Takaka Terrane Research Articles

Separation of U, Pb, Lu, and Hf from single zircons for combined U–Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

The U–Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U–Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed 176Lu– 180Hf and 233U– 205Pb tracers and then digested in HF–HNO 3. Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with Eichrom® Ln Spec resin. Using only ∼100 μl of resin for the separation keeps Pb blanks low (∼5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 μm (∼12pg of Pb and ∼3ng of Hf), can be analyzed with an external 176Hf/ 177Hf precision of ∼100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day 176Hf/ 177Hf of 0.282305 ± 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U–Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf.

Rb‐Sr age and strontium isotope characteristics of the Greenland Group, Buller Terrane, New Zealand, and correlations at the East Gondwanaland margin

Rb‐Sr whole‐rock isochron ages of early Paleozoic, Greenland Group (Buller Terrane) metasediments from 17 localities range from 458 ± 9 to 271 ± 26 Ma, in a pattern similar to, but frequently 20–30 m.y. older than, corresponding K‐Ar ages of slates. The ages reflect an original low‐grade regional metamorphism at 450 ± 10 Ma in Late Ordovician times and subsequent cooling, which has then been pervasively overprinted by younger thermal events, most probably associated with the emplacement of Devonian‐Carboniferous granitoids. Initial 87Sr/86Sr ratios (i) at the time of metamorphism (t) increase from c. 0.717 at c. 450 Ma to c. 0.722 at c. 350 Ma—this evolution reflecting a bulk sediment Rb/Sr ratio of c. 1.0. The (t)‐(i) data suggest that the Greenland Group source area is extensive granitoid (mostly S‐type) and older metamorphic complexes, similar to the Wilson Terrane of the Ross and Adelaide Foldbelts of Antarctica and Australia. The New Zealand early Paleozoic data follow the (t)‐(i) trend of other late Precambrian to early Paleozoic metagreywacke sequences at the East Gondwanaland margin, in North Victoria Land and Marie Byrd Land, Antarctica, and Victoria and New South Wales, Australia. This indicates a large uniform source, providing well‐mixed detritus, and deposition at a passive continental margin. Preliminary Rb‐Sr ages from Cambrian‐Ordovician Takaka Terrane metasediments show no evidence for the Late Ordovician metamorphism recorded in the Buller Terrane, and reflect only post‐Devonian thermal overprints. However, their (t)‐(i) characteristics follow the Greenland Group trend (but along the less radiogenic part), and indicate a generally similar continent‐derived provenance.

Base of the Devonian Baton Formation and the question of a pre‐Baton tectonic event in the Takaka Terrane, New Zealand

Previous interpretations of the Baton River area proposed a significant pre‐Baton (Late Silurian‐Early Devonian) tectonic event, during which Ordovician and Silurian sediments were folded, cleaved, and eroded before deposition of the Lower Devonian Baton Formation. This interpretation was based on an apparent lack of a penetrative cleavage in Baton sediments, and on the presence of a rudite at the base of Baton Formation in the Moran Creek/Skeet River area that was identified as a “basal conglomerate”. Current studies show a pre‐Baton event to be untenable and indicate that the Devonian was laid down in the same Takaka Terrane sedimentary basin as the underlying Ellis Formation and experienced the same deformation. Features of the Baton Formation rudite are consistent with a mass‐flow rather than with a “basal conglomerate” origin and the rudite is reinterpreted as a multistoried lobe of debris flows that laterally interfinger with mudstone. Baton Formation appears to conformably follow limestones or sandstones of Ellis Formation. The transition shows four sedimentary patterns. Pebbles in Ellis limestone and sandstone close to the base of the Baton Formation in Fowler Creek may represent a distal expression of the Moran Creek/Skeet River debris flows. Rocks preceding the Baton Formation are referred to as Ellis Formation rather than Hailes Quartzite as the lateral equivalency of these two formations is uncertain and under review. Fowler Formation, previously the highest part of the Ellis Group, is shown to be stratigraphically continuous with, and lithologically identical to, the Baton Formation, and is discarded. The base of Baton Formation appears to mark an abrupt increase in water depth with a change from mainly arenaceous shelf sedimentation to deeper water, finer grained lithologies. Structural studies show that Baton Formation is as deformed as the older rocks, and no structures can be attributed to a pre‐Baton event.

Pb, Nd, and Sr Isotopes and REE Systematics of Cambrian Sediments from New Zealand: Implications for the Reconstruction of the Early Paleozoic Gondwana Margin along Australia and Antarctica

The Takaka Terrane of New Zealand’s South Island contains a Middle to early Late Cambrian intraoceanic island arc assemblage that consists of the terrigeneous to volcanogenic Haupiri Group sediments and the predominantly mafic Devil River Volcanics. Pb, Nd, and Sr isotopes, and major and trace element data are presented here for the Haupiri Group sediments. These data have implications for (1) the stratigraphy of the Takaka Terrane, (2) the reconstruction of the Cambrian Gondwana margin along Australia/Antarctica, and (3) geochemical and isotope provenance studies in general. Nd isotopes, Th/Sc, and rare earth elements (REE) show that the volcaniclastic components in the Haupiri Group sediments reflect the concomitant island arc and back‐arc volcanism. These sediments can probably be linked with the Early Cambrian Kanmantoo Group and Ordovician Lachlan Fold Belt sediments in SE Australia. To explain the Nd isotope compositions, published detrital zircon populations, and paleocurrent data, an Antarctic source with Grenvillian and Ross‐Delamerian age granitoids, as well as a significant portion of reprocessed Paleoproterozoic and/or Archean crust, is required. We adopt a bipolar subduction model for the Cambrian Gondwana margin along Australia/Antarctica and support the previous suggestion that the Cambrian arc assemblages of the Takaka Terrane of New Zealand and the Antarctic Bowers Terrane belong to the same arc system with a paleogeographical position offshore the Antarctic continent.

Cambrian arc evolution along the SE Gondwana active margin: A synthesis from Tasmania‐New Zealand‐Australia‐Antarctica correlations

Belts of Cambrian rocks with arc affinities in eastern Australia, Tasmania, New Zealand, and Antarctica are part of a single convergent margin, active over 30–40 Ma from the latest Early Cambrian to the Late Cambrian. Two of the most complete sequences are exposed in western Tasmania and the northern South Island of New Zealand (Takaka Terrane). Throughout the Middle Cambrian, magmatism in these two regions, in the Lachlan Fold Belt (SE Australia) and the Bowers Terrane (Antarctica) is represented by intra‐oceanic arc and back arc sequences. In the mid‐Middle Cambrian, collision of these arc segments with the proto‐Gondwana continent is recorded by obducted boninite‐bearing ophiolites in Tasmania and SE Australia. Postcollisional magmatism of latest Middle to early Late Cambrian age (e.g., Mount Read Volcanics of Tasmania and Stavely Volcanic Complex of Victoria) terminates convergent tectonics in SE Australia and Tasmania. In contrast, no postcollisional volcanism is known from the Bowers Terrane in Antarctica and from New Zealand. In Antarctica, Cambrian igneous activity in the Wilson Terrane and Transantarctic Mountains formed in an active continental margin setting and lasted through the Middle and Late Cambrian. This suggests that the Wilson Terrane is the lateral continuation on continental basement of the Bowers and Takaka Terrane arcs. The differences between the Australian ‐ Tasmanian and New Zealand ‐ Antarctic arc segments may result from a change in subduction polarity along the arc chain, suggested by structural features, sedimentation patterns, and isotope systematics. In the SE Australian and Tasmanian arc segments the proto‐Gondwana plate subducted beneath the Pacific plate whereas subduction in the Antarctic and New Zealand segments was of opposite polarity. Common to most Cambrian fragments in SE proto Gondwana is the tectonic overprint by the Ross‐Delamerian orogeny from the Middle Cambrian to Early Ordovician, thus paleogeographically linking all the fragments by the end of the Cambrian. The presence of Early Cambrian intra‐oceanic rocks at the SE proto‐Gondwana margin suggests separation of Laurentia from Gondwana prior to the Early Cambrian.

The Isotope and Trace Element Budget of the Cambrian Devil River Arc System, New Zealand: Identification of Four Source Components

(all in wt %) to the mantle wedge is required to explain all The Takaka Terrane in the South Island of New Zealand magmatic compositions. Such mixing proportions are consistent contains a well-preserved Cambrian arc system (Devil River with those obtained for recent arc systems. Volcanics) that displays a complete assemblage of interbedded lowto high-K arc rocks, back-arc rocks and boninites. Most volcanic rocks are mafic. A coherent dataset was obtained including major elements, trace elements and Sr–Nd–Pb isotope

Pb-Nd Isotopes Indicate the Origin of Island Arc Terranes in the Early Paleozoic Pacific.

The Takaka Terrane in New Zealand is one of the best exposed arc fragments of the early Paleozoic Australian-Antarctic convergent margin and constitutes one of the most outboard terranes of this margin in paleogeographic reconstructions. Pb-Nd isotope compositions of clinopyroxenes from the Cambrian Devil River Volcanics of the Takaka Terrane enable identification of the location of the terrane in the Paleo-Pacific Ocean. The Devil River Volcanics, a suite of primitive arc and back-arc rocks, are interbedded with the partly continent-derived Haupiri Group sediments. Extremely radiogenic Pb and unradiogenic Nd compositions in the arc rocks cannot be explained by assimilation of the Haupiri Group sediments or a continental basement of such a composition. Pb isotope compositions of the Takaka Terrane sediments are much less radiogenic and overlap with crustal compositions of the Lachlan Fold Belt in Australia, suggesting that both units are derived from one source, the Australian-Antarctic Pacific margin. Pb-Nd isotope compositions in the Devil River Volcanics reflect contamination of their mantle sources by subducted sediments derived from Archean provinces in either Antarctica or Laurentia. Both provinces show characteristically high 207Pb/204Pb500 and were located at the Pacific rim in the Cambrian. Mixing between mantle and Proterozoic continental material from present western South America or eastern Laurentia cannot explain the high 207Pb/204Pb500 in the New Zealand rocks. As in New Zealand, extreme spreads in Pb-Nd isotope compositions in other Cambrian volcano-sedimentary sequences in southeast Australia and Tasmania can be explained by the same model, suggesting that all these fragments originated along the Australian-Antarctic Gondwana margin. Pb isotope compositions of arc rocks, therefore, provide a new tool for terrane analysis in the early Paleozoic Pacific ocean.

The Cambrian arc complex of the Takaka Terrane, New Zealand: An integrated stratigraphical, paleontological and geochemical approach

The Cambrian succession of the Takaka Terrane represents a volcano‐sedimentary arc complex, which was once part of the Pacific margin of Gondwana. The succession has been folded and disrupted by faulting during at least four tectonic events, bringing together at least 12 fault slices with dissimilar stratigraphy. A revised stratigraphy and regional models for lithofacies relationships, magmatic history, and tectonic setting are proposed. They are based on analysis of the lithostratigraphy, biostratigraphy, petrology, geochemistry, geochronology, and field relationships. Volcanic rock formations recognised are Benson Volcanics (arc), Mataki Volcanics (back‐arc), and Cobb Igneous Complex (boninites), which are all grouped in the Devil River Volcanics Group. Petrography and trace element geochemistry enable discrimination of the volcanic units of the Devil River Volcanics. Rocks of the Benson Volcanics are by far the most voluminous and are subdivided into nine (informal) volcanic suites. Arc activity lasted from middle Mid Cambrian (Undillan) to Late Cambrian ( Mindyallan or younger), about 20 m.y., constrained by ages based on trilobites, brachiopods, and conodonts in mterbedded sediments and U‐Pb and Ar‐Ar dates. This period of arc activity is different from those in Tasmania and southeast Australia, where post‐collisional volcanism commenced in the late Mid Cambrian. A separate igneous unit in the Takaka Terrane is the rift‐related Gendarme Dolerite of latest Cambrian to early Ordovician age. Sedimentary formations recognised are Tasman Formation, Mount Benson Sandstone (new), Ruby Saddle Formation (new), Lake Peel Formation (new), Lockett Conglomerate, Christmas Conglomerate, Salisbury Conglomerate, and Heath Creek beds, all comprising the Haupiri Group, and the Junction Formation (group unassigned). Rocks of the Haupiri Group were deposited in a back‐arc or intra‐arc setting.

Overview of the Median Batholith, New Zealand: a new interpretation of the geology of the Median Tectonic Zone and adjacent rocks

This paper proposes an alternative high-order, non-genetic classification of the basement rocks of medial New Zealand. More than 90% of the rocks in the Median Tectonic Zone are plutonic and can be included in part of a newly defined Carboniferous to Early Cretaceous, ca 10,200 km 2 composite regional batholith — the Median Batholith. The plutonic rocks of the batholith intrude the volcanic and sedimentary rocks of the Brook Street and Takaka Terranes (Eastern and Western Provinces, respectively). Emerging matches between the chronology of magmatism in the Median Batholith and batholiths in the Western Province also support probable in situ growth of most of the batholith. The internal and external contacts, and shape, of the batholith have been strongly modified by post-plutonic Cretaceous and Late Cenozoic tectonism, particularly within 50 km of the Alpine Fault. The Median Batholith represents a significant but previously little-recognised 250 Ma record of magmatism along the continental margin of South Gondwana, and invites comparison with other Cordilleran batholiths.

Nb/Ta fractionation in a Cambrian arc/back arc system, New Zealand: source constraints and application of refined ICPMS techniques

This study presents evidence for systematic Nb/Ta variations in an Cambrian island arc system of the Early Paleozoic Takaka Terrane, New Zealand. These volcanic rocks comprise an unusual variety of compositions ranging from back arc basin basalts through boninites to calc-alkaline basalts and andesites. They have been analyzed by ICPMS and display Nb contents between 0.15 and 30 ppm and Ta contents between 0.02 and 2 ppm. Nb/Ta ratios vary from 8 to 25. To monitor accuracy of the analytical procedure, ICPMS results for Nb and Ta in 20 geostandards are presented. Prior to analysis, HCl was added to the sample solutions and measurements were run within 24 h of dissolution to avoid precipitation of both elements. The constancy of Nb/Ta in samples of single volcanic suites at different H 2O and CO 2 contents and at different Mg# and Cr contents shows, that neither alteration processes nor crystal fractionation affected the primary Nb/Ta ratio of the samples. Nb/Ta in relatively primitive samples (Mg# 50–75) is positively correlated with Nb, Ta, La/Yb and Th/Yb below the chondritic Nb/Ta ratio (Low Nb/Ta trend). An inverse correlation is observed at higher ratios (High Nb/Ta trend). No correlations exist with ratios of moderately incompatible elements such as Ti/Zr and Zr/Hf, suggesting the Nb/Ta ratios not to be controlled by the depletion of the mantle wedge. Conversely, these systematics strongly suggest an important role of a residual mineral phase, most likely rutile, in mineral/melt (high Nb/Ta trend) and mineral/fluid systems (low Nb/Ta trend), respectively. The overall Nb–Ta depletion in the magmas, however, is best explained by extremely low partitioning of HFSE into slab equilibrated fluids as suggested by recent experimental work. The low Nb/Ta ratios in rocks that are extremely depleted in incompatible elements are due to rutile/fluid interaction. High Nb/Ta ratios in the medium- to high-K volcanic rocks reflect the interaction between rutile and siliceous magma produced by low degree melting of the slab. The subducting slab can be demonstrated here as the most likely locality for both Nb–Ta depletion and Nb/Ta fractionation, implying a small scale transfer of Nb and Ta from the slab to the mantle wedge by melts and fluids.

Isotope and Trace Element Patterns of Cambrian Sediments from New Zealand: A Record of Island Arc and SE-Australian Continental Provenance

In this study, we present a complete Sr-Nd-Pb data set from the Cambrian Haupiri Group Sediments of the Takaka Terrane, South Island of New Zealand. 24 samples of the Haupiri Group Sediments were analysed for major and trace elements by XRF. 12 samples were selected for Rb-Sr and Sm-Nd analyses and further trace element analyses (ICPMS). Additional Pb isotope data for seven samples is taken from Mtinker (1997). The data allows to identify continental provenance and mafic island arc

Detrital zircon age patterns and provenance in late Paleozoic–early Mesozoic New Zealand terranes and development of the paleo-Pacific Gondwana margin

Research Article| October 01, 1997 Detrital zircon age patterns and provenance in late Paleozoic–early Mesozoic New Zealand terranes and development of the paleo-Pacific Gondwana margin R. J. Wysoczanski; R. J. Wysoczanski 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar G. M. Gibson; G. M. Gibson 2University of Southern Queensland, Toowoomba, QLD 4350, Australia Search for other works by this author on: GSW Google Scholar T. R. Ireland T. R. Ireland 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information R. J. Wysoczanski 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia G. M. Gibson 2University of Southern Queensland, Toowoomba, QLD 4350, Australia T. R. Ireland 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (10): 939–942. https://doi.org/10.1130/0091-7613(1997)025<0939:DZAPAP>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. J. Wysoczanski, G. M. Gibson, T. R. Ireland; Detrital zircon age patterns and provenance in late Paleozoic–early Mesozoic New Zealand terranes and development of the paleo-Pacific Gondwana margin. Geology 1997;; 25 (10): 939–942. doi: https://doi.org/10.1130/0091-7613(1997)025<0939:DZAPAP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Permian-Triassic strata from Parapara Peak in the Takaka terrane, western province, New Zealand, are dominated by detrital zircon peaks at ca. 240 Ma comparable to the Torlesse terrane of the eastern province. This contrasts with lower Paleozoic strata (Onekaka schist), which is comparable to other lower Paleozoic terranes (including adjacent Buller terrane) from the Gondwana margin that are characterized by detrital zircon patterns with characteristic peaks at ca. 500–650 Ma and ca. 1000–1150 Ma, a less distinct peak around 1400–1700 Ma, and scattered Archean ages. These data indicate the close proximity of the eastern and western provinces by Permian time and a probable source region of Parapara Peak strata and Torlesse terrane in the New England fold belt of southeast Australia. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry of the Karamea Batholith, New Zealand and comparisons with the Lachlan Fold Belt granites of SE Australia

The Karamea Batholith in the Buller terrane of the South Island New Zealand forms part of an extensive Middle-Late Devonian belt of magmatic activity along, or close to, the Paleo-Pacific margin of Gondwana. The belt includes the I- and S-type granites of the Lachlan Fold Belt in SE Australia and coeval rocks in Antarctica. The northern half of the Karamea Batholith comprises five main intrusive phases: Zetland Diorite, Whale Creek Granite, Karamea Granite, O'Sullivans Granite and Dunphy Granite. To the east of the Karamea Batholith in the Takaka terrane, ultramafic-mafic Devonian igneous rocks are represented by the Riwaka Complex. The rocks forming the Karamea Batholith are a high-K calc-alkaline suite ranging in composition from metaluminous (ASI for Zetland Diorite = 0.8) to strongly peraluminous (ASI for Dunphy Granite = 1.2–1.3). Initial 87 Sr 86 Sr ratios exhibit a large range from 0.705 in the Zetland Diorite to 0.719 in the Dunphy Granite. The corresponding values for ϵ Nd are −0.3 and −9.2. There is a strong inverse correlation between ϵ Nd and initial 87 Sr 86 Sr , which suggests that the Karamea rocks were generated by a simple mixing process. The mafic end-member (with ϵ Nd = 0), which is itself probably derived from a mixed lithospheric source, is taken to be the Zetland Diorite/Riwaka Complex, and the crustal end-member is represented by Ordovician Greenland Group greywackes that form the country rocks to the batholith. Mixing is also supported by recent U-Pb zircon studies. The inherited zircon population in the granites matches the detrital zircon population in the Greenland Group greywackes. The Whale Creek Granite, Karamea Granite and O'Sullivans Granite can be modelled by 20–30% crustal material, whereas the Dunphy Granite appears to represent 65–85% crustal material. In terms of the I-S classification scheme developed for the Lachlan Fold Belt granites in SE Australia, both types are present in the Karamea Batholith. However, in New Zealand there appears to be a continuum from one extreme to the other, which is consistent with the mixing model presented here.

Reconnaissance sandstone geochemistry, provenance, and tectonic setting of the lower Paleozoic terranes of the West Coast and Nelson, New Zealand

Abstract Broad chemical characteristics of sedimentary rocks from the Buller and Takaka Terranes have been examined, and provenance and tectonic setting inferred, using whole‐rock major and trace element data from 208 samples. Buller Terrane rocks west of the Karamea Batholith (Greenland Group) have chemistry typical of turbidite sediments deposited at a weathered passive continental margin. Bulk chemical characteristics confirm earlier work, except that a small spatial chemical variation is seen, consistent with the presence of a small volcanogenic component in the south, decreasing to the north. East of the batholith, compositions of the Webb and Roaring Lion Formations overlap with those of the western Buller rocks, supporting equivalence (at least in part) with the Greenland Group. Passive margin chemical characteristics are maintained throughout the succeeding Ordovician formations. Within the Takaka Terrane there are sharp contrasts in chemistry between the Cambrian and post‐Cambrian (Ordovician—Sil...

Ion microprobe dating of Paleozoic granitoids: Devonian magmatism in New Zealand and correlations with Australia and Antarctica

Precise ion microprobe UPb zircon ages have been obtained from a representative set of Paleozoic igneous rocks from the Western Province of the South Island, New Zealand. Granitoid rocks forming the Karamea Batholith and related plutons in the Buller terrane all yield crystallisation ages that are indistinguishable within a ± 5 Ma uncertainty at 375 Ma (Middle-Late Devonian). Previous workers have suggested that the batholith was emplaced over a long time interval and comprises rocks ranging in age from 370 to 310 Ma, with the bulk in the Early Carboniferous. Granitoid rocks with Carboniferous ages (∼ 330 Ma) do occur further to the west and younger Cretaceous granitoids occur within the Karamea Batholith, but these are not volumetrically significant. A sample from the ultramafic-mafic Riwaka Complex in the adjacent Takaka terrane gave a crystallisation age of 376.9 ± 5.6 Ma (2σ), indicating emplacement coeval with the Karamea Batholith. The Paleozoic granitoids contain a large amount of inherited zircon, with distinct 390-, 500–600- and 1000-Ma components. The 390-Ma age corresponds to widespread plutonism in the Lachlan Fold Belt in SE Australia. The 500–600-Ma (Ross-Delamerian age) and 1000-Ma (Grenville age) age components have also been observed in granitoids from the Lachlan Fold Belt and in Ordovician metasedimentary rocks from SE Australia and New Zealand. The inherited zircons in the Karamea Batholith could be derived from continental basement at depth, from the incorporation of upper-crustal material in the granitoid magmas or both. The Devonian granitoids in New Zealand can now be correlated with rocks of similar age in northern Victoria Land (Admiralty Intrusives) and Marie Byrd Land (Ford Granodiorite) in West Antarctica, in NE Tasmania, and in the central part of the Lachlan Fold Belt in SE Australia. On a reconstruction of the SW Pacific margin of Gondwana, prior to later break-up, it is possible to trace out a semi-continuous magmatic belt in excess of 2000 km in length.

The Island Arc Setting of a New Zealand Cambrian Volcano-Sedimentary Sequence: Implications for the Evolution of the SW Pacific Gondwana Fragments

The Waingaro Fault Slice, one of several structural units of the early Paleozoic Takaka Terrane in Northwest Nelson, South Island, New Zealand, consists of six fault-bounded subslices that represent different sections of a late Middle to early Late Cambrian volcano-sedimentary succession. Three basaltic to basaltic-andesitic suites and their volcaniclastic equivalents (150-500 m thick; previously mapped as Devil River Volcanics) are intercalated in a nonvolcanic sequence of turbiditic sandstones and siltstones (approximately 1800 m thick). Compositional systematics of the relatively immobile high field strength elements (HFSE) and rare earth elements (REE) as well as the depositional facies provide strong evidence for an island arc setting for the three basaltic to basaltic-andesitic suites. The oldest (Benson volcanic suite) has incompatible trace elements similar to arc-tholeiites, whereas the two younger suites (Cobb Flat and Peel volcanic suites) show affinities with calc-alkaline and high-K suites. E...

Isotopic ages from the Nelson region of South Island New Zealand: crustal structure and definition of the Median Tectonic Zone

Abstract Plutonic rocks in the Rotoroa Complex and Drumduan Terrane of South Island, New Zealand yield zircon U/Pb dates of 156 and 142 Ma, respectively, that are interpreted as crystallization ages. Hornblende and biotite 40 Ar/ 39 Ar dates of 140-130 Ma from the Rotoroa represent either emplacement ages, cooling ages or a metamorphic resetting event. These two units crop out between the Brook Street Terrane and the Separation Point Batholith and lack any clear affinity with tectonostratigraphic terranes of the New Zealand Western or Eastern provinces. The Rotoroa Complex and Drumduan Terrane are interpreted as part of a series of dismembered Mesozoic volcanic-plutonic arc complexes that are sandwiched between terranes of the Western and Eastern provinces, occupying a structural position here referred to as the Median Tectonic Zone (MTZ). Correlative units in Fiordland on the opposite side of the Alpine Fault include the Mackay Intrusives, Darran Complex, Largs Terrane, Lochburn Formation and the Halfway Peak Gabbro. Farther south on Stewart Island the Anglern Complex and Paterson Group are part of the same structural belt. The MTZ is an extension of the original concept of the Median Tectonic Line put forth by Landis and Coombs (1967). Dismemberment and juxtaposition of arc magmatic assemblages in the MTZ with Western and Eastern Province terranes is related to large-scale transcurrent faulting in the Early Cretaceous. Its essential features as a regional tectonostratigraphic terrane were established by ~ 117 Ma as indicated by stitching of the Rotoroa Complex to the Takaka Terrane (Western Province) by the Separation Point Batholith (117-114 Ma). The Echinus Granite yields a 310 Ma U/Pb zircon crystallization age that suggests the granite and associated gneisses are part of the Western Province which may constrain the position of the western margin of the MTZ near Nelson City.

A review of the Median Tectonic Zone: Terrane boundaries and terrane amalgamation near the Median Tectonic Line

Abstract The pre‐Late Cretaceous basement rocks of New Zealand are divided into a Western Province and an Eastern Province along a tectonic boundary called the Median Tectonic Line. The two provinces comprise assemblages of terranes of fundamentally different origin. The Western Province attained continental‐type thickness and structure by the end of the Carboniferous. The Eastern Province developed as a result of convergent margin crustal processes in the Permian‐Cretaceous. The boundary between the two is drawn in one of two positions in Nelson, either east or west of the Drumduan Terrane. The evidence presented so far on the affinities of the Drumduan Terrane is equivocal, and the suggested eastward extension of the Tuhua Orogen is not well supported. To the south, similar difficulties of interpretation occur, and it seems that between the Takaka Terrane in the west and the Brook Street Terrane in the east there is a belt of enigmatic rocks best termed the Median Tectonic Zone. In a broader context, th...

Early Palaeozoic terranes in New Zealand and their relationship to the Lachlan Fold Belt

Two tectonostratigraphic terranes of Palaeozoic age are recognised in the Western Province of New Zealand, separated by the north-trending Anatoki Thrust. The Buller (western) terrane is composed largely of a relatively uniform suite of quartz-rich elastics (mainly turbiditic sandstone), siltstone and black shale and ranges from basal Ordovician to Late Ordovician in age. It is an equivalent of the Ballarat-Bendigo belt of Victoria. The Takaka (eastern) terrane contains a varied assemblage of Middle and Late Cambrian volcanics, volcaniclastics, carbonates and turbidites, Ordovician carbonate, shale and sandstone, and Silurian quartzite. The closest equivalent in Australia is Early Palaeozoic rocks of west Tasmania. The main deformational events, in the latest Ordovician or Silurian (Buller terrane), latest Silurian (Takaka terrane) and ?Middle Devonian (both terranes), can be equated with the Benambran, Bowning and Tabberabberan Orogenies of eastern Australia respectively. Palaeozoic granites are restricted to Buller terrane, and are spread over a belt 300–450 km wide. Age range (on sparse isotopic data) is 370−310 Ma (Late Devonian-Carboniferous), with the bulk in the Early Carboniferous. S-type granite predominates, with several A-type granites and a single I-type. The similarity in the nature and timing of the main events in the sedimentary, tectonic and volcanic history of New Zealand clearly link it with the Lachlan Fold Belt. On the other hand, the age and chemistry of the S-type New Zealand Palaeozoic granites make it unlikely that they are the direct equivalents of granite terranes of the Lachlan Fold Belt or Antarctica.

Medium-high-pressure metamorphic rocks of the Tuhua Orogen, western New Zealand, as lower crustal analogues of the Lachlan Fold Belt, SE Australia

Lower Palaeozoic rocks are widely distributed in the western province of New Zealand. They include: (1) Ordovician greywackes, slates and minor paragneiss in the Buller terrane; (2) weakly metamorphosed (subgreenschist- to greenschistfacies) Cambrian mafic volcanics, Ordovician limestones and Silurian siliciclastic rocks in the Takaka terrane; and (3) higher-grade rocks mainly in Fiordland, which include 350–380 Ma amphibolite-facies metasediments held to be more highly metamorphosed equivalents of the Buller and Takaka terranes. Metamorphic rocks derived from late Precambrian protoliths may also be present. Together with the Buller and Takaka terranes, these Fiordland rocks were regionally metamorphosed and deformed during the mid-Palaeozoic Tuhua Orogeny. Before late Mesozoic continental rifting and breakup of the Pacific margin of Gondwana, they were part of a much more extensive Palaeozoic orogen which included the Lachlan Fold Belt and extended from southern New South Wales through Victoria and Tasmania into New Zealand and Antarctica (Victoria Land). In common with the Lachlan Fold Belt, regional metamorphism in the western province occurred largely under low-pressure-high-temperature conditions (andalusite-sillimanite-facies series) such that in Fiordland and the higher-grade parts of the Western Province low pressure (0.3–0.5 GPa) sillimanite and sillimanite + potash feldspar ± cordierite assemblages developed in rocks of appropriate bulk chemical composition. Higher-pressure (0.7–0.9 GPa) rocks in which kyanite is the dominant aluminosilicate polymorph are almost entirely confined to Fiordland and represent the deepest exposed crustal levels within the western province. At these levels, migmatized pelitic schists containing kyanite and garnet are interstratified with calc-silicate and quartzofeldspathic rocks, tonalitic gneisses, metamorphosed anorthosites and granite orthogneisses. These rocks were exhumed from depths of 25–30 km and offer useful insights into the nature and composition of the continental crust beneath what was once part of eastern Gondwana. Exhumation itself was brought about by late Mesozoic crustal extension combined with more recent transpression along the mid-Tertiary Alpine Fault. Assuming plate reconstructions for Gondwana during the Palaeozoic are valid, the lower to middle crustal rocks of Fiordland may give some indication as to what lies at depth beneath the Lachlan Fold Belt.