ABSTRACT

Sm-Nd whole rock isochron ages have been determined for metadiabase and gabbro of the Ikoma Mass in the Kinki Ryoke Belt of the Southwest Japan Arc. The ages and initial ¹⁴³Nd/¹⁴⁴Nd ratios are as follows: hornblende gabbro-norite, 180±23 Ma, 0.512180±0.000024; metadiabase, 197±31 Ma, 0.512166±0.000035; anorthositic gabbro, 169±29 Ma, 0.512148±0.000027. The combined age, using hornblende gabbro-norite and metadiabase, is 192±19 Ma (0.512169±0.000021). Similar rock types collected from other masses plot around the isochrons defined by their counterparts from the Ikoma Mass. These age data indicate that the mafic igneous activity took place in the Ryoke Belt (a late Cretaceous granitic terrane) during the middle to early Jurassic. Sm-Nd and Rb-Sr whole rock and mineral isochron ages of hornblende gabbro are 97.7±5.5 Ma and 71.8±9.5 Ma, respectively, which indicate thermal resetting of the gabbro by intrusion of granitic plutons. Cortlanditic gabbros from the Ikoma Mass and other masses do not plot on the Sm-Nd whole rock isochrons for hornblende gabbro-norite, metadiabase and anorthositic gabbro. This suggests that cortlanditic gabbros evolved independently from other gabbros. Some Ryoke gabbroic masses, including the Ikoma Mass, consist of a complex array of cumulate which crystallized from several different mafic magmas, representing different ages and sources. Gabbroic rocks which formed in the lower crust have the same Sr and Nd isotopic character as the Ryoke gabbros.