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Book Chapter

Granitoid rocks of New Zealand — A brief review

By
A. J. Tulloch
A. J. Tulloch
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Published:
January 01, 1983

Granitoid rocks in New Zealand occur in two areas of the South Island, separated by 450 km displacement on the Alpine Fault. Biotite and biotite-muscovite granitoids predominate, with granites, granodiorites, and tonalites occurring in subequal proportions.

In west Nelson-Westland granitoid rocks dominate the basement and intrude upper Proterozoic gneisses, and lower to middle Paleozoic sedimentary rocks which generally do not exceed greenschist facies metamorphic grade, producing andalusite, cordierite, biotite, sillimanite and wollastonite, and possibly garnet, staurolite, chloritoid-bearing contact aureoles. Two N-NNE-trending batholiths are distinguished: the eastern Separation Point Batholith of Cretaceous age is relatively sodic and commonly contains hornblende in addition to biotite. The larger Karamea Batholith is composed chiefly of Paleozoic (430-280 Ma) biotite and/or muscovite granites, granodiorites, and tonalites of the Karamea Suite. Plutons are large (some 100–200 km2) and are commonly foliated. The Karamea Suite is discordantly intruded by small (< 30 km2), massive, high-level upper Mesozoic biotite granodiorite to muscovite granite plutons (Rahu Suite), which were locally extruded onto the surface. Minor but widespread Mo (Cu) mineralization is associated with this latter suite. Hornblende-bearing granitoid rocks are a minor constituent of the batholith; minor but widespread quartz diorites are intermediate in age between the Karamea and Rahu Suites.

Karamea Suite granitoids are relatively potassic and enriched in Fe, Mg, Ti, and Rb; have Sr0 0.709–10; and conform to S-type criteria of Chappell and White (1974); Separation Point Batholith rocks, on the other hand, are relatively sodic and enriched in Ca and Sr, with Sr0 0.704–5 (I-type). Rahu Suite plutons have Na/K approximately equal to 1 with Sr0 0.706–7 and are ambiguous in terms of I/S criteria. Reconnaissance dating indicates that Separation Point and Rahu granitoids are approximately coeval and are possibly related to the same plutonic event; the Rahu magmas possibly contain a greater contribution from the sub-Karamea Suite continental basement.

In Fiordland/Stewart Island, correlatives of west Nelson granitoids border an amphibolite-granulite facies metamorphic complex. In eastern Fiordland, sodic granites are correlated with the Separation Point Batholith of west Nelson; in southwest Fiordland correlatives of the Karamea Batholith occur in an area of relatively low metamorphic grade similar to that of the Paleozoic host rocks of west Nelson. In central and southern Stewart Island relatively sodic granitoids predominate. Less abundant potassic granites are associated with Sn-mineralization in metamorphic rocks similar to those of the Fiordland complex. Syntectonic granitoids also occur within central Fiordland.

Granitoid rocks are also associated with several mafic complexes that occur along the junction between the granitoid-bearing Tasman metamorphic belt in the west, and the Wakatipu belt to the east, which lacks granitoid rocks.

Samples from outlying islands and drillholes show that granitoid rocks are extensively distributed throughout the continental shelf to the south and north of the South Island of New Zealand, while a Cretaceous rhyolite has been dredged from the Lord Howe Rise, some 600 km west-northwest of west Nelson.

Granitoid plutonism is probably associated with Quaternary voluminous rhyolitic volcanism in the central North Island of New Zealand, related to the Tonga-Kermadec-New Zealand island arc, and minor quartz diorite-granodiorite plutons are associated with copper mineralization along the east coast of the North Island from the Coromandel Peninsula northwards.

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Contents

GSA Memoirs

Circum-Pacific Plutonic Terranes

J. A. Roddick
J. A. Roddick
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Geological Society of America
Volume
159
ISBN print:
9780813711591
Publication date:
January 01, 1983

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