Abstract
Abstract Gneisses and granitoids are widespread in Westland, mainly as isolated hills between the Alpine Fault and the coastal plain. On the current geological map ofNew Zealand, these rocks are designated as the Tuhua Group far convenience in regional mapping; in fact this Group includes at least two tectonically distinct units. The gneisses, which include small granitoid bodies (usually migmatitic or gneissose) make up a belt, up to 10 km wide but commonly much narrower, immediately west of the Alpine Fault, from the Grey River to Whataroa. The major granitoid bodies are intrusive into Greenland Group rocks, which make up a parallel belt to the west, but extend beyond Whataroa as far as the Arawata River and probably farther south. These two belts are separated by a major fault, probably a thrust (the Fraser Fault and its projection). The gneisses are high-grade metamorphics, consisting of quartz + plagioclase + biotite ± almandine ± hornblende ± sillimanite; the almandine is relatively Mg-rich (MgO 3.5...
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