Microsyenite from Lake Mykle, Oslo Rift: Subvolcanic rocks transitional between larvikite and nordmarkite

Abstract

A small body of trachyte or microsyenite cropping out east of lake Mykle in the Vestfold Graben of the Oslo Rift probably represents a subvolcanic intrusion emplaced at a fairly early stage of the intrusive history of the Siljan-Mykle area. The microsyenite shows chemical, petrographical and mineralogical features intermediate between larvikite and nordmarkite, suggesting that it represents a transitional magma type. The microsyenite shows pyroxene zonation patterns indicating a discontinous, two-stage crystallization history, whose initial stage resembles that observed in normal larvikite. At the final stage of pyroxene growth, the melt had developed to a more alkaline composition, resembling some of the less alkaline (alkali) syenites in the rift.