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1.
The stability of cordierite and garnet relative to their anhydrous breakdown products, i.e. hypersthene, sapphirine, olivine, spinel, sillimanite and quartz, has been studied experimentally in model pelitic compositions (system MgO-FeO-Al2O3-CaO-K2O-SiO2). Below 1000° C cordierite breaks down according to the divariant reaction cordierite garnet+sillimanite+quartz (1) for most values of the MgO/MgO + FeO ratio (X). At very high values of X (ca. X0.9) garnet in reaction (1) is replaced by hypersthene. The position and width of the divariant field (in terms of pressure and temperature) in which cordierite and garnet coexist, is a function of the MgO/MgO + FeO ratio. If this ratio is increased then the stability field of garnet is reduced and that of cordierite extended towards higher pressure. Compositions of coexisting cordierite and garnet in divariant equilibrium have been analysed by electron probe micro-analyser. These compositions are unique functions of pressure and temperature. Above ca. 1000° C the breakdown of cordierite involves the phases sapphirine and hercynite-rich spinel in Mg-rich and Fe-rich compositions respectively.  相似文献   

2.
Amphibolite facies mafic rocks that consist mainly of hornblende, plagioclase and quartz may also contain combinations of chlorite, garnet, epidote, and, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoamphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in addition to pressure, temperature and fluid composition. The mineral equilibria in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite, kyanite, staurolite and cordierite. Except in aluminous compositions, these assemblages are restricted to higher pressures. The iron to magnesium ratio (XFe), and to a lesser extent, sodium to calcium ratio, have important roles in determining which (if any) of the aluminous minerals occur under particular pressure–temperature conditions. Where aluminous minerals occur in amphibolites, the P–T–X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe‐rich amphibolites are typically dominated by garnet‐ and staurolite‐bearing assemblages, whereas their more Mg‐rich counterparts contain chlorite and cordierite. Assemblages involving staurolite–hornblende can occur over a wide range of pressures (4–10 kbar) at temperatures of 560–650 °C; however, except in the more aluminous, iron‐rich compositions, they occupy a narrow pressure–temperature window. Thus, although their occurrence in ‘typical’ amphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.  相似文献   

3.
In pelitic rocks, under conditions of low f O 2 and low f H 2 O, the stability of the mineral pair cordierite-garnet is limited by five univariant reactions. In sequence from high pressure and low temperature to high temperature and low pressure these are: cordierite+garnet hypersthene+sillimanite+quartz, cordierite+garnet hypersthene+sapphirine+quartz, cordierite+garnet hypersthene+spinel+quartz and cordierite+garnet olivine+spinel +quartz. In this sequence of reactions the Mg/Mg+Fe2+ ratio of all ferro-magnesian minerals involved decreases continuously from the first reaction to the fifth. The five univariant boundaries delimit a wide P-T range over which cordierite and garnet may coexist.Two divariant equilibria in which the Mg/Mg+ Fe2+ ratio of the coexisting phases are uniquely determined by pressure and temperature have been studied in detail. P-T-X grids for the reactions cordierite garnet+sillimanite+quartz and cordierite+hypersthene garnet+quartz are used to obtain pressure-temperature estimates for several high grade metamorphic areas. The results suggest temperatures of formation of 700–850° C and load pressures of 5–10 kb. In rare occasions temperatures of 950–1000° C appear to have been reached during granulite metamorphism.On the basis of melting experiments in pelitic compositions it is suggested that Ca-poor garnet xenocrysts found in calc-alkaline magmas derive from admixed pelitic rocks and did not equilibrate with the calc-alkaline magma.  相似文献   

4.
Aluminous parageneses containing gedrite, cordierite, garnet, staurolite, biotite, sillimanite, kyanite, quartz or spinel plus corundum are found as dark colored lenses in the polymetamorphic, multideformed Archean complex at Ajitpura in northwest peninsular India. Staurolite, like kyanite, is a relict phase of earlier metamorphism and is excluded as a paragenetic mineral in view of its incompatibility with quartz and gedrite and its lower X Mg values than for garnet of the assemblage. Its stability here is attributed to zinc content of up to 3 wt%. The XMg in other ferromagnesian minerals decreases in the order: cordierite, biotite, gedrite, garnet, as found elsewhere in high grade rocks.The textural criteria and systematic partitioning of Fe and Mg in the ferromagnesian phases, excluding staurolite, indicate attainment of equilibrium during the second metamorphism. From tie line configurations in the phase diagrams, X Mg ratios in the constituent minerals, and other petrographic criteria, it is suggested that gedrite — cordierite-garnet — sillimanite — biotite assemblage has been produced by the reactions: Biotite+Sillimanite+Quartz = Cordierite+Garnet+K-feldspar+Vapor (1) and Biotite+Sillimanite+Quartz = Cordierite +Gedrite+K-feldspar+Vapor (2) which occurred during partial melting of the rocks at fixed P and T conditions.By isothermal P-X(Fe-Mg) sections it has been demonstrated that release of FeO, SiO2 and other components modified the composition of the reactant biotite presumably by the substitution FeSi2 Al, whereby reaction 1 was replaced by reaction 2. Cordierite with higher X Mg was produced with gedrite instead of with garnet, whose X Mg is less than X Mg of gedrite. Reaction 2 has been tentatively located in T-P space from the intersection of some continuous loops in the P-X(Fe-Mg) diagram at 700°C and also by other constraints. The discontinuous reaction 2 is located about 1–2 kilobars higher than reaction 1, which implies that it is difficult to distinguish between effects of pressure and those of melting on the X Mg ratios of the reaction phases.The P-T calibrations of garnet — cordierite, garnet — biotite and garnet — plagioclase equilibria and the calibrations from other dehydration curves give temperatures near 700°C and pressure (assuming ) about 6 kilobars.  相似文献   

5.
Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al2O2−SiO2) under low fO2 and for various H2O/CO2 conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H2O, CO2 and H2O−CO2 (±CH4) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO2-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO2. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H2O and CO2 are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg2Al4SiO10 due to AlAl=MgSi substitutions [100(Al2O3/Al2O3+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.  相似文献   

6.
Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H2O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.  相似文献   

7.
Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to XMg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al2O3 to rims with up to 10.6 wt% Al2O3 adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al2O3–SiO2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet XMg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.  相似文献   

8.
The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (Mo) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M1) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M2) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (XAn=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinelss {XMg (Mg/Mg+Fe2+) = 0.23–0.26}, XAl (Al/Al+Fe3+)=0.71–0.81}-plagioclase (XAn=0.91)-cordierite {XMg (Mg/Mg+Fe2+) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M3), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M2 spinelss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).Editorial responsibility: W. Schreyer  相似文献   

9.
Garnet in silicic liquids and its possible use as a P-T indicator   总被引：1，自引：0，他引：1  
Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K D values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher value than coexisting liquid, but below 950 ° C the garnet value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids.Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains 10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically <4 % MnO) to an Mn-richer rim (typically >4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma.  相似文献   

10.
Cordierite occurs locally and sporadically in biotite-quartz-two feldspar paragneisses of the Precambrian Highlands complex in southeastern New York. Cordieritic and associated non-cordieritic gneisses were compared to determine the significance of cordierite for the metamorphic history of the complex.Microprobe analyses of the ferromagnesian phases show the following ranges in Fe/Mg (mol.): cordierite 0.19–0.43; biotite 0.33–0.73; garnet 1.98–3.56. Feldspar compositional ranges are: plagioclase An25–53; K-feldspar in microperthite Or62–87Ab12–37 An0–1. Garnet and plagioclase associated with cordierite are depleted in Ca relative to those in cordierite-free assemblages.Textural evidence, phase rule considerations and consistent distribution coefficients for FeO and MgO in coexisting garnet, cordierite and biotite from each locality examined suggest that all phases formed in at least local equilibrium during the hornblende-granulite subfacies metamorphism. The assemblages studied limit the conditions of metamorphism to between 700 and 750° C and 3.0 to 5.5. Kb. total pressure, with P T greater than . Differences in mineral compositions and partitioning coefficients among the sampled areas suggest slight local differences within these ranges.Mineral and modal analyses of cordierite-bearing and cordierite-free gneisses show the latter to be enriched in calcium and potassium and depleted in alumina relative to the former. We conclude that the rarity of cordierite in the Highlands paragneisses reflects a scarcity of parent rocks of suitable composition rather than unfavorable physical conditions.  相似文献   

11.
Theoretical stability relations have been derived between the phases cordierite (Cd), garnet (Ga), hypersthene (Hy), olivine (Ol), sapphirine (Sa), spinel (Sp), sillimanite (Si) and quartz (Qz) in the system MgO-FeO-Al2O3-SiO2. Natural rock data and experimental evidence suggest that the Mg/Mg+Fe2+ ratio (X) of coexisting ferromagnesian phases decreases as follows: X Cd>X Sa>X Hy>X Ol>X Sp>X Ga. By use of this information four stable invariant points are proposed involving the phases: Cd, Hy, Sa, Ga, Si, Qz; Cd, Sa, Ga, Sp, Si, Qz; Cd, Hy, Sa, Ga, Sp, Qz; Cd, Ga, Hy, Ol, Sp, Qz. All univariant curves in the system are nonterminal, representing the breakdown of a join rather than the stability limit of an individual phase. A detailed treatment of divariant equilibria involving two and three ferromagnesian solid solutions illustrates the potential of these equilibria as Pressure-Temperature indicators. Interactions between solid-solid reactions and dehydration reactions involving biotite in the system MgO-FeO-Al2O3-SiO2-K2O-H2O have been graphically analysed. The addition of biotite to anhydrous divariant assemblages does not affect the composition of coexisting phases at constant P and T but can affect their relative proportions.  相似文献   

12.
Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet-sillimanite-biotite-cordierite-spinel-ilmenite-rutile and garnet-orthopyroxene-cordierite-biotite, as well as quartz and K-feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P-T calculations suggest formation conditions of approximately 800d? C at 7-7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d? C for the cordierite-bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite-bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite-bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P-T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite-bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low-P/high-T post-peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica.  相似文献   

13.
Former idioblastic garnet crystals of an Archaean granulite with Mg/(Mg+Fe+Mn)-ratios (= M) near 0.40 were converted to symplectitic pseudomorphs consisting mainly of cordierite (M=0.61), orthopyroxene (M=0.40), and a relic garnet with a new composition (M=0.18), during a static metamorphism which is related in time to the Vredefort event. On the basis of experimental data for the continuous reaction garnet+quartz=orthopyroxene+cordierite the conditions of metamorphism were near 5 kb, 700 °C. Orthopyroxenes crystallized initially as minute, myrmekitic grains with metastable excess Al contents up to 13 mol% Al2O3 and, through grain growth and Al exsolution, transformed into dense aggregates of coarser crystals with equilibrated compositions near 3 mol% Al2O3. In the absence of free silica hercynitic spinel appears as an additional phase that coexists with cordierite, orthopyroxene, and garnet of more magnesian compositions than in the case of silica saturation.Pseudotachylite veins crosscutting the hornfelsed granulite consist of pyroxene of variable Al contents, two feldspars, opaques, and quartz, and are finely recrystallized. There is textural and mineral chemical evidence from both the Al-contents of the pyroxenes and the Mg/Fe distribution among the phases adjacent to the pseudotachylite that these veins were emplaced late during the static metamorphism, which was probably caused by a local, deep-seated magma diapir. On this basis, the meteorite impact hypothesis for the origin of the Vredefort Structure is considered less likely than a mechanism of internal origin which can be made responsible for the close link in time and space between static metamorphism and the high strain rate deformation that produced the pseudotachylite.  相似文献   

14.
A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (X Mg=78) — cordierite (X Mg=87) — garnet (X Mg=51) — sillimanite. Rare green spinel (X Mg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (X Mg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO 2 conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.  相似文献   

15.
Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts.  相似文献   

16.
The sub-solidus fields of crystallization of a spectrum of synthetic aluminous basic compositions (high-alumina basalt, anorthite-enriched high-alumina basalt, kyanite eclogite, grosspydite and gabbroic anorthosite) have been investigated at pressures of up to 36 kb. At low pressures the assemblages are characterized by abundant plagioclase, clinopyroxene and possibly minor olivine and orthopyroxene. These correspond to natural gabbroic and pyroxene granulite assemblages. As pressure is increased garnet appears and increases gradually in amount at the expense of other ferromagnesian minerals and plagioclase, until finally at pressures of >23 kb at 1,100° C, plagioclase disappears and high pressure clinopyroxene+garnet+kyanite±quartz assemblages equivalent to eclogite are obtained. In the eclogite stability field, with further rise in pressure, the ratio ga/cpx and the grossular content of the garnet increase.In the high-alumina basalt composition the transitional garnet granulite assemblage (clinopyroxene+plagioclase+garnet±quartz) is spread over a pressure interval of 11 kb at 1,100° C. This is a greater interval than observed for other basalt compositions and is important in considering the hypothesis that the Mohorovicic Discontinuity is a phase change from basalt to eclogite. It indicates that the change in V p would be spread over a significant depth range, and no sharp seismic velocity discontinuity could result.The first experimental synthesis of kyanite eclogite from both high-alumina basalt and kyanite eclogite compositions has been obtained, as well as synthesis of unusual grossular-clinopyroxene-kyanite assemblages (grosspydite) from grosspydite and gabbroic anorthosite compositions. The pressures needed to synthesize these assemblages are somewhat greater than the pressures needed to synthesize eclogite from basic compositions of lower alumina content at the same temperature. Experimental confirmation of the observation that there is a direct relation between Gross/Alm + Py ratio of garnet and the Jd/Di ratio of co-existing pyroxene in grosspydite and kyanite eclogite assemblages found in kimberlite pipes has also been obtained.  相似文献   

17.
Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (XMg>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.  相似文献   

18.
Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al2O3 (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al2O3), aluminous orthopyroxene (up to 8.93 wt% Al2O3), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al2O3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe2+-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).  相似文献   

19.
Abstract Phase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water-undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K-feldspar (Kfs), quartz (Qtz) and a water-undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) under low PH2O and fO2 conditions. Four compositions have been studied with 100 [Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P-T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000-1100°C, the pair Os-Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg-rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os-Opx-Sil-Kfs-Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high-T pair Spr-Qtz. When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe-rich phase in the run product. Our data support a theoretical P-T grid for the KFMAS system in which osumilite is stable outside the field of the high-T assemblage Spr-Qtz. Moreover, our grid indicates that Os-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than Os-Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg-rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust.  相似文献   

20.
The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites.A theoretical analysis of the phase relations involving osumilite in the chemical systems K2O-MgO-Al2O3-SiO2 and K2O-MgO-FeO-Al2O3-SiO2 is presented. In the pure Mg-system the lower temperature stability limit of Mg-osumilite is inferred to be defined with increasing pressure by the reactions OsCd+En+Kfeld+Qtz, OsSa+En+Kfeld+Qtz, OsSill+En+Kfeld+Qtz. In iron-bearing systems an important reaction involving osumilite is Os+GtCd+Hy+Kfeld+Qtz.At moderate temperatures and pressures, osumilite is limited to rocks which lie on the Mg-rich side of the Cd-Hy stable tie line on an AFM diagram. At higher pressures and temperatures osumilite occurs in a widerrange of rock compositions because of the stability of coexisting garnet and osumilite. Petrographic data, as well as chemographic relations indicate that for many common rock compositions, garnet, cordierite, hypersthene, sapphirine and sillimanite cannot coexist with both osumilite and K-feldspar.Published with the permission of the Director, Bureau of Mineral Resources  相似文献   

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