Geochemical and geochronological examination on Mesozoic continental lithosphere beneath SW Japan

日本西南部的长英质岩浆活动发生在晚白垩世（100 ~ 80 Ma），范围从良屋带到山阴带。此后，岩浆带宽度变窄，古新世早期岩浆活动仅发生在山阴带。始新世和渐新世岩浆活动发生在山阴带较局限的区域。与山阴岩带相比，柳屋岩带和山阳岩带南部的火成岩具有明显的地球化学特征。这可能与库拉-太平洋洋脊俯冲作用有关，前者具有高初始Sr和低Nd同位素比值，而后者则具有低初始Sr和高Nd同位素比值。混合和同化分离结晶模型的计算表明，这种地球化学差异，在两个带的火成岩之间不能归因于不同程度的岩浆与上地壳物质的污染。这可能是由于两带岩浆来源的地球化学差异所致。晚白垩世在良屋带和山阳带南部存在类似EMII的富集地幔的小陆块。山阴带的始新世和渐新世火成岩比白垩纪和古新世火成岩显示出更强烈的亏损性质，表明亏损的软流圈物质在始新世注入山阴带之下的地幔。

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.