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The solubilities in the system CaSO4–NaCl–H2O, at1 atm pressure and 35?, 50?, and 70?C, have been determinedby approaching the solubility curves from both the undersaturationand supersaturation sides. The experimental runs are of longduration, as much as 3 months; these rates should be commensuratewith those of the appropriate geological processes, and so theresults should be directly applicable to the interpretationof evaporite deposits. 相似文献
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Prehnite- and/or pumpellyite-bearing meta-igneous rocks arefound on the west side of the Appalachian metamorphic belt (1)near Jonestown, south-eastern Pennsylvania; (2) on RensselaerPlateau, eastern New York; (3) near Quebec City, Quebec; and(4) at Little Port, Humber Arm, western Newfoundland. The assemblagescritical to determining the conditions of metamorphism are (1)chlorite-epidote-hematite-pumpellyite-prehnite; actinolite-chlorite-hematite-pumpellyite-stilpnomelane;(2) actinolite-chlorite-epidote-stilpnomelane; chlorite-epidote-pumpellyite-stilpnomelane;chlorite-epidote-hematite-pumpellyite; (3) chlorite-epidote-hematite-pumpellyite-stilpnomelane;chlorite-epidote-pumpellyite-prehnite; and (4) chlorite-epidote-prehnite;chlorite-prehnite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite.One pumpellyite-bearing rock from western Newfoundland showsa later vein of analcime-calcite. All the assemblages also includequartz, sphene, calcite, K-mica, and albite. Analysis of the mineral assemblages by the Schreinemakers methodfor the phases actinolite-chlorite-epidote-hematite-prehnite-pumpellyite-stilpnomelaneshows that the different localities can be assigned differentmetamorphic grades. Though the detailed results of the Schreinemakersanalysis depend on the assumed source of ferric iron in epidote,the major conclusions are not affected. The thermodynamic roleof calcite is more problematic, but it appears that CO2 didnot behave as a boundary-value component during metamorphism.If calcite is treated as an excess phase, the Schreinemakersbundle decomposes to a net in a multisystem. Plotted on sucha net, the various localities again occupy different parts signifyingdifferent metamorphic grades. The occurrence of pumpellyite-bearing assemblages on the westflank of the northern Appalachian metamorphic belt might suggestthat these assemblages, contrary to the ideas of Miyashiro andof others, do not indicate high-pressure and low-temperaturetype of metamorphism. These assemblages, however, are compatiblewith an alternative interpretation as remnants of a high-pressure,low-temperature Taconic metamorphic regime, whose imprint withinmost of the Appalachian metamorphic belt has been obliteratedby later events. Such a reconstruction is compatible with thesuggestion that this zone, lying near the margin of the earlyPaleozoic craton, was an active subduction zone during the Taconicorogeny. 相似文献
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Aluminum Enrichment in Silicate Melts by Fractional Crystallization: Some Mineralogic and Petrographic Constraints 总被引：12，自引：0，他引：12
The degree of aluminum saturation of an igneous rock may bedescribed by its Aluminum Saturation Index (ASI) defined asthe molar ratio Al2O3(CaO + K2O + Na2O). One suggested originfor mildly peraluminous granites (ASI between 1 and about 1.1)is by fractional crystallization of subaluminous (ASI < 1)magmas; hornblende, having ASI < 0.5, could be a major drivingforce in such a fractionation process. The efficacy of the processdepends not only on precipitation of hornblende and its effectiveremoval from the reacting system, but on the composition andnature of other coprecipitating phases, weighted by their modalabundances in the reactive system. Precipitation of feldspar(ASI = 1), for instance, would retard or even prevent aluminumenrichment in the melt if the ASI of melt is < 1, but wouldenhance such evolution if the ASI of the melt is > 1. Discussionof the efficacy of any mineral must be made in the context ofthe total reacting system. For hornblende to effectively cause a melt to evolve into aperaluminous composition, it must be able to coexist with peraluminousmagmas. Experimental phase equilibrium data show that at pressure> 5 kb hornblende can coexist with strongly peraluminousmelts (ASI {small tilde} 1.5). Scantily phyric volcanic rocksshow that hornblende can coexist with granitic magma havingASI {small tilde} 1.1 –1.2. The aggregate ASI of last-stageminerals of a typical granite is less than this value; therefore,even after hornblende has reacted out, the residual magma maybe expected to continue to evolve toward more aluminous compositions. Potentials and problems of using coarse-grained granitic rocksto probe courses of magmatic evolution are illustrated by asuite of samples from the Grayling Lake pluton of southwesternMontana. Such rocks probably always contain a large cumulatecomponent in their texture and should not be used as primarymeans to test the occurrence or efficacy of a fractionationprocess that might lead to peraluminous melts. The process isunlikely to give rise to peraluminous plutons of batholithicdimensions. A simple differential equation is presented thatallows the direct use of petrographic data (mineral chemistryand modal abundance) to predict the path of incremental evolutionof a given magma. 相似文献
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