The “Charleston Gneiss” was such named by early European geologists to the region, based off the presumption that the rock they were looking at, with such noticeable foliation (a type of metamorphic layering), must be equivalent to the ancient regional metamorphic gneisses of the European Alps and Scottish Highlands. The name has stuck, but the rock is not what we would typically call a gneiss at all!
Instead, these types of rocks are termed “mylonites” and, in this case, are actually granites that have been sheared and deformed by fault movement, a great symbol of the Zealandia continent’s long and turbulent history along plate boundaries. The granites are much younger in age than typical gneisses (around 100 rather than over 400 million years old), and the foliation that has developed is a result of shear stress, not intense burial pressure. The foliation therefore also aligns with the direction of fault movement, not at right angles to the pressure direction, as in regionally metamorphosed rocks. This type of special metamorphism is called cataclastic or dynamic metamorphism, and results in a grinding down of the original rock and it’s minerals, rather than a growth or crystals and mineral bands, as typical in regional metamorphism. You can spot this “grinding down” by looking for feldspar crystals that have been rounded into oval blobs within the rock, instead of the nice clear rectangular shapes that they should grow as. These are termed “augen” which is the German word for “eye”! The word mylonite is derived from the Greek word for mill.
