Variations within a Scottish Lower Carboniferous lava flow

Abstract

SynopsisSystematic vertical gradational variations within a ‘hawaiite’ lava flow of Calciferous Sandstone age are investigated. The variations include an increase in the amount of augite, and in the size and anorthite content of plagioclase microphenocrysts upwards through the flow. There are corresponding variations in the chemical composition, particularly in CaO and systematic variations in the oxidation state. It is concluded that the major variations could not have resulted from post-eruptive processes and hence the flow must have been erupted in a differentiated condition. It is thought that the differentiation is most probably the result of selective fusion rather than crystal fractionation and that this mechanism could also explain the variations in the suite to which the differentiated flow belongs.