The alkaline rocks of Port Cygnet, Tasmania

The Port Cygnet alkaline complex consists of small scattered concordant and discordant intrusions of Cretaceous age emplaced in Jurassic dolerite and Permian fluvioglacial sediments in the vicinity of Port Cygnet, southern Tasmania. The alkaline rocks are miaskitic and fall into two main groups: one is a group of slightly oversaturated syenites with a porphyritic texture rich in phenocrysts of oligoclase (syenite porphyries), and the other consists of nepheline syenites, characteristically with a flow structure and containing phenocrysts of euhedral sanidine crystals, with feldspathoids in the groundmass, and occasionally as phenocrysts. These are the sanidine rocks or sanidine porphyries. The primary magma was a potash-rich upper mantle or lower crustal, partial melt (of a mafic alkali parent) probably equivalent ultimately to a tinguaite which began crystallizing in a magma chamber with subsequent successive injections of magma containing coarser sanidine phenocrysts. . This gave rise to the nepheline syenites. The intrusion of the potassic magma was accompanied by volatiles with evidence of the presence of water, carbon dioxide, sulphur dioxide and hydrogen sulphide, demonstrated by the presence of both primary and secondary reaction mineral products. The porphyritic syenites seem to have been derived by the mixing of a potassic upper mantle or lower crustal partial melt, with a crustal partial melt of the feldspathic matrix of an amphibolite followed by subsequent initial crystallization at depth before injection into the country rocks. The model is based on microscopic and textural characteristics, chemical compositions, mineralogy and initial strontium isotope ratios. Variation of the syenite porphyries due to addition of recrystallized amphibole from the parent amphibolite formed a horn-blende porphyry. Fractionation of the syenitic melt has resulted in a group of mixed feldspar phenocryst rocks showing potash feldspar reacting with plagioclase phenocrysts in a brown matrix. Assimilation of mineralized carbonates by the magma has produced one rock unique in having phenocrysts of spessartite and epidote. Sulphur from the country rocks was mobilized by this intrusion. At Regatta Point, on the western shore of Port Cygnet, the .potassic magma reacted with Jurassic dolerite to form hybrid rocks. These were most likely produced within the temperature range of 710 °C to 810 °C and a log £0 2 in the range -11 to -13. The alkaline intrusion was probably associated with the break-up of Gondwanaland.