3:00 PM - 3:15 PM
[BCG06-18] Geological significance of U-Pb and Lu-Hf isotope data from detrital zircons of the Shimanto accretionary complex in Shikoku, Japan
Keywords:accretionary complex , zircon, East Asia, Shimanto
Basement rocks of the Japanese archipelago are composed of the products of several Pacific-type orogenies that occurred after 500 Ma, which means that these rocks provide us with a comprehensive understanding of the geotectonic evolution in the oceanic plate convergent margin. However, the basement rocks have been affected by subsequent tectonic events, such as tectonic erosion, magma intrusion, and the Miocene opening of the Sea of Japan. As a result, most of the rocks that remain in Japan were formed in the post-Triassic. The 50-130 Ma granitoids account for more than 80% of all granitoids in Japan, and most of the accretionary complexes (AC) and intra-arc sediments (IAS) exposed at the surface are from the Jurassic to the Paleogene. Japan has developed in the eastern margin of Asia after 250 Ma. This means that there are limitations in discussing the tectonic evolution of the pre-Jurassic East Asian margin quantitatively from the exposed rocks in Japan. The best method to reveal the tectonic evolution under such limited conditions is detrital zircon geochronology and geochemistry separated from AC and/or IAS. This is because that clastic materials including zircons were derived from contemporaneous and/or reworked older orogenic components. The detrital zircons are expected to contain geological information about rocks lost from the basement in Japan, and a comparison of detrital zircon signatures obtained from AC and/or IAS in each era would give useful information for understanding secular change of the geotectonic evolution of East Asia in the oceanic plate convergent margin.
This study focused on the Cretaceous-Paleogene Shimanto AC in Shikoku, Japan. As mentioned above, this AC is expected to contain a lot of reworked materials of older orogenic components as terrigenous sediments. Detrital zircon U-Pb and Lu-Hf analyses of 12 sandstone samples collected from the tectonically upper (Douganaro Formation) to lower (Tanokuchi Formation) sections of the Shimanto AC revealed that the youngest U-Pb ages from each formation were consistent with or older than the depositional ages previously estimated from microfossils, and the ages were progressively younger descending tectonostratigraphic order. In terms of kernel density estimation and histogram based on the analyzed zircons, there was no great difference in their shape among samples, although there were slight differences in peak position and age frequency. In addition, a compilation of the obtained εHf(t) values clearly showed multiple patterns of increase and decrease from 250 Ma to 50 Ma, i.e., four maximum values plotted near the Hf isotopic evolution line of the depleted mantle and three minimum values around -20. In this session, the geotectonic development of East Asia in the oceanic plate convergent margin from 250 to 50 Ma will be discussed, based on our analytical results mentioned above and previous reports of zircon geochemistry and geochronology for East Asia.
This study focused on the Cretaceous-Paleogene Shimanto AC in Shikoku, Japan. As mentioned above, this AC is expected to contain a lot of reworked materials of older orogenic components as terrigenous sediments. Detrital zircon U-Pb and Lu-Hf analyses of 12 sandstone samples collected from the tectonically upper (Douganaro Formation) to lower (Tanokuchi Formation) sections of the Shimanto AC revealed that the youngest U-Pb ages from each formation were consistent with or older than the depositional ages previously estimated from microfossils, and the ages were progressively younger descending tectonostratigraphic order. In terms of kernel density estimation and histogram based on the analyzed zircons, there was no great difference in their shape among samples, although there were slight differences in peak position and age frequency. In addition, a compilation of the obtained εHf(t) values clearly showed multiple patterns of increase and decrease from 250 Ma to 50 Ma, i.e., four maximum values plotted near the Hf isotopic evolution line of the depleted mantle and three minimum values around -20. In this session, the geotectonic development of East Asia in the oceanic plate convergent margin from 250 to 50 Ma will be discussed, based on our analytical results mentioned above and previous reports of zircon geochemistry and geochronology for East Asia.