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Material Transfer and Local Equilibria in a Zoned Kelyphite from a Garnet Pyroxenite, Ronda, Spain 总被引：1，自引：2，他引：1
A zoned kelyphite after garnet, from a garnet pyroxenite layer,the Ronda peridotite. Spain, has been studied and the mechanismof kelyphite formation is discussed. The kelyphite is an extremelyfinegrained symplectitic mixture of orthopyroxene, spinel, olivine,plagioclase, and ilmenite. It is concentrically zoned, formingthree mineralogical subzones. They are, from adjacent to a garnetgrain toward a clinopyroxene side, zone I (orthopyroxene+spinel+ plagioclase), zone II (olivine+spinel+plagioclase), and zoneIII (olivine+plagioclase). The analysis of phase equilibriashows that this mineralogical zonation can develop stably asa result of the presence of chemical potential gradients. Onthe basis of microprobe chemical analyses for each zone, materialtransfer across the zone that took place during the kelyphitizationwas quantitatively evaluated, and by locating the initial grainboundary between garnet and clinopyroxene grains and by writingmetasomatic reactions for each zone boundary, a simple dynamicmodel for the kelyphite formation is proposed. The kelyphiteformation probably took place when the host Ronda peridotiteascended from the upper mantle to the crust. It involved a co-operativebreakdown of the garnet and aluminous clinopyroxene, being accompaniedby a material transfer across the zone boundaries. By examiningthe Fe-Mg partitioning between olivine, spinel, and orthopyroxenein the kelyphite and by examining the Al content of the orthopyroxene,an attainment of local equilibrium has been confirmed, and thephysical conditions of the kelyphite formation have been estimatedto be 620–700C and 4–8 kbar. 相似文献
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The Ronda Peridotite: Garnet-, Spinel-, and Plagioclase-Lherzolite Facies and the P-T Trajectories of a High-Temprature Mantle Intrusion 总被引：1，自引：0，他引：1
The Ronda peridotite is a high-temperature, alpine-type peridotiteemplaced in the internal Zone of the Betic Cordilleras, southernSpain. Using the mineral assemblages of the peridotite and maficlayers, the peridotite mass has been subdivided into 4 zonesof mineral facies: (1) garnet-lherzolite facies, (2) ariégitesubfacies of spinel-lherzolite facies, (3) seiland subfaciesof spinel-lherzolite facies, and (4) plagioclase-lherzolitefacies. It is proposed that this mineralogical zonation developedthrough a syntectic recrystallization of a hot (1100 to 1200°C),solid mantle peridotite during its ascent into the Earth's crust. Coexisting minerals from 12 peridotites covering all the mineralfacies above were analysed with an electron microprobe. Corecompositions of pyroxene porphyroclasts are constant in allmineral facies and indicate that the peridotite was initiallyequilibrated at temperatures of 1100 to 1200 °C and pressuresof 20 to 25 kb. In contrast, the compositions of pyroxene neoblastsand spinel grains (which appear to have grown during later recrystallization)are well correlated with mineral facies. They indicate thatthe recrystallization temperature throughout the mass is moreor less constant, 800 to 900 °C, but that the pressure rangesfrom 5–7 kb in the plagioclase-lherzolite facies to 12–15kb in the garnet-lherzolite facies. Therefore, variation inpressure appears to be primarily responsible for the four mineralfacies types. A pressure range of at least 5 kb appears to be too large tohave been maintained (at the same time) in a mass as small asthe Ronda peridotite. Dynamic cooling may explain the observedvariation in the recrystallization pressure; i.e. during theintrusion of the peridotite body, different parts of the bodyhave followed different P-T paths in response to different localcooling rates. Comparing the inferred P-T paths for the peridotitewith published melting temperature of peridotite and mafic rocks,it is concluded that the peridotite did not go through partialfusion during the ascent. A hypothetical, diapiric uprise thatcaused partial fusion and igneous differentiation of the mantleperidotite is considered to be a separate event prior to theascent that started from about 70 km depth in the upper mantle.Estimates of cooling rates and of Al diffusion rates in pyroxenessuggest that the ascent rate of the peridotite body was greaterthan 1 meter/year.
* Present address: Dement of Earth Sciences, Toyama University, Gofuku, Toyama 930, Japan. 相似文献
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MENZIES M.; OBATA M.; ARAI S.; BODINIER J.-L.; KOHLSTEDT D.; FREY F.; OZAWA K.; SHIMIZU N.; VISSERS R. 《Journal of Petrology》2004,45(2):221-222
The papers in this thematic issue of Journal of Petrology weredelivered at the Fourth International Workshop on Orogenic Lherzolitesand Mantle Processes, which was held in Samani, Hokkaido, Japan,between August 26 and September 3, 2002. Fifty oral presentationswere given and 45 posters were displayed during the meeting,and the research papers in this issue provide an 相似文献
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Transformation of Spinel Lherzolite to Garnet Lherzolite in Ultramafic Lenses of the Austridic Crystalline Complex, Northern Italy 总被引：7，自引：4，他引：7
Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 1100–1300?Cfor the formation of the primary spinel lherzolite, and 700–800?Cfor that of the fine-type peridotites. A pressure range of 16–28kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed. 相似文献
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A hypothetical model is proposed to explain the origin of compositionaldiscontinuities in the layering observed in orogenic lherzolites.The observed collinearity of the whole-rock peridotite compositionsis best explained in terms of partial melting and melt segregation.The presence of chemical discontinuities implies that melt segregationincludes an abrupt and discontinuous process. A key conceptin the model is the topological transformation of melt geometryin partially molten rocks responding to the equality and inequalityof the fluid pressure and solid pressure, which may be realizedin a gravitational field. It is emphasized that the percolationthreshold is a critical boundary, beyond which a rapid microstructuralchange occurs in response to the change of local fluid pressure,thus causing a rapid increase of permeability. The model impliesthat the mode of melting is closer to batch melting than tofractional melting in the upper mantle. KEY WORDS: critical phenomenon; partial melting; percolation threshold; Horoman peridotite; melt segregation 相似文献
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