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1.
Segregations that are darker than their host basalt are locally common in the Late Pleistocene Toomba Basalt flow northwest of Charters Towers. At Basalt Corner this flow has been inflated to a thickness of 17 m and segregations are displayed in pipes, sheets, veins and vesicles. Their chemical compositions are distinct from the host hawaiite and least‐square mixing solutions provide close models involving crystallisation of olivine, plagioclase, augite and magnetite in the cooling host basalt and gas‐pressure extraction of residual melt to form the segregations. Trace‐element concentrations are consistent with this mechanism and can, in general, be modelled using mineral partition coefficients from the literature. Segregations may concentrate near the edges of the inflation structures.  相似文献   

2.
Varioles and matrix have been studied in the typical globular rocks??variolites of the Yalguba Range and Suisari Island in the Onega structure, Central Karelia. It was determined that the cores of the varioles are significantly enriched in silica, Na, K, Rb, Cl, and Ba, and have lower K/Na ratio as compared to matrix. In addition, varioles strongly differ from matrix in oxygen isotope composition (??18Ov-??18Om varies from 1.6 to 2.6??). The consideration of possible mechanisms of the formation of the Onega variolites with allowance for available isotopic and geochemical data (major element composition, REE and trace element distribution) demonstrated that the observed geochemical characteristics of matrix and globules of the Yalguba Range and Suisari Island variolites cannot be formed by melt mixing. Such processes as greenstone alteration and crystallization of spherulites during melt overcooling also did not define the isotopic and geochemical peculiarities of the Onega variolites, but played only subordinate role. Except for significant oxygen isotope shift in the variole-matrix system, the obtained data are best consistent with liquation model. However, few available experimental data on the distribution of trace elements and oxygen isotopes during liquid immiscibility make it impossible to reach decisive conclusion concerning liquation genesis of the Onega variolites.  相似文献   

3.
The Lewisian complex of the Scourie-Badcall area is composed predominantly of banded tonalitic gneiss which intrudes layered gabbro-ultramafic complexes. Intrusive into both gabbro and tonalitic gneiss are homogeneous acid sheets which are trondhjemitic to granitic in composition. All rocks were subjected to granulite facies metamorphism. Smooth continuous trends on chemical variation diagrams suggest that the evolution of these rocks was dominated by fractional crystallisation. A scheme is proposed whereby a tonalitic melt was parental to trondhjemite and granite. Variation within tonalites was a function of the fractional crystallisation of hornblende and plagioclase, and trondhjemite was derived from tonalite by the fractional crystallisation of hornblende and/or plagioclase. Granite and granodiorite represent residual liquids which evolved along the quartz-feldspar cotectic surface; they were derived by the fractional crystallisation of plagioclase from a trondhjemite liquid. Some trondhjemitic sheets are quartz-plagioclase residues from which a granitic melt was removed. The associated gabbros and ultramafic rocks are not directly related to the proposed fractional crystallisation scheme and are not crystal residues removed from the tonalitic melt. Tonalites were probably derived from a basaltic source by partial melting or fractional crystallisation with either hornblende and/or garnet as residual phases.  相似文献   

4.
Tholeiitic pillow basalt from the South Andaman island, an integral part of the outer sedimentary arc of the Sunda-Burmese double chain arc system in the Bay of Bengal, is characterized by the occurrence of several morphologies of quenched crystals of plagioclase and pyroxene. Plagioclase shows a swallow tail, belt-buckle, rosette and closely spaced fan-spherulites pattern while pyroxene has elongate parallel chain, dendritic, spherulitic and finely ornamented feathery spherulitic habit. Most of these textures are identical to those reported from submarine basalts, lunar basalts, spinifex textured rocks and experimentally produced textures. The occurrence of these quench textures in the Andaman basalt suggests that they were formed by rapid cooling at 30–70°C/h in a submarine environment.  相似文献   

5.
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.  相似文献   

6.
Columbia River Basalt Group dikes invade biotite–hornblendetonalite to granodiorite rocks of the Wallowa Mountains. Mostdikes are strongly quenched against wallrock, but rare dikesegments have preserved zones of partial melt in adjacent wallrockand provide an opportunity to examine shallow crustal melting.At Maxwell Lake, the 4 m thick wallrock partial melt zone containsas much as 47 vol. % melt (glass plus quench crystals) aroundmineral reaction sites and along quartz–feldspar boundaries.Bulk compositional data indicate that melting took place underclosed conditions (excepting volatiles). With progressive melting,hornblende, biotite, and orthoclase were consumed but plagioclase,quartz, and magnetite persisted in the restite. Clinopyroxene,orthopyroxene, plagioclase, and Fe–Ti oxides were producedduring dehydration-melting reactions involving hornblende andbiotite. Reacting phases became more heterogeneous with progressivemelting; crystallizing phases were relatively homogeneous. Progressivemelting produced an early clear glass, followed by light (high-K)and dark (high-Ca) brown glass domains overprinted by devitrification.Melts were metaluminous and granitic to granodioritic. Thermalmodeling of the partial melt zone suggests that melting tookplace over a period of about 4 years. Thus, rare dikes withmelted margins represent long-lived portions of the ColumbiaRiver Basalt dike system and may have sustained large flows. KEY WORDS: Columbia River Basalt dike; crustal melting; dehydration-melting; tonalite–granodiorite; thermal model  相似文献   

7.
Variation in the primary textures of “Cordilleran” granitic rocks is described relative to three identifiable stages of the crystallisation interval; namely: (1) crystallisation in suspension; (2) growth of a touching crystal framework; (3) interstitial crystallisation. Crystals that initially grow in isolation will start to impinge and form small clusters as crystallisation proceeds and the volume of solid material increases, eventually forming a continuous interconnected crystal framework. Subsequent crystallisation involves solidification of the melt occupying the interstices of the framework, and therefore shows similarities to the way in which the porosity occludes in sedimentary systems. A case study of textural development in Cordilleran granitic rocks from the zoned Linga superunit of the Peruvian Coastal Batholith, reveals that compositional zonation from granodiorite through to syenogranite is accompanied by a systematic variation in the textures, specifically those of the three felsic phases (plagioclase, quartz and alkali feldspar). Plagioclase was the first phase to appear on the liquidus, and was joined by the other two phases as crystallisation proceeded and the melt evolved. The melt fraction at which quartz and alkali feldspar started to crystallise influenced the early growth of plagioclase, and the way in which the texture developed through each stage of the crystallisation interval. The geometry of plagioclase progressively changes from a touching framework of crystals in the granodiorite, to small aggregates or isolated crystals suspended in an equant mosaic of the other felsic phases in the syenogranite. This variation can be explained by an earlier evolution of the melt to the cotectic (i.e. at higher melt fractions) as the rocks become more acidic, and hence a greater contribution of alkali feldspar and quartz to the growth of the framework at the expense of plagioclase and the mafic phases. Textural observations are comparable to the crystallisation pathways of the felsic phases modelled in the quaternary An-Ab-Or-Qz system from the bulk compositions. All compositions lie in the plagioclase volume, and evolved to three-phase saturation on the cotectic via either the quartz/plagioclase divariant surface (granodiorites) or the alkali feldspar/plagioclase divariant surface (monzogranite and syenogranite).  相似文献   

8.
We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.  相似文献   

9.
Dehydration (vapour absent) partial melting reactions in the Earth's crust produce a hydrous granitic melt phase, new anhydrous minerals that are mostly pyroxenes, and new plagioclase more calcic than the initial plagioclase. These solid phases of the melt reaction are restite. If the restite is carried to high levels in the crust as a component of the magma, cooling and crystallisation to granite will result in back reactions in which the H2O in the melt phase is consumed and is not then available to form a hydrothermal solution. Even in magmas in which some restite has been removed there will be some back reaction and again less H2O. Only fractional crystallisation will enrich the H2O in the magma in sufficient amounts to form a substantial quantity of hydrothermal solution and possible mineralisation.  相似文献   

10.
A pseudotachylite vein network crosscutting late Hercynian foliated tonalites can be observed along the Copanello cliffs (Calabria, Southern Italy). Pseudotachylites formed during the Oligocene–Miocene at intermediate crustal levels (ca. 10 km). They show variable thickness ranging from few mm up to 10 cm, as observed in injection veins branching from the fault plane. Microscopic observations indicate that pseudotachylite matrix mainly consists of plagioclase (An46–An58) and biotite microlites. Rounded clasts of quartz, plagioclase or of plagioclase–quartz lithic fragments are disseminated in the matrix. Intergranular, flow and spherulitic textures are commonly observed. Microstructural features are consistent with rapid crystallisation from melt. EDS analyses of rare and tiny glass veins indicated a trachyandesite or An50 plagioclase melt composition.The conditions for pseudotachylite formation were reproduced by an analytical model taking into account the heat released by friction along a horizontal fault plane during a seismic event. The model is based on a three-stage rupture history that includes nucleation, propagation and stopping. In addition, by means of a numerical approach, the model reproduces cooling that follows the stopping stage.According to previous studies, the thermal perturbation induced by fault displacement is very intense. In fact, temperatures exceeding the tonalite and even An50 plagioclase liquidus (1470 °C) are reproduced by small amount of slip (≤ 6 cm) in suprahydrostatic regime. On the other hand, the thermal perturbation is strongly localised and of short duration. Peak temperatures abruptly decrease at a short distance from the fault plane (typically in few millimetres). In these conditions a thin film of melt can be produced. Therefore, the presence of cm-scale pseudotachylite veins can be only explained assuming an efficient and fast melt migration towards dilatant sites, such as pull-apart structures and injections veins. Results of the model may be useful to predict the thermal disturbance produced by earthquakes of low intensity.  相似文献   

11.
Major element composition ranges of closely associated basalt glass-whole rock pairs from individual small cooling units approach the total known range of basalt glass and whole rock compositions at IPOD sites 417 and 418. The whole rock samples fall into two groups: one is depleted in MgO and distinctly enriched in plagioclase but has lost some olivine and/or pyroxene relative to its corresponding glass; and the other is enriched in MgO and in phenocrysts of olivine and pyroxene as well as plagioclase compared to its corresponding glass. By analogy with observed phenocryst distributions in lava pillows, tubes, and dikes, and with some theoretical studies, we infer that bulk rock compositions are strongly affected by phenocryst redistribution due to gravity settling, flotation, and dynamic sorting after eruption, although specific models are not well constrained by the one-dimensional geometry of drill core.Compositional trends or groupings in whole rock data resulting from such late-stage processes should not be confused with more fundamental compositional effects produced in deep chambers or during partial melting.Lamont-Doherty Geological Observatory Contribution Number 3243.Woods Hole Oceanographic Institution Contribution Number 4838  相似文献   

12.
Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters.The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%).In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills.FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel.The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present.The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.Contribution n ° 480 du Départment Scientifique, Centre Océanologique de Bretagne  相似文献   

13.
鲁西泰山岩群柳杭岩组中,典型变质枕状玄武岩出露在济南章丘的团员沟村西一带。本文对变质枕状玄武岩的野外地质和岩相学特征,作了详细描述。电子探针分析表明,岩枕内部既有原生斜长石(An>30),又有绿片岩相变质形成的钠长石(An<10)。侵入柳杭岩组的片麻状奥长花岗岩中的岩浆锆石SHRIMP U-Pb年龄为2707±9Ma,它限制了泰山岩群形成时代不晚于2.71Ga。  相似文献   

14.
Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel+cordierite+plagioclase±corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about 1,000°C at a pressure of about 650 bars and at an oxygen fugacity about 0.2–0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum+mullite+sillimanite+ plagioclase+spinel+rutile±tridymite±cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An55 close to the basalt to An90 close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel+cordierite+plagioclase+quartz+K-feldspar +magnetite+ilmenite at about 800°C.  相似文献   

15.
This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

16.
The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai–Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin.  相似文献   

17.
早元古代滹沱群变玄武岩岩石地球化学及构造环境   总被引：1，自引：0，他引：1  
早元古代滹沱群存在先后两期火山岩，早期青石村组变玄武岩呈灰黑色，致密块状构造，主要岩类为玄武岩，部分为玄武安山岩，玄武粗安岩，具有与火山弧玄武相似的成分特点；晚期河边村组变玄武岩呈灰绿色。片理发育，主要岩类为碱玄岩为碧玄岩。具有典型板内玄武岩的成分特点。  相似文献   

18.
Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.  相似文献   

19.
我们对采自于加勒比海地区小安德列斯岛弧(Lesser Antilles Arc)Kick’em Jenny(KEJ)海底火山玄武岩中的斜长石斑晶进行了矿物形态和成分分析。利用电子探针(EMPA)和LA-ICP-MS测定了具有环带结构的斜长石斑晶中主量元素的空间分布,同时也利用LA-ICP-MS分析了斜长石中Sr的分布。结果表明,在不同的矿物斑晶中,元素含量均表现出和环带结构相联系的空间分布变化。斜长石斑晶中最主要的结构为韵律环带以及熔蚀结构,所测定的矿物边缘都存在An值从由内向外迅速降低的致密韵律环带,可能反映了快速结晶时的不平衡;而晶体内部的稀疏韵律环带结构是由岩浆填充或对流活动导致的。部分斜长石的熔蚀层An值由内向外升高,反映了高Ca岩浆填充的过程。这说明斜长石斑晶的矿物形态和元素环带可以用来制约俯冲带海底火山岩浆从源区上升到岩浆房再到喷发的复杂过程,包括岩浆演化、熔体多次填充、熔体与结晶矿物之间的反应、以及矿物再熔融等。这对于理解海底火山的喷发以及岛弧岩浆岩的演化有重要意义。  相似文献   

20.
Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.  相似文献   

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