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Coexisting white micas and plagioclase were studied by electronmicroprobe (EMP), and transmission and analytical electron microscopy(TEMAEM) in greenschist- to amphibolite-grade metabauxitesfrom Naxos. The TEMAEM studies indicate that sub-micronscale (0.011.0 µm thick) semicoherent intergrowthsof margarite, paragonite and muscovite are common up to loweramphibolite conditions. If unrecognized, such small-scale micainterlayering can easily lead to incorrect interpretation ofEMP data. Muscovite and paragonite in M2 greenschist-grade Naxosrocks are mainly relics of an earlier high-pressure metamorphism(M1). Owing to the medium-pressure M2 event, margante occursin middle greenschist-grade metabauxites and gradually is replacedby plagioclase + corundum in amphibolite-grade metabauxites.The margarite displays minor IVAl3 VI(Fe3+, Al) Si-3 VI--1 andconsiderable (Na, K) SiCa-1Al-1 substitution, resulting in upto 44 mol% paragonite and 6 mol % muscovite in solution. Thecompositional variation of muscovite is mainly described byVI(Fe2+, Mg) Si VI Al-1VI Al-1 and VI(Fe3+Al-1) exchanges, thelatter becoming dominant at amphibolite grade, Muscovite issignificantly richer in Fe than margarite or paragonite. CaNaKpartitioning data indicate that margarite commonly has a significantlyhigher Na/(Na+ K+Ca) value than coexisting muscovite or plagioclase.Exceptions are found in several greenschist-grade rocks, inwhich M1-formed mussovite may have failed to equilibrate withM2 margarite. The sluggishness of K-rich micas to recrystallizeand adjust composidonally to changing P-T conditions is alsoreflected in the results of mus-covite-paragonite solvus thermometry.Chemical data for CaNa micas from this study and literaturedata indicate that naturally coexisting margariteparagonitepairs display considerably less mutual solubility than suggestedby experimental work. The variable and irregular Na partitioningbetween margarite and muscovite as observed in many metamorphicrocks could largely be related to opposing effects of pressureon Na solubility in margarite and paragonite and/or non-equilibriumbetween micas. KEY WORDS: CaNaK mica; margarite; metabauxite; Naxos; sub-micron-scale mica interlayering 相似文献
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The mutual solubility in the system corundumhematite[-(Al, Fe3+)2O3] was investigated experimentally using bothsynthetic and natural materials. Mixtures of -Al2O3 and -Fe2O3(weight ratios of 8:2 and 10:1) were used as starting materialsfor synthesis experiments in air at 8001300°C withrun times of 734 days. Experiments at 840 kbarand 4901100°C were performed in a piston-cylinderapparatus (run times of 0·87·4 days) usinga natural diasporite consisting of 6070 vol. % diasporeand 2030 vol. % Ti-hematite. During the diasporitecorunditetransformation, the FeTiO3 component (1218 mol %) ofTi-hematite only slightly increased, implying that oxygen fugacitywas maintained at high values. Run products were studied byelectron microprobe and X-ray diffraction (Rietveld) techniques.An essentially linear volume of mixing exists in the solid solutionwith a slight positive deviation at the hematite side. Up to1000°C, corundum contains <4 mol % Fe2O3 and hematite<10 mol % Al2O3; at 1200°C these amounts increase to9·3 and 17·0 mol %, respectively. At 1300°Chematite was no longer stable and coexists with the orthorhombic phase . The present results agree with corundum (solvus) compositions obtained inprevious studies but indicate a larger solubility of Al in hematite.The miscibility gap in the solution can be modelled with anasymmetric Margules equation with interaction parameters (2uncertainties): ; ; ; . Application of the corundumhematite solution as a solvus geothermometer is limited because of thescarcity of suitable rock compositions. KEY WORDS: corundum; hematite; corundumhematite miscibility gap; experimental study; Margules model; metabauxite 相似文献
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Porphyroblastic garnet schists from northern Samos contain in their matrix the assemblage Ca‐rich garnet + phengite + paragonite ± chloritoid equilibrated at ~530 °C and ~19 kbar during early Tertiary metamorphism. These high‐pressure/low‐temperature (HP‐LT) metapelitic rocks also exhibit mineralogical and microstructural evidence of an older, higher temperature metamorphism. Large, centimetre‐sized Fe‐rich garnet showing growth zoning developed discontinuous, <0.5 mm thick, Ca‐rich and Mn‐poor overgrowths, compositionally matching small (<1 mm) high‐P matrix garnet. Because the discontinuous garnet rims are in textural and chemical equilibrium with Alpine high‐P minerals, the central parts of the garnet porphyroblasts were found to have formed prior to the Tertiary metamorphism. This is supported by electron microprobe U‐Th‐Pb dating of monazite inclusions yielding partly reset Variscan ages between 360 and 160 Ma. Monazite‐xenotime and garnet‐muscovite thermometry applied to inclusions in the pre‐Alpine garnet yielded temperatures of 600–625 °C (at 3–8 kbar). Prismatic Al‐rich pseudomorphs, possibly after kyanite/sillimanite, and inclusions in garnet composed of white K‐Na mica + quartz ± albite ± K feldspar, interpreted as possible replacements of an intermediate K‐Na feldspar, further support Variscan amphibolite facies conditions. The Samos metapelites thus experienced higher temperatures during the Variscan than during Alpine metamorphism. Diffusional relaxation was very limited between pre‐Alpine garnet and Alpine garnet; both were filled with Alpine garnet along overgrowths and fractures. Fluid‐mediated intergranular element transport, enhanced by deformation, appears crucial in transforming the Variscan garnet into a grossular richer composition during Alpine subduction‐zone metamorphism. At such conditions, dissolution–reprecipitation appears to be a much more effective mechanism for modifying garnet compositions than diffusion. Amphibolite facies conditions are typical for Variscan basement relics exposed in central Cycladic and Dodecanese islands as well as in eastern Crete. The Samos metapelites studied comprise a north‐eastern extension of these basement occurrences. 相似文献
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