Thematic section: Special topics in 4th <scp>IGS</scp> ‘Precambrian World 2’

The 4th International Geoscience Symposium (4th IGS) was held at Kyushu University, Fukuoka on March 4th–6th 2017, followed by a 7 days field trip of Kyushu Island and Satsuma Iwo Jima island. The IGS was organized by ‘Project A research group’, which focuses on early Earth history (main organizer was S. Kiyokawa, T. Ito, M. Ikehara, and K.E. Yamaguchi), and have taken place every 3–4 years; The 1st IGS took place in 2008 at Kyushu University and Kitakyushu National Science Museum, Fukuoka. The 2nd IGS was in 2011 at National Taiwan University, Taiwan and the 3rd IGS was in 2014 at KIGAMU, Korea. The title of the 4th IGS was ‘Precambrian World 2’. It mainly focused on Earth system science in the Precambrian, but also featured studies of the Phanerozoic, recent environmental change, and the modern Earth system. During the Precambrian World 2, a total of 28 oral talks were given, including 5 key note talks (40 min), and 7 invited talks (20 min). Thirty-three posters were presented during a dedicated poster session. Five keynote speakers, J. Kirschvink (CALTEC: USA), A. Hofmann (University of Johannesburg: South Africa), C. Rosiere (UFMG: Brazil), W. Bleeker (Geological Survey of Canada: Canada), A. Lelpi (Royal Museum for Central Africa: Belgium), and F. Inagaki (JAMSTEC: Japan) provided insights into breakthrough and new theories of Earth's history. Many other talks and posters also represented innovative findings and suggestive ideas for understanding Earth's history. A total of 151 people including 40 non-Japanese scientists, 35 Japanese scientists, 40 students, and several representatives from industry joined this meeting. Topics covered many geoscience disciplines, Stratigraphy, Sedimentology, Structural geology, Geochemistry, Age dating, Paleo-oceanography, Microbiology, Marine and Resource Geology, which were performed in various regions, such as Canada, USA, Brazil, South Africa, Ghana, Congo, Egypt, Australia, Antarctica, Japan and modern ocean (Pacific, Sea of Japan, Southern Atlantic Ocean, Bering sea, Izu Bonin island, Japan Trench and Nankai Trough etc.). A post conference field trip investigated a transect of Kyushu island to Satsuma Iwo-Jima island during March 6–10th. It included the Nippon Steel ironworks, Hiraodai Mitsubishi Material limestone quarry, Taioh Gold mine, Aso Caldera-Mountain, Japan Limonite mine, Sakurajima, Aira caldera and Satsuma Iwo-Jima island. This symposium was supported by Kyushu University, JSPS, GSJ, Mishmi Village, JAMSTEC, Nippon Steel, Mitsubishi Material, Windy Network, Kyuhel Co., Aso Volcano Museum, Taioh Gold Mine, Japan Limonite and Fukuoka Convention and Visitors Bureau. We are pleased to publish some selected researches from 4th IGS ‘Precambrian World 2’ in a special issue of Island Arc (this volume). We briefly introduce nine articles in the ‘Precambrian World 2’ special issue, which covers a long period of Earth's history form 3.1 Ga to 220 Ma. Kiyokawa, Aihara, Takehara, and Horie (2019) provide important new age data for sedimentation of the Cleaverville Formation, coastal Pilbara terrane, Australia, which is one of the most important Banded Iron formation (BIF) records of the Mesoarchean world which is formed 6 million years ago before Great Oxidation Event (GOE). Onset of the Cleaverville deposition was newly dated as 3114 ± 14 Ma, 40–90 Ma older than previous estimates. After sedimentation, this area was affected by deformations (D1, D2 and D3: Kiyokawa, Taira, Byrne, Bowring, & Sano, 2002). The first gave rise to accretion and collision deformation at about 3070–3050 Ma, concomitant with shallow marine sedimentation (Prinsep Orogeny). The second included left-lateral transtensional deformation with shallow marine pull-apart basin at about 3000–2930 Ma. This is a crustal scale shear deformation event in the Pilbara region. This work emphasizes the Mesoarchean timing of BIF sedimentation and later deformations of the coastal Pilbara terrane. Rosière, Bekker, Rolim, and Santos (2019) reports the development of ferruginous conditions in an intracratonic basin along the eastern margin of the São Francisco protocraton, straddling the Orosirian-Statherian boundary after the termination of the 2.1–2.0 Ga Trans-Amazonian orogeny, with the occurrence of Fe-rich sequences that extend from the northwestern cratonic border to the Southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group). The presence of iron formations of this age indicates the development of an unusual setting in a large basin after the Great Oxidation Event. It was proposed that a mantle plume event led to the opening of the Espinhaço, Pirapora, and Paramirim rifts and caused magmatism that supplied hydrothermal Fe to the basin. Vafeas, Viljoen, and Blignaut (2019) identifies a new Mn enrichment event in the Kalahari Manganese Field, which is petrographically defined by the development of significant abundances of massive to euhedral arsenic-bearing fluor-apatite. The manganese ore was identified within the Hotazel Formation that has been tectonically thrust to near surface depths. This newly characterized supergene alteration event, referred to as the ‘Mukulu Enrichment’, is manifested through the downward movement of acidic supergene fluids percolating through a carbonate-poor, oxide-rich manganese ore. This process resulted in the residual enrichment of manganese ore in this world class manganese deposit. Ibrahim et al. (2020) studied the petrography and geochemistry of manganiferous rocks in the Mankwadzi area in the southernmost portion of the Kibi-Winneba metavolcanic belt, one of several Mn occurrences in the Palaeoproterozoic in Birimian of Ghana. Ores are hosted in hornblende schist and amphibolite rocks that are, in places, intruded by a hornblende dyke. In outcrop, the manganiferous rocks appear to be conformable with the host rocks, are macroscopically dark, fine-grained and structurally massive to distinctly banded. The similarity in geo-tectonic setting, possible presence of precursor cherty (Si-rich) bands and high Mn contents in the manganiferous rocks suggest similarity of formation with other manganese rocks in Birimian terrains. Motomura, Kiyokawa, Ikehara, Tanaka, and Sano (2020) made detailed lithological observations and organic carbon analysis of a Mesoproterozoic sandstone-shale sequence, Embry Lake Formation in the Flin Flon Belt, Canada. This area was situated collided with the North Rae terrane and the Superior terrane at 1800 Ma during the Trans Hudson orogeny. The Embry Lake Formation is distributed along the Flin Flon Formation (Island arc volcanic sequence) and the Missi Formation (subaerial sedimentary basin) with strike-slip fault boundary. The main lithology of the Embry Lake Formation is a continuous sandstone-shale turbidite sequence, which was previously identified as an oceanic sequence. However, detailed observations, organic carbon and sulfur ratio, and their isotope data suggest this sedimentary sequence formed as freshwater-lacustrine condition, which formed strike-slip basin during the orogeny. Igisu et al. (2019) investigated four types of morphologically different organic microfossils (one filamentous type and three coccoidal types), diffuse organic matter (OM), and one irregularly shaped structure obtained from ~ 810 Ma Fifteenmile Group, in Yukon, Canada by conventional and synchrotron radiation-based (SR) Fourier transform infrared microspectroscopy (micro-FTIR). Results obtained by comparing CH3/CH2 peak height ratios of the microfossils support the morphology-based assignments of the microfossils and diffuse OM. The comparison results also suggest that the irregularly shaped structure may represent a eukaryotic cell. SR micro-FTIR measurements of two coccoid types indicate that they are chemically and morphologically different from each other. Micro-FTIR characterization and morphological analysis of organic microfossils can provide new insights into their biological affinities. Horie and Hidaka (2019) report a suitable reference material collected from Faraday Mine, Bancroft, Canada (206Pb/238U = 0.1647) and calibration method for U–Pb uraninite isotopic dating using secondary ion mass spectrometer. Correlation between 251(235U16O)+/235U+ and 206Pb+/235U+ obtained by a sensitive high-resolution ion microprobe (SHRIMP) was adopted for the calibration from secondary ion ratios (Pb+/U+) to the atomic abundance ratios (Pb/U). The established calibration method using Faraday Mine uraninite is useful for U–Pb isotopic dating for fine-grained material of a few micrometers to tens of micrometers, which makes it possible to obtain the accurate age of uraninite. Takehara and Horie (2019) report U–Pb ages and oxygen isotope data of zircons collected from a granitic mylonite and an undeformed granite in the Kamioka area in the Hida Belt, southwest Japan. The zircon oxygen isotope ratios suggest that these rocks are derived from different magmas, and the timing of the mylonitization in the Funatsu Shear Zone is constrained to be 242.6–199.1 Ma. The U–Pb zircon ages of the banded gneiss in the Kamioka area also reveals that the sedimentary protolith deposited at approximately 256 Ma, and regional metamorphism occurred at 245.0 ±6.6 Ma. The data give new insight into the relationship between the Hida Belt and the eastern margin of the Asian continent: the Funatsu Shear Zone is comparable with the Cheongsan Shear Zone, the center of the Ogcheon Belt, in the Korean Peninsula. Soda and Onoue (2018) report chemical compositional changes of Middle Triassic (Anisian) siliceous shales in the Mino Belt (central Japan), which accumulated in a pelagic deep seafloor environment of the Panthalassic Ocean. Geochemical proxies detected compositional changes of the terrigenous materials, coeval with several oceanic anoxic events during the middle to late Anisian. These stratigraphic trends suggest that a relatively humid climate was a triggering mechanism for the oceanic anoxic events during the middle Anisian. In addition, the geochemical profiles indicate that the termination of Superanoxia in the Panthalassic Ocean was associated with a relatively arid climate during the middle to late Anisian.

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