Occurrences and chemical compositions of ultrapotassic mafic dyke rocks from Skallevikshalsen and Rundvågshetta, Lützow-Holm Complex, East Antarctica

Abstract

Melanocratic dyke rocks were found at Skallevikshalsen and Rundvågshetta in the Lützow-Holm Complex. The rocks are commonly holocrystalline and aphyric, and consist mainly of alkali feldspar, biotite, augite, quartz, apatite, and titanite, with minor amounts of plagioclase. Almost all the mineral compositions tended to be homogeneous, whereas the composition of apatite sometimes showed intragranular heterogeneity in the F and Cl components. The melanocratic dyke rocks at Skallevikshalsen have an ultrapotassic mafic composition and resemble the compositional features of the lamproitic to minettic dyke rock previously found at Innhovde, located in the western region of the Lützow-Holm Complex. The dyke rocks at Rundvågshetta are considered to be a mixture of ultrapotassic mafic magma and high-Cl/F intermediate to felsic magma. Considering their occurrence and the results of Rb-Sr mineral dating, the time of intrusion was just after the metamorphism of the Lützow-Holm Complex, and the igneous activity was thought to have been caused by the collision between East and West Gondwana.