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1.
The chemistry and phase relations of calcic and sodic amphiboles in the Ouégoa blueschists are investigated. The first appearance of sodic amphiboles is controlled by bulkrock chemistry. Sodic amphibole appears first in weakly-metamorphosed pumpellyite metabasalts prior to the crystallization of lawsonite but does not crystallize in pelitic schists until the middle of the lawsonite zone; sodic amphibole continues as an apparently stable phase in rocks of all bulk compositions into, and throughout, the highest-grade rocks in the district. Calcic amphibole is widespread in metabasalts of the lawsonite and epidote zones and also occurs in metasediments of appropriate composition. Coexisting pairs of calcic and sodic amphiboles are common in metabasalts but they have also been found in some metasediments. A grunerite-riebeckite pair is described.Electron-probe analyses of 120 amphiboles from representative rock-types are presented in graphical form. Sodic amphiboles show an increased Mg/(Mg+Fe) ratio with increasing metamorphic grade. Sodic amphiboles in pelitic schists are ferroglaucophane in the lawsonite zone and crossite and glaucophane in the epidote zone. Sodic amphiboles in metabasalts are iron-rich crossites in weakly-metamorphosed rocks and more-magnesian crossites and glaucophanes in the lawsonite and epidote zones. The abrupt increase in Mg/(Mg+Fe) ratio in sodic amphiboles at the epidote isograd is attributed to the crystallization of epidote and almandine which take the place of lawsonite and spessartine of the lawsonite zone. Calcic amphiboles are fibrous actinolites in the lawsonite zone and grade with increasing Al and Na/Ca ratio into prismatic blue-green hornblendes (barroisites) in the upper epidote zone. In calcic amphiboles, increasing metamorphic grade effects the coupled substitution of (Na+Al) for (Ca+Mg) and a small increases in Fe/Mg ratio; octahedrally and tetrahedrally coordinated Al increases in an approximately 11 ratio. Both the calcic and the sodic amphiboles show an increase in A-site occupancy with increasing metamorphic grade. In two-amphibole assemblages Ti, Mn and K are concentrated in the calcic amphibole.The textural and chemical relations between coexisting calcic and sodic amphiboles are discussed. If the calcic and sodic amphiboles are an equilibrium pair then the data collected from the Ouégoa amphiboles gives a picture of a very asymmetric solvus in the system glaucophane-actinolite-hornblende, i.e. steep-sided to glaucophane and with a gentle slope to the calcic amphibole field; there is no indication of any termination of the solvus under the pressure-temperature conditions of crystallization of the Ouégoa schists.  相似文献   

2.
Glaucophane-lawsonite facies blueschists representing a metamorphosed sequence of basic igneous rocks, cherts and shales have been investigated northeast of the district of Tav?anli in Northwest Turkey. Sodic amphiboles are rich in magnesium reflecting the generally high oxidation states of the blueschists. Lawsonite has a very uniform composition with up to 2.5 wt.% Fe2O3. Sodic pyroxenes show an extensive range of compositions with all the end-members represented. Chlorites are uniform in their Al/(Al+Fe+Mg) ratio but show variable Fe/ (Fe+Mg) ratios. Garnets from metacherts are rich in spessartine (>50%) whereas those from metabasites are largely almandine. Pistacite rich epidote is found in metacherts coexisting with lawsonite. Phengites are distinctly higher in their Fe, Mg and Si contents than those from greenschist facies. Hematites with low TiO2 are ubiquitous in metacherts. Fe2+/Mg partitioning between chlorite and sodic amphibole is strongly controlled by the calcium content of the sodic amphibole and ranges from 1.1 for low calcium substitution to 0.8 for higher calcium substitution. The Al/Fe3+ partition coefficient between sodic amphibole and sodic pyroxene is 2.1. A model system has been constructed involving projections from lawsonite, iron-oxide and quartz onto a tetrahedron with Na, Al, Fe2+ and Mg at its apices. Calcite is treated as an indifferent phase. The model system illustrates the incompatibility of the sodic pyroxene with chlorite in the glaucophanelawsonite facies; this assemblage is represented by sodic amphibole. Sodic amphibole compositions are plotted in terms of coexisting ferromagnesian minerals. Five major areas on the sodic amphibole compositional field are delineated, each associated with one of the following minerals: chlorite, stilpnomelane, talc, almandine, deerite.  相似文献   

3.
The western part of the Hidaka Metamorphic Belt, Hokkaido, consistsof primary pyroxene gabbro and lesser amounts of olivine gabbrothat have been dynamically metamorphosed to metagabbro—gabbroicamphibolite-amphibolite-epidote amphibolite during uplift andshearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphiboleand plagioclase in the same rock indicate incomplete reactionand non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuousoverall compositional range from actinolite to dark green hornblende(with 100 mg/(Mg+Fe2++Fe3+Mn) ratios varying from 89.5 to 32.0)marked by increasing Al, Fe, Ti, and Na. A compositional gapis usually present between relic and recrystallized amphibolesin any one rock which becomes more prominent with increasingshearing. In addition to host rock chemical control, amphibole compositionis largely dependent on the An content of coexisting plagioclase.Locally epidote and sphene exert a strong influence on bothamphibole and plagioclase compositions. Amphibole Ti and Mncontents decrease with shearing and Fe enrichment of the hostrocks largely as a result of the incoming of rutile, sphene,and Fe-Ti oxides. Analysis of host rock oxidation ratio andamphibole compositions indicates that the rocks essentiallybehaved as closed systems to oxygen during metamorphism. Al1V-AlIV, AlIV-Fe3+, and AlIV-(Na, K)A are the main substitutionsin the amphiboles. Within any one rock the recrystallized amphibolesare enriched in Al, Fe, Ti, and Na relative to the relice amphiboles.Increasing metamorphism results in a progressive change of amphiboles(recrystallized) to more Fe and Si (rather than Al) rich compositionsreflecting the trend towards greenschist where Fe-actinolite(+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of SiAlreplacement and the compositional limits of the early stagereaction rim and replacement amphiboles in the host olivineand pyroxene metagabbros.  相似文献   

4.
KUBO  K. 《Journal of Petrology》2002,43(3):423-448
Dunite formation processes in highly depleted peridotites arediscussed based upon a detailed study of the Iwanaidake peridotite,Hokkaido, Japan, which consists mainly of harzburgite with asmall amount of dunite. In the harzburgites, the Mg# [= 100x Mg/(Mg + Fe2+)] of olivine ranges from 91·5 to 92·5,and the Cr# [= 100 x Cr/(Cr + Al)] of spinel from 30 to 70;in the dunites, the Mg# of olivine ranges from 92·5 to94 and the Cr# of spinel from 60 to 85, respectively. The NiOwt % of olivine in harzburgites ranges from 0·38 to 0·44,and in dunites from 0·35 to 0·37. The Mg# andCr# are higher and NiO wt % is lower in the dunites than inthe harzburgites surrounding the dunites. The Mg# and Cr# exhibitnormal depletion trends expected from simple partial melting,whereas the NiO wt % shows an abnormal trend. On the basis ofmass balance calculations, dunites are considered to be derivedfrom the harzburgites by a process involving incongruent meltingof orthopyroxene (orthopyroxene olivine + Si-rich melt). Hydrousconditions were necessary to lower the solidus, and thus meltingof harzburgite was probably triggered by the introduction ofhydrous silicate melt. The dunite in this massif may have formedin the mantle wedge above a subduction zone. KEY WORDS: depleted peridotite; hydrous melt; incongruent melting; residual dunite; Iwanaidake peridotite  相似文献   

5.
Modal mantle metasomatism, involving the re-enrichment of depletedmantle by the introduction or production of new hydrous phases,apatite and other minerals, has been proposed as a criticalprecursor to alkaline volcanism. The merits of the modal metasomatismmodel are evaluated by examining whole-rock 100 Mg/(Mg+Fe2+)ratios and the abundances of TiO2, K2O and P2O5 in mafic volcanicsspanning the mafic alkaline-subalkaline compositional spectrum.Upper mantle amphiboles and micas are also discussed becausethey would be major donors of Ti, Fe, and K to melts duringanatexis of either modally metasomatized depleted mantle orundepleted mantle. Compared with tholeiitic and calc-alkaline basalts and andesites,basanites and alkali basalts and alkali andesites are neitherdistinctive nor unique by virtue of persistant or well-definedhigher abundances of TiO2, K2O, and P2O5 or lower 100 Mg/(Mg+Fe2+)ratios, features which might reflect precursor modal metasomatismof the alkaline sources. Some basanites and alkali basalts dohave higher abundances of TiO2, K2O, and P2O5 than some tholeiitesbut these abundances may be the result of lower degrees of meltingof similar undepleted mantle sources for both magma types. The most widespread mantle phases of inferred metasomatic originare interstitial amphiboles and micas in Group I spinel peridotitexenoliths. These have high 100 Mg/(Mg+Fe) ratios ({small tilde}90) and high Cr2O3 and low TiO2 abundances, and the K2O/Na2Oratios of the amphiboles (chromian pargasites) are low, generallyless than 0?3. Interstitial amphiboles and micas developed asa result of near-isochemical hydration reactions which largelyinvolved Cr-spinel and Cr-diopside. Their formation was probablyinduced in many instances by fluids derived from crystallizingmafic magmas. Metasomatized Group I xenoliths with interstitialhydrous phases remain depleted in TiO2, K2O, and P2O5, and theyretain the high 100 Mg/(Mg+Fe) ratios characterizing depletedGroup I xenoliths. Together with the low K2O/Na2O ratios, thesefeatures preclude such peridotites as suitable sources of mostalkaline (and subalkaline) volcanics. It is suggested that modalmetasomatism plays an insignificant role in the genesis of mostmantle-derived mafic volcanics. Compared with the interstitial phases, kaersutitic amphibolesand titaniferous micas from vein, Group II inclusion and megacrystupper mantle parageneses have lower 100 Mg/(Mg+Fe) ratios andCr2O3 contents, and much higher TiO2 abundances. K2O/Na2O ratiosof the Ti-amphiboles are also much more wide-ranging (0?3 togreater than 1?0). These Fe, Ti-rich amphiboles and micas areneither widespread nor pervasive phases in metasomatized mantle.They are directly related to alkaline magmatism in the uppermantle where they may be associated with incompatible elementenrichment of peridotite wallrocks in the immediate vicinityof frozen conduits of alkaline mafic magmas. The varying K2O/Na2O ratios of mafic volcanics (MORB constitutea major exception) indicate that the principal K-bearing phasesin undepleted mantle are kaersutitic amphibole and titaniferousmica, in varying proportions. The former is probably the majorsource of Ti and K for low K/Na volcanics (K2O/Na2O < 0?5)and also many medium K/Na types (0?5 < K2O/Na2O < 1?0),whereas mica is more likely to be the major K-bearing phasein the source regions of high K/Na extrusives (K2O/Na2 >1?0). Experimental data indicate that kaersutitic amphibole,mica and apatite probably coexist in undepleted spinel- andgarnet lherzolites at pressures up to 25 kb, with mica persistingto pressures as high as 50 kb. It is proposed that undepleted asthenospheric mantle is heterogeneouswith respect to its amphibole, mica, and apatite contents (andhence TiO2, K2O, and P2O5 abundances and K2O/Na2O ratios), andalso with respect to 100 Mg/(Mg+Fe2+ ) ratios which may be significantlyless than the ratios generally assigned to undepleted mantle,namely 88–90.  相似文献   

6.
Calculated phase equilibria among the minerals sodic amphibole, calcic amphibole, garnet, chloritoid, talc, chlorite, paragonite, margarite, omphacite, plagioclase, carpholite, zoisite/clinozoisite, lawsonite, pyrophyllite, kyanite, sillimanite, quartz and H2O are presented for the model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH), which is relevant for many greenschist, blueschist, amphibolite and eclogite facies rocks. Using the activity-composition relationships for multicomponent amphiboles constrained by Will and Powell (1992), equilibria containing coexisting calcic and sodic amphiboles could be determined. The blueschist–greenschist transition reaction in the NCFMASH system, for example, is defined by the univariant reaction sodic amphibole + zoisite = calcic amphibole + chlorite + paragonite + plagioclase (+ quartz + H2O) occurring between approximately 420 and 450 °C at 9.5 to 10 kbar. The calculated petrogenetic grid is a valuable tool for reconstructing the PT-evolution of metabasic rocks. This is shown for rocks from the island of Samos, Greece. On the basis of mineral and whole rock analyses, PT-pseudosections were calculated and, together with the observed mineral assemblages and reaction textures, are used to reconstruct PT-paths. For rocks from northern Samos, pseudomorphs after lawsonite preserved in garnet, the assemblage sodic amphibole-garnet-paragonite-chlorite-zoisite-quartz and the retrograde appearance of albitic plagioclase and the formation of calcic amphibole around sodic amphibole constrain a clockwise PT-path that reaches its thermal maximum at some 520 °C and 19 kbar. The derived PT-trajectory indicates cooling during exhumation of the rocks and is similar to paths for rocks from the western part of the Attic-Cycladic crystalline complex. Rocks from eastern Samos indicate lower pressures and are probably related to high-pressure rocks from the Menderes Massif in western Turkey. Received: 8 July 1997 / Accepted: 11 February 1998  相似文献   

7.
The Crossite Content of Ca-Amphibole as a Guide to Pressure of Metamorphism   总被引：10，自引：0，他引：10  
A correlation between the crossite component (NaM4) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H2O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe2+/Mg and Fe3+/Al. Ca-amphiboleshould have fixed NaM4, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM4 is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe2+/Mgand Fe3+/Al in the amphibole have relatively little effect onNaM4, but in high pressure amphiboles NaM4 varies inverselywith Aliv. Ca-amphiboles from high pressure areas have substantially moreNaM4 (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM4 content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks.  相似文献   

8.
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 1·2–1·4; Na2O < 2·95and normative corundum contents of 1·9–4·0per 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 (An78–80) 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 (40–43 mol. per cent SiO2) and sillimanite(49 mol. per cent SiO2) are Fe2O3 and TiO2 rich (up to 6·1–0·84and 3·6–0·24 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 66–69 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) = 76·5–69·4),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) = 64–69), 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) FeAl2O4—66·6–45·0;MgAl2O4—53·0–18·7; Fe3O4—6·9–28·1;MnAl2O4—1·2–1·5; Fe2TiO4—0·6–6·2.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(5·1–7·5 and 8·5–13·5wt. 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 615–710°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 2–3kb 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   

9.
Amphiboles are the main mafic minerals in most metaluminous to peralkaline alkali-feldspar granites and syenites, and they usually preserve an important record of the compositional evolution of the melts from which they crystallize. In the alkaline association of the Serra da Graciosa A-type Granites and Syenites (southern Brazil), amphibole compositions span a large range, including calcic, sodic–calcic, and sodic amphiboles. Calcic amphiboles are typically observed in the metaluminous rocks, while sodic amphiboles are characteristic of the more strongly peralkaline rocks; sodic–calcic amphiboles are found in intermediate varieties. Compositional variations record the differentiation trends within two petrographic series of the alkaline association. The overall evolution of amphibole compositions is similar in both: they reveal a progressive increase in Na and Fe3+ with differentiation (increase in alkalinity of the magmas), a characteristic shared by undersaturated peralkaline (or agpaitic) differentiation trends. In detail, however, the evolutions of the amphibole compositions in the two series are distinct. In Alkaline series 1, the cores of the crystals form a continuum from calcic to sodic compositions, with the exception of a small compositional gap within the sodic–calcic amphiboles. The rims, however, show compositions that diverge from this main trend; this divergence results from increasing amounts of the oxy-amphibole component, and reflects more oxidizing conditions at the final stages of magmatic crystallization. In Alkaline series 2, these oxidation trends are much more subtle and a reverse trend is observed in the sodic amphiboles. Sodic–calcic amphiboles are in several cases replaced by intergrowths of post-magmatic sodic amphibole and Al-poor (“tetrasilicic”) biotite.  相似文献   

10.
Seventeen co-existing blue and blue-green amphibole pairs fromamphibolites of zones I, II and III of Ile de Groix have beenanalysed by electron microprobe. The size of the miscibilitygap, related to variations of compositions of blue and associatedblue-green amphiboles in well defined P and T conditions, asa function of whole rock compositions, is shown. Blue amphibolesare glaucophanes of relatively homogeneous composition (0.018< Ca/(Ca + Na) < 0.102); associated blue-green amphibolesshow larger substitutions, from actinolitic hornblendes to pargasitichornblendes (0.43 < Ca/(Ca + Na) < 0.81). A comparisonwith pairs crystallized in other physical conditions shows variationsin blue amphiboles with P, in blue-green amphiboles with T,inside glaucophane schist facies. The miscibility gap seemsto be the narrowest for glaucophane schist facies conditions,which are near the greenschist facies boundary: P 8 kb, T 450 °C.  相似文献   

11.
Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair. Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted. Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe2O3-rich metabasites. The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM4 of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.  相似文献   

12.
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   

13.
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   

14.
Chemical analyses are given for actinolitic hornblendes of tonalitic rocks from the Hercynian belt of Northern Portugal. The distribution of elements between amphibole and co-existing biotite is studied. The composition of the amphiboles is analysed in the light of experimental data on amphiboles and the physical conditions of crystallization inferred from the study of the biotite and rock series. The data on the biotites lead to the definition of a temperature of 800°C for the crystallization of actinolitic hornblendes with Mg/(Mg + Fe) ratios of 0·72-0·61 at pressures of about 3 Kb and fO2 defined by FMQ.  相似文献   

15.
Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO2 and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. 87Sr/ 86Sr(i) = 0· 70298–0· 70316,Nd(t) = +3· 8 to +6· 3 and 206Pb/ 204Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China  相似文献   

16.
Harzburgite and lherzolite tectonites from the Horoman peridotite complex, Hokkaido, northern Japan, contain variable amounts of secondary phlogopite and amphibole. Phlogopite-rich veinlets parallel to the foliation planes usually cut olivine-rich parts of the rocks; single-grained interstitial phlogopites are usually associated with orthopyroxene grains. Amphiboles are disseminated in rocks or sometimes occur in the phlogopite-rich veinlets. Within individual veinlets, phlogopites show extensive inter-grain variations in K/(K + Na) ratio (0.96–0.75), generally decreasing from the central (usually the thickest) part towards the marginal parts of veinlets. In contrast, Ti contents are nearly constant in Ti-poor veins or decrease slightly with decreasing K/(K + Na) in T-rich veins. Variation of Ti in phlogopites is very large (0.1–6.8 wt%) and is inversely correlated with Mg/(Mg + Fe*) (Fe*, total iron) atomic ratios, which vary from 0.96 to 0.88. Intra-vein variation of phlogopite chemistry (especially K/(K + Na) ratio) could be achieved by in situ fractional crystallization of trapped fluids; variation of Ti, however, cannot be explained by in situ fractionation of the fluids, indicating various Ti contents of the parent fluids. It is suggested that fluids responsible for the formation of the Horoman phlogopites and amphiboles were magmatic volatiles successively released from evolving alkali basaltic magmas. Individual fluids trapped within peridotites were fractionated, precipitating phlogopites successively poorer in K. When the fluids became rich enough in Na, amphiboles co-precipitated with phlogopites. Similar fractional crystallization of phlogopites and amphiboles is expected in the upper mantle on a larger scale if fluids move upwards. This process may control, at least partly, a lateral K/Na distribution in the upper mantle; K and Na may be concentrated in deeper and shallower parts, respectively.  相似文献   

17.
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 2·5–3 mwide dehydration zone (650–700°C; 800 MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet)overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet).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 Fe–Mgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO2; brines; localized dehydration; Söndrum  相似文献   

18.
The following article presents constraints of the stability of Mg-rich (Mg/(Mg + Fe2+) > 0.5) calcic amphibole in both calc-alkaline and alkaline magmas, testing of previous thermobarometers, and formulation of new empirical equations that take into consideration a large amount of literature data (e.g. more than one thousand amphibole compositions among experimental and natural crystals). Particular care has been taken in choosing a large number of natural amphiboles and selecting quality experimental data from literature. The final database of experimental data, composed of 61 amphiboles synthesized in the ranges of 800–1,130°C and 130–2,200 MPa, indicates that amphibole crystallization occurs in a horn-like P–T stability field limited by two increasing curves (i.e. the thermal stability and an upper limit), which should start to bend back to higher pressures. Among calcic amphiboles, magnesiohornblendes and tschermakitic pargasites are only found in equilibrium with calc-alkaline melts and crystallize at relatively shallow conditions (P up to ~1 GPa). Kaersutite and pargasite are species almost exclusively found in alkaline igneous products, while magnesiohastingsite is equally distributed in calc-alkaline and alkaline rocks. The reliability of previous amphibole applications was checked using the selected experimental database. The results of this testing indicate that none of the previous thermobarometers can be successfully used to estimate the P, T and fO2 in a wide range of amphibole crystallization conditions. Multivariate least-square analyses of experimental amphibole compositions and physico-chemical parameters allowed us to achieve a new thermobarometric model that gives reasonably low uncertainties (T ± 23.5°C, P ± 11.5%, H2Omelt ± 0.78wt%) for calc-alkaline and alkaline magmas in a wide range of P–T conditions (up to 1,130°C and 2,200 MPa) and ∆NNO values (±0.37 log units) up to 500 MPa. The AK-[4]Al relation in amphibole can be readily used to distinguish crystals of calc-alkaline liquids from those of alkaline magmas. In addition, several chemometric equations allowing to estimate the anhydrous composition of the melts in equilibrium with amphiboles of calc-alkaline magmas were derived.  相似文献   

19.
Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na2O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(XFe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO2 values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al2O3content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al2O3 of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al2O3 greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures.  相似文献   

20.
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   

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