J. M. Palin

Metasomatized ancient lithospheric mantle beneath the young Zealandia microcontinent and its role in HIMU‐like intraplate magmatism

There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time-integrated U/Pb (HIMU)-like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the subcontinental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene-Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton-like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb ± Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf-Nd isotope composition than the investigated spinel facies, or was mixed with asthenosphere-derived melts containing less radiogenic Hf.

LA‐ICP‐MS U‐Pb zircon ages from Mesozoic plutonic rocks in eastern Fiordland, New Zealand

Abstract Laser ablation inductively coupled plasma mass‐spectrometry (LA‐ICP‐MS) 206Pb‐238U zircon dates are reported for eight plutonic rocks from eastern Fiordland. The oldest samples are Jurassic (granodiorite, 160.6 ±1.4 Ma; quartz diorite, 153.0 ± 0.8 Ma; granodiorite, 152.5 ± 0.7 Ma; errors at 2 a) and collectively belong to the Hunter Intrusives member of the Darran/Median Suite. The LA‐ICP‐MS age for the younger granodiorite agrees with a concordant multigrain TIMS U‐Pb zircon age previously obtained from the same sample. Xenoliths of Hunter Intrusives occur within the Early Cretaceous (127.9 ± 1.1 Ma) granitic Fowler Pluton, also of the Darran/Median Suite. There was a short lull in plutonism until emplacement of leucogranite dikes (123.5 ± 1.2 Ma) and granitoids of the Refrigerator Orthogneiss (120.7 ± 1.1 Ma), Puteketeke Pluton (120.8 ± 0.9 Ma), and West Arm Leucogranite (116.3 ± 1.2 Ma), all of which are members of the Separation Point Suite.

Emplacement, metamorphism, deformation and affiliation of mid-Cretaceous orthogneiss from the Paparoa Metamorphic Core Complex lower-plate, Charleston, New Zealand

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating and whole-rock geochemistry of plutonic rocks from coastal exposures of the Paparoa Metamorphic Core Complex (PMCC) lower-plate has revealed their suite affinity and magmatic and metamorphic ages. Emplacement and metamorphism of the Charleston Orthogneiss occurred at 118±2 Ma and 107±2 Ma, respectively. Metamorphism of the Charleston Orthogneiss was coeval with Buckland Granite intrusion, suggesting a link between the two. Field relations between the post-metamorphic Doctor Bay dike (DBD) and Charleston Orthogneiss indicate intrusion of the dike at 105±2 Ma was syn-tectonic. Deformation of the Charleston Orthogneiss occurred during mid-crustal continental extension and PMCC formation in the New Zealand region of the Pacific Gondwana margin. Fabric development in the Charleston Orthogneiss may therefore be related to mid-crustal deformation associated with extension. The Charleston Orthogneiss and DBD are assigned to the I-/S-type Rahu Suite on the basis of their geochemistry, mid-Cretaceous ages, inherited zircon populations and inboard location.

Petrology and geochronology of the volcaniclastic and volcanogenic Mesozoic Loch Burn Formation in eastern Fiordland, New Zealand

Abstract The Loch Burn Formation in eastern Fiordland is the metamorphosed and eroded effusive product of a long‐lived Jurassic to Early Cretaceous volcanic arc. Relict sedimentary features within meta‐volcaniclastic components indicate sedimentation in a mostly terrestrial or shallow‐water environment that was fed by debris flows from proximal granitic and volcanic high points. In the Murchison Mountains, deposition is constrained by a c. 342 Ma granite, which is unconformably overlain by the Loch Burn Formation, and a c. 158 Ma quartz diorite that intrudes the Loch Burn Formation. This latter age is 8–9 m.y. older than a volcanic clast from Loch Burn Formation and a sandstone horizon previously dated from the Stuart Mountains, and therefore supports previous suggestions that the Loch Burn Formation is a long‐lived and time‐transgressive unit. The Carboniferous basement provides a potential source for detrital zircon in metasediment, inherited zircon in the intrusive quartz diorite, and some granitoid clasts within the formation. Geochemical similarities between the quartz diorite, volcanic clasts within the Loch Burn Formation, and nearby Jurassic to Early Cretaceous Darran Suite plutons imply that the Loch Burn Formation is the volcanic equivalent of the Darran Suite. The distinctive lithological, geochemical, age, and internal relationships of the Loch Burn Formation are also seen in the volcano‐sedimentary Largs Group of northern Fiordland and Paterson Group of Stewart Island, suggesting that these three units are lithological and chronological equivalents to one another. The Loch Burn Formation provides a comprehensive record of the tectonic evolution of eastern Fiordland, with several episodes of uplift and burial. These are: uplift and erosion of a Carboniferous plutonic basement by c. 195 Ma; deposition of the older part of the Loch Burn Formation sequence in the Jurassic before burial and intrusion by Darran Suite plutons at 158 Ma; deposition continuing until at least 148 Ma; metamorphism of the entire Loch Burn Formation at greenschist and amphibolite facies conditions; uplift, erosion, and then deposition of the overlying Eocene sediments; reburial to zeolite facies depths beneath the Tertiary Te Anau and Waiau Basins; final uplift in the Pliocene.

The sub-Antarctic Antipodes Volcano: a <0.5 Ma HIMU-like Surtseyan volcanic outpost on the edge of the Campbell Plateau, New Zealand

Abstract The sub-Antarctic Antipodes Islands group, located on the edge of the Campbell Plateau, is composed of Surtseyan-like tuff cones, porphyritic lavas and dykes, overlain by scoriaceous cinder cones. The youthful nature is indicated by well-preserved cones and craters, raised boulder beaches, pollen and spores comparable to the present-day vegetation in peat underlying one lava flow, and published Ar/Ar ages of ≤0.5 Ma. The alkaline crystalline rocks record a fractionation lineage that was controlled by progressive removal of clinopyroxene and olivine at shallow depths with minimal interaction with continental crust. A compilation of isotopic data confirms that the magmas were derived from a mantle source containing a high-time integrated U/Pb (HIMU)-like component. Although this source component has been recognised in many Zealandia intraplate volcanoes, the Antipodes Volcano lavas contain consistently high 206Pb/204Pb and low 87Sr/86Sr ratios, making the geochemical suite an excellent baseline for HIMU studies. The distinctive isotope ratios and chemistries are comparable with those of some anhydrous Pb- and K-depleted peridotitic mantle xenoliths from intraplate volcanoes in Zealandia. If the lithospheric mantle contributed to the source of Antipodes Volcano, the Pb and K anomalies in the lavas may not require residual amphibole/phlogopite and/or sulphide in their source region. Furthermore, comparison with the peridotite suite indicates that the Zealandia lithospheric mantle is isotopically heterogeneous and thus not all the Zealandia Cretaceous–Cenozoic intraplate volcanoes need have tapped a HIMU-like reservoir.

Ultra‐fast early Miocene exhumation of Cavalli Seamount, Northland Plateau, Southwest Pacific Ocean

Abstract We present new photographic, petrological, geochronological, and isotopic data for gneissic and granitic rocks obtained from six sample stations on Cavalli Seamount during two cruises in 2002. These data lead to revision of earlier conclusions based on two dredges of schist in 1999. Based on c. 100 Ma ages of zircon cores, and whole rock petrochemistry and tracer isotopes, we interpret the protoliths of paragneisses and orthogneisses to probably have been sedimentary and plutonic correlatives of the Late Cretaceous Houhora Complex. U‐Pb dating of low Th/U zircon rims confirms an earliest Miocene high‐grade metamorphic episode. A cooling history based on Ar‐Ar K‐feldspar dating indicates ultra‐rapid cooling (c. 2000°C/m.y.) and vertical exhumation (c. 100 mm/yr) of the rocks at 19.9 Ma. Our preferred tectonic model relates the amphibolite facies metamorphism to Northland Allochthon emplacement and the rapid exhumation to dextral transtension along the Vening Meinesz Fracture Zone system and/or a rapidly retreating Pacific trench.

Late Jurassic detrital zircons from the Haast Schist and their implications for New Zealand terrane assembly and metamorphism

The youngest detrital zircon age groups from three samples within the Haast Schist in northwest Otago are Late Jurassic (154, 155, 160 Ma), as determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of U–Pb isotopes in individual zircons. It is inferred that this is the maximum age of sedimentation for these samples, which is within the range of ages for Haast Schist metamorphism (145–180 Ma). This maximum sedimentation age is at least 50 Ma younger than the previously inferred depositional ages for the Caples and Rakaia terranes which are the protoliths of Haast Schist. The zircon age populations within the samples are also different from those found within the Rakaia and Caples terranes, implying different sedimentary sources and possibly a different terrane. The detrital zircon populations are comparable to those found within the Waipapa Terrane in the North Island.

Beehive Diorite: A Late Jurassic two‐pyroxene pluton at Lake Manapouri, Fiordland

Abstract Beehive Diorite is a small but distinctive two‐pyroxene pluton exposed amongst the heterogeneous Hunter Intrusives on the shoreline of Lake Manapouri in Fiordland, New Zealand. This 149 Ma pluton has been partially re‐hydrated and deformed to amphibole‐bearing meta‐diorite, but contains relict hypersthene‐augite primary assemblages in low strain domains. Mafic and felsic dikes that intrude Beehive Diorite recrystallised under amphibolite and then greenschist facies conditions. Geochemical composition indicates that Beehive Diorite is a calc‐alkaline cumulate derived from a primitive source in a volcanic arc setting, and provides a baseline for Darran Suite mafic magmatism at Lake Manapouri. The crosscutting mafic dikes also belong to the Darran Suite, but the felsic dikes belong to the Separation Point Suite. Beehive Diorite is petro graphic ally and geochemically similar to Hollyford Gabbronorite, and diorite at Mt Underwood in northern Fiordland. This correlation indicates that subduction‐related processes in the Outboard Median Batholith remained dominant from 149 to 138 Ma, but contrasts with slightly younger and older A‐type magmatic pulses, which suggest short periods of slab rollback or stalling. Amphibolite facies metamorphism of Beehive Diorite and the crosscutting dikes occurred between 124 and 116 Ma, after the cessation of Darran Suite and initiation of Separation Point Suite magmatism.

Geology and geochronology of the Sub-Antarctic Snares Islands/Tini Heke, New Zealand

The first comprehensive geological map, a summary of lithologies and new radiogenic isotope data (U–Pb, Rb–Sr) are presented for crystalline rocks of the Sub-Antarctic Snares Islands/Tini Heke, 150 km south of Stewart Island. The main lithology is Snares Granite (c. 109 Ma from U–Pb dating of zircon), which intrudes Broughton Granodiorite (c. 114 Ma from U–Pb zircon) on Broughton Island. Rafts of schist within Snares Granite are common on the outlying Western Chain islets, and rare on North East and Broughton islands. Zircon grains extracted from one schistose raft on Broughton Island are prismatic and yield an essentially unimodal age population of c. 116 Ma that is within error of the granodiorite. These properties suggest that the dated raft represents a meta-igneous rock despite its mica-rich nature. Some schistose rocks on the Western Chain contain coarse relict plagioclase phenocrysts and appear to have an igneous protolith. No conclusive metasedimentary rocks have been identified, although sillimanite-bearing mica-rich schist occurs on Rua. Deformation of the crystalline rocks occurred after Snares Granite intrusion and before cooling below muscovite K–Ar closure at 400 ± 50 °C at 95 Ma. This period overlaps the age of extensional ductile shear zones on Stewart Island. The discovery of several basaltic dykes, which cut across fabrics and are unmetamorphosed, indicates that volcanic rocks are associated with all Sub-Antarctic island groups. The larger of the islands are overlain by peat, which on North East Island also contains gravel deposits.

A possible Late Oligocene–Early Miocene rocky shoreline on Otago Schist

Duntroonian–Waitakian limestone at Kokonga in the Maniototo district, Central Otago, New Zealand is interpreted to be a rocky shoreline facies formed near the time of maximum Cenozoic marine transgression across Zealandia. The limestone, which is correlated with the Otekaike Limestone of North Otago, contains shallow and warm water indicators such as coralline algae, oysters, echinoderms, bryozoa and a foraminiferal assemblage composed almost entirely of Amphistegina. Textural zone 2B Otago Schist cobbles, pebbles and sand within the limestone, often adjacent to invertebrate fossil and coralline algal fragments, were sourced from proximal schist exposures and may be indirect evidence of the limestone having formed near to a low-lying Otago Schist coastline. However, because the precise age of limestone deposition cannot be constrained by fossil assemblages or Sr isotopes and the timing of peak marine transgression is imprecisely known, it is not possible to state whether Otekaike Limestone at Kokonga formed before, during or after the peak of maximum Cenozoic marine transgression.