西南日本花崗岩・火山岩の包括的なジルコンU-Pb年代測定から明らかになった、白亜紀～古第三紀におけるパルス状の珪長質地殻形成

<p>Convergent plate margins, such as the Mesozoic-Cenozoic circum-Pacific orogenic belts, are regarded as one of the plausible candidates for the post-Archean continental crust formation, as they are associated with abundant calc-alkaline I-type batholiths. However, the fundamental tectonic processes that triggered these voluminous granitic crust formations have remained largely unresolved due to the lack of precise temporal constraints on the granitic magmatism. We have conducted a comprehensive geochronological study of the granitic and silicic volcanic rocks exposed in the Southwest Japan Arc, which is typical of the circum-Pacific orogenic belts, utilizing zircon U-Pb geochronology. Newly obtained zircon ages reveal three clear pulses of granitic crust formation at 85, 60 and 35 Ma separated by approximately 25 million year intervals. The ca. 85 Ma magmatism was the most voluminous and was distributed in a broad zone that extends ~120 km across-strike, whereas the magmatism at 60 and 35 Ma were focused on the northern margin of the SW Japan Arc. Furthermore, the magmatism at 85 Ma involved sediment-incorporated, ilmenite series granitic rocks, while the magmatism at 60 and 35 Ma involved more juvenile, mantle-derived, magnetite series rocks. Thus, not only did the silicic magmatism in SW Japan occur in pulses, there was also a spatial and compositional transition in the magmatism through time. The ocean basin reconstruction model by Müller et al. (2008, Science) revealed pulsed oceanic crust production at the mid-oceanic ridges in the Pacific Plate during the Jurassic to Paleogene, also showing approximately 25 million year interval, indicating that convergence rate of the Pacific Plate around the circum-Pacific region has experienced fluctuation of similar time interval. This suggests that pulsed Cretaceous-Paleogebe silicic crust formation revealed in the SW Japan can potentially be attributed to the enhanced subduction zone magmatism.</p>