C. J. Adams

Provenance comparisons of Permian to Jurassic tectonostratigraphic terranes in New Zealand: perspectives from detrital zircon age patterns

U–Pb detrital zircon ages (LAM-ICPMS) are reported for 20 greywackes and sandstones from seven major tectono-stratigraphic terranes of the Eastern Province of New Zealand (Cretaceous to Carboniferous) to constrain sediment provenances. Samples are mainly from three time horizons: Late Permian, Late Triassic and Late Jurassic. Age datasets are analysed as percentages in geological intervals, and in histogram and cumulative probability diagrams. The latter discriminate significant zircon age components in terms of terrane, sample stratigraphic age, component age, precision and percentage (of total set). Zircon age distributions from all samples have persistent, large Triassic–Permian, and very few Devonian–Silurian, populations, features which exclude a sediment provenance from the early Palaeozoic, Lachlan Fold Belt of southeast Australia or continuations in New Zealand and Antarctica. In the accretionary terranes, significant Palaeozoic (and Precambrian) zircon age populations are present in Torlesse and Waipapa terranes, and variably in Caples terrane. In the fore-arc and back-arc terranes, a unimodal character persists in Murihiku and Brook Street terranes, while Dun Mountain–Maitai terrane is more variable, and with Caples terrane, displays a hybrid character. Required extensive Triassic–Permian zircon sources can only be found within the New England Fold Belt and Hodgkinson Province of northeast Australia, and southward continuations to Dampier Ridge, Lord Howe Rise and West Norfolk Ridge (Tasman Sea). Small but significant Palaeozoic (and Precambrian) age components in the accretionary terranes (plus Dun Mountain–Maitai terrane), have sources in hinterlands of the New England Fold Belt, in particular to mid-Palaeozoic granite complexes in NE Queensland, and Carboniferous granite complexes in NE New South Wales. Major and minor components place sources (1) for the older Torlesse (Rakaia) terrane, in NE Queensland, and (2) for Waipapa terrane, in NE New South Wales, with Dun Mountain–Maitai and Caples terrane sources more inshore and offshore, respectively. In Early Jurassic–Late Cretaceous, Torlesse (Pahau) and Waipapa terranes, there is less continental influence, and more isolated, offshore volcanic arc sources are suggested. There is local input of plutonic rock detritus into Pahau depocentres from the Median Batholith in New Zealand, or its northward continuation on Lord Howe Rise. Excepting Murihiku and Brook Street terranes, all others are suspect terranes, with depocentres close to the contemporary Gondwanaland margin in NE Australia, and subsequent margin-parallel, tectonic transport to their present New Zealand position. This is highlighted by a slight southeastward migration of terrane depocentres with time. Murihiku and Brook Street terrane sources are more remote from continental influences and represent isolated offshore volcanic depocentres, perhaps in their present New Zealand position.

K‐Ar ages of early Miocene arc‐type volcanoes in northern New Zealand

Abstract Understanding the temporal and spatial development of the early Miocene Northland Volcanic Arc is critical to interpreting the patterns of volcanic activity in northern New Zealand through the late Cenozoic. The northwesterly trending arc is considered to have developed above a southwest‐dipping subduction system. The distribution of its constituent eruptive centres is described in terms of an eastern belt that extends along the eastern side of Northland and a complementary broad western belt which includes subaerial and submarine volcanic edifices. Critical examination of all 216 K‐Ar ages available, including 180 previously unpublished ages, and their assessment against tectonic, lithostratigraphic, seismic stratigraphic, and biostratigraphic constraints, leads us to deduce a detailed chronology of periods of activity for the various early (and middle) Miocene arc‐type volcanic complexes and centres of northern New Zealand: Waipoua Shield Volcano Complex (19–18 Ma, Altonian); Kaipara Volcanic Complex (23–16 Ma, Waitakian‐Altonian); Manukau Volcanic Complex (c. 23–15.5 Ma, Waitakian‐Clifdenian); North Cape Volcanic Centre (23–18 Ma, Waitakian‐Altonian); Whangaroa Volcanic Complex (22.5–17.5 Ma, Waitakian‐Altonian); Taurikura Volcanic Complex (22–15.5 Ma, Otaian‐Clifdenian); Parahaki Dacites (22.5–18 Ma, Waitakian‐Altonian); Kuaotunu Volcanic Complex (18.5–11 Ma, Altonian‐Waiauan). In general, volcanic activity does not show geographic migration with time, and the western (25–15.5 Ma) and eastern (23–11 Ma) belts appear to have developed concurrently.

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.

Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia∗

In easternmost Australia, the New England Orogen contains a geological record dominated by subduction-related rocks, with plate convergence during the Late Devonian to Triassic being related to a west-dipping subduction system, assuming present-day orientation, at the boundary of eastern Gondwanaland and the Panthalassan Ocean. A well-preserved Late Paleozoic accretionary wedge contains deep-marine turbidites deposited as trench fill, plus infaulted slices of oceanic crust. The turbidites are mostly first-cycle, immature, quartz-poor, volcanic-derived sedimentary rocks, some of which contain detrital hornblende, along with less-common quartz-rich sandstones to the east. In this study, detrital zircons from sandstones in various tectonic blocks of the New England Orogen are dated by the U–Pb SHRIMP and LA-ICPMS techniques and detrital hornblendes by the Ar–Ar technique to constrain the age and provenance of sedimentary rocks in the accretionary wedge. All samples, except two quartz-rich sandstones from the northern Shoalwater Formation, have maximum depositional ages of 355–316 Ma, indicating that the accretionary wedge evolved over a period of at least 40 Ma, with principal sources from a contemporaneous active continental margin volcanic arc. Quartz-rich sandstones from the easternmost part of the accretionary wedge (Shoalwater Formation and eastern Beenleigh Block) contain a greater range of individual detrital zircon ages from Late Paleozoic to Archean (several individual grains >3000 Ma). These ages indicate that, although detritus from Carboniferous volcanic arc sources was involved, quartz-rich detritus mostly derived from the continental interior dominated the depocentres. We suggest that these quartz-rich sandstones accumulated from longitudinal transport along the trench, like the modern-day Barbados Ridge accretionary wedge, along with breaching of the marginal arc by streams draining the continental interior.

Zealandia: Earth’s Hidden Continent

A 4.9 Mkm2 region of the southwest Pacific Ocean is made up of continental crust. The region has elevated bathymetry relative to surrounding oceanic crust, diverse and silica-rich rocks, and relatively thick and low-velocity crustal structure. Its isolation from Australia and large area support its definition as a continent—Zealandia. Zealandia was formerly part of Gondwana. Today it is 94% submerged, mainly as a result of widespread Late Cretaceous crustal thinning preceding supercontinent breakup and consequent isostatic balance. The identification of Zealandia as a geological continent, rather than a collection of continental islands, fragments, and slices, more correctly represents the geology of this part of Earth. Zealandia provides a fresh context Nick Mortimer, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Hamish J. Campbell, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Andy J. Tulloch, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Peter R. King, Vaughan M. Stagpoole, Ray A. Wood, Mark S. Rattenbury, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Rupert Sutherland, SGEES, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand; Chris J. Adams, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Julien Collot, Service Géologique de Nouvelle Calédonie, B.P. 465, Nouméa 98845, New Caledonia; and Maria Seton, School of Geosciences, University of Sydney, NSW 2006, Australia in which to investigate processes of continental rifting, thinning, and breakup.

Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand

Abstract Fifty‐six previously unpublished K‐Ar ages for the Dunedin Volcanic Group and previously published K‐Ar and 40Ar/39Ar ages demonstrate that activity in the centrally situated Dunedin Volcano (here given formal lithostrati‐graphic status) lasted from 16.0 ± 0.4 to c. 10.1 Ma, and that of the surrounding Waipiata Volcanics lasted from 24.8 ± 0.6 to 8.9 ± 0.9 Ma. Apart from a gap at c. 20 Ma, recorded Waipiata activity climaxed at c. 16–14 Ma when activity of the Dunedin Volcano was beginning; it outlasted that of the Dunedin Volcano by c. 1 m.y. The total volume erupted by the Dunedin Volcano may have exceeded that of the largely monogenetic Waipiata Volcanics by an order of magnitude. New major‐ and trace‐element analyses are given for 87 whole‐rock samples and kaersutite. The whole‐rock data demonstrate the exclusively alkalic nature of the group, the Waipiata Volcanics being more strongly alkalic than most of the mafic members of the central volcano. This fractionated to give a much greater volume of phonolitic differentiates than the Waipiata Volcanics. As for other intraplate Cenozoic volcanism in the New Zealand region, ranging overall from tholeiitic to highly alkalic, major‐ and trace‐element patterns support an origin from a garnet‐bearing ocean island basalt source region with high U/Pb mantle characteristics.

Detrital zircon ages and geochemistry of sedimentary rocks in basement Mesozoic terranes and their cover rocks in New Caledonia, and provenances at the Eastern Gondwanaland margin.

Geochemical and Sr–Nd isotope data for Mesozoic greywackes of New Caledonia terranes, indicate a forearc tectonic environment at the Eastern Gondwanaland margin, but they support only minor continental influences. Detrital-zircon U–Pb age patterns for the greywackes in these terranes similarly reflect an active-margin tectonic environment of Late Triassic, Late Jurassic, and in particular mid-Cretaceous, depocentres which comprise much contemporaneous volcanic detritus, but also include minor sediment inputs from Precambrian–Early Paleozoic continental clastic rocks. The contemporary volcanic sources are probably now hidden within a former hinterland to New Caledonia, such as Lord Howe Rise or Marion Plateau. The older, continental sediment sources were probably in northeasternmost Queensland, and beyond the northern extremity of the New England Orogen. Such sediments could have been supplied on long rivers, and submarine longshore current systems outboard of the orogen. Alternatively, the depocentres could have been consolidated close to the contemporary Gondwanaland margin and then tectonically transported, as suspect terranes, southwards in Early Cretaceous times to their present New Caledonia position.

Discovery of Early Cretaceous Rocks in New Caledonia: New Geochemical and U-Pb Zircon Age Constraints on the Transition from Subduction to Marginal Breakup in the Southwest Pacific

New U-Pb dating of detrital zircon and geochemical features of Permian-Mesozoic arc-derived volcanic rocks and volcaniclastic turbidites (graywackes), when compared with those of the volcanic rocks associated with unconformable Late Cretaceous shallow-water sediments, reveal that subduction in New Caledonia, once thought to be extinct in the Late Jurassic (ca. 150 Ma), was still active at least from ca. 130 to 95 Ma. The accumulation of volcanic arc-derived sediments during the late Early Cretaceous suggests that, as in New Zealand, active-margin activity went on for a short time in spite of the assumed subduction jamming by the Hikurangi Plateau at ca. 100 Ma. Meanwhile, the rift-related magmatic activity that preceded the marginal breakup migrated eastward from ca. 130 Ma (130–95 Ma) in eastern Australia, to 110 Ma (110–82 Ma) in New Zealand, and, finally, to ca. 89 Ma (89–83 Ma) in New Caledonia and generated large volumes of silicic magma. In contrast, marginal basins opened synchronously at ca. 83 Ma when the stretched continental crust finally broke out. In general, intraplate and volcanic arc signatures coexisted in Cretaceous syn-rift magmas. Therefore, the Australian marginal breakup appears to be the final effect of continuous southward unzipping of Gondwana that interfered with the subduction-modified mantle wedge of the Mesozoic active margin. The occurrence of lateral flow of the upper asthenospheric mantle due to the rapidly eastward-migrating Australian plate margin possibly prevented the formation of a volcanic arc at the eastern end of the system.

Tracing the Caples Terrane through New Zealand using detrital zircon age patterns and radiogenic isotope signatures

Abstract Rb‐Sr isochron ages and associated initial 87Sr/86Sr ratios of metasedimentary rocks of Caples Group in Southland and Otago, Pelorus Group in Nelson and Marlborough, and their minor North Island correlates in Northland and King Country, reveal the essential continuity of a long Caples Terrane throughout New Zealand. The Rb‐Sr ages, in the range 251–117 Ma, record post‐metamorphic, Early Triassic to Early Cretaceous uplift and cooling, whilst their associated range of initial 87Sr/86Sr ratios, 0.7039–0.7053, allows a discrimination from analogous datasets of neighbouring Torlesse and Waipapa Terranes. Some petrographically and geochemically anomalous Caples Terrane rocks in East Otago, and Otago Schists of the Aspiring Terrane, all with initial 87Sr/86Sr ratios >0.7055, compare best with dataseis of Waipapa Terrane of the North Island. Detrital zircon U‐Pb ages of greywackes from Caples Terrane metasediments have youngest zircon age components from 251 to 215 Ma, and a comparison of dataset patterns from fossiliferous examples suggests that these probably originate from contemporary volcanic sources. Caples volcanism and associated depocentres thus span much of the Triassic. However, their major zircon component, invariably in the range Early Triassic to latest Permian, is most persistent, and constitutes an enormous and enduring zircon source thought to be in the New England Orogen of eastern Australia. Minor, late Paleozoic (295–340 Ma) zircon components are restricted to (Early‐Middle Triassic) Caples Terrane metasediments with older, minimum zircon age components, perhaps originating in nearby primary sources in the northeasternmost Lachlan Fold Belt and southern part of the New England Orogen. More scattered early Paleozoic‐Precambrian zircons are present throughout the Caples Terrane and reflect reworking of zircons from Paleozoic metasediments and plutonic rocks within the New England Orogen (and its immediate hinterland). Caples Terrane metasediments thus represent an offshore, Triassic component of the accretionary prism terranes (Late Carboniferous to Early Cretaceous) of the Eastern Province of New Zealand. Their depocentres were more isolated from the Gondwanaland continental m argin than Torlesse and Waipapa Terrane counterparts, and more influenced by substantial contemporary volcanic centres, perhaps located along the Lord Howe Rise.

The mid-Cretaceous transition from basement to cover within sedimentary rocks in eastern New Zealand: evidence from detrital zircon age patterns

Detrital zircon U-Pb ages for 30 Late Jurassic and Cretaceous sandstones from the Eastern Province of eastern New Zealand, combined with previously-published geochronological and palaeontological data, constrain the time of deposition in the Pahau and Waioeka terranes of the Cretaceous accretionary margin of Zealandia, and their adjacent cover strata. The zircon age patterns also constrain possible sediment source areas and mid-Cretaceous geodynamic models of the transition from basement accretionary wedge to passive-margin cover successions. Pahau Terrane deposition was mainly Barremian to Aptian but continued locally through to late Albian time, with major source areas in the adjacent Kaweka and Waipapa terranes and minor inputs from the inboard Median Batholith. Waioeka Terrane deposition was mainly Albian, with distinctive and exclusive sediment sources, principally from the Median Batholith but with minor inputs from the Western Province. Alternative tectonic models to deliver such exclusive Median Batholith and Western Province-derived sediment to the mid-Cretaceous Zealandia continental margin are: (1) the creation of a rift depression across Zealandia or (2) sinistral displacement of South Zealandia with respect to North Zealandia, to expose Western Province rocks directly at the Zealandia margin. Detrital zircon age patterns of Cretaceous cover successions of the Eastern Province of eastern New Zealand demonstrate purely local sources in the adjacent Kaweka and Waipapa terranes. Cretaceous zircon components show a decline in successions of late Early Cretaceous age and disappear by late Late Cretaceous time, suggesting the abandonment or loss of access to both the Median Batholith and Western Province as sediment sources.