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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.
Orthopyroxene-Corundum in Mg-Al-rich Granulites from the Oygarden Islands, East Antarctica 总被引:1,自引:0,他引:1
High-MgAl, 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 FeMg ratios. The assemblage orthopyroxenecorundumis shown to exist in only a very restricted range of bulk compositionsand PT 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 clockwisePTt evolution. Conditions of P > 910kbar and T 相似文献
3.
Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain,Manitoba, Canada 总被引:1,自引:0,他引:1
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
4.
Silica-undersaturated sapphirine, spinel and kornerupine granulite facies rocks, NE Strangways Range, Central Australia 总被引:6,自引:0,他引:6
B. GOSCOMBE 《Journal of Metamorphic Geology》1992,10(2):181-201
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
5.
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
6.
J. A. BALDWIN R. POWELL M. BROWN R. MORAES R. A. FUCK 《Journal of Metamorphic Geology》2005,23(7):511-531
A new quantitative approach to constraining mineral equilibria in sapphirine‐bearing ultrahigh‐temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse‐grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature (P–T) path of this locality involves decompression across the FeO–MgO–Al2O3–SiO2 (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two‐stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in aH2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P–T conditions. Such grids are effectively constant aH2O sections in FMAS‐H2O (FMASH), for which the corresponding aH2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on aH2O is that local variations in aH2O may explain the formation of cordierite without significant changes in P–T conditions, particularly without marked decompression. 相似文献
7.
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
8.
The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (XMg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (XMg Grt = 0.60), sapphirine (XMg Spr = 0.87), cordierite (XMg Crd = 0.92) and biotite (XMg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al2O3) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust. 相似文献
9.
Philippe Bertrand David J. Ellis David H. Green 《Contributions to Mineralogy and Petrology》1991,108(1-2):55-71
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
10.
Equilibration and reaction in Archaean quartz-sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa 总被引:1,自引:0,他引:1
J. B. DAWSON S. L. HARLEY R. L. RUDNICK & T. R. IREL 《Journal of Metamorphic Geology》1997,15(2):253-266
Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11 kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P–T path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T estimates from garnet-orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7 kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83 Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes. 相似文献
11.
Sapphirine and spinel phase relationships in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 in the presence of quartz and hypersthene 总被引:1,自引:0,他引:1
Sapphirine and spinel can accommodate significant ferric iron and therefore the mineral equilibria involving these phases must be sensitive to a(O2). In this paper we examine the theoretical phase relationships involving sapphirine and spinel in addition to sillimanite, garnet, cordierite, rutile, hematite-ilmenite solid solution (henceforth ilmenite), and magnetite-ulvospinel solid solution (henceforth magnetite), in the presence of quartz and hypersthene in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 (FMASTO), with particular reference to the topological inversion in P-T postulated by Hensen (Hensen 1986). Documented natural associations suggest that the appropriate topology for assemblages involving magnetite and ilmenite is Hensen's higher a(O2) one, while, in contrast, the topology for assemblages involving ilmenite and rutile is the lower a(O2) one. The exact configuration of the inversion between these two topologies remains uncertain because of uncertainties in the ferric/ferrous iron partitioning between sapphirine and spinel-cordierite at high temperatures. By comparison with experimental data and natural occurences, the sillimanite-sapphirine-cordierite-garnet-hypersthene-quartz assemblage is in equilibrium at about 1000°–1020° C and 7–8 kbars, while sapphirine-cordierite-spinel-garnet-hypersthene-quartz occurs at temperatures in excess of those attainable during crustal metamorphism, for ilmenite-rutile buffered assemblages. This implies that sapphirine-rutil-ehypersthene-quartz assemblages, as found in the Napier Complex, Antarctica, can only occur at > 1000° C. Also, spinel-rutile-hypersthene-quartz assemblages should not be found in rocks because temperatures in excess of 1100° C are expected to be involved in their formation. The temperatures of formation of spinel-sillimanite-sapphirine-garnethypersthene-quartz, sapphirine-spinel-cordierite-sillimanite-hypersthene-quartz, and sillimanite-spinel-cordieritegarnet-hypersthene-quartz in assemblages buffered by magnetite and ilmenite are less well constrained, but are likely to be in the range 900°–1000° C. These conclusions apply to rocks with compositions close to FMASTO; the perturbing effects of substantial concentrations of additional components, in particular Ca, mainly in garnet, and Zn and Cr, mainly in spinel, may invalidate these conclusions. 相似文献
12.
Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures 总被引:1,自引:0,他引:1
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
13.
A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system 总被引:15,自引:0,他引:15
Due to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H2O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H2O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria. 相似文献
14.
Divariant and trivariant reaction line slopes in FMAS and CFMAS: theory and applications 总被引:1,自引:0,他引:1
Leo M. Kriegsman 《Contributions to Mineralogy and Petrology》1996,126(1-2):38-50
The theory of divariant reactions is investigated in some detail and approaches involving simultaneously operating reactions and X Mg isopleths are combined. Using the Gibbs' method (Spear et al. 1982), a general equation is derived for X Mg isopleth slopes in FMAS and CFMAS, which may be applied to any divariant assemblage in these systems. Isopleth slopes are calculated for a number of divariant FMAS and CFMAS reactions inferred to have operated in Sri Lankan granulites, giving more rigorous constraints on the local P-T path. Isopleth slopes are also calculated for the trivariant CFMAS reaction orthopyroxene + plagioclase = garnet + quartz. The resulting dP/dT range from +8 to +18 bar/K. Application to Sri Lankan granulites confirms an earlier hypothesis (Faulhaber and Raith 1991) that a retrograde path with a dP/dT of ~10–15 bar/K may cause garnet growth in charnockites and garnet breakdown in metabasites. It is shown here that dP/dT depends primarily on the Ca contents of garnet and plagioclase. Large variations in isopleth and reaction line slopes are possible, due to the thermodynamic effect of additional components in solid solutions, e.g. albite in plagioclase. This has a good potential for the assessment of the exact form of P-T paths. Equations to calculate isopleth slopes could be included in software packages aimed at defining P-T estimates, enabling geologists to derive realistic P-T paths. 相似文献
15.
The Mg–Al granulites from Ganguvarpatti consist of orthopyroxene–sillimanite–garnet ± quartz as peak assemblage, with a few porphyroblasts of cordierite and sapphirine. These assemblages were strongly overprinted by late symplectites and coronas. Orthopyroxene inclusions in garnet and porphyroblast cores have the highest X
Mg (0.80) and Al2O3 content (10.7 wt%). The estimated near-peak metamorphic conditions (1,000±50°C and 11 kbar) using garnet–orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid. The proposed multi-stage evolution process implies an initial decompression, deduced from multi-phase symplectites, followed by cooling during biotite formation. Further late decompression is explained from the orthopyroxene rims on biotite. This proposed P–T path thus suggests a unique and complex evolution history for the UHT granulites of southern India. Present results are comparable with similar adjacent terranes in the Gondwana supercontinent, but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain. 相似文献
16.
Al-Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K-feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that the P–T–t evolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, this P–T–t evolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P-T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F-bearing system. 相似文献
17.
Silica-deficient sapphirine-bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south-east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.
Fe2+ /(Fe2+ + Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.
Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene-sillimanite-sapphirine-cordierite-corundum developed during a high-pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium-pressure granulite facies generated various sapphirine-bearing assemblages. The diagnostic assemblage of this stage is sapphirine-cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.
The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H2 O-poor conditions. 相似文献
Fe
Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene-sillimanite-sapphirine-cordierite-corundum developed during a high-pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium-pressure granulite facies generated various sapphirine-bearing assemblages. The diagnostic assemblage of this stage is sapphirine-cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.
The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H
18.
First Report of Sapphirine+Quartz Assemblage from Southern India: Implications for Ultrahigh-temperature Metamorphism 总被引:1,自引:0,他引:1
We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al2O3), and (iii) in symplectitic intergrowth with orthopyroxene (Al2O3= 5.9–6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (XMg = 0.79–0.80) and low in Si content (1.55–1.56 pfu) as compared with those associated with orthopyroxene and cordierite (XMg= 0.77–0.79, Si = 1.59–1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 °C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al2O3 content of up to 9.7 wt.%, suggesting T = 1040–1060°C and P = 8 kbar. FMAS reaction of sapphirine+quartz→garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950–1000°C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (±sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India. 相似文献
19.
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks. 相似文献
20.
P. Tropper I. Deibl F. Finger R. Kaindl 《International Journal of Earth Sciences》2006,95(6):1019-1037
The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit. 相似文献