The geological beginnings of New Zealand were simply the volcanism and following sedimentation at the edge of a subduction zone, located along the southern offshore of supercontinent Gondwana.
The rocks now seen in Northwest Nelson and Te Wahipounamu, Land of the Greenstone otherwise known as Fiordland National Park showcase the beginning of New Zealand’s history. These rocks form what is known as the Western Province, which used to be connected but are now separated by the Alpine Fault. The forming of these rocks began during the mid Cambrian.
Fiordland’s primary bedrock contains diorite, granodiorite, gabbro and gneisses. It is also home to the famous pounamu, otherwise known as jade. Stay tuned on another post specifically about jade in New Zealand and its geological background as well as its uses for the Maori.
Within Milford Sound I saw mostly granitic type rocks and mafic volcanics. Diorite, gabbros and granodiorites littered themselves along the beach of Milford, as seen below. I saw heavy amounts of purple-grey plagioclase here.
Heading further inland and to the southeast, I came across mudstones and siltstones along some hiking trails. These are the pre-existing build of of the sea floor sediments before being folded and pushed up into a mountain range. These plutonic rocks of Milford being formed due to the back-arc movements during the building process. Note there is more metamorphism to take place, and this phase 1 is only the beginning of what these rocks have to go through!
Milford Sound, glacially carved plutonics and metamorphics.
There are two main terranes around the NW Nelson area, the Buller (west) and Takaka (east). Buller represents the offshore sedimentary environment, while Takaka are the oceanics and volcanics. The most distinctive in this area are the ultramafics, seen near the Takaka River, that have all since been highly serpentinized. Most volcanism took place during the Cambrian and had dissipated to sedimentation during the Ordovician. This sedimentation process continued, mainly in the east, until the Tuhua orogeny.
The Tuhua orogeny was the first of several major upheavals for New Zealand, occuring in the mid Devonian. In the Nelson region this resulted in the kyanite-bearing Onekaka Schist, and many of the banded sediments can be seen beautifully folded along the highways as psammites and pelites.
Folded semi-pelites, west of Takaka, backpack for scale.
Two major batholiths are associated with this orogeny, the Karamea Granite (west) and the Riwaka Gabbro (east). I spent a lot of time in the more felsic Karamea Granite kayaking and hiking in Abel Tasman National Park and Marine Reserve. As evidenced by the coarse, crystalline nature of the monzo-syenogranite, with beautiful sub to rarely euhedral white-pink feldspar phenocrysts. These rocks give way to beautiful white sandy beaches and the turquoise waters of the paradise you see below!
More on the seds front, all those Ordovician limestone platforms building up in the shallow seas around the volcanoes off Gondwana are now beautiful exposed marble around Takaka, thanks to the Tuhua orogeny. There is some great climbing in the area, unfortunately the weather was a little too wet for me to check out some of the sweet crags!
I also spent some time in the karst caves, one in particular called the Ngarua Caves. These caves were formed due to rainwater and have merged into many cave systems. These karst caves swallow 5% of the sheep population every year. In conjunction, there are many underground spring systems in this area. One particularily special to the Maori is Te Waikoropupū, or PuPu Springs near Takaka. I spent one of my mornings visiting this sacred place.
Stalactites and stalagmites galore!!
And now onto phase II we go…