On the Spilitic Rocks

• Published in: Geological Magazine
• Main Author: Sundius, N.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1930
• Subjects: Geology; Kiruna
• Online Access: http://dx.doi.org/10.1017/s0016756800002582; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756800002582

An average analysis is made of an altered intrusive greenstone occurring in the zone of the Kiruna greenstones with the special object of proving whether any local change of chemical composition is occasioned during the alteration processes by the uneven distribution of epidote, as was earlier supposed by the writer. The analysis confirms this, but it also shows that chemical inhomogenities must originally have occurred in the rock-mass. The relations found thus do not indicate great local changes of the chemical composition during the mineral alteration processes still less a change of the bulk composition of the rocks. A comparison of a number of analyses of spilites and related An-richer rocks affords the following facts: among them there occurs a small group of An-rich members, which are designated here as diabases. Most of the analyses refer to rocks richer in albite, which are here called spilites in the proper sense. In the felspar diagram the projection points of the calculated felspar mixtures of spacing the rocks form a narrow zone close to the plagioclase line extending from 7 per cent An to 58 per cent An. As the dividing point between the diabases and the spilitic members a mixture with about 40 per cent An is accepted. When adding to the diagram the keratophyric rocks associated with the spilites, the whole distance from about 60 per cent of An to the immediate vicinity of the Ab-corner would be filled up. Between the spilites and other rocks of equal femic composition and of about equal SiO 2 content there exists in the felspar diagram an area in which very few rocks are represented. Petrographically the spilites are thus distinguished as a special rock-type. The boundary towards the ordinary basalts and diabases at the An-rich end of the series on account of the relations of the felspars alone is more dubious, as the individual basalts or diabases may be very poor in potash, but a difference seems to exist here also. Other chemical qualities characteristic of the spilites are deficiency in ...