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1.
The Bjerkreim-Sokndal (BKSK) layered intrusion belongs to the Rogaland anorthosite province in southern Norway. The northwestern part of BKSK consists of a ca. 6 km-thick Layered Series, made up of macrocyclic units (MCU) arranged in a syncline. Each MCU, which resulted from the crystallization of a major-magma influx, can be subdivided into a series of cumulate zones. The MCU III/IV boundary has been studied in seven profiles across its strike length of 24 km. Massive piC1 at the base of MCU IV overlies laminated and modally layered phimC in the central part of the chamber and phimacC towards the flanks; there is a discordance of between 2 and 6° between the base of MCU IV and phase layering in MCU III. The MCU IV piC is overlain by 75–100 m of massive poiC (the Svaalestad unit of Michot 1960; a similar olivine-bearing unit occurs near the base of MCU III) which has more primitive compositions than the underlying piC. This is followed by laminated and modally layered phiC, phimC and phimacC. The reversal to more primitive mineral assemblages across the MCU III/IV boundary is accompanied by a cryptic reversal; plagioclase and Ca-poor pyroxene have compositions of about An 44/Mg no. 71 at the top MCU III and about An 52/Mg no. 77 near the base of MCU IV. Olivines in the MCU IV poiC vary unsystematically from Fo 66 to 76. Macrocyclic units III and IV crystallized from monzonoritic parental magma. The BKSK magma chamber had a broad saucer-like shape with a small thickness to breadth ratio. The magma in the chamber during crystallization of MCU III was compositionally zoned and crystallized on an inward-sloping floor by down-dip accretion. Just before the major-magma influx at the base of MCU IV, phimC was crystallizing from the basal-magma layer at the centre of the chamber, while phimacC was crystallizing towards the flanks. The new, dense magma fountained into and mixed with the basal-magma layers already in the chamber. This hybrid magma crystallized during continued influx to produce massive piC at the base of MCU IV. This hybrid unit is thickest near the centre of the chamber and smoothed out the floor to an essentially horizontal surface. Continued influx resulted in the dense, primitive magma ponding on the floor; this crystallized fairly rapidly to produce the massive poiC unit. The return of normal fractional crystallization conditions is marked by the overlying sequence of modally and cryptically layered cumulates which duplicate the succession in MCU III. The variation in thickness of the upper part of MCU IV indicates that crystallization of the BKSK Layered Series was accompanied by sinking of the floor at a greater rate near the centre of the chamber than towards the flanks. This was accompanied by compaction of the underlying cumulates, promoting the development of lamination and the expulsion of intercumulus melt to encourage the development of adcumulates.
1
p
plagioclase
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i
ilmenite
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h
Ca-poor pyroxene
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o
olivine
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m
magnetite
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a
apatite
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c
Ca-rich pyroxene
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C
cumulate 相似文献
2.
We present compositional data on a 1,250-m-thick sequence of sparsely porphyritic lavas that comprise the Geikie Plateau Formation, part of the ~55-Ma break-up-related flood basalts in East Greenland. Major element compositions are relatively restricted (6.3–7.6 wt% MgO; 2.2–2.4 wt% TiO2), with two excursions to more evolved compositions (2.4–3.4 wt% TiO2) that are similar to the inferred parental magma of the nearby Skaergaard Intrusion. Major and trace element calculations show that fractional crystallisation is the principal control on magma compositions, and the cyclical sequential variations imply regular magma chamber replenishment events. Isotopic data indicate minor crustal assimilation, but with different contaminants for the main group (amphibolitic gneiss) and evolved cycles (granulitic gneiss). Rifting episodes may have allowed more primitive magmas to ascend to shallow crustal levels and subsequently fractionate to more evolved compositions in a separate chamber, which was perhaps similar to the source of the Skaergaard Intrusion.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsibility: I. Parsons 相似文献
3.
Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series 总被引：5，自引：1，他引：5
The Ultramafic series of the Stillwater Complex has been dividedinto two major zones: a Peridotite zone formed of 20 macro-rhythmicunits of dunite-harzburgite-orthopyroxenite, and an overlyingOrthopyroxenite zone. The stratigraphic section has been determinedat Mountain View (2065 m) and at Chrome Mountain (840 m). TheMountain View section apparently formed in a subsiding basinwhereas the rocks at Chrome Mountain accumulated in a relativelystable, higher area of the chamber floor. In both sections,Mg/(Mg + Fe) in cumulus mafic minerals increases with stratigraphicheight in the lower 400 m, then remains relatively constantthrough the rest of the series. The base of the series is marked by the first appearance oflaterally extensive olivine-rich cumulates. The accretion ofthe cumulates and the growth of the chamber proceeded throughperiodic injections of olivine-saturated mafic magma. The lowercontact of the cycles represents a hiatus in crystallizationand a return to a more primitive magma composition. Althoughhotter, the primitive magma was more dense, so it entered thechamber at or near the floor and did not immediately mix withthe more differentiated orthopyroxene-saturated magma alreadypresent. As it cooled by transfer of heat across its upper surface,the primitive magma crystallized olivine and differentiatedin situ to form the lower dunite. With the accumulation of olivinenear the base, the crystal/liquid ratio, and thus the density,decreased at the top of the layer eventually resulting in mixingand the formation of harzburgite. After removal of olivine byresorption and settling from the hybrid magma, orthopyroxenealone crystallized forming an orthopyroxenite. Chromitite layersprobably formed by the mixing of primitive olivine± chromite-saturatedmagma and narrow layers of orthopyroxene-saturated magma trappedunderneath. The Mg-enrichment trend in the lower 400 m resulted from reactionof cumulus olivine and/or orthopyroxene with progressively decreasingvolumes of intercumulus liquid. As heat loss through the floordecreased, accumulation rate approached a steady state, thefraction of trapped liquid remained more or less constant andvariation in Mg/(Mg + Fe) was governed dominantly by cumulusprocesses. The constant NiO abundances in olivine throughoutthe section are consistent with the model for the formationof the macro-rhythmic units. Depletion of NiO was dampened byrepeated additions of parental magma, localized equilibriumcrystallization, mixing, and the effect of postcumulus equi-librationwith varied amounts of trapped liquid. Discordant dunite bodies, which are common at Chrome Mountain,formed by the replacement by olivine of earlier formed cumulates.The replacement involved the incongruent dissolution of ortho-pyroxeneat near-solidus temperatures by a late-stage, hydrous vaporprobably derived from the magma. The vapor phase migrated alongfractures formed by the readjustment of the cumulate pile. 相似文献
4.
The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Rogaland area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Layered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/Fe---Ti oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclase-hypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of K-feldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites.
There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44–48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and Fe---Ti oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in Fe---Ti oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift.
The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the Sr-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx ceased, olivine-bearing rocks began to crystallize at the base of zone IVb. The leucotroctolites at the base of this zone are the most primitive rocks in the entire intrusion. The systematic increase in Sr-isotope ratios up through zone IVb resulted from progressive mixing between new and resident magma. This mixing either took place during magma influx or by the progressive mixing of overlying resident magma layers during crystallization.
Calculations based on geochemical modelling, the thickness of cumulate stratigraphy repeated and Sr-isotope ratios indicate that the new magma influx had a thickness of 350–500 m in the Storeknuten section and that the leucotroctolites of zone IVb represent about 20–30% crystallization of this influx. 相似文献
5.
The Hyllingen Series comprises the southern part of the Caledoniansynorogenic Fongen- Hyllingen layered mafic intrusion, whichoccupies an area of 160 km2, southeast of Trondheim, Norway.Large, raft-like inclusions form an important part of the HyllingenScries. Most of these are of fine-grained, equigranular rocksof basaltic composition with lithologies matching those of theadjacent country rocks. The rafts, which compose up to 22% ofthe lower part of the Hyllingen Series, are broadly concordantwith modal layering in the host gabbroic rocks. Individual bodiescan be up to 1500 m long and over 100 m thick. Some of the raftsare branching, and appear locally to form a threedimensionalnetwork. Impact structures are associated with small metabasicinclusions but not with the large rafts. The Hyllingen magma chamber is believed to have developed asa southerly expanding, thin wedge, forming the upper part ofthe Fongen chamber. The magma was compositionally zoned andcrystallized along the inclined floor of the wedge-shaped chamber.The wedge expanded as a result of the influx of dense, primitivemagma in the northern part of the chamber. The highly evolvedmagma at the top of the chamber penetrated along fractures inthe roof and spread laterally to form sill-like bodies. Theroof zone consisted of a network of veins and sills penetratingan interconnected framework of metabasic hornfels. Continuedcrystallization at the floor, while the magma chamber expanded,finally resulted in the interconnected rafts being engulfedby the crystallization front. Fragments detached from the roofsank to the floor to cause the observed impact structures. Thelarge, raft-like, fine-grained, granular, gabbroic bodies areconsidered to be in situ country rock inclusions.
Reprints available from J. R. Wilson 相似文献
6.
Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications 总被引：3，自引：1，他引：3
R. Stephen J. Sparks Herbert E. Huppert 《Contributions to Mineralogy and Petrology》1984,85(3):300-309
The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma. 相似文献
7.
Stephen R. Tait 《Contributions to Mineralogy and Petrology》1988,100(4):470-483
The spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr.B.P. tephra deposit of Laacher See Volcano. The melt showed vertical chemical zonation immediately prior to eruption, and throughout most of the magma volume only a small fraction of crystals were present. The eruption also ejected crystal-rich nodules, ranging from mafic to felsic in composition, which are samples of the materials crystallising at the boundaries of the chamber. New data on nodule petrography and chemical compositions of whole-rocks, minerals and interstitial glasses are presented. Volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol%, i.e. the crystals interlock. One exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol%. Bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt. Felsic nodules show much less pronounced differences with their interstitial melt. Felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls. Mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor. Modal mineralogy, glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber, a broader range of mineral compositions are often found. Mineral assemblages were especially diverse in the upper part of the chamber. It is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption, that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor, that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls, and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and/or time. 相似文献
8.
Petrographic, mineral chemical and whole-rock major oxide data are presented for the lavas of the Main Volcanic Series of Patmos, Dodecanesos, Greece. These lavas were erupted about 7 m.y. ago and range in composition from ne-trachybasalts through hy-trachybasalts and trachyandesites to Q-trachytes. To some extent, the ne-trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc-alkaline lavas of destructive plate margins. Major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz. olivine, plagioclase, clinopyroxene and Ti-magnetite in the mafic lavas, plagioclase, clinopyroxene, mica and Ti-magnetite in the evolved lavas. Apatite, which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed. However, mass balance calculations indicate that the chemistry of the hy-trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80-77% ne-trachybasalt with 20–23% trachyandesite. It is estimated that the mixing event preceded eruption by a period of 12 h-2 weeks suggesting that mixing triggered eruption. Combined fractionation and mixing cannot explain the relatively low MgO contents of the hy-trachybasalts and it is concluded that assimilation also occurred. Assimilation, and especially addition of volatiles to the magmas, may be responsible for the evolutionary trend from ne-normative to hy-normative magmas and was probably facilitated by intensified convection resulting from mixing. A model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot primitive magma into the base of the chamber facilitates assimilation, but eventually mixing yields the hy-trachybasalts and finally the ne-trachybasalts are erupted. 相似文献
9.
The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes. 相似文献
10.
Jonathan H. Berg 《Contributions to Mineralogy and Petrology》1980,72(4):339-351
The snowflake troctolite (SFT) in the Hettasch intrusion is a thin (0–10 m) zone of melatroctolite concordant with the normal leucotroctolites on one limb of the Hettasch intrusion. The textures of this unit are strikingly different from the normal cumulate textures of the Hettasch leucotroctolites and include comb-layered plagioclase, skeletal megacrysts of plagioclase, and spherulitic plagioclase 5–15 cm in diameter (snowflakes). These supersaturation plagioclase morphologies are set in a matrix of fine-grained, layered and unlayered melatroctolite. Because the mineral compositions are more primitive in the SFT than in the surrounding Hettasch cumulates, crystallization models involving supersaturation of the basaltic Hettasch magma fail. However, over a short distance below the texturally-defined SFT and a shorter distance above it, the mineral compositions of the surrounding cumulates gradually merge with those of the SFT. It is concluded, therefore, that the SFT formed from a separate magma that irrupted onto the floor of the Hettasch magma chamber and was supercooled by the basaltic Hettasch magma. Thus the SFT magma is inferred to have been more primitive than basalt. Assuming that fractionation in the SFT was minimal or that no fractionated material has escaped the SFT, an estimate of the bulk composition of the unit should approximate the magma composition. A picritic magma is thereby inferred, and a search for comparable rocks or magma types indicates that these rocks have the most similarities with picritic rocks of the Brito-Arctic province which formed during the early opening of the North Alantic. This similarity supports the hypothesis that the anorthositic Nain complex in Labrador also formed in a rifting environment about 1.4 Gyr ago. 相似文献
11.
Magmatic zoning in apatite: a monitor of porosity and permeability change in granites 总被引：1，自引：0，他引：1
Apatites from the Shap Granite, northern England, are strongly zoned, reflecting multiple generations of growth and dissolution. Such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using LA-ICP-MS analyses. The zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber. Early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries. The main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning, followed by the growth of a thin rim of relatively uniform composition. The chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets.Editorial responsibility: I. Parsons 相似文献
12.
Compositional relations among natural glasses in basalts recovered by Legs 45 and 46 (DSDP) provide powerful constraints on their differentiation histories. Residual glass compositions in the moderately evolved aphyric and abundantly phyric basalts within each site demonstrate that none of the units is mutually related to any other or to a common parent by simple fractional crystallization. At Site 396, where clinopyroxene phenocrysts are absent, progressively more evolved liquids (lower Mg/ (Mg+Fe) and higher TiO2) are characterized by lower calcium-aluminum ratios, which can only be generated by clinopyroxene fractionation. This paradox is amplified by some melt inclusions in olivine phenocrysts that have higher CaO/Al2O3 and lower TiO2 than any residual glasses. The occurrences of these distinctive compositions are correlated with the highly magnesian character of the host olivines (Fo90–89), and the melts are interpreted as trapped primitive liquids, parental to the more fractionated derivatives.Melt inclusions intermediate in composition between the residual glasses and the most primitive olivine melt inclusions are present in the cores of some plagioclase phenocrysts that have had a history of resorption. On the basis of a petrographic and microprobe analysis of the zoning relations in these phenocrysts, the inclusions are inferred to be melts entrapped at the time of extensive corrosion of the host crystals.Interpreted in conjunction with other mineral and geochemical data, the compositional trends in the glasses indicate that magma mixing has played a major role in the genesis of the Leg 45 and 46 basalts. The reality of mixing is demonstrated by extensive disequilibrium textures in the plagioclase phenocrysts and the presence in evolved lavas of refractory plagioclase and olivine phenocrysts bearing primitive melt inclusions. The chemical imprint of clinopyroxene fractionation despite the absence of clinopyroxene phenocrysts is believed to be accomplished by plating of gabbro on to the upper walls of the subvolcanic magma chamber as it evolves between mixing events. Repeated influxes of primitive magma batches will move the resultant hybrids alway from clinopyroxene saturation and generate olivine-plagioclase cotectic magmas. This model provides a physical buffering mechanism that accounts for the volumetric dominance of moderately evolved basalts among ocean floor tholeiites. Major and trace element models based on the combination of mixing and fractional crystallization also explain heretofore enigmatic geochemical characteristics of MORB.Lunar and Planetary Institute Contribution no. 326After August 1, 1978: Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USAThe Lunar and Planetary Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration 相似文献
13.
Antonio Castro Jesús D. de la Rosa W. Edryd Stephens 《Contributions to Mineralogy and Petrology》1990,106(1):9-26
The mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high-level subvolcanic setting in the Spanish Hercynian at Gerena, near Seville. The compositions involved are calc-alkaline and the situation is one of tonalite-quartz diorite synplutonic dykes injected into a granitic magma chamber. The resulting hybrids include dykes, pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite. The present investigation is based on field and petrographic observations of hybridization textures, the identification of different stages in the crystallisation history of the tonalite through mineral textures, and the characterization of mineral compositions at these various stages. Proportions of the end-member magmas involved were obtained by major-oxide mixing models and tested satisfactorily with trace elements. A mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber. After high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions. It is argued that such high-energy flow conditions are a necessary requirement for effective hybridization in this environment in contrast to most large-scale magma chamber settings where mixing is driven by thermal and buoyancy contrasts. 相似文献
14.
Successive mixing and mingling of magmas in a plutonic complex of Northeast Brazil 总被引：19，自引：0，他引：19
Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300–500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: (1) coarse-grained granite; (2) porphyritic granite and (3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite.
The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and majorelements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiO2 in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr.
The diorites contain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Abundant enclaves of diorite in the porphyritic granite, despite their textural diversity, are typically devoid of chilled margins, and were therefore formed relatively early in the crystallisation history of the granite. They are interpreted as liquid droplets separated from the heterogeneous hybrid magma through convection currents and incorporated in the, crystallising granitic magma.
Subsequently, during the crystallisation of the porphyritic granite, mafic magma supply to the batholith continued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of stromatitic-like structures, with alternating bands of mutually contaminated granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled finer-grained margins. 相似文献
15.
Volcán Tequila is an extinct stratovolcano in the western Mexican Volcanic Belt that has erupted lavas ranging from andesite to rhyolite during the last 0.9 Ma. Following an early period of rhyolitic volcanism, the main edifice of the volcano was constructed by central vent eruptions that produced 25 km3 of pyroxene-andesite. At about 0.2 Ma central activity ceased and numerous flows of hornblende-bearing andesite, dacite, and rhyodacite erupted from vents located around the flanks of the volcano. Bimodal plagioclase phenocryst rim compositions in lavas from both the main edifice and the flanks indicate that magma mixing commonly occurred shortly prior to or during eruption. Compositions of endmember magmas involved in mixing, as constrained by whole-rock major and trace element abundances, phenocryst compositions, and mineral-melt exchange equilibria, are similar to those of some lavas erupted from the central vent and on the flanks of the volcano. Estimated pre-eruptive temperatures for hornblende-bearing lavas (970°–830°C) are systematically lower than for lavas that lack hornblende (1045°–970°C), whereas magmatic H2O contents are systematically higher for hornblende-bearing lavas. In addition to stabilizing hornblende, high magmatic water contents promoted crystallization of calcic plagioclase (An70–82). Frequent injections of magma into the base of the subvolcanic plumbing system followed by eruption of mixed magma probably prevented formation of large volumes of silicic magma, which have caused paroxysmal, caldera-forming eruptions at other stratovolcanoes in western Mexico. The later stages of volcanic activity, represented by the flank lavas, indicate a change from a large magma storage reservoir to numerous small ones that developed along a NW-trending zone parallel to regional fault trends. Sr and Nd isotopic data for lavas from the Tequila region and other volcanoes in western Mexico demonstrate that differentiated calc-alkaline magmas are formed primarily through crystal fractionation of mantle-derived calc-alkaline basalt coupled with assimilation of crustal material.
Present Address:Department of the Geophysical Sciences The University of Chicago, Chicago IL, 60637, USA 相似文献
16.
浙东晚白垩世酸性岩浆的自混合作用及其意义 总被引：1，自引：1，他引：0
岩浆混合作用是造成火成岩多样性的主要原因之一,也是诱发火山喷发的重要机制。以往的研究多集中于基性和酸性岩浆之间的混合作用,但近年来酸性岩浆之间的混合作用受到越来越多的关注和研究。本文报道了浙东小雄破火山一个次级火山口内粗面质和流纹质两种酸性岩浆之间的混合现象。野外调查及岩相学研究显示,粗面质岩浆多呈大小不一的条带状以及透镜体状分布于流纹质岩浆内,局部发生扩散,粗面岩中斑晶大多为粗大的正长石斑晶,强烈熔蚀且聚斑结构普遍;在副矿物聚晶(由钛磁铁矿+磷灰石+锆石组成)的周围常可见反应边结构。流纹岩的斑晶主要由正长石、透长石及石英组成,晶体粒径较小,且熔蚀现象不发育。全岩主、微量元素特征及其他地质证据均显示,两种酸性岩浆之间以机械混合为主,其地球化学成分变化趋势主要受结晶分异过程控制。粗面质及流纹质岩浆在矿物组成、结构等方面的差异表明两者来源于同一层状岩浆房内的不同部位,其中粗面质岩浆应代表岩浆房底部及边部富晶体、贫熔体的粥状层部分(正长石+磁钛铁矿+锆石+磷灰石);而分异程度较高的流纹质岩浆则聚集于岩浆房上部形成富熔体、贫晶体的部分。两种酸性岩浆的混合现象是它们在地壳浅部层状岩浆房内自混合的结果,这一过程可能受岩浆房底部基性岩浆的聚集作用所控制,当更热、更基性的岩浆聚集时,岩浆房下部晶粥区内的粗面质岩浆迅速升温、活化,从而向上运移并与上部富熔体贫晶体的流纹质岩浆发生自混合作用。这一发现为我们理解中国东南沿海地区晚中生代大规模酸性火山喷发及岩浆演化机制、岩浆房结构提供了重要的参考,同时也为认识地壳浅部岩浆房内岩浆之间的自混合作用提供了可靠的例证。 相似文献
17.
Abyssal tholeiites from the Oceanographer Fracture Zone 总被引：17，自引：0，他引：17
David Walker Tsugio Shibata Stephen E. DeLong 《Contributions to Mineralogy and Petrology》1979,70(2):111-125
Unusual effects have been discovered in the major element phase relations of basalts from the Oceanographer Fracture Zone
(OFZ) which suggest that magma mixing of primitive basalt with the differentiation residue of a previous batch of primitive
magma has occurred. These effects include a reversal in mineral crystallization sequence which cannot happen during normal
differentiation processes or be explained by any plausible change in physical conditions. This unusual effect is encountered
as a result of curvature in composition space of the liquid-line-of-descent equilibria involving olivine, plagioclase, and
high-calcium clinopyroxene. Mixing of magmas at different stages of their evolution produces mixtures that do not lie on the
curved liquid-line-of-descent. Observation of such anomalous compositions in the OFZ suite supplements accumulating petrographic
and trace-element geochemistry evidence that magma mixing is an important petrogenetic process.
Mixing of fractionated residual liquids can produce mixtures which are either superheated or supercooled depending on the
sense of ‘thermal curvature’ of the liquid-line-of-descent. Both senses are encountered in the tholeiitic system, and this
effect may exert a qualitative control on the crystallization texture of the mixture.
A comparison of approximately 2,000 abyssal tholeiite compositions to the experimental liquid-line-of-descent reveals that
erupted differentiates which would be expected from advanced fractionation are scarce. Just this sort of phenomenon (the ‘perched’
steady state) was proposed by O'Hara (1977) as an earmark of the operation of a continuously fractionating magma chamber into
which fresh magma is periodically remixed. 相似文献
18.
Andesites from northeastern Kanaga Island,Aleutians 总被引：1，自引：0，他引：1
James G. Brophy 《Contributions to Mineralogy and Petrology》1990,104(5):568-581
Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO2 contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO2 content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO2 contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO2 contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO2 content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO2) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO2 that increase the thickness and lowers the overall SiO2 content of the upper silicic zone. It is suggested that the maximum SiO2 content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall. 相似文献
19.
The 2011 eruption of Nabro volcano, Eritrea, produced one of the largest volcanic sulphur inputs to the atmosphere since the 1991 eruption of Mt. Pinatubo, yet has received comparatively little scientific attention. Nabro forms part of an off-axis alignment, broadly perpendicular to the Afar Rift, and has a history of large-magnitude explosive silicic eruptions, as well as smaller more mafic ones. Here, we present and analyse extensive petrological data obtained from samples of trachybasaltic tephra erupted during the 2011 eruption to assess the pre-eruptive magma storage system and explain the large sulphur emission. We show that the eruption involved two texturally distinct batches of magma, one of which was more primitive and richer in sulphur than the other, which was higher in water (up to 2.5 wt%). Modelling of the degassing and crystallisation histories demonstrates that the more primitive magma rose rapidly from depth and experienced degassing crystallisation, while the other experienced isobaric cooling in the crust at around 5 km depth. Interaction between the two batches occurred shortly before the eruption. The eruption itself was likely triggered by recharge-induced destabilisation of vertically extensive mush zone under the volcano. This could potentially account for the large volume of sulphur released. Some of the melt inclusions are volatile undersaturated, and suggest that the original water content of the magma was around 1.3 wt%, which is relatively high for an intraplate setting, but consistent with seismic studies of the Afar plume. This eruption was smaller than some geological eruptions at Nabro, but provides important insights into the plumbing systems and dynamics of off-axis volcanoes in Afar. 相似文献
20.
Pre-eruption history of phyric basalts from DSDP legs 45 and 46: Evidence from morphology and zoning patterns in plagioclase 总被引：3，自引：0，他引：3
Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal.It is proposed that the plagioclase cores formed during injection of primitive magma into a previously existing magma chamber, that the mantle formed during mixing of a partially mixed magma and the remaining magma already in the chamber, and that the inner rim formed when the mixed magma was in a sheeted dike system. The large plateau at the outside of the mantle may have formed during the injection of the next batch of primitive magma into the main chamber, which may trigger an eruption. This model is consistent with fluid dynamic calculations and geochemically based magma mixing models, and is suggested to be the major mechanism for generating the disequilibrium conditions in the magma. 相似文献