High-grade basement gneisses and granitoids in Westland, New Zealand

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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 3.5-11 %). Isotopic dating indicates an age of 600-700 Ma, which was probably the peak of metamarphism. The chemical composition and mineralogy of these gneisses suggest a protolith of mainly greywacke composition. Granitoid is used here in a very general sense, for all quartz-bearing intrusive rocks. Specifically, these rocks range from tonalite (quartz +plagioclase + biotite) through granodiorite to adamellite, with increase in K-feldspar. Granitoids of similar composition are present both in the gneisses and in Greenland Group rocks. No ages are available for granitoids in the gneiss terranes, but they may weil be the product of local melting and migmatization at the metamorphic peak. Granitoids intrusive into Greenland Group rocks are post-Ordovician, probably Devonian, although some may be younger. Preliminary isotopic data suggest that the granitoids have a considerable component from the Proterozoic basement gneisses; some may have been formed almost entirely from the melting of these rocks. These Westland rocks show that the New Zealand microcontinent has, at least in part, a gneiss basement of Precambrian age, overlain by a Lower Paleozoic sedimentary sequence which underwent a Devonian orogenic episode (Tuhua Orogeny) accompanied by granitoid intrusion. These Westland terranes are comparable with sequences of similar age distribution on Campbell Island and Marie Byrd Land, and with those in North Victoria Land and southeast Australia, although the paleogeographic relationship of these areas to each other and to the Gondwana supercontinent is uncertain.