U-Pb ages of detrital zircons from the Sanbagawa Belt in western Shikoku: Additional evidence for the prevalence of Late Cretaceous protoliths of the Sanbagawa Metamorphics

The Sanbagawa Metamorphic Belt in Japan is one of the best studied high-pressure, low temperature metamorphic belts. Recent work applying new dating techniques has challenged the previously accepted temporal framework for the evolution of the belt, as it was shown that large parts of the belt contain detrital zircons of Late Cretaceous age (younger than 100 Ma), i.e. they have protolith ages younger than the previously accepted age of metamorphism at ca. 110 Ma. A 2000 m bore hole from north-western Shikoku provided an excellent opportunity to further evaluate the areal extent of Late Cretaceous protoliths as the drill hole was drilled in an area considered to be part of the Jurassic to Early Cretaceous part of the Sanbagawa Belt. Dating of single zircon grains using the LA-ICP-MS U-Pb dating method shows that all but one sample contain zircons younger than 100 Ma and thus the protoliths are younger than the previously accepted age of metamorphism of the Sanbagawa Belt. The single sample that contains only zircons dated at 136 ± 3 Ma, apparently is of volcanic origin and could be a clast representing the source of 130-140 Ma zircons of the sample taken about 120 above this sample. In addition, three surface samples were analysed. Two of these also contain zircons younger than 100 Ma, whereas the third sample contains only zircons older than 159 Ma. The zircons from this sample also exhibit an age spectrum different from that exhibited by the other samples. The exact significance of this sample is not clear as yet.