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1.
Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole 总被引:4,自引:0,他引:4
Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·380·40),42 vol. % plagioclase (An2629), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 24 wt % H2O. This reflectsthe fact that conditions of low aH2O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH2O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt 相似文献
2.
In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structuralmicrotexturalrelations and thermobarometry indicate that syn-deformationalsegregationcrystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO2 , P2O5 , Rb, Zr and total rare earth elements(REE), and higher SiO2 , K2O, Ba and Sr than the protolith andthe mesosome, whereas Na2O and Al2O3 abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; PT conditions 相似文献
3.
HARLOV DANIEL E.; JOHANSSON LEIF; VAN DEN KERKHOF ALFONS; FORSTER HANS-JURGEN 《Journal of Petrology》2006,47(1):3-33
A localized dehydration zone, Söndrum stone quarry, Halmstad,SW Sweden, consists of a central, 1 m wide granitic pegmatoiddyke, on either side of which extends a 2·53 mwide dehydration zone (650700°C; 800 MPa; orthopyroxeneclinopyroxenebiotiteamphibolegarnet)overprinting a local migmatized granitic gneiss (amphibolebiotitegarnet).Whole-rock chemistry indicates that dehydration of the graniticgneiss was predominantly isochemical. Exceptions include [Y+ heavy rare earth elements (HREE)], Ba, Sr, and F, which aremarkedly depleted throughout the dehydration zone. Systematictrends in the silicate and fluorapatite mineral chemistry acrossthe dehydration zone include depletion in Fe, (Y + HREE), Na,K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presenceof a fluid containing CO2, NaCl, and H2O components. Water activitiesin the dehydration zone average 0·36, or XH2O = 0·25.All lines of evidence suggest that the formation of the dehydrationzone was due to advective transport of a CO2-rich fluid witha minor NaCl brine component originating from a tectonic fracture.Fluid infiltration resulted in the localized partial breakdownof biotite and amphiboles to pyroxenes releasing Ti and Ca,which were partitioned into the remaining biotite and amphibole,as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, andF. At some later stage, H2O-rich fluids (H2O activity >0·8)gave rise to localized partial melting and the probable injectionof a granitic melt into the tectonic fracture, which resultedin the biotite and amphibole recording a diffusion profile forF across the dehydration zone into the granitic gneiss as wellas a diffusion profile in Fe, Mn, and Mg for all FeMgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO2; brines; localized dehydration; Söndrum 相似文献
4.
This study explores the origin and geochemical evolution ofapatite, monazite, and xenotime along two metamorphic traverses.The first, from the Kigluaik Mountains, Seward Peninsula, Alaska,consists of a localized (85 cm) orthopyroxeneclinopyroxene-bearingdehydration zone. The second consists of orthopyroxene ±clinopyroxene-bearing granulite facies metabasite layers interlayeredwith metapelites over a 34 km traverse, along the ValStrona, IvreaVerbano Zone, Northern Italy (IVZ). In bothdehydration zones small Th- and U-poor inclusions of monaziteand/or xenotime occur in the apatite. These inclusions are metasomaticallyinduced and nucleated within the apatite via the coupled substitutionsNa+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+.These are not present in apatite from the original amphibolitefacies gneiss. Apatite, in both dehydration zones, also showsa relative increase in both F and Cl compared with apatite fromthe amphibolite facies zone. Granulite facies metabasites inthe IVZ also contain isolated monazite grains, which range fromuniform to complexly zoned in Th the (1330·1 mol% ThSiO4). These are the product of breakdown and subsequentmobilization of the lanthanides and actinides from monazite-(Ce)in the metapelite layers into the metabasite layers at the startof granulite facies metamorphism. KEY WORDS: apatite; monazite; xenotime; KClNaCl brines; metasomatism; phosphate minerals; charnockiteenderbite; granulite facies metamorphism 相似文献
5.
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
6.
Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists 总被引:2,自引:0,他引:2
Pressuretemperature grids in the system Na2OCaOK2OFeOMgOAl2O3SiO2H2O and its subsystems have been calculatedin the range 1545 kbar and 550900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressuretemperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H2O-carriersin subducting slabs. Depending on bulk composition and pressuretemperaturepath, amphibole may or may not be a major H2O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H2O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists 相似文献
7.
Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs4595Py02and from the orthopyroxene-bearing rocks is Grs1015Py3643.Peak PT metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·31·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from EqAn= 0·07, XCl =0·99 to EqAn = 0·61, XCl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield XCO2 = 0·030·15.XNaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatitebiotite and amphibolebiotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO2 and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan 相似文献
8.
DASGUPTA SOMNATH; SENGUPTA PULAK; EHL JUERGEN; RAITH M.; BARDHAN S. 《Journal of Petrology》1995,36(2):435-461
Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinelquartzcordierite,(II) spinelquartzcordieritegarnetorthopyroxenesillimanite,(III) spinelcordieriteorthopyroxenesillimanite,(IV) spinelquartzsapphirinesillimanitegarnet,(V) spinelquartz-sapphirinegarnet and (IV) rhombohedral(FeTi) oxidecordieriteorthopyroxenesillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. XMg in the coexisting phasesdecreases in the order cordieritebiotitesapphirineorthopyroxenespinelgarnet(FeTi)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeOMgOAl2O3SiO2K2OH2Oat high fo2, has been constructed and the effects of ZnO andFe2O3 on this grid have been explored Combining available experimentaland natural occurrence data, the high fo2 invariant points inthe partial grid have been located in PT space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise PT trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry
Corresponding author 相似文献
9.
High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications 总被引:55,自引:1,他引:55
ZHAO GUOCHUN; CAWOOD PETER A.; WILDE SIMON A.; LU LIANGZHAO 《Journal of Petrology》2001,42(6):1141-1170
Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitictrondhjemiticgranodioriticgneisses 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) PT conditions of 13·415·5kbar and 770840°C for the high-pressure granulitesand 911 kbar and 820870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The PT conditions of pyroxene+ plagioclase symplectite and corona (M3) were estimated at 相似文献
10.
Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil
Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M1) grew during fabric formation inthe rocks (D1) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M2)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D2 foldingof the rocks at T = 750800°C and P = 7 kbar. M3 reactiontextures overprint the M2 assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M2 PT vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise PT path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; PT pseudosection 相似文献
11.
Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H2Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from KD(Mg/Fe, chl&) < 1 to KD > 1, the partitioning betweenbiotite and hornblende changed from KD (Mg/Fe, bio/amp) <1 in chlorite-zone samples to KD > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An55and An80. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H2O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions. 相似文献
12.
Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions 总被引:2,自引:0,他引:2
HOLTZ FRANCOIS; SATO HIROAKI; LEWIS JARED; BEHRENS HARALD; NAKADA SETSUYA 《Journal of Petrology》2005,46(2):319-337
Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100300 MPa, 775875°C,various water activities, and fO2 buffered by the NiNiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 19911995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H2O. The phase relations at 300 MPa and in the temperaturerange 870900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H2O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H2Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H2O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study 相似文献
13.
The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP T aH2O XFe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H2O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and XH2O (assuming PH2O +PCO3 = Ptotal) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H2O-rich. A flux of H2O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions. 相似文献
14.
A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (Mo) is represented by an inclusion assemblageof hornblende1 + ilmenite1 + plagioclase1 ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M1)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressuretemperature conditions of 910 kbarand 750800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M2), andpost-decompression cooling (M3) events. In mafic granulites,the former resulted in the development of early clinopyroxene2Ahornblende2Aplagioclase2Asymplectites at 8 kbar and 775°C (M2A stage), synchronouswith D2 and later anhydrous clinopyroxene2Bplagioclase2Bilmenite2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M2Bstage) and post-dating D2. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M2at 6·4 kbar and 700°C. During M3, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende3plagioclase3 symplectitesembaying garnet in mafic granulites were formed. PT estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M3. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined PT path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continentcontinent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise PT path; continental collision; metabasite 相似文献
15.
Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage PT history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirinegarnetorthopyroxenesillimanitequartzK-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated PT of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al2O3in orthopyroxene are consistent with a peak temperature above1150°C. The PT path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnetsapphirinequartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar PT paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism 相似文献
16.
Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India 总被引:2,自引:0,他引:2
Chemical trends from north (amphibolite facies) to south (granulitefacies) along a 95 km traverse in Tamil Nadu, Southern India,include: whole-rock depletion of Rb, Cs, Th and U, enrichmentin Ti and F, and depletion in Fe and Mn in biotite and amphibole;increases in Al and decreases in Mn in orthopyroxene; enrichmentof fluorapatite in F. K-feldspar blebs are widespread alongquartz–plagioclase grain boundaries, and could indicateeither partial melting or metasomatism. In the northernmostportion of the traverse the principal rare earth element (REE)-bearingminerals are allanite and titanite. South of a clinopyroxeneisograd, monazite grains independent of fluorapatite are themajor REE- and Th-bearing phase. Further south independent monaziteis rare but Th-free monazite inclusions are common in fluorapatite.During prograde metamorphism, independent monazite was replacedby REE-rich fluorapatite in which the monazite inclusions laterformed. The loss of independent monazite was accompanied bya loss of whole-rock Th and possibly a small depletion in lightREE. Most mineralogical features along the traverse can be accountedfor by progressive dehydration and oxidation reactions. Trace-elementdepletion is best explained by the action of an externally derivedlow H2O activity brine migrating from a source at greater depth,possibly preceded or accompanied by partial melting. KEY WORDS: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite 相似文献
17.
A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron
microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins
along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing
gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins
are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association
appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz
= orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this.
These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of
amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures
>500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have
formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths
and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression
in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked
with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies
metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and
quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures
estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet,
orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or
alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if
fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point
to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies
metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions
involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and
H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse
in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade,
low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for
the Mafic Formation; and that this dehydration event did not involve partial melting.
Received: 15 February 2000 / Accepted: 26 June 2000 相似文献
18.
Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites 总被引:5,自引:0,他引:5
Petrogenetic grids in the system NCKFMASH (Na2OCaOK2OFeOMgOAl2O3SiO2H2O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the PT range 730kbar and 450680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H2O in excess. These grids, together with calculated compatibilitydiagrams and PT and TXCa and PXCa pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different PT conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and PTconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in PT and PXCapseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry 相似文献
19.
Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland 总被引:1,自引:1,他引:1
The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedPT estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordieriteK-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and aH2O < 0·3. The pressure estimate impliesan emplacement depth of 相似文献
20.
The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO2relative to the NiNiO solid buffer), between 800 and660°C and for melt H2O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckitearfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H2O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H2O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 5060°C,and that of amphibole by 30°C. Riebeckitearfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO2 lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H2O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H2O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO2 at or below the fayalitequartzmagnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria 相似文献