The majority of arc-type andesites in the Tongariro Volcanic Centre are highly porphyritic, hornblende-free, two-pyroxene andesites. An exception is tephras from the c. 10,000 ka Pahoka-Mangamate event. Magmas of these Plinian eruptions bypassed the extensive crustal mush columns under the central volcanoes and sequentially derived a series of almost aphyric rocks spanning a compositional range from dacite to basaltic andesite. Mineral composition, trace element and isotopic data suggest that this eruptive series tapped a mid-crustal magma reservoir, resulting in the initial eruption of an hydrous dacitic magma and several following eruptions characterised by less-evolved and less-hydrous compositions at progressively higher temperatures and substantially lower 87Sr/86Sr ratios. Systematic changes in magma chemistry are also reflected in a sequential change in phenocryst content starting with an early hornblende–plagioclase-dominated assemblage to a late olivine–plagioclase-dominated assemblage.
The majority of arc-type andesites in the Tongariro Volcanic Centre are highly porphyritic, hornblende-free, two-pyroxene andesites. An exception is tephras from the c. 10,000 ka Pahoka-Mangamate event. Magmas of these Plinian eruptions bypassed the extensive crustal mush columns under the central volcanoes and sequentially derived a series of almost aphyric rocks spanning a compositional range from dacite to basaltic andesite. Mineral composition, trace element and isotopic data suggest that this eruptive series tapped a mid-crustal magma reservoir, resulting in the initial eruption of an hydrous dacitic magma and several following eruptions characterised by less-evolved and less-hydrous compositions at progressively higher temperatures and substantially lower 87Sr/86Sr ratios. Systematic changes in magma chemistry are also reflected in a sequential change in phenocryst content starting with an early hornblende–plagioclase-dominated assemblage to a late olivine–plagioclase-dominated assemblage.
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.
The Glenroy Complex consists of retrogressed monzodioritic granulites (Woodham Orthogneiss) and heterogeneous metasedimentary rocks (Davis Creek Paragneiss). U–Pb zircon dating shows that the Woodham Orthogneiss comprises rocks ranging in age from 121.8 to 126.6 Ma that are correlatives of the geochemically similar Western Fiordland Orthogneiss Worsley Pluton in northern Fiordland. Permian–Jurassic detrital zircon age peaks in the Davis Creek Paragneiss indicate a maximum depositional age of c. 227 Ma, suggesting correlation with George Sound Paragneiss in northern Fiordland and the paleo-Pacific Gondwana margin cover sequence. Metamorphism of the Davis Creek Paragneiss is constrained to 120.6 ± 0.9 Ma by U–Pb dating of low Th/U zircon overgrowths. The McKnee Intrusives were intruded in two discrete episodes at 225.0 ± 2.7 Ma and 137.8 ± 0.7 Ma which, together with their geochemistry, suggests correlation with northern Fiordland Late Triassic and Early Cretaceous Darran Suite plutons. The HiSY Mt Cann Pluton has an age of 113.8 ± 1.0 Ma and is correlated with the Separation Point Granite. The Glenroy Complex and McKnee Intrusives were likely exhumed during at least two stages: initially in the Cretaceous prior to displacement from their northern Fiordland correlatives by the Alpine Fault; and during emplacement as tectonic blocks in the Matakitaki foreland thrust system at the Big Bend.
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.
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.
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.
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.
Abstract Laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) U–Pb zircon dating combined with whole-rock geochemical analysis indicate that the Redjacket Granite was emplaced at 350.9±2.3 Ma (2σ) and belongs to the S-type Karamea Suite. This is the first report of Karamea Suite-type magmatism occurring after 368 Ma in the Nelson block and the first confirmation of Karamea Suite granitoids in the lower-plate of the Paparoa Metamorphic Core Complex.
Abstract A small occurrence of molybdenite (MoS2) mineralisation has been discovered in the weakly A-type Pomona Island Granite on the shorelines of Lake Manapouri in eastern Fiordland. The disseminated appearance of molybdenite and the absence of quartz veins indicates that mineralisation is probably the product of magmatic and/or hydrothermal activity related to pluton crystallisation at c. 157 Ma, and not younger (c.128–116 Ma) shear zone-related mesothermal mineralisation as has been recently described from the Murchison Mountains to the north and Stewart Island to the south. Although apparently not of economic grade, the Pomona Island Granite locality is regionally important because it is the first direct pluton-related Mo-mineralisation event to be recognised in eastern Fiordland. This occurrence adds to the growing number and known styles of base metal occurrences within the Jurassic–Cretaceous magmatic arc (Outboard Median Batholith) that formed on or near the New Zealand Gondwana margin. Furthermore, the wide distribution of essentially uninvestigated A-type granites in the Outboard Median Batholith means that there may be further Mo-mineralised localities awaiting discovery.
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