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The Idea of Contrasted Differentiation

Published online by Cambridge University Press:  01 May 2009

Arthur Holmes
Arthur Holmes
Affiliation:
University of Durham.
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Extract

As early as 1915, it was made clear by Bowen that, given continuous separation of crystals from the successive residual liquids of a consolidating magma which was initially basaltic in composition, the inevitable result would be a gabbroic rock in depth, a granitic one above, and “various intermediate types in the intermediate layers”. As a necessary condition to the evolution of a magmatic residuum of granitic composition, this process of progressive crystallization differentiation involves the generation of intermediate rock-types from residual liquids of intermediate composition. Certain investigators, however, led by the authors of the Mull memoir (1924), have entirely overlooked the necessity of this condition. Faced with the association of contrasted acid and basic rocks, which is a conspicuous characteristic of many continental central complexes, they have attempted to account for the facts by introducing a modified conception of crystallization differentiation for which Nockolds has recently proposed the name contrasted differentiation. According to this conception, the residual liquid of consolidating basaltic magma is of granitic composition and can be separated in bulk from a crystalline gabbroic phase.

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Type
Original Articles
Information
Geological Magazine , Volume 73 , Issue 5 , May 1936, pp. 228 - 238
Copyright
Copyright © Cambridge University Press 1936

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References

page 228 note 1 Nockolds, S. R., “The Production of Normal Rock Types by Contamination and their Bearing on Petrogenesis,” Geol. Mag., LXXI, 1934, 31–9.CrossRefGoogle Scholar

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page 229 note 1 Holmes, A., and Harwood, H. F.., “The Age and Composition of the Whin Sill and the related Dikes of the North of England,” Min. Mag., xxi, 1928, 508, 511; “The Tholeiite Dikes of the North of England,“ Min. Mag., xxii, 1929, 49.Google Scholar

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page 233 note 2 Incontrovertible evidence that quartz-dolerite, leucocratic “segregations” and granophyric veins have been generated by the action of olivine-dolerite magma on sediment incorporated within it has recently been recorded by E. D. Mountain in a paper on “Syntectic Phenomena in Karroo Dolerite at Coedmore Quarries, Durban” (Trans. Geol. Soc. South Africa, xxxviii, 1936, 93–112). From the lower contact of the sill a band of Table Mountain Sandstone is traced into the dolerite as a “vein” which, 13 feet from the base has become transformed into micropegmatite containing disseminated quartz grains (p. 101). In the neighbourhood of such remnants of sandstone the dolerite “varies from a normal quartz-dolerite to a granophyre” (p. 104). It is concluded that this “pale dolerite” (quartz- and/or micropegmatite-bearing) “is in all cases here produced by silicification of the dolerite-magma through incorporation of T.M.S.” (p. 107).Google Scholar

page 233 note 3 Richey, J. E.., British Regional Geology: Scotland: The Tertiary Volcanic Districts, 1935, 67.Google Scholar

page 234 note 1 In discussion of Tomkeieff, S. I., and Marshall, C. E., “The Mourne Dyke Swarm,” Quart. Journ. Geol. Soc., xci, 1934, 290.Google Scholar

page 234 note 2 Krokström, T.., “On the Ophitic Texture and the Order of Crystallization in Basaltic Magmas,” Bull. Geol. Inst., Upsala, xxiv, 1932, 213.Google Scholar

page 234 note 3 Holmes., A., and Harwood, H. F., “The Age and Composition of the Whin Sill and the Related Dikes of the North of England,” Min. Mag., xxi, 1928, 511.Google Scholar

page 235 note 1 Tomkeieff, S. I., “A Contribution to the Petrology of the Whin Sill,” Min. Mag., xxii, 1929, 117.Google Scholar

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page 236 note 1 The term distillation is used here for simplicity. We do not yet sufficiently understand the nature of magmatic behaviour to describe its manifestations accurately. In addition to intrinsic volatility, which does not appear to be of general importance by itself in petrogenesis, such phenomena as gaseous solution and atomic or ionic mobility are likely to be involved in processes of differential migration. The appearance of “contrasted differentiation” may be simulated by the effects of migration of certain constituents from magmas into xenolithic material: cf. the footnote-reference to Mountain's work on page 233. Mountain points out that whereas Daly (Igneous Rocks, 1933, p. 434) refers to acid veins cutting Karroo dolerite in connection with the origin of acid rocks by syntexis, the same phenomena are cited by Nockolds (1934, p. 35) as emphasizing the idea of contrasted differentiation. Both authors give the same reference. The observed facts, as described by du Toit (15th Ann. Rep. Geol. Comm. Cape of Good Hope, 1911, pp. 120, 122, and 133; and Trans. Geol. Soc. S. Af., 23, 1921, p. 16), clearly foreshadow the proof of syntexis established by Mountain.Google Scholar

page 237 note 1 Fenner, C. N., “Some Magmatic Problems,” Journ. Wash. Acad. Sci., xxiv, 1934, 113–124.Google Scholar