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Precambrian Para-Gneisses of Southern Eyre Peninsula, South Australia

Published online by Cambridge University Press:  01 May 2009

C. E. Tilley
C. E. Tilley
Affiliation:
Emmanuel College, Cambridge
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Summary

1. Garnet gneisses of sedimentary origin are described from the oldest group of sediments recognized in Southern Eyre Peninsula, South Australia. The high-grade metamorphism which these rocks have undergone has resulted in the development of felspar, garnet (almandine), sillimanite, and green spinel. 2. The mineralogical association of these constituents is described, and their genetic relations are discussed. 3. A sedimentary origin of these gneisses is attested by their interbanding with undoubted sediments, their mineralogical composition, and lastly by chemical analysis. 4. In the garnet gneisses developed in the Grenville series of North America, notably those of the Adirondack region, and in the province of Quebec, these para-gneisses find their most striking analogues.

Type
Original Articles
Information
Geological Magazine , Volume 58 , Issue 7 , July 1921 , pp. 305 - 312
Copyright
Copyright © Cambridge University Press 1921

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References

page 306 note 1 Harker, , Pres. Address Geol. Soc., 1918, p. lxxix.Google Scholar

page 308 note 1 Clarke, , Bull. U.S.G.S., No. 419, p. 211.Google Scholar

page 309 note 1 In these equations the quantitative proportions of orthoclase to garnet, sillimanite, and spinel will obviously depend on the ratio H: K. in the mica involved. In this, and following equations, the mica must be regarded as mechanical mixtures of sericite, and biotite of the composition shown in equation (1). For simplicity the gross composition of these mixtures is shown. Whether Clarke's view, or the hypothesis of Tschermak for the constitution of the micas is adopted, it is quite clear that the main types muscovite and biotite form mix crystals only to a very limited degree. In a recent paper in a study of a large number of analyses of the mica group, H. E. Boeke (“Die Granzen der Mischkristallbildung in Muscovit und Biotit”: Neues Jahrbuch, 1916, pp. 83–117) shows that the compositions of muscovite and biotite are to be represented in two distinct and separate fields, each of which is quite limited in extent.

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