Plagioclase porphyroblasts from an amphibolite paleozome

Abstract

Plagioclase porphyroblasts from the amphibolite paleozome of the Valle granite, Setesdal in South Norway have been studied in detail petrographically. Each porphyroblast is composed of a mosaic of nearly one thousand plagioclase grains with interstitial quartz and some microline. Both plagioclase and quartz in the porphyroblastic domain have distinct crystallographic regularities such as twin relationships and graphic intergrowth textures indicating that the porphyroblastic domains were formed under high vapour pressure and higher temperature than the granitic minimum in an unstrained environment such as that postulated for segregation pegmatites. This is an example of static porphyroblastesis under the metamorphic conditions of the amphibolite facies.