The Significance of Clouded Plagioclase in the Basic Dykes of Mysore State, India

A preliminary account is given in this paper of the results of a statistical study made in Mysore State on the distribution of about 500 basic dykes, with special reference to the occurrence in them of clouded plagioclase feldspars.

The basic dykes are of different geological ages and of various petrographic types. They are distributed all over the State in a fairly uniform manner.

Attention is drawn to the important fact that the clouded dykes are concentrated in the southern half of Mysore State, and that they occur not only in the charnockite region but in a zone adjoining it. Two kinds of charnockites have been recognized in Mysore: an older gneissic type caused by an earlier regional metamorphism, and a younger granitic variety produced by subsequent palingenetic fusion. The two metamorphisms which were responsible for these differing types of charnockites are separated by the period during which the basic dykes were intruded.

Clouding of plagioclase feldspars can be caused by thermal metamorphism. In Mysore, the basic dykes have chilled against the first type of charnockites and transect the foliation of the country rocks. They are consequently younger in age. It is suggested that the later formed charnockites were responsible for the subsequent clouding of the plagioclase in those dykes.

Clouded feldspars in the basic dykes are, therefore, of great significance in recording the metamorphic history of some aspects of Precambrian geology in Mysore State.