Brook Street Research Articles

Geochemistry and Nd‐isotope systematics of chemical and terrigenous sediments from the Dun Mountain Ophiolite, New Zealand

Two sedimentary associations closely related to temporally discrete ophiolitic magma suites occur within the Early Permian Dun Mountain Ophiolite Belt (DMOB), New Zealand. These are: (1) a suite of diverse chemical sediments and turbidite argillites (TA) that bear an intimate depositional relationship to early‐formed pillow lavas (back‐arc basin basalts); and (2) a younger, lithic‐dominated, bimodal, coarse sandstone‐ophiolitic rudite assemblage of proximal turbidite/mass‐flow origin, rich in clasts of infant‐arc magmas which comprise the bulk of the ophiolite. There are four groups of DMOB chemical sediments. Red hematitic chert (group 1) fills interstices between basalt pillows, and black nodular Fe‐Mn deposits (group 2) occur along pillow lava/sediment interfaces. These facies are overlain by red mudstones (group 3), and mottled orange‐olive brown mudstones (altered hyaloclastites; group 4). Geochemical features (including REE contents, Fe/Ti and transition metal ratios) indicate that the cherts reflect silica and metalliferous contributions to sea water, promoted by low‐temperature hydrothermal alteration of glassy basalt, while group 4 muds represent residual components in halmyrolytically altered volcanic glass. Transition metal and REE enrichments in group 2 nodules (with high Ce/Ce*, Ni/Fe, and Cu/Fe ratios) reflect hydrogenous chemisorption to a hydothermal component (with high Ba and Sr). The nodules possess ϵ Nd(T) values (c. 0) identical to those calculated for Permian sea water. Group 3 red muds have lower ϵNd(T) = ‐1 to ‐2, and Nd model ages (T NdDM) that indicate contributions from continentally derived fluvial particulate fallout of mean Proterozoic age. For the nodules and red muds, strong negative correlations between Mn/Fe, Nd, Ce*/Ce, and ϵNd(T) are attributed to increasing diagenetic influence in the muds. ϵNd(T) values (c. +2) in group 4 muds are transitional toward higher values in their (hyaloclastite) basalt glass precursors. Metalliferous contributions to red and green TA in the overlying terrigenous sedimentary sequence also link these facies to early DMOB back‐arc eruptives. The red and green TA pass upwards into ungraded Atomodesmid‐bearing grey TA, implying turbidite deposition in a shallowing marine environment. The (mainly andesitic) TA show systematic trends of decreasing Eu/Eu* and ϵNd(T) with increasing SREE, La/Y, Th/Sc, and SiO2. Th abundances and Th/Sc ratios (up to 1.6) in the TA are akin to those of calc‐alkaline magmas in continental arcs, and are significantly greater than island‐arc or ophiolitic volcanics. A narrow range of positive ϵNd(T) values (+0.5 to +2.0) for the TA suggests a young differentiated continental arc source, less dissected than the quartzofeldspathic plutonic provenance for the Torlesse (ϵNd(T) ?2). The exclusively terrigenous, younger DMOB sedimentary suite is compositionally bimodal, with coarse mafic sandstones rich in clinopyroxenes identical to those in DMOB infant‐arc magmas, and felsic, matrix‐supported breccias that contain abundant clasts of infant‐arc rocks, including silicic plagiogranites and rare ultramafics. Low Th/Sc ratios (<0.3), and slight LREE enrichment (CeN/YbN= 2.5–2.8) in the sandstones resemble island‐arc tholeiites (IAT). The breccias have low Eu/Eu* (c. 0.72), CeN/YbN = 1.6, Th/Sc (0.1–0.2), and TbN/YbN c. 1.0, features transitional between the sandstones and DMOB plagiogranites. Importantly, the sandstones and breccias show high positive initial Nd ratios (ϵNd(T) = +5 to +8) akin to values for Brook Street IAT (ϵNd(T) = +9). This implies juvenile (mantle‐derived) source rocks for both the mafic and felsic younger terrigenous sediments, compatible with crustal residence (mean provenance) ages less than for the TA. Little continental input is permitted by Nd‐isotopic data for these rocks. Whereas the early DMOB pillow basalt ‐chemical sediment ‐ TA assemblage was emplaced in a back‐arc setting, with influx of detritus in part from an active continental margin (possibly within the New England Orogen), the younger DMOB terrigenous sediment association shows a close kinship with bimodal DMOB magmatism related to infant arc volcanic centres in an extensional forearc regime. Turbidite sands and mass‐flow deposits were shed into proximal fault‐bound basins isolated from continental clastic sources. The data are compatible with a recently reassessed model for the tectonomagmatic evolution of the DMOB, involving initial back‐arc seafloor generation, and later voluminous extensional magmatism during nascent subduction, probably related to initiation of the Brook Street arc.

Provenance of Permian‐Triassic volcaniclastic sedimentary terranes in New Zealand: Evidence from their radiogenic isotope characteristics and detrital mineral age patterns

Detrital mineral age patterns, Rb‐Sr age, and initial strontium isotope ratio variations are reported for metasediments of the Brook Street, Dun Mountain‐Maitai, and Murihiku Terranes (informally grouped as “central arc terranes") of New Zealand. U‐Pb SHRIMP detrital zircon age populations in Late Permian Tramway Formation, sandstones of the Dun Mountain‐Maitai Terrane in Nelson and Southland, New Zealand, show restricted Late Permian (253–268 Ma) and broader Devonian‐Carboniferous (330–373 Ma) age components in about equal proportions, with a minor (<20%) proportion of early Paleozoic and Mesoproterozoic ages. The Permian component is related to contemporaneous volcanic inputs, whereas the older Devonian‐Carboniferous component is derived from a mixture of exhumed continental crust of Lachlan Fold Belt, and possibly New England Fold Belt age characteristics. A detrital zircon age population of Jurassic Barretts Formation sandstone of the Brook Street Terrane in Wairaki valley, Southland, carries a major (75%), dominantly Middle Jurassic to Late Triassic (170–230 Ma) component derived from contemporary volcanic inputs of uncertain origin, and a minor (<25%) Middle Triassic to Late Permian (229–254 Ma) component. Rb‐Sr isochron ages (t) of Brook Street, Dun Mountain‐Maitai, and Murihiku Terrane metasediments (and their initial 87Sr/86Sr ratios (i) at the time of metamorphism), reveal cooling age patterns consequent upon low‐grade metamorphism in latest Triassic and earliest Jurassic times. The (t)‐(i) data define a central arc terranes group consistently less radiogenic than Permian and Triassic time correlatives in the Torlesse Supergroup of the Rakaia Terrane. This feature suggests a provenance that includes sediment detritus dominantly from less radiogenic I‐type igneous sources in both Lachlan and New England Fold Belts in similar proportions, and with little contribution from their Precambrian and early Paleozoic hinterlands.

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

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.

Reassessment of the origin of the Dun Mountain Ophiolite, New Zealand: Nd‐isotopic and geochemical evolution of magma suites

Magmatic suites with contrasting isotopic and geochemical compositions, sequentially emplaced in different tectonic regimes, comprise the Dun Mountain Ophiolite Belt (DMOB), New Zealand. At D'Urville Island, the northernmost exposure of the DMOB, earliest erupted (stage 1) pillow basalts have ϵNd(T) = +6.3 to +7.5, and are incompatible element enriched, like basalts from geo‐chemically anomalous ridge segments. Overlying stage 2 basalts (sheeted flows) show a narrow range of ϵnd(T) = ‐8.3 ± 0.2, with chemical characteristics of depleted back‐arc basin basalts. These rocks are intruded by mafic to silicic stage 3 magmas, which have high uniform initial 143Nd/144Nd ratios (ϵNd(T) = +9.3 ± 0.2) over a wide range of 147Sm/144Nd values (yielding a precise Early Pemian Ndisotope age of 278 ± 4 Ma (MSWD = 0.48)). Stage 3 magmas show pronounced subduction‐related geochemical signatures similar to island arc tholeiites (IAT) from immature arcs. They are closely analogous to some (boninite)‐IAT magmas which characterise “infant arc” eruptive activity in forearc basins of present‐day Western Pacific island arc systems. A wide range of stage 3 magma compositions, ranging from near‐primary basaltic dikes (Mg# = 74) to extremely fractionated silicic plagiogranites with uniformly very depleted isotopic ratios, is consistent with slow spreading rates which gave rise to polybaric, closed‐system fractionation of magmas and periodic chamber abandonment. Some stage 3 rocks with SiO2 levels in the andesite range have low‐TiO2 contents and high Mg#, and may be fractionated equivalents of boninites. High ϵNd(T) values of stage 3 magmas indicate a lack of subducted sediment with inherited crustal residence signatures, and reflect the extent of supra‐subduction zone (SSZ) mantle wedge depletion. DMOB stage 3 magmas may represent forearc magmatism that was the precursor to normal subduction‐related volcanism established by c. 265 Ma in the Brook Street Arc and derived from a SSZ mantle source with identical Nd‐isotope characteristics. Less isotopically depleted stage 1 and 2 basalts (and rocks of the Lee River Group in general) may represent fragments of pre‐existing mafic ocean crust sensu stricto, as suggested by significantly older Nd‐isotopic ages for the basaltic seafloor volcanic component of the DMOB. Upon initiation of subduction, pre‐existing geochemically anomalous (possibly back‐arc basin) ocean crust (stage 1 and 2 magmas) was intruded by the “infant arc” (stage 3) magmas, as proposed for older seafloor remnants in forearc regimes of the Izu‐Bonin‐Mariana and Tonga arc systems.

HIGH-Mg ARC-ANKARAMITIC DIKES, GREENHILLS COMPLEX, SOUTHLAND, NEW ZEALAND

Minor dikes 8–70 cm thick of ankaramitic composition were emplaced late in the crystallization of the Greenhills Complex, a layered dunite – wehrlite – hornblende gabbronorite intrusion of Permian age in the volcanic arc-derived Brook Street Terrane of southern New Zealand. Mg contents range from 10 to 28 wt% MgO. The interior of a hornblende ankaramite dike at Steep Head contains phenocrysts of olivine Fo 93–82 , in some cases skeletal, and scattered phenocrysts of diopside, set in a groundmass of magnesiohornblende. The chilled margins of this dike consist almost wholly of amphibole (91%) and anorthite, An 99–94 (9%). In other dikes, crystallization of amphibole followed that of olivine, diopside, enstatite, and overlapped with plagioclase. Olivine in these dikes varies in composition from Fo 90 to Fo 72 . Diopside contains oriented blebs of magnesiohornblende and edenite. We believe that the components of these amphibole inclusions were in part introduced and in part derived from the host pyroxene by a process of reactive exsolution. Spinel varies from chromite containing 53 wt% Cr 2 O 3 through compositions at first increasingly Al-rich, and then through chromian magnetite to virtually Cr-free magnetite. Amphiboles range from magnesiohornblende to edenite, pargasite, and tschermakite, with minor late-stage high-Si magnesiohornblende. The aphyric margin of the Steep Head dike contains 17.4 wt% MgO on a volatile-free basis. Some of the melts may have exceeded this MgO content. Dike centers are more Mg-rich than margins, reaching 28.3% in one case, in part an effect of concentration of phenocrysts by flow differentiation, and in part a result of changing composition of the melt. The presence of abundant magmatic amphibole, especially in the most magnesian rocks, indicates a relatively hydrous magma. Various geothermometers suggest extensive low-temperature re-equilibration. Even where the dikes consist of almost monomineralic amphibole, chemically they are arc-ankaramites with CaO/Al 2 O 3 >1, REE contents less than twice primitive mantle, low abundances of high-field-strength elements and of incompatible elements, highly calcic plagioclase, and Cr-rich spinels with Ti, Mg, and Cr contents typical of low-K, high-Mg ankaramitic arc-tholeiitic suites. They have some chemical similarities to komatiites, but have lower Ni/MgO, Cr/MgO, and Ti/V values. The most highly magnesian dike-rocks have olivine in excess of diopside, whereas in dikes with lower Mg contents, the proportion of diopside phenocrysts exceeds that of olivine. Progressive fractionation of these two minerals lowered the bulk CaO/Al 2 O 3 ratio. Below about 10% MgO, CaO/Al 2 O 3 was no longer greater than one, the ankaramitic nature was lost, and evolution continued along an arc-tholeiitic trend. The dike compositions support the hypothesis that the Greenhills Complex is the remnant of an upper-crust magma chamber from which arc-ankaramites, tholeiitic basalts, and more evolved effusive volcanic products in the Brook Street Terrane were erupted. The highly magnesian ankaramitic dikes and their effusive analogues provide another example of an association increasingly recognized in Phanerozoic island-arc systems.

Geology and thermochronometry of the east edge of the Median Batholith (Median Tectonic Zone): a new perspective on Permian to Cretaceous crustal growth of New Zealand

Abstract We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre‐Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (87Sr/86Sri = 0.702 to 0.703; ɛNdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (87Sr/86Sri = 0.703 to 0.704; ɛNdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra‐batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K‐feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post‐accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford‐Eglinton area is indicated by a new 140 ± 2 Ma U‐Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K‐feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite.

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.

Permian‐Jurassic strata at Productus Creek, Southland, New Zealand: Implications for terrane dynamics of the eastern Gondwanaland margin

An area in the Wairaki Hills lying to the east of the Takitimu Mountains and extending from the vicinity of the Wairaki River south to near Ohai is described. The area straddles the faulted contact between the Brook Street Terrane and the Murihiku Terrane. Sparsely fossiliferous volcaniclastic strata of the 15 km thick Takitimu Group (Early Permian) comprise the oldest rocks of the region. Youngest Takitimu rocks, the newly recognised Caravan Formation, contain ankaramitic dikes and pyroxene‐rich marine volcaniclastics. These are conformably overlain by richly fossiliferous sandstone and impure limestone of Mangarewa Formation (120 m) and atomodesmatinid prism limestones of Glendale Limestone (400 m), which together constitute the newly restricted Productus Creek Group of latest Early to early Late Permian age. Takitimu and Productus Creek Group rocks are intruded by Triassic to Early Jurassic igneous rocks ranging from andesite to diorite and olivine monzonite. Barretts Formation, a newly recognised Middle Jurassic conglomerate‐sandstone unit (200 m), unconformably overlies the Takitimu Group, Productus Creek Group, and the intrusive bodies. All of these rocks are included in the Brook Street Terrane. Triassic strata of the Murihiku Terrane comprising a fore‐arc or back‐arc basin assemblage have been thrust westward (Letham Ridge Thrust) over Barretts Formation and underlying rocks of the Brook Street Terrane. Slices of melanged olistostrome (Hawtel‐Coral Bluff Melange) are developed along this thrust. Rocks incorporated within the melange include the entire Wairaki Breccia, Trig Z, and Nemo Formations and parts of units originally defined as Hawtel, Letham, Mangarewa, and Wether Hill Formations. All these rocks have previously been mapped as components of the Productus Creek Group but are here excluded from that unit. Petrographically, Barretts Formation resembles Murihiku sediments of Jurassic age exposed in southeastern Otago. In contrast, Early and Middle Triassic Murihiku sandstones show distinctly different framework compositions. On the basis of sandstone petrography, paleomagnetic data, regional structure, and the unconformable contact of the Jurassic Barretts Formation on Middle Permian Caravan Formation, we conclude that some degree of spatial proximity between the Brook Street Terrane and the Murihiku arc was achieved during the Jurassic. There is no evidence for close proximity before that time. Accretion of the New Zealand suite of oceanic terranes (i.e., Eastern Province) onto the Gondwanaland margin was probably accomplished during latest Jurassic to middle Cretaceous time.

Relationships between the brook street Terrane and Median Tectonic Zone (Median Batholith): Evidence from Jurassic conglomerates

U‐Pb zircon ages of 237–180 Ma and c. 280 Ma of seven granitoid clasts from the Rainy River Conglomerate which lies within the eastern Median Tectonic Zone (Median Batholith) in Nelson, and the Barretts Formation of the Brook Street Terrane in Southland, constrain the depositional ages of both units to be no older than c. 180–200 Ma (Early Jurassic). The minimum age of the Rainy River Conglomerate is constrained by the 147 +2 ‐1 Ma (latest Jurassic) emplacement age of the One Mile Gabbronorite (new name: previously western Buller Diorite). The ages and chemistry of five of the granitoid clasts are broadly compatible with derivation from rocks that are now represented by Triassic plutons of the Median Tectonic Zone (Median Batholith), although ages as young as 180 Ma are slightly outside the range of the latter as currently exposed in New Zealand. The age (273–290 Ma, 237 ± 3 Ma) and chemistry of the other two clasts (one each from Rainy River Conglomerate and Barretts Formation) suggest derivation from the Brook Street Terrane. Similarity in stratigraphic age, depositional characteristics, granitoid clast ages and composition between Rainy River Conglomerate and Barretts Formation suggests that they are broadly correlative and collectively overlapped a combined Brook Street Terrane ‐ Median Batholith (MTZ) before the Late Jurassic (147+2 ‐1 Ma). Sedimentary overlap may also have continued across to Middle Jurassic conglomeratic strata in the Murihiku Terrane to the east of the Brook Street Terrane. A U‐Pb zircon age of 261 ± 2 Ma is reported for Pourakino Trondhjemite of the Brook Street Terrane.

Basement geology from Three Kings Ridge to West Norfolk Ridge, southwest Pacific Ocean: evidence from petrology, geochemistry and isotopic dating of dredge samples

We present new petrographie, X-ray fluorescence, electron microprobe, Ar-Ar radiometric, fission track, and Sr, Nd and Pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest Pacific Ocean. The dredge samples can be divided into four groups: (1) Permian to Cretaceous gabbroids, granitoids and hornfelses from the West Norfolk and Wanganella Ridges; (2) mainly Cretaceous mafic igneous rocks from the Reinga Ridge and Vening Meinesz escarpment; (3) subduction-related Late Oligocene and Early Miocene basalts and shoshonites from the Three Kings and southern Norfolk Ridges; and (4) back-arc basin and intraplate basalts of Early Miocene age in the South and North Norfolk Basins, respectively, and intraplate basalts of Pliocene age in the Reinga Basin. Our dredge data, combined with regional magnetic and gravity data, provide much-needed tests of earlier tectonic interpretations of the region. Correlatives of the Permian Brook Street Terrane and Mesozoic Median Tectonic Zone of onshore New Zealand clearly extend along the West Norfolk and Wanganella Ridges. The Three Kings Ridge was active at least in the 19- to 20-Ma period, and was probably generated above a west-dipping subduction zone. The Norfolk Basin was a back-arc basin to this arc, and had opened by 18–20 Ma. The oldest Cenozoic volcanic rocks in the region are 26-Ma subduction-related volcanic breccias on the southern Norfolk Ridge; they probably predate opening of the Norfolk Basin and associated eastward migration of the Three Kings Ridge.

Basement geology of Taranaki and Wanganui Basins, New Zealand

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 Cretaceous (cf. Separation Point Suite). Our interpretation of sub‐basin geology is compatible with previously observed onland relationships in the North and South Islands.

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

Abstract Clast compositions of Waiho‐1 conglomerate horizons record dextral movement of the borehole site past different basement terranes on the opposite side of the Alpine Fault. Metavolcanic and volcanogenic metagreywacke clasts in sediments aged c. 4.5 Ma indicate deposition adjacent to exposures of Caples, Dun Mountain—Maitai, or Brook Street basement terranes. Sediments aged c. 4 Ma contain Torlesse Terrane and Haast Schist clasts. Relocation of the Waiho‐1 borehole site using published finite rotations, combined with the observed change in provenance at c. 4.5 Ma, requires that the source rocks had relationships similar to those observed at the present day, that is, that most curvature of basement terranes in southern South Island, New Zealand, was already in place at 4.5 Ma. Major increases in sedimentation rate recorded by Waiho‐1 borehole at c. 12 and 5 Ma are linked to changes in the Australia‐Pacific plate vector. Subsidence before 12 Ma may have been related to thermal subsidence of the Emerald Basin, or/and middle Miocene foreland basin development. Clast compositions of middle—late Miocene (c. 12–5 Ma) sediments record reverse movement on the South Westland Fault Zone and uplift southeast of the Alpine Fault in areas where meagreywacke basement rocks were exposed. Fiordland was low lying and contributing little sediment to the basin in middle—late Miocene time. Large volumes of Pliocene—Quaternary (c. 5–0 Ma) sediment derived from southeast of the Alpine Fault reflect widespread uplift of the Southern Alps. Schist clasts in an early Pliocene sample indicate that amphibolite facies Alpine Schist was exposed by c. 4 Ma.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Brook street and Murihiku terranes of New Zealand in the context of a mobile South Pacific Gondwana margin

Data from geochemistry, isotope studies and paleomagnetism suggest that both the Permian arc-dominated Brook Street terrane and Triassic-Jurassic Murihiku forearc basin terrane are unrelated to each other and that both are allochthonous with respect to the Gondwana margin. The Murihiku terrane lies farther from the Gondwana margin but its fauna and flora have strong Gondwana affinities and closely resemble those of autochthonous epi-Gondwana basins in Australia and Argentina. All terranes within New Zealand are elongate parallel to the Gondwana margin but very few can be clearly identified in the Australian and Antarctic continental crust that abuts New Zealand in Cretaceous Gondwana reconstructions. Similarly, some Australia terranes do not appear to continue into New Zealand or Antarctica. This suggests that breakup was influenced significantly by old terrane boundaries and also that there was considerable terrane mobility in the active Gondwana margin before the Cretaceous. A systematic comparison of arc chemistry, geochronology, biota, and paleomagnetism is desirable to constrain the development of the South Pacific margin between Australia and South America in the Mesozoic.

Tectonism, magmatism and sedimentary basin development, Paleozoic to Paleogene, New Caledonia

Three pre-Cretaceous basement terranes are recognised in New Caledonia: 1. Karagreu­ Boghen Terrane - an old accretionary complex, metamorphosed to greenschist facies, of uncertain age but probably Early Permian or older and tentatively correlated with Mid-Late Paleozoic complexes of New England Fold Belt. 2. Mt. Canala-Teremba Terrane - arc type volcanics and volcaniclastics of Upper Permian to Early Jurassic age and correlated with the Brook Street Terrane of New Zealand. 3. Ponerihouen-Baie St. Vincent Terrane - volcaniclastic and terrigenous sandstones and argillites of Mid Triassic to Jurassic age and correlated with the Murihiku and Waipapa Terranes of New Zealand. The Karagreu-Boghen and Mt. Canala-Teremba Terranes were probably juxtaposed by the Late Triassic. The three basement terranes were stitched together by late Mesozoic intrusives and Rangitata (Early Cretaceous) metamorphism and deformation. Late Cretaceous-Paleocene extensional sedimentary basins with bimodal volcanism were developed to the N (Diahot Basin) and S (Noumea Basin). The Cretaceous basalts of the Diahot basin are ocean floor type while those of the Noumea Basin are arc type and represent the final extensional stage of Mesozoic subduction. The allochthonous Cretaceous-Eocene West Coast basalts, believed to be New Caledonia Basin ocean floor, are normal MORB-type tholeiites with some arc affinities. The West Caledonian Fault post dates Early Oligocene obduction of the New Caledonian ultramafic sheet and has 55 km of right lateral transcurrent movement. Vertical movement on this fault has also played a major role in Miocene uplift particularly in the north of the island.

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...

Evidence of discrepant commercial ISI assignment

The author is with the Manchester Thrombosis Research Foundation, 4a Waterloo Park, Upper Brook Street, Stockport SK1 3BP, UK.

Communication: On the correlation of flash points

Fire and MaterialsVolume 16, Issue 3 p. 153-154 Research Article Communication: On the correlation of flash points E. Metcalfe, E. Metcalfe School of Biological and Chemical Sciences, Thames Polytechnic, Wellington Street, London, SE18 6PF, UKSearch for more papers by this authorA. E. M. Metcalfe, A. E. M. Metcalfe The Judd School, Brook Street, Tonbridge, Kent, UKSearch for more papers by this author E. Metcalfe, E. Metcalfe School of Biological and Chemical Sciences, Thames Polytechnic, Wellington Street, London, SE18 6PF, UKSearch for more papers by this authorA. E. M. Metcalfe, A. E. M. Metcalfe The Judd School, Brook Street, Tonbridge, Kent, UKSearch for more papers by this author First published: July/September 1992 https://doi.org/10.1002/fam.810160307Citations: 20AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume16, Issue3July/September 1992Pages 153-154 RelatedInformation

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.