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Chlorite-mica stacks in low-strain rocks from central Wales

Published online by Cambridge University Press:  01 May 2009

J. Craig ,
W. R. Fitches  and
A. J. Maltman
J. Craig
Affiliation:
Department of Geology, University College of Wales, Aberystwyth, Wales
W. R. Fitches
Affiliation:
Department of Geology, University College of Wales, Aberystwyth, Wales
A. J. Maltman
Affiliation:
Department of Geology, University College of Wales, Aberystwyth, Wales
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Summary

Weakly deformed, low grade, Lower Palaeozoic metasediments from central Wales contain abundant stack-like intergrowths of chlorite and white mica that closely resemble stacks described from the Devonian Hunsruckschiefer of West Germany; the Ordovician Martinsburg Slate, New Jersey, U.S.A.; and elsewhere. Several theories have been proposed to explain the origin of such stacks, including a detrital origin; strain-controlled growth of chlorite on a detrital mica nucleus; and strain-controlled intergrowth during metamorphism. None of these satisfactorily explains the central Wales stacks. A detrital origin is precluded by the presence of many stacks with shapes too delicate to have survived transportation, and a lack of hydrodynamic equivalence between the stacks and the clastic host grains. Features inconsistent with strain-controlled growth are constant alignment parallel to bedding but non-systematic orientation with respect to tectonic cleavage, their common occurrence in undeformed rocks, and petrographic evidence that they precede the tectonic cleavage. It is proposed that the stacks formed during diagenesis and low-grade metamorphism, and before the onset of deformation, through mimetic growth on a primary bedding fabric composed of clay minerals.

Type
Articles
Information
Geological Magazine , Volume 119 , Issue 3 , May 1982 , pp. 243 - 256
Copyright
Copyright © Cambridge University Press 1982

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