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The Skaergaard Layered Series. Part VI. Excluded Trace Elements 总被引：1，自引：2，他引：1
In contrast to the smooth trends of major elements and mineralcompositions, the excluded trace elements in the SkaergaardLayered Series have an irregular distribution that does notconform to the normal trends of Rayleigh-type fractionation.Their concentrations are about constant or even decline throughthe Lower and Middle Zones before increasing sharply to reachmaximum concentrations 100–200 m above the Sandwich Horizon.As in the case of included elements, the relative concentrationsof excluded elements in coexisting phases deviate widely fromthose predicted by experimentally determined partition coefficientsunder presumed magmatic conditions. This is seen most clearlyin the immiscible melanogranophyres and conjugate ferrogabbros.Although the major elements conform to the experimentally determinedrelations for immiscible liquids, the trace elements do not;they follow a totally independent trend. The abrupt increasein the concentrations of excluded elements in the upper partof the intrusion could plausibly be attributed to an additionof new magma or to a density inversion that resulted in upwardmigration of a late liquid or fluid, but these possibilitiesare inconsistent with the compositional and spatial relationsof the upper parts of the intrusion. Although a late residualliquid certainly migrated upward, the most likely explanationfor the observed distribution of excluded elements is that thepartition coefficients were altered by volatile components,which gradually increased during the early stages of crystallizationthen began to exsolve near the top of the Middle Zone. KEY WORDS: igneous differentiation; Skaergaard intrusion 相似文献
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Re-examination of the Skaergaard Layered Series in the lightof more extensive field work and sampling shows that the lithologiczones vary laterally as well as vertically, in both their bulkchemical composition and their mineralogical assemblages. Themargins of the zones differ from both the central part of theLayered Series and Marginal Border Series in being richer inFeO*, TiO2, K2, P2O2, and most excluded elements. Mafic mineralstend to be more abundant and more iron-rich, plagioclase ismore albitic and more strongly zoned, and apatite and biotiteare more abundant near the margins. When the average compositions of successive zones are compared,the abundances of most excluded components are seen to declineupward as far as Middle Zone then reverse their trends and increasethrough Upper Zone. P2O5 and K2O are negatively correlated inUpper Zones B and C, owing, perhaps, to separation of immisciblefelsic liquids from the iron-rich magma. No evidence has beenfound for introduction of a new batch of less differentiatedmagma. Layered rocks have an average composition that is more maficthan that of homogenous rocks at the same level. Blocks thatfell from the roof have the opposite relation; they are greatlyenriched in felsic components compared to the original compositionsof the Upper Border Series from which they came. Although some of the compositional variations may be consistentwith differing degrees of fractionation of trapped liquids,no consistent relation has been found between the degree offractionation and rates of crystal accumulation or cooling atthe walls. Contamination with the metamorphic wall rocks, eitherby assimilation or by hydrothermal fluids, seems to have hadonly local effects and cannot account for the large-scale variations.At least some of the compositional differences must have resultedfrom late-stage processes that redistributed certain componentsafter the intrusion reached advanced stages of solidification. 相似文献
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The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes 总被引：2，自引：0，他引：2
The initial isotopic ratios of strontium and neodymium in theSkaergaard Layered Series vary both vertically and laterally,on every scale from the intrusion as a whole down to coexistingminerals in a single rock. The magma that filled the Skaergaardchamber was contaminated to various degrees with the metamorphicrocks through which it rose and was never completely homogenizedafter being intruded. The contamination was most pronouncedin contact zones and aureoles around rare xenoliths. The greaterconcentrations of lithophile trace elements in the Upper BorderSeries was previously attributed to assimilation of buoyantfragments of gneiss that collected under the roof, but mostof the rocks of the Upper Border Series are isotopically indistinguishablefrom those of the Layered Series. It is doubtful, therefore,that this part of the intrusion assimilated much more of themetamorphic basement than did the rest of the magma. Similarly,the marked increase in the concentrations of excluded elementsin the upper part of the Layered Series is not matched by achange in the isotopic character of the rocks and cannot beattributed to a later influx of new magma. Analyses of mineralsseparated from rocks with exceptionally mafic or felsic modalcompositions revealed marked inhomogeneities in the isotopiccompositions of their constituent minerals. For example, coexistingplagioclase and pyroxene from closely associated anorthositesand pyroxenites have very different initial isotopic ratiosof both strontium and neodymium. The same is true of mafic andfelsic layers in modally graded gabbros. These differences areunrelated to the low-temperature alteration shown by oxygenisotopes. They must have been introduced when the original gabbrowas largely crystallized and underwent local metasomatic replacementby nearly mono-mineralic mafic and felsic assemblages. KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes 相似文献
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