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Electron-microprobe analyses are presented for pyroxene, amphibole,and biotite from monzonite, granodiorite, and granite at Finnmarka,Norway. Compositional trends measured in biotite, present inall three rock types, and in amphibole, present in the monzoniteand granodiorite, are markedly atypical and are interpretedas reflecting crystallization under progressively more oxidizingconditions. The average Fe/Fe + Mg for biotites from successivelymore silicic rock types changes from 0.64 0.35 0.28, and foramphiboles changes from 0.58 in the monzonite to 0.29 in thegranodiorite. Analyses of selected areas within amphibole grains in the granodioriteshow marked chemical variations, although single-crystal X-rayphotographs are sharp and do not reveal multiple phases. Onthe basis of 33 such analyses, four coupled substitutions areidentified as operative; the most unusual finding is the relationof 1 Ti cation to 4 AlIV cations in the unit cell. Variationswithin individual amphibole grains of the granodiorite resemblechanges noted in evolution of amphibole composition from monzoniteto granodiorite and are interpreted as reflecting progressiveoxidation. Consideration of these data for the mafic silicates, data forthe opaque oxides, and the extensive formation of sphene inthe granodiorite, has allowed development of schematic reactionsand an overall picture of magmatic environment and evolutionat Finnmarka. Crystallization apparently took place at PH2oof 1000 bars or less and a temperature of about 700 °C.The trend of oxidation during differentiation is more extremethan any heretofore reported. Amphiboles, as well as biotites,may participate in oxidation reactions and may reflect the oxidation-reductionprocesses that occurred during magmatic evolution. 相似文献
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Rare Earth Element Evidence for the Petrogenesis of the Banded Series of the Stillwater Complex, Montana, and its Anorthosites 总被引：1，自引：0，他引：1
LOFERSKI PATRICIA J.; ARCULUS RICHARD J.; CZAMANSKE GERALD K. 《Journal of Petrology》1994,35(6):1623-1649
A rare earth element (REE) study was made by isotope-dilutionmass spectrometry of plagioclase separates from a variety ofcumulates stratigraphically spanning the Banded series of theStillwater Complex, Montana. Evaluation of parent liquid REEpatterns, calculated on the basis of published plagioclase-liquidpartition coefficients, shows that the range of REE ratios istoo large to be attributable to fractionation of a single magmatype. At least two different parental melts were present throughoutthe Banded series. This finding supports hypotheses of previousworkers that the Stillwater Complex formed from two differentparent magma types, designated the anorthositic- or A-type liquidand the ultramafic- or U-type liquid. On the basis of our data,one melt has a REE pattern with a distinctive shallow slopeand is represented by samples from the thick, massive Anorthositezones I and II (AN I and AN II) of the Middle Banded series.Although samples from AN I and AN II are separated by as muchas 1400 m stratigraphically, they have remarkably similar calculatedparent liquid characteristics, with (Ce/Sm)n = 1.7–1.9,(Nd/Sm)n = 1.3–1.4 and (Ce/Yb)n = 2.9–4.6 (wheren denotes chondrite-normalized). These calculated liquids areprobably close to representing A-type magma. In addition, plagioclase-bronzitecumulates from Norite zones I and II (N I and N II), althoughthought to be U-type cumulates, contain plagioclase that hasA-type REE characteristics, implying that A-type magmas wereinjected into the magma chamber during formation of those zones.In contrast, calculated parent liquids of cumulus augite-bearingrocks have REE patterns that display distinctly steeper slopesthan the A-type REE pattern. The extreme is the calculated parentliquid of a plagioclase-bronzite-augite cumulated with (Ce/Sm)n= 2.9, (Nd/Sm)n = 1.7, and (Ce/Yb)n = 10.1. Analysis of published REE and Nd isotopic data for Stillwatercumulates reveals similarities between AN I, AN II, and otherthin plagioclase cumulate layers in the Lower and Upper Bandedseries, which supports the notion that they were all derivedfrom similar (A-type) parent melts. In contrast, plagioclaseseparates from cumulus augite-bearing rocks display light REEand Nd isotopic characteristics that are similar to U-type cumulatesfrom the Ultramafic series as described by previous studies.Thus far, the only cumulates from the Banded series that displayU-type REE and Nd isotopic characteristics are those that containcumulus augite. Therefore, cumulus augite appears to be an importantindicator of magmatic parentage. The REE and Nd isotopic ratios show erratic variation with stratigraphicposition, indicating that the magmas from which the Banded seriescrystallized were injected at various levels into the magmachamber. Different cumulate types crystallized from discreteliquids, as indicated by the correlation between REE signatureand cumulate type. Samples from Olivine-bearing zones III andIV (stratigraphically between AN I and AN II) display a rangein REE ratios; e.g., (Ce/Sm)n = 1.8–2.8 and (Ce/Yb)n =3.9–6.1, results that rule out the crystallization ofthe Middle Banded series from a single magma type. Furthermore,the possibility that AN I and AN II are directly related tothe underlying Ultramafic series, either as flotation cumulatesor as crystallization products of expelled liquids, is not substantiatedby the REE data because the calculated parent magma of AN Iand AN II was different from that of the Ultramafic series asdefined by previous studies. The REE data of this study further constrain interpretationsof published Pb isotopic data (Wooden et al., 1991) and indicatethat the magmas from which the Stillwater Complex formed werederived from two sources that had only small differences inPb isotopic composition. The REE and isotopic data, as wellas crystallization sequences of the two main parental magmas,indicate that the magmas were probably derived from two closelyrelated upper-mantle sources, one harzburgitic and the otherlherzolitic in composition, resulting in the U-type and A-typemagmas, from which orthopyroxene crystallized before and afterclinopyroxene, respectively. Both sources had been enrichedin large-ion lithophile elements, probably owing to mantle metasomatism. 相似文献
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Microprobe analyses of the oxide mineral assemblages from monzonitic,granodioritic, and granitic units of the Finnmarka igneous complex,Oslo area, Norway, disclose a remarkable trend of Mn enrichmentin ilmenite with differentiation. Ilmenite-pyrophanite solidsolutions containing up to 63 mole per cent MnTiO3 have developedunder apparently unusually oxidizing conditions, a conclusiondeduced in part from the fact that Fe2+ is more readily oxidizedthan Mn2+ and was thus preferentially extracted. Several differentmineral reactions may have been involved in this process. Geothermometricapplications of oxide Mn contents must be made with caution. Additional mineralogic complexities, e.g. ilmenite lamellaein magnetite, magnetite lamellae in ilmenite, and, in the granite,the breakdown of ilmenite to rutile+hematite, all provide insightinto the conditions of emplacement and cooling history of thecomplex. 相似文献
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