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A study of okaite and associated rocks near Oka, Quebec Davidson, Anthony

Abstract

The Oka complex is the most westerly of the Monteregian alkaline intrusions. It is a composite intrusion formed by the introduction of basic alkaline and carbonatite rocks along arcuate fractures. There are two overlapping ring complexes; these are situated along the crest of the north-west trending anticlinal Beauharnois axis. Precambrian gneissic and granitic rocks have been fenitized within a few hundred feet of contact with the complex. Okaite is found only within the north-west half of the Oka complex. It is associated with rocks that are closely related to it in mineralogy and in mode of occurrence; among these are jacupirangite and calc-silicate carbonatite. Other rocks within the complex, such as ijolite, urtite, and melteigite, do not appear to be related to okaite in the same way. Okaite and related rocks were studied mainly by thin section examination. Rock forming minerals were identified, in part by X-rays, and the relations between them were established. Melilite is the main rock forming mineral, and varying amounts of titanaugite, hauyne, nepheline, biotite, and calcite are present. The accessory minerals are magnetite, perovskite, and apatite. Fresh rocks have a granitic texture. Microscopic structures such as magmatic reaction rims and exsolution intergrowths are conclusive evidence of igneous origin. On the basis of mineral relations, the rocks are divided into the rock types jacupirangite, pyroxene okaite, okaite, melilite okaite, melilite-calcite rock, and calc-silicate carbonatite. These rock types constitute what has here been called the okaite series. Melilite, titanaugite, and the opaque minerals from rocks of the okaite series were studied in detail. Optical properties of melilite were determined, and are compared with those of melilite from similar occurrences. The chemical composition of natural melilite can be represented by a proportion of the three molecular types sodium melilite, gehlenite, and akermanite. It is found that melilite in okaite and in similar rocks from other localities contains these type molecules in the approximate ratio 35: 10: 55 respectively. Optical properties of titanaugite were determined; chemical analysis shows that titanaugicte from jacupirangite is a highly aluminous pyroxene containing more calcium than is normal. The opaque minerals were studied in polished sections. Exsolution intergrowths between magnetite and a spinel, identified as hercynite, are present in all rocks of the okaite series, and suggest a high temperature origin. Titanium minerals are not exsolved from magnetite which is therefore assumed to be a low-titanium variety. Many of the okaitic rocks are altered. This is primarily due to hydrothermal replacement of melilite and hauyne by a variety of secondary minerals. Alteration was accomplished by hydrothermal fluids derived from crystallizing magma. Four types of alteration are described; these four assemblages are gradational and represent differing temperature conditions. Little or no chemical changes were involved in alteration. Common secondary minerals are vesuvianilte, diopside, calcite, cebollite, zeolite, and garnet. The okaite intrusion is compared with other occurrences of melilite bearing rocks. The rocks at Iron Hill, Colorado, at Turja, Kola Peninsula, and near Gwasi, Kenya, are similar to those at Oka both chemically and in mode of occurrence. There is a notable similarity between the rocks of the okaite series and the basic alkaline rocks that are known to have been formed by limestone syntexis at Scawt Hill, Northern Ireland. Consideration of the stability of melilite leads to the conclusion that the okaite series magma crystallized at temperatures between 700°C. and 800°C. at a depth of less than two kilometers. Temperatures at which alteration took place varied from above 600°C. to below 300°C. The sequence of intrusion is not altogether clear; it appears that specific rock types were not necessarily associated in time. The okaite series does not seem to be directly related to ijolite, urtite, and melteigite; these rocks may represent fenites that have undergone rheomorphism. It is postulated that the okaite series was derived from a gabbro or an essexite magma that assimilated limestone, first by reaction with, and then by solution of, calcium carbonate. It is possible that the ultimate effect of this process was to produce a carbonatiite magma.

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