Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)
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Keywords:
Chromite
Layered intrusion
Pigeonite
Pyroxene
Trace element
Incompatible element
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Pigeonite
Troilite
Pyroxene
Ilmenite
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Abstract The Somerset Dam Layered Basic Intrusion is probably a sub‐volcanic magma chamber, and it consists of 20 saucer‐shaped layers composed of troctolites, olivine gabbros, ferrigabbros, and leucogabbros. The layered sequence is 1650 ft (500 m) thick, and comprises several repetitions of a standard pattern termed a zone. Each zone is generally composed of four layers, and successive mineral assemblages from the base upwards are: plagioclase‐olivine, plagioclase‐augite‐olivine, plagioclase‐augite‐magnetite‐ilmenite, and plagioclase‐uralite. Pronounced modal and textural changes define the boundaries between these layers. Within a zone, systematic variation in the proportions and compositions of minerals is thought to be the result of a slight decrease in temperature, and an increase in the partial pressures of water vapour and oxygen from the base to the top. Repetition of zones is explained by a mechanism involving periodic renewals of magma. Lack of progressive changes in mineral compositions and proportions from the base to the top of the layered sequence is also a consequence of the magma composition, the control of water vapour pressure, and the limited range of crystallisation temperatures. Gravitational settling of early minerals does not explain the variation within a zone, or the small‐scale rhythmic layering that is locally developed, and it is concluded that diffusion has been an important control.
Layered intrusion
Ilmenite
Xenolith
Breccia
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Chromite
Troilite
Ordinary chondrite
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The estimated initial plagioclase compositions and plagioclase/liquid partitions for $$X_{An}$$ in the Kiglapait intrusion and Makaopuhi lava lake are the same: $$An_{67}$$ crystal, $$An_{57}$$ liquid. The Kiglapait residuum amounts to 0.01% by volume at $$An_{11.5}$$, whereas the Makaopuhi residuum amounts to 3% at $$An_{6.4}$$. The large difference in secular plagioclase variation can be ascribed only to augite, which is abundant in the lava lake and causes strong depletion of Ca. The effective secular fractionation for $$X_{An}$$ can be described as a function of CIPW-normative di/(di + fsp), here denoted $$F'_{di}$$, by $$D^{eff} = 1.81F'_{di}+ 1.04$$, where $$D^{eff}$$ represents the effective distribution coefficient $$X^{plag}_{An}/X^{lig}_{An}$$. This relation holds for systems ranging at least from olivine tholeiite to high alumina basalt in bulk composition. Plagioclase fractionation alone accounts for a bulk D of only about 1.17. Augite is the major cause of extreme An depletion. These relations can be used to estimate the stage of fractionation at several points in the Skaergaard intrusion, and they account for the small amount of plagioclase variation typically seen in augite-poor massif anorthosites.
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Abstract Experimental data from the system CaO-MgO-Na 2 O-Al 2 O 3 -SiO 2 and from the system CaO-MgO-FeO-Al 2 O 3 -SiO 2 are used to show the nature of the changes in composition, with temperature and with pressure, of liquids in equilibrium with augite, plagioclase, and olivine as MgO in the system CaO-MgO-Al 2 O 3 -SiO 2 is partially replaced by FeO and as Na 2 O replaces CaO. As differentiation of tholeiitic basalts proceeds, Fe/Mg increases, Na/Ca increases, and normative hypersthene increases and these effects alter the solubilities, and hence the proportions of augite, plagioclase, and olivine which co-precipitate. In particular olivine becomes more soluble (less is precipitated) as Fe/Mg increases; plagioclase becomes more soluble but augite less soluble as Na/(Na + Ca) increases; augite also becomes less soluble as normative hypersthene increases, and as pressure is reduced during ascent. Experimental data from remelted ocean tholeiites encounter the same equilibrium and are presented and compared with the data from synthetic systems. Some natural examples of daughter liquids related to parents by crystallization of augite, plagioclase, olivine, are combined with the experimental data and with some model calculations to demonstrate that only a very limited range of proportions of augite : plagioclase: olivine (approximately 30 : 50 : 15 to 20 : 60: 20) produces daughter liquids which lie on the major element variation of the tholeiitic basalt series. Other proportions lead to daughter liquids which are not basalts. In the light of these restrictions, several recent publications are reinterpreted as examples of fractionation of augite, plagioclase, and olivine, rather than examples of partial melting. The recognition of the small effect that massive (50 to 83%) crystallization has on basaltic chemistry by contrast with the large effect that small accumulations of phenocrysts can have is particularly emphasized in this reinterpretation. Comparisons and possible relationships are suggested between some low-K 2 O, low-TiO 2 tholeiites; some calcic-, aphyric-, dykes of some tholeiitic provinces; some chilled margins of some layered intrusions; some basalts from Iceland, and some basalts from the ocean ridges.
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Summary Petrofabric analysis of five specimens of laminated gabbro from the Centre 3 igneous complex, Ardnamurchan, was carried out by measuring orientations of grains of plagioclase, olivine, and augite with a Universal-stage. Linear alignments of primary precipitate mineral grains within the laminar fabrics were revealed by plots of data for coexisting plagioclase and olivine or augite. The plagioclase grains involved have tabular forms elongated parallel to their a -axes (rather than their c -axes). The olivine grains have flattened ovoid forms and the augite grains have sub-ophitic forms developed about prismatic primocrysts. In all specimens plagioclase displayed alignment of long grain-axes directed approximately down the dip of the lamination, save for one case in which a weak oblique alignment was observed. These lineations were reproduced by accompanying olivine and augite. The alignments are tentatively ascribed to flowage of a viscous boundary zone to the magma body in which crystal growth was taking place.
Phenocryst
Anorthosite
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