Metamorphic Rocks

Schist

With the exception of the far western mountains, most of the basement rock of Otago is schist. It is a coarsely flakey material composed mainly of layers of white quartz and mica. To the north and south the rocks are less metamorphosed and grade out into greywacke, a kind of hard sandstone. This suggests that mica of the Otago or Haast Schist as it is now called, originated in greywacke which was subjected to great pressure and heat through being completely buried for a long period. This caused the constituent minerals to separate into layers of different composition (foliation) and gave it the ability to split along the layers (schistosity) (from the ORMC site). This metamorphism of Greywacke occurred between about 200 to 65 million years ago.