共查询到20条相似文献,搜索用时 171 毫秒
1.
Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents 总被引:3,自引:2,他引:3
GREW E. S.; CHERNOSKY J. V.; WERDING G.; ABRAHAM K.; MARQUEZ N.; HINTHORNE J. R. 《Journal of Petrology》1990,31(5):1025-1070
Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB2O3, whereas the ion microprobe Li2O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li2O=00?19 wt.%; BeO=00?032 wt.%;B2O3=04?01 wt.%; and F=0?070?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li2O, O066 wt.% BeO, and 4?72 wt.% B2O3. Thesum B+Al+Fe3++Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?10?85 wt.% B2O3) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B2O3 contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and Kkrn-BtD=(F/OH)Krn/(F/(OH)Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B2O3 content is a measureof B2O3 activity in associated metamorphic fluid, whereas sillimaniteB2O3 content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe3+ content.In Fe3+-poor kornerupines, b increases with Mg and with (Mg+ Fe2+) but the effect of Mg on b via the substitution VIMg+IVSi=VIAl+IVAloverwhelms the effect of Fe2+=Mg substitution. 相似文献
2.
ACOSTA-VIGIL ANTONIO; LONDON DAVID; MORGAN GEORGE B. VI; DEWERS THOMAS A. 《Journal of Petrology》2006,47(2):231-254
We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H2O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 1201488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H2O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineralglassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al2O3, Na2O, andK2O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al2O3/[Na2O + K2O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylindersquartz and albite, quartzand K-feldspar, or albite and corundumtested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·02·8) x 1015m2/s and (0·62·4) x 1014 m2/s, respectively.Minimum diffusivities of alkalis [(39) x 1011m2/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 23 mm undergoinghydrostatic, H2O-saturated melting at 800°C (infinite heatsupply) could produce 2030 vol. % of homogeneous meltin less than 110 years. Slower diffusion in H2O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis 相似文献
3.
The configurational heat capacity, shear modulus and shear viscosity of a series of Na2O–Fe2O3–Al2O3–SiO2 melts have been determined as a function of composition. A change in composition dependence of each of the physical properties
is observed as Na2O/(Na2O + Al2O3) is decreased, and the peralkaline melts become peraluminous and a new charge-balanced Al-structure appears in the melts.
Of special interest are the frequency dependent (1 mHz–1 Hz) measurements of the shear modulus. These forced oscillation measurements
determine the lifetimes of Si–O bonds and Na–O bonds in the melt. The lifetime of the Al–O bonds could not, however, be resolved
from the mechanical spectrum. Therefore, it appears that the lifetime of Al–O bonds in these melts is similar to that of Si–O
bonds with the Al–O relaxation peak being subsumed by the Si–O relaxation peak. The appearance of a new Al-structure in the
peraluminous melts also cannot be resolved from the mechanical spectra, although a change in elastic shear modulus is determined
as a function of composition. The structural shear-relaxation time of some of these melts is not that which is predicted by
the Maxwell equation, but up to 1.5 orders of magnitude faster. Although the configurational heat capacity, density and shear
modulus of the melts show a change in trend as a function of composition at the boundary between peralkaline and peraluminous,
the deviation in relaxation time from the Maxwell equation occurs in the peralkaline regime. The measured relaxation times
for both the very peralkaline melts and the peraluminous melts are identical with the calculated Maxwell relaxation time.
As the Maxwell equation was created to describe the timescale of flow of a mono-structure material, a deviation from the prediction
would indicate that the structure of the melt is too complex to be described by this simple flow equation. One possibility
is that Al-rich channels form and then disappear with decreasing Si/Al, and that the flow is dominated by the lifetime of
Si–O bonds in the Al-poor peralkaline melts, and by the lifetime of Al–O bonds in the relatively Si-poor peralkaline and peraluminous
melts with a complex flow mechanism occurring in the mid-compositions. This anomalous deviation from the calculated relaxation
time appears to be independent of the change in structure expected to occur at the peralkaline/peraluminous boundary due to
the lack of charge-balancing cations for the Al-tetrahedra. 相似文献
4.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2 = 43.745.7 wt.per cent, A12O3 = 1.6O8.21 wt. per cent, CaO = 0.708.12wt. per cent, alk = 0.100.90 wt. per cent and Mg/(Mg+Fe2+)= 0.940.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H2O,CO2, and H2. The vapor-saturated peridotite solidi are 50200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al2O3.Conversely, lowering CaO/Al2O3 reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H2O in the vapor (XH2Ov). The data alsoshow that some CO2 may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of XH2Ov = 0.50.25 (XCO2v= 0.50.75). Such activities of H2O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite. 相似文献
5.
Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany 总被引:3,自引:0,他引:3
Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-fo2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An17·3Ab82·3Or0·4-An20·0Ab78·1Or1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H2O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO2 between 71 and 77 wt. per cent;A/(CNK) ratios of 1·21·4; Na2O < 2·95and normative corundum contents of 1·94·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi3O8 component accompanied by a simultaneous increasein CaAl2Si2O8 (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An7880) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H2O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (4043 mol. per cent SiO2) and sillimanite(49 mol. per cent SiO2) are Fe2O3 and TiO2 rich (up to 6·10·84and 3·60·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al2O3) occurs with corundumwhich has high Fe2O3 and TiO2 (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO2 contents of 6669 percent and normative corundum of 4 per cent. Sanidine (An1·9Ab26·0Or72·1-An1·3Ab15·9Or82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K2O from 8·6 to less than 1 wt. per cent and oxidetotals (less H2O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An2·0Ab34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO(total iroq) and up to 10 percent normative corundum. Near liquidus biotite with higher Al2O3and TiO2 than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO2) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·569·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 6469), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl2O466·645·0;MgAl2O453·018·7; Fe3O46·928·1;MnAl2O41·21·5; Fe2TiO40·66·2.Rims are generally enriched in the Fe3O4 component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe3+, Al and decreasing Ti4+, Fe2+ inilmenite, and increasing Fe2+, Ti4+ and decreasing Fe3+ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·17·5 and 8·513·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al2O3, TiO2, FeO, Fe2O3, and MgO that resultsby removal of a granitic melt, and/or by interactionwith the magma, to occur. T °C-fo2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. fo2 values of metamorphismand melting were between the Ni-NiO and Fe2O3-Fe3O4 buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 23kb Pload = Pfluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers. 相似文献
6.
An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models 总被引:14,自引:7,他引:7
DIXON JACQUELINE EABY; STOLPER EDWARD M.; HOLLOWAY JOHN R. 《Journal of Petrology》1995,36(6):1607-1631
Experiments were conducted to determine the solubilities ofH2O and CO2 and the nature of their mixing behavior in basalticliquid at pressures and temperature relevant to seqfloor eruption.Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°Cwith pure H2O at pressures of 176717 bar and H2OCO2vapor at pressures up to 980 bar. Concentrations and speciationof H2O and CO2 dissolved in the quenched glasses were measuredusing IR spectroscopy. Molar absorptivities for the 4500 cm1band of hydroxyl groups and the 5200 and 1630 cm1 bandsof molecular water are 067±003, 062±007,and 25±3 l/mol-cm, respectively. These and previouslydetermined molar absorptivities for a range of silicate meltcompositions correlate positively and linearly with the concentrationof tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturatedbasaltic melt is similar to that determined by Silver &Stolper (Journal of Petrology 30, 667709, 1989) in albiticglass and can be fitted by their regular ternary solution modelusing the coefficients for albitic glasses. Concentrations ofmolecular water measured in the quenched basaltic glasses areproportional to f H2O in all samples regardless of the compositionof the vapor, demonstrating that the activity of molecular waterin basaltic melts follows Henry's law at these pressures. Abest fit to our data and existing higher-pressure water solubilitydata (Khitarov et al., Geochemistry 5, 479492, 1959;Hamilton et al., Journal of Petrology 5, 2139, 1964),assuming Henrian behavior for molecular water and that the dependenceof molecular water content on total water content can be describedby the regular solution model, gives estimates for the Vo, mH2Oof 12±1 cm3/mol and for the 1-bar water solubility of011 wt%. Concentrations of CO2 dissolved as carbonate in the melt forpure CO2-saturated and mixed H2O-CO2-saturated experiments area simple function of fCO2 These results suggest Henrian behaviorfor the activity of carbonate in basaltic melt and do not supportthe widely held view that water significantly enhances the solutionof carbon dioxide in basaltic melts. Using a Vo, mr of 23 cm3/mol(Pan et al., Geochimica et Cosmochimica Acta 55, 15871595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 05 p.p.m. Our revised determination of CO2solubility is 20% higher than that reported by Stolper &Holloway (Earth and Planetary Science Letters 87, 397408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology 相似文献
7.
Volcan Popocatepetl, Mexico. Petrology, Magma Mixing, and Immediate Sources of Volatiles for the 1994-Present Eruption 总被引:1,自引:0,他引:1
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + FeTi oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 14 kbar). Magma mixed at 46 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles 相似文献
8.
The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H2O-CO2vapour. Starting materials were glasses of intermediate composition(65 wt.% Si02 representing a metaluminous I-typedacite and a peraluminous S-type greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH2O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675700?C at high aH2O, 725750?Cat aH2O0?5, and 800875?C at aH2O0?25, corresponding to<6070% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH2O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H2O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH2O in the dacite. At aH2O0?25 and temperatures 825850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?31?6 wt.% normative Cwithin therange of peraluminous TVZ rhyolites), whereas, at aH2O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?52?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH2O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH2O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH2O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH2O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component. 相似文献
9.
Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9monthinterval in 1982 83, some 305 106 m3 of mediumK,calcalkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An6075)and twopyrozene bearing andesites with 58% SiO2 to finalplagioclase (An8590), diopside, and olivine (Fo8590)bearing primitive magnesium basalts with 47% SiO2 Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of highMg parental melt. Theeruptive volumes of 35 106 m3 andesite, 120 106 m3 maficandesite, and 150 106 m3 basalt are consistent with the ideathat the 1982 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have 18O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with 18Oplagpxvalues of + 0.4 to + 0.6o, indicating internal Oisotopeequiliburium at temperature of 1100850 C. The magenesianbasalts have magmatic 18O/ 16O ratios similar to those of midoceanridge basalt, and the Oisotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by 18Orich crust during differentiation.Andesites and transitional mafic and sites have a more variableOisotope character, with laves and phenocrysts havingboth higher and lower 18O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 198283eruption. 相似文献
10.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?745?7 wt. percent, Al2O3=1?6O8?21 wt. per cent, CaO=0?708?12wt. per cent,alk=0?100?90 wt. per cent and Mg/(Mg+Fe2+)=0?940?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite. 相似文献
11.
Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (lxl7middot;5 km) introducing Upper Jurassic sediments,Marlborough, New Zealand. The ultrabasic-gabbroic rocks containlenses of kaersutite pegmatite and sodic syenite pegmatite andare intruded by ring dykes of titanaugite-ilmenite gabbro andlamprophyre. The margin of the intrusion is defined by a ringdyke of alkali gabbro. The plutonic rocks are cut by a swarmof hornblendebiotite-rich lamprophyre dykes. Thermal metamorphismhas converted the sediments to a hornfels ranging in grade fromthe albite-epidote hornfels facies to the upper limit of thehornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo8274),endiopside (Ca45Mg48Fe7Ca36Mg55Fe9), titanaugite (Ca40Mg50Fe10Ca44Mg39Fe17),plagioclase (An7318), and ilmenitetitaniferous magnetite,with various amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between endiopside and titanaugitewith the coupled substitution Ry+2+Si;;(Ti+4+Fe+3+Al+3 and asympathetic increase in CaAl2SiO6 (0·210·2percent) and CaTiAl2O6 (2·18·1 per cent)with fractionation. Endiopside shows a small, progressive Mgenrichment along a trend subparallel to the CaMgSi2O6Mg2Si2O6boundary, and titanaugite is enriched in Ca and Fe+2+Fe+3 withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1·057middot;7 per cent TiO2) kaersutite (6·4per cent TiO2) Fe-rich hastingsite (18·019·1per cent FeO as total Fe). Biotite is high in TiO2 (6·67·8per cent). Ilmenite and titaniferous magnetite (3·510·6per cent TiO2) are typically homogeneous grains; their compositioncan be expressed in terms of R+2RO3:R+2O:R2+3O4. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe2O3, TiO2, MnO, and V; almost linear increase in A12O3and late stage increase in alkalis and P2O3, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high PH2Oand fairly high, constant 相似文献
12.
Experimental Interaction of Granitic and Basaltic Magmas and Implications for Mafic Enclaves 总被引:4,自引:0,他引:4
We have performed time series experiments for periods rangingfrom 3 min to 44 h on the interaction of granite melt and partiallymolten basalt at 920C and 10 kbar, in the presence of 5 wt.%water. With time, the assemblage of the basalt domain changesfrom predominantly amphibole+plagioclase to clinopyroxene+garnet;the melt fraction increases from {small tilde}25 to 40%;and between the two domains, the melt compositions progressivelyequilibrate. Initially in each run, melts of the basalt domainhave uniform plateau concentrations for SiO2, Al2O3, CaO, MgO,and FeO because the activities of these components are regulatedby the mineral assemblage, but at advanced stages of reaction,no such control is evident. We have derived analytical expressionsto describe and simulate the diffusion profiles. The concentrationprofiles for SiO2, Al2O3, CaO, and Na2O in the granite, emanatingfrom the basaltgranite interface, have been used to estimateeffective diffusivities. The values from the shorter runs arecompared with those of the experiment of longest duration forwhich we assumed finite couples in our calculations. In thediffusion calculations for K2O the difference in melt fractionbetween the two domains is accounted for. The resulting values(in cm2/s) are: DNa2O=6 107, DK2O=3 107, DMgO=9 108, DCaO=(46) 108, and DSiO2 and DAl2O3=(306) 108. They are in reasonable agreement with values fromother studies. On the basis of our experiments we calculatethat mafic enclaves of magmatic origin should equilibrate toa large degree with their host magma in slowly cooling non-convectinggranitic plutons. Enclaves approaching complete re-equilibrationretain distinctly higher modal amounts of mafic minerals. Theydo not compositionally resemble binary magma mixtures, but aremore like host magma with accumulated crystals. We show thatthe modal differences between enclave and host are indicativeof the temperature of homogenization and that, in principle,this temperature can be deduced from equilibrium phase diagrams.
* Present address: Mineralogisch-Petrologisches Institut, Universitt Gttingen, Goldschmidtstrasse 1, 3400 Gttingen, Germany 相似文献
13.
Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing 总被引:4,自引:1,他引:3
The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO2 and K2O (7274 wt.% and5?46?4 wt.%, respectively), moderate P2O5 and REE (0?220?24%and 85?095?7 ppm), and low Fe2O3* and Zr contents (1?31?5%and 8090 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (525%),complexly zoned plagioclase, and high Fe2O3 (2?726?99%),CaO (0?561?95%), Zr (101213 ppm), and Ce (39?898?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (3060% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention. 相似文献
14.
Corella marbles in the Mary Kathleen Fold Belt were infiltratedby fluids during low-pressure (200-MPa) contact metamorphismassociated with the intrusion of the Burstall granite at 17301740Ma. Fluids emanating from the granite [whole-rock (WR) 18O=8.18.6%]produced Fe-rich massive and banded garnetclinopyroxeneskarns [18O(WR)=9.111.9%]. Outside the skarn zones, marblemineralogies define an increase in temperature (500 to >575C) and XCO2 (0.05 to >0.12) towards the granite, andmost marbles contain isobarically univariant or invariant assemblagesin the end-member CaOMgOAl2O3SiO2H2OCO2system. Marbles have calcite (Cc) 18O and 13C values of 12.324.6%and 1.0 to 3.9%, respectively. A lack of down-temperaturemineral reactions in the marbles suggests that pervasive fluidinfiltration did not continue after the thermal peak of contactmetamorphism. The timing of fluid flow probably correspondsto a period of high fluid production and high intrinsic permeabilitiesduring prograde contact metamorphism. The petrology and stableisotope geochemistry of the marbles suggest that these rockswere infiltrated by water-rich fluids. If fluid flow occurredup to the peak of contact metamorphism, the mineralogical andisotopic resetting is best explained by fluids flowing up-temperaturetoward the Burstall granite. However, if fluid flow ceased beforthe peak of regional metamorphism, the fluid flow directioncannot be unambiguously determined. At individual outcrops,marble 18O(Cc) values vary by several permil over a few squaremetres, suggesting that fluid fluxes varied by at least an orderof magnitude on the metre to tens-of-metre scale. Fluids werefocused across lithological layering; however, mesoscopic fracturesare not recognized. The focusing of fluids was possibly viamicrofractures, and the variation in the degree of resettingmay reflect variations in microcrack density and fracture permeability.The marbleskarn contacts represent a sharp discontinuityin both major element geochemistry and 18O values, suggestingthat, at least locally, little fluid flow occurred across thesecontacts. 相似文献
15.
We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 10501075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO2, <1 wt % Al2O3, and<0·1 wt % TiO2). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (3443 wt % SiO2)with high TiO2 (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO2). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO2-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO2systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents,and CaO/Al2O3 ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO2 and FeO* and low Al2O3, that have not been observedfrom partial melting of peridotite ± CO2. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts 相似文献
16.
Metamorphic Evolution of Neoproterozoic Manganese Formations and their Country Rocks at Otjosondu, Namibia 总被引:2,自引:0,他引:2
Manganiferous chemical sediments of Neoproterozoic age in Namibiawere subjected to high-Tlow-P metamorphism during theDamara Orogeny and display unique phase assemblages. The manganeseformations are embedded in iron formations and siliciclasticcountry rocks. This sequence is petrographically subdividedinto restricted lithotypes which bear specific mineral assemblagesand compositions depending on their protolith type. In puremanganese ores the critical assemblage braunite + haematite+ jacobsite + rhodonite is frequently developed, whereas interlayeredimpure silicate ores bear various proportions of spessartine,Mn3+-bearing andraditecalderite and andradite garnets,rhodonite, manganoan aegirineaugite, aegirine, BaKNa-feldspars,barite and rare kinoshitalite. Petrological constraints derivedfrom country rock lithologies indicate peak metamorphic conditionsof 660700C at estimated pressures of 3545 kbar.Numerous Ba-rich pegmatitic veins restricted to the ore horizonstestify to the production of partial melts from siliciclasticstrata within the manganese formations. They are correlatedwith peak pressure conditions between 5 and 6 kbar, accompanyingthe main deformation event and pre-dating the thermal peak.An early H2O-rich generation of fluid inclusions is interpretedas a manifestation of prograde dehydration reactions in theore horizons. This caused hydraulic fracturing of the ores and,subsequently, triggered the formation of partial melts whichintruded the fracture planes in situ. Peak metamorphism thenoccurred under strainfree conditions allowing equilibrium recrystallizationof all minerals to develop. Phase relationships of manganeseoxides and silicates modelled in the system MnFeSiOreveal variable chemical compositions of braunites, jacobsitesand haematites depending on their paragenesis. They indicatevery restricted oxygen reservoirs within specific strata ofthe manganese ores and eliminate a prominent mass exchange evenon a small scale. This is supported by 18O analyses of silicateassemblages which further exclude mass transfer between manganeseores and country rocks, and indicate preservation of the exchangeequilibria during cooling. The uplift path of the sequence canbe constrained using different decrepitation patterns of H2Ofluid inclusions and a syn-to late-metamorphic CO2-rich fluidinclusion population, which indicate high geothermal gradientsof 70C/km and more. The PTD evolution of thishigh-Tlow-P metamorphic belt conforms with the palaeotectonicsetting of the study area at the southernmost part of the CongoCraton, representing the continental buttress colliding withthe Kalahari Craton during the Pan-African orogeny. KEY WORDS: manganiferous sediments; Damara Orogeny; Namibia; metamorphism; oxygen isotopes; fluid inclusions
*Corresponding author. Present address Institut fr Geowissenschaften und Lithosphrenforschung, Senckenbergstrasse 3, D-35390 Giessen, Germany 相似文献
17.
Thermodynamic calculations based on addition of mass balanceequations to the Gibbs Method (Spear, 1986) are used to modelthe cordierite-producing reaction in pelitic gneiss from theMcCullough Range, southern Nevada. Calculations which treatthe model paragenesis as a system open to transfer of H2O areconsistent with textural relations. Results indicate that cordieritegrew by the continuous net-transfer reaction: 0?76 BIO+1?72 SILL+3? 55 QTZ+0?27 PLG+0?005 GRT +0?06Al2R2+1Si1[BIO]1?02 KSP+0?76 H2O +0?30 FeMg1[CRD]+0?15FeMg1[BIO]+0?0005 FeMg1[GRT] +0?005 CaNaAl1Si1[PLG] with decreasing P, decreasing T, and increasing aH2O The steepretrograde dP/dT path for these low-pressure granulites contrastswith isobaric cooling paths typical of higher pressure granulites,and suggests uplift and erosion were active during Proterozoicgranulite-grade metamorphism in this area. 相似文献
18.
GURENKO ANDREY A.; TRUMBULL ROBERT B.; THOMAS RAINER; LINDSAY JAN M. 《Journal of Petrology》2005,46(12):2495-2526
Glass inclusions in plagioclase and orthopyroxene from daciticpumice of the Cabrits Dome, Plat Pays Volcanic Complex in southernDominica reveal a complexity of element behavior and LiBisotope variations in a single volcanic center that would gounnoticed in a whole-rock study. Inclusions and matrix glassesare high-silica rhyolite with compositions consistent with about50% fractional crystallization of the observed phenocrysts.Estimated crystallization conditions are 760880°C,200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (whereFMQ is the fayalitemagnetitequartz buffer). Manyinclusion glasses are volatile-rich (up to 6 wt % H2O and 2900ppm Cl), but contents range down to 1 wt % H2O and 2000 ppmCl as a result of shallow-level degassing. Sulfur contents arelow throughout, with <350 ppm S. The trace element compositionof inclusion glasses shows enrichment in light rare earth elements(LREE; (La/Sm)n = 2·56·6) and elevatedBa, Th and K contents compared with whole rocks and similaror lower Nb and heavy REE (HREE; (Gd/Yb)n = 0·51·0).Lithium and boron concentrations and isotope ratios in meltinclusions are highly variable (2060 ppm Li with 7Li= +4 to +15 ± 2; 60100 ppm B with 11B = +6 to+13 ± 2) and imply trapping of isotopically heterogeneous,hybrid melts. Multiple sources and processes are required toexplain these features. The mid-ocean ridge basalt (MORB)-likeHREE, Nb and Y signature reflects the parental magma(s) derivedfrom the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlationsin inclusion glasses indicate coupled enrichment in stronglyfluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,which can be explained by fractional crystallization of plagioclase,orthopyroxene and FeTi oxides. The 7Li and 11B valuesare at the high end of known ranges for other island arc magmas.We attribute the high values to a 11B and 7Li-enriched slabcomponent derived from sea-floor-altered oceanic crust and possiblyfurther enriched in heavy isotopes by dehydration fractionation.The heterogeneity of isotope ratios in the evolved, trappedmelts is attributed to shallow-level assimilation of older volcanicrocks of the Plat Pays Volcanic Complex. KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion 相似文献
19.
Trace Element and Sr-Pb-Nd-Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites 总被引:3,自引:0,他引:3
DAVIES G. R.; STOLZ A. J.; MAHOTKIN I. L.; NOWELL G. M.; PEARSON D. G. 《Journal of Petrology》2006,47(6):1119-1146
A phase of Mesozoic extension associated with the terminationof continental collision at the southern margin of the AldanShield produced ultrabasic lamproites in a discontinuous belt500 km long and 150 km wide. The lamproites, locally poorlydiamondiferous, were emplaced as dykes, sills and pipes. AllAldan lamproites have primitive chemical characteristics (e.g.MgO up to 22·7 wt %) and are ultrapotassic (K2O up to8·3 wt %) and peralkaline with K2O + Na2O/Al2O3 in therange 0·61·16. A distinctive feature ofthese rocks is their low TiO2 content (0·51·4wt %). Aldan lamproites are moderately light rare earth element(LREE) enriched with (La/Yb)N ranging from 10 to 47. Heavy rareearth element (HREE) abundances are lower than for all otherlamproites by up to a factor of five. Therefore, the combinedmajor and trace element characteristics of the Aldan samplesare not typical of other lamproite occurrences. Large ion lithophileelement concentrations are high (100800 x Primitive Mantle)but the high field strength elements (HFSE; Nb, Ta, Ti) plusTh and U display unusually low concentrations for rocks of thistype. The style of trace element enrichment recorded by theAldan Shield lamproites is comparable with that of subduction-relatedmagmatism. The Aldan lamproites have among the most extremeinitial isotopic ratios yet recorded from mantle-derived magmas;Ndi = 10·3 to 22·3, 87Sr/86Sri =0·70550·7079, Hfi = 7·6 to29·4 and 206Pb/204Pbi = 16·617·4.When interpreted in terms of multi-stage Pb isotope evolution,the Pb isotope data require fractionation from a Bulk Earthreservoir at 3·0 Ga and subsequent evolution with second-stageµ values between 6·4 and 8·0. The inferredArchaean age of the lamproite source is consistent with Nd andHf model ages, which range from 1·5 to 3·0 Ga.Aldan lamproites have Hf values that range from +3 to 7.Trace element and SrNdPbHf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative Hf as a result of metasomatism by slab-derivedhydrous melts that left rutilegarnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotopetrace element correlations and the lowREE contents. Although high-quality trace element data (e.g.HFSE) are not available for most lamproites, it appears thatmany of their source regions contain a component of recycledoceanic crust, possibly including subducted sediment. The sourcesof the Aldan and many other lamproites are distinct from oceanisland basalt mantle sources. This suggests that the long-termstorage of trace element enriched lamproite sources occurredin the sub-continental lithospheric mantle and not at depthwithin the convecting asthenosphere. KEY WORDS: potassic volcanism; isotope geochemistry; fluid enrichment 相似文献
20.
Experimental Crystallization of Leucogranite Magmas 总被引:25,自引:8,他引:17
Both crystallization and melting experiments have been carriedout on two natural, biotite-muscovite (DK) and tourmaline-muscovite(GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO2 =FMQ05, aH2O = 1003, and at five temperaturesbetween 803 and 663C H2O contents of the quenched glasses wereanalysed by ion microprobe. Plagioclase and biotite are theliquidus phases for reduced melt H2O contents and H2O-rich conditions,respectively. H2O saturation limits range from 8 to 10 wt%.DK has a wider crystallization interval than GB (150 vs 80Cfor conditions close to H2O saturation), and a slightly higherH2O-saturated solidus (645 compared with 630C for GB). Tourmalinenever crystallized spontaneously from the melt. Tourmaline seedsalways reacted out to biotite in the biotite-muscovite sample,whereas they remained stable in the tourmaline-muscovite sample.Biotite is replaced by hercynite as the main ferromagnesianphase at high temperature and reduced aH2O. Muscovite crystallizationis restricted to near-solidus conditions. The compositions ofplagioclase, alkali feldspar, biotite and muscovite are givenas a function of bulk composition, temperature and aH2O. Glasscompositions are richer in normative quartz than the 4 kbarH2O-saturated QzAbOr eutectic, and become moreperaluminous and less mafic with increasing fractionation. Biotitecrystallization in peraluminous liquids is favoured by elevatedFe, Mg and Ti contents. Muscovite crystallization is not promotedunder H2O-saturated conditions. Tourmaline stability is stronglydependent on aH2O. For GB, tourmaline is present at elevatedtemperatures for intermediate values of aH2O (803 C, 07),but not above 650C for H2O-saturated conditions. Comparisonof the natural crystallization sequence with experiments suggestsinitial water contents between 5 and 75 wt % for the DK magma,and > 7 wt% for the GB magma. Plagioclase core compositionsgive minimum temperatures of 700C for GB and 750C for DK,consistent with an emplacement of these HHL as almost entirelyliquid bodies. The restricted occurrence of biotite in the GBgranite suggests that it reacted out during the magmatic evolution,owing to a marked change in fO2 toward more oxidizing conditions.Tourmaline leucogranites can be generated from biotite leucogranitesby fractional crystallization under conditions of increasingdegree of oxidation. KEY WORDS: leucogranite; melting experiments; crystlization experiments; Himalayas; phase relations
*Corresponding author 相似文献