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Western metamorphic and sedimentary basement rocks

Western Metamorphic and Sedimentary Basement Map

In Northwest Nelson, the West Coast and southern Fiordland are remnants of the Gondwana supercontinent that once lay close to their counterparts in southern Australia prior to the opening of the Tasman Sea around 80 million years ago. These rocks are mostly sedimentary, some with substantial volcanic rock included, with varying degrees of metamorphism superimposed.

In some parts of the West Coast these sedimentary rocks have been metamorphosed to the verge of melting. In the Cobb valley are New Zealand’s oldest known rocks; sandstone, mudstone and conglomerate around 505 million years old. Distinctive grey marble, seen at Mt Owen and Mt Arthur, represents part of a relatively quiet period following a turbulent history at a very old plate boundary. The marble was once quarried and used as a building stone for Parliament Buildings and Nelson cathedral.

Much of Fiordland is metamorphosed igneous rock, plutons of diorite, gabbro, granodiorite and granite in particular that had new minerals grown during burial. In some cases minerals such as omphacite and kyanite record rapid burial to extraordinary 80 km depths during plate boundary activity around 115 million years ago. These rocks were brought back almost as quickly close to the surface only five million years later.

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