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1.
Experimental melting studies were conducted on a nepheline mugearitecomposition to pressures of 31 kbar in the presence of 0–30%added water. A temperature maximum in the near-liquidus stabilityof amphibole (with olivine) was found for a water content of3·5 wt % at a pressure of 14 kbar. This is interpretedto have petrogenetic significance for the derivation of nephelinemugearite magmas from nepheline hawaiite by amphibole-dominatedfractional crystallization at depth within the lithosphericmantle. Synthetic liquids at progressively lower temperaturesrange to nepheline benmoreite compositions very similar to thoseof natural xenolith-bearing high-pressure lavas elsewhere, andsupport the hypothesis that continued fractional crystallizationcould lead to high-pressure phonolite liquids. Independent experimentaldata for a basanite composition modeled on a lava from the sameigneous province (the Newer Basalts of Victoria) permit theinference that primary asthenospheric basanite magmas undergopolybaric fractional crystallization during ascent, and mayevolve to liquids ranging from nepheline hawaiite to phonoliteupon encountering cooler lithospheric mantle at depths of 42–50km. Such a model is consistent with the presence in some evolvedalkalic lavas of both lithospheric peridotite xenoliths indicativeof similar depths and of megacryst suites that probably representdisrupted pegmatitic segregations precipitated from precursoralkalic magmas in conduit systems within lithospheric mantle. KEY WORDS: experiment; high pressure; alkalic magmas; amphibole; nepheline mugearite; basanite; lithosphere 相似文献
2.
Kalsilite—a typical mineral of ore-bearing zones of the Khibiny and Lovozero plutons—was formed after low-Si and high-K nepheline
in one of three ways: (1) by relatively high-temperature replacement of Na with K; (2) due to orthoclase-kalsilite poikiloblastesis
in foidolites and overlapping foyaites; or (3) by replacement of nepheline with zeolite. 相似文献
3.
Mawpyut igneous suite in Jaintia Hills of Meghalaya plateau comprises differentiated suite of ultramafic–mafic rocks. The
complex differs from other ultramafic–alkaline–carbonatite igneous emplacements of Shillong plateau and Mikir Hills like Jesra,
Sung, Samchampi complexes, by the absence of alkaline–carbonatite rocks as major litho-units. Melanite garnet-bearing nepheline
syenite, occurs as late phase minor intrusion in Mawpyut igneous complex, posseses alkaline character and shows inubiquitous
relation with the host ultramafic–mafic rocks. On the other hand, this alkaline intrusive bodies of the Mawpyut igneous complex
shows chemico-mineralogical resemblance with garnet-bearing nepheline syenite, ijolite litho-members of Jesra, Sung, Samchampi
complexes of the region. It is interpreted that melanite garnet-bearing nepheline syenite intrusion in Mawpyut is contemporaneous
with Jesra, Sung, Samchampi ultramafic–alkaline–carbonatite complexes and the host rocks of Mawpyut complex is an earlier
magmatic activity possibly from a comparatively least enriched source. 相似文献
4.
I. E. Smith 《Contributions to Mineralogy and Petrology》1972,34(2):167-176
This paper presents petrographic, major element, and trace element data from high-potassium ‘shoshonitic’ rocks of Miocene
age which intrude Eocene submarine basalts in southeastern Papua. The intrusives fall into two distinct but overlapping groups,
a ‘near-saturated’ group ranging from gabbro to syenite with regular petrographic and chemical variations, which is either
slightly nepheline normative or quartz normative, and a nepheline normative ‘undersaturated’ group which shows wide variations
in texture, modal mineralogy, and chemistry. Biotite-bearing pyroxenites are associated with the intrusives but their genetic
relationship to the intrusives is unknown. The intrusion of shoshonitic rocks at the beginning of a period of major tectonic
activity in southeastern Papua shows that high-potassium magmas can be generated in areas of active tectonism and may form
part of the island arc ‘magmatic’ association. 相似文献
5.
Polymineralic inclusions which consist of a few grains of diopside, enstatite, jadeite, nepheline, albite, pargasite, phlogopites
and olivine were found in chromian spinel in a chromitite pod and in troctolite from Hess Deep, equatorial Pacific. The inclusion
mineral suite in chromitite is characterized by Na-Al silicates, such as jadeite, nepheline and albite. Jadeite and nepheline
commonly coexist with enstatite, and tend to occur as interstitial grains between subhedral enstatite (or other minerals)
and host spinel. Albite, diopside and enstatite occur as equant inclusions. The mafic minerals in the inclusions have similar
chemistry to those found in the troctolite and dunite. The modes of occurrence and mineral chemistry of the inclusions are
controlled by magmatic precipitation, and subsequent reequilibration due to decrease of temperature in the uppermost mantle.
The mafic minerals in spinel inclusions were crystallized from a melt enriched in Cr and some incompatible components formed
by melt-mantle interaction process mixed to various extent with subsequently supplied more primary melt. Albite and nepheline
could also be formed at the magmatic stage. Jadeite was formed by a subsolidus reaction of albite and nepheline at low temperatures
(250–300 °C) at slightly less than 3 kbar. This requires a remarkable temperature decrease, at least locally, of the uppermost
mantle and crust. The Hess Deep rocks were formed in the uppermost mantle beneath a spreading-ridge axis at more than 1000 °C,
and were transposed outwards from the axis by corner flow. At the off-ridge conditions, the rocks were cooled and serpentinized
by circulation of sea water at the mantle depth to form jadeite in chromitite. The serpentinized portion could have risen
as a kind of serpentinite diapir through the thin crust up to the ocean floor.
Received: 24 January 1997 / Accepted: 6 November 1997 相似文献
6.
D. de Ligny P. Richet E. F. Westrum Jr J. Roux 《Physics and Chemistry of Minerals》2002,29(4):267-272
From heat capacities measured adiabatically at low temperatures, the standard entropies at 298.15 K of synthetic rutile (TiO2) and nepheline (NaAlSiO4) have been determined to be 50.0 ± 0.1 and 122.8 ± 0.3 J mol−1 K, respectively. These values agree with previous measurements and in particular confirm the higher entropy of nepheline
with respect to that of the less dense NaAlSiO4 polymorph carnegieite.
Received: 23 July 2001 / Accepted: 12 October 2001 相似文献
7.
Tanja Waterwiese Niranjan D. Chatterjee Ivana Dierdorf Jörg Göttlicher Herbert Kroll 《Contributions to Mineralogy and Petrology》1995,121(1):61-73
Internally consistent thermodynamic datasets available at present call for a further improvement of the data for nepheline
(Holland and Powell 1988; Berman 1991). Because nepheline is a common rock-forming mineral, an attempt has been made to improve
on the present state of knowledge of its thermodynamic properties. To achieve that goal, two heterogeneous reactions involving
nepheline, albite, jadeite and a-quartz in the system NaAlSiO4-SiO2 have been reversed bylong duration runs in the range 460 ≤ T(°C) ≤ 960 and 10 ≤ P(kbar) ≤ 22. Given sufficiently long run times, thealbite run products approach internal equilibrium with respect to their Al,Si order-disorder states. Using appropriate thermochemical, thermophysical, and volumetric data,
Landau expansion for albite, and the relevant reaction reversals, a refined thermodynamic dataset (ΔfHi0 and Si0) has been derived for nepheline, jadeite, a-quartz, albite, and monalbite. Our refined data agree very well with theircalorimetric counterparts, but have smaller uncertainties. The refined dataset for ΔfHi0 and Si0, including their uncertainties and correlation, help generate the NaAlSiO4-SiO2 phase diagram including 2a confidence interval for eachP-T curve (Fig. 5).
Editorial responsibility: W. Schreyer 相似文献
8.
The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion
∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the
Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones
200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture
of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment
in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents
(1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios),
nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved
product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique
composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation.
Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of
prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals
with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal
stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive
decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and
occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison
with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit
is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th,
and thus can be used in various fields of application. 相似文献
9.
A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nepheline. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K3NaAl4Si4O16. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the giant leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nepheline phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution. 相似文献
10.
Monchique is a sizeable subvolcanic ?laccolith, unusual amongalkaline complexes in invading sediments and lacking apparentconnection with either rifting or orogeny; it may however relateto the opening of the North Atlantic(age = 76 m.y.). The intrusionis predominantly miaskitic syenite, varying irregularly fromfoyaite to pulaskite with fine-grained nepheline-poor marginsbut showing no rhythmic or cryptic layering. Minor rock-typesinclude early masses of olivine-free kaersutitetheralites(berondrites) and essexites, bodies of igneous breccias, maligniteand agpaite, veins of foyaitepegmatite and shonkinite,and dykes of lamprophyres and peralkaline tinguaite. Coevaldykes outside the main intrusion include quartztrachytes,normal (olivine-bearing) basanites and amphibolepicrites. The whole suite may have derived from a basanitic parent undermoderately oxidizing conditions. Geochemistry is apparentlycontinuous along the trend BerondriteEssexiteFoyaite(Malignite)PulaskiteQuartz trachyte. As far asFoyaite this parallels the normal oceanic basanitephonolitetrend with major and trace elements and minerals (except olivine)behaving as in normal fractionation; the absence of nephelinemonzonites,creating a Daly Gap, may merely reflect high fractionation efficiency.The apparent evolution across the thermal barrier in NeKsQtz,with a reversal in some major element trends, however, can beexplained neither by fractionation nor country-rock assimilation.The enigmatic pulaskites cannot be related directly to the foyaitesbut might have formed from the same parent under lower pressureconditions; they themselves fractionated to the peralkalinetinguaites. The quartztrachytes probably originated wherefoyaite magmas lost alkalis to the siliceous country-rocks,became oversaturated, and then fractionated feldspars. Liquidimmiscibility might explain some anomalous monzonitic rocksbut otherwise contributed little to the evolution of the complex. 相似文献
11.
L. M. Lyalina Ye. E. Savchenko E. A. Selivanova D. R. Zozulya 《Geology of Ore Deposits》2010,52(7):641-645
The detailed study of the mineral composition of the nepheline syenite pegmatite from the Saharjok Intrusion has resulted
in the finding of behoite and mimetite, a mineral species identified in the Kola region for the first time. The pegmatite
body at the contact between nepheline syenite and essexite is unusual in textural and structural features and combination
of mineral assemblages including unique beryllium mineralization. Behoite Be(OH)2 is an extremely rare beryllium mineral. It occurs as powderlike aggregates in the leaching cavities between euhedral pyroxene
crystals. Behoite was identified by comparison of X-ray powder diffraction data of the studied mineral phase and behoite from
the Be-bearing tuff in the type locality of this mineral (Utah, United States). Mimetite was found in the same pegmatite of
the Saharjok intrusion. It forms unusual parallel-fibrous aggregates with individual fibers as long as ∼1 mm and only ∼1 μm
across. X-ray powder diffraction data and the chemical composition characterize the mineral as hexagonal phase Pb5[AsO4]3Cl. Both behoite and mimetite are the products of late hydrothermal alteration of primary minerals (meliphanite, galena, arsenopyrite,
and loellingite). The secondary phases freely crystallized in the cavities remaining after the leached nepheline. 相似文献
12.
Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO2, aq + K+ + H2Oor alternatively beryl Kfeldspar + H2O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimaniteKfeldspar, muscoviteKfeldsparsillimanite,and annitemagnetitespinelsillimanitegarnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at PT conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H+ = chrysoberyl + quartz+ Na+ + H2O or alternatively beryl + albite + H2O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H+ =chrysoberyl + quartz + sillimanite + Na+ + K+ + H2O or alternativelyberyl + albite + muscovite + H2O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H2O = chrysoberyl + phenakite + melt at PTconditions between the KfeldsparquartzH2Osolidus and the Kfeldsparalbitequartz-H2Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high PT conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO2-poor country rocks, or of the assimilationof Al2O3-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites. 相似文献
13.
TUCCILLO M. E.; MEZGER K.; ESSENE E. J.; VAN DER PLUIJM B. A. 《Journal of Petrology》1992,33(6):1225-1259
Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (PTt)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H2O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. GarnetAl2SiO5plagioclasequartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnetplagioclaseilmeniterutilequartzand garnetclinopyroxeneplagioclasequartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 14 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the PTtdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince. 相似文献
14.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Nilanjan Chaterjee 《Journal of Earth System Science》2010,119(5):675-699
The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,
Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development
of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite
and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives
of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton.
Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.
Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase
in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of
the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field
in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite,
which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s
pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C
for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for
ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric
data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite,
alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration
pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation
index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field
geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly
suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics. 相似文献
15.
In the albite-orthoclase-nepheline-kalsilite-water system theassemblage analcite-melt (±nepheline albite and orthoclase)can only exist in the pressure range 5 to 13 Kbars and at temperaturesbetween 640 and 600 °C. Primary igneous analcite will havea similar restricted PT range which probably means that interstitialanalcite found in many high-level dike rocks is secondary. 相似文献
16.
Nepheline-alkali feldspar equilibria with alkali chloride aqueoussolutions have been determined for the temperature range 400to 700 °C at 1000 bars pressure. Nepheline-alkali feldsparequilibria with alkali chloride melts have been determined forthe temperature range 800 to 1100 °C at approximately 6bars pressure. (1) NaAlSiO4 + KCl(aq) = NaCl(aq) + KAlSiO4 (2) NaAlSiO4 + KCl(melt) = NaCl(melt) + KAlSiO4 (3) NaAlSi3O8(high) + KCl(aq) = NaCl(aq) + KAlSi3O8(San) (4) NaAlSi3O8(low) + KCl(aq) = NaCl(aq) + KAlSi3O8(Mic) (5) NaAlSi3O8(high) + KCl(melt) = NaCl(melt) + KAlSi3O4(San) (6) NaAlSi3O8(low) + KCl(melt) = NaCl(melt) + KAlSi3O8(Mic) From these, two diagrams of phase relationships were derivedfor the following exchange equilibria: (7) NaAlSiO4 + KAlSi3O8(San) = NaAlSi3O8(high) + KAlSiO4; (8) NaAlSiO4 + KAlSi3O8(Mic) = NaAlSi3O8(low) + KAlSiO4. The effect of pressure on these equilibria has been determinedby comparing the experimental data for 1000 and 5000 bars (t= 500 °C) and thermodynamic calculations. It has also beenshown that the effect of excess silica in nepheline solid solutionon the KNa distribution between nepheline and alkalifeldspar is substantial and opposite to that of temperature.In the high temperature region an increase in silica contentin nepheline of 2 wt. per cent eliminates the effect on theredistribution of a temperature increase of 100 °C. Thesecation exchange data and unit cell data for the crystal phasesare used to calculate thermodynamic mixing properties of nephelinesolid solution and alkali feldspar solid solution for a widerange of temperature and pressure. 相似文献
17.
A Model of Magmatic Crystallization 总被引:2,自引:0,他引:2
A computer model simulating fractional crystallization at oneatmosphere pressure incorporates nine broadly-defined mineralsmagnetite,olivine, hypersthene, augite, quartz, plagioclase, orthoclase,leucite, and nepheline. The crystallization temperature of eachmineral is considered to be a smooth function of the compositionof the magmatic liquid. These mineral temperature equationsare obtained by multiple linear regression analysis of informationfrom published silicate systems and rock melting experiments.The nine equations are solved for any primary liquid, withinthe broad range of common magma types, to select the crystallizingmineral or minerals. Partition ratios from published experimentsand analyses of lavas and phenocrysts permit calculation ofthe composition of the crystallizing mineral assemblage. Subtractionof a small amount of that composition from the primary liquidyields a new liquid, which may be recycled to yield a sequenceof liquids during fractional crystallization. The crystallizationmodel handles assemblages of co-precipitating minerals, andcan trace progressive saturation in new minerals, substitutionof a new mineral for an old mineral, and cessation of crystallizationof a mineral. The sequences of minerals and liquids derivedfrom a broad set of primary liquids are geologically realistic,so the model is useful in predicting phenocrysts in volcanicrocks and events during crystallization of shallow intrusions. 相似文献
18.
BECCALUVA L.; BARBIERI M.; BORN H.; BROTZU P.; COLTORTI M.; CONTE A.; GARBARINO C.; G. C.B. GOMES; M. ACCIOTTA; MORBIDELLI L.; RUBERTI E.; SIENA F.; TRAVERSA G. 《Journal of Petrology》1992,33(6):1371-1404
The Juqui circular intrusion, which is Cretaceous in age (130135Ma), crops out in the Precambrian gneissic basement in Brazilover an area of 14 km2. It consists of olivine clinopyroxen-itecumulates (with minor olivine gabbros) in the northeastern sector(74 vol.%), whereas ijolites-melteigites-urtites (4%) and nephelinesyenites with minor essexites and syenodiorites (21%) outlinesubannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trendsindicate that the origin of the complex can be largely accountedfor by shallow-level fractional crystallization of a carbonatedbasanitic parental magma. Such a magma was generated deep inthe subcontinental lithosphere by low-degree partial meltingof a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimatespermit definition of several fractionation stages of the magmaticevolution under nearly closed-system conditions, with inwarddevelopment of zonally arranged side-wall cumulates. These stagesinvolved: (1) fractionation from basanite to essexite magma(liquid fraction F = 335%) by crystallization of olivineclinopyroxenite plus minor olivine alkali gabbro cumulates;(2) derivation of the least differentiated mafic nepheline syenite(F = 55 %) from essexitic magma by subtraction of a syenodioriteassemblage; (3) exsolution of a carbonatite liquid (5%) froma CO2-enriched mafic nepheline syenite magma, which also underwentcontinuous fractionation giving rise to ijolite-melteigite-urtitecumulates. The proportion of cumulus clinopyroxene and biotiteand intercumulus nepheline and alkali feldspar in these lastrocks, as well as the absence of alkalis in carbonatite, maybe attributed, at least in part, to loss of alkali-rich hydrousfluids released during and after the unmixing formation of thetwo conjugate liquids. The KD values determined for Mg-carbonatite/nepheline syeniteare lower (1429) for light rare earth elements(LREE) than for REE from Eu to Yb (4678),in contrast to recent experimental results (Hamilton et al.,1989). A possible explanation is that Juquia Mg-carbonatiterepresents an alreadydifferentiated magma, which underwent extensivefractionation of LREE-enriched calcite. In this way, the highvariability of K0 REE patterns observed in several alkaline-carbonatitecomplexes can also be accounted for. The remarkably constant initial 87Sr/86Sr ratios (mostly between07052 and 07057) support the interpretation ofthe intrusion as having been generated by fractrional crystallizationand liquid immiscibility from a common parental magma. Iligherisotopic ratios (0706007078), found mainlyin dykes and in the border facies of the intrusion, may be dueto contamination by the gecissic basement. 相似文献
19.
The Lower Zone of the Eastern Bushveld Complex in the OlifantsRiver Trough reaches 1584 m in thickness and is divisible intoBasal subzone, Lower Bronzitite, Harzburgite subzone, and UpperBronzitite. The Lower Zone is directly and conformably overlainby the Critical Zone; there is no break between the two. The principal cumulus minerals in the Lower Zone are bronziteand olivine. Chromite is an accessory cumulus mineral in peridotites,especially in the Harzburgite subzone, and cumulus plagioclaseoccurs in two thin units in the Basal subzone. Elsewhere plagioclase,with or without chromian augite, is postcumulus in origin. Electron microprobe analyses show that the range in En and Focontents of bronzite and olivine, respectively, is only a fewper cent over the entire rock sequence. Highest values of bothare found in the Harzburgite subzone. From modal and mineralanalyses the bulk composition of the Lower Zone (wt. per cent)is calculated as SiO253.94, TiO20.08, Cr2O30.55,V2O30.01, Al2O32.64, NiO0.09, FeO (totalFe as FeO)9.62, MnO0.20, MgO31.72, CaO1.48,K2O0.1, Na2O0.13. This composition is unlike thatof any magma type, indicating that the Lower Zone is indeeda pile of crystal cumulates. From the data for the Lower Zone, together with available datafor the Critical, Main, and Upper Zones, the average MgO contentof the Eastern Bushveld Complex is calculated as about 13 percent, the Cr content as in excess of 1000 ppm. Even if the Complexformed from a single body of magma, the magma cannot have beentholeiitic, but rather olivine tholeiitic or picritic. An hypothesis of evolution of the Lower Zone is presented. Shiftsin total pressure are inferred to have been a major factor inproducing the succession of rock types and in producing theextraordinarily persistent chromitites of the overlying CriticalZone. It is suggested that the extraordinary richness in chromiteof the Bushveld is related to its formation not from tholeiiticmagma, but from more Mg-rich, chromium-rich magma drawn froma deeper level of the mantle than that which has yielded thetholeiitic basalts. 相似文献
20.
A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nephe-line. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K3NaAl4Si4O16. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the giant leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nephe-line phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution. 相似文献