Journal of Petrology | Volume 1 | Number 1 | Pages 178-217 | 1960
© Oxford University Press 1960
research-article |
Pelitic Gneisses with varying Ferrous/Ferric Ratios from Glen Clova, Angus, Scotland
Department of Mineralogy and Petrology, University of Cambridge Washington, D.C.; and the the Geophysical Laboratory, Carnegie Institution of Washington Washington, D.C.
ABSTRACT
The influence of varying contents of ferric and ferrous iron on the mineral assemblages developed in regionally metamorphosed pelitic gneisses from Glen Clova, Angus, is considered on the basis of petrographic, chemical, and X-ray data. The rocks described are essentially quartz-oligoclase-muscovite-biotite-garnet-kyanite-oxide minerals gneisses, with oxidation ratios 
varying from 6 to 75; rocks of widely varying oxidation ratio may be found intimately interbedded.
Increasing oxidation ratios in these rocks are accompanied by increasing amounts of muscovite and iron oxides, and decreasing amounts of biotite and garnet. The relationship between interbedded layers of varying oxygen content may be represented by the equation
Fe³³ eastonite (biotite)³almandine³O muscovite-iron oxides³quartz.
However, the magnesium content of the rocks is contained almost exclusively in biotite, and the manganese content in garnet. Diminishing amounts of these minerals with increasing rock oxidation ratios are thus accompanied by major increases in the MgO/FeO ratios of the biotites, and in the MnO/FeO ratios of the garnets.
The rocks may be more precisely classified on the basis of the oxide minerals which they contain: (1) ilmenite-magnetite-bearing assemblages, embracing rocks of oxidation ratio 0–37; (2) ilmenite-magnetite-hematite-bearing assemblages, developed in rocks of oxidation ratio approximately 40; (3) magnetite-hematite-bearing assemblages, embracing rocks of oxidation ratio in excess of approximately 43. (1) and (3) are interpreted as ‘univariant’ assemblages at constant T and P; for every rock oxidation ratio there is a specific composition for each mineral present, and it is in (3) that the major increase in biotite MgO/FeO ratio and in garnet MnO/FeO ratio is found. (2) is interpreted as an assemblage ‘invariant’ at constant T and P, in which over a limited range of rock oxidation ratios the compositions of the minerals are constant, variations in rock oxygen content being accommodated by varying proportions of magnetite, ilmenite, hematite, and silicate minerals.
The confinement of rocks of varying oxidation ratio to well-defined sedimentary bands suggests that the differences in oxygen content are of premetamorphic, diagenetic origin; a correlation between oxidation ratio and both the manganese and total iron contents of the rocks is believed to support this. This conclusion is in harmony with recent suggestions that during regional metamorphism rocks in general behave as narrowly defined units ‘closed’ to oxygen, the oxygen partial pressure in each unit being determined by the mineral assemblage, and hence the original oxygen content, rather than being externally imposed. It is suggested that the failure of adjacent layers of differing oxygen content to reach equilibrium with each other is the result of a relatively low ‘oxidizing’ or ‘reducing’ capacity of the vapour phase, due to the mass of the solids far exceeding that of the vapour present.
Examples of reduction of ferric iron in many thermal aureoles indicate that the thermal metamorphic environment, unlike the regional, is generally ‘open’ to oxygen. It is suggested that this difference is due to a greater mobility of ‘water’ within the abrupt thermal gradients of an aureole, and to the control of the oxygen/hydrogen content of this ‘water’ by the iron-bearing minerals of the intruded melt.
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