Geology of the southern Longwood Tops, western Southland


Rombouts, Michael Jozef.


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The objective of this thesis was to determine the rock types present and their distribution in an unforrested area on the top of the Longwood Range. One of the principal aims was to confirm or otherwise the interpretation given to the geology of the area by Challis and Lauder (1977) as shown in their regional geological map of the entire Longwood Range (NZGS Misc. Series Map 11). Their map necessarily involved much less detailed mapping of the area of the thesis.

For the purpose of accuracy the whole area, except for some areas of thick scrub in the north and south, was mapped on a grid basis and was heavily sampled, especially where there was marked variation in rock types over short distances and where the outcrop distribution allowed for it. All samples collected were thin sectioned and petrographically examined. Whole rock and mineral chemistry was determined for selected samples.

The rock types were classified on the basis of their cumulate mineralogy and primary mineralogy into three subseries the cumulate olivine-bearing Troctolitic Subseries (mafic to leucotroctolite, troctolitic gabbro, gabbro) and Olivine Gabbro Subseries (plagioclase wherlite, olivine gabbro) and what is believed to be a predominantly noncumulate, often recrystallized, olivine-free Gabbronorite Subseries (gabbronorite).

Taking into account differences in terminology and classification most of these rock types are the same as those recognized by Challis and Lauder (1977) however their distribution is not as shown in the Challis and Lauder map. Geochemically it can be confirmed that the rocks can be classified as arc complex rocks with a chemistry intermediate between calc-alkaline and island arc tholeiite. The magma was essentially anhydrous as evidenced by the lack of primary hornblende and high modal plagioclase.

Structurally the rocks are not thought to be part of a layered complex but, in the model adhered to here, to consist of a number of cumulate plutons, often zoned with a mafic-ultrabasic core, that have been intruded into a noncumulate gabbronorite - diorite batholith-like complex.

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xxiii, 582 p. : ill. (some col.), maps (some col.) ; 30 cm.


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POLYGON ((167.825483471000098 -46.282835346999946,167.828755305000072 -46.243493141999977,167.847975648000102 -46.22917584399994,167.870437228000014 -46.230083023999953,167.869618675000083 -46.239143336999973,167.847126650000064 -46.257603331999974,167.849249351000026 -46.271087609999938,167.838655062000043 -46.28329955199996,167.825483471000098 -46.282835346999946))




Rombouts, Michael Jozef., “Geology of the southern Longwood Tops, western Southland,” Otago Geology Theses, accessed May 22, 2024,

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