Alkali Feldspars from Gneisses and Granites of the Västervik Area, SE. Sweden

Abstract

Metamorphosed Precambrian sedimentary rocks and associated late-kinematic granites of the Västervik area in southeastern Sweden contain abundant potassium feldspar. The alkali feldspar from the metasediments shows zero to low obliquity and contains from 8 to 10 % albite in solid solution. In the granitic rocks, and in the accompanying pegmatites and aplites, the potassium feldspar is microcline, containing 1 to 3 % albite The potassium feldspar of contact rocks and inclusions in granite is also microcline The formation of monoclinic potassium feldspar in the metasediments is the result of regional metamorphism in the upper part of the amphibolite facies. The widespread association with sillimanite suggests a temperature of formation above that of the triple point of andalusite—sillimanite—kyanite. In the granites, a monoclinic ancestry of the potassium feldspar is indicated, but transformation to the triclinic form has taken place, and unmixing has resulted in a nearly pure potassium feldspar. Solid-state transformations in the granitic potassium feldspars may have been facilitated by abundance of volatile constituents.