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1.
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   

2.
Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe2O3 in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al2O3–Fe2O3–SiO2–H2O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe3+ content and a high XMg= (Mg/ (Mg+Fe2+)) under these P–T conditions.  相似文献   

3.
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   

4.
The stability of cordierite and garnet has been studied experimentally in complex, silica oversaturated compositions (in the systems MgO-FeO-Al2O-CaO3-Na2O-K2OSiO2) in which the molecular ratio Al2O3/FeO+MgO<1. Compositions with 100 Mg/Mg+Fe2+ ratios (X) of 0, 30, 50, 70 and 100 have been used to investigate the role of this ratio in determining phase assemblages and P, T coordinates of reactions. The minimum pressure for appearance of garnet at a given temperature is strongly dependent on X total rock.The X-values of co-existing phases (chiefly garnet, cordierite, hypersthene) in divariant equilibrium are a function of temperature and pressure and have been experimentally determined at 900° C, 1000° C and 1100° C. At high temperature (>1050° C) the phases sapphirine and spinel are stable with quartz in Mg-rich and Fe-rich compositions respectively. Experiments in the system MgO-FeO-Al2O3-SiO2 show that for a given X-value and temperature the pressure required to produce Ca-free garnet from hypersthene-cordierite assemblages is 1–2 kb greater than that required to produce garnet containing 6±2 mol percent grossular solid solution in the more complex Ca-bearing system.  相似文献   

5.
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   

6.
The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for buffered conditions): Sp+Qz=Ga+Hy+Si+O2 (Sa, Cd), Cd+Sp+Qz=Hy+Si+O2 (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O2 (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O2 (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites.  相似文献   

7.
Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. XMg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al2O3–SiO2–K2O–H2Oat high fo2, has been constructed and the effects of ZnO andFe2O3 on this grid have been explored Combining available experimentaland natural occurrence data, the high fo2 invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author  相似文献   

8.
Osumilite, approximate composition K(Mg,Fe)2 Al5Si10O30, has been reported recently from two granulite localities. The mineral has been synthesised in a model pelitic composition at 1000 and 1100 ° C and 3.6–6.3 kb under conditions of low water and oxygen fugacity. Osumilite coexists, apparently stably, with hypersthene, cordierite and quartz (?) thus duplicating the mineral assemblage of one of the natural occurrences. Osumilite is in a divariant reaction relationship with cordierite and hypersthene i.e. osumilite ? cordierite + hypersthene + orthoclase+quartz. This reaction runs to the right with increasing pressure. Experimental data and field observations suggest that the joins osumilite-garnet and osumilite-sillimanite are not stable. It is suggested osumilite is involved in an invariant point in the system K2O-MgO-FeO-A12O3-SiO2 with the phases cordierite, hypersthene, sapphirine, spinel, orthoclase and quartz. The invariant point should occur at 1000 ± 100 °C and 7± 2kb.  相似文献   

9.
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   

10.
Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The XMg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al2O3-SiO2-(H2O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history.  相似文献   

11.
The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py37)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py37)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py30) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py42) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-Tmax of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.  相似文献   

12.
Mineral textures in metapelitic granulites from the northern Prince Charles Mountains, coupled with thermodynamic modelling in the K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) model system, point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde cooling (i.e. an anticlockwise P–T path). Textural evidence for the increasing temperature part of the path is given by the breakdown of garnet and biotite to form orthopyroxene and cordierite in sillimanite‐absent rocks, and through the break‐down of biotite and sillimanite to form spinel, cordierite and garnet in more aluminous assemblages. This is equated to the advective addition of heat from the regional emplacement of granitic and charnockitic magmas dated at c. 980 Ma. A subsequent increase in pressure, inferred from the break‐down of spinel and quartz to sillimanite, cordierite and garnet in aluminous rocks, is attributed to crustal thickening related to upright folding dated at 940–910 Ma. The terrane attained peak metamorphic temperatures of c. 880 °C at pressures of c. 6.0–6.5 kbar during this event. Subsequent cooling is inferred from the localised breakdown of cordierite and garnet to form biotite and sillimanite that developed in the latter stages of the same event. The textural observations described are interpreted via the application of P–T and P–T–X pseudosections. The latter show that most rock compositions preserve only fragments of the overall P–T path; a result of different rock compositions undergoing mineral assemblage changes, or changes in mineral modal abundance, on different sections of the P–T path. The results also suggest that partial melting during granulite facies metamorphism, coupled with melt loss and dehydration, initiated a switch from pervasive ductile, to discrete ductile/brittle deformation, during retrograde cooling.  相似文献   

13.
High-Mg–Al, silica-undersaturated metapelites from theOygarden Group of islands, East Antarctica, preserve clear evidencefor the stable coexistence of the assemblage orthopyroxene +corundum in natural rocks. The quartz-absent metapelite occursas pods and isolated layers within a high-strain zone relatedto deformation during the c. 0·93 Ga Rayner StructuralEpisode. Assemblages that include orthopyroxene, corundum, sapphirine,sillimanite, cordierite, garnet and kornerupine are developedacross a pre-existing compositional zoning, leading to contrastingmineral Fe–Mg ratios. The assemblage orthopyroxene–corundumis shown to exist in only a very restricted range of bulk compositionsand P–T histories. Simplified qualitative FMAS grids havebeen constructed for kornerupine-absent and -present systems,illustrating MAS terminations and divariant equilibria thathelp to describe the mineral assemblage and reaction history.Reaction textures that include coronas of sapphirine and sillimaniteseparating orthopyroxene and corundum, and symplectites of orthopyroxene+ sapphirine ± cordierite/plagioclase, orthopyroxene+ sillimanite ± cordierite/plagioclase and orthopyroxene+ sapphirine + sillimanite embaying garnet, imply a clockwiseP–T–t evolution. Conditions of P > 9–10kbar and T  相似文献   

14.
Small pods of silica-undersaturated Al-rich and Mg-rich granulite facies rocks containing sapphirine, pleonastic spinel, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene-rich aluminous granofels and metapelitic gneisses containing sapphirine, spinel or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper. Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900–950 °C and 8–9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the sequential crossing of the following reactions in the system FMAS: cordierite + spinel + corundum = sapphirine + sillimanite, cordierite + spinel = orthopyroxene + sapphirine + sillimanite, sapphirine + spinel + sillimanite = orthopyroxene + corundum, sapphirine + sillimanite = cordierite + orthopyroxene + corundum. Phase stability relationships in FMAS and MASH indicate an anticlockwise P–T path terminated by isobaric cooling. Such a path is exemplified by early low-P mineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase in aH2O and aB2O3 with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1 P–T path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.  相似文献   

15.
Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 793 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel.The consistent topology of the observed mineral assemblages with reference to the system MgO-FeO-Al2O3-SiO2-H2O, systematic partitioning of Mg and Fe in the coexisting phases, and textural relations are in accord with attainment of chemical equilibrium during the regional metamorphism. The sequence of metamorphic reactions has been deduced as a function of changing MgO/(MgO+FeO) ratio in the bulk composition and it has been shown that the topology in the SiO2-(FeO+MgO)-Al2O3 diagram for more magnesian compositions is consistent with experimental data in the system MgO-SiO2-Al2O3-H2O. The stable occurrence of the spinel-cordierite and spinel-sillimanite joins in less magnesian bulk compositions suggests that these tie lines are stable to higher pressures and lower temperatures respectively as compared to the Fe-free synthetic system.Using experimental and theoretical data on garnet-cordierite, garnet-biotite, garnet-plagioclase and orthopyroxene-clinopyroxene equilibria, PT conditions have been estimated to be 750 °±50 °C/ 5±0.5 Kb for the thermal peak of metamorphism.It is suggested that the unusual bulk composition of the sapphirine-bearing rocks viz. higher Al2O3, MgO and MgO/(MgO+FeO) and lower SiO2, Na2O and CaO compared to the parent cordierite-bearing granulitic gneisses may be attributed to partial melting and removal of granitic melt. This is in agreement with the variation of oxides in the restites and parent rock. The restites consisted of the assemblage cordierite-spinel—corundum which then reacted to form sapphirine. This reaction, which in the synthetic Fe-free system is confined to extremely low-pressures (300 bars), is likely to occur at higher pressures with MgFe substitution.  相似文献   

16.
The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980 °C, based on the stability of this assemblage in the simplified chemical system FeO–MgO–Al2O3–SiO2 (FMAS) system. However, the potential for sapphirine to contain significant Fe3+ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz‐bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O (FMASTO), expansion into K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (KFMASHTO) allows the effect of the stability of the additional phases, biotite, K‐feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using P–T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P–T pseudosections, and the appropriate H2O and O contents to use in the modelled compositions are investigated using T–MH2O and T–MO pseudosections. The peak P–T estimates from a sapphirine + quartz‐bearing sample are ～960 to 935 °C at ～10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz ± garnet‐bearing samples. Whereas the sapphirine + quartz‐bearing sample is more Fe‐rich than the orthopyroxene + sillimanite‐bearing sample on an all‐Fe‐as‐FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.  相似文献   

17.
In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.  相似文献   

18.
Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as
  1. spinel+rutile+quartz+MgFe–1=sapphirine+ilmenite
  2. cordierite=sapphirine+quartz+MgFe–1
  3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe–1
  4. orthopyroxene+sapphirine+quartz=garnet+MgFe–1
  5. orthopyroxene+sillimanite=garnet+quartz+MgFe–1
  6. orthopyroxene+sillimanite+quartz+MgFe–1=cordierite.
Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from Pmax and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.  相似文献   

19.
The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO2 (0.80–0.95 g/cm3) as the dominant fluid phase, with traces of probable CH4 (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO2 and CO2-H2O (±CH4?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics.  相似文献   

20.
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   

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