaroles suggest growth of the ovoids at relatively shallow depth, at conditions of delayed nucleation and consequent undercooling, resulting from accumulation and retention of fluid. Development of the ovoids is independent of plagioclase mantling.  相似文献   

3.
The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.  相似文献   

4.
Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is pumped into the reaction volume from a silica reservoir, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.  相似文献   

5.
The syenitic layered series in the Klokken intrusion is surrounded by a zone (500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro (200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series.Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at 1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at 900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.CRPG contribution 729  相似文献   

6.
The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An96, An61, An54, An23, and An22) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few m was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal.Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An90) and sodic (about An50), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.  相似文献   

7.
Summary The cooling history of the feldspars of a hypersthene-augite trachyte lava of Oki-Dogo island, Japan was investigated by optical microscope, electron microscope and X-ray microanalyzer. Anti-Rpakivi mantled feldspars in the alkaline volcanic rocks consist of anhedral plagioclase cores and subhedral to euhedral sanidine mantles. The interfaces between the cores and mantles are wavy, saw-tooth-like, or comb-like under the optical microscope, suggesting sanidine overgrowth after plagioclase was partially resorbed. Perthitic lenses or lamellae of plagioclase are also observed in the sanidine near the interfaces. After the formation of such mantled feldspars at the magmatic stage, perthitic lamellae were produced in sanidine due to subsolidus exsolution. Periodicity of the perthitic lamellae is below 10 nm.[/ab]
Zusammenfassung Die Abkühlungs-Geschichte von Feldspäten einer Hypersthen-Augit-Trachyt-Lava von der Insel Oki-Dogo, Japan, wurde mikroskopisch, elektronenmikroskopisch und mit der Mikrosonde untersucht. Anti-Rapakivi-Feldspate in den Alkali-Vulkaniten bestehen aus anidiomorphen Plagioklas-Kernen and idiomorphen Sanidin-Rändern. Die Grenzen zwischen Kernen and Randern erscheinen unter dem Mikroskop gewellt, sägezahnartig oder kammartig. Dies deutet darauf hin, daß Überwachsung mit Sanidin nach teilweiser Resorption von Plagioklas stattgefunden hat. Perthitische Linsen oder Lamellen von Plagioklas im Sanidin wurden im Grenzbereich der beiden Feldspäte beobachtet. Perthit-Lamellen im Sanidin entstanden durch Subsolidus-Entmischung nachdem die Feldspäte wahrend der magmatischen Phase gebildet worden waren. Die Periodizität der Perthit-Lamellen liegt unter 10 nm.
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8.
Argon diffusion domains in K-feldspar I: microstructures in MH-10   总被引：1，自引：1，他引：0  
Samples of MH-10, a K-feldspar thought to contain discrete diffusion domains for argon, have been examined by light and TEM microsopy to search for domain boudaries. We examined both unheated K-feldspar separates and samples heated in vacuum between 750 and 1150°C in order to characterize the initial microstructure as well as any changes resulting from laboratory heating. The MH-10 rock shows almost no evidence of plastic deformation, and the K-feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses. Three classes of substructure are present in the K-feldspar, but only the third appears to have been affected by laboratory heating: (1) Cross-hatched extinction in light microscope is common but variably developed. TEM reveals almost no albite/pericline twinning but tweed microstructure is ubiquitous. (2) Turbid zones occur with about 5 volume % abundance. Micropores, <1 to 2m in diameter, characterize these regions, which often also contain bleds of albite, up to 40 m indiameter, TEM shows these turbid zones to be very complex with intricate twin and tweed structures at the sub-micron scale and numerous dislocation and strain features. (3) Albitic exsolution lamellae (<1 m thick, 20 m long and separated by 1 m) are rarely seen in the light microscope but TEM shows that 0.01 m thick by 0.2–1 m long lamellae, separated by 0.1 to 0.5 m, occur in about 20% of the sample. These lamellae are disk-shaped, have a semi-coherent or coherent relationship to the host K-feldspar, and seem to disappear upon heating to 750°C. Extensive diffusion studies have been conducted on HM-10, and the results analyzed in terms of distribution of sizes of diffusion domains. The preferred result of the analyses was a distribution consisting of three distinct sizes. The largest domain size identified in this way is approximately 50 to 100 m, and this corresponds to blocks of K-feldspar defined by the network of fractured/turbid zones. The smallest domain size inferred from diffusion analysis is 0.1 m which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways. Alternatively, the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients. Even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses, at least two questions remain unanswered. Firstly, no clear candidate has been identified for a domain of intermediate size. Secondly, the diffusion analysis has been formulated with independent gas release from all domains, whereas the observed structures are likely to be nested (i.e., small domains exist inside bigger domains) with the network of semi-coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values.  相似文献   

9.
An extremely differentiated suite of unaltered volcanic rocks dredged from the Galapagos Spreading Center ranges in 18O from 5.7 to 7.1 At 95°W, low K-tholeiites, FeTi-basalts, andesites and rhyodacites were recovered. Their lithologic and major element geochemical variation can be accounted for by crystal fractionation of plagioclase, pyroxenes, olivine and titanomagnetite in the same proportions and amounts needed to model the 18O variation by simple Rayleigh fractionation. More complicated behaviour was observed in a FeTi-basalt suite from 85°W. This study shows that 90% fractionation only enriches the residual melt by about 1.2 in 18O. It also implies that the magma chambers along parts of the Galapagos Spreading Center were static and isolated such that extreme differentiation could occur.  相似文献   

10.
Summary The Degana pluton hosts one of the few known tungsten deposits in India It is an epizonal, moderately high silica pluton emplaced during the Proterozoic in a posttectonic setting. Though homogeneous in composition, it displays textural heterogeneity from coarse-grained hypidiomorphic to fine-grained porphyritic to hypabyssal granite porphyry. Genetically related rhyolites are also present. Coherency of geochemical and mineralogical attributes in the Degana pluton can be explained by fractional crystallisation. Complex variety of hydrothermal and pneumatolytic features is also present. At shallow depths, emanation differentiation has led to progressive enrichment of Li, Rb, and W. Both the plutonic and volcanic phases of the magma show development of rapakivi texture and other diagnostic characteristics of the rapakivi granites.The Degana granite is a specialised granite and classified as an A-type intraplate anorogenic granite of mantle plume origin. The mineralogy and chemistry of the Degana pluton compares well with the various rapakivi granites of south-eastern Fennoscandia. Chemical and textural characteristics of the Degana pluton provide a constraint on the formation of the rapakivi texture when interpreted in terms of experimentally determined phase equilibria. The mantling process is interpreted as a result of pressure fluctuations due to escape and recharging of volatiles (e.g., H2O and F) accompanying the emplacement of the magma.
Geologie und Geochemie des Degana-Plutons—ein proterozoischer Rapakivi Granite in Rajasthan, Indien
Zusammenfassung Der Degana Pluton enhält eine der wenigen in Indien bekannten Wolfram-Lagerstätten. Es handelt sich hier um einen epizonalen Pluton mit höheren Si-Gehalten, der während des Proterozoikums in ein posttektonisches Setting intrudiert wurde. Obwohl er in seiner Zusammensetzung homogen ist, zeigt er Heterogenität auf dem texturellen Bereich, die von grobkörnig hypidiomorph bis feinkörnig porphyritisch und schließlich bis zu hypabyssischen Granitporphyren reicht. Genetisch verwandte Rhyolite kommen im Untersuchungsgebiet auch vor. Übereinstimmende geochemische und mineralogische Parameter können auf fraktionierte Kristallisation zurückgeführt werden. Eine komplexe Vielfalt von hydrothermalen und pneumotolytischen Erscheinungen ist bemerkenswert. In geringen Tiefen hat die Emanations-Differentiation zu einer progressiven Anreicherung von Li, Rb und W geführt. Sowohl die plutonischen als auch die vulkanischen Erstarrungsprodukte des Magmas zeigen die Entwicklung von Rapakivi-Texturen und anderen diagnostischen Eigenschaften der Rapakivi-Granite.Der Degana-Granit ist ein spezialisierter Granit und ist als ein anorogener Intraplattengranit des A-Typs zu klassifizieren, der auf einen mantle plume zurückgeführt wird. Die Mineralogie und Chemie des Degana-Plutons läßt sich gut mit der verschiedener Rapakivi Granite im südöstlichen Fennoskandien vergleichen. Chemische und texturelle Eigenheiten des Degana Plutons ermöglichen eine Eingrenzung der Bildung von Rapakivi Texturen, sofern sie im Sinne experimentell bestimmter Phasen-Gleichgewichte interpretiert werden. Die Entstehung von Überwachsungen einzelner Kristalle wird als Resultat von Druckschwankungen interpretiert, die auf das Entweichen und die Neuzufuhr von volatilen Phasen (i.e. H2O und F) im Gefolge der Platz nahme des Magmas zurückzuführen sind.
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11.
The epizonal, Tertiary Lilloise layered intrusion (5–9 km diameter) has a well developed amphibolitised basalt aureole, several hundred metres wide. The sequence of main cumulate minerals is olivine, augite, plagioclase, amphibole (Ti-hornblende to hastingsite), with amphibole first appearing as an intercumulus mineral about 2000 m below the present top of the intrusion. The range of D and 18O values (SMOW) of amphiboles and biotites from early intercumulus, and cumulus minerals and late pegmatites is very narrow, -70 to -88 and +4.3 to 5.5 respectively. Amphiboles or whole rocks from the basaltic country rocks change from D= -85 near the contact to -116 in the outer part of the aureole, and to -118 in the non-metamorphosed basalts up to 9km away. All basalts are depleted in 18O relative to normal values with the largest depletion (up to 2–3) in the outer part of the aureole. Meteoric water did not interact with either the magma or the hot plutonic rocks. This is in contrast to the results from most other plutonic complexes in the North Atlantic Tertiary igneous province (Skaergaard, Kangerdlugssuaq, Skye, etc.) many of which were similarly emplaced into basaltic country rocks. A meteoric-hydrothermal convective system was established in the basalts including the inner contact zone prior to the complete crystallisation of the Lilloise magma. The inner part of the aureole was finally modified by magmatic-hydrothermal fluids which evolved non explosively from the Lilloise magma. The meteoric-hydrothermal system in the outer part of the aureole, with integrated water/rock ratios of about 0.2 (atom % oxygen), did not collapse in on the hot pluton or inner contact metabasalts. Several factors may have reduced the permeability of the basalt country rocks. The emplacement of dykes prior to the Lilloise intrusion and possible associated weak hydrothermal activity could have restricted circulation of meteoric water; major fracture permeability was not generated during the quiet non-explosive crystallisation history of the Lilloise single magma pulse; metasomatism and alteration accompanying the magmatic-hydrothermal fluids may have reduced permeabilities in the inner aureole. Meteoric waters, however, did enter locally, at a very late stage, during low temperature serpentinisation of periodotite.  相似文献   

12.
Feldspar chemical variations in representative leucite-bearing and related rocks from well-known localities in Italy, Germany, Uganda and Australia demonstrate that phenocrystal core to rim variations may not represent the feldspar crystallization trend in the host lava and only the groundmass feldspar zoning trend is a reliable indicator of crystal-liquid relationships. Textural relationships indicate that coexisting plagioclase and alkali feldspar crystallized sequentially, the latter after the former, rather than cotectically.Groundmass alkali feldspar show Ca-, Na-depletion and K-enrichment zoning trends. Plagioclase crystallization follows Ca-depletion, Na and K-enrichment trends. Typically, Sr and Ba solid solubility is significant, particularly in groundmass feldspar.The alkali feldspar variation trend from groundmass assemblages is not consistent with the theoretical phase relationships in the system NaAlSiO4-KAlSiO4 CaAl2Si2O8-SiO2 (The phonolite pentahedron) proposed by Carmichael et al. (1974).Factors believed to be important in controlling feldspar crystallization trends are the Sr-Ba feldspar components, the role of the coexisting pyroxene and the presence of F, Cl and/or their alkali compounds.  相似文献   

13.
Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that 29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that 55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers.  相似文献   

14.
小兴安岭东南端晚石炭世大岭环斑花岗岩成因   总被引：4，自引：1，他引：3       下载免费PDF全文
在小兴安岭东南端的鹤岗—伊春市交界处大岭一带的晚石炭世弱片麻状中粒似斑状二长花岗岩中发育环斑结构长石,多以呈自形宽板状或宽板柱状的碱性长石内核和斜长石外薄壳组成,少量为不发育斜长石外壳的卵球状、球状,大小为1.5￣3.5cm,其特征与典型的环斑结构在岩相学上是相同的。另外岩体中普遍发育暗色微细粒闪长质包体,与环斑钾长石在时空上紧密相伴;包体具典型的岩浆结构及针状磷灰石,含寄主岩的钾长石、石英巨晶;包体形态多呈浑圆的外形,显示出明显的塑性流变特点,与寄主岩常呈明显的接触关系,有时呈过渡状、雾迷状;以上充分说明了包体为岩浆混合成因(MME)。通过对岩体地质、环斑结构钾长石似斑晶、暗色微细粒闪长质包体等特征及岩体的岩石化学、地球化学研究表明大岭环斑花岗岩岩体为岩浆混合成因,产于造山环境,其形成时代、产出构造背景均不同于典型环斑花岗岩。  相似文献   

15.
The island of Kod Ali is a small diatreme cone lying close to the Ethiopian coast at latitude 13° 57 N, longitude 41° 49 E. This volcanic structure, produced by geologically Recent volcanic activity, consists, almost entirely, of pyroclastic debris. Evidence of a liquid phase is confined to the thin veneers of markedly alkalic olivine basalt which coat many of the abundant gabbroic, pyroxenitic and lherzolitic fragments found on the island. As silicic inclusions are completely absent and as the alkalic liquid phase and the tholeiitic inclusions are seemingly genetically unrelated, it is proposed that the latter might have been derived from a tholeiitic plutonic layered sequence that underlies the tholeiitic basalts of an oceanic Red Sea floor.  相似文献   

16.
Co-existing plagioclase and alkali feldspars of the Sierra Nevada granites and plagioclases of the mafic inclusions have been analysed using an ARL EMX electron microprobe analyser. Each Sierran rock type contains co-existing feldspar pairs within specific compositional ranges. Core plagioclase compositions of the mafic inclusions are only slightly higher or lower in anorthite than the host rock plagioclases and cluster between An30 and An40. The chemical inhomogeneity of the Sierran potash feldspars and this effect on the Barth k value prohibits the use of the feldspars as geothermometers for these particular rocks. Results of the electron microprobe, x-ray, and petrographic study and the experimental hydrothermal investigation of the granites suggest but do not prove that both the plagioclase composition and the mafic inclusion mineralogy can be explained in terms of a model which considers the inclusions to be the refractory residue left over from the partial melting of crustal material.Submitted to the Faculty of the Department of the Geophysical Sciences, The University of Chicago, in partial fulfillment for the degree of Doctor of Philosophy.  相似文献   

17.
Micrographic quartz/K-feldspar intergrowths occur in quartzitic pebbles of the metamorphosed conglomerate from Buri-Rashicha/Ula-Ulo, S. Ethiopia. All transition phases and stages are observed from the interlocking quartzitic texture to the granophyric (the K-feldspar being metasomatically introduced).Metasomatic, replacement and infiltration processes are considered to have been responsible for the metamorphism and granitisation of the conglomerate and for the micrographic textures.  相似文献   

18.
Plutonic rocks associated with the Latir volcanic field comprise three groups: 1) 25 Ma high-level resurgent plutons composed of monzogranite and silicic metaluminous and peralkaline granite, 2) 23–25 Ma syenogranite, and alkali-feldspar granite intrusions emplaced along the southern caldera margin, and 3) 19–23 Ma granodiorite and granite plutons emplaced south of the caldera. Major-element compositions of both extrusive and intrusive suites in the Latir field are broadly similar; both suites include high-SiO2 rocks with low Ba and Sr, and high Rb, Nb, Th, and U contents. Moreover, both intermediateto siliciccomposition volcanic and plutonic rocks contain abundant accessory sphene and apatite, rich in rare-earth elements (REE), as well as phases in which REE's are essential components. Strong depletion in Y and REE contents, with increasing SiO2 content, in the plutonic rocks indicate a major role for accessory mineral fractionation that is not observed in volcanic rocks of equivalent composition. Considerations of the rheology of granitic magma suggest that accessory-mineral fractionation may occur primarily by filter-pressing evolved magmas from crystal-rich melts. More limited accessory-mineral crystallization and fractionation during evolution of the volcanic magmas may have resulted from markedly lower diffusivities of essential trace elements than major elements. Accessory-mineral fractionation probably becomes most significant at high crystallinities. The contrast in crystallization environments postulated for the extrusive and intrusive rocks may be common to other magmatic systems; the effects are particularly pronounced in highly evolved rocks of the Latir field. High-SiO2 peralkaline porphyry emplaced during resurgence of the Questa caldera represents non-erupted portions of the magma that produced the Amalia Tuff during caldera-forming eruption. The peralkaline porphyry continues compositional and mineralogical trends found in the tuff. Amphibole, mica, and sphene compositions suggest that the peralkaline magma evolved from metaluminous magma. Extensive feldspar fractionation occurred during evolution of the peralkaline magmas, but additional alkali and iron enrichment was likely a result of high halogen fluxes from crystallizing plutons and basaltic magmas at depth.  相似文献   

19.
The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial 87Sr/86Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial 87Sr/86Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and 87Sr/ 86Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in 87Sr/86Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.  相似文献   

20.
The Origin of Rapakivi Texture   总被引：9，自引：0，他引：9  
The mantling of large ovoids of K-feldspar by a rim of plagioclasehas been investigated in the rapakivi granites from the Mid-ProterozoicWiborg batholith of SE Finland. The formation of rapakivi texture,in this the type area, was examined using a variety of techniquesincluding isotopic analyses of mineral separates from specifictextural sites. Cathodoluminescence combined with microprobeanalysis points to the pulsed development of the mantles involvinggrowth of successive plagioclases of composition An30, An25,and An3, the last being in optical continuity with perthiticplagioclase exsolved from the K-feldspar. Plagioclase mantleshave high 18O and 87Sr/86 signatures relative to K-feldspar,which indicate the presence of a late, low-temperature componentthought to represent albite exsolved from the K-feldspar andredistributed onto the ovoid margin. Oligoclase components ofthe mantles are formed by a similar, although higher-temperaturemagmatic process. This involves the subsolvus re-equilibrationof alkali feldspar compositions with evolving melt conditions.Redistribution of the exsolved plagioclase from the alkali feldsparphenocrysts is linked to high fluorine contents of rapakivi-typemagmas, and this major reconstruction of the feldspar phenocrystsgenerates their distinctive ovoidal shape.  相似文献   

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