Yusuke Sawaki

Imbricated ocean-plate stratigraphy and U-Pb zircon ages from tuff beds in cherts in the Ballantrae complex, SW Scotland

Ocean-plate stratigraphy (basalt, chert, clastics) of the Balcreuchan Group in the Ballantrae complex is repeated by layer-parallel thrusts to form duplex structures south of Bennane Head, ~5 km north of Ballantrae. Five tuff beds, all in chert, have similar U-Pb zircon ages of ca. 470 ± 10 Ma. The geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the Ballantrae complex. Northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast, and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the Ballantrae complex.

Rock magnetism of tiny exsolved magnetite in plagioclase from a Paleoarchean granitoid in the Pilbara craton

Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse-grained, magnetically unstable oxides. In this study, the first example of tiny, needle-shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First-order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.

U-Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551, Japan Department of Chemistry, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo, 171-8588, Japan Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan WiscSIMS, Department of Geoscience, University of Wisconsin, 1215, W. Dayton Street, Madison, Wisconsin, 53706, USA Geochemistry Research Center, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551, Japan Correspondence Hisashi Asanuma, Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan. Email: asanuma.h.aa@m.titech.ac.jp Funding information Japan Society for the Promotion of Science, Grant/Award number: 16J07060; Ministry of Education, Culture, Sports, Science and Technology, Grant/Award numbers: 26106002, 26800259, 16J07060 Abstract Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic-ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U– Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre-ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (Pb/Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (Pb/Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a Pb/Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre-ablation technique has the potentials to identify nonmetamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.