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Thomas Fockenberg - One of the best experts on this subject based on the ideXlab platform.
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The enthalpy of formation and internally consistent thermodynamic data of Mg-Staurolite
American Mineralogist, 2002Co-Authors: Klaus D. Grevel, Thomas Fockenberg, Alexandra Navrotsky, Juraj MajzlanAbstract:The enthalpies of drop solution in lead borate (2 PbO.B 2 O 3 ) of four Mg-Staurolite samples, synthesized at 720 °C and pressures between 2 and 5 GPa, were measured by high-temperature oxide-melt calorimetry at 702 °C. Staurolite compositions, determined by electron microprobe analysis, Karl-Fischer titration, and thermogravimetry, are: Mg 3 . 7 1 Al 1 8 . 1 7 Si 7 . 6 0 O 4 4 . 3 1 (OH) 3 . 6 9 , 9 , Mg 3 . 8 7 Al 1 7 . 6 5 Si 7 . 7 5 O 4 3 . 6 8 (OH) 4 . 3 2 , Mg 3 . 6 6 Al 1 7 . 7 6 Si 7 . 6 8 O 4 3 . 3 1 (OH) 4 . 6 9 , and Mg 3 . 5 8 Al 1 8 . 0 5 Si 7 . 4 3 O 4 3 . 0 1 (OH) 4 . 9 9 . The enthalpy of drop solution of the bulk samples (as well as the calculated values for the enthalpy of formation from the elements of Mg-Staurolite) are strongly correlated to the H content of the samples. The enthalpy of formation from the elements is best described by the linear relation Δ f H 0 2 9 8 (Mg-Staurolite) = (-25357.58 + 87.35 N) kJ/mol, where N = number of H atoms per formula unit in Mg-Staurolite. The enthalpy of drop solution of two partially dehydrated Mg-Staurolite samples is in a good agreement with the linear relation. Phase-equilibrium data for Mg-Staurolite (Fockenberg 1998) were recalculated using the stoichiometric formula Mg 3 . 5 Al 1 8 Si 7 . 7 5 O 4 4 (OH) 4 . Based on these mineral equilibria and the internally consistent data set of Berman (1988), a mathematical programming analysis of the thermodynamic data of Mg-Staurolite gave Δ f H 0 2 9 8 [Mg 3 . 5 Al 1 8 Si 7 . 7 5 O 4 4 (OH) 4 ] = -25005.14 kJ/mol, and S 0 2 9 8 [Mg 3 . 5 Al 1 8 Si 7 . 7 5 O 4 4 (OH) 4 ] = 937.94 J/(K.mol). Thus, for the first time reliable thermodynamic data for Mg-Staurolite, based on experimental constraints, are provided.
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an experimental investigation on the p t stability of mg Staurolite in the system mgo al 2 o 3 sio 2 h 2 o
Contributions to Mineralogy and Petrology, 1998Co-Authors: Thomas FockenbergAbstract:The pressure-temperature stability field of Mg-Staurolite, ideally Mg4Al18Si8O46(OH)2, was bracketed for six possible breakdown reactions in the system MgO-Al2O3-SiO2-H2O (MASH). Mg-Staurolite is stable at water pressures between 12 and 66 kbar and temperatures of 608–918 °C, requiring linear geotherms between 3 and 18 °C/km. This phase occurs in rocks that were metamorphosed at high-pressure, low-temperature conditions, e.g. in subducted crustal material, provided they are of appropriate chemical composition. Mg-Staurolite is formed from the assemblage chlorite + kyanite + corundum at pressures <24 kbar, whereas at pressures up to 27 kbar Staurolite becomes stable by the breakdown of the assemblage Mg-chloritoid + kyanite + corundum. Beyond 27 kbar the reaction Mg-chloritoid + kyanite + diaspore = Mg-Staurolite + vapour limits the Staurolite field on its low-temperature side. The upper pressure limit of Mg-Staurolite is marked by alternative assemblages containing pyrope + topaz-OH with either corundum or diaspore. At higher temperatures Mg-Staurolite breaks down by complete dehydration to pyrope + kyanite + corundum and at pressures below 14 kbar to enstatite + kyanite + corundum. The reaction curve Mg-Staurolite = talc + kyanite + corundum marks the low-pressure stability of Staurolite at 12 kbar. Mg-Staurolite does not coexist with quartz because alternative assemblages such as chlorite-kyanite, enstatite-kyanite, talc-kyanite, pyrope-kyanite, and MgMgAl-pumpellyite-kyanite are stable over the entire field of Mg-Staurolite.
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An experimental investigation on the P-T stability of Mg-Staurolite in the system MgO-Al 2 O 3 -SiO 2 -H 2 O
Contributions to Mineralogy and Petrology, 1998Co-Authors: Thomas FockenbergAbstract:The pressure-temperature stability field of Mg-Staurolite, ideally Mg4Al18Si8O46(OH)2, was bracketed for six possible breakdown reactions in the system MgO-Al2O3-SiO2-H2O (MASH). Mg-Staurolite is stable at water pressures between 12 and 66 kbar and temperatures of 608–918 °C, requiring linear geotherms between 3 and 18 °C/km. This phase occurs in rocks that were metamorphosed at high-pressure, low-temperature conditions, e.g. in subducted crustal material, provided they are of appropriate chemical composition. Mg-Staurolite is formed from the assemblage chlorite + kyanite + corundum at pressures
Barbara L. Dutrow - One of the best experts on this subject based on the ideXlab platform.
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Tourmaline-rich pseudomorphs in sillimanite zone metapelites: Demarcation of an infiltration front
American Mineralogist, 1999Co-Authors: Barbara L. Dutrow, C. T. Foster, Darrell J. HenryAbstract:Textural features combined with mineral chemistry preserved in metamorphic rocks provide insights into metamorphic reaction mechanisms as well as open vs. closed system processes. Prograde tourmaline-rich muscovite pseudomorphs after Staurolite develop in sillimanite zone metapelites adjacent to peraluminous granitoid intrusives in NW Maine. Tourmalines occur in discrete domains restricted to central regions of muscovite-rich, quartz-poor pseudomorphs with biotite-rich margins. These tourmaline grains are relatively large ( ≤1.0 mm), lack detrital cores and exhibit only minor compositional zoning, in sharp contrast to matrix tourmaline from other samples. These features suggest fluid-infiltration as the causative mechanism for the formation of these tourmaline-rich mica pseudomorphs after Staurolite. Irreversible thermodynamic models of local reactions and material transport in combination with mineral chemistry allow evaluation of reaction mechanisms that produced these pseudomorphs. Thermodynamic models in the NKCMTFASHOB system mimic the observed textural features if a threestage process is used. Stage 1: Staurolite replacement is initiated by infiltration of an aqueous phase that adds K+Na+H2O to the rock with the concomitant removal of Al+Fe. Because the system is initially undersaturated with respect to tourmaline, a pseudomorph containing muscovite with minor biotite develops at the expense of Staurolite. Stage 2: With continued infiltration, concentration of B increases, tourmaline saturation is exceeded, tourmaline nucleates and grows. Local material transport constraints mandate that tourmaline precipitation be spatially restricted to regions of Staurolite dissolution. Consequently, tourmaline forms in clusters at sites containing the last vestiges of Staurolite in the pseudomorph core, also evidenced by Staurolite inclusions within several tourmaline grains. Resultant domains of Staurolite replacement during this stage contain about equal amounts of muscovite and tourmaline. Typical Staurolite poikiloblast pseudomorphing reactions require silica transport, matrix quartz dissolves from the surrounding host resulting in a local enrichment of biotite and plagioclase at the pseudomorph margin. Stage 3: Small amounts of sillimanite nucleate and grow throughout the rock. Late-stage aqueous fluids from the adjacent monzonitic intrusive are likely to be the primary B source. Theoretical, textural, and compositional modeling combined with observational data indicate that boron must have been derived externally from the rock, that the modal amount of tourmaline is very sensitive to the B content of the fluid, that tourmaline is stable throughout the sillimanite zone depending on other cation activities and pH of the fluid, and that these pseudomorphs provide insight into B contents of metamorphic fluids and the timing of the B influx. The outer geographic extent of the tourmaline-bearing pseudomorphs marks the boundary of a reactive geochemical front, and thus defines an advective isograd. Interpretation of subtle textural features preserved in the rock in conjunction with irreversible textural modeling provides a powerful tool with which to understand the chemical evolution of metamorphic rocks and the fluids involved.
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Thermodynamic properties of stoichiometric Staurolite H2Fe4AllsSis04s and H6Fe2AllsSis048
1995Co-Authors: M J Holdaway, Biswajit Mukhopadhyay, Barbara L. DutrowAbstract:Recent studies have shown that all natural and synthetic Staurolite crystals are nonstoichiometric. In an Fe-Al-Si-O-H (FASH) system, the predominant hypothetical stoichiometric end-members are H2Fe4AlisSis04s (2H) and H6Fe2AlisSis04s (6H). End-member molar volume (V), specific heat (Cp), and calorimetric entropy (S) are presented on the basis of previous crystallographic and calorimetric work. Compositional data, including H content, are not available for any experimental Staurolite. Because of the number of variables involved, it is necessary to use a trial and error approach to enthalpy (H) and entropy (S) retrieval. Using (1) the above values for V and Cp, (2) thermodynamic data for additional phases in the prograde Staurolite + quartz reaction, (3) experimental data, including new data on the reaction of Staurolite + quartz to sillimanite + almandine, (4) mole fraction models and composition estimates, and (5) natural data from Black Mountain, New Hampshire, and Maine, we have determined Mlr (- 23961.25 :t 20 kJ/mol, - 23992.86 :t 40 kJ/mol), S [918 :t 20 J/(mol. K), 850 :t 40 J/(mol. K)] for 2H and 6H Staurolite. We have also determined H content in Staurolite as a function of P, T, and coexisting phases. These values of S are substantially lower than corrected calorimetric values. These combined thermodynamic data for stoichiometric Staurolite reproduce most of the experimental reversals, and show that synthetic and natural Staurolite near Staurolite + quartz breakdown conditions vary from H = 2.8 to >4.6 apfu with increasing P. Within the Staurolite + quartz stability field, the composition of Staurolite coexisting with almandine can be contoured for H content, which increases with P. Likewise, Staurolite coexisting with aluminum silicate can be contoured for H content, which increases with P and decreases with T. For excess-Si02 rocks at any given P and T, the total range of FASH Staurolite composition is between that which occurs with almandine and quartz and that which occurs with aluminum silicate and quartz. Those experimental data points that do not fit the calculated curve result mainly from use ofH-poor Staurolite as starting material under conditions at which H-rich Staurolite is stable. Discrepancies between individual experimental data points and the curve calculated from the thermodynamic data are thus mainly the result of problems with experimental studies. These results provide an improved data set for calculation of Staurolite equilibria.
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A new perspective on Staurolite crystal chemistry: Use of stoichiometric and chemical end-members for a mole fraction model
American Mineralogist, 1991Co-Authors: M J Holdaway, Barbara L. Dutrow, Biswajit Mukhopadhyay, M. D. Dyar, Douglas Rumble, J.a. GramblingAbstract:Recent contributions to the crystal chemistry of Staurolite have been aided by Miissbauer studies on 35 natural and synthetic samples and structure determinations on 42 single crystals. The combination of these data with chemical studies enables us to understand better the crystal chemistry of this mineral relative to its use as a petrogenetic indicator. In reduced rocks about 3.50/o f Fe is Fe3*, and in oxidized rocks about 7olo is Fe3*. It is now possible to recast a Staurolite chemical analysis in terms of site occupancies using reasonable guidelines and intersite partitioning. Whereas the formula for hypothetical stoichiometric end-member iron Staurolite is HrFeoAl,8Si8Oos, chemical end-member formulas such as HrFeorrAl,re'Sir65oos are useful for retrieving thermodynamic data from experimental studies on simple systems. Other formulas may be written to account for appropriate amounts of the R'z*H-, substitution. In the absence of analyses for H, stoichiometry may be estimated from chemical analyses by assuming Si + Al h Li + 2/t Ti + Fe3* : 25.55 ions pfu. In normal Staurolite Li may be assumed to be 0.2. H may then be estimated by subtracting total cation charge from 96. H estimates for any group of related natural Staurolite samples are normally a function of mineral assemblage. The preferred thermodynamic mole fraction (activity) model for phase equilibrium calculations takes into account dilution on all cation sites except H. Applied to Staurolite-chloritoid pairs, this mole fraction model does not provide an explanation for the variable Ko values between these two minerals. Knowledge of the ferric content, formula, and mole fraction model of Staurolite enables meaningful retrieval and use of thermochemical quantities from experimental studies.
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The effects of Al and vacancies on Li substitution in iron Staurolite: A synthesis approach
American Mineralogist, 1991Co-Authors: Barbara L. DutrowAbstract:Synthesis of a variety of single phase iron lithium Staurolite samples (at720 'C, 30 kbar, /", : IW) demonstrates that a saturation level of Li is attained in iron Staurolite at - 1.5 Li ions per 48 O via the substitution t4lli+ + 0.33161413- : t4lFe2+ + 0.331611. Synthesis experiments designed to investigate the substitution Li* + Al3* : 2Fe2+ produced substantial amounts of corundum which implies that this substitution is not the primary mechanism responsible for Li incorporation into Staurolite. Because Al is charge coupled with Li, the Li maximum appears to be dependent on the vacancies available for the concomitant Al incorporation. Synthesis of two compositional iron Staurolite series with differing amounts of Si, 7.5 and 8 pfu, but similar vacancy contents, produced similar results. Utilizing several simplifoing assumptions, A(3A, 38) octahedra appear to be firlly occupied atLi: -1.5 pfu; site occupancy and bond strengths prohibit additional Li substitution. At more Li-Al-rich compositions, Al and Li phases form in addition to Staurolite producing the assemblage Staurolite * corundum * a-spodumene, at these P-7"-X conditions. Lattice parameters of the synthetic Staurolite vary linearly as a function of Fe-Li composition with the most pronounced variation occurring in the D dimension. The 060 diffraction peak also shifts systematically as a function of Li substitution. Its position potentially provides a method to estimate Li contents in natural Fe-rich Staurolite after compensating for shifts caused by substitution of other ions. The maximum Li contents in Staurolite will displace the upper thermal stability of the equilibrium of st * qtz : alm + sill + fluid approximately 40 'C. The displacement to higher ?n will, however, be moderated in most natural settings because of the nonideal FeLi mixing in Staurolite, the partitioning of Li into coexisting phyllosilicates and the addition of other components.
Toshiaki Tsunogae - One of the best experts on this subject based on the ideXlab platform.
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metamorphic p t evolution of garnet Staurolite biotite pelitic schist and amphibolite from keffi north central nigeria geothermobarometry mineral equilibrium modeling and p t path
Journal of African Earth Sciences, 2017Co-Authors: Emmanuel Nwachukwu Ugwuonah, Toshiaki Tsunogae, Smart C ObioraAbstract:Abstract We report the first detailed petrological and pressure-temperature data from the pelitic schists and amphibolites of the Keffi area, north-central Nigeria, which is located on the eastern flank of the western Nigerian based, Great Nigerian Schist Belt. In one fresh exposure of the schist, Staurolite-bearing and Staurolite-absent, garnet-rich assemblages occur. All pelitic samples contain garnet, quartz, biotite, plagioclase, chlorite and ilmenite, but the Staurolite-bearing assemblage contains euhedral to subhedral Staurolites and very subordinate retrograde chlorites in addition. Mineral compositions applied to calculate metamorphic P–T conditions using different approaches reveal a temperature range of 570–630 °C for the garnet-biotite geothermometry. P–T pseudosection analyses calculated using THERMOCALC software for the suitable rock types, constrain garnet/Staurolite equilibration within the range of 6.4–7.7 kbar and 610–630 °C. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the study area. The result of geothermobarometry (peak conditions) from this study is consistent with previous P–T estimations for the Pan-African episode on several areas within the Trans-Saharan Belt. All evidences point towards a magmatic arc tectonic setting for this area of study.
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sapphirine quartz corona around magnesian xmg 0 58 Staurolite from the palghat cauvery suture zone southern india evidence for high pressure and ultrahigh temperature metamorphism within the gondwana suture
Lithos, 2010Co-Authors: Yuki Nishimiya, Toshiaki Tsunogae, M. SantoshAbstract:Abstract We report the first finding of equilibrium sapphirine + quartz assemblage from the Palghat-Cauvery Suture Zone (PCSZ) in southern India providing unequivocal evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions associated with the collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. The sapphirine and quartz occur as coronas around Mg-rich ( X Mg ~ 0.58) Staurolite within poikiloblastic garnet in an Mg–Al-rich rock, suggesting the progress of the prograde dehydration reaction: Staurolite + garnet → sapphirine + quartz + H 2 O. Although the occurrence of Mg-rich Staurolite was previously noted from some localities in the PCSZ, no sapphirine + quartz direct association has yet been reported. The available experimental studies on Mg-rich Staurolite indicate that the mineral is stable at P > 15 kbar, and suggest that the complex texture that we report here might correspond to a peak high-pressure regime prior to the UHT event. The symplectic sapphirine + quartz around Staurolite probably implies decompression from P > 15 kbar toward the stability of sapphirine + quartz at temperatures of c. 1000 °C along a clockwise P–T path. The prograde high-pressure metamorphism and following UHT event correlate with the subduction–collision tectonics associated with the final stage of amalgamation of Gondwana supercontinent.
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High P-T phase relation of magnesian (Mg0.7Fe0.3) Staurolite compositon in the system FeO-MgO-Al2O3-SiO2-H2O: Implications for prograde high-pressure history of ultrahigh-temperature metamorphic rocks
American Mineralogist, 2009Co-Authors: Kei Sato, M. Santosh, Toshiaki TsunogaeAbstract:High-pressure and high-temperature phase relation on Mg 0.74 Fe 0.26 -Staurolite composition with excess water in the system FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O was experimentally determined at 12-19 kbar and 850-1050 °C, with a focus on the lower-pressure stability limit of magnesian Staurolite. The experimental results show that the stability field of Staurolite in the system shifts to higher temperatures as compared to that of pure-Mg Staurolite. At 950 °C, the lower-pressure stability limit is located between 15 and 14 kbar. This study demonstrates that Staurolite breaks down with isothermal decompression by a series of reactions at 950 °C: St → Opx + Crn + Melt and St + Opx → Spr + Melt. The Mg' [=Mg/(Fe+Mg)] of Staurolite in the former reaction is 0.7. When Staurolite coexists with orthopyroxene in run products, the Staurolite Mg' is decreased to 0.6-0.5. Moderate-Mg Staurolite (Mg' = ∼0.5) in natural occurrences has been reported as inclusion minerals in poikiloblastic garnets within ultrahigh-temperature (UHT) metamorphic rocks from major collisional orogenic belts in southern India and southern Africa. The experimental data presented in our study suggest the possibility that the Staurolite-bearing UHT metamorphic rocks had experienced high-pressure metamorphism and/or UHT extreme metamorphism during the prograde metamorphic stage.
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corundum quartz and mg Staurolite bearing granulite from the limpopo belt southern africa implications for a p t path
Lithos, 2006Co-Authors: Toshiaki Tsunogae, Dirk D. Van ReenenAbstract:Abstract A new occurrence of the rare corundum + quartz assemblage and magnesian Staurolite has been found in a gedrite–garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian Staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe–Mg ratios of Staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H2O to give the corundum + quartz assemblage. Peak conditions of 890–930 °C at 9–10 kbar are obtained from orthopyroxene + sapphirine and garnet + Staurolite assemblages. A clockwise P–T path is inferred, with peak conditions being followed by retrograde conditions of 4–6 kbar and 500–570 °C. The presence of unusually magnesian Staurolite (Mg / [Fe + Mg] = 0.47–0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons.
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Corundum + quartz and Mg-Staurolite bearing granulite from the Limpopo Belt, southern Africa: Implications for a P–T path
Lithos, 2006Co-Authors: Toshiaki Tsunogae, Dirk D. Van ReenenAbstract:Abstract A new occurrence of the rare corundum + quartz assemblage and magnesian Staurolite has been found in a gedrite–garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian Staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe–Mg ratios of Staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H2O to give the corundum + quartz assemblage. Peak conditions of 890–930 °C at 9–10 kbar are obtained from orthopyroxene + sapphirine and garnet + Staurolite assemblages. A clockwise P–T path is inferred, with peak conditions being followed by retrograde conditions of 4–6 kbar and 500–570 °C. The presence of unusually magnesian Staurolite (Mg / [Fe + Mg] = 0.47–0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons.
Jochen E. Mezger - One of the best experts on this subject based on the ideXlab platform.
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Mimicking syntectonic growth: cordierite overgrowth of earlier rotated Staurolite porphyroblasts, strain caps and deflected foliation
Journal of Structural Geology, 2010Co-Authors: Jochen E. MezgerAbstract:Monoclinic shape fabrics and inclusion trail geometries of porphyroblasts are regarded as reliable shear-sense indicators, provided that timing and sequence of growth can be established. In the southern Bossost dome of the Central Pyrenees, Staurolite and cordierite porphyroblasts in mica schists contain inclusion trails oblique to, but continuous with the external foliation, indicating porphyroblast rotation. Straight inclusion trails in Staurolites record growth between formation of the main schistosity and subsequent shear, i.e. an intertectonic origin. Strain caps and deflection of foliation around porphyroblasts show a distinct asymmetry with uniform sense of shear. Cordierite porphyroblasts are significantly larger than Staurolites and contain curved inclusion trails which would suggest syntectonic growth and a similar shear of sense. However, Staurolite and cordierite do not belong to the same paragenesis and textural evidence, corrosion of Staurolite rims and relict inclusions in cordierites, suggest partial consumption by cordierite. Complete overgrowth of Staurolite with preservation of its inclusion trails and adjacent foliation deflection results in curved inclusion trails that mimic syntectonic growth of cordierite. Actually, the larger cordierites statically overgrew rotated Staurolites during post-tectonic contact metamorphism.
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comparison of the western aston hospitalet and the bossost domes evidence for polymetamorphism and its implications for the variscan tectonic evolution of the axial zone of the pyrenees
Journal of The Virtual Explorer, 2005Co-Authors: Jochen E. Mezger, Martinlutheruniversitat HallewittenbergAbstract:The Bossost and the western Aston-Hospitalet structural and metamorphic domes hav e sim ilar metamorphic characteristics. Textural observations show garnet and Staurolite as earlier phases, andalusite, cordierite and sillimanite as later phases. Metastable Staurolite-cordierite-biotite-muscovite assemblages found in both domes indicate a decompressional path from medium pressure-medium temperature regional metamorphism producing garnet and s taurolite to a low pressure-higher temperature contact metamorphism resulting in the growth of andalusite, cordierite and sillimanite. In the Bossost dome these two metamorphic events are separated by a phase of strong non-coaxial deformation which is preserved in a prominent shear zone. The shear zone is inferred to have facilitated de compression as an extensional shear z one. I n the Aston-Hospitalet dom es, the similarity of the metamorphic characteristics with the Bossost dome is not matched with the mircostructural evidence. This may be due to the lack of a suitable lithology which prevented subsequent metamorphic phases to form, so that earlier fabrics had been completely obliterated by later thermal and tectonic events. The presence of large migmatite zone along the western Aston gneiss may record such an event. The tectonic conclusions derived from the Bossost dome can not readily be applied to the Aston-Hospitalet domes.
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metastable Staurolite cordierite assemblage of the bossost dome late variscan decompression and polyphase metamorphism in the axial zone of the central pyrenees
Comptes Rendus Geoscience, 2004Co-Authors: Jochen E. Mezger, Cees W Passchier, J. L. RégnierAbstract:Abstract A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossost dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and Staurolite porphyroblasts following regional metamorphism M 1 , while coaxial conditions prevailed during later contact metamorphic M 2 growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–Staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–Staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–Staurolite assemblage preserves conditions during M 1 , estimated by AFM diagrams and P – T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that Staurolite is not a stable phase in cordierite–andalusite assemblages of M 2 , suggesting polyphase metamorphism and decompression along a clockwise P – T path for the Staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).
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Metastable Staurolite–cordierite assemblage of the Bossòst dome: Late Variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees
Comptes Rendus Geoscience, 2004Co-Authors: Jochen E. Mezger, Cees W Passchier, J. L. RégnierAbstract:Abstract A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossost dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and Staurolite porphyroblasts following regional metamorphism M 1 , while coaxial conditions prevailed during later contact metamorphic M 2 growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–Staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–Staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–Staurolite assemblage preserves conditions during M 1 , estimated by AFM diagrams and P – T pseudosections to be 5.5 kbar and 580 °C, respectively. Pseudosections also indicate that Staurolite is not a stable phase in cordierite–andalusite assemblages of M 2 , suggesting polyphase metamorphism and decompression along a clockwise P – T path for the Staurolite–cordierite–andalusite assemblages. This concurs with proposed extensional tectonics along the regional shear zone. To cite this article: J.E. Mezger et al., C. R. Geoscience 336 (2004).
M. Santosh - One of the best experts on this subject based on the ideXlab platform.
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sapphirine quartz corona around magnesian xmg 0 58 Staurolite from the palghat cauvery suture zone southern india evidence for high pressure and ultrahigh temperature metamorphism within the gondwana suture
Lithos, 2010Co-Authors: Yuki Nishimiya, Toshiaki Tsunogae, M. SantoshAbstract:Abstract We report the first finding of equilibrium sapphirine + quartz assemblage from the Palghat-Cauvery Suture Zone (PCSZ) in southern India providing unequivocal evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions associated with the collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. The sapphirine and quartz occur as coronas around Mg-rich ( X Mg ~ 0.58) Staurolite within poikiloblastic garnet in an Mg–Al-rich rock, suggesting the progress of the prograde dehydration reaction: Staurolite + garnet → sapphirine + quartz + H 2 O. Although the occurrence of Mg-rich Staurolite was previously noted from some localities in the PCSZ, no sapphirine + quartz direct association has yet been reported. The available experimental studies on Mg-rich Staurolite indicate that the mineral is stable at P > 15 kbar, and suggest that the complex texture that we report here might correspond to a peak high-pressure regime prior to the UHT event. The symplectic sapphirine + quartz around Staurolite probably implies decompression from P > 15 kbar toward the stability of sapphirine + quartz at temperatures of c. 1000 °C along a clockwise P–T path. The prograde high-pressure metamorphism and following UHT event correlate with the subduction–collision tectonics associated with the final stage of amalgamation of Gondwana supercontinent.
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High P-T phase relation of magnesian (Mg0.7Fe0.3) Staurolite compositon in the system FeO-MgO-Al2O3-SiO2-H2O: Implications for prograde high-pressure history of ultrahigh-temperature metamorphic rocks
American Mineralogist, 2009Co-Authors: Kei Sato, M. Santosh, Toshiaki TsunogaeAbstract:High-pressure and high-temperature phase relation on Mg 0.74 Fe 0.26 -Staurolite composition with excess water in the system FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O was experimentally determined at 12-19 kbar and 850-1050 °C, with a focus on the lower-pressure stability limit of magnesian Staurolite. The experimental results show that the stability field of Staurolite in the system shifts to higher temperatures as compared to that of pure-Mg Staurolite. At 950 °C, the lower-pressure stability limit is located between 15 and 14 kbar. This study demonstrates that Staurolite breaks down with isothermal decompression by a series of reactions at 950 °C: St → Opx + Crn + Melt and St + Opx → Spr + Melt. The Mg' [=Mg/(Fe+Mg)] of Staurolite in the former reaction is 0.7. When Staurolite coexists with orthopyroxene in run products, the Staurolite Mg' is decreased to 0.6-0.5. Moderate-Mg Staurolite (Mg' = ∼0.5) in natural occurrences has been reported as inclusion minerals in poikiloblastic garnets within ultrahigh-temperature (UHT) metamorphic rocks from major collisional orogenic belts in southern India and southern Africa. The experimental data presented in our study suggest the possibility that the Staurolite-bearing UHT metamorphic rocks had experienced high-pressure metamorphism and/or UHT extreme metamorphism during the prograde metamorphic stage.
