Three Distinct Shallow Marine Cretaceous Units in Western Shikoku and the U–Pb Age Spectra of Their Detrital Zircons:

Abstract

To reconstruct the detailed paleogeographic configuration of the Cretaceous arc-trench system in East Asia, shallow marine sandstones from the Ryoke, Sanbagawa, and Chichibu belts in western Shikoku are investigated with age spectra by U-Pb dating detrital zircons with LA-ICPMS. The mid-Cretaceous Shuki and Nigyu formations, unconformably covering the preCretaceous accretionary complex of the Chichibu belt, contain abundant detrital zircons from the Jurassic to Early Cretaceous ages, with small quantities of Permian and Triassic detrital zircons. These age spectra are almost identical to those previously obtained from other coeval formations elsewhere in the Chichibu belt （the Monobegawa Group） , which represent the Cretaceous forearc setting. The common age spectra suggest that the provenance of the Cretaceous fore-arc domain had ubiquitous compositions of rocks for nearly 1,000 km along the arc, and that Jurassic to Early Cretaceous granitoids were predominant, with associated small quantities of older pre-Jurassic granitoids. From the Ryoke belt, the Campanian （Late Cretaceous） Yamanouchi Formation of the Izumi Group, unconformably covering the mid-Cretaceous Ryoke granitoids, also has a similar age spectrum. This confirms that the provenance of the fore-arc remained more or less the same at least until the Late Cretaceous. The most intriguing age spectrum was obtained from the Maana Formation, which occurs as a klippe sitting on top of the southern Sanbagawa belt. As well as other dated sandstones, Maana sandstone contains Early Cretaceous to Jurassic zircons; nonetheless it accompanies not only Permo-Triassic zircons but also abundant Paleoproterozoic （2400-1600 Ma） grains. This unique age spectrum is correlative solely with those from the Tetori/Jinzu groups in the Hida belt, whose depositional setting has featured provenance with the Precambrian basement. This suggests that the Maana Fm was primarily deposited at the continent side of all the Paleozoic accretionary complexes and their highpressure metamorphosed equivalents of SW Japan; i.e., the back-arc side of the Cretaceous arctrench system in East Asia. As to the age spectrum of detrital zircons, in addition to lithofacies,

Submarine slope–fan sedimentation in an ancient forearc related to contemporaneous magmatism: The Upper Cretaceous Izumi Group, southwestern Japan

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