The Somerset Dam Layered basic intrusion, Southeastern Queensland
Titel:
The Somerset Dam Layered basic intrusion, Southeastern Queensland
Auteur:
Mathison, C. I.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 14 (1967) nr. 1 pagina's 57-86
Jaar:
1967
Inhoud:
The Somerset Dam Layered Basic Intrusion is probably a sub-volcanic magma chamber, and it consists of 20 saucer-shaped layers composed of troctolites, olivine gabbros, ferrigabbros, and leucogabbros. The layered sequence is 1650 ft (500 m) thick, and comprises several repetitions of a standard pattern termed a zone. Each zone is generally composed of four layers, and successive mineral assemblages from the base upwards are: plagioclase-olivine, plagioclase-augite-olivine, plagioclase-augite-magnetite-ilmenite, and plagioclase-uralite. Pronounced modal and textural changes define the boundaries between these layers. Within a zone, systematic variation in the proportions and compositions of minerals is thought to be the result of a slight decrease in temperature, and an increase in the partial pressures of water vapour and oxygen from the base to the top. Repetition of zones is explained by a mechanism involving periodic renewals of magma. Lack of progressive changes in mineral compositions and proportions from the base to the top of the layered sequence is also a consequence of the magma composition, the control of water vapour pressure, and the limited range of crystallisation temperatures. Gravitational settling of early minerals does not explain the variation within a zone, or the small-scale rhythmic layering that is locally developed, and it is concluded that diffusion has been an important control.
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