Formation and emplacement of the Northland ophiolite, northern New Zealand: SW Pacific tectonic implications

Journal of the Geological Society, Mar 2005 by Whattam, Scott A, Malpas, John, Ali, Jason R, Lo, Ching-Hua, Smith, Ian E M

Abstract:

Petrological, geochemical, geochronological and palaeomagnetic data for rocks of the Northland ophiolite terrane of northern New Zealand suggest that it formed in a suprasubduction-zone setting between c. 29 and 26 Ma. at c. 35°S. close to its Late Oligocene obduction site. Cretaceous igneous rocks formerly considered to be part of the ophiolite probably represent the basement upon which the ophiolite was emplaced. and are probably part of the Mount Camel arc-related terrane. The ophiolite is believed to have been generated in the southeastern South Fiji Basin, close to a NW-SE-oriented transform fault located to the SW of the Vening Meinesz Fracture Zone, and was probably emplaced in response to the collision of the Hikurangi Plateau with eastern New Zealand at the end of the Oligocene. This collision would have involved a major adjustment on the transform fault, thereby allowing a portion of the upper-crustal section of the southern South Fiji Basin to be emplaced southwestward onto northern New Zealand as well as the coeval emplacement of the East Cape Allochthon to the south. Concomitant subduction of the lower crust-mantle section led to the initiation of arc volcanism that resulted in the Northland Lower Miocene volcanic-plutonic suite.

Keywords: SW Pacific, Northland ophiolite, tectonics, palaeomagnetism, absolute age.

The 100-0 Ma tectonic evolution of the SW Pacific region has been complex (Yan & Kroenke 1993: Hall 2002). In the Early Cretaceous, the eastern edge of the Australian Plate formed a convergent boundary against which various 'Pacific' oceanic terranes were accreted. From about 80 to 55 Ma, eastern Gondwana experienced large-scale extension and fragmentation, and a number of ribbon-like slivers of Australian basement simultaneously separated from the margin extending the Australian Plate >2000km eastward (Falvey & Mutter 1981; Gaina et al. 1998). In the Palaeogene, the eastern edge of the plate became a convergent boundary and oceanic terranes were accreted against the Australian Plate at various stages in eastern New Guinea, New Caledonia and northern New Zealand (Malpas et al. 1992, 1994). Since the beginning of the Neogene, the system has continued to evolve, particularly in the formation of the three-ridge spreading system in the West Fiji Basin and with development of the Lau (back-arc) Basin.

Geological setting

The Northland ophiolite of northern New Zealand (Fig. 1) comprises massifs of mainly basaltic volcanic rocks, which form the upper thrust slices of the Northland Allochthon. It represents a key fragment in a commonly accepted regional model (Aubouin et al. 1977; Walcott 1978; Parrot & Dugas 1980), where it forms the youngest emplaced segment of an almost continuous Late Cretaceous-Eocene oceanic belt that collided with eastern Gondwana diachronously, starting in the north in New Guinea in the Palaeocene (Davis 1971) followed by New Caledonia in the latter part of the Eocene (AIi & Aitchison 2000). However, to understand the role of the Northland ophiolite in the development of the SW Pacific, a number of points need to be addressed, including the tectonic setting in which it formed, when and where it formed, and how it relates to other regional tectonic elements.

Despite the efforts of various workers (Malpas et al. 1992; Cassidy 1993; Nicholson et al. 2000a, b) a clear consensus on the origin of the Northland ophiolite has not yet emerged. One of the biggest questions concerns its age. Traditionally, it has been accepted that the ophiolite formed in the Late Cretaceous to Palaeocene, based on micro- and macrofaunas recovered from a number of massifs (Farnell 1973; Brook et al. 1988; Larsen & Spörli 1989; Mollis & Hanson 1991). Until recently, radiometric dates have been based on K-Ar dating methods, but the large spread of ages (100-42Ma) (Brothers & Delahoye 1982) has made their interpretation and synthesis difficult in a tectonic model. The problem with a c. 80-55 Ma fossil-age spread for the generation of the ophiolite is that it is difficult to develop a plate model that places such old oceanic crust adjacent to northern New Zealand in the Late Oligocene (Malpas et al. 1992). In an attempt to resolve this conundrum, we have carried out multidisciplinary research involving field studies, radiometric dating, geochemistry, palaeontology and palaeomagnetism. (For example, geochemical data clearly indicate that all rocks mapped as ophiolite formed in suprasubduction-zone environments (Hopper & Smith 1996; Thompson et al. 1997; Whattam 2003). The geochemical data, however, when placed in a regional context and matched with other nearby volcanic bodies (e.g. Mount Camel Volcanics, and possibly the (?)(Cretaceous Hikurangi Plateau basalts) (Kamp 1986), together with our radiometric data, suggest that the Northland ophiolite, as it is currently mapped, represents two distinct units; one relatively minor group of rocks that formed in the Late Cretaceous and the main part of the allochthonous igneous terrane that formed in the Late Oligocene).

Field exposures

Northland ophiolite

Outcrops consist primarily of basaltic pillow lavas and less common sheet flows. Basalt dominates with subordinate diabase and gabbro, together with minor alkalic rocks (Larsen & Parker 1989; Malpas et cil. 1992; Thompson et al. 1997). Rare ultramafic rocks crop out at North Cape (Bennett 1976) (Fig. 1) and include a variety of lithologies including serpentinite, cumulate harzburgite and Iherzolite, olivine clinopyroxenite, wehrlite dykes and hornblendite (Malpas et al. 1992). In addition, minor intermediate and acid intrusive rocks that include diorite, quartz diorite and plagiogranite form part of the terrane (Malpas et al. 1992; Thompson et al. 1997). The plagiogranites are present as irregular dykes and micro-sills intruded into the high-level gabbros of the ophiolite.