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Flow direction and groundmass shear zones in dykes, Shimane Peninsula, Japan

Published online by Cambridge University Press:  01 May 2009

J. V. Smith ,
Y. Miyake  and
S. Yamauchi
J. V. Smith
Affiliation:
Geology Department, Shimane University, Matsue, Shimane 690, Japan
Y. Miyake
Affiliation:
Department of Geology, Shinshu University, Matsumoto, Nagano 390, Japan
S. Yamauchi
Affiliation:
Geology Department, Shimane University, Matsue, Shimane 690, Japan
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Abstract

The groundmass of andesitic dykes at Sezaki, southwest Japan, has trachytic texture and contains microscopic shear zones. The shear zones comprise a conjugate pair formed by flattening of the solidifying dyke rock, probably caused by the magma pressure of the still molten part of the dyke. This pressure shortened the solidifying rock perpendicular to the dyke margins and caused it to extrude parallel to the magma flow direction. The groundmass shears indicate that locally the magma flowed 60° upward in the dykes. It is concluded that while groundmass shears are a useful indicator of flow direction in dykes, phenocryst alignment in dykes is strongly influenced by magma-pressure flattening and thus may be a poor indicator of flow direction

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 1 , January 1993 , pp. 117 - 120
Copyright
Copyright © Cambridge University Press 1993

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