Structure and origin of the Dun Mountain ultramafic massif, New Zealand NIKOLAS I. CHRISTENSEN* Department of Geological Sciences and Graduate Program in Geophysics, University of Washington, Seattle, Washington 98195 ABSTRACT contrast to the dark green bush and trees of the be of igneous origin but suggested that it formed surrounding hills. as a differentiate within a deep-level magma Dun Mountain, the type locality of dunite, Early studies proposed an igneous origin for chamber underlying Permian volcanoes. forms part of a major belt of predominantly the dunite. McKay (1879) suggested that the Coleman (1962, 1966), from observations of mafic and ultramafic rocks in South Island, New ultramafic rocks originated as submarine lava the mineral assemblages at the ultramafic con- Zealand. The core of the mountain consists of flows, whereas Park (1887) and Benson (1926) tact, estimated P-T conditions of low tempera- unserpentinized dunite and harzburgite tecton- favored an intrusive origin. Turner (1942) con- ture and high pressure, which implies an origin ite. Foliations in the ultramafic rocks strike ap- cluded, on the basis of petrofabric studies, that other than igneous intrusion or load metamor- proximately northeast-southwest, parallel to the the olivine from Dun Mountain did not origi- phism. He proposed that Dun Mountain was elongation of the crestal plateau. Lineations are nate from crystal settling in a large magma emplaced into its present location as a cold tec- horizontal and northeast-southwest. Texture is chamber. Instead, he proposed an origin as an tonic intrusion. Blake and Landis (1973) further usually porphyroclastic, with olivine elongated inclined sill injected along a major fault zone. developed this hypothesis and concluded that parallel to lineations. Petrofabric analyses show Turner (1942) presented evidence for postem- Dun Mountain is an allochthonous block in a strong preferred orientations of olivine a crystal- placement deformation of the ultramafic, as did mélange that forms part of an ophiolite suite. lographic axes subparallel to foliations and line- Battey (1960), who suggested that deformation Coombs and others (1976), Hunt (1978), and ations. Shear senses within the ultramafic are features and preferred olivine orientation in Dun Davis and others (1980) also concluded that dextral, indicating that the northern portion of Mountain are related to regional folding. Lauder Dun Mountain is the ultramafic portion of an Dun Mountain has been displaced to the east (1965), on the other hand, interpreted the oli- ophiolite. relative to the southern portion. Undulatory ex- vine orientation obtained by petrofabric anal- This paper presents new data on the internal tinction of olivine, resulting from kinking and yses as originating from crystal settling within a structure of Dun Mountain and evaluates the associated slip on (010) [100] and {Okl} [100] volcanic pipe. In support of this origin, he pro- data in terms of the origin of the ultramafic slip systems, is interpreted as originating within posed that Dun Mountain is composed of con- rocks. the upper mantle beneath oceanic crust that centric zones of dunite and harzburgite a few formed at a spreading ridge or in a marginal hundred metres in width formed by deposition GEOLOGIC SETTING basin. Seismic anisotropy of the Dun Mountain of crystals on the walls of the pipe during succes- ultramafic is similar to observed anisotropy in sive pulses of magma injection. Challis (1965) The Dun Mountain ophiolite belt (Fig. 1), the upper oceanic mantle. also considered the Dun Mountain ultramafic to consisting of mafic and ultramafic rocks, is a major geologic feature of the South Island of INTRODUCTION New Zealand. It is divided into two segments separated by the Alpine Fault, a northern or Dun Mountain, located —10 km southeast of Nelson Belt and a southern or Otago Belt. The Nelson on South Island, New Zealand, is the two segments, considered to once have been type locality of dunite. The rock was originally continuous, have been separated by a 480-km named by von Hochstetter (1864) after the dun- dextral shift on the Alpine Fault (Wellman, brown-colored mountain, which in places is 1948). Geophysically, both segments are charac- largely devoid of vegetation. The fresh rock is terized by strong positive magnetic anomalies light olive green in color, whereas a thin weath- (Hatherton, 1969). ering rind common on the rock exposures has a The Nelson segment of the ophiolite belt con- characteristic reddish-brown color. This, as well sists of a band of serpentinite, dunite, and harz- as the presence of tussock, a tan grass that covers burgite, varying in width from 0 to ~8 km; this the shallower slopes, is responsible for the dun is bordered to the west by discontinuous seg- color of the mountain, which stands out in direct ments of gabbro and mafic volcanics. In the northernmost portion of the Nelson segment at •Present address: Department of Geosciences, Figure 1. The Dun Mountain ophiolite belt, D'Urville Island, the ultramafic rocks consist of Purdue University, West Lafayette, Indiana 47907. South Island, New Zealand. a serpentinite mélange (Coleman, 1966). Al- Geological Society of America Bulletin, v. 95, p. 551-558,11 figs., May 1984. 551 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/5/551/3444891/i0016-7606-95-5-551.pdf by guest on 01 October 2021 552 N. I. CHRISTENSEN tered dolerites and mafic volcanics are common on the western border of the serpentinite. Dun Mountain is one of two relatively unserpentin- ized sections of the Nelson segment, the other being Red Hills, which is cut off on its southern margin by the Wairau Fault, a branch of the Alpine Fault (Fig. 1). With more than 1 km of exposed vertical section and an outcrop area of -110 km2, the Red Hills massif is the largest and best-exposed ultramafic of the Nelson seg- ment. At Red Hills, numerous faults bound and crosscut the ultramafic massif (Walcott, 1969) and gravity studies (Malahoff, 1965) indicate a total thickness of <4 km, which suggests that the base of the massif is also in fault contact. To the south and on the opposite side of the Alpine Fault, the Otago segment of the Dun Mountain ophiolite belt consists of several ul- tramafic massifs, often with associated mafic rocks. The northernmost ultramafic massif is that of the Cascade River-Martyr Hill region, which, like Red Hills, is truncated by the Alpine Fault (Fig. 1). Samples collected by the author from this region show variable serpentinization and strong tectonite fabrics. Red Mountain (Coombs and others, 1976; Sinton, 1977), lo- cated south of the Cascade River-Martyr Hill massifs, is the best-preserved ophiolite section of the Dun Mountain belt. The mafic sequence contains pillow basalts, a dike complex which is not sheeted, and a gabbro section often in fault contact with ultramafic rocks consisting primar- ily of harzburgite and dunite. Similar rocks occur —30 km south at Cow Saddle. Farther south, the ophiolite belt consists of a series of melanges containing inclusions of spi- lite, diabase, gabbro, and metasedimentary rocks in a matrix of argillite, spilite, and serpentinite. sandstone, siltstone FAULT In the Bald Hill area, an attenuated crostai sec- MAITAI GROUP and limestone tion consisting of spilitic lavas, a dike complex, basalt, volcanic breccia ^Q FOLIATION, and gabbro and massive gabbro is believed to have under- LEE RIVER y LAYERING gone tectonic thinning (Coombs and others, dunite, harzburgite , 1976). Southeast of Mossburn (Fig. 1), the DUN MOUNTAIN and serpentinite melange is) ophiolite belt consists of a tectonic mélange of spilite, serpentinite, argillite, diabase, spilite, gabbro, sedimentary rocks, and PATUKI MELANGE conglomerate and amphibolile felsic igneous rocks in a serpentinite matrix f I l sandstone, siltstone (Coombs and others, 1976). PELORUS GROUP L^J and mudstone The geology in the vicinity of Dun Mountain Figure 2. Geology in the vicinity of Dun Mountain. modified from maps of Lauder (1965), Cole- man (1966), Hunt (1978), and Davis and others northwest of the serpentinite and on the south- sisting of massive gabbro, microgabbro, and (1980) is shown in Figure 2. Dun Mountain east side of Wooded Peak, there is a belt of metamorphosed mafic dikes and sills, and the (elevation 1,129 m) contains relatively good ex- mafic igneous rocks that with the ultramafic Glennie Formation, containing mctabasalt of posures, over an —2 km2 area, of fresh harz- rocks forms the ophiolite belt. The mafic rocks greenschist-facies grade. Unconformibly overly- burgite and dunite surrounded by serpentinite were mapped as the Little Twin Spilite by ing, and northwest of, the Lee River Group, (Reed, 1959). On the northwest flank of Dun Lauder (1965) and were included as pait of the there are late Permian limestones, siltstones, and Mountain, a narrow band of serpentinite, —1 Te Anau Group. Waterhouse (1964) and Wal- sandstones of the Maitai Group (Fig. 2). km in width, extends to the northeast and cott (1969) referred to these as part of the Lee Bounded on the northwest by the Wooded southwest. Foliations in the serpentinite are River Group. Davis and others (1980) subdi- Peak Fault and bordering Dun Mountain on its generally parallel to the strike of the band and vided the Lee River Group in the vicinity of southwestern, southern, and eastern Hanks, there dip steeply to the southeast or northwest. To the Dun Mountain into the Tinline Formation, con- is a belt of mélange, generally 2 to 4 km wide, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/5/551/3444891/i0016-7606-95-5-551.pdf by guest on 01 October 2021 DUN MOUNTAIN MASSIF, NEW ZEALAND 553 that trends northeast-southwest and consists of a sheared serpentinite matrix and blocks of argil- lite, spilite, conglomerate, and amphibolite. The inclusions were described by Bell and others (1911), Waterhouse (1959), and Coleman (1966).