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1.
The join tremolite (Tr)-pargasite (Pa) has been studied in the temperature range 750 °–1,150 ° C under a water vapor pressure of 1 and 5 kbar. There is a continuous solid solution series between the compositions Tr85Pa15 and TroPa100 at 850 ° C and 5 kbar. Tremolite and pargasite are separated by a solvus at 1 kbar and the field of tremolitic amphibole +pargasitic amphibole+vapor is present in the region between Tr90Pa10 and Tr10Pa90 at 800 ° C. The phase assemblages at 850 ° C and 1 kbar change as follows with increasing pargasite component; clinopyroxene +orthopyroxene+quartz+vapor, tremolitic amphibole+vapor, tremolitic amphibole+clinopyroxene +forsterite+plagioclase+vapor, tremolitic amphibole+pargasitic amphibole+vapor, and pargasitic amphibole+vapor. The petrological significance of amphibole pairs in metamorphic rocks is discussed on the basis of the experimental results.  相似文献   

2.
The join tremolite (Tr)—tschermakite (Ts) was studied at temperatures of 450 to 900° C under water vapour pressure of 2 kbar. Solid solution between the end members is restricted to composition range Tr100-Tr45. Reconnaissance runs at 800°C and 10 kbar indicated that no further substitution of Al in the tremolite structure is possible by an increase of pressure. In the composition range Ts55-Ts100 tremolite-tschermakite solid solution Tr45Ts55 is formed with anorthite, forsterite and enstatite above 700°C and with anorthite and chlorite below 700° C. No amphibole could be synthezised from a material of composition Ts100. Materials of composition Ts100 crystallized to anorthite, enstatite and frosterite above 700°C and to anorthite and chlorite below 700°C. The high temperature breakdown curve for tremolite-tschermakite solid solutions drops from 870°C for pure tremolite to 826°C for Tr45Ts55. Additional experiments at 1 and 3 kbar indicate that the pressure effect on breakdown temperatures amounts to about 35°C/kbar. The formation of natural amphiboles belonging to the tremolite-tschermakite series is discussed in the light of the experimental data.  相似文献   

3.
Dehydration melting of tonalites. Part II. Composition of melts and solids   总被引：6，自引：0，他引：6  
 Dehydration melting of tonalitic compositions (phlogopite or biotite-plagioclase-quartz assemblages) is investigated within a temperature range of 700–1000°C and pressure range of 2–15 kbar. The solid reaction products in the case of the phlogopite-plagioclase(An45)-quartz starting material are enstatite, clinopyroxene and potassium feldspar, with amphiboles occurring occasionally. At 12 kbar, zoisite is observed below 800°C, and garnet at 900°C. The reaction products of dehydration melting of the biotite (Ann50)-plagioclase (An45)-quartz assemblage are melt, orthopyroxene, clinopyroxene, amphibole and potassium feldspar. At pressures > 8 kbar and temperatures below 800°C, epidote is also formed. Almandine-rich garnet appears above 10 kbar at temperatures ≥ 750°C. The composition of melts is granitic to granodioritic, hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure-temperature conditions within the continental crust. The melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and a H 2O = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing temperature, the melt becomes enriched in Al2O3 and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900°C at 5 kbar whereas at higher temperatures it increases as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na2O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900°C approaches that of the starting assemblage. The melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion increases from 19.8 to 22.3 vol.% (850°C and 5 kbar). With increasing temperature from 800 to 950°C (at 5 kbar), the increase in melt fraction is from 11 to 25.8 vol.%. The effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850°C from 19.8 vol.% at 5 kbar to 15.3 vol.% at 15 kbar. Selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration. Received: 27 December 1995 / Accepted: 7 May 1996  相似文献   

4.
 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na2O– CaO–MgO–Al2O3–SiO2–H2O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An50)+1.5 enstatite+H2O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An50)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An50). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled NaA-AlT1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG 0 f ,Pg=−12022.11±5.2 kJ mole-1, and S 0 Pg=591.7 ±7.9 JK-1 mole-1. Received: 31 July 1995/Accepted: 3 June 1996  相似文献   

5.
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6.
The stability field of pargasitic amphibole in a model mantle composition (MORB pyrolite) has been experimentally determined for a fixed water content. A solidus for a pargasite-bearing lherzolite has been defined at pressures below the limit of amphibole stability of 30 kbar at T = 925 °C. The maximum temperature for pargasitic amphibole in MORB pyrolite occurs at 1075 °C between P = 18 and 25 kbar. This maximum lies between that determined for a fertile peridotite composition (Hawaiian pyrolite) and a depleted peridotite composition (Tinaquillo lherzolite). A comparison of the new results with those from earlier studies suggests that the stability for a particular bulk H2O content is mostly controlled by alkali content of the lherzolite composition. The systematic compositional variation of pargasitic amphibole as a function of pressure and temperature can be represented as an increase of the richterite component with increase in both pressure and temperature. For a given pressure the tschermakite component increases with increasing temperature. The compositions of coexisting clinopyroxenes also show a systematic variation with pressure and temperature. The phase relationships in MORB pyrolite combined with the modal abundance of coexisting phases show that the breakdown reactions of pargasitic amphibole occur continuously throughout the subsolidus region studied. The temperature stability limit of pargasitic amphibole coincides with the water-undersaturated solidus (amphibole-dehydration solidus) at pressures below 30 kbar. The experimental results are applicable to pargasitic amphibole-bearing natural peridotites. Cooling and decompression paths and heating events observed in natural peridotites can be interpreted from changes in the composition of pargasitic amphibole. The data are also applicable to a model for peridotite melting and hydration process in the subduction environment. Received: 27 October 1997 / Accepted: 6 November 1998  相似文献   

7.
 The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An45) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions of biotite varying from iron-free phlogopite to iron-rich Ann70, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase (An45) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and 730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar). The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration melting of biotite (Ann50) + plagioclase (An45) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene + clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f O 2 in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium, and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann50) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann70-bearing starting composition begins at 730°C, and in the Ann25-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of biotite (Ann25 and Ann50) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5 kbar). Received: 27 December 1995 / Accepted: 7 May 1996  相似文献   

8.
Metabasaltic rocks in the Klamath Mountains of California with ‘komatiitic’ major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentrations preserve relict igneous trends and suggest that the rocks are not komatitic basalts, but immature arc rocks and within-plate alkalic lavas. Correlation of ‘excess’ MgO with the volume per cent hornblende (±clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions. Early submarine alteration produced regional δ18O values of +10±1.5%° and shifts in Al2O3, Na2O, and K2O concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350–550°C, c. 3 kbar) southward to prehnite–actinolite facies (200–400°C, ≤3 kbar), but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclase + microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these phases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearly stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatible with P–T conditions in a magmatic arc. Later contact metamorphism at 2–2.9±0.5 kbar and at peak temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3–4–km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, muscovite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatism during contact metamorphism produced further increases in bulk-rock δ18OSMOW of as much as +6%°. Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metasomatism and the presence of magnesian cumulus phases.  相似文献   

9.
Experiments in the system high-A1 basalt (HAB)-water have been conducted in the melting range at pressures between 1 atm. and 10 kbar, defining the amphibole stability field and the composition of liquids which coexist with this amphibole. Plagioclase is the anhydrous liquidus phase between 1 atm. and 10 kbar but in the hydrous runs this role is taken by olivine at <7 kbar and then by clinopyroxene at higher pressures. Because amphibole is never on the high-A1 basalt liquidus it is not likely that andesite is derived from primary basalt by pure fractional crystallisation, although as we discuss, other mechanisms including equilibrium crystallisation might implicate amphibole. If primary basaltic magma undergoes closed-system equilibrium crystallisation, then the amphibole field will be intersected at between 50 and 100°C below the liquidus. The compositions of melts coexisting with amphibole alone do not match those of any of the natural andesite or dacitic lavas associated with the particular high-A1 basalt investigated. Like natural andesites, they become rapidly silica enriched, but they also become far more depleted in TiO2 and MgO. However, the compositions of liquids lying directly on the divariant amphibole-out reaction zone, where amphibole +liquid coexist with clinopyroxene or olivine (±plagioclase), do resemble those of naturally occurring low-silica andesites. With increasing temperature pargasitic amphibole breaks down via incongruent melting reactions over a narrow temperature range to form a large volume of relatively low-silica basaltic andesite liquid and a crystalline assemblage dominated by either clinopyroxene or olivine. Our important conclusion is that basaltic andesite liquid will be the product of reaction between cooling, hydrous mafic liquid and anhydrous ferromagnesian phases. The solid reactants could represent earlier cumulates from the same or different magma batches, or they could be peridotite wall-rock material. Because the amphibole-out boundary coexisting with liquid is one of reaction, it will not be traversed so long as the phases on the high temperature side remain. Thus, the assemblage amphibole+clinopyroxene±olivine±plagioclase+liquid is one in which the liquid is buffered (within limits), and results reported here indicate that this buffering generates melts of low-silica andesite composition. When tapped to lower pressures these liquids will rise, eventually to fractionate plagioclase-rich assemblages yielding silicarich andesite and dacite melts. Conversely, the partial melting of hornblende pyroxenite, hornblende peridotite or hornblende gabbro can also yield basaltic andesite liquids. The phase relationships suggested by these experiments are discussed in the light of naturally occurring phenocryst and xenolith assemblages from the east Sunda Arc. Primary magmatic additions to the lithosphere of volcanic arcs are basaltic and voluminous upper crustal andesite in these terranes, complemented by mafic and ultramafic crystalline deposits emplaced in the lower crust or close to the Moho. Together these components constitute total arc growth with a basaltic composition and represent the net accreted contribution to continental growth.  相似文献   

10.
A calibration is presented for an activity–composition model for amphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O (NCFMASHO), formulated in terms of an independent set of six end‐members: tremolite, tschermakite, pargasite, glaucophane, ferroactinolite and ferritschermakite. The model uses mixing‐on‐sites for the ideal‐mixing activities, and for the activity coefficients, a macroscopic multicomponent van Laar model. This formulation involves 15 pairwise interaction energies and six asymmetry parameters. Calibration of the model is based on the geometrical constraints imposed by the size and shape of amphibole solvi inherent in a data set of 71 coexisting amphibole pairs from rocks, formed over 400–600 °C and 2–18 kbar. The model parameters are calibrated by combining these geometric constraints with qualitative consideration of parameter relationships, given that the data are insufficient to allow all the model parameters to be determined from a regression of the data. Use of coexisting amphiboles means that amphibole activity–composition relationships are calibrated independently of the thermodynamic properties of the end‐members. For practical applications, in geothermobarometry and the calculation of phase diagrams, the amphibole activity–composition relationships are placed in the context of the stability of other minerals by evaluating the properties of the end‐members in the independent set that are in internally consistent data sets. This has been performed using an extended natural data set for hornblende–garnet–plagioclase–quartz, giving the small adjustments necessary to the enthalpies of formation of tschermakite, pargasite and glaucophane for working with the Holland and Powell data set.  相似文献   

11.
Low‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.  相似文献   

12.
The stability and partial melting of synthetic pargasite in the presence of enstatitic orthopyroxene (opx), forsterite, diopsidic clinopyroxene (cpx), plagioclase (An50), and water has been studied in the range of 0.4–6.0 kb and 750–1000°C in the system Na2O-CaO-MgO-Al2O3-SiO2-H2O with a fixed bulk composition of pargasite+5 opx. The addition of orthopyroxene effectively reduces the stability field of pargasite by approximately 200°C at 1 kb. The invariant point involving pargasite coexisting with water-saturated liquid and anhydrous phase shifts from about 0.85 kb and 1025°C to 2.5±0.5 kb and 925±25°C with the addition of opx. Based on the solidus mineral assemblage and direct chemical analysis of quenched glass, the vapor-saturated liquid has a composition close to that of intermediate plagioclase. A layered silicate, interpreted to be Na-phlogopite, has an upper-thermal stability that nearly equals that of pargasite in the field of partial melting and coexists with liquid, pargasite, cpx, and forsterite at 6 kb, 1000°C. These results support the hypothesis that mantle metasomatism could involve formation of pargasitic amphibole from a silicate melt at depths as shallow as 8–10 km.  相似文献   

13.
The stability relations of lawsonite, CaAl2Si2O7(OH)2H2O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H2O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al2O3-SiO2-H2O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO3(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg3Al2Si3O12] show that at 10 GPa the reaction lawsonite=Gr-Pyss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr20Py80. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H2O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H2O to an even greater depth in the mantle.  相似文献   

14.
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   

15.
Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H2O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.  相似文献   

16.
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 = 0·38–0·40),42 vol. % plagioclase (An26–29), 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 2–4 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   

17.
The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.  相似文献   

18.
《Lithos》1986,19(2):153-163
Amphiboles approached edenite (NaCa2Mg5Si7AlO22(OH)2), richterite (Na2CaMg5Si8O22(OH)2), tremolite (□Ca2Mg5Si8O22(OH)2) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H2O. For the stoichiometric edenite bulk composition + excess H2O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad Pfluid-T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed35Rc50Tr15.In the system edenite + 4 quartz + excess H2O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole Pfluid-T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO2.Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high aSiO2; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.  相似文献   

19.
The stability of synthetic armalcolite of composition (Fe0.5Mg0.5Ti2O5 was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O2) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down (dTdP = 20°C/kbar), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way (dTdP = 30°C/kbar) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO2 appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O2 was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe0.5Mg0.5)-Ti2O5, is stable over a range ?O2 from about 10?9.5to 10?10.5 atm. Below this range to at least 10?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti3+. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.  相似文献   

20.
The El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe‐oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH2O (<740 °C for aH2O = 0.5; <790 °C for aH2O = 1). Furthermore, but not pervasive, re‐equilibration under greenschist facies P–T conditions is documented by retrograde epidote and chlorite. The resulting counterclockwise P–T path consists of progressive, nearly isobaric cooling from the igneous stage down to the granulite, amphibolite and greenschist stage.  相似文献   

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