Jurassic
A major intrusion of dolerite occurred in the Jurassic. This was a widespread phenomena covering over one third of Tasmania, and possibly more in the past. This intrusion also affected Antarctica, Argentina and South Africa at 183 million years ago. This has been called the Karoo-Ferrar large igneous province. Three to five million cubic kilometers of magma were intruded overall, being the planet's fourth largest known magma intrusion. This may have been the cause of the Toarcian extinction through a consequent oceanic anoxic event. Henrik Svensen claimed that the magma baked coal and oil shale producing up to 27.4 teratonnes of carbon dioxide, some of which entered the Earth's atmosphere.
Tasmania has the largest exposure of dolerite in the world of 30000 km2 and a volume of 15000 km3. In Tasmania the rock is characteristic of many mountains with its columnar joining and dark blue grey colour. The composition is 40% plagioclase, 20% clinopyroxene, 20% quartz, 5% ilmenite and small percentages of potassium feldspar and amphibole. The rock is altered by water to smectite and kaolinite with quartz being left unaltered. The soil surface concentrates zirconium and titanium. The soils on dolerite also contain nodules of hematite and may contain a buried layer of stones called a stone line.
Most of the intrusions are in the form of sills up to 500 m thick. Mostly the sills are in the Parmeener Super Group rocks. There are also stepped sills, inclined sheets, cones and some dykes. Closely adjacent country rocks were metamorphosed to hornfels. The upper parts of sills may be more coarsely grained. Dolerite is crushed to use as road metal, and aggregate.
Mount Anne, Mount Mueller, and Mount Wedge in the south west are capped in dolerite, where it also makes contact with Precambrian rocks.
It appears that the magma came from the crust rather than the mantle. Solidification occurred at 174.5 ± 8 million years ago.
A Jurassic forest was buried in an andestic volcanic eruption at Lune River. Here beneath the lava flow is mudstone with fossil wood and leaves.
On King Island, there was an intrusion of a biotite lamprophyre dyke at 143 million years ago. Xenoliths of granulite-facies metamorphic rock resemble those found in eastern Antarctica.
Read more about this topic: Geology Of Tasmania, Geological History