The petrology of the Cambrian volcanic rocks of Tasmania

A spilitic suite comprising picrite basalts, spilites, keratophyres, albite dolerites and associated pyroclastics reputed to be Middle Cambrian in age forms an interesting petrographic province on the West Coast of Tasmania. The spilites are considered to have been normal basalts which later suffered soda metasomatism as a result of eugeosynclinal orogeny. Like basalts from other orogenic regions these are rich in alumina but the presence of excess alumina in this suite is regarded as being secondary and not magmatic. Fresh diopsidic augite is a feature of some of the lavas. Hydrothermal alteration has given rise to a variety of rock types which includes porphyries, kerntophyres, variolite‚ÄövÑvp, spherulitic quartz rock and jasper. Petrological-similarity exists in basaltic lavas in Tasmania, South Australia and Western Australia but the Tasmanian lavas which are eugeosynclinal show advanced metasomatic alteration whereas the Western Australian, which were erupted on to the stable shield are unaltered. The South Australian, which are miogeosyncinal, are partially metasomatised.