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Dolerites Associated with the Karroo System, South Africa

Published online by Cambridge University Press:  01 May 2009

Reginald A. Daly  and
Tom F. W. Barth
Reginald A. Daly
Affiliation:
Harvard University, Washington.
Tom F. W. Barth
Affiliation:
Geophysical Laboratory, Washington.
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Summary

—The field impression of a high degree of uniformity in the composition of the intrusive Karroo dolerites over a vast area is confirmed by new chemical and microscopical studies. The pyroxene characteristic of the dolerites is a typical pigeonite, which, however, does not display any twinning on (100). The plagioclase is potash-free, usually a labradorite, but it has been observed that a homogeneous crystal of plagioclase may be in twin position to another homogeneous crystal of a very differently composed plagioclase. The close resemblance of the doleritic liquid to the artificial liquid taken by Bowen to represent basaltic liquid is noted, and his conclusion that pyroxene and feldspar should crystallize simultaneously in natural basaltic liquid is confirmed.

Conceivably the doleritic liquid was quite original and not a differentiate of any earlier liquid. However, analogies like those with the tholeiites and similar hypabyssal rocks of Great Britain suggest that in South Africa, as in Great Britain, the liquids of these hypabyssal rocks were derived from the slightly more femic plateau-basalt.

On that assumption the question of the mode of differentiation arises. The fractional crystallization of plateau-basalt, as now described by Bowen and other leaders, does not appear competent to explain the abnormally low soda of one of the analyzed dolerites, nor the excess of soda and total alkalies in plateau-basalt respectively over the soda and total alkalies of the Karroo dolerite. Further, the settling-out of early olivine does not explain the excess of (total) FeO in plateau-basalt over the (total) FeO in the Karroo dolerite. The actual relations indicate the need of renewed examination of the theory of magmatic differentiation by crystal-fractionation. In any case many more data are required before it is possible to decide upon the precise relation of the Karroo dolerite to a parental magma. Possibly such additions to knowledge may annul present difficulties in the way of accounting for the composition of the dolerite.

Type
Original Articles
Information
Geological Magazine , Volume 67 , Issue 3 , March 1930 , pp. 97 - 110
Copyright
Copyright © Cambridge University Press 1930

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References

page 97 note 1 The name “dolerite” is here used in deference to the tradition of the South African geologists. Yet the rocks considered are fresh diabases in the sense given this word in America, and also on the continent of Europe, now that the age criterion and also the alteration criterion have there been so generally given up. The Committee on British Petrographic Nomenclature (Miner. Mag., vol. xix, 1921, p. 139) recommends the complete disuse of “diabase”, but this suggestion has not been accepted by American writers or by those of continental Europe. Is there not some advantage in retaining “diabase” as the name for the intrusive rocks with the universally recognized diabasic composition and texture, while restricting “dolerite” to similar rocks of extrusive character?Google Scholar

page 98 note 1 du Toit, A. L., The Geology of South Africa, Edinburgh and London, 1926, p. 283.Google Scholar

page 98 note 2 Pigeonite is a pyroxene rich in clino-enstatite and/or clino-hypersthene. It was so named by Winchell, A. N. (Amer. Geol., vol. xxvi, 1900, p. 199).Google Scholar See also Winchell, N. H. and Winchell, A. N., Elements of Optical Mineralogy, 2nd ed., New York, pt. ii, 1927, p. 181.Google Scholar

page 102 note 1 Prior, G. T., Ann. Natal Museum, vol. ii (2), 1910, p. 149. Its norm, copied from Washington's Tables, is shown in col. 5, Table III.Google Scholar

page 104 note 1 Bowen, N. L., The Evolution of the Igneous Rocks, Princeton, 1928, p. 66.Google Scholar

page 107 note 1 Bailey, E. B. and others, “Tertiary and Post-Tertiary Geology of Mull, etc.,” Mem. Geol. Survey Scotland, 1924, p. 31.Google Scholar Bowen, N. L., The Evolution of the Igneous Rocks, Princeton, 1928, pp. 75, 316, etc.Google Scholar

page 107 note 2 Washington, H. S., Bull. Geol. Soc. America, vol. xxxiii, 1922, p. 797.Google Scholar

page 108 note 1 Fenner, C. N., Amer. Journ. Science, vol. xviii, 1929, p. 225.CrossRefGoogle Scholar

page 109 note 1 Pure crystal-fractionation of basaltic liquid, implying increase of the alkalies in the liquid during the process, does not obviously account for the relations of the alkalies in average diabase (90 analyses averaged) to those of average quartz-diabase (12 analyses averaged), the two kinds of rock being supposed to represent successive liquids in the differentiation. While the average diabase gives 3·13 per cent of soda and 4·11 per cent of total alkalies, the quartz-diabase average gives only 2·64 and 3·83 per cent respectively. The corresponding proportions are therefore the reverse of those deduced from the ruling theory of fractional crystallization.

Again, the average quartz-diabase has 5·13 per cent of Fe2O3 and 8·92 per cent of FeO, while the average diabase has only 3·91 and 7·93 per cent respectively. Similarly the quartz-diabase of the Palisades sheet is richer in iron than the mean rock of the sheet, one of the few bodies where crystal-fractionation of basaltic liquid can be assumed with some confidence. Whatever be the explanation, this parallel appears to strengthen Fenner's conclusion that processes other than pure crystal-fractionation have been at work, or else to indicate the need of modifying the usual statement of the early crystallization of basaltic liquid.

page 109 note 2 That some iron oxide did accompany the olivine of the Palisades (New Jersey) sheet, when the olivine-diabase layer was developed in that sheet by the gravitative concentration of olivine-substance, is suggested by the excess of totalized FeO in the layer over that in the chill-phase of the sheet, which doubtless represents the original magma closely. Similarly, the picrite-basalt (oceanite) of Hawaii is richer in totalized FeO than the dominant Hawaiian basalt, from which the picrite-basalt seems to have been derived. So far as they go, these facts do not support the contention of Fenner, C. N. (Amer. Journ. Science, vol. xviii, 1929, p. 238) that the early crystal-fractionation of basaltic magma should enrich the residual liquid with iron; for obviously this liquid and the layer of concentration of early-formed olivine cannot both be enriched in iron.Google Scholar