High initial Sr-isotopic ratios of gabbro and metadiabase in the Ryoke Belt, Southwest Japan.

Abstract

Rb and Sr contents and Sr-isotopic ratios as well as Si02 and K20 contents were determined for basic rocks (gabbro, cortlandite, anorthosite and metadiabase) from the Ryoke Belt in the Kinki and Setouchi Inland Sea districts, Southwest Japan. The basic rocks are characterized by lower K20 and Rb contents and by higher initial Sr-isotopic ratios than the mantle-derived igneous rocks in the oceanic region. The high initial Sr-isotopic ratios of the basic rocks are explained by the high isotopic ratios of their source materials.

Sm-Nd isochron ages of mafic igneous rocks from the Ryoke Belt, Southwest Japan: Remains of Jurassic igneous activity in a late Cretaceous granitic terrane.

“…The gabbros are extremely depleted in K20 and Rb, and have high initial 87Sr/86Sr ratios, which are distinctly different from mantle derived igneous rocks from oceanic regions (Kagami et al, 1985;Iizumi et al, 1990). Metadiabase has slightly higher K20 and Rb than the gabbro.…”

“…Rb and Sr concentrations, and Sr isotopic data for most samples have been reported by Kagami et al (1985). Brief petrographic de scriptions were given in the text and Table 1 of Kagami et al (1985). Chemical compositions of some analyzed samples are shown in the Appen dix.…”

“…On the other hand, sedimentary and mafic igneous rocks of the Ryoke Belt were thermally affected by intrusion of vast granitic plutons. The mafic igneous rocks are divided into two rock types; gabbros, and fine-grained massive, metamorphic granular rocks which are tentatively called metadiabase (Kagami et al, 1985). Few geochronological studies of these mafic rocks have been carried out, although the existing age data have been referred to by many geologists.…”

“…Several authors have attempted to determine the intrusive ages of these rocks in the Kinki Ryoke Belt. Kagami et al (1985) measured their Sr isotopic ratios, but the ages were unreliable because of the extremely low and restricted ranges in Rb/Sr. Kutsukake and Nakai (1983) reported a fission track zircon age of 101 ± 7 Ma for a gabbroic pegmatite.…”

Two contrasting types of Rb-Sr isotope systems for the Funatsu granitic rocks in the northwestern part of the Hida belt, central Japan.

“…Consequently high Sr concentrations of samples from the Shokawa intrusion (Fig. 4) This ten dency is recognized in many granitic rocks with Cretaceous to Paleogene ages widely exposed in the Inner Zone of Southwest Japan (Shibata and Ishihara, 1979;Kagami et al, 1985 , 1977;Fleck and Criss, 1985), British Caledonian granites and the Cretaceous-Tertiary granitic rocks in Japan (Shibata and Ishihara, 1979).…”