Petrology of the Grimstad granite II. Petrography, geochemistry, crystallography of alkali feldspars and genesis.

Abstract

The Precambrian Grimstad granite, South Norway, has been studied by petrographical, chemical and x-ray crystallographical analysis. It has a semi-circular outline and is partly rimmed by an agmatite. Eight different rock types are found within the granite, but the chemical variation trends mostly form concentric patterns. Extreme K\/Rb ratios are found in some samples, that may have been derived from charnockitic rocks (arendalites). The alkali feldspars are triclinic, but random disorder x-ray patterns and monoclinic phases - indicative of assimilation and rapid growth of alkali feldspars - are occasionally found. The granite seems to have formed by partial melting of a granulitic substratum and the amount of granitic melt in the emplacing granite magma may have been relatively small. The different rock types have formed more or less intense granitization of foreign material, and the granite seems to have intruded by buoyancy.