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1.
The hydration of peridotites modelled by the system H2O-CaO-MgO-Al2O3-SiO2 has been treated theoretically after the method of Schreinemakers, and has been investigated experimentally in the temperature range 700°–900° C and in the pressure range of 8–14 kbar. In the presence of excess forsterite and water, the garnet- to spinel-peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865±5° C and 15.2±0.3 kbar. At lower pressures, a model spinel lherzolite hydrates to both chlorite- and amphibole-bearing assemblages at an invariant point located at 825±10° C and 9.3±0.5 kbar. At even lower pressures the spinel-to plagioclase-peridotite transition boundary intersects the dehydration curve for amphibole+forsterite at an invariant point estimated to lie at 855±10° C and 6.5±0.5 kbar.Both chlorite and amphibole were characterized along their respective dehydration curves. Chlorite was found to shift continuously from clinochlore, with increasing temperature, to more aluminous compositions. Amphibole was found to be tremolitic with a maximmum of 6 wt.% Al2O3.The experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting, plagioclase, talc, anthophyllite, and antigorite to obtain a petrogenetic grid applicable to peridotites, modelled by the system H2O-CaO-MgO-Al2O3-SiO2, that covers a wide range of geological conditions. Direct applications of this grid, although quite limited, can be made for ultramafic assemblages that have been extensively re-equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism.  相似文献   

2.
Experimental results up to 10 kb pressure are presented on thestability of amphibole in the quartz-excess CaO+MgO+Al2O3 (CMASH)system under H2O)-excess and H2O deficient conditions. Amphiboleis stable above the solidus under H2O-excess conditions whereasunder H2O-deficient conditions dehydration melting of amphibole-bearingassemblages defines the solidus. The successive appearance ofamphibole, talc, and zoisite with increasing pressure considerablymodifies the plagioclase-pyroxene-garnet-kyanite reactions documentedexperimentally in the CaO+MgO+Al2O3+SiO2 system for gabbro-granulite-eclogitetransitions. Although both clino pyroxene and cordierite (withanorthite+orthopyroxene+quartz) may melt eutectically at oneatmosphere to form diopside-normative and corundum-normativemelts respectively, at higher pressures under H2O-excess conditionsthe peritectic melting of mafic rock compositions produces corundum-normativeliquids together with either clinopyroxene or amphibole. Dehydrationmelting produces melts which are not corundum-normative. Thesedata are used to discuss the origins and evolution of contrastingbasalt-andesite-dacite-rhyolite volcanic suites and graniticplutons, many of whose silicic variants are corundum-normativein character, such as the Toba luff ignimbrites, Indonesia (Beddoc-Stephenset al., 1983) and I-type granite minimum melts (White &Chappell, 1977). In contrast, it is proposed that for the Cascadesbasalt-andesite-dacite-rhyolite suite the ortho pyroxene-plagioclase-quartzthermal divide was maintained up to rhyolite compositions, therebyprohibiting the derivation of corundum-normative rocks fromdiopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole,plagioclase, quartz, and liquid are used to explain the absenceor extreme scarcity of hydrous phases in some hydrous magmas.These phase relations can also explain the development of laterplagioclase overgrowths on resorbed plagioclase cores in graniticintrusives, and the general absence of resorption and overgrowthsin chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearingassemblages in the CaO+MgO+Al2O3+SiO2+H2O system shows thatunder H2O-excess conditions partial melting may generate corundum-normative(but low SiO2) melts from a peridotite source at shallow depths.  相似文献   

3.
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   

4.
A calculated petrogenetic grid for the system CaO-MgO-Al2O3-SiO2-CO2-H2O (CaMASCH), incorporating Tschermak's substitutions in amphibole, chlorite, talc and clinopyroxene, is used to examine phase relationships in aluminous marbles. A series of diagrams illustrating the effect upon stable mineral assemblages of increasing the aluminium content of a bulk composition is used to show the way aluminous minerals enter mineral assemblages in progressively more aluminous rocks. The effects of changing pressure and the incorporation of Fe into the bulk composition on the stable mineral assemblages are also examined. The calculated equilibria are shown to be in reasonable agreement with natural assemblages, and the incorporation of new experimental data on amphiboles into the existing dataset is shown to improve the agreement between observed and natural amphibole compositions.  相似文献   

5.
A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H2O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H2O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.  相似文献   

6.
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
  1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
  2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
  3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
  4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
  5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
  6. pumpellyite + chlorite + quartz = epidote + actinolite + H2O
  相似文献   

7.
Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al2O3–SiO2–CO2–H2O (CaF-MASCH), with chlorite and H2O–CO2 fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg-1 and (Mg, Fe)SiAl-1Al-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.  相似文献   

8.
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.  相似文献   

9.
Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H2O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H2O concentration in the liquid changes the crystallization sequence. Low H2O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H2O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm3. The combination of high pressure and high H2O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO2, Al2O3) of igneous crust in arcs.  相似文献   

10.
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   

11.
Local fluid migration through a serpentine melange caused successive carbonation of a metabasite block (about 80 meter in diameter) during the uplift stage of the glaucophanitic metamorphic rocks, the Nishisonogi metamorphics, southwest Japan. The block shows a zonal sequence as follows. Zone 1: original greenschist (Am+Ep+ Chl+Ab+Sph+Qtz). Zone 2: epidote disappears by the reaction Ep+Am+CO2+H2O=Chl+Cc+Qtz. Zone 3: balc appears by the reaction Am+CO2+H2O=Ta+Cc+ Qtz. Zone 4:sphene breakdowns by the reaction Sph+ CO2=Rt+Cc+Qtz. Zone 5: amphibole disappears by the two simultaneous reactions, Am+CO2=Do+Ta+Qtz and Ta+Cc+CO2=Do+Qtz+H2O. Zone 7: albite is replaced by chlorite, calcite, dolomite and quartz, and the assemblage of Do+Cc+Chl+Rt+Qtz is stable. Analyses of phase relations indicate an introduction of CO2-rich fluid into the greenschist body during regional metamorphism. The CO2-rich fluid may have formed by devolatilization reactions between serpentinite and graphite-bearing metasediments. The fluid migrated within the melange through a channelized pathway and into the greenschist body from a deeper part of the melange.  相似文献   

12.
《Ore Geology Reviews》2008,33(3-4):629-650
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

13.
The stability of the high-pressure phase Mg-sursassite, previously MgMgAl-pumpellyite, in ultramafic compositions has been determined in experiments in the system MgO-Al2O3-SiO2-H2O (MASH). The breakdown of Mg-sursassite + forsterite + enstatite to pyrope + vapour with increasing temperature was bracketed at 6.0 and 7.0 GPa. Below 6.0 GPa, Mg-sursassite + forsterite + vapour reacts to chlorite + enstatite. This reaction provides a mechanism for transfer of water from chlorite- to Mg-sursassite-bearing assemblages. At pressures of 7.0 GPa and above, the assemblage Mg-sursassite + phase A + enstatite was found. Phase relations involving Mg-sursassite and phase A are considered. For bulk compositions with a low water content, the vapour-absent reaction Mg-sursassite + forsterite = pyrope + phase A + enstatite determines the upper-pressure stability of Mg-sursassite, and provides a mechanism for the complete transfer of water from Mg-sursassite to phase A-bearing assemblages. Mg-sursassite plays an important role in peridotite compositions in the subducting slab because, at temperatures below 700 °C, it can transfer water from hydrous phases such as antigorite and chlorite to high-pressure stable phases such as phase A.  相似文献   

14.
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&amp) < 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   

15.
Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al2O3-Fe2O3 (+Na2O + SiO2+ H2O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the XFe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe3+ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions.  相似文献   

16.
Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H2O, chlorite + quartz = talc + cordierite + H2O, and talc + kyanite + quartz = cordierite ± H2O by reversed runs in the system MgO-Al2O3-SiO2-H2O (MASH). These reaction curves intersect at an invariant point (IP1) at PH2O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H2O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H2O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg4.85Al1.15[Al1.15Si2.85O10](OH)8 over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg2.94Al0.06[Al0.06Si3.94O10](OH)2 at PH2O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H2O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K2O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H2O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe2+ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.  相似文献   

17.
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

18.
The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO2-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO2, with minor H2S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO2-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.  相似文献   

19.
The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite, Tibet, in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction. The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures. Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock (~10 m × 30 m), and they record wide variations in bulk composition, mineralogy, and texture. The mineral assemblages, from center to margin, vary from (1) relics of primary clinopyroxene (Cpxr) and primary amphibole (Ampr) + newly formed late amphibole (Act) + primary plagioclase (Plr) + clinozoisite + prehnite + albite + chlorite + titanite + ilmenite (R1 rodingite), through (2) relics of primary clinopyroxene (Cpxr) + newly formed late clinopyroxene (Cpxn) + primary and late amphiboles (Ampr + Act) + clinozoisite + prehnite + albite + chlorite + titanite (R2 rodingite), to (3) newly formed late clinopyroxene (Cpxn) and amphibole (Act) + clinozoisite + prehnite + albite + chlorite + titanite (R3 rodingite). As a result of the metasomatic process of rodingitization, the content of CaO in the whole rock chemical composition from R1 to R3 increases, SiO2 decreases, and Na2O + K2O is almost completely removed. Mass-balance diagrams show enrichments in large ion lithophile elements such as Rb, Cs, Ba, and Pb as well as Ni during rodingitization. The central part of the rodingitized intrusion (R1 rodingite) was only slightly affected by metasomatism. On the other hand, the contents of the rare earth elements (REEs), high field strength elements (HFSEs; e.g. Zr, Nb, Ta, Hf, and Y), and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization. Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H2O-saturated, CO2-rich environment. The estimated conditions of metamorphism were ~281–323 °C and 0.4–3.9 kbar, representing the subgreenschist facies. In this environment, REEs and HFSEs were soluble in the fluids and partly removed. Moreover, these prehnite rodingites formed in a progressively reducing and less alkaline environment, as indicated by decreases in f(O2) and bulk-rock Fe3+/Fe2+ ratios, and the records of fluid ΔpH from the center to the margin of the studied rodingitized intrusion.  相似文献   

20.
The following equilibrium among tremolite forsterite, diopside, and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials: Ca2Mg5Si8O22(OH)2+Mg2SiO4 =2 CaMgSi2O6+5MgSiO3+H2O A significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite. For example, in nearly pure H2O with the H2 content of the fluid buffered by nickel-bunsenite at one kilobar (108 pascals), the breakdown of the assemblage with synthetic amphibole occurs at 708±20° C. The breakdown of the assemblage with natural amphibole, Ca2.16Mg4.94Fe0.03Si7.92 Al0.01O22(OH)2F0.03 occurs at 841±47° C. The shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments. The reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the NB, OH (XG, COH) buffer. Analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160.92 joules/degree is used. The resulting enthalpy of reaction is 113.96±1.82 kilojoules with the natural amphibole and 104.83±0.12 kilojoules with synthetic tremolite. Deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability.  相似文献   

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