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Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland 总被引：4，自引：1，他引：4
MARKL GREGOR; MARKS MICHAEL; SCHWINN GREGOR; SOMMER HOLGER 《Journal of Petrology》2001,42(12):2231-2257
The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofXFemafic mineral, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from 相似文献
2.
MARKS MICHAEL; VENNEMANN TORSTEN; SIEBEL WOLFGANG; MARKL GREGOR 《Journal of Petrology》2003,44(7):1247-1280
The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +Fe–Ti oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950–750°C,aSiO2 = 0·7–1 and an fO2 of 1–3 log unitsbelow the fayalite–magnetite–quartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at fO2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirine–augiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcite–siderite)ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to 相似文献
3.
The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with aSiO2 = 0·8 and fO2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with aSiO2 = 0·4 and log fO2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF 相似文献
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MARKL GREGOR; ABART RAINER; VENNEMANN TORSTEN; SOMMER HOLGER 《Journal of Petrology》2003,44(6):1097-1120
In Central Dronning Maud Land, East Antarctica, rare metre-sizedlenses of spinel peridotite are enclosed in high-grade metamorphicrocks. The rocks experienced a medium-P granulite-facies metamorphismat 575 Ma and a low-P amphibolite-facies overprint at 530 Ma.The latter is probably related to extensive granitoid magmatismbetween 530 and 500 Ma, which produced large volumes (abouthalf of the outcrops today) of granitic to syenitic rocks aswell as abundant K-feldspar–quartz pegmatites. One ofthe spinel peridotite lenses in the Schirmacher Oasis of CentralDronning Maud Land is crosscut by several small (up to 10 cmwide) veins with a characteristic zoned sequence of mineralassemblages, which was formed by reaction of a hydrous, SiO2-saturatedfluid or pegmatitic melt with the peridotite. The zoned sequenceconsists of the following mineral assemblages (from the centreof the vein towards the outer margin): zone 0, plagioclase +quartz; zone 1, green biotite intergrown with zircon + clinoamphibole;zone 2, cummingtonite + dark brown biotite intergrown with rutile+ clinoamphibole; zone 3, cummingtonite + light brown biotite+ spinel; zone 4, olivine + orthopyroxene + spinel ±clinopyroxene (unaltered peridotite). This sequence was investigatedwith respect to its conditions of formation, modal mineralogy,mineral chemistry, fluid inclusions, and oxygen and hydrogenisotope compositions of selected minerals. Based on the stabilityof cummingtonite and on equilibrium calculations in the MgO–SiO2–H2Osystem and on quartz–biotite oxygen isotope thermometry,the reaction vein formed at 650°C, which is in accord withtypical pegmatite crystallization temperatures. The pegmatiteof zone 0 is interpreted to have formed in an open fissure whereas,on textural grounds, zone 3 replaces former peridotite. On thebasis of mass balance constraints, the boundary between zones1 and 2 is interpreted to approximately represent the formerboundary between peridotite and the open fissure before reaction.Oxygen isotope systematics show that the infiltrating fluidhad an isotopic composition of 9–10 SMOW. All mineralsof the reaction vein with the exception of the inherited spineland olivine in the adjacent peridotite are in equilibrium withsuch a fluid. Spinel in the peridotite is depleted in 18O comparedwith coexisting olivine, which suggests isotopic disequilibrium.Spinel in zone 3 has a distinctly different isotopic compositioncompared with that in the peridotite, apparently approachingbut not reaching equilibrium. The combination of mineral chemistryand mass balance constraints of the modal mineralogy constrainsthe volume change during metasomatism and the direction of elementaldiffusion. It is indicated that Mg, Cr and Ni always diffusedtowards the vein, whereas Si, Al, K, Na, H2O and possibly Fediffused into the peridotite. KEY WORDS: peridotite; metasomatism; pegmatite; diffusion; reaction 相似文献
5.
Reactions occurring during cooling of charnockitic intrusives on the Lofoten Islands produce characteristic diffusion-controlled textures around fayalite and Fe–Ti oxides. Thermobarometry indicates the corona textures formed at 780–840 °C and pressures of 4–10 kbar, whereas the magmatic assemblage of the charnockite (clinopyroxene–olivine–quartz) crystallized at about 850–870 °C and 4 kbar. The succession olivine|orthopyroxene+magnetite|orthopyroxene+garnet and olivine|orthopyroxene+magnetite|amphibole developed where olivine reacted with adjacent plagioclase or K-feldspar, but the modes and the thicknesses of the corona textures vary according to the feldspar type, indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began. Simultaneously, in other parts of the rock, primary magmatic clinopyroxene reacted to amphibole and Fe–Ti oxides reacted to orthopyroxene+garnet coronas or to amphibole. Textures demonstrate significant Al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of H2O probably related to water-release during the last stages of magmatism. Calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand-specimen scale with respect to all elements except for Na and H2O. The larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions, and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the Lofoten Islands. The following set of relative diffusion coefficients (Li/LFe) reproduces the observed textures in the Lofoten rocks: Si=0.82, Mg=0.59, Mn=0.05, Na=0.38, K=0.39, Al=0.05 and Ca=0.07. 相似文献
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The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility 总被引：1，自引：0，他引：1
HALAMA RALF; VENNEMANN TORSTEN; SIEBEL WOLFGANG; MARKL GREGOR 《Journal of Petrology》2005,46(1):191-217
The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCN–YbCN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has 18O values of 7·8 to 8·6and 13C values of –3·9 to –4·6. 18Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in 18O betweencalculated syenitic melts and carbonatites, and the overlapin Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite 相似文献
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