Geochemical and geochronological examination on Mesozoic continental lithosphere beneath SW Japan

晚白垩世（100ma ~ 80ma），日本西南部长英质岩浆活动发生在从良轭带到三印带的广阔区域。此后岩浆带宽度变窄，早古新世岩浆活动仅发生在三隐带。始新世和渐新世岩浆活动发生在三隐带较局限的地区。黎阳及三洋带南部火成岩的地球化学特征与三音带形成鲜明对比。这可以用库拉-太平洋脊的俯冲来解释。前者具有高初始Sr和低Nd同位素比值，后者具有低初始Sr和高Nd同位素比值。混合同化分馏结晶模型计算表明，两带火成岩的地球化学差异不能归因于岩浆与上地壳物质污染程度的差异。这可能与两带岩浆来源的地球化学差异有关。在晚白垩世时期，在良轭带和三洋带南部一定存在一个类似EMII的富含地幔的小型大陆块体。三印带始新世和渐新世火成岩比白垩纪和古新世火成岩表现出更强的衰竭性质，表明始新世时，贫软流圈物质注入了三印带地幔。

Felsic magmatism in SW Japan took place in a wide area from the Ryoke Belt to the San'in Belt during the late Cretaceous (from 100 Ma to 80 Ma). After that width of the magmatic zone became narrow, and the early Paleocene magmatism occurred only in the San'in Belt. Eocene and Oligocene magmatism occurred in more restricted area in the San'in Belt. Igneous rocks in the Ryoke and southern part of the Sanyo Belts show contrasting geochemical characteristic to those of the San'in Belt. This may be explained by subduction of the Kula-Pacific ridge.The former have high initial Sr and low Nd isotope ratios, whereas the latter show low initial Sr and high Nd isotope ratios. Mixing and assimilation fractional crystallization model calculations indicate that this geochemical difference, between the igneous rocks in both belts cannot be ascribed to difference in the degree of contamination of the magma with upper crustal materials. This could be attributed to geochemical difference in the source of the magmas in both belts. A small continental blocks with enriched mantle similar to EMII must have existed beneath the Ryoke and southern part of the Sanyo Belts during the late Cretaceous. Eocene and Oligocene igneous rocks in the San'in Belt show more depleted nature than the Cretaceous and Paleocene igneous rocks, suggesting that depleted asthenospheric materials injected into the mantle beneath the San'in Belt in the Eocene.