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Neodymium isotopic compositions of Silurian Yakuno metagabbros.

1992; Japan Association of Mineralogical Sciences; Volume: 87; Issue: 7 Linguagem: Inglês

10.2465/ganko.87.272

1881-3275

Sakae Sano,

Geochemistry and Elemental Analysis

Radiometric age and origin of metagabbros and the related plagio-granites constituting "Yakuno ophiolite" in Maizuru Tectonic Belt, southwest Japan, have been examined by using the Nd isotopic compositions. It has been considered that the metagabbros are equivalent to lower parts of unusually thick oceanic crust, because the metamorphism corresponds to amphibolite ?? granulite facies. Based upon K-Ar and Rb-Sr mineral age determinations, the range of radiometric age of 240-280 Ma has been reported. The Sm-Nd ages of the metagabbros and the related rocks are 409-426 Ma, and the initial 134Nd/144Nd ratio is 0.512265-0.512300 (εNd=+3.4-3.7). The obtained ages are enough older than results of previous works. These ages must represent igneous event such as the solidification from a magma. Ages including clinopyroxene which is product of metamorphism of granulite facies condition, are also same with that of isochron of whole rocks. Ages reported from previous workers maybe represent the events that the minerals such as amphiboles and micas are recrystallized during metamorphism of any later stages, or passed through the closure temperatures of the system. Therefore, metamorphism of granulite facies should be followed to the magmatic process. The ages of 409-426 Ma of the gabbros and the related rocks of the "Yakuno ophiolite" is similar to that of age (about 400 Ma) of metagabbros in the Sangun-Renge Belt where locates on different tectonic unit. Based on age similarity the gabbros of the "Yakuno ophiolite" may at least belong to member of mafic-ultramafic complexes in the Sangun-Renge Belt. The obtained εNd values of the gabbros at 409-426 Ma are +3.4-+3.7. These values are quite lower than those from recent oceanic basalts. On the other hand, it has been reported that the metabasalts from the Ibara district belonging to the Maizuru Tectonic Belt show higher εNd values of +7.7, and younger age of about 300 Ma. Furthermore, it has been preliminary reported that Rb-Sr whole rock radiometric age of metabasalts in the Yakuno district is 285 Ma. It means that there is no genetic relations between the metagabbros in the Yakuno district and metabasalts in both Yakuno and Ibara districts. Metagabbros and the related rocks in the Yakuno ophiolite must represent lower crustal materials showing less depleted Nd isotopic signature at Silurian Epoch.