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Reconstitution processes in shales, slates, and phyllites

Published online by Cambridge University Press:  14 March 2018

Alfred Brammall
Alfred Brammall*
Affiliation:
Imperial College of Science and Technology, London
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Extract

It is the author's good fortune to collaborate with Dr. J. W. Evans in describing the rock specimens which he collected on the expedition to Caupoliean in Bolivia during 1901-2. These specimens contain a very large proportion of shales, slates, and phyllites, and in their investigation certain petrographic methods devised for a detailed study of the slates of North Wales, Devon, and Skiddaw were followed. The present paper is intended primarily to describe these methods as applied particularly r the Bolivian rocks and to a Skiddaw slate facies which, for purposes of comparison and contrast, is of special interest. It is not suggested that between groups of rocks from places so widely separated as Bolivia and Skiddaw close correlation is possible; the products of the reconstitution processes which the two groups record differ considerably in detail; but as providing data for the study of the mechanism of these processes the two groups can be conveniently examined together.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 19 , Issue 94 , September 1921 , pp. 211 - 224
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1921

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References

page 211 note 1 Evans, J. W., Expedition to Caupolican, Geographical Journal, Bolivia. 1903, vol. 22, pp. 601–642, mapGoogle Scholar.

page 220 note 1 Leith, C. K. and Mead, W. J., Metamorphic Geology, 1915, p. 63Google Scholar.

page 222 note 1 The data thus brought to bear upon the problem of the genesis of these ilmenite porphyroblasts, correlated with corresponding data obtained in an investigation of pyritous slates, particularly those from Penrhyn slate quarry, Carnarvonshire, suggest a somewhat similar mechanism for the genesis of porphyroblastie pyrites and epidote: when reconstitution-trend is in the direction of chlorite, the residual ehloritie matter tends to become enriched in ferrous oxide; and if sulphates are also undergoing reduction to sulphides, porphyreblasts of pyrites may develop. Magnesia thus liberated would become an increment available for evolving chlorite. But lime is not markedly assimilable to chlorite, and like ferrous oxide it tends to be relegated to the residual chloritic matter; in the latter, a hydrous system, it may react with ferrous oxide and triad-oxides to form epidote:

In the case of the Calvario phyllite, titanite may conceivably develop in preference to ilmenite at some loci, and ilmenite constituted at a stage when the limeconcentration in the chlorit, ic matter was low may become liable to alteration to titanite as the lime-concentration increases.

The wedges and prisms of quartz within the selvage of the ihnenite crystals described, and similar quartz structures about porphyroblasts of pyrites (e. g. in the Penrhyn slate), are in the main disposed normal to the crystal wall or matrix wall of the selvage: they recall the structures assumed by quartz crystals sessile on a cavity wall. A reduction of volume is achieved by the chemical combinations yielding porphyroblastic ores; tendencies towards a similar reduction of volume are operative around foci of re-crystallization in the groundmass and may continue to be operative after ores have separated. It is suggested that tension within the groundmass may partly detach the latter from the ores and tend to produce around them a cavity which is ultimately infilled with quartz. It is proposed to discuss this phenomenon more generally in a later paper.

page 222 note 2 Rastall, R. H., The Skiddaw granite and its metamorphism. Quart. Journ. Geol. Soc., 1910, vol. 66, p. 126 CrossRefGoogle Scholar.

page 223 note 1 Hutchings, W. M., Geol. Mag., 1890, pp. 264 CrossRefGoogle Scholar, 316; 1891, p. 164; 1892 pp. 154, 218 ; 1894, pp. 36, 64 ; 1896, p. 309.