抄録
The northern Shimanto belt in southeastern Tokushima prefecture contains many in-situ basaltic to andesitic greenstones which were intruded into and extruded onto Cretaceous terrigenous clastic sediments. The greenstones show varying degrees of alteration, with a range from weakly to strongly altered samples observed at single outcrops. Analyses of major and trace elements in 25 samples show that although K2O, Rb and Ba in the more altered greenstones are strongly depleted, original abundances of total alkalis (Na2O+K2O) and the remaining elements are retained. The greenstones are classified into three groups: (1) high-P2O5 alkaline rocks, (2) high-Ce alkaline rocks and (3) low-Ce alkaline rocks and high-alkali tholeiites. On MORB-normalized spidergrams all samples show the high LILE/HFSE ratios and distinct negative Nb anomalies characteristic of subduction related magmas.
Cretaceous in-situ greenstones also occur in the Naze Formation on Amami Island. They consist of both intraplate and subduction-zone alkaline types, along with high-alkali tholeiites (Yoshida et al., 1994). These magmatic suites are similar to Miocene igneous complexes located in the southernmost part of the outer zone of Southwest Japan which formed on a structural high in a forearc basin. High alkali contents of the greenstones indicate that the primitive magma from which they were derived were segregated from relatively deep mantle (>50-70 km) in a subduction-zone. Our results suggest that the in-situ greenstones were formed by igneous activity on the Cretaceous forearc structural high in a Ryukyu-type subduction zone.
