Geology of the eastern Manapouri province

Staite, Christopher Gilbert, 1974-

The Eastern Manapouri Province of Turner (1937a) consists of at least two identifiable generations of plutonism; a 303±4.8 Ma granite, and a 160-155 Ma suite of gabbroic to granitic composition. These can be divided into three main groups: (1) The Pohuruhuru Granite (new name), anI-type Carboniferous granite probably representing local basement to the MTZ. (2) The Holmwood Island Gneisses and the Beehive Epidiorite, ea. 160 Ma gabbros and diorites metamorphosed to amphibolite fades conditions in an event correlated with the metamorphism of the Fraser Complex of Westland. Some of these rocks are magnetic, and are regarded as the cause of the Manapouri magnetic anomaly. (3) The Matarua Quartz Monzonite (new name), the Western Orthogneiss and the Pomona Island Granite, a suite of I-type granitic rocks intruded syn- and post-metamorphically. The 155±2 Ma Pomona ~land Granite emplacement represents the final event in this sequence. Most lithologies occur as xenoliths from metre to kilometre scale within later rock types. Holmwood Island Gneiss and Beehive Epidiorite in particular occur as 'sheets' within later granites. A vertica~possibly active oblique slip fault scarp with a 100 m cataclasite zone, previously interpreted as the western boundary of the Eastern Fiordland Belt (Smith, 1993) is present, but regionally insignificant displacement suggests that it is not an important geological boundary. Geochemical analysis shows a prevailing alkali-calcic continental island arc trend for these rocks, and linear trends on Harker discrimination diagrams demonstrate a common tectonic environment for all lithologies . • All units (except the Pohuruhuru Granite) formed in the same intrusive event, between 160 and 155 Ma. Formation was at depth (at least 22 km) with successive intrusions into the base of an island arc, with each intrusive rock type picking up xenoliths of earlier rocks.

77 leaves : ill. (some col.), maps (some col.) ; 30 cm.

1995Staite

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