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1.
Petrogenetic grids in the system NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the P–T range 7–30kbar and 450–680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H2O in excess. These grids, together with calculated compatibilitydiagrams and P–T and T–XCa and P–XCa pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different P–T conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and P–Tconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in P–T and P–XCapseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry  相似文献   

2.
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (fHOxides=-168.7Dž.4 kJ mol-1, or (fH0298=-4,872.5dž.0 kJ mol-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al2O3-SiO2-H2O (CASH). The following data for lawsonite and zoisite were obtained: (fH0298 (lawsonite)=-4,865.68 kJ mol-1 , S0298 (lawsonite)=229.27 J K-1 mol-1 , (fH0298 (zoisite)=-6,888.99 kJ mol-1 , S0298 (zoisite)=297.71 J K-1 mol-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.  相似文献   

3.
Using an internally consistent thermodynamic dataset and updatedmodels of activity–composition relation for solid solutions,petrogenetic grids in the system NKFMASH (Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O)and the subsystems NKMASH and NKFASH have been calculated withthe software THERMOCALC 3.1 in the P–T range 5–36kbar and 400–810°C, involving garnet, chloritoid,biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite,phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesiteand H2O in excess. These grids, together with calculated AFMcompatibility diagrams and P–T pseudosections, are shownto be powerful tools for delineating the phase equilibria andP–T conditions of Na-bearing pelitic assemblages for avariety of bulk compositions from high-P terranes around theworld. These calculated equilibria are in good agreement withpetrological studies. Moreover, contours of the calculated phengiteSi isopleths in P–T pseudosections for different bulkcompositions confirm that phengite barometry is highly dependenton mineral assemblage. KEY WORDS: phase relations; HP metapelite; NKFMASH; THERMOCALC; phengite geobarometry  相似文献   

4.
In situ eclogitic schist lenses occur in the coherent low-gradeepidote-zone Ward Creek metabasite unit of the Central Franciscanbelt. They contain almandine garnet, clinopyroxene, and rutile.They have slightly higher Mn content (0–5–1–0wt.%) than the coexisting Type III metabasites (0–12–0–25wt%) which contain epidote + glaucophane + actinolite + chlorite+ omphacite + quartz + sphene ? aragonite? lawsonite ? pumpellyite+ albite. The in situ eclogitic schists (130–140 Ma) canbe distinguished from older tectonic eclogites (150–160Ma) in Ward Creek as follows: (1) they are medium grained, whereasType IV tectonic eclogites are coarse grained; (2) they haveunaltered spessartine-rich idioblastic (0–4–10 mm)garnets, whereas Type IV tectonic eclogites have larger xenoblasticto hypidiomorphic spessartine-poor garnets which were corrodedand chloritized along the rim during retrograde metamorphism;(3) clinopyroxenes are chloromelanite in in situ eclogitic schistsbut omphacite in Type IV tectonic eclogites; (4) barroisiticamphiboles occur both as inclusions in garnets and as matrixminerals in Type IV tectonic eclogites but not in in situ eclogiticschists; (5) albite is present in in situ eclogitic schistsbut not in Type IV tectonic eclogites; and (6) the estimatedP-T condition of in situ eclogitic schists is 290 ?C < T<350 ?C, P = 8–9 kb, whereas that of Ward Creek Type IVtectonic eclogites is 500?C< r<540?C, P< 10–11–5kb. Medium-grained eclogites occur as individual blocks in WardCreek; they are different from Type IV tectonic eclogites butare very similar to in situ eclogitic schists. They have unalteredidioblastic garnet with high almandine and spessartine content(Alm47Sp23Gr20Py10), and they have chloromel-anitic clinopyroxeneand quartz but no barroisite. Paragonite is also stable in theseeclogites. The blocks formed at 380 ?C< r<400?C, and 9–5kb<P< 14 kb. They are presumably in situ eclogites formedat the highest-temperature part of the Ward Creek metabasiteunit and may be younger than Type IV tectonic eclogites. Such low-temperature occurrences of eclogitic assemblages aredue to the compositional effect on reactions between blueschistand eclogite that are insensitive to pressure and shift towardslower temperatures as bulk-rock MnO content and XFe/(Fe+Mg)increase. The Mn/(Mn + Fe) ratio of bulk rock is an importantfactor in controlling the P-T positions of these reactions attemperatures below 450 ?C, whereas the Fe/(Fe + Mg) ratio ofbulk-rock becomes important at temperatures higher than 450?C.  相似文献   

5.
Medium‐temperature ultrahigh pressure (MT‐UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core–mantle increasing grossular (Xgr) and decreasing pyrope (Xpy) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the Xpy and Xgr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core–rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post‐peak stages. The maximum Si contents are mostly 3.60–3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT‐UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the Xpy in garnet mostly depends on bulk compositions, whereas the Xgr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the Xgr and Si isopleths in pseudosections, the peak P–T conditions were estimated to be 40 kbar/730 °C for the Jinheqiao, 41 kbar/726 °C for the Shuanghe, and 37–52 kbar and 700–830 °C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite ± phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early‐stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite‐bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid‐present conditions, it is difficult to recover real peak P–T conditions for UHP eclogites with lawsonite. This may be a reason why the P–T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late‐stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite‐absent MT‐UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting.  相似文献   

6.
Pseudosections calculated with thermocalc predict that lawsonite‐bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid‐saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk‐rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type‐L) and glaucophane–epidote eclogites (Type‐E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote‐bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H2O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane‐rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca‐rich rocks and blueschist assemblages most readily in Ca‐poor rocks. This distinction in bulk‐rock composition can account for the co‐existence of low‐T eclogite and blueschist in high‐pressure terranes.  相似文献   

7.
Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ～27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ～70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.  相似文献   

8.
The Bixiling mafic-ultramafic metamorphic complex is a 1•5km2 tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H2O-bearing and CO2-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism *Corresponding author  相似文献   

9.
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   

10.
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   

11.
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   

12.
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   

13.
The reaction 2 zoisite + CO2 = 3 anorthite + calcite + H2O hasbeen reversed experimentally in cold-seal pressure vessels usingnatural phases and H2O–C02 fluids generated by water-silveroxalate mixtures. Equilibrium has been determined at 5000 50bars, 599 9 °C and 0–075 ± 0–010 XCO2.Extrapolation using the MRK equation of Kerrick & Jacobs(1981) gives an equilibrium curve of negative T–X slopeconsistent with bracketing runs at 500, 550 and 650 °C.The curve agrees only with a new bracket of Nitsch (in Hoschek,1980), and is at higher XCo2 than all other experimental determinationsand at lower XCO2 than those calculated from the thermodynamicdata of Helgeson et al. (1978). Discrepancies are attributedto differences in starting materials and small errors in thethermodynamic properties of the phases. Reaction direction and equilibrium have been determined by observingsurface textures of run products by SEM. Growth and solutiontextures are non-equivalent, permitting unequivocal determinationof reaction direction even where the extent of reaction is small,an advantage over conventional and insensitive XRD methods whichmeasure bulk changes in the charge. Dissolution features ofanorthite and zoisite are defect-related indicating controlby surface reaction, whereas calcite dissolves by both surfacereaction and diffusion controlled processes. Margarite forms in most runs below 585 °C. Textural features,its restriction to the margarite stability field and comparisonwith feldspar solubility data demonstrate it is an equilibriumphase formed by incongruent solubility of anorthite and zoisitein H2O-CO2 fluids. Quench phases formed from the solute areconsequently silica-rich, with implications for metasomaticprocesses in feldspar–epidote–bearing rock and fluidsystems. Absence of margarite from runs with anorthite, zoisiteand calcite in the zoisite stability field is apparently dueto the fast growth rate of zoisite. The full equilibrium assemblageis zoisite–anorthite–calcite–margarite atthese temperatures, and the degeneracy of the model system isunobtainable in experiments, and presumably, in nature.  相似文献   

14.
Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c . 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H2O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite-zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.  相似文献   

15.
The 456 ± 4 Ma Skattøra migmatite complex in thenorth Norwegian Caledonides consists of migmatitic nepheline-normativemetagabbros and amphibolites that are net-veined by numerousnepheline-normative anorthositic and leucodioritic dykes. Plagioclase(An20–50) is the dominant mineral (85–100%) in thedykes and the leucosome, but amphibole is generally presentin amounts up to 15%. The following observations strongly suggestformation of the anorthositic magma by anatexis of the surroundinggabbro in the presence of an H2O-bearing fluid phase: (1) themigmatites have plagioclase-rich (anorthositic) leucosomes andamphibole-rich restites; (2) crystallization of amphibole inthe anorthositic and leucodioritic dykes suggests high H2O activity;(3) the presence of coarse-grained to pegmatitic dykes and miaroliticcavities indicates a fluid-rich magma; (4) hydration zones thatsurround many anorthosite dykes suggest that the magma probablyexpelled H2O-rich fluids during crystallization. Water-saturatedmelting experiments at 0·5–1·5 GPa and temperaturesfrom 800 to 1000°C have been performed on a nepheline-normativegabbro to test the proposed petrogenesis of the Skattøraanorthosites. The glasses produced close to the solidus aretonalitic in composition, but they become richer in plagioclaseat higher temperatures. At and below 1·0 GPa, the residuesare composed of amphibole. Experiments above 1·0 GPaproduced residual garnet and/or zoisite in addition to amphibole,suggesting that the anorthositic dykes in the Skattøramigmatite complex formed below 1·25 GPa. The experimentsshow that the high Na2O content of the anorthosite dykes canonly be produced if Na is added to the charges. The glass thatbest fits the composition of the Skattøra dykes was producedat 1·0 GPa and 900°C with 2 wt % Na(OH) added. KEY WORDS: anorthosite; dyke swarm; anatexis; experimental petrology  相似文献   

16.
LIOU  J. G. 《Journal of Petrology》1971,12(2):379-411
Hydrothermal investigation of the bulk composition CaO.Al2O3.4SiO2+excessH2O has been conducted using conventional techniques over thetemperature ranges 200–450 °C and 500–6000 barsPfluid. A number of reactions have been studied by employingmineral mixtures consisting of reactants and products in about9: 1 and 1: 9 ratios. The phase relations were deduced fromrelatively long experiments by observing which seeded assemblagedisappeared or decreased markedly in one of the paired run charges. Laumontite was synthesized in the laboratory, probably for thefirst time. Laumontite was grown from seeded wairakite to over99 per cent using a weak NaCl solution. The refractive indicesof the synthetic material are about = 1.504 and = 1.514. Theaverage unit cell dimensions are a0 = 14.761±0.005 Å;b0 = 13.077±0.005 Å; c0 = 7.561±0.003 Å;and ß = 112.02°±0.04°. Within the errorof measurement, the optical properties and cell parameters arein good agreement with those of natural laumontite. The equilibriumdehydration of laumontite involves two reactions: (1) laumontite= wairakite+2H2O, passing through about 230 °C at 0.5 kb,255±5 °C at 1 kb, 282±5 °C at 2 kb, 297±5°C at 3 kb and 325±5 °C at 6 kb; and (2) laumontite= lawsonite+2 quartz+2H2O, taking place at about 210 °Cat 3 kb and 275 °C at 3.2 kb. Above 300 °C, the equilibriumcurve for the solid-solid reaction (3) lawsonite+2 quartz =wairakite passes through 305 °C, 3.4 kb and 390 °C,4.4 kb. Equilibrium has been demonstrated unambiguously forthe above three reactions. The hydrothermal decomposition ofnatural laumontite above its own stability limit appears tobe a very slow process. Combined with previously published equilibria determined hydrothermallyfor wairakite, the phase relations are further investigatedby chemographic analysis interrelating the phases, laumontite,wairakite, lawsonite, anorthite, prehnite+kaolinite, and 2 pumpellyite+kaolinitein the system CaAl2Si2O8-SiO2-H2O. This synthesis allowed theconstruction of a semiquantitative petrogenetic grid applicableto natural parageneses and the delineation of the physical conditionsfor the various low-grade metamorphic facies in low µCO2environments. The similar stratigraphic zonations, consistentlyfound in a variety of environments, are recognized to be a functionof burial depth, geothermal gradient, and mineralogical andchemical composition of the parental rocks. Departures fromthe normal sequences are believed to be due to the combinationsof mineralogical variations, availability of H2O, differencesin the ratio µCO2/µH2O, and the rate of reaction.The possible P-T boundaries for diagenesis, the zeolite facies,the lawsonite-albite facies, the prehnite-pumpellyite facies,and the adjacent metamorphic facies are illustrated diagrammatically.  相似文献   

17.
The northern part of the Cycladic island of Sifnos (Greece)is formed by a coherent sequence of interlayered acid and basicmetavolcanic rocks and metasediments, which underwent a high-pressureblueschist facies metamorphism during the Eocene. The metabasicrocks, including eclogites, blueschists, and actinolite-bearingrocks, are discussed in terms of their mineral assemblages,and bulk-rock and mineral chemistries. Metamorphic conditionsof 470 ? 30 ?C and 15 ? 3 kb are indicated by garnet-omphacitegeothermometry and by the development of deerite in meta-ironstonesand jadeite +quartz in meta-acidites.Mineral textures and systematicelement distributions between coexisting minerals suggest attainmentof chemical equilibrium. A new projection from garnet, epidote,quartz and vapour onto the NaAlO2-Al2O3-CaMgO2 plane is usedto illustrate equilibrium phase relations between omphacite,glaucophane, actinolite, paragonite, and chloritoid. It is demonstratedthat eclogites, blueschists, and actinolite-bearing metabasitesrepresent different bulk-rock compositions that recrystallizedunder the same fluid pressure and temperature conditions. Eclogitescontaining hydrous minerals such as glaucophane, actinolite,phengite, or paragonite in equilibrium with garnet and omphacitecan occur together with blueschists in high-pressure terraneswithout indicating different metamorphic conditions.  相似文献   

18.
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   

19.
The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H2O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na2O–MgO–Al2O3–SiO2–H2O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H2O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (?(CaNa)(Mg4Al)Si8O22(OH)2) and barroisite (?(CaNa)(Mg3Al2)(AlSi7)O22(OH)2). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H2O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fetotal/(Fetotal+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.  相似文献   

20.
Lawsonite eclogites are crucial to decipher material recycling along a cold geotherm into the deep Earth and orogenic geodynamics at convergent margins. However, their tectono‐metamorphic role and record especially at ultrahigh‐pressure (UHP) conditions are poorly known due to rare exposure in orogenic belts. In a ~4 km long cross‐section in Muzhaerte, China, at the western termination of the HP‐UHP metamorphic belt of western Tianshan, metabasite blocks contain omphacite and lawsonite inclusions in porphyroblastic garnet, although matrix assemblages have been significantly affected by overprinting at shallower structural levels. Two types of lawsonite eclogites occur in different parts of the section and are distinguished based on inclusion assemblages in garnet: Type 1 (UHP) with the peak equilibrium assemblage garnet+omphacite±jadeite+lawsonite+rutile+coesite±chlorite±glaucophane and Type 2 (HP) with the assemblage garnet+omphacite±diopside+lawsonite+titanite+quartz±actinolite±chlorite+glaucophane. Pristine coesite and lawsonite and their pseudomorphs in Type 1 are present in the mantle domains of zoned garnet, indicative of a coesite‐lawsonite eclogite facies. Regardless of grain size and zoning profiles, garnet with Type 1 inclusions systematically shows higher Mg and lower Ca contents than Type 2 (prp4–25grs13–24 and prp1–8grs20–45 respectively). Phase equilibria modelling indicates that the low‐Ca garnet core and mantle of Type 1 formed at UHP conditions and that there was a major difference in peak pressures (i.e., maximum return depth) between the two types (2.8–3.2 GPa at 480–590°C and 1.3–1.85 GPa at 390–500°C respectively). Scattered exposures of Type 1 lawsonite eclogite is scatteredly exposed in the north of the Muzhaerte section with a structural thickness of ~1 km, whereas Type 2 occurs throughout the rest of the section. We conclude from this regular distribution that they were derived from two contrasting units that formed along two different geothermal systems (150–200°C/GPa for the northern UHP unit and 200–300°C/GPa for the southern HP unit), with subsequent stacking of UHP and HP slices at a kilometre scale.  相似文献   

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