N. Mortimer

Cretaceous felsic volcanism in New Zealand and Lord Howe Rise (Zealandia) as a precursor to final Gondwana break-up

Abstract We report new, highly precise, U–Pb and Ar/Ar ages for seven Cretaceous rhyolites, tuffs and granites from across Zealandia spanning a 30 Ma period from arc magmatism to continental break-up. Combined with previously published data, these reveal a strong episodicity in Cretaceous silicic magmatism outside the Median Batholith. 112 Ma tuffs are known only from the Eastern Province in association with a Cretaceous normal fault system. Both 101 and 97 Ma groups of rhyolites and tuffs occur across the entire width and half the length of Zealandia from near the palaeotrench to the continental interior, indicating widespread and effectively instantaneous extension. We attribute an increase in A-type character with time (112–101–97–88–82 Ma) to the progressive thinning of the Zealandia continental crust whereby, with time, there is less opportunity for crustal contamination. Extension directions associated with 101, 97 and 82 Ma magmatism and associated core complex exhumation across Zealandia are all oriented c. 30° oblique to the margin. These observations suggest Zealandia rifting was controlled by either >83 Ma capture of Zealandia by the Pacific Plate and/or <83 Ma Zealandia–West Antarctica spreading, rather than by laterally migrating triple junctions, slab windows or plume heads.

Basement geology of Taranaki and Wanganui Basins, New Zealand

Abstract We present a revised interpretation of the basement geology beneath Late Cretaceous to Cenozoic Taranaki and Wanganui Basins of central New Zealand, based on new petrographic, geochemical, and geo‐chronological data from 30 oil exploration wells. Recently published structural and magnetic interpretations of the area assist in the interpolation and extrapolation of geological boundaries. Torlesse and Waipapa Terranes have been identified in Wanganui Basin, and Murihiku Terrane in eastern Taranaki Basin, but Maitai and Brook Street Terrane rocks have not been recognised. Separation Point Suite, Karamea Suite, and Median Tectonic Zone igneous rocks are all identified on the basis of characteristic petrography, geochemistry, and/or age. SHRIMP U‐Pb zircon measurements on igneous samples from western Taranaki wells do not give precise ages but do provide useful constraints: Motueka‐1 granite is latest Devonian — earliest Carboniferous; Tangaroa‐1 and Toropuihi‐1 are Carboniferous; and Surville‐1 is Cr...

Age and isotopic characterisation of metasedimentary rocks from the Torlesse Supergroup and Waipapa Group in the central North Island, New Zealand

Abstract Detrital zircon U‐Pb ages in grey wackes and initial 87Sr/86Sr ratios are reported for low‐grade metasedimentary rocks from Torlesse Supergroup, Waipapa Group, and Kaimanawa Schist in the central North Island, New Zealand. The data reveal the presence of a hitherto unsuspected, areally extensive, Jurassic part of the Torlesse composite terrane in the Kaimanawa, Kaweka, and Ruahine Ranges, which we name the Kaweka Terrane. Kaweka rocks have initial 87Sr/86Sr ratios (at metamorphism) and detrital zircon patterns that in part are transitional between Rakaia and Pahau Terrane rocks and in part similar to Waipapa Terrane rocks. Combined detrital zircon age data for all Torlesse and Waipapa Terrane data reveal an essential unity, with a long persistence (260–120 Ma) of predominant Permian‐Triassic sources in the form of a major Cordilleran‐style batholith, a decline in major early Paleozoic‐Precambrian sources between 260 and 220 Ma, and presence of minor Early Carboniferous to Late Devonian sources between 180 and 120 Ma. Rb‐Sr and K‐Ar ages indicate latest Triassic to Early Cretaceous metamorphism in an evolving accretionary wedge.

Geology of the Permian Kuriwao Group, Murihiku Terrane, Southland, New Zealand

Abstract Kuriwao Group rocks form a small inlier (5×2 km) of calcareous and volcaniclastic Permian rocks in Southland, surrounded by Triassic and Jurassic Murihiku Supergroup. Previously reported Kuriwao Group field relationships, macrofauna, leaf fossils, and palynoflora have been re‐examined and interpretations are made of these and new data. A Late Permian age for sandstones and limestones of the Kuriwao Group is confirmed and no evidence is found that the limestones are olistoliths. Field relationships and palynological data suggest that Early‐Middle Triassic Wairuna Peak Group (Murihiku Supergroup) strata lie in conformable sedimentary contact on the Kuriwao Group. The petrological composition of Kuriwao Group sandstones closely resembles quartz‐poor sandstones in overlying Wairuna Peak Group and differs from broadly coeval Permian sandstones in the Brook Street and Maitai Terranes. Kuriwao Group should be regarded as part of the Murihiku tectonostratigraphic terrane. With the age of Murihiku Terrane extending back into the Permian, some 120 m.y. of geological history are represented in an aggregate thickness of c. 16 km of little‐deformed strata, although not in any one section. Permian rocks are now known from seven New Zealand terranes.

Continuation of the New England Orogen, Australia, beneath the Queensland Plateau and Lord Howe rise

Greywacke, argillite, greyschist and hypabyssal igneous rocks have been obtained from an Ocean Drilling Program core on the Queensland Plateau and from xenoliths in a volcanic breccia dredged from the crest of the Lord Howe Rise. Low to intermediate detrital quartz contents, 260 – 240 Ma K – Ar ages, and only moderately radiogenic Sr and Nd isotope compositions, suggest a correlation with the New England Orogen of eastern Australia, rather than with Australias Lachlan Orogen or other adjacent geological provinces. Our results indicate that the New England Orogen terranes continue towards New Zealand at least as far as the southern Lord Howe Rise. The projected offshore boundaries of the major east Australian orogens are now known with more confidence, and do not appear to require any major cross-orogen offsets.

New insights into the origin and evolution of the Hikurangi oceanic plateau

Oceanic plateaus and continental flood basalts, collectively referred to as large igneous provinces (LIPs), represent the most voluminous volcanic events on Earth. In contrast to continental LIPs, relatively little is known about the surface and internal structure, range in age and chemical composition, origin, and evolution of oceanic plateaus, which occur throughout the worlds oceans [e.g., Mahoney and Coffin, 1997]. n nOne of the major goals of the R/V Sonne SO168 ZEALANDIA expedition (depart Wellington, 3 December 2002, return Christchurch, 15 January 2003) was to investigate the Hikurangi oceanic plateau off the east coast of New Zealand.

Petlab: New Zealand’s national rock catalogue and geoanalytical database

ABSTRACT The Petlab database is New Zealand’s online national rock and analytical database, and provides an accessible, structured and permanent data repository for the local and global geoscience communities. The database contains locations, descriptions and analyses of rock and mineral samples from onland New Zealand, the offshore New Zealand region, Antarctica and worldwide. It is operated by GNS Science and, as a nationally significant database, currently receives core funding from central government. The major data contributors are GNS Science and Auckland, Massey, Waikato, Victoria, Canterbury and Otago universities, all of whom use the database as a digital catalogue for their rock and mineral collections. Petlab sample information and geoanalytical data can be uploaded, queried, viewed and downloaded at http://pet.gns.cri.nz. Petlab is a valuable tool for geoscientists in research and industry.

U–Pb geochronology of Permian plutonic rocks, Longwood Range, New Zealand: implications for Median Batholith–Brook Street Terrane relations

We present U–Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages from gabbros and associated intrusive rocks of the Longwood Range, Southland. U–Pb zircon ages of 257.6 ± 2.5 Ma, 256.5 ± 1.8 Ma and 251.6 ± 2.0 Ma (2σm) have been obtained from the layered, platiniferous Hekeia Gabbro. These are 5–10 Ma older than cooling ages previously obtained by Ar–Ar dating of hornblendes. A revised U–Pb zircon age of 252.8 ± 2.7 Ma is reported for a sample of western Pourakino Trondhjemite [previously dated at 292 ± 8 Ma by multicrystal 207Pb/206Pb thermal ionization mass spectrometry (TIMS) methods] and a dike of the trondhjemite that cuts the Hekeia Gabbro is dated at 254.8 ± 2.6 Ma. Our new zircon ages show that the Hekeia Gabbro and Pourakino Trondhjemite were intruded within the same, relatively narrow interval from 261 to 252 Ma. This age range is substantially younger than Early Permian (c. 276–288 Ma) ages inferred from fossil evidence of major Takitimu Group volcanism in the adjacent Brook Street Terrane. We make the novel suggestion that the Hekeia and Pourakino bodies, along with plutons at Bluff and Oraka Point, constitute a new Longwood Suite of the Median Batholith that is spatially and temporally distinct from allochthonous Brook Street Terrane volcanic magmatism. A U–Pb zircon age of 232.3 ± 1.5 Ma from a western Longwood Range granodiorite is the oldest Darran Suite pluton so far recognized and represents the start of a major, isotopically more evolved, pulse of the Median Batholith.

Paleocene MORB and OIB from the Resolution Ridge, Tasman Sea

Altered lavas have been dredged from three locations on the Resolution Ridge, west of New Zealands South Island. On the basis of whole-rock geochemistry, Sr, Nd and Pb isotope data and Ar–Ar ages, they can be divided into two suites: 62–60 Ma enriched mid-ocean ridge basalt (E-MORB), and 57 Ma trachybasalt and trachyandesite of ocean island basalt (OIB) affinity. The E-MORBs from the Resolution Ridge are only the second place from which Tasman Sea abyssal oceanic crust has ever been sampled, they have Indian MORB-like isotope compositions, and their ages support a recent interpretation of a 100 km sinistral offset of the southern part of the Tasman Sea spreading ridge. The slightly younger OIB suite erupted shortly after oceanic crust formation and has FOZO to HIMU source characteristics similar to the well-known SW Pacific Diffuse Alkaline Magmatic Province (DAMP). The close occurrence and isotopic mixing relationships of both Paleocene volcanic suites on the Resolution Ridge may be explained by a heterogeneous upper mantle in which the more fertile OIB component was extracted during a later melting event away from the spreading ridge. The dredged lavas predate formation of Southeast Tasman oceanic crust that borders the Resolution Ridge to the south.

Triassic–Jurassic granites on the Lord Howe Rise, northern Zealandia

We present U–Pb zircon ages from a phosphate-cemented pebbly sandstone dredged from the central Lord Howe Rise and a 97 Ma rhyolite drilled on the southern Lord Howe Rise. Four granitoid pebbles from the sandstone give U–Pb ages in the range 216–183 Ma. Most detrital zircons in the bulk sandstone are also Late Triassic–Early Jurassic, but subordinate populations of Late Cretaceous and Precambrian zircons are present. The pebbly sandstones highly restricted Late Triassic–Early Jurassic zircon population indicates the nearby occurrence of underlying basement plutons that are the same age as parts of the I-type Darran Suite, Median Batholith of New Zealand and supports a continuation of the Early Mesozoic magmatic arc northwest from New Zealand. Zircon cores from the southern Lord Howe Rise rhyolite do not yield ages older than 97 Ma and thus provide no information about older basement.