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Browsing by Author "Tani, K."
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Item Generation of Silicic Melts in the Early Izu-Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea(Wiley, 2017) Barth, Andrew P.; Tani, K.; Meffre, S.; Wooden, J. L.; Coble, M. A.; Arculus, R. J.; Ishizuka, O.; Shukle, John T.; Department of Earth Sciences, School of ScienceA 1.2 km thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation, and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U, and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.Item Marine Volcaniclastic Record of Early Arc Evolution in the Eastern Ritter Range Pendant, Central Sierra Nevada, California(Wiley, 2018) Barth, Andrew P.; Wooden, J. L.; Riggs, N. R.; Walker, J. D.; Tani, K.; Penniston-Dorland, S. C.; Jacobson, C. E.; Laughlin, Jennifer A.; Hiramatsu, Reina; Earth Sciences, School of ScienceMarine volcaniclastic rocks in the Sierra Nevada preserve a critical record of silicic magmatism in the early Sierra Nevada volcanic arc, and this magmatic record provides precise minimum age constraints on subduction inception and tectonic evolution of the early Mesozoic Cordilleran convergent margin at this latitude. New zircon Pb/U ages from the Ritter Range pendant and regional correlations indicate arc inception no later than mid‐Triassic time between 37 and 38°N. The regional first‐order felsic magma eruption rate as recorded by marine volcanic arc rocks was episodic, with distinct pulses of ignimbrite emplacement at ca. 221 to 216 Ma and 174 to 167 Ma. Ignimbrites range from dacite to rhyolite in bulk composition, and are petrographically similar to modern arc‐type, monotonous intermediate dacite or phenocryst‐poor, low‐silica rhyolite. Zircon trace element geochemistry indicates that Jurassic silicic melts were consistently Ti‐ and light rare earth‐enriched and U‐depleted in comparison to Triassic melts of the juvenile arc, suggesting Jurassic silicic melts were hotter, drier, and derived from distinct lithospheric sources not tapped in the juvenile stage of arc construction. Pulses of ignimbrite deposition were coeval with granodioritic to granitic components of the underlying early Mesozoic Sierra Nevada batholith, suggesting explosive silicic volcanism and batholith construction were closely coupled at one‐ to two‐million‐year time scales.