Shigeki Hada

Kurosegawa Terrane in Southwest Japan: Disrupted remnants of a Gondwana-derived Terrane

Abstract The Kurosegawa Terrane is an anomalous, disrupted, Paleozoic and Mesozoic lithotectonic assemblage characterized by fragments of continent and continental margins. It is located in Southwest Japan where it lies between two Mesozoic subduction complex terranes. The Kurosegawa Terrane is an exotic and far-travelled geologic entity with respect to its present position. Limestones of the Kurosegawa Terrane formed along a continental margin yield fusulinacean fossils Cancellina, Colania and Lepidolina. Accordingly, the Kurosegawa Terrane was once situated within the Colania-Lepidolina territory in the East Tethys-Panthalassa region at a palaeo-equatorial latitude, possibly close to the eastern margin of the South China and/or Indochina-East Malaya continental blocks. These blocks had rifted from Gondwana by late Devonian. They drifted northwards, passing through the Colania-Lepidolina territory in mid-Permian time, and amalgamated with the proto-Asian continent during the late Triassic. Subsequently, during the Cretaceous, parts of the allochthonous continental blocks and their associated tectonic collage were transpressed, dispersed, and displaced from the southeastern periphery of Asia towards the north. As a result, the Kurosegawa Terrane is formed as a disrupted allochthonous terrane, characterized by a serpentinite melange zone, lying between the adjoining Mesozoic subduction complex terranes.

Ages of Silurian radiolarians from the Kurosegawa terrane, southwest Japan constrained by U/Pb SHRIMP data

Abstract Radiolarians are abundant in mid-Paleozoic tuffs of the Kurosegawa terrane of southwest Japan. Well preserved Silurian radiolarian faunas recovered from several localities are described herein and include four new species: Ceratoikiscum kurosegum n. sp., Fusalfanus? konomoriensis n. sp., Futobari? jingamoriensis n. sp., and Futobari? tosaensis n. sp. The absence, within the same strata, of fossils belonging to other taxonomic groups has presented difficulties in determining the precise ages of the Kurosegawa radiolarian faunas. U/Pb SHRIMP ages of pyroclastic zircons from tuff units within the succession indicate Wenlockian [427.2 ± 7.6] and Pridolian [408.9 ± 7.6] ages and remove ambiguity regarding the Silurian age of tuffaceous rocks in the Kurosegawa sequence.

Permian fragmentation, accretion and subsequent translation of a low-latitude Tethyan seamount to the high-latitude east Gondwana margin: evidence from detrital zircon age data

Ion microprobe analyses of detrital zircons in the Te Akatarawa Terrane, New Zealand, reveal that the age of unfossiliferous turbidites overlying a fusuline- and coral-bearing limestone block olistostromal melange is no older than 255±4 Ma (Late Permian). This is approximately 15 m.y. younger than the Kungurian age of the fusulinid limestone. We interpret this to indicate collapse of a Permian oceanic seamount as it entered a subduction zone along the Pacific margin of Gondwana. These turbidites differ markedly in composition from adjoining Permian to Middle Triassic sand-stones of the Torlesse Terrane. Detrital zircon age data indicate predominantly Permian and Carboniferous ages for source rocks supplying the Te Akatarawa turbidites, but also reveal significant earlier Palaeozoic and Proterozoic components, ranging back to 1.9 Ga. The warm-water setting of limestone blocks and the short 15 m.y. time period between sedimentation and accretion onto a continental margin require the limestone to have formed in a low-latitude position probably off the northeast Australian (New Guinea) margin of Gondwana. Zircons within the sample underwent re-crystallization at around 230±11 Ma which may be related to alteration during accretion in a subduction zone environment. Over a period of 100 to 150 m.y. from 255 Ma the terrane underwent more than 5000 km translation along the continental margin southward to its current location as an exotic mini-terrane enclosed within the New Zealand Torlesse Terrane.

The Chrystalls Beach‐Brighton block, southeast Otago, New Zealand: Petrography, geochemistry, and terrane correlation

Abstract The Chrystalls Beach‐Brighton coastal block in southeast Otago has commonly has been placed in Caples Terrane, but has recently been described as a geochemically anomalous area of uncertain terrane affinity. Data points on discriminant diagrams occupy fields centred between those for type Caples Group and Torlesse Terrane, overlapping both. The psammites average 71.9% SiO2, closely comparable to Torlesse Terrane psammites, in contrast to the majority of type Caples Group psammites (av. 64.3%) and Waipapa Terrane psammites (64.4%). QFL plots show the Chrystalls Beach psammites as a petrofacies distinct from those described hitherto for Torlesse Terrane (lithic feldsarenites) and Caples Group and Murihiku Terrane (volcarenites).

Tectonic Setting of the Permo-Triassic Chiang Khong Volcanic Rocks, Northern Thailand Based on Petrochemical Characteristics

Abstract The inferred Permo-Triassic Chiang Khong volcanic belt is composed of felsic to mafic volcanic rocks and their pyroclastic equivalents. Almost all the least-altered mafic volcanic rocks are lava flows; a few might have occurred as dykes. These mafic volcanic rocks are non-foliated to weakly foliated, and mostly have porphyritic textures. The phenocrysts/microphenocrysts in porphyritic samples are commonly plagioclase, and may include clinopyroxene, olivine, Fe-Ti oxide, apatite and amphibole. The matrix of lava flows ranges texturally from felty to trachytic but a few samples show felty to ophitic/subophitic, and glassy textures, whereas that of possible dyke samples is holocrystalline. The primary matrix constituents are largely plagioclase and variable proportions of clinopyroxene, Fe-Ti oxide, amphibole, olivine, apatite, quartz, alkali feldspar and/or glass. All the studied samples have been subjected to greenschist-facies regional metamorphism. Chemically, the samples show narrow ranges of least-mobile incompatible-element ratios and range compositionally from dacite to basalt of tholeiitic series. These samples are chemically analogous to those of the Tertiary andesite from Sardinian Rift, Sardinia, Italy, particularly in terms of least-mobile incompatible-element ratios. Accordingly, the studied mafic volcanic rocks are interpreted to have formed in a continental volcanic arc. However, the problem related to the geometry of plate convergence, giving rise to the continental volcanic arc, still exists.

New radiolarian age information for the Chrystalls Beach Complex, southwest of Dunedin, New Zealand

Abstract On the basis of lithology, structural style, and fossil evidence, northern and southern units are recognised within the Chrystalls Beach Complex exposed on the coast south of Akatore Creek, southwest of Dunedin, South Island, New Zealand. Radiolarians including Spongopallium sp. cf. S. contortum Dumitrica, Kozur & Mostler, Eptingium(?) sp., Norispongus(?) sp., and Pseudostylosphaera(?) sp. have been discovered in phosphatic nodules in metamorphosed argillite of the northern unit of the complex. They are regarded as characteristic species of the Middle Triassic (Anisian‐Ladinian). It has previously been known that the more weakly metamorphosed southern unit of the Chrystalls Beach Complex contains moderately well preserved Middle Triassic (Early Ladinian) radiolarian faunas. Together with forms of Tethyan affinity, the faunas in the southern unit contain non‐Tethyan forms including Glomeropyle spp. Aita & Bragin, possibly of Southern Hemisphere high‐latitude origin. These non‐Tethyan forms are also known from the Mahinepua section of the Waipapa Terrane in Northland. Distinctive non‐Tethyan forms such as Glomeropyle Aita & Bragin have not yet been found in the northern unit, the fauna of which includes species that are known from the European Tethys area and which also occur in the southern unit.