首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 93 毫秒
1.
‘Country-rock eclogite’ pods occur enclosed withtectonic contacts within heterogeneous amphibolite-facies gneissesin the Basal Gneiss Region of Western Norway. Sixty-nine newmicroprobe analyses for garnets, clino-and orthopyroxenes, olivine,clinoamphiboles, biotite and carbonates from a number of orthopyroxeneeclogite pods in the Selje District are presented. The firstfour minerals are primary whilst the others, of which the amphibolesare described in some detail, formed during a subsequent butstill early stage in eclogite history. The primary minerals have a wider range of compositions thanorthopyroxene eclogites from other geological environments;jadeite-rich clinopyroxene and unusually grossular-poor garnetsare described from this environment for the first time. Sidero-magnesiteoccurs in apparent equilibrium with primary eclogite minerals.The early amphiboles have apparently grown at the expense of,but nevertheless in equilibrium with, primary minerals throughreactions involving OH, K+, Na+, and possibly Mg-bearing fluids.Magnesio-cummingtonite intergrown with actinolite is recognizedas an early phase in one eclogite pod. The early amphibolescan be distinguished from the symplectitic amphiboles by thelower Allv, Alvl, Ti and alkali contents and Fe/Fe + Mg ratiosand higher Si content of the former minerals. The symplectiticamphiboles form, together with plagioclase, during the stilllater amphibolitization of the eclogites. Fe/Mg distribution coefficients are affected by the Na contentsof clinopyroxenes and probably also by the Fe/Mg contents ofthe bulk assemblages. The former is due largely to increasingacmite content in jadeite-rich clinopyroxenes whilst the latteris tentatively attributed to lower closure temperatures of Fe-richassemblages. The Ca content in garnet is significantly relatedto both of these Na and Fe/Mg factors. Nevertheless a rangeof different distribution coefficients, including the Ca/Ca+ Mg ratio in coexisting pyroxenes, suggests a very limitedrange of temperatures of equilibration, the best estimate ofwhich is 700–850 °C. Pressures of equilibration are more difficult to assess. Onemodel, based upon the assumption of the stable occurrence ofamphiboles together with primary minerals and upon the minimumpressures necessary to transform a range of rock types to eclogite,suggests pressures of 15–28 kb at 700–850°C.A second model, based upon the Al2O3 content of primary orthopyroxeneand upon the association of sidero-magnesite with pyroxenes,suggests higher pressures (30–45 kb) over the same temperaturerange. Amphiboles are not stable under these conditions andare considered to form during a subsequent lower pressure (15–28kb) event when the low Al2O3 orthopyroxenes and sidero-magnesitesurvive metastably during an essentially isothermal history. One eclogite pod contains minerals with coarse exsolution lamellae:orthopyroxene exsolving garnet and clinopyroxene exsolving orthopyroxene.These imply high T-P processes, roughly estimated at 1200–1370°C, 30–40 kb, and hence suggest eclogite generationby igneous fractionation processes. Four T-P regimes (A, B, C, D) of mineral equilibration are recognizedin the history of the Selje district orthopyroxene eclogites,between their prior origin, presumably in the upper mantle,and their present surface exposure. This initial eclogite fractionation(regime A) occurred in an olivine-poor rather than olivine-richupper mantle environment, followed by cooling, exsolution, recrystallizationand re-equilibration (regime B) in a Precambrian tectonic environment.Subsequent history involved mineral reaction, metasomatism,and probably chemical redistribution through the medium of amphibole-formingfluids (regime C) and finally Caledonian tectonic transportinto poly-metamorphic continental basement where their survivalis thought to be due to a low activity of water. Marginal symplectiticamphibolitization (regime D), due to localized fluxing of metamorphicfluids, was the last significant petrological event prior touplift and exposure. The processes of tectonic transport aretentatively considered to represent deep level obduction processesrelated to continent/continent collision.  相似文献   

2.
We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe2+. A mixture of PbO with about 55 mol per cent PbF2 provedvery effective as a flux. The experimental results do not suggest any significant dependenceof KD on Fe/Mg ratio at T 1000?C. The lnKD vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to KD and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr3+ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe2+ in garnetand about 1 wt. per cent Al2O3 in orthopyroxene.  相似文献   

3.
In a bimineralic eclogite xenolith (sample JJG41) from the Roberts Victor kimberlite, compositional gradients in clinopyroxene are related to garnet exsolution. Two principal reactions involving clinopyroxene and garnet occur: (i) The net-transfer Al2Si-1Mg-1 which is responsible for garnet growth according to the equation 2Di+Al2Si-1Mg-1=Grossular+MgCa-1 (reaction 1). This has created substantial compositional gradients in Al, Si and Mg within clinopyroxene. (ii) The exchange of Fe–Mg between garnet and clinopyroxene (reaction 2). During the stage of garnet growth (reaction 1) the lamellae crystallized sequentially as a result of a temperature decrease from around 1400 to 1200° C. This exsolution growth-stage was under the control of Al diffusion in clinopyroxene and at around 1200° C further growth of garnet lamellae became impeded by the sluggishness of Al diffusion in the clinopyroxene host. However, reaction 2 continued during further cooling down to about 1000° C; this temperature being inferred from the constant Fe–Mg partitioning at clinopyroxene-garnet interfaces for the whole set of lamellae. The initial clinopyroxene in JJG41 was probably formed by crystallization from a melt in Archaean time. The cessation of Fe–Mg exchange between garnet and clinopyroxene at about 1000° C may well predate the eruption of the eclogite in kimberlite at around 100 Ma. Kinetic models of reaction are examined for both reactions. Modelling of reaction 1, involving both diffusion and interface migration, allows several means of estimating the diffusion coefficient of Al in clinopyroxene; the estimates are in the range 10-16-10-20 cm2/s at 1200° C. These estimates bracket the experimentally determined data for Al diffusion in clinopyroxene, and from these experimental data a preferred cooling rate of about 300° C/Ma is obtained for the period of growth of garnet exsolution lamellae. A geospeedometry approach (Lasaga 1983) suitable for a pure-exchange process (reaction 2) is used to estimate the cooling rate in the later stages of the thermal history (after garnet growth); values 4–40° C/Ma are consistent with the shape of the Fe-diffusion gradients in the clinopyroxene. The extensive thermal history recorded by JJG41, including probable melt involvement at ca. 1400° C, demonstrates the complex evolution of rocks within the mantle. Whilst the notion of formation of mantle eclogites from subducted oceanic crust has become fashionable, it is clear that tracing eclogite geochemical and P-T characteristics backwards from their nature at the time of xenolith eruption, through high-temperature mantle events to the characteristics of the original subducted oceanic crust, will be very complex.  相似文献   

4.
Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (XMg =0·83) to rim (XMg = 0·6–0·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py23–27, Alm41–43,Grs29–33) has two different compositional domains, onerelatively rich in Mg (Py27–30) and the other rich inCa (Grs35–38) with a low Mg (Py20–25) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs40–57,Py2–7, Alm46–51). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached P–Tconditions of 550–583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps  相似文献   

5.
The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al2O3–FeO–SiO2–CO2–O2in T–aCO2–fO2 space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO2 are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO2, however,behaves as a dependent variable of CO2 in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria  相似文献   

6.
The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al2O3 in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al2O3—SiO2(FMAS). These results have been extended into the CaO—FeO—MgO—Al2O3—SiO2(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al2O3 contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters Goy= –5436 + 2.45T cal mol–1,and WM1FeA1= –920 cal mol–1. The volume change ofreaction, Vo, the entropy change, So970 and the enthalpy changeHo1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term WgaCaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al2O3 contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.  相似文献   

7.
Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism  相似文献   

8.
Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M1) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M2) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M3), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M4), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M2) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M3) were estimated at  相似文献   

9.
INUI  M.; TORIUMI  M. 《Journal of Petrology》2004,45(7):1369-1392
Chemical zoning of garnet is often used to deduce P–Tpaths of rocks by inverse calculation. To validate the derivedP–T paths, it is desired to establish a method to predictthe chemical compositions of garnet theoretically. This studyproposes a new forward calculation of the formation of Mg–Fe–Mngarnet from chlorite, which solves the non-linear simultaneousequations using nested iterative calculations. Growth of garnetconsuming chlorite and quartz was modelled in a MnO–FeO–MgO–Al2O3–SiO2–H2Osystem, using the most recent thermodynamic data for the minerals.The prograde P–T history of the Sambagawa metamorphicbelt, SW Japan, was modelled. To reproduce growth zoning, crystallizedgarnet was removed step by step from the system; perfect diffusionwas assumed for chlorite. The proposed model derived the evolutionof molar amounts and chemical compositions of Mg–Fe–Mnchlorite and garnet. It successfully reproduced the shape ofthe observed chemical profile of garnet, although the temperaturecondition was higher than general observations. The Mn contentof the garnet core was generally high, and Mg/Fe ratio alwaysstarted rising rapidly after Mn was depleted. Thermodynamicproperties of minerals, initial chlorite composition, P–Tpath, H2O partial pressure, and Ca content in garnet were variedto test the behaviour of the system. The properties of Mn phasesinfluenced only the chemical composition of the garnet core.The temperature range in which garnet grew depended on the H2Opartial pressure or the Ca content in garnet. KEY WORDS: chemical equilibrium; chemical zoning; garnet; forward modelling; Sambagawa metamorphic belt  相似文献   

10.
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=0–0?19 wt.%; BeO=0–0?032 wt.%;B2O3=0–4?01 wt.%; and F=0?07–0?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?1–0?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   

11.
BREY  G. P.; K?HLER  T. 《Journal of Petrology》1990,31(6):1353-1378
On the basis of experiments presented in Part I of this series,most of the published thermobarometers relevant to four-phaseperidotites are tested here for their ability to reproduce experimentalconditions. They were rejected if any systematic discrepancyin either pressure or temperature was discernible. This testcautions against the use of all published versions of thermometersbasad on the compositions of coexisting ortho- and clinopyroxenesand the use of existing barometers based on the Al content oforthopyroxene axxisting with garnet. Therefore, we formulatednew versions of the two-pyroxene thermometer and the Al-in-opxbarometer: with and is in degress Kelvin and P is in kilobars. Our new barometer is of the form (C1–C3) and site occupancies are given in the text. Temperatures may also be calculated from the Ca content of opxalone: This thermometer can be applied both to the CMS and the naturalsystem experiments, which may indicate that Fe and Na have counter-balancingeffects on the Ca content of opx. The partitioning of Na between opx and cpx can also serve asa useful thermometer, and was calibrated from natural rock data: where T is in degrees Kelvin, P is in kilobars, and DNa=Naopx/Nacpx. The following three published thermobarometers based on furtherexchange reactions are capable of reprducing experimental conditions:
  1. exchangeof Ca between olivine and clinopyroxene as a barometer(PKB),
  2. exchange of Fe and Mg between garnet and clinopyroxene asathermometer (TKrogh),
  3. exchange of Fe and Mg between garnetand olivine as a thermometer(TO'Neiii).
Our tests also show that the most accurate pressure and temperatureestimates arc obtained from the following combinations of thermometersand barometers:
  1. TBKN+PBKN,
  2. TBKN+PKB,
  3. TKrogh+PBKN,
  4. TO'Ne$$$ll+PBKN.
  相似文献   

12.
The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks   总被引：33，自引：10，他引：33  
Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO2 vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe2+/(Mg + Fe2+) (the Cr–Al trend); increasingFe3+, Fe2+/(Mg + Fe2+) and T iO2 at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe2+/(Mg + Fe2+); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel  相似文献   

13.
Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO4(OH) for CaTiSiO5. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A12O3, TiO2,and Na2O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na2O and TiO2. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs2 and indicating low fo2. Ratios Mg: Fe* (Fe* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving KD =(Mg/Fe*) actlnolIte/(Mg/Fe*)chlorle = 1.72. Mg/Fe* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A12 in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A12O3 contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe2O3, FeO, and MgO as separate componentsand it may also be necessary to regard CO2 as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow Xco2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco2 at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe3+)+chlorite+quartz+H2=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.  相似文献   

14.
Distribution of Ferric Iron in some Upper-Mantle Assemblages   总被引：16，自引：5，他引：11  
The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using 57Fe Mssbauer spectroscopyto determine the Fe3+/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe3+ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe3+. Whole-rock Fe3+ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe3+ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe3+ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe3+ between mantle phases resultsin complicated patterns of the activities of the Fe3+ -bearingcomponents, and thus in calculated equilibrium fO2, which showlittle correlation with whole-rock Fe3+ or degree of depletion.Whether Fe3+ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose fO2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe3+ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry *Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada  相似文献   

15.
Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and XSpa, (from 0•2 to 0•05), XGra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with XAn increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O.Plagioclase is consumed to provide Ca for growing garnet, andXAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. * Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487  相似文献   

16.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2 = 43.7–45.7 wt.per cent, A12O3 = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe2+)= 0.94–0.85) 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 50–200?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 = 0.5–0.25 (XCO2v= 0.5–0.75). 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   

17.
Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation where Gij* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. Xio = Xi/(Xi+Xj))and Xi is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (Wij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K–1 mol –1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T–2.The Wij data are: Opx: W12 = W21 = 25 W13 = (13?1–0-015T),W31 = (3?37–0?005T), W23 = 20, W32 = 16, W24 = 5, W42= 7, W34 = 15, W43 = 15; Cpx: W12 = (25?484+0?0812P), W21 =(31?216–0?0061P),W31 = W13 = 0W14 = (93?3–0?045T), W41 = (–20?0+0?028T),W23 = 24, W32 = 15, W24 = 12, W42 = 12, W34 = (16?941+0?00592P),W43 = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed.  相似文献   

18.
The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria,enclose abundant xenoliths of spinel lherzolites, many of whichcontain amphibole ± apatite ± phlogopite. Thexenolith suite also includes cumulate wehrlites, spinel metapyroxenitesand garnet metapyroxenites. All xenolith types contain abundantlarge CO2-rich fluid inclusions. Microstructural evidence forthe exsolution of spinel, orthopyroxene, garnet and rare plagioclasefrom complex clinopyroxenes suggests that all of the metapyroxeniteshave formed from clinopyroxene (± spinel ± orthopyroxene)cumulates by exsolution and recrystallization during coolingto the ambient geotherm. Pyroxene chemistry implies that a rangeof parental magma types was involved. Garnet pyroxenites showa series of reactions to successively finer-grained, lower-Pmineral assemblages, which imply a relatively slow initial upwardtransport of the xenoliths in the magma, prior to explosiveeruption. The same process has allowed crystallization of phenocrystsfrom small patches of interstitial melt within xenoliths oflherzolite, wehrlite and metapyroxenite. Critically selected P-T estimates for 16 garnet websteritesare consistent with published experimental studies of the spinel/garnetpyroxenite transition, and define a geotherm from 900 °C,11 kb to 1100 °C, 16 kb. Other published data extend thecurve down to c. 7 kb and up to 25 kb. This elevated geothermsuggests that the high regional heat flow is related to convectiveheat transfer by dike injection accompanying the vulcanism.T estimates for the lherzolites range from 850–1050 °C;comparison with the derived geotherm implies that the spinellherzolites are derived from depths of 30–55 km. Thiszone has low seismic velocities (Vp = 6.8–7.8 km/sec)and has thus previously been regarded as a thick, largely maficlower crust. The xenolith data show that this Mower crust' isdominantly ultramafic, with layers, dikes and some large bodiesof pyroxenites and mafic granulites. The anomalously low Vpmay be due to the high T, the high proportion of fluid-filledpore volume, and the magnesian composition of the lherzolites.The seismically defined Moho (Vp >8.0 km/sec) coincides withthe experimentally determined position of the spinel lherzolite-garnetlherzolite transition.  相似文献   

19.
The Bixiling mafic-ultramafic metamorphic complex is a 1•5km2 tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H2O-bearing and CO2-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism *Corresponding author  相似文献   

20.
The aluminous enclaves occur in gedrite-cordierite-gneissesof the Middle Ordovician Ammonoosuc Volcanics, and are composedof combinations of the aluminous minerals sillimanite (Sill),kyanite, corundum (Cor), staurolite (St), sapphirine (Sa), andspinel (Sp), which are set in a matrix of cordierite (Crd) orplagioclase (Plag). Generally, where plagioclase is present,both it and the aluminous minerals are separated from gedrite(Ged) and rare hornblende (Hbl) by cordierite. The enclavesarc interpreted to have formed near the peak of Acadian (Devonian)metamorphism at sillimanite-staurolite-muscovite grade by reactionsthat were encountered during the pressure decrease which accompaniedthe rise of gneiss domes in the region. The enclaves are divided into two main types: (1) enclaves ofcordierite surrounding aluminous minerals; and (2) enclavesof cordierite and plagioclase surrounding aluminuous minerals.Sapphirine grains contain between 9?2 and 9?3 Al atoms per formulacalculated to 14 cations. Staurolites from the enclaves areMg-rich and have (Fe2++ Mn)/(Fe2++Mn+Mg) ratios of 0-59–0?64. The textures and mineralogy of the enclaves suggest that theserocks originally consisted of Ged+Sill?Qz?Hbl?Sp?Plag. Theseminerals reacted to form Crd+Aluminous Minerals?Plag. The mineralogyof both main types of enclaves can be explained by two analogoussets of continuous Fe-Mg reactions:The structure of the enclavessuggests that the mineral growth by the above reactions wasdiffusion controlled, which would have resulted from oversteppingthe above reactions (i.e. the P change exceeded the reactionrate). Therefore, chemical potential gradients (relative mobilityof diffusing components) between gedrite and sillimanite controlledthe location of mineral growth. The Fe-Mg ratio of the bulkcomposition and the proportions of non-Fe-Mg minerals (quartzand sillimanite) appear to determine which continuous Fe-Mgreactions were encountered. Examples of mineral sequences in the cordierite enclaves are:Sill (core)/St+Crd/Ged (matrix); Cor+Crd (core)/Ged (matrix),and Sill (core)/St+Crd/Sa+Crd/Ged (matrix). Examples of themineral sequences in the cordierite-plagioclase enclaves are:Sill (core)/St+Plag/Plag+Crd/Hbl+Ged (matrix); Cor+Plag (core)/St+Plag/Sa+Plag/Ged+ Hbl (matrix); and St+Plag (core)/Plag+Crd/Ged+Hbl (matrix). P–µFeMg–1 diagrams proved to be an importanttool for understanding and illustrating the development of theenclaves. These diagrams allow one to view simultaneously allthe discontinuous and continuous Fe-Mg reactions along a P–µH2O(or T) rock path. With this information it is possible to determinequalitatively which reactions and what sequence of reactionsmight be encountered by bulk compositions with variable Fe-Mgratios and modal proportions of phases.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号