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1.
Layering in which one or more of the component minerals has grown perpendicular to layer boundaries occurs, under a variety of names, in volcanic, hypabyssal nad plutonic igneous rocks. The most recent and best name is comb layering. The oriented minerals are elongate, are commonly branching and may be curved. Experimental crystallization of plagioclase and ternary feldspar melts confirms that a substantial degree of supercooling 01 a significant cooling rate is necessary to produce the curved or branching crystal morphologies typical of comb layering. The more viscous the melt, the less the supercooling required. Changes in the water content or confining pressure are mechanisms for inducing supersaturation in deep-seated magmas, that are consistent with field and experimental evidence. The change from modal dominance by a single elongate crystal phase in one comb layer to dominance by another phase in a contiguous comb layer is explained by the presence of constitutional supercooling ahead of the growing crystals of a given layer.  相似文献   

2.
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   

3.
The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An68-An60), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (87Sr/86Sri: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An60-An38). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm3 during early fractionation of troctolites and then decreased continuously to 2.16 g/cm3 with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An69-An60) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An60) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.  相似文献   

4.
Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.  相似文献   

5.
The anorthite content of plagioclase grains (XAn) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An66 at the base of the layered series (LZa) to An32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An50 ± 1) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at XAn ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite.  相似文献   

6.
Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An18?CAn64 in the cores to An17?CAn29 in the rims, while that of the host rocks varies from An17 to An55 in the cores to An07 to An33 in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups?? biotites suggest that the Horoz magma possibly crystallised at fO2 conditions above the nickel?Cnickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe3+ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3?kbar, T >730°C.  相似文献   

7.
Chemical and structural zoning in plagioclase can develop in response to a number of different magmatic processes. We examine plagioclase zonation formed during the transfer of plagioclase from a granodioritic host to a monzodioritic enclave to understand the development of different zonation patterns caused by this relatively simple magma mixing process. The transferred plagioclase records two stages of evolution: crystallization of oscillatory plagioclase in the host granodioritic magma and crystallization of high An zones and low An rims in the hybrid enclave magma. High An zones (up to An72) are formed only in the hybrid enclaves after plagioclase transfer. Plagioclase from a primitive enclave, showing no or only minimal interaction with the host, is An30–43. The implication is that high An zones crystallize only from the hybrid magma and not from the primitive one, probably because of an increase in water content in the hybrid magma. Complex interactions between the two magmas are also recorded in Sr content in plagioclase, which indicates an initial increase in Sr concentration in the melt upon transfer. This is contrary to what is expected from the mixing of low Sr enclave magma with a high Sr granodiorite one. Such Sr distribution in the plagioclase implies that the transfer of the plagioclase took place before the onset of plagioclase crystallization in the enclave magma. Therefore, the mixing between high Sr granodiorite magma and low Sr enclave magma was recorded only in plagioclase rims and not in the high An zones.  相似文献   

8.
Crystal-rich materials (scoriae and lava flows) emitted during the 1985–2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO2 49.2–50.9 wt% and K2O 1.96–2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An62 and An88 and is characterized by alternating, <10–100 m thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An68. The labradoritic layers (An62–70) show small-scale (1–5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An70-An88) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth.A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An90 to An75. In the light of these observations, we propose that input of H2O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level.Editorial responsibility: T.L. GroveAn erratum to this article can be found at  相似文献   

9.
Olivine metagabbros from the Adirondacks usually contain both clear and spinel-clouded plagioclase, as well as garnet. The latter occurs primarily as the outer rim of coronas surrounding olivine and pyroxene, and less commonly as lamellae or isolated grains within plagioclase. The formation of garnet and metamorphic spinel is dependent upon the anorthite content of the plagioclase. Plagioclase more sodic than An38±2 does not exhibit spinel clouding, and garnet rarely occurs in contact with plagioclase more albitic than An36±4. As a result of these compositional controls, the distribution of spinel and garnet mimics and visually enhances original igneous zoning in plagioclase. Most features of the arrangement of clear (unclouded) plagioclase, including the shells or moats of clear plagioclase which frequently occur inside the garnet rims of coronas, can be explained on the basis of igneous zoning. The form and distribution of the clear zones may also be affected by the metamorphic reactions which have produced the coronas, and by redistribution of plagioclase in response to local volume changes during metamorphism.Authors listed alphabeticallyPublished by permission of the Director, New York State Museum, Journal Series Number 299  相似文献   

10.
We report new field and petrographic observations, and mineral-chemical data, on the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, which occur in different tectonostratigraphic terranes within the Archaean gneiss complex of SW Greenland. The Buksefjorden body [from the Akulleq (middle) terrane] is dominated by plagioclase and Ca-amphibole, but shows widespread effects of retrograde hydration (epidote, chlorite). Most plagioclase compositions are in the An60–82 range, with the majority of samples showing average core compositions ∼An76, whereas rims or recrystallized margins are ∼An65. Most grains in the An70–82 range display optically visible Huttenlocher intergrowths. Amphiboles at Buksefjorden are mainly magnesio-hornblende with X Mg ranging from 0.70 to 0.45. The Nordland anorthosite [from the Akia (northern) terrane] is also dominated by plagioclase and Ca-amphibole, but contains additional clinopyroxene (∼Ca47Mg38Fe15) as well as minor orthopyroxene (∼En68), spinel and corundum. Plagioclase at Nordland shows an equilibrated, equigranular texture, consistent with prolonged slow cooling from high temperatures. Despite this textural equilibration, plagioclase at Nordland shows a striking range of compositions from An28 to An97, most of which is found in single thin sections. A distinctive feature is the presence of discrete anorthite (+ spinel ± corundum) domains in some samples. Although a number of explanations may apply, we consider these domains to result from prograde mass transfer reactions involving Ca-amphibole and plagioclase. Amphibole compositions at Nordland show similar X Mg to those at Buksefjorden, but are more aluminous, alkalic, and titanian. This shift to more pargasitic compositions is consistent with the contrasts in metamorphic grade between the two anorthosite bodies. At Buksefjorden, there is no correlation between the amount of modal Ca-amphibole and plagioclase composition, which would be expected if amphibole was produced solely through metamorphism. Our results suggest, alternatively, that the primary igneous mineralogy of these rocks may have been plagioclase (∼An76) + hornblende + pyroxene + magnetite. The primary mineralogy at Nordland is less certain, but it is noteworthy that no rocks contain anorthite of unambiguous igneous origin, in contrast to some other occurrences of Archaean anorthosites. Received: 17 January 1996 / Accepted: 12 March 1997  相似文献   

11.
The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An62), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.  相似文献   

12.
<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.  相似文献   

13.
We studied the chemical composition of rock-forming minerals in gabbroids from the Chirii outcrop and the evolutionary features of parental basic melt during the crystallization of these rocks. Results were compared with data for basanites from pipes of the North Minusa depression. The mineralogical composition and thermobarogeochemical data of the gabbroids were examined in detail, and chemical analyses of rock-forming minerals (clinopyroxene, plagioclase, amphibole, biotite, titanomagnetite, and apatite) were carried out. Based on the homogenization temperatures of primary melt inclusions, we established the minimum temperatures and sequence of mineral crystallization in the gabbroids: clinopyroxene (>1160 °C), plagioclase, magnetite → amphibole (>950 °C) → biotite. The rock crystallization proceeded at shallow depths. Thermometric data are confirmed by results of modeling of equilibrium gabbroid crystallization. The crystallization of parental basic melt was accompanied by the accumulation of SiO2, Al2O3, alkalies, and Cl and depletion in femic components. The melt evolved to granodiorite and alkali-syenite compositions. Compared with basanites from pipes, the parental melt had a longer evolution. The geochemical features of the gabbroids indicate that they, like basanites, crystallized from intraplate alkali-basaltoid magmas. But in petrochemistry and mineralogy the Chirii gabbroids differ considerably from the pipe basanites.  相似文献   

14.
斜长岩体中Fe-Ti-P矿床的特征与成因   总被引：3，自引：0，他引：3       下载免费PDF全文
岩体型斜长岩为由90%以上斜长石组成的岩浆岩,具变压结晶的特点,仅形成于元古宙(2.1～0.9Ga),常赋存有Fe-Ti-P矿床。Fe-Ti-P矿体既呈整合层状也呈透镜状和席状等不规则形式产出;矿石类型有块状和侵染状,前者矿石矿物含量>70%,后者矿石矿物含量为20%～70%;矿物组成上,不同矿床稍有差别:部分矿床的Fe-Ti氧化物以钛磁铁矿为主、钛铁矿次之,而其他矿床则以赤钛铁矿为主、磁铁矿次之。一些矿床磷灰石含量较高,出现仅由Fe-Ti氧化物和磷灰石组成的铁钛磷灰岩。研究表明,Fe-Ti-P矿床由富Fe、Ti的岩浆演化形成,其母岩浆是在深部岩浆房中大量结晶斜长石后的残余岩浆。部分学者认为不同矿石经正常的结晶分异作用并堆晶形成,但该机制很难解释呈不规则状产出的矿石;其他学者则认为不混熔作用对矿石的富集(尤其是脉状、席状的铁钛磷灰岩)有重要作用,但该机制缺乏岩相学和地球化学方面的证据。河北大庙Fe-Ti-P矿体呈透镜状、席状等不连续地分布于斜长岩中,矿体不发育明显岩浆分层,但仍出现不同矿石的相带。依据详细的岩相学、矿体中矿物含量和成分的变化规律以及全岩地球化学特征,我们判断大庙矿床中不同矿石为堆晶矿物和晶隙流体的混合产物,它们由铁闪长质岩浆经结晶分异和堆晶作用形成,与不混熔作用关系不大。矿体不规则状产出的特点可能与岩浆动力分异作用有关,并伴随有小范围的亚固相迁移。  相似文献   

15.
Plagioclase from a progressively metamorphosed granodiorite changes as the metamorphic grade increases. Lower grade plagioclase are chemically inhomogeneous, with zoned rims containing distinct compositional levels of An0?3, An17, and An25. As grade increases the plagioclase becomes more chemically homogeneous with An0?3 rims dominating. Microcline inclusions are controlled by internal defects at lower grades and grain boundaries at higher grades. Myrmekite rims are developed at the highest grade. Rims are dependent on surface energy factors and occur at triple points, high angle lattice misfits and other high energy surfaces. At low grades, rims form at plagioclase-plagioclase contacts and at higher grades, at plagioclase-microcline contacts. These changes are due to impurity segregation and grain boundary migration, and an increase of the letter process at higher grades.  相似文献   

16.
An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H2O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H2O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H2O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H2O content can notably increase the An content of plagioclase, up to 10 mol % for H2O-undersaturated melts, and 20 mol % for H2O-saturated melts. Exceptionally calcic plagioclases (up to An100) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H2O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na2O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An94) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na2O = 12–15, or from melts with exceptionally high Al2O3(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H2O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An95 in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na2O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994  相似文献   

17.
Dioritic plugs (< 1 km across) are common associates of the late Caledonian, post-tectonic granites of the Scottish Highlands. These contain a very wide range of rock types from ultramafic through mesocratic diorites to anorthositic and granitic. These rocks form steeply dipping, wall-parallel layers and zones within the plugs. Outer layers are shown to form first, inner layers later. Mafic zones are composed of the minerals seen as phenocrysts in chilled margins and have the chemical characteristics of cumulates. The inner margins of the mafic zones reflect the onset of plagioclase crystallization. Core zones also show inward variations in mineralogy and texture which define vertical, wall-parallel cylinders. These variations are ascribed to fractional crystallization accompanying accretion of rock onto the pipe walls. Cores in different intrusions may be mafic, mesocratic, or leucocratic, which indicates vertical zonation in the bodies. The mafic cores additionally show that recharge with less evolved magma occurred in some cases. The few larger (> 1 km diameter) intrusions show examples of layering and lamination dipping at low to moderate angles. These indicate the beginning of a change from wall-dominated to floor-dominated crystallization as intrusion diameter increases. A comparison is made with the nature and origin of layering in other intrusions.  相似文献   

18.
Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~?An65) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~?An20–45) or more calcic (>?An70), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (<?~?An45), have 87Sr/86Sr ratios that are either mantle-like (~?0.7030) or crustal-like (~?0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.  相似文献   

19.
新疆百口泉闪长岩中高An值斜长石的成因及岩石学意义   总被引：1，自引：0，他引：1  
陈博  朱永峰 《岩石学报》2015,31(2):479-490
岩浆成因的高An值斜长石(An80~100)是玄武质岩浆早期结晶的产物,由于其形成条件较为苛刻,它对于限定寄主岩浆条件和探讨岩石成因有重要的指示作用。在新疆西准噶尔百口泉地区发育含高An值斜长石(An80~90)的闪长岩,高An值斜长石呈不规则包裹体形式存在于闪长岩主体斜长石中(An40~60)。锆石SHRIMP定年显示百口泉闪长岩形成于316.9±2.9Ma(MSWD=1.5),闪长岩主、微量元素和Sr-Nd同位素与区域内同时期岩浆作用产物具有相似的演化趋势。根据闪长岩岩石学特征,结合高An值斜长石的实验研究成果,本文认为百口泉闪长岩中的高An值斜长石不是捕虏晶,也不是闪长岩的结晶产物,而是与闪长岩成岩过程相关的循环晶。依据角闪石铝压力计的估算,闪长岩结晶于1.2~2.9kbar,与实验条件下获得的高An值斜长石产出的压力范围相符。高An值斜长石的产出表明原始岩浆具有富水、亏损的特征,对于探讨整个岩浆系统的演化过程有重要指示意义。  相似文献   

20.
Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo64 to Fo70, average Fo67), 27% pigeonite (En60Fs24Wo16 to En67Fs27Wo6), 11% augite (En50Fs17Wo33 to En47Fs13Wo40), 2% orthopyroxene (En70Fs26Wo4), 11% plagioclase (An80 to An94), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.  相似文献   

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